Papers by Konstanze Stübner
The Pamir forms the northwestern tail of the Tibetan plateau and is a first-order tectonic featur... more The Pamir forms the northwestern tail of the Tibetan plateau and is a first-order tectonic feature of the Cenozoic Indo-Eurasian collision. The nature of the topographic uplift and orogenic growth of the entire northwestern margin of the Pamir is poorly constrained; however, this history can provide important constraints that are required to test geodynamic models of the tectonic evolution of the Pamir. Here we focus on the uplift history of the western and northwestern unglaciated margin of the Northern Pamir, the Darvaz and the Peter-the-First Ranges. These three ranges were formed by three major fault systems: the Main Pamir Thrust (MPT), the Darvaz and the Vakhsh fault zones (DFZ, VFZ). To assess the impact of tectonic uplift on the geomorphic evolution, we analyzed geomorphic characteristics of the topography, the longitudinal river profiles and the relief. To better constrain the regional crustal cooling history and uplift, we obtained thermochronologic cooling ages from the t...
Die Spaltspuren-Datierung als wesentliche Methode aus dem Bereich der Niedrigtemperatur-Thermochr... more Die Spaltspuren-Datierung als wesentliche Methode aus dem Bereich der Niedrigtemperatur-Thermochronologie basiert auf der Zählung und Messung geätzter Spuren unter dem Mikroskop. Für eine akkurate Altersbestimmung ist daher das Verständnis der Ätzung von größter Bedeutung. Ein atomistisches Modell und eine Monte-Carlo Computersimulation erklären Ätzgruben-Formen und deren Größenwachstum. Thermochronologie wird in zwei Fallstudien angewendet: eine umfassende Studie über die tektonische Entwicklung Zentralamerikas seit dem Paläozoikum zeigt, wie Geo- und Thermochronologie, Strukturgeologie und Petrologie zusammenarbeiten können, um >400 Ma einer komplexen tektonischen Geschichte zu enträtseln. Eine thermochronologische Studie im Pamir, Tadschikistan betont vor allem die Möglichkeiten, die sich aus der Anwendung der Thermochronologie auf dem Gebiet der Geomorphologie und Neotektonik eröffnen.
Quaternary Science Reviews, 2021
Abstract Several hundred thousand year old moraines preserved in the semi-arid environment of Hig... more Abstract Several hundred thousand year old moraines preserved in the semi-arid environment of High Mountain Asia attest to Middle Pleistocene glaciations, but the regional correlation of glacial stages and the spatial extent of the glacial advances remain poorly constrained. We examined glacial landforms and Quaternary sediments in the Bartang valley, northwestern Pamir, a region with no previous quantitative glacial chronology. Using cosmogenic 10Be exposure ages, we dated glacially polished bedrock, moraines, and mass wasting deposits. Our data show that the northwestern Pamir was heavily glaciated in the Middle Pleistocene (≥220 ky) with large valley glaciers occupying some of the major valleys in the western Pamir. During the penultimate glacial cycle (191–130 ky) these valleys may have been largely ice free. Catastrophic mega debris flows with volumes ≥0.05 km3 occurred after the ice retreat and reflect paraglacial destabilization of glacial sediments. The age of the best-dated mega debris flow (81 ± 4 ky) is similar to moraine ages ∼70–80 ky documented throughout the Pamir, demonstrating that remobilized sediments may provide valuable age constraints on glacial histories. In order to facilitate regional comparison of glacial chronologies, we developed a Gaussian separation algorithm, which determines a moraine age from a distribution of boulder exposure ages based on the assumption that post-depositional processes prevail over inheritance, and that the oldest boulder ages best represent the timing of moraine formation. We compiled moraine boulder exposure ages from the Pamir and adjacent regions and provide a summary of Middle and early Late Pleistocene glacial cycles of western High-Mountain Asia.
