T11B-1856 INVITED
Tectonic microplates: laying it down on wax
We present a wax analogue model of sea-floor spreading that produces rotating, growing microplates. Wax
microplates are kinematically similar to sea-floor tectonic microplates in terms of spreading rate and growth
rate. Furthermore, their spiral pseudofault geometry is quantitatively consistent with Schouten's oceanic
microplate model. These results suggest that Schouten's edge-driven microplate model captures the
kinematics of tectonic microplate evolution on Earth. We propose a theory for the formation of microplates.
http://www.iop.org/EJ/abstract/1367-2630/7/1/037/
T11B-1857
Complex Pull-apart Structure Evolving at Northern Explorer Ridge, Northeast Pacific
The Explorer Spreading Center (ESC) bounding the western edge of the Explorer microplate has undergone substantial reorganization over the past few million years as the microplate has rotated in response to increasing resistance to the subduction of its young crust. The northern Explorer ridge (NER) has evolved into a complex compound structure consisting of several rift basins bounded by half-graben and arcuate shaped faults with a superimposed pattern of rhombohedral grabens and horsts. This pattern is similar to structures on rift zones and pull-apart basins formed along major continental transforms and rift zones and contrasts with the ridge-parallel faults formed at seafloor spreading centers. However, initiation of faulting in the NER appears to have occurred along inactive off-axis faults generated by seafloor spreading. The NER region appears to be evolving into a larger pull-apart-like structure. As extension continues, individual basins have widened with some of the larger arcuate boundary faults linking up to form accommodation zones between adjacent depressions. If diffuse rifting continues in the NER the nascent pull- apart structure will grow longer as it accommodates the changing regional strain field induced by the diminishment of the Explorer plate. Alternatively, a strike-slip fault could propagate through the area and join the two master faults. Although there's no well-defined though-going fault with the strike of the Pacific-North American vector (about 340°), there is some indication from that such a structure is beginning to break through.
T11B-1858
Microplates vs Siberian Craton, History of the Patom Belt
The Central Asian Orogenic Belt (CAOB) also known as the Altaides is supposed to result from the accretion of volcanic arcs and microcontinents against the Siberian craton. An interdisciplinary approach including stratigraphic and structural study and a field survey led in the Patom area allows us to precise the geodynamic history of the region. The observed sedimentary succession confirms the existence of a passive margin setting in the Late Riphean (around 900 Ma), followed during the Vendian (650-600 Ma) by the obduction of the Baikal-Muya ophiolites belt and a foredeep inversion. This first accretion stage does not induce significant deformations and rather appears as a soft-docking event. Then, a period of relative tectonic quiescence is recorded until the main collision stage (post Devonian) attested by new age constraints. The regional geodynamics is thus marked by a slow evolution with very long stable periods of more than 250 Ma between "convergent pulses". Moreover, we observe limited deformation and low shortening rate which seem to be characteristic of a pericratonic context and we conclude that accretionary orogens appear not mature. On another hand, apatite fission tracks analysis were carried out on samples from the Siberian platform, Patom range and Cenozoic Baikal rift basins. These new data reveal relative vertical movements and clearly show a late Jurassic – early Cretaceous tectonic reactivation of ancient Paleozoic boundaries. These results evidence successive stages of tectonic collage and terranes accretion, from Neoproterozoic to Mesozoic times, and raise the question of stabilization or cratonization of accretionary orogens and microplates jigsaws.
T11B-1859
Transtensional Analyses of Fault Patterns and Strain Provinces of the Eastern California Shear Zone-Walker Lane on the Eastern Margin of the Sierra Nevada Microplate, California and Nevada
Substantial work on the theory of transtensional strain at various scales has shown that transtension produces triaxial non-plane constrictional strains. At the plate boundary scale, fault geometries predicted by transtensional theory better explain observed fault patterns in the transtensional Eastern California shear zone-Walker Lane than 2D plane strain pure or simple shear solutions. Seemingly heterogeneous fault patterns and strain styles within structural provinces along the higher strain corridor adjacent to the eastern Sierra Nevada microplate margin are reconciled by non-plane strain evaluation, and in the context of triaxial, 3D strain partitioning characteristic of transtensional deformation. Among these structural provinces, which include the Honey Lake-Pyramid Lake region, the Lake Tahoe region, the Mono Lake-Long Valley region, Owens Valley, and the Coso region, all are found to be undergoing coaxially dominated transtension, except for the Mono Lake-Long Valley region which has possibly formed by noncoaxially dominated strain, and the northern Honey Lake-Pyramid Lake region, where the Sierra Nevada microplate boundary zone curves west toward the Mendocino triple junction. The local geometry of the transtensional zone boundary and the microplate transport direction determine the dominant type of non-plane strain. Fault orientations predicted by application of transtensional theory to identify instantaneous strain axes are consistent with those observed in each structural province, and comparatively, are not adequately explained by plane strain kinematic models. The orientations and amount of shortening and elongation of the finite strain ellipsoid axes for each province indicate smaller amounts of shortening, elongation and rotation of axes, and overall less ellipticity, in the coaxially dominated strain areas, and greater shortening, elongation, and rotation, and overall greater ellipticity, in the noncoaxially dominated areas. Exceptions are the Owens Valley and Coso regions which have larger amounts of shortening and elongation of the finite strain axes without substantial rotation, caused by the narrowness of the high strain corridor in these two regions. The variation of coaxial to noncoaxial strain ratios among predominantly coaxially dominated strain provinces results in variations in shape among finite strain ellipsoids. K-values for each structural province plot above the k=1 diagonal on the logarithmic Flinn (Ramsay) diagram, indicating prolate non-plane transtensional strain. Plate tectonic reconstruction of Sierran microplate motion over the last 3 My shows characteristic occurrence of areas of zone boundary parallel transport with intervening pull-aparts that correspond to centers of volcanism, magmatism, and subsidence. The nature of rotation of the microplate around its Euler pole, and adjacent tectonic block geometry and behavior are possible mechanisms for systematic widening of the higher strain transtensional corridor to the north. Collectively, these theoretical applications and structural-tectonic observations in the ECSZ-WL have significant implications for the kinematics of brittle deformation and evolution of this actively developing transtensional plate boundary.
