T23C-2040
Dynamic coupling and coeval contraction/extension in and around orogenic plateaux
Once an orogenic plateau has reached a critical thickness and thermal structure, it flows under its own weight. This gravitational flow may compete with far-field contraction (continued plate convergence), go along with far-field extension (slab retreat, etc.), or work against stationary lowland. In this study, we explore the dynamics of flow from orogenic plateaux (thick/hot crust) toward lowland regions for a wide spectrum of far- field boundary velocities, from contraction to extension. In particular, we investigate the consequences of plateau centrifugal flow on structures in the lowland upper crust (thrusts, normal faults) and lower crust (channel of low-viscosity crust flowing laterally beneath the lowland), as well as structures in the plateau, like the style of flow of lower crust, and the presence/absence of core complexes that "intrude" the upper crust. We use Ellipsis 2D numerical modeling to construct a plateau and a lowland region as a starting condition. Boundary velocities are applied for various cases of contraction and extension, including a velocity of zero (fixed boundary). In all cases, we input a heterogeneity (dipping fault) in the upper crust of both the plateau and the lowland to help localize deformation. For a far-field velocity of zero, material from the plateau is transferred to the lowland, i.e. normal faulting in the plateau upper crust is accommodated by thrust faulting in the lowland. Flow of lower crust from plateau to lowland may take place if the lowland crust is relatively weak. When an extensional velocity is applied at the model boundary, thrust faulting in the lowland is still activated because the gravitational flow velocity exceeds the extension velocity. When boundary extension is sufficiently fast, lowland upper crust thrusting is inhibited, yet extension of the plateau persists and core complexes cored by low viscosity crust develop. When boundary contraction is applied, the lowland is characterized by distributed thrusting while the plateau still develops extension structures and core complexes. This modeling demonstrates that lowlands and orogenic plateaux can have coeval contraction and extension dynamics that are not readily related to boundary velocities. In many orogens, both active and exhumed, foreland thrusting is likely coeval with plateau extension during the transition from far-field contraction to extension.
T23C-2041
Role of Erosion law and Rocks Erodability on Orogens Evolution from 2D Thermo- Mechanical Modeling.
Orogen landscapes are shaped mainly by a competition between erosion and tectonic deformation. However
the role of erosion laws used in numerical modeling is still a matter of debate. Here we test commonly used
erosion laws including slope dependent erosion, diffusion and river incision. In particular we focus our
approach on the effect of the long time scale (around 10 Ma) evolution of the exhumed rocks strength
during an orogen.
To compare the localization and amount of erosion, deformation and uplift for each
erosion law we use a 2D finite element model ADELI, which has been modified to include surface remeshing.
Mechanical layering as well as thermal properties of continental lithosphere are taken into account to model
evolution of orogenic landscapes. Our results shows that the assumed erosion law has a primary effect on
the localization of deformation as well as on the P-T-t paths of exhumed rocks, and on the topographic profile
of a range.
On the other hand, erodability of rocks at the surface is also suspected to have a major impact
on erosion efficiency. We have in particular tested the scenario of having the erodability evolving through the
history of a mountain range. To simplify our approach surface rock strength is described by only one
parameter called erodability, which includes some rheological properties of exhumed lithologies. We
hypothesize that the progressive exhumation of high grade metamorphic rocks leads to an increase of
surface rocks strength, i.e. to a decrease of its erodability. Our numerical results reveal that this temporal
evolution of erodability can greatly affect the way an orogen deforms and evolves, and they suggest that
such time variation of erodability must be considered to interpret the evolution of an orogen from its
denudation history.
T23C-2042
Crustal Structure Along the Transantarctic Mountain Front Using Receiver Functions
This study investigates crustal thickness variation along the front range of the Transantarctic Mountains using teleseismic waveforms recorded by the Transantarctic Mountain Seismic Experiment (TAMSEIS). This experiment deployed 44 three-component broadband seismometers from 2000-2003 and was divided into three arrays: Coastal, East-West, and North-South. Receiver functions have been computed for stations that are on bedrock along the Coastal and East-West arrays to avoid complications caused by reverberations in the ice layer. Estimates of crustal thickness and bulk Vp/Vs ratio have been obtained through the H- κ stacking method of Zhu and Kanamori (2000). The P-to-S converted phase across the crust-mantle boundary (Ps) is clearly observed in all the stations, but the reverberations in the crustal layer (PpPs, PsPs+PpSs) are difficult to identify. The lack of multiples likely indicates that the crust-mantle boundary is gradational, and it makes it difficult to estimate bulk Vp/Vs ratios accurately. Variations in earth structure around the stations have also been investigated, but no dependence of crustal properties with back-azimuth has been found. Preliminary results indicate the crust beneath the crest of the Transantarctic Mountains is ~39 km thick, in contrast to thicknesses between 30 and 35 km along the mountain front.
T23C-2043
A Geophysical Characterization of the Orogenic Rifting to Seafloor Spreading Transition in Western Woodlark Basin, Papua New Guinea
The Woodlark Basin, forming as the mountains of Papua New Guinea collapse, provides a unique opportunity to capture the complete orogenic cycle as continental collision gives way to orogenic collapse, rifting and finally, seafloor spreading. Within the basin itself is the only place on Earth where the instant of spreading center initiation has been fully imaged geophysically. Ring dikes and high-acoustic backscatter on the seafloor are coincident with a high amplitude positive magnetization anomaly. Multichannel seismic lines image intrusions that represent a nascent spreading center, offsetting the two sides of the shallow angle normal fault (~30°) bounding the north side of Moresby Seamount. Through an analysis of the bathymetry, magnetic, gravity, and reflection seismic data the shape of the shallow angle fault as it interacts with the nascent spreading center has been fully characterized and the volume of material accreted estimated. A number of questions are addressed: 1) what is the stress state at the transition zone; 2) how does the transfer from strain accommodation by faulting to strain accommodation by accretion takes place; 3) what is the stratigraphic response to the transition; and 4) how do the intrusives interact with the syn-rift sediments. A model is presented that describes in detail the process by which a nascent seafloor spreading center nucleates in a rift basin as orogenic rifting draws to a close and a new ocean basin is born.
T23C-2044
Constraints on the Low Temperature Thermal History of the Tsagan Subarga and Tavan Har Basement Blocks of the East Gobi Fault Zone, Southeastern Mongolia, and Tectonic Implications
The East Gobi Fault Zone (EGFZ) records multiple episodes of intracontinental deformation associated with a protracted Mesozoic and Cenozoic tectonic history. We present preliminary results from an ongoing study focused on constraining the low temperature thermal history of fault-bound basement blocks within the EGFZ. The Tsagan Subarga and Tavan Har basement blocks expose northeast-striking metamorphic tectonites largely in fault contact with variably deformed Late Paleozoic and Mesozoic age rocks. These tectonites record sinistral deformation ca. 225 Ma at amphibolite-facies conditions. The ductile shear zone can be traced for ~250 km where the exposed structural level appears to deepen to the northeast. The boundary faults that define each basement block have a complex history including activity as: (1) normal faults during Late Jurassic - Early Cretaceous rift basin formation, (2) thrust faults during post-rifting inversion, (3) and sinistral strike-slip faults during the Cenozoic. Previous 40Ar/39Ar work suggests that tectonites began cooling during the Late Triassic, likely in response to regional exhumation. At ~300 °C the cooling histories for rocks exposed at Tsagan Subarga and Tavan Har diverge with minimum K-feldspar ages suggesting final cooling through ~150 °C during the Late Triassic at Tsagan Subarga and Middle - Late Jurassic at Tavan Har. Thermal modeling of apatite fission track (AFT) data from samples collected at Tavan Har support the 40Ar/39Ar results and suggest relatively rapid cooling through the partial annealing zone (110-60 °C) during the Late Jurassic - Early Cretaceous. When combined with the observation that Late Jurassic - Early Cretaceous rift basins generally deepen to the northeast from Tsagan Subarga to Tavan Har, the 40Ar/39Ar and AFT results suggest that differential exhumation, resulting from a northeastward increase in throw during normal slip on the basement block boundary faults is, in part, responsible for exhumation of the tectonites from depth and likely the controlling factor for the deeper exposed structural level of the ductile shear zone to the northeast.