Tectonics, 2018
Despite remarkable tectonostratigraphic similarities along the Himalayan arc, pronounced topograp... more Despite remarkable tectonostratigraphic similarities along the Himalayan arc, pronounced topographic and exhumational variability exists in different morphotectonic segments. The processes responsible for this segmentation are debated. Of particular interest is a 30-to 40-km-wide orogen-parallel belt of rapid exhumation that extends from central Nepal to the western Himalaya and its possible linkage to a midcrustal ramp in the basal décollement, and the related growth of Lesser Himalayan duplex structures. Here we present 26 new apatite fission track cooling ages from the Beas-Lahul region, at the transition from the Central to the Western Himalaya (~77°-78°E) to investigate segmentation in the Himalayan arc from a thermochronologic perspective. Together with previously published data from this part of the orogen, we document significant lateral changes in exhumation between the Dhauladar Range to the west, the Beas-Lahul region, and the Sutlej area to the east of the study area. In contrast to the Himalayan front farther east, exhumation in the far western sectors is focused at the frontal parts of the mountain range and associated with the hanging wall of the Main Boundary Thrust fault ramp. Our results allow us to spatially correlate the termination of the rapid exhumation belt with a midcrustal ramp to the west. We suggest that a plunging anticline at the northwestern edge of the Larji-Kullu-Rampur window represents the termination of the Central Himalayan segment, which is related to the evolution of the Lesser Himalayan duplex.
Earth and Planetary Science Letters, 2018
The Himalayan thrust belt comprises three in-sequence foreland-propagating orogen-scale faults, t... more The Himalayan thrust belt comprises three in-sequence foreland-propagating orogen-scale faults, the Main Central thrust, the Main Boundary thrust, and the Main Frontal thrust. Recently, the Munsiari-Ramgarh-Shumar thrust system has been recognized as an additional, potentially orogen-scale shear zone in the proximal footwall of the Main Central thrust. The timing of the Munsiari, Ramgarh, and Shumar thrusts and their role in Himalayan tectonics are disputed. We present 31 new zircon (U-Th)/He ages from a profile across the central Himachal Himalaya in the Beas River area. Within a ∼40 km wide belt northeast of the Kullu-Larji-Rampur window, ages ranging from 2.4 ± 0.4 Ma to 5.4 ± 0.9 Ma constrain a distinct episode of rapid Pliocene to Present exhumation; north and south of this belt, zircon (U-Th)/He ages are older (7.0 ± 0.7 Ma to 42.2 ± 2.1 Ma). We attribute the Pliocene rapid exhumation episode to basal accretion to the Himalayan thrust belt and duplex formation in the Lesser Himalayan sequence including initiation of the Munsiari thrust. Pecube thermokinematic modelling suggests exhumation rates of ∼2-3 mm/yr from 4-7 to 0 Ma above the duplex contrasting with lower (<0.3 mm/yr) middle-late Miocene exhumation rates. The Munsiari thrust terminates laterally in central Himachal Pradesh. In the NW Indian Himalaya, the Main Central thrust zone comprises the sheared basal sections of the Greater Himalayan sequence and the mylonitic 'Bajaura nappe' of Lesser Himalayan affinity. We correlate the Bajaura unit with the Ramgarh thrust sheet in Nepal based on similar lithologies and the middle Miocene age of deformation. The Munsiari thrust in the central Himachal Himalaya is several Myr younger than deformation in the Bajaura and Ramgarh thrust sheets. Our results illustrate the complex and segmented nature of the Munsiari-Ramgarh-Shumar thrust system.