T11B-1860
Cutting Through the Plate: Rift Interaction North of the Galapagos Triple Junction
The Galápagos triple junction features the Cocos-Nazca Ridge (CNR) propagating towards the East Pacific Rise (EPR) without connecting to it. Instead, secondary rifts like the Incipient Rift initiate from the EPR and delimitate a distributed deformation zone encompassing the Galápagos microplate. To the north of the triple junction, a succession of large scarps indicates that the Incipient Rift is but the latest in a series of short-lived rifts that quickly cease their activity and are rafted away from the triple junction with the Cocos plate. Other ridge-ridge-ridge triple junctions like the Rodriguez, Azores, and Bouvet triple junctions feature secondary ridges, microplates, and transform faults that link the major ridges rather than a direct connection between those ridges. To understand the origin of secondary rifts, we modeled the stress field in the vicinity of the Galápagos triple junction in the presence of cracks representing the EPR and the CNR. The presence of the CNR increases the tensile stress along the EPR away from the triple junction. The Incipient Rift initiates exactly where the tensile stress is maximum along the EPR, at 1.4 D from the triple junction, where D is the distance between the tip of the CNR and the EPR. Therefore, we propose that the Incipient Rift is the result of cracking of the Cocos plate initiated at the EPR due to the nearby presence of the CNR. Once a rift is formed at this location, plate motion carries it away from the maximum tension region, so that its activity ceases and a new rift is subsequently generated at 1.4 D. The region ahead of the CNR tip is under reduced tension, so that propagation of the rift toward the EPR is not favored. If a ridge-ridge-ridge triple junction exists, it is kinematically stable. However, mechanical arguments may prevent such a triple junction to form, producing instead a complex pattern of rifts and intervening microplates.
T11B-1861
Sr-Nd-Pb Isotopic Signatures of High-Ti Basalts From the Pacific Rivera Plate, Western Margin of Mexico.
Knowledge of the chemical and isotopic composition of ocean floor basalts is crucial for petrogenetic modeling of continental arc magmas. However, for Mexico few information is available, which was primarily that obtained from the DSDP perforations front of Acapulco and in the mouth of the Gulf of California. In this contribution we present new geochemical and geochronological analyses of Mid Ocean Ridge basalts, dredged at a depth of approx 2.800 m, offshore Manzanillo. The sampled area lies at the southeastern edge of the Rivera Plate close to its poorly defined boundary with the Cocos Plate and the Middle America Trench. Hand sized pillow fragments are glass-free and characterized by a fresh interior zone with porphyritic texture and a slightly more altered outer rim. Four tholeiitic samples were analyzed. Two generations of olivine phenocrysts were observed. Large grains without chrome spinel and smaller grains with chrome spinel and other inclusions. Both occur together with microphenocrysts of uninverted (monoclinic) pidgeonite and plagioclase (An60-80) in the fresh interior zone whereas the outer rim zone shows more alterated olivines, smectite, and secondary carbonates in vesicles. SiO2 contents are between 48.6 and 50.5 wt.% and TiO2 is substantially elevated with 2.5 to 3.9 wt.%. On the other hand, MgO is heterogeneous at relatively low levels (3.2 to 6.2 wt.%; Mg # 32-45). 87Sr/86Sr and epsilon Nd values are quite homogeneous with MORB-like ratios of 0.7025 and +11.8, respectively. Pb isotopic ratios, on the other hand, are somewhat lower than typical Pacific MORB values but lie well in the depleted mantle field. Within error, consistently young 39Ar/40Ar ages of 1.3 +/- 0.3 Ma and 1.4 +/- 0.7 Ma were obtained from the interior zone and the outer rim. More than 99% of the Ar was of atmospheric origin, which was responsible for the relatively large errors. Our high-Ti basalts are best interpreted as resulting from a propagating rift magmatism, probably as a consequence of subduction related stress in the Pacific in front of the North American Plate. This observation is also supported by magnetic anomalies and the relief of the ocean floor in the East Pacific Rise –- Pacific Cocos Segment.
T11B-1862
Forearc Basin Location Originating From Tectonic Inversion Along an old Ophiolite Suture : the Gulf of Guayaquil-Tumbes Basin (Ecuador-Peru Border)
The Gulf of Guayaquil-Tumbes basin (GGTB) located along the Andean forearc (Ecuador-Peru border) developed in the tectonic wake of the coastwise, northward migrating North Andean block (NAB). The Industrial multichannel seismic and well data (Witt and Bourgois, in press) document that E-W trending low- angle (10-20°) detachment normal faults accommodated the main basin subsidence steps during the Late Pliocene-Quaternary times (1.8-1.6 Ma to Present). It includes the Posorja Jambeli and the northward dipping Tumbes Zorritos detachment systems (PJDS and TZDS) located respectively along the northern and southern edge of the basin. A major transfer system, the N-S trending Inner Domito Banco Peru fault system bounds the detachment systems to the West. The right lateral transcontinental strike-slip system of the Dolores Guayaquil Megashear bounds the basin to the East. Since the PJDS and TZDS extend 80 to 120 km at seafloor they must penetrate the brittle continental crust, far below the 6-8 km thick sediment accumulation at basin depocenters. We assume that detachments extend deep into the 8-10 km thick brittle crust down to the Nazca-South America plate interface at less than ~20 km beneath sea bottom at site. The active TZDS, which connects landward with the continental structures assumed to be part of the eastern frontier of the NAB is the master detachment fault system controlling the basin evolution through time. Gravimetric and geologic data show that depocenters are located along the 80-60 Ma obduction bounding at depth the Cretaceous ophiolite of northern Andes from the westward underthrusted South America continental basement (Bourgois et al., 1987). Because inference suggests the obduction megathrust to branch upward to the TZDS, we hypothesized that tectonic inversion occurred along the ophiolite suture during the GGTB evolution, at least for the past 1.8-1.6 Myr. The 80-60 Ma ophiolite suture is an old zone of weakness, which reactivation from the NAB northward drift controlled the GGTB location. Bourgois, J., Toussaint, J-F, Gonzales, H., Azema, J., Calle, B., Desmet, A., Murcia L.A., Acevedo, A.P., Parra, E., and Tournon, J., 1987, Geological history of the Cretaceous ophiolitic complexes of Northwestern South America (Colombia Andes): Tectonophysics, v. 143, p. 307-327. Witt, C. and Bourgois, J., Forearc basin formation in the tectonic wake of a collision-driven, coastwise migrating crustal block: the example of the North Andean block and the extensional Gulf of Guayaquil-Tumbes basin (Ecuador-Peru border area): Geological Society of America Bulletin, in press.