T23C-2045
How Cylindrical is the Main Himalayan Thrust? Insights from Low - Temperature Thermochronology and Numerical Modelling
We study the recent dynamics of the Himalayan orogen in central Nepal with the specific goal of quantifying the onset of activity and the deformation history recorded by the different major thrusts along the Himalayan range, and propose a structural and kinematic model of the major crustal Himalayan thrust, the MHT. We report 27 new apatite fission-track (AFT) ages collected along north - south transects in western and eastern – central Nepal (Kali Gandaki and Trisuli Rivers). AFT ages are consistently young (< 3 My) along both N-S transects in the MCT zone and increase (4 to 6 My) toward the south in the Lesser Himalaya. We present and compare 2 age - elevation transects, one in the MCT zone and one in the outer Lesser Himalaya, and interpret them in terms of exhumation rate that we use to constrain the geometry of the MHT ramp. The Himalayan range is commonly presented as a cylindrical structure from west to east. However, geological structures, topography, precipitation rate, convergence rates and low - temperature thermochronological ages all vary significantly along strike. Here, we focus on the interpretation of thermochronological datasets in term of cylindricity in geometry and kinematics of the MHT along the Himalayan range. We compare our new data to published low-temperature thermochronological datasets for western - central Nepal, eastern - central Nepal and the Bhutan Himalaya. We use these data to perform numerical thermal- kinematic modelling with a modified version of the PECUBE code, in order to constrain potential along-strike variations in the kinematics of the Himalayan range. Our results show that lateral variations in geometry of the MHT (in particular the presence or absence of a major ramp) strongly control the kinematics and exhumation history of the orogen.
T23C-2046
Quantifying Relationships Between Mid-Crustal Metamorphism and Exhumation-Erosion. A Case Study From the Greater Himalayan Sequence of the Everest-Makalu Massif, East Nepal
Metapelites and anatectic granites from the upper structural levels of the Greater Himalayan Sequence (GHS) in the Everest-Makalu area of east Nepal record a history of prograde burial metamorphism, decompression melting and exhumation-erosion. This area presents itself as an ideal case study for understanding the links between these processes. Peak sillimanite-grade metamorphism in metapelitic migmatites is associated with crustal anatexis and is quantified at 713±107 °C and >5.9±1.8 kbar. However, a mapped area of the metapelite with a cordierite grade metamorphic overprint is observed along the uppermost levels of the GHS. This secondary overprint is quantified at 618±58 °C with pressures as low as 2.1±0.9 kbar and is associated with additional crustal anatexis producing cordierite leucogranites related to decompression. U-Pb monazite and xenotime geochronology on both metapelites and granites date this decompression part of the P-T path at 18.7±0.7 Ma to 16.1±0.2 Ma despite melting and high-grade metamorphic conditions being prevalent prior to this time. Such rapid decompression is thought to be the result of flow of ductile mid crust from beneath Tibet (see Law et al, 2006, and references therein). This forms a 'channel flow' system between a lower thrust fault (Main Central Thrust) and an upper low-angle normal fault (South Tibetan Detachment). Decompression to 2.1 kbar would have brought rocks to within ~8 km of the surface, yet their temperatures remain relatively elevated at this point. Zircon and Apatite Fission Track data from samples collected at a range of altitudes between 860m and 6500m then reveal a much slower, protracted cooling of these rocks from 16 Ma onwards. We interpret this to represent slower exhumation largely due to erosion. This demonstrates the transition from mid-crustal gravity driven ductile deformation (channel flow) to brittle-ductile exhumation-erosion implying that ductile flow had ceased by then, at least in rocks now along the Himalayan crest. Law, R. D., Searle, M. P. and Godin, L. (Editors) 2006. Channel Flow, Ductile Extrusion and Exhumation in Continental Collision Zones. Geol. Soc. London, Special Publication 268.
T23C-2047
Polyphase Extension at the Southern Margin of the Tibetan Plateau, Ama Drime Range, Tibet
The physiographic transition at the southern margin of the Tibetan Plateau is broadly coincident with the
trace of the major detachments of the South Tibetan fault (STF) system. The STF system has been assigned
an Early-Middle Miocene initiation age based on its relationship with leucogranites in the Greater Himalaya,
but recent geomorphic and structural studies of this transition suggest that extensional deformation may have
persisted along the STF system into the Pliocene and Pleistocene. Our recent work in the Ama Drime Range
of southern Tibet (ca. 28°30'N-28°50'N; 87°20'E- 87°45'E) adds to this growing
body of evidence.
The range itself is a N-S transverse feature bound on the E and W by high-angle normal faults that display
Quaternary scarps but probably initiated in Early Pliocene time (Jessup et al., 2008). These extensional
features cut ca. 16 Ma E-W-striking STF detachments (Hodges et al., 1994), which are preserved in both the
hanging wall and footwall of the range-bounding faults. We have found strong evidence for a third major
structure, also E-W striking, that occurs SE of the Ama Drime range and is truncated by the high-angle
Quaternary normal fault on the east side of the range. The structure in question separates Greater
Himalayan sequence rocks on either side of a WNW-ESE-trending valley that is controlled by the fault.
Although we have found abundant tectonite fabrics in these rocks consistent with the existence of a major
structure in the valley, the fault surface itself is not exposed in the areas we have been able to reach during
the early stages of fieldwork. The principal evidence for the character of the fault is that the rocks on either
side of the structure have distinctive single crystal (U-Th)/He zircon and apatite cooling ages that are
consistent with down to the north normal-sense displacement after ca. 7 Ma.
There is some evidence that another E-W-striking structure related to a very young phase of STF
deformation occurs at the southern end of the Ama Drime Range: 1) large river knickpoints coinciding with 2)
loss of north-south trending boundary fault traces in digital imagery and 3) an E-W trending array of zircon
(U-Th)/He ages (< 2.8 Ma) that are younger than many of the apatite (U-Th)/He ages previously reported
from the interior of the range. If this feature exists, it would be the youngest STF structure in the region and
would extend the known age range of STF deformation to the Late Pliocene or Pleistocene Epoch.
Hodges et al. (1994) Contrib Min Pet v117
Jessup et al. (2008) Geology v36
T23C-2048
Preliminary fission track ages of detrital zircons from modern fluvial sediments along Yarlung-Tsangpo, South Tibet
The Tibetan plateau has long been recognized as the most active orogen in the world due to its spectacular elevation and extent. Studies from various disciplines have suggested that the plateau reached its current altitude not only separately but also dischronously. It can be roughly illustrated by three major phases: (1) the N-S shortening stage started at 50-55 Ma, in which thrust fault systems have been developed in the front of the Indian Plate; (2) the lithosphere thinning stage during the Miocene, which have allowed the thick crust to rise up to its modern elevation before ~8 Ma; (3) the slow E-W extension stage, which has reduced the crustal thickness and may have decreased the elevation of the plateau. The scenario of three-stage uplift can be delineated by the topographic features and mineral cooling histories; however, these relevant studies have demonstrated a discrepancy between erosion rates among north, central and south Tibet. To further solve this problem, this study particularly aims at the timing and rates of the relief changes. Modern detrital minerals collected for fission track dating have been collected along the Yarlung-Tsangpo and its two tributaries, Lhasa River and Nyang River. The preliminary results of zircon fission track (ZFT) from the sample collected near Lhasa display fairly confined young age peaks that spans from 10 Ma to 6 Ma with a pooled age ~8 Ma. The ZFT ages are much younger than the surrounding strata of Lhasa terrain (Proterozoic to Mesozoic) and the intrusive Gangdese belt (Mesozoic to Mid-Miocene); therefore they indicate denudation age rather than intrusion age. This recent exhumation signal agrees with the sedimentary strata recorded in the drill core of Bengal Fan, suggesting a rapid exhumation phase in 10-6 Ma.
T23C-2049
Collisional versus coastal accretionary prism in the Andes at 35° S: a comparison
At the Pacific convergent margin of South America around the latitude 35° S two fundamental types of accretionary systems occur: (1) the Paleozoic collisional accretionary prism of the Guarguaraz Complex at the suture between the Cuyania and Chilenia microplates in W Argentina (longitude 69° W) and (2) the late Paleozoic coastal accretionary complex within the Coastal Cordillera of central Chile (longitude 72° W). Both complexes are composed of continent-derived, high-pressure metasediments with intercalations of disrupted oceanic crust. In the coastal accretionary complex two modes of accretion characterized by specific PT-conditions evolved with time: Early frontal accretion at 4-6 kbar, 250-300° C changed to basal accretion at 7-10 kbar, 350- 400° C with pronounced near isobaric thermal equilibration after emplacement at maximum depth. Exhumation rates are slow (around 0.5 mm/a) with erosion as prime exhumation factor. Rare rocks were incorporated in the coastal accretionary complex that show higher pressures than 10 kbar and hence originated in a subduction channel, where forced flow is the predominating mass flow process. By contrast, the predominating PT-conditions in the collisional accretionary prism of the Guarguaraz Complex are higher at 13-14 kbar, 450-550° C with fast near-isothermal exhumation following short thermal equilibration at maximum depth. By analogy, forced flow is envisaged as predominant mass flow process in the collisional accretionary prism. The contrasting evolution of both tectonic environments is important for unravelling the history of continent collisions.