Lithosphere, 2017
Competing hypotheses suggest that Himalayan topography is sustained and the plate convergence is ... more Competing hypotheses suggest that Himalayan topography is sustained and the plate convergence is accommodated either solely along the basal décollement, the Main Himalayan thrust (MHT), or more broadly, across multiple thrust faults. In the past, structural, geomorphic, and geodetic data of the Nepalese Himalaya have been used to constrain the geometry of the MHT and its shallow frontal thrust fault, known as Main Frontal thrust (MFT). The MHT flattens at depth and connects to a hinterland mid-crustal, steeper thrust ramp, located ~100 km north of the deformation front. There, the present-day convergence across the Himalaya is mostly accommodated by slip along the MFT. Despite a general agreement that in Nepal most of the shortening is accommodated along the MHT, some researchers have suggested the occurrence of persistent out-of-sequence shortening on interior faults near the Main Central thrust (MCT). Along the northwest Himalaya, in contrast, some of these characteristics of central Nepal are missing, suggesting along-strike variation of wedge deformation and MHT fault geometry. Here we present new field observations and seven zircon (U-Th)/He (ZHe) cooling ages combined with existing low-temperature data sets. In agreement with our previous findings, we suggest that the transect of cooling age patterns across the frontal Dhauladhar Range reveals that the Main Boundary thrust (MBT) is a primary fault, which has uplifted and sustained this spectacular mountain front since at least the late Miocene. Our results suggest that the MBT forms an ~40-km-long fault ramp before it soles into the MHT, and motion along it has exhumed rocks from depth of ~8-10 km. New three-dimensional thermokinematic modeling (using Pecube finite-element code) reveals that the observed ZHe and apatite fission track cooling ages can only be explained by sustained mean MBT slip rates between ~2.6 and 3.5 mm a-1 since at least 8 Ma, which corresponds to a horizontal shortening rate of ~1.7-2.4 mm a-1. We propose that the MBT is active today, despite a lack of definitive field or seismogenic evidence, and continues to accommodate crustal shorting by out-of-sequence faulting. Assuming that present-day geodetic shorting rates (~14 ± 2 mm a-1) across the northwest Himalaya have been sustained over geologic time scales, this implies that the MBT accommodated ~15% of the total Himalayan convergence since its onset. Furthermore, our modeling results imply that the MHT is missing a hinterland mid-crustal ramp further north.
Tectonics, 2018
The deformation processes at work across the eastern margin of the Tibetan Plateau remain controv... more The deformation processes at work across the eastern margin of the Tibetan Plateau remain controversial. The interpretation of its tectonic history is often polarized between two deformation models: ductile flow in the lower crust and shortening and crustal thickening accommodated by brittle structures in the upper crust. Many geological investigations on this plateau margin focused on the Longmen Shan, at the western edge of the Sichuan Basin. However, the Longriba fault system (LFS) located 200 km northwest and parallel to the Longmen Shan structures provides an opportunity to understand the role of hinterland faults in eastern Tibet geodynamics. For this reason, we investigate the exhumation history of rocks across the LFS using (U-Th)/He and fission track ages from apatite and zircon. Results show a significant contrast in cooling histories across the Maoergai fault, the southernmost fault of the LFS. South of the Maoergai fault, the bedrock records a rapid increase in exhumation rate since ~10-15 Ma. In contrast, the area north of the fault has experienced steady cooling since ~25-35 Ma. We attribute this cooling contrast to ~2 km of differential rock uplift across the Maoergai fault, providing the first evidence of activity of the LFS in the Late Cenozoic. Our results indicate that deformation of the eastern Tibetan margin has been partitioned into the LFS and the Longmen Shan over an ~200 km wide block, which should be incorporated in future studies on the region's deformation, and in both above-mentioned deformation models.
Geology, 2018
Tectonics and climate are major contributors to the topographic evolution of mountain ranges. Her... more Tectonics and climate are major contributors to the topographic evolution of mountain ranges. Here, we investigate temporal variations in exhumation due to the onset of Pleistocene glaciation in the Olympic Mountains (Washington State, USA). We present 29 new apatite and zircon (U-Th)/He ages (AHe and ZHe), showing a decrease in ages toward the interior of the mountain range for both thermochronometric systems. Young AHe ages (<2 Ma) can be found on the western side and the interior of the mountain range. Thermokinematic modeling of sample cooling ages suggests, that ZHe ages can be explained by an ellipse-shaped exhumation pattern with lowest/ highest rates of ~0.25 and 0.9 km/m.y. These rates are interpreted as tectonically driven rock uplift, where the pattern of rates is governed by the shape of the subducted plate. However, the youngest AHe ages require a 50-150% increase in exhumation rates in the past 2-3 m.y. This increase in rates is contemporaneous with Pliocene-Pleistocene alpine glaciation of the orogen, indicating that tectonic rock uplift is perturbed by glacial erosion.