T11B-1863 INVITED
Microplates in the Greater Indonesian Region
The Indonesian region is an active orogenic belt but poorly known compared to others of similar size. Its development involved multiple small plates with boundaries that changed frequently during the Cenozoic. The region is commonly used to provide analogues and models that often reflect inadequate knowledge rather than realistic scenarios. It is impossible to understand its history without adequate reconstruction of small plates and ignoring them leads to erroneous conclusions. At present our data are largely limited to surface observations, combined with seismicity. Tomography generally resolves only the big plates and may never image the small ones. However, surface geology may yield a biased upper crustal view. How are microplates recognised? The classic features used to establish continental drift cannot be used. Some are easy to define – others not, something particularly problematical in deforming continental regions. GPS and seismicity on one hand, and the geological record on the other, do not always tell the same story. Analyses of active data often confuse the two. In the longer term, some microplates are short lived and may leave no record. Where do they form? In Indonesia boundaries are typically older structures or edges of thickened crust. Almost all new subduction zones developed at arc/ocean crust boundaries and never at young spreading centres. Old faults were repeatedly reactivated. Boundaries of some microplates are easily defined in places but elsewhere impossible to identify. Is this because the boundary is new, and not yet established (e.g. a developing fracture) or is it because deformation continues to be accommodated in a wide zone? Some microplates have spreading centres and subduction boundaries. With others it is not clear if they are genuinely plates, or if they have plate-like boundaries at the surface but are detached at shallow levels in the lithosphere. What drives small plate motions? Extension, contraction and strike-slip movements resulted from plate coupling and subduction hinge movements. Sometimes it is clear that larger plates drive small plates, e.g. by slab pull, or are partly coupled, and motions are partitioned in plate boundary zones. But do strike-slip faults really cut the entire lithosphere? East Indonesia is an example of microplate complexities, in this case driven by subduction.
T11B-1864
Surface Uplift and Disequilibrium Fluvial Geomorphology of Metamorphic Core Complexes in the D'Entrecasteaux Islands and Dayman-Suckling Massif, Papua New Guinea
Structural, thermobarometric, geochronological, and thermochronological evidence suggests that the D'Entrecasteaux Islands (DEI) and the Dayman-Suckling massif (DSM), in southeastern Papua New Guinea, are metamorphic core complexes, which have been exhumed from depths as great as 90 km over the past 2- -8 Ma, possibly because of microplate rotation. The dome-shaped DEI and DSM reach elevations of ~2500 m and ~3500 m, respectively, however little is known about their kinematic histories since their emergence above sea level in the Plio-Pleistocene. Detachment faults are commonly corrugated or dip- slope surfaces occurring in various states of dissection. The question of whether faults bounding the DEI domes are active today remains debated, yet has implications for what mechanisms have exhumed the core complexes as well as where and how active rifting in the Woodlark rift is accommodated. In order to provide a regional overview and direct future tectonic geomorphic studies, we conducted a stream profile analysis of the DEI and DSM using a 3-arc-second Shuttle Radar Topography Mission digital elevation model. Footwall stream profiles in the study area are characterized by large knickpoints across which stream gradients steepen downstream by a factor of 2. These knickpoints are not typically associated with mapped lithologic contacts or faults and occur in all lower plate lithologies. Therefore, we interpret the knickpoints as transients formed as the result of a Quaternary region-wide increase in rock uplift rate or a decrease in stream erodibility. Model profiles extrapolated from relict reaches above knickpoints project to former base levels 40- -1600 m above sea level, indicating similar amounts of incision and rock uplift. Although the timing of knickpoint initiation is not tightly constrained and may vary throughout the region, estimates of profile uplift measured this way correlate linearly with normalized stream steepness index (ksn) below knickpoints, suggesting that observed variations in ksn are related to variations in rates of rock uplift. Furthermore, ksn measured above and below knickpoints increases from east to west across the DEI and DSM, suggesting that rates of active rock uplift similarly increase from east to west, a result that is consistent with low-temperature thermochronological data in the DEI. Disequilibrium stream profiles indicate that erosion has not kept pace with rock uplift and thus there has been surface uplift of the domes. Reorganized drainage networks provide complementary evidence for recent tectonism. In the DSM, barbed tributaries, wind gaps, and captured streams incised <50 m into the detachment surface resulted from dome growth and surface uplift after exhumation of the detachment fault. In the DEI, beheaded and captured streams are also common where aggressively headward eroding streams intersect more slowly eroding relict portions of adjacent drainage basins. Yet, dome amplification is not readily explained as the consequence of steady fault slip and flexural footwall uplift, lower crustal flow, or magmatic underplating. In light of seismic evidence for a shallow Moho and hence mantle support of the DEI metamorphic core complexes, surface uplift and an increase in rock uplift rate of domes region-wide both in the Quaternary and towards the west likely reflect the role of mantle dynamics on topography and microplate boundary processes. This profound topographic change may also be related to the spontaneous spreading center reorientation in the Woodlark Basin at ~70 ka.