T23C-2050
Can Protracted Tectonic Constraints Associated to a Slow Cooling Rate Prevail for 35 My ? The Study Case of the Quartzo Syenite of Ibituruna (SE-Brazil)
The Aracuai belt (south-East of Brazil) is composed of a stack of High Temperature-Low Pressure (HT-LP) nappes and results from the late amalgation of the Gondawana supercontinent. The Ibituruna quartz-syenite is a laccolith localized between a mylonitic allochtonous unit to the west and a large tonalitic domain to the east (the central domain of the belt). Previous studies have classified this batholith as post-orogenic and intrusive, however our field observations with: i) a strong and consistent magmatic fabric that matches the one of the country rocks, ii) a lack of tectonic extension or other post-orogenic clues and iii) an absence of contact metamorphism, led us to reconsider the emplacement of the Ibituruna massif coevally with its country rocks. To settle on the good scenario, we performed a magnetic study using the Anisotropy of Magnetic Susceptibility (AMS) coupled with a magnetic characterization using AARM and pAARM in complement to field observations. In addition, we estimated the P-T conditions in the country rocks on thin rock-sections, and finally we extracted U/Pb ages (LA-ICPMS and SHRIMP) and 40Ar-39Ar ages from several minerals collected in the syenite and its host rocks. The magnetic foliations and lineations in the syenite match well with the magnetic and field measurements acquired in the country rocks and comfort our first hypothesis of a coeval deformation of the syenite and its host rocks. Ages found in the syenite and in synkinematic leucocratic veins in the country rocks are distinct of about 35 My with 535 My and 570-580 My respectively. These ages are more consistent with a late emplacement of the syenite in the country rocks rather than syn to late emplacement. Here we point out a paradoxal situation. While the syenite displays all aspects of a laccolith deformed with its host rocks, ages record in the different units show a discordance of about 35 My and suggest a late intrusive emplacement. From our point of view, this situation could be explained by a very slow cooling rate on the whole area or a protracted deformation, or more probably, a mixing of the two phenomena. A few clues indicate a low cooling rate such as: i) equilibrated microstructure in the syenite (triple junction) ii) thermal overgrowths on zircons iii) post- kinematic grain growth of quartz in the mylonites. Using 40Ar-39Ar ages extracted from minerals of the different units, we are able to deduce the primary stages of the thermal history which clearly describe a low cooling rate, with 3 degrees C per My between 570 and 500 My, and 5 degrees C per My between 500 and 460 My. While combining AMS, field and thin rock sections observations with isotopic dating and thermochronolgy, it is still difficult to decipher on the mystery of the Ibituruna syenite emplacement. However, through this study we demonstrate the necessity to use and cross as much as possible techniques in order to clear up hidden zones. Such results might be characteristic to particulars regimes associated to hot orogens with HT-LP conditions with a slow cooling rate and protracted tectonic constraints or multiple deformation stage at High Temperature.
T23C-2051
U-Pb and K-Ar Geochronological Constraints on the Tectonothermal Evolution of the Precambrian Terranes of the SE São Paulo State, Brazil
The Ribeira belt, the major tectonic unit of the Mantiqueira Province, southeastern Brazil, records the interface between the Congo-São Francisco, Kalahari, La Plata cratons and the Paranapanema cratonic block. The belt is made up of many terrains of different nature, and was generated and affected by the tectonomagmatic episodes of the Brasiliano orogenic cycle, the South American equivalent of the Pan- African Orogeny. Aiming at the characterization of the thermal history of Pre-Cambrian terranes of the southeastern of São Paulo state, specific geochronological studies were carried out on mylonitic and granitic rocks. This sector of Ribeira Belt consists of four major tectonic domains limited by significant shear zones, associated with Neoproterozoic events. The Embu Domain, north of the Cubatão Shear Zone (CSZ), is composed of metasedimentary rocks, and peraluminous granites, that yielded U-Pb ages of ca.790 Ma, 620 Ma and 600 Ma. Gneiss-migmatite rocks (640-620 Ma) and intrusive granites (580 Ma) predominate in the Mongaguá Domain, which is limited by Cubatão and Itariri shear zones. The Registro Domain, between Cubatão - Itariri Shear System (CISS) and the Serrinha Shear Zone (SSZ), is formed by metasediments and granitic rocks with migmatitic features represents a Paleoproterozoic terrane (1.9-2.2 Ga) strongly deformed during the Neoproterozoic (750-580 Ma). Rocks of the Iguape Domain, limited to the north by the SSZ, include granites (ca. 600 Ma) and low grade metasediments. The Itariri and Cubatão mylonites occur as high- and low-temperature varieties, formed in amphibolite and greenschist facies conditions, respectively, and the Serrinha mylonites developed under amphibolite facies. U-Pb zircon and monazite geochronological evidence indicates a short time interval at the end of Neoproterozoic for the blocks juxtaposition. From the compilation of geochronological data and apatite fission-track thermochronology, a temperature vs time correlation diagram was obtained and both sequences of processes with slower and faster cooling rates along the geological history could be identified.
T23C-2052
New SHRIMP U-Pb and 40Ar/39Ar constraints on the crustal stabilization of southern South America, from the margin of the Rio de Plata (Sierra de Ventana) craton to northern Patagonia
Two models exist to explain the late Paleozoic tectonic history for southern South America: an accretionary model of crustal growth through magmatism and a collisional model involving pre-existing continental elements, namely, the Rio de Plata craton and the possibly allochthonous terrane(s) of Patagonia, the Northern Patagonia Massif and the Deseado Massif. We report new U-Pb and 40Ar/39Ar results from rocks within a posited collision zone between the SW edge of the Rio de Plata craton and the northern margin of the Northern Patagonia Massif. Igneous basement samples from the Sierra de Ventana region, Buenos Aires province, were dated by ion microprobe (SHRIMP) analysis of zircon. A previously unrecognized occurrence of Paleoproterozoic basement indicates that the Rio de Plata craton extends ca.250 km farther west than considered. The majority of the basement rocks are shallow mid-Cambrian granitoids and rhyolites, including the rocks of the Cerro Colorado granite, which is intrusive into the sediments of the Curamalal Gp, signifying that these mature quartzites and conglomerates are older than early Cambrian in age, possibly correlated with the low-grade sedimentary rocks of the Tandilia Range that includes the La Tinta Fm. The 40Ar/39Ar ages from biotite, muscovite, and sericite from three different sheared basement localities demonstrates deformation in the latest Permian (265-260 Ma), ca. 20 Ma after the foreland deposition of the synorogenic Tunas Fm. in the upper Pilahuinco Gp, constrained by 282.4 ± 2.8 Ma zircon ages in volcanic ashbeds. Farther south, along the northern margin of the Northern Patagonian Massif, late Ordovician 40Ar/39Ar cooling ages of granites intrusive into the Cambro-Ordovician Nahuel Niyeu Fm. are consistent with the presence of Ordovician magmatism along the W edge of the Rio de Plata craton. These ages alternate with late Permian 40Ar/39Ar cooling ages from undeformed granites and pegmatites, as well as early Jurassic cross-cutting dykes that are likely related to opening of the South Atlantic. Overall, the cooling age heterogeneity indicates that the thermal effects of igneous activity were localized, typical of higher crustal levels, whereas age homogeneization in the wake of crustal thickening and thermal relaxation should occur along the inferred collision zone. Based on this preliminary data, we suggest that the foreland propagation of deformation and discrete magmatism during the late Paleozoic are more consistent with accretionary tectonic models.