Earth and Planetary Science Letters, 2017
Glacial chronologies provide insight into the evolution of paleo-landscapes, paleoclimate, topogr... more Glacial chronologies provide insight into the evolution of paleo-landscapes, paleoclimate, topography, and the erosion processes that shape mountain ranges. In the Pamir of Central Asia, glacial morphologies and deposits indicate extensive past glaciations, whose timing and extent remain poorly constrained. Geomorphic data and 15 new 10 Be exposure ages from moraine boulders and roches moutonnées in the southwestern Pamir document multiple Pleistocene glacial stages. The oldest exposure ages, 113 ± 10 ka, underestimate the age of the earliest preserved glacial advance and imply that the modern relief of the southwestern Pamir (peaks at ~5000-6000 m asl; valleys at ~2000-3000 m asl) already existed in the late Middle Pleistocene. Younger exposure ages (~40-80 ka, ~30 ka) complement the existing Central Asian glacial chronology and reflect successively less extensive Late Pleistocene glaciations. The topography of the Pamir and the glacial chronologies suggests that, in the Middle Pleistocene, an ice cap or ice field occupied the eastern Pamir high-altitude plateau, whereas westward flowing valley glaciers incised the southwestern Pamir. Since the Late Pleistocene deglaciation, the rivers of the southwestern Pamir adjusted to the glacially shaped landscape. Localized rapid fluvial incision and drainage network reorganization reflect the transient nature of the deglaciated landscape. 3 Field description and sampling We present 15 new 10 Be exposure ages from roches moutonnées and moraine boulders from five locations in the SW Pamir (Figure 1B; Table 1). The roches moutonnées rise ≥110 m above the valley floor. The topmost surfaces were sampled, some of which preserve glacial striations. The lateral and hummocky moraine boulder samples were collected from ~1-4 m high boulders. They showed no sign of tilting, surface erosion, or frost shattering. Sample lithologies comprise granites and felsic gneiss. Each sample consists of 2-3 cm thick chips from the topmost near-horizontal surface. For each sample, we measured topographic shielding with a clinometer and compass every 60° (after Balco et al., 2008).
Earth and Planetary Science Letters, 2017
Abstract This study presents the first comprehensive dataset of low-temperature thermochronology ... more Abstract This study presents the first comprehensive dataset of low-temperature thermochronology from 43 bedrock samples collected north of the active Yakutat–North American plate boundary. Our apatite and zircon (U–Th)/He and fission-track data reveal the cooling history of the inboard Wrangellia Composite Terrane that is dominated by rapid cooling after Late Jurassic to Early Cretaceous arc magmatism followed by very little cooling and exhumation until today. Deformation resulting in rock exhumation due to the collision of the Yakutat microplate is spatially very limited (20–30 km) and is concentrated mainly in the Chugach–Prince William Terrane and rocks near the Border Ranges Fault. Focused exhumation from greater depths of ca. 10 km with very high rates (>5 km/Myr) is localized at the syntaxis region, starting ca. 10 Ma and shifted south through time. The rapid exhumation rates are explained by the development of strong feedbacks between tectonically driven surface uplift and erosion, which started already before glaciation of the area. The shift in the location towards the south is a consequence of continuous readjusting between tectonics and climate, which is changing on local and global scales since the Late Miocene.