T11B-1865
Low-Temperature Constraints on the Evolution of Metamorphic Core Complexes of the Woodlark Rift System, Southeastern Papua New Guinea
Subduction of the Woodlark microplate at the New Britain trench is driving seafloor spreading in the Woodlark Basin and, to the west of the basin, extension of continental crust in the Woodlark rift. Seafloor spreading in the Woodlark Basin initiated in the east by ~6 Ma, after which it has propagated westward. To the west, in the Woodlark rift, one of the most rapidly extending places on earth, exhumation has been locally both profound and rapid (cm/yr rates). In the NW part of the D'Entrecasteaux Islands, HP and UHP rocks have been unroofed from depths ≥90 km in the last 2-8 Ma. As part of a multidisciplinary study to understand the tectonic evolution of this region and to constrain how rifting is exhuming the world's youngest HP/UHP rocks, low temperature thermochronology is being applied to the metamorphic core complexes of the D'Entrecasteaux Islands, as well as to continental fragments along the rifted conjugate margins of the oceanic Woodlark Basin farther to the east. Within the Woodlark Basin and Woodlark rift, apatite fission track (AFT) ages generally decrease from east to west, from ~7.5 Ma in the lower plate of the Misima metamorphic core complex, to ~3 Ma at Moresby Seamount, to between ca. 1.5 and 0.5 Ma in the D'Entrecasteaux Islands. Precision of AFT ages is poor because of low [U] and hence very few tracks are present. There is considerable variation amongst apatite (U-Th)/He (AHe) single-grain ages but not with respect to [eU], indicating that the radiation damage trapping model is likely not responsible for the observed single grain age distribution. Because of the extremely rapid cooling, anomalous older apparent AHe ages are likely due to external 4He implantation from adjacent minerals. We report ranges of minimum AHe ages as these have been shown to be reliably closer to actual AHe ages. AHe single grain ages decrease from east to west, from 3-6 Ma at Misima Island, although AHe ages may be younger on the western end of the island, to between 2.0 and 0.3 Ma within the D'Entrecasteaux Islands. Age variations or trends within the individual D'Entrecasteaux Islands, or among fault-bounded domes within the islands, are difficult to constrain because of the poor age precision and single-grain AHe age variation. Nonetheless, both AFT and AHe ages are youngest (<1 Ma) on Goodenough Island, the western-most of the D'Entrecasteaux Islands. Interpretation of both AFT and AHe data is improved by comparison with much higher precision 40Ar/39Ar ages on a variety of minerals (e.g., K-feldspar). The general trend of ages decreasing to the west is consistent with rapid cooling due to tectonic exhumation of the lower plates of these core complexes in the late Miocene and Pliocene to Holocene as continental extension operated hundreds of km ahead of the propagating seafloor-spreading rift tip in the Woodlark Basin.
T11B-1866
Exhumation and brittle to ductile deformation of the Suckling-Dayman core complex along an active microplate boundary, eastern Papua New Guinea
The ~1500 km2 twin domes of the Suckling-Dayman massif is arguably the world's only active core complex with elevations ranging from 2850 to 3676 m. Its northeastern edge is a greater than 1000-m-high fault scarp with a surface slope of 18-21 degrees that sharply defines part of the 1400-km-long transtensional Owen Stanley fault zone (OSFZ) separating the Australia plate and the Woodlark microplate. The fresh, linear scarp of the OSFZ elevates Holocene reefs on its footwall at rates up to 3.4 mm/yr and exposes shallowly dipping mylonitic (S1) rocks on the flanks of Mounts Dayman and Suckling. Down dip fault slip on the OSFZ is indicated by (i) megamullions observed in shuttle radar topography mission (SRTM) data and (ii) mineral stretching lineations (L1) and kinematic indicators observed at outcrop and thin section scale. Field relationships show a cross cutting sequence of structures that includes: (i) ductile S2 folia with ESE-plunging riebeckite mineral stretching lineations; (ii) narrow steeply dipping ductile D2 shear zones; and (iii) semi-brittle to brittle fault zones. Kinematic vorticity analysis of the highest-grade ductile deformation indicates a kinematic vorticity number (Wk) between 0.34 and 0.56, suggesting general shear for the early stage of deformation (D1). Metamorphic mineral assemblages of the metabasite ductile mylonitic rocks record greenschist facies conditions. The presence of pumpellyite-actinolite facies assemblages in the core of the complex indicates peak metamorphic pressures of 6-9.5 kbar, demonstrating exhumation of the core from 20- 30 km depth. 3.3 Ma granite and monzonite intrusions cut the mylonitic fabric. A SE-dipping, Mio-Pliocene sedimentary sequence (Gwoira Conglomerate) occupies the hanging wall of the metamorphic core complex. Petrography of the clasts within the sedimentary rocks indicates that metabasite rocks were the dominant source. The sedimentary unit is in low-angle fault contact with the metabasite footwall across prehnite- bearing D3 brittle fault zones. Erosion pattern across the dome suggests continuing recent fault movement and uplift.