T23C-2053
Thermal History of the Central Pyrenees: Combining Low-Temperature Thermochronology and Thermal Modeling to Constrain the Formation of Convergent Orogens
Constraining the timing of thrusting is fundamental to understanding the tectonic evolution of convergent orogens. Without external constraints, it is difficult to directly tie low-temperature thermochronology to thrust fault activity in convergent orogens. This is largely because thrust faults do not exhume, and therefore do not directly cool, rocks. Topography generated in convergent orogens can cool rocks, either via lateral heat loss from the flanks of orogens ("topographic cooling") or more typically by increasing erosion rates. Neither of these scenarios can be simply applied to the Pyrenees. Prior to the Late Cretaceous initiation of convergence between the Iberian and European plates, the region that makes up the Pyrenees was part of a broad shallow marine basin. Many of the major thrust faults that were active during convergence are inverted low-angle extensional faults. Therefore, a significant amount of convergence and thrust faulting was accommodated prior to emergence above sea-level of the proto-Pyrenees, and hence there was no significant erodable topography. Topographic cooling is unlikely to be significant in the Pyrenees due to their relatively long wavelength and moderate amplitude. However, rocks in the footwall of thrust faults will record the onset of faulting as heating events. In the Pyrenees we are applying 40Ar/39Ar K-feldspar multi-diffusion domain (MDD) modeling and apatite fission track (AFT) thermochronology to examine the onset and magnitude of thrust faulting in the Axial Zone of the central and west-central Pyrenees. Both MDD modeling and AFT thermochronology can record heating events and can therefore provide quantitative constraints on the timing and rate of thrust fault activity in convergent orogens. MDD thermal models from granitic massifs in the Orri thrust sheet (the footwall of the Gavarnie Thrust) suggest reheating beginning at 65-60 Ma. This is interpreted to reflect burial due to thrusting beneath the Nogueres thrust sheet (hangingwall of the Gavarnie Thrust) and the onset of thrust fault activity in the Pyrenean Axial Zone. These samples within the Orri thrust sheet reached maximum temperatures (up to 280°C) at 45-50 Ma, after which they were rapidly cooled. Rapid cooling at this time is consistent with AFT data and thermal models, as well as apatite (U-Th)/He ages. Modeled maximum temperatures suggest that the Orri thrust sheet was buried more deeply in the east than the west, consistent with geologic constraints that indicate a westward decrease in total convergence and shortening throughout the Pyrenees. MDD thermal models from the Neouvielle pluton in the Nogueres thrust sheet are not consistent with reheating, but instead suggest extended residence at relatively high temperatures (>230°C) prior to rapid cooling beginning ~50 Ma. The Eocene-Oligocene onset of rapid cooling is recorded in both the Orri and Nogueres thrust sheets, and is interpreted to represent rapid erosional exhumation, following thrusting that created topographic relief. AFT age data and thermal models, as well as apatite (U-Th)/He ages suggest that erosion and exhumation occurred asynchronously across the orogen. Throughout the study area, AFT and (U-Th)/He ages do not record thrust-fault activity, but instead reflect cooling during erosional exhumation. In the case of the central and west-central Pyrenees, the onset of thrust fault activity predates rapid erosional exhumation by as much as 30 m.y.
T23C-2054
Metamorphic Rates in Northern Part of the Central Alps
Understanding the thermal structure evolution during the development of collisionnal orogeny requires studies combining petrology, geochronology and tectonic evolution. Early metamorphic rates are difficult to quantify, simply because few chronometers preserve isotopic signatures of prograde metamorphism, without adjustment upon further heating. In this study, we address the potential of REE minerals to record the timing and the heating rates of early collisionnal stages. Samples from a section across the low- and medium-grade northern part of the Central Alps were studied with the aim of unravelling the dynamics of the Barrovian metamorphic evolution. Detailed petrological work, aided by thermodynamic modeling, allow us to recognize the prograde sequence of assemblages involving REE-minerals. In metapelites with low CaO/Na2O ratios, relic allanite formed at 430-450°C (Ctd-in) are partly replaced by monazite at temperatures above 560-580°C (postdate garnet growth). In individual samples, dating of coexisting allanite and monazite delimit in time successive stages of the Barrovian metamorphism. In situ SHRIMP U-Th-Pb dating of allanite (29.2-35.5 Ma) and monazite (18.0-19.1 Ma) constrains the time elapsed between 430-450°C and 560–580°C, which implies an average heating rate of 8–15 °/m.y. This heating rate pertains to a T-interval, in which major reaction progress occurs in meta¬clastic rocks (e.g. garnet growth). Combined with new fission track ages (zircon: 10–9 Ma; apatite: 7.5–6.5 Ma), metamorphic rates of the entire orogenic cycle, from prograde to final cooling, can be reconstructed.
T23C-2055
Thermal Modeling of an Area N-W of the Larderello Geothermal Field, Italy.
A wide area enclosed between the ancient Etruscan town of Volterra and the northern rim of the Larderello high enthalpy geothermal field (Tuscany, Italy) shows thermal features which suggest further investigations aimed at mid-low enthalpy geothermal energy exploitation. Thermal gradients are in the range 75 - 100 C°/km, while surface heat flow spans between 100 - 150 mW/m2. Numerical simulations were performed to predict the spatial distribution of temperature and fluid circulation paths, constrained by field data. Temperature control based on real data is allowed by a few deep exploratory geothermal wells along with several shallower gradient wells, down to a maximum of about 3 km. The model domain extends over an area 20 by 10 km; thickness is 6 km. Local geology is simplified in four different terrains, according to the generalized stratigraphy of the area. Several sets of simulations were carried out running SHEMAT and TOUGH2 numerical codes, considering various boundary conditions, inner geometries and hydraulic permeabilities. The model was realized by means of unsteady forward simulations, under the assumptions of impervious and isothermal top and bottom boundaries, lateral adiabatic faces and variable internal physical properties. The results indicate that the present temperature and pressure distribution of hot fluids with depth in the northern border area of the Larderello field allows to hypothesize a fruitful exploitation of the medium- enthalpy geothermal resources, possibly with low-boiling point fluids binary plants for electricity generation.
T23C-2056
Detrital Zircon U-Pb Ages of Late Silurian and Early Devonian Sedimentary Sequences From Northwestern Svalbard, Implications for Regional Correlations in the Arctic Caledonides
In the North Atlantic region, Caledonian and Grenvillian terrains are exposed on Svalbard, Greenland, Great Britain, Canada and Scandinavia. Reconstructing the original configuration of these terrains depends partly on determining the age and provenance of the sedimentary rocks overlying the basement. Provenance investigations using detrital zircon U-Pb ages are a powerful tool to explore the palaeogeography and can yield important information how the North Atlantic region evolved through the Grenvillian and Caledonian orogenies. U-Pb laser ablation ICP-MS provenance study of zircons from Late Silurian and Early Devonian coarse siliciclastics of NW Svalbard indicate an east Greenland provenance. The decrease in 410-440 Ma ages and 2600-2800 Ma ages, combined with a large increase in 950-1000 Ma ages up-section suggests that sediment detritus was initially derived from exhumation of the Caledonian Orogen and subsequently from the Grenvillain Orogen.
T23C-2057
Paleozoic and Mesozoic Stratigraphy and Detrital Zircon Geochronology of Wrangel Island, Arctic Russia: Rosetta Stone for Arctic Reconstructions?
Wrangel Island represents a unique exposure of Neoproterozoic basement and its Paleozoic-Mesozoic cover sequence surrounded by the vast waters of the unexplored Siberian Shelf. Its geology is critical for testing the continuity of units, facies and structure from offshore Alaska to Arctic Russia across the Chukchi Sea and for placing constraints on various proposed plate tectonic reconstructions of the Arctic Ocean. Strata on Wrangel Island are highly deformed (tight to isoclinal folds at all scales) and metamorphosed to greenschist facies. Despite deformation, its Paleozoic lithologic succession appears to be a good match to the Hannah Trough section on Alaska's Chukchi Shelf. Dev(?)-Miss(?) conglomerates and sandstone overlie basement dated as 630-700 Ma (Kos'ko et al., 1986) and are overlain conformably by late Paleozoic limestone, shale and lesser clastic rocks deposited in a shelf basin setting. Like its counterparts on mainland Arctic Russia, the thick Triassic siliciclastic section was deposited in a series of rift basins, but dolerite dikes and sills are absent on Wrangel. Detrital zircon suites from Paleozoic strata on Wrangel Island are similar through the upper Paleozoic, reflecting a platformal setting with stable sources for siliciclastic detritus. Basement detritus is present in all sandstones but decreases upsection, replaced by Ordovician zircons (~ 490-440 Ma). All Pz units contain lesser Precambrian zircons in the 1000-2000 Ma interval. A significant change in provenance occurs at the base of the Triassic where 1000-1500 Ma sources disappear in concert with the arrival of upper Paleozoic sources (320-250 Ma zircons). Zircon ages in the range 215-235 Ma validate their previously inferred age. Comparison of Pz and Mz detrital zircon populations of Wrangel to the Russian mainland and to new data from Lisburne Hills, Alaska, suggests basin continuity between these three regions throughout the Pz-Mz, establishing more firmly the continuity of the Arctic Alaska-Chukotka microplate. These data also narrow the possibilities for plate tectonic reconstructions of the Cretaceous Amerasian Basin. The new data suggest restoration of Wrangel/Chukotka/N.Alaska back to the Lomonosov Ridge where it was once continuous with Taimyr and/or Barents Shelf region in order to receive Caledonian as well as Baltic shield detritus in the upper Paleozoic and to share its Permo-Triassic rift history with Siberia.