Terra Nova, 2017
The timing of the closure of the Bangong Ocean between the Lhasa and South Qiangtang Terranes in ... more The timing of the closure of the Bangong Ocean between the Lhasa and South Qiangtang Terranes in central Tibet and the resulting crustal thickening are still under debate. We integrate published apatite fission track and (U-Th)/He thermochronometer data with new zircon (U-Th)/He ages from eight samples and with structural profiles to document that the South Qiangtang Terrane experienced slow exhumation between 200 and 150 Ma, associated with the opening of the Bangong Ocean. Accelerated exhumation (around 0.2-0.3 mm/a) of the South Qiangtang Terrane was initiated at around 150 Ma. This exhumation event is interpreted to reflect collision between the Lhasa and South Qiangtang Terranes after closure of the Bangong Ocean, associated with crustal thickening via thick-Accepted Article This article is protected by copyright. All rights reserved. skinned folding and thrusting within the South Qiangtang Terrane. The amalgamation of the Lhasa and South Qiangtang Terranes recorded here may represent the first stage of crustal thickening in the central Tibetan Plateau.
Tectonophysics, 2017
We report two 40 Ar/ 39 Ar illite ages from fault gouge directly above the current trace of the A... more We report two 40 Ar/ 39 Ar illite ages from fault gouge directly above the current trace of the Alpine Fault in New Zealand at Gaunt Creek (1.36±0.27 Ma) and Harold Creek (1.18±0.47 Ma), and one 40 Ar/ 39 Ar illite age from fault gouge from the Two Thumbs Fault on the east side of the Southern Alps. Metamorphic muscovite clasts inherited into the Alpine Fault gouge yielded 40 Ar/ 39 Ar ages of 2.04±0.3 Ma at Gaunt Creek and 11.46±0.47 Ma at Harold Creek. We also report Rb-Sr muscovitebased multimineral ages of Alpine Schist mylonite adjacent to the dated fault gouge at Harold Creek (13.1±4.3 Ma) and Gaunt Creek (8.9±3.2 Ma). 40 Ar/ 39 Ar muscovite ages from the Gaunt Creek mylonite yielded plateau ages of 1.47±0.08 Ma and 1.57±0.15 Ma. Finally, we report zircon fission track (0.79±0.11 and 0.81±0.17 Ma) and zircon (U-Th)/He ages (0.35±0.03 and 0.4±0.06 Ma) from Harold Creek. We interpret the fault gouge ages to date growth of newly formed illite during gouge formation at temperatures of ~300-350°C towards the base of the seismogenic zone. Simple backcalculation using current uplift/exhumation and convergence rates, and dip angles of 45-60° at the Alpine Fault support that interpretation. We infer that the fault gouge ages record faulting and gouge formation as the rocks passed very rapidly through the brittle-ductile transition zone on their way to the surface. Rb-Sr and 40 Ar/ 39 Ar ages on muscovite from Alpine Schist mylonite date muscovite growth at ~11 Ma together with a younger phase of cooling/shearing at ~1.5-2 Ma. Our ages from the Alpine Schist indicate extremely rapid cooling exceeding 200°C/Ma. The fault gouge age from the Two Thumbs Fault is significantly too old to have formed as part of the late Neogene/Quaternary Southern Alps evolution.
Tectonics, 2017
Geo-thermochronologic data outline the temperature-deformation-time evolution of the Muskol and S... more Geo-thermochronologic data outline the temperature-deformation-time evolution of the Muskol and Shatput gneiss domes and their hanging walls in the Central Pamir. Prograde metamorphism started before ~35 Ma and peaked at ~23-20 Ma, reflecting top-to-~N thrustsheet and fold-nappe emplacement that tripled the thickness of the upper ~7-10 km of the Asian crust. Multimethod thermochronology traces cooling through ~700-100 °C between ~22-12 Ma due to exhumation along dome-bounding normal-sense shear zones. Synkinematic minerals date normal sense shear-zone deformation at ~22-17 Ma. Age-versuselevation relationships and paleoisotherm spacing imply exhumation at ≥3 km/Myr. South of the domes, Mesozoic granitoids record slow cooling and/or constant temperature throughout the Paleogene, and enhanced cooling (7-31 °C/Myr) starting between ~23-12 Ma and continuing today. Integrating the Central Pamir data with those of the East (Chinese) Pamir Kongur Shan and Muztaghata domes, and with the South Pamir Shakhdara dome implies i) regionally distributed, Paleogene crustal thickening; ii) Pamir-wide gravitational collapse of thickened crust starting at ~23-21 Ma during ongoing India-Asia convergence; and iii) termination of doming and resumption of shortening following northward-propagating underthrusting of the Indian cratonic lithosphere at ≥12 Ma. Westward lateral extrusion of Pamir Plateau crust into the Hindu Kush and the Tajik depression accompanied all stages. Deep-seated processes, e.g., slab breakoff, crustal foundering, and underthrusting of buoyant lithosphere, governed transitional phases in the Pamir, and likely the Tibet crust.