T11B-1867
Metamorphic Core Complex Formation on Misima Island during Miocene-Pliocene Rifting and Seafloor Spreading in the Woodlark Basin, Papua New Guinea
The Misima Island metamorphic core complex (MCC), located on the southern rifted conjugate margin of the Woodlark Basin is bisected by a major low angle fault that dips ~25° to the NE. Lower plate amphibolite-facies felsic and mafic gneisses are juxtaposed against upper plate greenschist-facies schists and overlying unmetamorphosed sedimentary and volcanic rocks. A thermochronologic and geochemical study of the Misima Island MCC was undertaken to assess possible protoliths, and the timing and process(es) of rock exhumation. REE patterns for lower plate rocks are flat when normalized to the average composition of shales and are geochemically similar to rocks exhumed in the D'Entrecasteaux Islands MCCs that lie to the west. Hf and Nd isotopic compositions of these samples are variable and could reflect protolith differences. The Pb isotopic compositions of these samples lie within the field of overlap between Pacific MORB and marine sediments, and are broadly similar to previously reported Pb isotopic compositions from this region. Ion microprobe U-Pb analyses revealed a population of 90-150 Ma inherited zircons indicating lower plate gneisses were derived in part from Early Cretaceous protoliths. Amphibole yielded complex spectra with 40Ar/39Ar apparent ages ranging from 15.1 to 9.8 Ma, and one sample exhibited partial argon loss at ~4 Ma. These data are interpreted to indicate cooling through Ar closure in amphibole and exhumation from depths of >15km during the mid-late Miocene. Biotite from lower plate gneisses yielded relatively flat 40Ar/39Ar spectra with apparent ages ranging from 8.0-7.6 Ma and are interpreted to reflect continued cooling and exhumation in the Late Miocene. Biotite 40Ar/39Ar ages are concordant with previously reported U-Pb zircon crystallization ages on the Boiu granodiorite indicating Late Miocene synextensional intrusion into the lower plate. Low temperature thermochronometers, including 40Ar/39Ar K-feldspar, AFT, and apatite (U-Th)/He data (7.5-3 Ma), indicate Late Miocene to Early Pliocene differential cooling within the lower plate and final exhumation to shallow crustal levels. Hydrothermal alteration and associated mineralization occurred during the final phase of exhumation to shallow crustal levels. Greenschist facies lithologies comprising the Misima upper plate were challenging from which to derive any thermochronologic information. One sample yielded a complex amphibole Ar loss profile with oldest 40Ar/39Ar apparent ages of 9.7 Ma. In summary, mid-late Miocene cooling and exhumation of the Misima MCC commenced prior to the earliest known record of seafloor spreading in the Woodlark Basin (i.e., ~6 Ma). The normal fault that presently bisects the island contains a >3 m-thick layer of gouge with a NNE slip direction that is parallel to the Australian–Woodlark plate vector between 0.5-3.5 Ma calculated from seafloor spreading data. Fault movement can be directly related to extension within the Woodlark rift and has been active until at least 3 Ma.
T11B-1868 INVITED
Rapid Microplate Rotations at the Transition From Subduction to Collision
The majority of rapidly rotating microplates on Earth occur in the forearcs of subduction zones, where there is an abrupt, along-strike transition from normal subduction to collision or partial subduction of a buoyant, bathymetric high. We review geophysical and geological data from 23 active and ancient plate boundaries to document a compelling spatial and temporal relationship between this transition from collision to subduction, rapid rotation of a forearc microplate, plate boundary curvature, and the occurrence of coeval, back-arc rifting. In particular, we will show examples of contemporary microplate rotation at collision/subduction transitions from several locations, including: Papua New Guinea, the Mediterranean, Vanuatu, New Zealand, the Marianas, and Tonga. Our observations support a conceptual model where an along-strike change from subduction to collision of a bathymetric high exerts a torque on forearc microplates, leading to rapid vertical axis rotations, and marked plate boundary curvature. Our global compilation also reveals that most active back-arc rifts are bounded on one edge by a rapidly rotating forearc block or microplate, indicating that collision-induced microplate rotation may play a major role in the episodic opening of back-arc rifts. Finite element modelling simulating the presence of an indentor or choke point (e.g., collision) in the subduction system produces rapid rotation of the forearc about a nearby pole as seen in the natural examples. In numerical models, the rate of forearc rotation depends on the incoming buoyant indentor velocity, and can be greatly enhanced by slab rollback and the presence of a low-viscosity back-arc. Where viscosity of the back-arc is low, forearc microplate rotation dominates; where back-arc viscosity is high, the formation of strike-slip faults and tectonic escape dominates. Our observational and model-derived results show that forces produced by collision or partial subduction of bathymetrically high and buoyant features at subduction margins are a fundamental cause of rapid microplate rotation. Processes like slab rollback will enhance microplate rotation and back-arc rifting, but rollback is not always a pre-requisite for rotation and rifting.
T11B-1869
Late Quaternary Activity and Seismogenic Potential of the Gonave Microplate: Plantain Garden Strike-Slip Fault Zone of Eastern Jamaica
At the longitude of Jamaica, Caribbean (Carib)-North America (Noam) plate motion of 19 ± 2 mm/a is carried by two parallel, left-lateral strike-slip faults, the Oriente fault zone, immediately south of Cuba, and the Enriquillo-Plantain Garden fault zone (EPGFZ), which lies 100-150 km further south. It has been postulated that the lithosphere between these faults constitutes an independent Gonave microplate that has formed in response to the ongoing collision between the leading edge of Carib in Hispaniola and the Bahama carbonate platform. GPS measurements in Jamaica and Hispanola is supportive of the microplate hypothesis and indicates that roughly half of Carib-Noam plate motion (8-14 mm/a) is carried by the EPGFZ of southern Hispaniola and eastern Jamaica. This study applies geomorphic and paleoseismic methods as a direct test of the activity and amount of microplate motion carried on the Plantain Garden fault segment of eastern Hispaniola and how this motion is distributed across a large restraining bend that has formed the island of Jamaica since the late Miocene. The EPFZ curves gently to the northeast and forming a steep mountain front to the Blue Mountains restraining bend with elevations up to 2200 m. Geomorphic fault-related features along the mountain front fault zone include left-laterally deflected rivers and streams, but no small scale features indicative of Holocene activity. River and stream deflections range from 0.1 to 0.5 km. We identified and trenched the most active trace of the mountain front fault at the Morant River where the fault is characterized by a 1.5-m-wide sub-vertical fault zone juxtaposing sheared alluvium and fault Cretaceous basement rocks This section is overlain by a 6-m-thick fluvial terrace. Trenching in the unfaulted terrace immediately overlying the fault trace revealed radiocarbon and OSL ages ranging from 20 to 21 ka that are consistent with a prominent unfaulted alluvial fan along the projection of this fault 1.5 km to the east. Channel profiles constructed for 11 rivers and streams crossing the fault show concave-up profiles indicating a dominance of erosion over active tectonic uplift. We conclude that motion on the EPGFZ is highly episodic and therefore may not be the source of large, destructive earthquakes that caused widespread destruction and landslides in eastern Jamaica in 1692 and 1907.