T23C-2058
Records of Sedimentation and Palaeofluid Events in Detrital Zircon U-Th-Pb Ages From Northern Greeland.
The Neoproterozoic (1.0 Ga to 550 Ma) is an important interval of dramatic change in the global environment. Several fragments of Neoproterozoic crust occur in the circum-Artic region and are thought to have been dispersed during the break-up of Rodinia/Pannotia. The correlation between these fragments and the models for their dispersion however remains controversial. We present U-Th-Pb detrital zircon age profiles from turbidite trough sediments (the Cambrian Polkorridoren and Vølvedal, and the Silurian Sydgletscher groups) of Northern Greenland and use the results to assess the source(s) of these sediments. These results allow us to test possible correlation(s) with dispersed circum-Arctic Neoproterozoic fragments. U-Th-Pb zircon ages from the two younger units (Vølvedal and Sydgletscher groups) show a significant number of discordant analyses. These are routinely ignored in detrital zircon studies, but when plotted on a standard inverse Concordia diagram define a pattern of radiogenic Pb-loss which occurred at ca. 400 Ma. Indeed, one zircon from the Sydgletcher Group yields a re-set pseudo-concordant age of 407 ± 13 Ma. Partial or total resetting of zircon U-Pb systematics at low temperatures (<200°C) is well- documented, particularly in U rich grains. The stratigraphically confined nature of the reset units, coupled with the presence of quartz/calcite veining, suggests that, rather than regional metamorphism, a major palaeofluid event at approx. 400 Ma was responsible for this resetting. We propose that this fluid event was driven by uplift of the Caledonian Orogen to the east.
T23C-2059
The Mount Kinabalu Granite of North Borneo: Result and Cause of Orogenic Deformation
Mount Kinabalu is a granite body in north Borneo that intrudes rocks deformed in the Early Miocene Sabah Orogeny following subduction of the South China continental margin beneath the north Borneo margin. Kinabalu is the highest mountain in SE Asia at 4100m and ice action during Pleistocene glaciations has resulted in excellent exposure of the summit area. The granite has previously been interpreted as a compositionally zoned, steeply sided pluton with a central biotite granodiorite, surrounded by hornblende granite and a marginal porphyritic facies. New zircon U-Pb SHRIMP ages record emplacement and crystallization. Zircon fission track data and apatite (U-Th)/He dates record the development and exhumation of the orogen. U-Pb SHRIMP analyses of concentric growth zones in zircon date crystallisation of the granite at between 7.85 and 7.22 Ma. The ages support models relating the Kinabalu granite to anatexis not subduction. Inherited zircon ages suggest melting of deep crust, including South China continental crust and arc basement rocks. SHRIMP dating provides insight into rates of magmatic processes. The entire pluton was emplaced and crystallised within a period of less than 700,000 years, with at least four pulses of magmatism, each lasting about 100,000 yrs. Zircon fission track data record post-crystallisation cooling but abundant dislocations make apatite fission track data unreliable. Apatite (U-Th)/He ages have a broadly concentric pattern. The thermochronological data indicate cooling of the granite was a response to growth of topography and rapid exhumation of the orogen. We interpret all these data to indicate that Kinabalu has a sheet-like character with the oldest biotite granite near the summit. The porphyritic facies represents the last and deepest major intrusive pulse. There was significant topographic expression by around 6 Ma with subsequent NE-SW trending extensional faulting on the south side of the body. The granite is the product of collision-related thickening in the Sabah orogeny, but is now itself driving fold and thrust deformation offshore after rapid uplift and exhumation following loss of a deep lithospheric root.
T23C-2060 [WITHDRAWN]
Testing Tectonic Models in the Southeastern Canadian Cordillera
The Selkirk allochthon infrastructure, which is part of the Shuswap Complex of southeastern Canadian Cordillera, is a 10-15 km thick panel of migmatitic metasediments sandwiched in between low-grade rocks of the overlying suprastructure and high-grade basement rocks of the underlying Monashee Complex. A literature review of structural, metamorphic and geochronologic data from this panel indicate protracted, east-directed high-temperature ductile flow in the Cretaceous. Furthermore, a compilation of zircon dates from the two shear zones bounding the infrastructure revealed a similar distribution of dates from ~120 to 60 Ma that point to synchronous deformation „Ÿ at least during Late-Cretaceous„Ÿ of top-to-the-west shear strain at its top, and top-to-the-east shear strain at its base. This contribution also presents a detailed Pressure-Temperature-time-deformation path for the base of the Selkirk allochthon infrastructure, which was reconstructed by combining results of microstructural observations, conventional thermobarometry, thermodynamic forward modeling and laser ablation U-Pb dating of zircon and titanite from a syn-shearing leucosome. The path indicates high-temperature exhumation from a depth of ~29 km to less than 10 km between 83 and 57 Ma during top-to-the-east reverse-sense shearing. From this path and other data taken from the literature, five depth-time paths were derived and clearly demonstrate that the Selkirk allochthon infrastructure was exhumed to upper crustal levels in the Late-Cretaceous while basement rocks of the Monashee Complex were being buried. Those findings have major implications because they are incompatible with popular tectonic models of exhumation by gravitational collapse and large-magnitude extension of a crustal welt in the Eocene. The results are rather compatible with channel flow and ductile extrusion of the Selkirk allochthon infrastructure during underthrusting of basement rocks of the Monashee Complex.
T23C-2061
Lu/Hf dating of garnet constrains timing of metamorphism and deformation, Prince Rupert Area, British Columbia
We present new Lu-Hf garnet ages that constrain the timing of deformation and metamorphism in the Western Metamorphic Belt (WMB), near Prince Rupert, British Columbia. We examined four samples of grt- bearing schist collected within the aureole to the Ecstall Pluton in the WMB. Garnets were separated from these rocks and dated using the Lu-Hf method at Washington State University. We determined geologically meaningful ages from three of these samples. The tectonic history of the Prince Rupert area is marked by phases of transpressive deformation, which included the development of crustal scale strike-slip shear zones and thrust slices with inverted metamorphic gradients. The Grenville Channel shear zone (GCSZ) is a crustal-scale sinistral-slip shear zone over 300 km long that strikes NW with a steep dip and shallow lineation. The GCSZ cuts through the WMB, a ductile fold and thrust belt composed of gneiss and schist with an inverted metamorphic sequence. Index minerals range from: chl and chd-grade units at the bottom of the sequence, str-bearing rocks in the middle, and ky-grt schist and local migmatites at the top of the thrust stack. The WMB was deformed and intruded by the Ecstall Pluton after the inverted metamorphic sequence had formed. The Ecstall is an epi-bearing hbl-qtz diorite emplaced between 91 and 93.5±1 Ma (Butler et al., 2001). Sample G-16A from Kumeleon Inlet (W of the Ecstall pluton) is a schist containing grt+biot+musc+qtz+epi+amph+sil, with small (<1 mm) euhedral grt. Kinematic indicators, including grt porphyroclasts, indicate left-lateral, top to the south, strike-slip shear. This sample yields a Lu-Hf age of 102±3.6 Ma (2σ, MSWD=1.5) based on seven grt and three whole-rock fractions, and a P-T estimate of 5.5±1 kbar and 590°±50° C from garnet-biotite thermobarometry. Sample 98-114A from Ridley Island (NW of Ecstall pluton) is a schist containing musc+biot+qtz+grt+ky+plag+chl+ill and with syn-tectonic euhedral garnet (1 cm). Grt contains sigmoidal inclusion trails that suggest rotation during left lateral shear, consistent with shear bands and C-S fabrics developed in the matrix of the sample. Grt in the sample produced a twelve-point isochron of 107.3±2.6 Ma (2σ, MSWD=1.6), indicating deformation and metamorphism at this time. Sample 06B-57, a garnet amphibolite migmatite from the inner aureole of the Ecstall pluton, contains grt up to 4 cm in diameter concentrated in leucosome layers. Qtz inclusion trails are consistent with rotation during reverse shear (pluton-side up), in addition to meso-scale folds, shear bands and dike arrays in adjacent rocks. This sample had complex systematics that record an older age of ~105 Ma with a younger overprinting of 90- 94 Ma during pluton emplacement. Pegmatite dikes contained within the Ecstall occur at high angles to the magmatic foliation, normal to the pluton margins, and indicate that the pluton was not folded after the pegmatite dikes were intruded. These new ages directly date garnet growth during metamorphism and deformation in the Prince Rupert area, and show that development of the inverted metamorphic sequence predated emplacement of the Ecstall pluton by 10 to 15 Ma. The data further indicate that left lateral strike slip shearing occurred between 107 and 102 Ma, at the same time much of the North American Cordillera was undergoing major contractional deformation.
