T21E-01
Timing and Rates of Neogene Exhumation During Orogen-Parallel Extension in the Central Alps
Different structural levels of the crust, recording both brittle and ductile deformation, are often juxtaposed across major low-angle normal faults. However, there is a controversy regarding whether the observed field relationships are the result of a single continuous process or of two separate events, with the later brittle detachment exhuming a fossil ductile shear zone from depth. These potential models have been investigated with detailed geochronology on the Simplon Fault, located in the Central Alps. This well exposed low-angle detachment developed as the result of orogen-parallel extension that has affected the Alps since at least the Oligocene. This normal fault zone developed during continued convergence, resulting in a complex interplay between large-scale backfolds, mylonites, and the brittle detachment. Different geochronological methods are used to constrain the temporal and spatial evolution of the Simplon fault system and quantify the amount of exhumation involved. We present new apatite and zircon fission tracks ages and 40Ar/39Ar ages on white mica and biotite. Samples used are structurally controlled and come from mylonites developed under different metamorphic grades. In the high grade mylonites, 40Ar/39Ar dating of recrystallized muscovites yield cooling ages indicating shear zone activity from 20 to 14 Ma. 40Ar/39Ar formation ages of new white micas that grew synkinematically in the mylonites varies from 14 Ma in the SE to 10 Ma in the NW, indicating a lateral and temporal evolution of distributed deformation within the Simplon Fault. Zircon and apatite fission track ages indicate a slower cooling through the brittle transition at around 8-12 Ma but continuing until 5Ma. 40Ar/39Ar dating of biotite overprinted by the axial plane schistosity of the backfolds constrains this event to ca. 10-13Ma, which corresponds approximately to the time of transition to brittle fracture on the Simplon Fault. Overall the new data argue for a single continuous transition from ductile shearing to a more localized zone of brittle deformation over a period of ca. 15 Ma during the Neogene, while the Alpine convergence was still active. 2D numerical thermo-kinematic models (Pecube, Braun 2003) coupled with a formal inversion algorithm (Neighbourhood Algorithm) are used to quantify the relative displacement and rates of exhumation related to the Simplon Fault.
T21E-02 INVITED
Laserprobe 40Ar/39Ar Dating Of Strain Fringes: An Example From The Interior Of The Sevier Orogen
Determining the absolute ages of deformation fabrics in metamorphic rocks is critical to understanding the nature of orogenesis in deeply exhumed ancient mountain belts. In situ isotopic analysis of fibrous strain fringes is particularly advantageous because the relationships between the analyzed mineral, deformation fabric, and isotopic age can be established, and mineral growth can be linked to the progressive strain history. Furthermore, when isotopic closure temperatures (Tc) are higher than crystallization temperatures, isotopic ages reflect deformation ages. We have applied in situ 40Ar/39Ar UV and CO2 laser techniques to dating phlogopite in quartz-phlogopite-calcite and muscovite in quartz-muscovite fibrous strain fringes in greenschist-facies metamorphic rocks from the Grouse Creek Mountains (Utah) and Black Pine Mountains (Idaho). In both cases, syntectonic mica growth occurred below the minerals Tc. Phlogopite 40Ar/39Ar ages for individual strain fringes in the Grouse Creek Mountains range from 92 Ma to 110 Ma, with the most reliable ages ranging from 101 Ma to 110 Ma (mean age, 105.0 plus or minus 5.8 Ma). Muscovite 40Ar/39Ar ages for individual strain fringes in the Black Pine Mountains range from 97 Ma to 112 Ma (mean age, 104.7 plus or minus 5.8 Ma). Strain fringes are associated with a sub-horizontal foliation and a generally N-trending elongation lineation exhibiting components of top-to-the-north simple shear and coaxial strain accommodating N-S extension and sub-vertical shortening. The new dates confirm prior correlation of fabrics between these ranges and yield a mid-Cretaceous age for the earliest ductile fabric preserved in the Raft River-Albion-Grouse Creek metamorphic core complex, in the interior (hinterland) of the retroarc Sevier orogenic belt. Mid-crustal northward flow at 105 (plus or minus 6) Ma within the interior of the Sevier orogen, coeval with east-directed shortening in the foreland and with plate convergence, records orogen-parallel synconvergent extension. Gravitational relaxation of structural culminations resulting from focused crustal shortening may be the driving mechanism for orogen-parallel flow.