T11B-1870
Late Quaternary Activity and Seismogenic Potential of the Gonave microplate: South Coast Fault Zone of Southern Jamaica
The South Coast fault zone (SCFZ) strikes east-west and forms a scarp as high as 600 m along the southern coast of Jamaica. It has been postulated that this fault acts as a left-lateral, strike-slip 'bypass' fault that truncates the large, right-stepping restraining bend formed between the Plantain Garden fault zone of southeastern Jamaica and the Duanvale-Walton fault zone of northwestern Jamaica. GPS measurements near the SCFZ show anomalously rotated vectors consistent with active left-lateral shear. Anomalous topography along the trace of the SCFZ includes two, doubly plunging anticlines: Kemp's Hill (119 m), an isolated high in the otherwise flat Vere Plain, and Round Hill (333 m), a larger high directly adjacent to the coast. Field work identified the most active trace of the SCFZ in a notch along the north flank of Round Hill; this trace can be extrapolated to the west along the coast and east that locally defines a low scarp in alluvium. Channel profiles constructed for six rivers and streams crossing the projected trace of the SCFZ show convex-upward morphologies, consistent with dominance of tectonic uplift over river downcutting. To better define the subsurface location of the SCFZ beneath the Vere Plain, a gravity survey network consisting of 327 stations and covering an areas of 500 km2 was performed using a Lacoste and Romberg G-meter. Differential GPS allowed centimeter-level elevation control for each station. Gravity corrections (elevation, latitude, instrument drift, and earth tides) were made using QC Tool software, and topographic and terrane corrections were made using both local topographic measurements and high-resolution SRTM data. An ~20 mgal negative gravity anomaly on the otherwise flat gravity field of the Vere Plain corresponds with the projected trace of the SCFZ across the Vere Plain and the locations of one river offset. We interpret that the SCFZ has down-to-the-south throw, which has led to thickening of Quaternary sediments south of the fault. Further work, including geodetic surveys and fault trenching, will better constrain the amount of late Quaternary motion on the SCFZ.
T11B-1871
The Jan Mayen Microcontinent and the Evolution of the NE Atlantic Region
Breakup and seafloor spreading between Greenland and Eurasia established a series of new plate boundaries in the North Atlantic region since the Late Paleocene. A regional kinematic model from the pre- breakup to present day is assuming that Eurasia and Greenland moved apart as a two plates system. However, new regional geophysical datasets and quantitative kinematic parameters show that this system suffered several adjustments since its inception and suggest that additional temporal plate boundaries existed in the NE Atlantic. Among the consequences of numerous plate boundary relocations is the formation of a highly extended or even fragmented Jan Mayen microcontinent (JMMC) and subsequent deformation of its margins and surrounding regions. The new plate kinematic model and preliminary interpretations of potential field and seismic data indicate that the JMMC experienced a significantly longer and more complex tectonic evolution than has previously been considered. Several separate tectonic blocks within the JMMC have been interpreted, and we suggest that the southernmost extended, fragmented character of the JMMC is a product of several failed ridge propagation attempts of the Kolbeinsey Ridge. In addition, we have identified several compressional events SE and NE of the JMMC that are partially ground truth by geophysical evidences. Our model implies a series of failed ridges offshore the Faroe Islands, a northern propagation of the Aegir Ridge NE of JMMC, and a series of triple junction and/or propagators in the southern Greenland Basin. The propagation of plate boundaries within the Greenland plate led to episodic magmatic events, as suggested by recent studies.
T11B-1872
The evolution of paleostress fields in the Central European Basin System reconstructed from fault-slip data
The Central European Basin System (CEBS) reveals a complex structure resulting from a polyphase deformation history since the Permian. The basin system is framed by two major NW-striking fault systems, the Elbe Fault System (EFS) in the south and the Tornquist Zone (TZ) in the north. We investigate the kinematics of faults on the outcrop scale to estimate the diversity of paleostress states responsible for the observed strain. The method used to estimate the reduced stress tensors for the measured fault populations integrates graphical and numerical approaches of fault-slip analysis. This technique facilitates the separation of heterogeneous data sets and guarantees each estimated stress state to fulfil both the criterion of low misfit angles and the criterion of high shear-to-normal stress ratios. For the basin-wide reconstruction of paleostress fields, the orientations of more than 850 faults with known slip directions have been sampled from outcrops across the EFS, where Upper Carboniferous, Upper Permian, Middle Triassic, Upper Jurassic, and Upper Cretaceous rocks are exposed. In addition, more than 4600 fault-slip data from the Oslo Graben area north of the TZ have been sampled from rocks of Precambrian to Permian ages. For both study areas, a polyphase paleostress history is established. The most prominent paleostress field reconstructed for the EFS is characterised by a horizontal N-S- to NE-SW-directed maximum compression combined with a relatively low stress ratio. This stress field can clearly be assigned to a phase of basin inversion which is known to have affected the entire CEBS in the Late Cretaceous-Early Tertiary. The signs of earlier phases of deformation are widely overprinted in the study area. On the contrary, the most prominent paleostress field reconstructed for the Oslo Graben area corresponds to radial tension and is related to the phase of rifting and graben formation during the Late Carboniferous and Early Permian. The distribution of preserved traces of deformation indicates that the crustal domains south of the TZ have intensely been affected by late Mesozoic and Cenozoic stress fields as opposed to areas north of the TZ where the effects of Paleozoic and early Mesozoic stress fields are preserved.