T23C-2062
Ages of Sevier thrusting from dating of metamorphic garnet using the Lu-Hf method
Combined thermodynamic modeling of garnet growth zoning and Lu-Hf dating of garnet yield well-constrained pressure-temperature-time (PTt) paths. Here we present PTt paths from amphibolite-facies pelitic garnet from the Raft River-Albion-Grouse Creek metamorphic core complex that constrain the timing of thrusting in the hinterland of the Sevier thrust belt. Three general times of thrust burial are indicated: 150, 138, and 85 Ma. Lu-Hf garnet dating of burial-related garnet growth in the Raft River Mountains yielded a Late Jurassic age of 149.9 ± 1.2 Ma (2σ, MSWD = 1.1) based on three garnet fractions and a whole rock. A PT path from the schist of Mahogany Peaks in the Albion Range, Idaho, records an isothermal pressure increase indicating growth during thrusting. Lu-Hf dating of garnet from the same rock yielded an Early Cretaceous age of 138.7 ± 0.7 Ma (2σ, MSWD = 1.6) based on seven garnet fractions. An additional PT path from a nearby outcrop also records an isothermal pressure increase and a similar Lu-Hf garnet age of 132.1 ± 5.1 Ma (2σ, MSWD = 9.5) based on three garnet fractions and a whole rock. PT paths of multiple garnet grains from the schist of Stevens Spring in the Grouse Creek Mountains, Utah, exhibit isothermal pressure increases and yielded a Lu-Hf garnet age of 85.5 ± 1.9 Ma (2σ, MSWD = 3.9) based on five garnet fractions and a whole rock. The Late Jurassic burial event recorded in the Raft River Mountains is older than the ages of inception of thrusting of the western thrusts of the Sevier fold-thrust belt including the Canyon Range and Paris – Willard thrusts, but consistent with an eastward progression in initial shortening in the orogenic wedge and development of an inferred thrust load responsible for the retroarc Morrison Formation basin. Early Cretaceous hinterland burial recorded in the Albion Range is permissively coeval with activity on the Willard and Canyon Range thrusts. Finally, renewed hinterland thrust burial during the Late Cretaceous, as documented in the Grouse Creek Mountains, is consistent in timing with previous interpretations of major thrusting in frontal thrust systems of the Sevier belt, such as the development of the Absaroka thrust.
T23C-2063
High-precision Dating of Metamorphism and Melt Segregation in a Convergent Margin Setting: the North Cascades Continental Magmatic Arc
Convergent plate margins represent areas where the crust has undergone intense physical and chemical changes that may be tracked through the use of accessory mineral chronometers. The Skagit Gneiss is located at the southernmost extent of the > 1500 km long Coast Plutonic-North Cascades arc system. The Skagit has experienced a protracted thermal and deformational history with the emplacement of plutons from ca. 96 to 45 Ma that overlaps a transition from transpression (ca. 73 to 58 Ma) to transtension (55-45 Ma). Migmatitic metapelites in the core of the Skagit record metamorphism during significant crustal thickening, heating, and possibly during decompression, with peak pressure-temperature conditions of 8-10 kbar and 650-725 °C. Electron backscatter diffraction (EBSD) was utilized to investigate the fabric of the leucosomes located throughout the Skagit core. The results show that the leucosomes were affected by an intense low-temperature deformation post-melt crystallization, with quartz results yielding basal-a and prism-a slip. In order to better understand the timescales of metamorphism, deformation and partial melting in the Skagit, both monazite and zircon were dated from leucosomes representing a variety of textures (stromatic/discordant; fine-grained/pegmatitic) and from the host metapelite. Zircons from the metapelite commonly yield Cretaceous dates, with a youngest date of ca. 60 Ma. Leucosomes yield zircon with concordant dates that range from 68 Ma to 47 Ma. In comparison, monazite from individual leucosomes yield a variety of dates, with one group clustering near 48 Ma and a second set of older dates from 69 to 65 Ma. The latter monazite dates are consistently older than the zircons from the same leucosome, consistent with the possibility that the older monazites record the timing of prograde to possibly peak metamorphism in the Skagit. Similar monazite dates have been revealed from the metapelite. The Cretaceous zircon results may either represent the timing of melt crystallization or may reflect inheritance from the melt source rock. The Eocene zircon and monazite dates are at the young end of the age spectrum for the North Cascades arc system and overlap with zircon dates from tuffs in adjacent transtensional basins, suggesting that the arc remained at high-temperature and was undergoing partial melting during at least the initial stages of transtension. Moreover, the abundance and the duration of magmatism, and the young metamorphic ages reveal that the Skagit underwent a high-grade thermal history throughout the Late Cretaceous to Eocene with major deformation outlasting melt crystallization.
T23C-2064
Thermochronology applied to the Chiapas mountains, Mexico.
The Chiapas area is located at a triple junction between the Caribbean, North American and Cocos plates. It is an ideal area to investigate how tectonics influence landscape development and surface processes in the context of erosion, sediment routing and sedimentary basin evolution. At present the long-term evolution of the Chiapas Mountain is not well understood especially regarding the timing and magnitude of tectonic phases that led to formation of the Chiapas Mountains and its present-day topography. This area is mainly formed by the Permian Chiapas batholith (~250 Ma) and the Chiapas sierra. Important stratigraphic changes in nature and thickness lead authors to consider that up to six tectonic 'phases' participated in mountain formation and evolution. In this way, each major change in sediment nature, from marine to continental, has been related to uplift of the batholith and increase of sediment input. The onset of deformation along the region results from the northward migration of the left-lateral strike slip systems which is connected, further south, to the Polochic-Motagua fault system (i.e. limit between the North America and Caribbean plates). In this tectonic setting deformation along the Chiapas area results from the transpressive component of deformation related to the relative motion of the North American and Caribbean plates. A regional thermochronological study has been initiated to constrain the location, timing and magnitude of rock uplift associated with this transpressive regime. Thermochronometry will also help to constrain links to sedimentary basin development by identifying possible source areas (based on the assumption that exhumation of uplifted blocks is accompanied by erosion) and constrain sediment routing. U-Pb results obtained on the Palaeocene-Miocene series of the Sierra shows an important component from Greenvillian- type basement rocks (i.e. 0.8-1Ga) that have not been identified along the Chiapas region. The most likely source for is probably from a northern (Oaxaca and Xolapa blocks) or a southern (Chortis block) area, where Greenvillian basement has been widely documented. Our preliminary results show that the proposed relationships between batholith uplift and continental sedimentation along the sierra needs to be revised.