T21E-03
Progression from South-Directed to Orogen-Parallel Mid-Crustal Flow on the Southern Margin of the Tibetan Plateau, Ama Drime Massif, Tibet
In the South Tibetan Himalaya, two major detachment systems are exposed in the Ama Drime and Mount Everest Massifs. These structures represent a fundamental shift in the dynamics of the Himalayan orogen, recording a progression from south-directed to orogen-parallel mid-crustal flow and exhumation. The South Tibetan detachment system (STDS) accommodated exhumation of the Greater Himalayan series (GHS) until the Middle Miocene. A relatively narrow mylonite zone that progressed into a brittle detachment accommodated exhumation of the GHS. Northward, in the down-dip direction (Dzakaa Chu and Doya La), a 1-km-wide distributed zone of deformation that lacks a foliation-parallel brittle detachment characterizes the STDS. Leucogranites in the footwall of the STDS range between 17-18 Ma. Previously published 40Ar/39Ar ages suggest that movement on the STDS ended by ~ 16 Ma in Rongbuk Valley and ~ 13 Ma near Dinggye. This once continuous section of the STDS is displaced by the trans- Himalayan Ama Drime Massif and Xainza-Dinggye graben. Two oppositely dipping normal faults and shear zones that bound the Ama Drime Massif record orogen-parallel extension. During exhumation, deformation was partitioned into relatively narrow (100-300-m-thick) mylonite zones that progressed into brittle faults/detachments, which offset Quaternary deposits. U(-Th-)Pb geochronology of mafic lenses suggests that the core of the ADM reached granulite facies at ~ 15 Ma. Leucogranites in the footwall of the detachment faults range between 12-11 Ma: significantly younger than those related to movement on the STDS. Previously published 40Ar/39Ar ages from the eastern limb of the Ama Drime Massif suggest that exhumation progressed into the footwall of the Nyüonno detachment between ~ 13-10 Ma. (U-Th)/He apatite ages record a minimum exhumation rate of ~ 1mm/yr between 1.5-3.0 Ma that was enhanced by focused denudation in the trans-Himalayan Arun River gorge. Together these bracket the timing (~ 12 Ma) of a transition from south-directed to orogen-parallel mid-crustal flow and associated graben formation and exhumation along the southern margin of the Tibetan Plateau.
T21E-04
Thermal durations and heating behaviour for the Barrovian metamorphism, Scotland
Published U/Pb ages for the syn-metamorphic gabbros and granites of the Grampian Terrane, Scotland, that provided heat for the classical Barrovian metamorphism, suggests that they were emplaced between 473.5 and 470 Ma. New SHRIMP U/Pb ages of 472.2 ± 5.8 Ma and 470.4 ± 6.1 Ma for peak metamorphism in the highest-grade units of the Barrovian metamorphic series are consistent with a 473.5 to 470 Ma heating episode in the highest-grade units. U/Pb-calibrated 40Ar/39Ar ages for white mica from the Barrovian metamorphic series vary from c. 465 Ma for the biotite zone to c. 461 Ma for the sillimanite zone and suggest that the Barrovian thermal episode lasted less than 8.5 million years in the biotite zone and less than 12.5 million years in the sillimanite zone. The lowest-grade units of the Barrovian metamorphic series retain detrital ages in white mica 40Ar/39Ar step-heating spectra, while units metamorphosed to temperatures of 475°C or more yield Grampian 40Ar/39Ar plateau ages. Forward modelling of Ar diffusion from white mica grains was carried out for different grain sizes and thermal histories to match the position of the across-metamorphic-grade transition from detrital 40Ar/39Ar patterns to Grampian 40Ar/39Ar plateau ages. The results of Ar diffusion modelling are consistent with thermal durations of between one and 4.5 million years for the Barrovian metamorphism of the biotite zone. Microstructural observations suggest that peak metamorphism and cooling occurred earliest in the lowest-grade units of the Barrovian metamorphic series and metamorphism in the higher-grade units continued for longer. We propose metamorphic durations of between 3.5 and eight million years for the Barrovian metamorphism of the sillimanite zone. Geochemical textures preserved within high-grade garnets from the Barrovian metamorphic series record evidence of Mn diffusion over c. 1000 μm lengthscales during the Barrovian metamorphism. In addition, sillimanite-grade garnets from the Barrovian metamorphic series preserve c. 100 μm diffusion textures between sillimanite-grade rim domains and lower-grade cores. Timescales for Fickian diffusion processes increase with the square of the diffusion lengthscale. Lengthscales of diffusion are considered within the context of 3.5- to eight-million-year duration for the Barrovian thermal event. Heat associated with regional metamorphism appears to have accumulated within the metamorphosed units following numerous, short- timescale (tens of thousands of year) heating events. Shear zones that occur in the highest-grade parts of the Barrovian metamorphic series provide a suitably narrow heating region for regional metamorphism over a several million years and, with episodic movement histories, can account for self-similar heating behaviour (by mechanical work and/or the introduction of magmas and hot fluids).