T11B-1873
Break-up of Pangaea and Tectonic History of the Adria Microplate
A new kinematic model is proposed for the tectonic evolution of the western Tethys and the Adria microplate, constrained by the Atlantic plate kinematics and on land geologic evidences. The model defines thirteen tectonic phases, spanning the time interval from the late Ladinian (230 Ma) to the present. During the first phase, from the late Ladinian (230 Ma) to the latest Rhaetian (200 Ma), rifting proceeded along the eastern margin of North America, the northwest African margin, the High, Saharan and Tunisian Atlas, determining the formation of a separate Moroccan microplate at the interface between Gondwana and Laurasia. To the East, the break-up of Pangaea proceeded through a new phase of spreading in the central Mediterranean (Lagonegro basin) and the Tethys Ocean, while Adria remained attached to the southern Eurasian margin. During the second phase, from the latest Rhaetian (200 Ma) to the late Pliensbachian (185 Ma), oceanic crust started forming between the East Coast and Blake Spur magnetic anomalies, while the Morrocan Meseta simply continued to rift away from North America. During this time interval the Atlas rift reached its maximum extent. In the western Tethys region, cessation of spreading in the Lagonegro and Ionian Basins was accompanied by the formation of a new plate boundary which rifted Adria from Eurasia. The third phase, from the late Pliensbachian (185 Ma) to chron M21 (147.7 Ma), was triggered by the northward jump of the main plate boundary connecting the central Atlantic with the Tethys area. Therefore, as soon as rifting in the Atlas zone ceased, plate motion started along complex fault systems between Morocco and Iberia, while a rift/drift transition occurred in the northern segment of the central Atlantic, between Morocco and the conjugate margin of Nova Scotia. Sea floor spreading also started in the Mediterranean area, forming the Ligurian and Alpine Tethys oceans. The next two phases, from chron M21 (147.7 Ma) to chron M0 (120.4) describe the process of formation of an independent Iberian plate and the opening of the Valais oceanic seaway. The sixth phase, from chron M0 (120.4) to the Albian-Cenomanian boundary (Ĕ100 Ma), is associated with the early Alpine collision. During the seventh phase, from the Albian-Cenomanian boundary (Ĕ100 Ma) to chron C34ny (83.5 Ma), a relatively large block comprising Adria, Greece, and Turkey separated from Africa, determining a new spreading event in the eastern Mediterranean and further East in the Tethys. The next four phases, from chron C34ny (83.5 Ma) to chron C13 (33.1 Ma), describe the Pyrenean and Alpine collisions, the subduction of the Ligurian and Valais oceans, the progressive internal deformation of Adria, and the assembly of the modern Anatolian block. In phase 12, from chron C13 (33.1 Ma) to chron C6n (19 Ma), are included the formation of the Atlas mountain belt and the onset of rifting and sea-floor spreading in the western Mediterranean. During this time interval Morocco was an independent plate with a distinct spreading rate with respect to North America. Finally, during the last phase passive subduction of the last remains of southern Liguride, Ionian, and Pennine oceans determined the opening of the Alboran, Tyrrhenian, and Pannonian basins, accompanied by trench retreat. Thirteen plate tectonic reconstructions and a computer animation are proposed to illustrate the major phases of plate motions in the western Tethys region during and after the breakup of the Pangaea supercontinent.
T11B-1874
The Tectonic Evolution of Kinematic Blocks Along Major Plate Boundaries: the Case History of the Hyblean Region (Central Mediterranean)
The Hyblean Region (SE Sicily) in the Central Mediterranean represents a crustal block, which is entrapped at the junction between two major plate boundaries. The former consists of the E-W oriented Nubia-Eurasia boundary and the latter is represented by an incipient, roughly N-S trending, divergent margin, which has propagated from the Italian Peninsula through the African continental domains of Sicily. Usually interpreted as part of the stable African platform, the Hyblean region, during the Quaternary, has actually played the role of an independent crustal block, if framed in the larger scale plate motion. Since about 1.5 Ma, the Hyblean Block, in fact, was isolated from the rest of the Africa margin, as consequence of the propagation of the divergent margin through the SE Sicily. Since the Middle Pleistocene (¡Ö0.8Ma), the Hyblean Block was accreted to the Nubia-Eurasia boundary, as the result of a sudden east-ward jumping of the divergent margin, to its present location. The Quaternary evolution of the Hyblean Block has emphasized some peculiar tectonic, kinematic and dynamic features that, being potentially diagnostic also for the identification of microplates, can be summarised as follow: 1. Occurrence of main regional Quaternary (<1.5 Ma) tectonics that are incongruent with the stress-in-situ measurements, focal mechanisms and geodetic data; 2. Evidence of very recent positive tectonic inversion of the Early Quaternary structures, coupled with sudden change in the displacement rate, not accompanied by variation in the larger scale plate motion; 3. Distribution of high- magnitude seismicity (6¡ÜM¡Ü7) along the tectonic boundaries of the crustal block; 4. Distribution of low- magnitude seismicity within the block, mostly independent from the geometry of the major Quaternary tectonics. The recognition of the Hyblean Block represents a key constraint in modelling the large scale deformation of the Central Mediterranean region. The correction of the GPS vectors measured in the Hyblean region (e.g. Noto site), due to its relative motion with major plates, has consequence on the resulting kinematics of the E-W oriented segment of the Nubia-Eurasia boundary, crossing the northern Sicily. Along this alignment, a prevalent right-lateral motion, at rate of about 1.7 mm/a, would result instead of the NW-SE oriented convergence at rate of 1.9 mm/a, predicted by previous model.