T23C-2065
Determining the origin of enigmatic bedrock structures using apatite (U-Th)/He thermochronology: Alabama and Poverty Hills, Owens Valley, California
The Alabama and Poverty Hills are enigmatic, topographic highs of crystalline basement surrounded by Neogene sediments in Owens Valley, California. The 150-km long Owens Valley, the westernmost graben of the Basin and Range Province, initiated at about 3 Ma, creating ~2-4 km of vertical relief from the Sierra Nevada and White/Inyos crests to the valley floor. Along the valley, the active right-lateral Owens Valley Fault Zone (OVFZ) accommodates a significant portion of Pacific-North American plate motion, creating an oblique dextral fault zone, with localized transpression along minor left-stepovers. The dominantly granitic Mesozoic rocks of the Alabama Hills are bounded by the OVFZ to the east, and the granitic and metavolcanic Mesozoic rocks of the Poverty Hills are located along an apparent 3-km left stepover of the OVFZ. The tectonic origin and geodynamic significance of both these structures are not known, but previously published hypotheses include: 1) transpressional uplifts as OVFZ-related flower structures; 2) down-dropped normal fault blocks; and 3) giant landslides from adjacent ranges. We measured apatite (U-Th)/He ages on 15 samples from the Alabama and Poverty Hills to understand the history of shallow crustal exhumation of these structures, and to potentially correlate them to rocks from adjacent ranges. Apatite He dating typically yields cooling ages corresponding to closure temperatures of ~55-65 °C, corresponding roughly to depths of ~2-3 km in the crust. The majority of apatite He ages from the Alabama Hills ranged from 58-70 Ma, but the far eastern, and lowest elevation sample showed ages of 51-55 Ma. The Poverty Hills shows younger ages of 40-65 Ma and no recognizable spatial pattern. Although the data do not conclusively rule out a transpressional uplift origin of the Poverty Hills, the rocks within them could not have been exhumed from depths greater than ~2-3 km in Owens Valley. Data from both structures are most consistent with down-dropping from adjacent ranges. Apatite He ages in the Alabama Hills correlate with He ages of rocks about 2.5-3 km higher, near Mt. Whitney in the adjacent Sierra Nevada. This, coupled with the spatial pattern of ages, strongly suggests that the Alabama Hills are a down-dropped normal fault block along the Sierra Nevada frontal fault zone or a related fault. A structural reconstruction using tilt-corrected Sierran apatite He age-elevation correlations requires 2.6 km of vertical, and 1.5 km of eastward motion for the Alabama Hills. The proximity of this extensive down- dropped basement block, directly east of the highest topography in the Sierra Nevada, suggests the possibility of localized isostatic response as a cause for locally high elevation in the Mt. Whitney area.
T23C-2066
New Constraints on the Extent of Paleoproterozoic and Archean Basement in the Northwest U.S. Cordillera
The Laurentian basement west of the Wyoming craton in southwest Montana and northern Idaho has been interpreted as a collage of Archean and Proterozoic terranes which accreted to the North American craton and incorporated into Laurentia at ~ 1.86 Ga [1]. This basement and the geometry of the Archean and Proterozoic crust are poorly understood due to coverage by metasediments of the Belt-Purcell Supergroup and are further obscured by Mesozoic magmatism (Idaho Batholith, sensu lato). Exposures of the basement are rare but have been documented in a few regions including the Priest River Complex in northern Idaho and the Sevier fold and thrust belt just northwest of the Wyoming craton in the Great Falls tectonic zone (Foster et al. 2006). New ages and isotopic data from orthogneisses in north-central Idaho provide evidence for previously undocumented exposures of both Paleoproterozoic and Archean basement that may place important constraints on the reconstruction of Laurentia and its tectonic setting. The orthogneisses analyzed in this study (all previously mapped as deformed Cretaceous plutons) fall into two distinct age groups of 1.86 Ga and 2.67 Ga. The zircons from both the Archean and Proterozoic rocks have simple systematics. The zircons from three Archean samples have εHf(i) values of 2.4 ± 2.1, 3.8 ± 1.8, and 5.2 ± 3.5 (average values based on 6 individual zircon Hf analyses per sample). Zircons from the Paleoproterozoic gneisses have different but internally consistent εHf(i) values of -8.0 ± 0.9 and -0.6 ± 1.4. In contrast, both Hf and Nd whole rock data are highly scattered in these samples especially in the Archean samples in which εHf(i) varies from -25 to +21 and εNd(i) varies from -8 to +11. These extreme values are implausible for initial compositions and indicate open system behavior in both Lu-Hf and Sm-Nd in the whole rocks. The zircons, in contrast, appear to be closed to significant Hf mobility on the scale of the laser analyses. The data from this project provides evidence for new exposures of Paleoproterozoic and Archean basement in North Central Idaho. The presence of Archean basement in this region suggests unrecognized complexities in the Selway Terrane possibly obscuring the boundary between this and the older Priest River block [1]. The positive εHf(i) values of the Archean gneisses at ~ +4 are consistent with depleted mantle compositions in the Late Archean and suggest that this is juvenile crust derived from the mantle at ~ 2.67 Ga. On the other hand, the Paleoproterozoic gneisses, with εHf(i) values of ~ -1 and -8, clearly represent the products of reworked pre-existing crust. The whole-rock Lu-Hf and Sm-Nd isotopic data from both the Paleoproterozoic and (certainly) the Archean samples clearly record a major isotopic disturbance. The timing of this disturbance in the Archean rocks is not yet clear but may have happened during Paleoproterozoic magmatism (1860 Ga), widespread metamorphism in the Mesoproterozoic (1.1 Ga; [2]), or tectonic thickening and magmatism in the Cretaceous (~ 80 Ma). [1] D.A. Foster et al., Can. J. Earth Sci., 43, 1601 (2006). [2] J.D. Vervoort, et al., Geol. Soc. Amer. Abstr, 36 (2005).
T23C-2067
Retrograde T-t Histories From Pelitic Migmatites Reflect Structural Distance From the Gwillim Creek Shear Zone, Valhalla Complex, British Columbia
Diffusion-zoned garnets from pelitic migmatites from the Valhalla metamorphic core complex, southeastern B.C. record relatively fast cooling rates that vary with distance above the Gwillim Creek shear zone (GCSZ). Fast cooling was caused by thrusting onto a cold footwall, where rocks closest to the fault began at the highest temperatures and conductively cooled at the fastest rate. Pelitic migmatites adjacent to and structurally above the GCSZ contain garnet with core to rim zoning of Fe/(Fe+Mg), controlled mainly by diffusion during progress of the retrograde net transfer reaction (ReNTR) Grt + Kfs + Melt = Bt + Sil + Plg. Where biotite is present in contact with garnet, retrograde Fe/Mg exchange has resulted in additional increased Fe/(Fe+Mg). A strongly foliated pelitic migmatite with pervasive 2-5mm-thick leucosomes from ~1.5 km structurally above the GCSZ contains garnet with Fe/(Fe+Mg) ranging from 0.72 to 0.93 and Xsps zoning with a uniform core (~0.02) and a near rim increase (to ~0.04). Interdiffusion of Fe+Mg was modeled with garnet radius varying linearly with temperature. This finite difference model calculates Fe-Mg interdiffusivity and generates a diffusion profile based on changing garnet rim boundary conditions governed by the ReNTR. Model fits to measured profiles require a 1-5 m.y. period of slow to moderate cooling (5-25°C/ m.y.) followed by a brief period (<1m.y.) of relatively fast cooling (60-500°C/m.y). In contrast, rocks adjacent to the GCSZ show a very short initial slow cooling step (generally <1m.y.), followed by a short period of fast cooling (100-1000°C/m.y.). The difference in the duration of the initial slow cooling steps between the two localities reflects the time scale of thermal conduction when the complex is thrust onto an effective heat sink. The late cooling history (below 600°C), which is poorly constrained by the diffusion profile due to very slow diffusion at these temperatures, is interpreted to represent unloading by displacement on the Slocan Lake normal fault. 2-D thermal modeling of a low angle thrust ramp yields different T-t histories for hanging wall rocks based on their distance from the thrust. Model results show that transport on the order of cm/yr up a 10-20°-dipping thrust fault can produce cooling rates that are consistent with those calculated from garnet diffusion, and that the duration of initial slow cooling increases with distance from the fault.
T23C-2068
4D Quantification of Stress and Strain Tensors at Sheep Mountain Anticline (Wyoming, USA) Using Calcite Twin Analysis
We use the calcite twin analysis to investigate the relationship between fold development, stress and strain
distribution. We chose for this study the Sheep Mountain Anticline (Wyoming, USA) as a natural laboratory.
Because it's asymmetric and basement-cored fold, this anticline was formed during the Laramide orogeny in
the Early Tertiary.
The calcite twin have been measured in folded Lower Carboniferous to Permian age
carbonates and sandstones. Calcite twin recorded both in the matrix and in the veins, highlight three different
tectonic stages: the first phase is a pre-folding compression parallel to fold axis, a second one is also pre-
folding compression but it's perpendicular to the future fold axis and the third stage is also perpendicular to
the fold axis but it's a post-folding compression. Furthermore, calcite twin data provide information about the
evolution of stress (Etchecopar's method) and strain (Groshong's method) through time and space. Both pre-
folding and post-folding NE-directed compressional stress and/or shortening were recorded within pre-folding
veins (set I) as well as in fold-related veins (sets II and III). Set III veins also recorded outer rim extension
along the fold hinge line.