T21E-05 INVITED
Quantitative Thermochronology in Orogenic Settings
We review recent developments in thermal modeling methods - from simple one-dimensional solutions of the heat transport equation to sophisticated three-dimensional finite element models that include the effect of an evolving, finite amplitude topography or the movement of crustal blocks along an array of faults - that have led to a more quantitative interpretation of thermochronological datasets in orogenic settings. We will show the importance of incorporating the effects of the advection of heat due to rock exhumation and lateral tectonic translation, as well as the perturbation caused by surface topography. We will also focus on the establishment and relaxation of the thermal state of the crust at the onset and end of a tectonic event. We will show how different thermochronoligical datasets can be modeled, including multi-system and detrital datasets. We will demonstrate how modeling the thermal structure of the crust strongly alters our interpretation of thermochronological data, and how numerical models combined with appropriate inverse methods can not only provide quantitative constraints on the parameters that we have often arbitrarily chosen to describe a given tectonic and geomorphic scenario, but also help refine the parameterization or define appropriate targets for further data collection. These points will be illustrated by a wide range of case studies.
T21E-06
Dynamics of Metamorphic Core Complexes Inferred From Modeling and Metamorphic Petrology
Orogenic collapse involves extension and thinning of thick, hot, and in some cases partially molten, crust, leading to the formation of metamorphic core complexes (MCC) that are commonly cored by migmatite domes. 2D numerical modeling predicts that the geometry and P-T-t history of MCC varies as a function of the presence/absence of a partially molten layer in the deep crust; the nature of heterogeneities that localize the MCC (e.g. normal fault in upper crust vs. point-like anomaly in the deep crust); and extensional strain rate. The presence of melt in particular has a significant effect on the thermal and structural history of MCC because the presence of partially molten crust or magma bodies at depth enhances upward advection of material and heat. At high extension rate (cm/year in the region of the MCC), partially molten crust crystallizes as migmatite and cools along a high geothermal gradient (35-65 C/km); material remains partially molten during ascent, forming a migmatite dome when it crystallizes at shallower crustal levels (e.g. cordierite/sillimanite stability field). At low strain rate (mm/yr in the MCC region), the partially molten crust crystallizes at high pressure (e.g. kyanite zone); this material is subsequently deformed in the solid-state along a cooler geothermal gradient (20-35 C/km) during ascent. MCC that develop during extension of partially molten crust may therefore record distinct crystallization versus exhumation histories as a function of extensional strain rate. The mineral assemblages, metamorphic reaction histories, and structures of migmatite-cored (Mc) MCC can therefore be used to interpret the dynamics of MCC formation, e.g. "fast" McMCC in the northern N American Cordillera and Aegean regions.
T21E-07 INVITED
Radiometric dating of brittle fault rocks; illite polytype age analysis and application to the Spanish Pyrenees.
A variety of approaches have been available to indirectly date the timing of deformation and motion on faults, but few approaches for direct, radiometric dating of shallow crustal fault rocks were available until recently. The growing recognition of clay neomineralization at low temperatures in many fault rocks, particularly the 1Md illite polytype, allows the successful application of Ar dating to these K-bearing phases. In this presentation we will discuss our recent illite age analysis approach (sampling, treatments, analytical methods), and present new results from fault dating along the Spanish Pyrenean orogenic front as an example. X-ray quantification of polytype ratios in three or more size fractions is used to define a mixing line between (1Md illite) authigenic and (2M illite) detrital end-member phases that constrain the fault age and host rock provenance/cooling age for each fault. The common problem of recoil in clays is addressed by encapsulating samples before irradiation. Nine fault gouge ages in the south-central and south-eastern Pyrenees support several contractional pulses in the Pyrenean orogen: 1) Late Cretaceous thrusting (Boixols), 2) Latest Paleocene-Early Eocene deformation (Nogueres Zone and Freser antiformal stack), 3) Middle-Late Eocene deformation (Ripoll syncline, Vallfogona, Gavernie, Abocador and L'Escala thrusts), and 4) Middle Oligocene thrusting in the central portion of the Axial Zone (Llavorsi-Senet). The late Paleocene-Early Eocene and Middle-Late Eocene events may or may not be one single phase, due to slightly overlapping error estimates. The outboard thrusts give Hercynian ages for the detrital component of the fault rock, while the inboard thrusts, which juxtapose metamorphic units, give Cretaceous ages for the non-authigenic component, reflecting the cooling age of the adjacent wallrocks. Based on our latest work, the illite polytype dating method complements previously developed illite-smectite dating (van der Pluijm et al., 2001), and can be applied to fault rocks with temperatures up to ~150 degrees C, allowing broad application of illite age analysis to shallowly-exhumed orogenic systems around the world.