T11B-1875
The Nestos shear zone in the southern Rhodopes (Northern Greece/Southern Bulgaria) – a late Eocene to Oligocene mid-crustal detachment
The Nestos shearzone in the Rhodopes (Northern Greece/Southern Bulgaria) separates a composite gneis unit, here called mixed gneiss unit, in the hangingwall from the Pangaion-Pirin complex in the footwall. The mixed gneiss unit consists of various levels of mingled continental and oceanic basement rocks, intruded by granitic bodies of Cretaceous and Tertiary age. The entire unit underwent at least amphibolite facies conditions during the alpine orogenic cycle and several localities with preserved high-pressure and/or ultrahigh-pressure rocks have been found. The age of metamorphism is ambiguous and several Mesozoic and Tertiary cycles have been inferred for that unit. The lowermost level immediately on top of the Nestos shear zone, the so-called Sidironero unit, appears to show the youngest reported high-grade metamorphism of Eocene age (including ultra-high-pressure conditions and a subsequent migmatic stage). The underlying Pangaion-Pirin unit consists of marbles and gneisses intruded by Tertiary granitoids. This unit experienced only blueschist facies followed by upper greenschist facies conditions during the alpine cycle. The Nestos shear zone is defined by top-to-the-southwest-directed mylonites formed under upper greenschist facies conditions. So far, it has been viewed as a thrust. We present structural and petrological data suggesting that the Nestos-shear zone is instead a major mid-crustal detachment related to late Eocene-Oligocene extension in the Rhodopes. Mylonitisation along the shear zone was associated with exhumation of the footwall from blueschist facies to greenschist facies conditions. The shear zone formed between about 40 Ma and 30 Ma as indicated by the age of migmatites in the hangingwall and the age of the Xanthi pluton, which crosscuts the mylonites. During this time, pronounced extension and basin formation took place in the hangingwall of the Nestos shearzone. We propose that the well-known brittle Ribnovo detachment, which bounds the Mesta Graben to the East, roots into the Nestos shearzone.
T11B-1876
Relation Between Tectonics and Intrusions in the Western Rhodope Mountains, SW Bulgaria
We present an E-W profile through the Western Rhodopes (Bulgaria). It runs through basement rocks, granitic intrusions therein and Tertiary extension-related structures such as the Strouma and the Mesta Graben. Two major alpine units are exposed in the basement, the Pangaion-Pirin complex below and a mixed gneiss unit above. The Pangaion-Pirin unit consists mainly of marbles, variscan orthogneisses, and Tertiary ganitic intrusions and underwent only upper greenschist facies conditions during the alpine cycle. The mixed gneiss unit is made up of different continental and oceanic high-grade metamorphic rocks, including preserved HP and UHP relicts at several localities. U/Pb zircon geochronology was carried out on samples from three granitic intrusions, the Spanchevo, Teshovo and Dolno Dryanovo plutons using LA-ICP-MS at GEUS in Copenhagen. The Teshovo pluton is located in the Pangaion-Pirin complex. Apart from a weak magmatic foliation, it is undeformed and thus intruded posttectonically. Zircons within the assayed samples are single-phased and give well defined crystallization ages of around 32 Ma. The Spanchevo and Dolno Dryanovo plutons are partly deformed bodies within the mixed gneiss unit. Single-phase magmatic zircons and magmatic rims yield ages of around 56 Ma, whereas inherited cores display ages of around 146 Ma. We propose that (1) ductile deformation in the Pangaion-Pirin complex ceased before 32 Ma and (2) penetrative deformation lasted beyond 56 Ma in the mixed gneiss unit above. Due to the lack of inherited variscan cores in zircons from the Spanchevo and Dolno Dryanovo plutons, we infer that the upper complex was not yet placed upon the Pangaion-Pirin complex at 56 Ma. Hence, the southwestward thrusting of the upper complex over the Pangaion-Pirin complex took place between 56 and 32 Ma.
T11B-1877
Structural Geology of Daba Shan and its Tectonic Relationships with the Sichuan Basin and Central China Orogen, China
Daba Shan is a fold-and-thrust belt located on the northeastern margin of the Sichuan Basin, central China orogen. It is the transitional zone between the Sichuan Basin and Qinling orogenic belt, and it is located in the middle part of the Mianlu suture zone which is the boundary between the Qinling orogenic belt and Yangtze block. Numerous faults and fault-related folds are well preserved in Daba Shan. It is a natural laboratory to carry out fold-thrust belt research on relationships between the Qingling orogen and subsidence in the Sichuan basin. In this talk, I will introduce the general geologic background about and around Daba Shan, including the geologic history of the central China orogen, the formation and development of the Mianlue suture, and the most popular ideas about the geodynamic evolution of Daba Shan, as well as its geologic position between the Sichuan Basin and North China craton and its relative geodynamic relationship with Mianlue suture zone. Field investigations have shown the different fault-related structure styles, e.g. fault-bend fold, fault-propagation fold, duplex structure across the orogen. In addition, a major extensional detachment fault, the Chengkou fault, crops out in the center of the orogen and dips beneath northern Daba Shan fold-thrust belt and Mianlue suture. It is so impressive of the typical and complex geological structure scenarios there, which were mostly caused by the collisional and post-collisional activities between Qinling micro-continent and Yangtze block since mid-Triassic time. Daba Shan has very important tectonic and economic significance in China. Although geoscientists have been working on the Sichuan Basin and central China orogen for many years, Daba Shan has gained little attention. These years, with gas and oil exploration development in foreland basin and fold-thrust belt areas, especially after the discovery of carbonate strata in Daba Shan, its economic potential has become more prominent. This research will be significant for geoscientists to better understand the geologic features of Daba Shan. For the further research, I will complete construction of balanced cross-sections, calculation of the shortening amount, and determine the timing of the faulting in Daba Shan with the interpretation of seismic-reflection profiles interpretation and remote sensing images to better understand the geologic structures of Daba Shan. By studying the formation and evolution histories, the tectonic relationships among Daba Shan, Yangtze platform and Qinling orogenic belts will be more obvious.