Besides, calcite twin analysis allow us to quantify stress and strain. Our results
point out both temporal and spatial evolution of stress and strain tensors. Spatially, we notice that both strain
and particularly differential-stress in the backlimb are higher than in the forelimb. We are also able to show
that differential-stress drops both in the backlimb and in the forelimb between pre-folding and post-folding
stages. Our new dataset should putting better constrains on numerical models in order to increase our
knowledge on fold mechanics.
T23C-2069
Stress and Temperature Dependence of Calcite Twinning: New Experimental and Field Constraints
In low-grade metamorphic terrains at temperatures < 300° C e-twinning of calcite is common. The width and density of e-twins have been suggested to indicate stress and temperature representing robust paleopiezometers and geothermometers. To evaluate the stress- and temperature dependence of e-twins in calcite we have performed a series of deformation experiments on specimens of Carrara marble. 14 experiments were performed at 100-400 MPa confining pressure and T < 350° C in a Paterson-type gas deformation apparatus. Seven samples were deformed in axial compression test at strain rates from 10-4 -10-6s-1. Seven samples were deformed in torsion tests to shear strains γ < 1.8. After testing, thin sections of all samples were prepared for optical inspection of twin density and twin width. Twin density varied between 10 and 500 [twins/mm] at stresses up to 280 MPa. At given conditions and with increasing strain twin density increased significantly. No clear dependence of twin density on temperatures up to 300° C was found in experiments. The experimentally deformed samples were compared to naturally-deformed low-grade calcite rocks from different fault zones. From optical thin sections of 20 samples deformation temperatures were estimated based on twin width and fluid inclusion data. These samples were subsequently used for optical measurements of twin density. Twin density varied between 10 and 100 [twins/mm] up to temperatures of 300° C. Using published calcite piezometers (Rowe and Rutter 1990) we estimated paleostresses. From our experiments and field data we did not observe a clear relation between twin width and temperature up to 250° C. Above 250° C, temperatures estimated from calcite twin widths in naturally deformed calcite samples do correspond to temperatures estimated using other methods (e.g. fluid inclusion analysis, vitrinite reflection, conodont colour alteration index). Using the existing paleopiezometres, differential stresses inferred from twin density in the experimentally deformed samples are commonly overestimated by up to a factor of 2 at low stress. Towards higher stresses > 150 MPa predicted and experimental values converge for samples deformed to similar strains. The experimental data clearly shows a strong dependence of twin density on strain for all tested temperatures.
T23C-2070
Using SHRIMP Zircon Geochronology to Characterise the Evolution of the Proterozoic Mount Isa Inlier, Australia
The Proterozoic Mount Isa Inlier of northern Australia records an extensive record of basin evolution between 1800 Ma and 1575 Ma, and contains a number of world-class Pb-Zn-Ag, U and Iron Oxide Cu-Au deposits. Understanding the timing and nature of basin development is a critical component in understanding these mineral systems. The integration of U-Pb zircon SHIRMP geochronology with structural and facies analysis has allowed basin packages across this area to be divided into three superbasins; the Leichhardt, Calvert and Isa Superbasins. Detrital zircon geochronology of stratigraphic units within these basins has been used in conjunction with syn-sedimentary volcanics to constrain depositional ages, and to identify and characterise changes in provenance through time. Sedimentation between 1790 Ma and 1740 Ma associated with the Leichhardt Superbasin is characterised by fluvial to shallow marine sandstones deposited in half-grabens. Between 1690 Ma and 1670 Ma, deep-water turbidites in the eastern-most parts of the inlier were deposited during an interval of missing rock record on the platform to the west, and are coincident with the initiation of a break-up unconformity. Sedimentation between 1790 Ma and 1670 Ma is also associated with voluminous felsic and mafic magmatism, and mafic rocks emplaced during this time period record a change in geochemical signature from continental flood basalts to oceanic tholeiites. We interpret these changes to be consistent with an evolution in tectonic setting from intercontinental rifting to near passive margin development.
T23C-2071
Re-Os systematics of shale-hosted Cu-Au mineralization at the Bidjovagge deposit in northern Norway (Finnmark)
Black shales may serve as hosts for base and precious metal deposits and contribute to the metal inventory of those deposits. Re-Os dating and tracer studies assist in determining the source of metals and establishing the time of their mobility and fixation as sulfide. We have obtained a telling suite of drill core samples from the well-described Bidjovagge Cu-Au deposit in Finnmark to examine sulfide-shale relationships using the Re-Os isotopic system. The data narrate a complex and evolving history associated with the Svecofennian orogeny. Our results contribute not only to the understanding of Cu-Au mineralization, but also to the geologic record leading to overmature and highly altered black shale in the vicinity of ore mineralization. The Bidjovagge Cu-Au deposit is located in the Kautokeino greenstone belt in northern Norway. The deposit is hosted in carbonaceous shale deposited in a Paleoproterozoic marine rift basin filled with carbonates and mafic volcanic rocks, and intruded by diabase sills. This basin fill has been metamorphosed to upper greenschist ?lower amphibolite facies during the Svecofennian orogeny. At Bidjovagge, the units are isoclinally folded into an upright, N-S trending, gently N-plunging antiformal structure, cut by N-S shear zones. The stratabound Cu-Au ores occur exclusively in the shale (graphitic schist) or its oxidized equivalent (albitized felsite). In the vicinity of Cu-Au mineralization the graphitic schist is highly altered (albitization) and nearly white. Previous U-Pb and Sm-Nd dating of davidite yields imprecise ages of about 1885 Ma. U-Pb dating of uraninite yields a more constrained age of about 1835 Ma. Both ages confirm the association of the mineralization with the Svecofennian orogeny. The stratabound nature of the ore bodies, their association with altered (albitized) schist, and their juxtaposition with carbonaceous (graphitic) schist suggests that black shales played a role in the ore-forming process. Several generations of syn- to late-deformational ore-bearing veins are present. Most of the veins, especially the early, gold-rich veins, display ductile deformation following the fabric of the country rocks. Later veins, especially the gold-poor, Cu-rich veins, fill brittle fractures and small breccia zones that crosscut earlier fabrics. Thus, determination of the ages for the sulphides constrains the timing of alteration and associated metamorphism and deformation of the black shales. We have acquired Re-Os dates on chalcopyrites and pyrites from different host settings that reveal a complex history of Svecofennian ore deposition.
T23C-2072
Hydrocarbons and Au Deposits (and Their Pyrite) - Is There a Link?
Non-economic and isolated occurrences of hydrocarbon are known in the rock record. Among these is the somewhat common association of hydrocarbon veins and breccia fillings observed at Au deposits. This association begs investigation of the genetic association between hydrocarbon and Au deposition. To address this question we obtained a suite of Au-associated pyrites and cross-cutting hydrocarbon occurrences (referred to as coalblende in the vernacular) that are typical of the ore at the Eidsvoll Au deposit in SE Norway. The Au mineralization at Eidsvoll has long been attributed to a Late Sveconorwegian continental-scale terrane boundary along the SW margin of Fennoscandia, known as the Mylonite Zone (1000-970 Ma). Eidsvoll is located in the MZ but ore-hosting structures clearly crosscut the MZ fabric. Re- Os dating of euhedral cubic pyrites provides a well-constrained 8-point isochron with a corresponding age of c. 870 Ma. Further, a second generation of fine-grained cataclastic pyrite with 187Re/188Os ratios of greater than 4000 yields early Caledonian ages of c. 440 Ma. We attribute the c. 870 Ma age to extensional reactivation of the Kristiansand-Porsgrunn shear zone separating the Bamble and Telemarkia terranes SW of the intervening Permian Oslo rift. The younger c. 440 Ma pyrite may represent cataclasis of earlier cubic pyrite leading to liberation of Au and creation of associated chalcopyrite. The c. 440 Ma age may reflect lithospheric deformation in response to loading by the encroaching Caledonian thrust sheets to the west. Late hydrocarbon fracture coatings, veins, and breccia cements are markedly present at the Eidsvoll Brustad mine where we obtained our samples. Surprisingly, Re-Os model ages for simple fracture coatings of coalblende yield Miocene ages, indicating that Au and hydrocarbon deposition are neither temporally nor genetically related. We interpret the Miocene ages as recording hydrocarbon migration into Eidsvoll rocks, likely expelled from Paleozoic source rocks in S Norway and offshore equivalents. South Norway is host to numerous non-economic occurrences of hydrocarbon, and Neogene hydrocarbon migration has been recorded offshore. Dating of hydrocarbon at the Eidsvoll Au deposit has directed our attention to the potential for young hydrocarbon in S Norway and offshore.