Geological Magazine, 2016
A balanced cross-section spanning the Eastern Cordillera and Subandean Zone of southern Peru (13–... more A balanced cross-section spanning the Eastern Cordillera and Subandean Zone of southern Peru (13–15°S) constrains ~130 km (38%) of Cenozoic orogen-normal SW–NE Andean deformation accommodated by thick- and thin-skinned retro–arc fold–thrust belt shortening that overprinted pre-Andean Triassic normal faults. Zircon and apatite (U–Th)/He ages demonstrate continuous Oligocene to Miocene cooling of the Permo-Triassic Coasa pluton in the Eastern Cordillera. Zircon (U–Th)/He ages (~34–18 Ma) are reset and define a steep age versus elevation relationship. Apatite (U–Th)/He results reveal reset ages that define two spatially separated groups with ages of ~30–26 Ma and ~17–11 Ma. Detrital zircon U–Pb geochronologic results from Cretaceous–Cenozoic siliciclastic rocks from the Altiplano/Eastern Cordillera record Andean fold–thrust belt and magmatic-arc sediment sources. Correlative Subandean Zone rocks preserve a cratonic sediment contribution, with minor Andean sediment appearing in some Cen...
Natural Hazards and Earth System Sciences Discussions, 2015
Earthquakes represent the highest risk in terms of potential loss of lives and economic damage fo... more Earthquakes represent the highest risk in terms of potential loss of lives and economic damage for Central Asian countries. Knowledge of fault location and behavior is essential in calculating and mapping seismic hazard. Previous efforts in compiling fault information for Central Asia have generated a large amount of data that are published in limited-access journals with no digital maps publicly available, or are limited in their description of important fault parameters such as slip rates. This study builds on previous work by improving access to fault information through a webbased interactive map and an online database with search capabilities that allow users to organize data by different fields. The data presented in this compilation include fault location, its geographic, seismic and structural characteristics, short descriptions, narrative comments and references to peer-reviewed publications. The interactive map displays 1196 fault segments and 34 000 earthquake locations on a shaded-relief map. The online database contains attributes for 122 faults mentioned in the literature, with Quaternary and geodetic slip rates reported for 38 and 26 faults respectively, and earthquake history reported for 39 faults. This work has implications for seismic hazard studies in Central Asia as it summarizes important fault parameters, and can reduce earthquake risk by enhancing public access to information. It also allows scientists and hazard assessment teams to identify structures and regions where data gaps exist and future investigations are needed.
Chemical Geology, 2015
Measurements of alpha-recoil-track densities in mica as a basis for geological dating depend on a... more Measurements of alpha-recoil-track densities in mica as a basis for geological dating depend on an etch model relating the number of tracks per unit volume (N RT) to the counted number of etched tracks per unit area (ρ RT). The model of Gögen and Wagner (2000) implies that ρ RT increases linearly with etch time (t E), so that N RT can be calculated from the slopes or intercepts of step-etch functions ρ RT (t E). This model rests on the assumption that the etch rate of the mica surface (v V) and the horizontal etch pit growth rate (v H) are both constant, in contradiction with experimental results and computer simulations of mineral dissolution. We present results of four different experiments aimed at relating etch pit densities to volumetric track densities. Step-etch data are vulnerable to observation-related artefacts at increasing t E and lack the resolution to confirm or refute the supposed linear increase of ρ RT with t E. The intercepts of regression lines fitted to step-etch data are imprecise and perhaps inaccurate. Intercept estimates based on mirror-image counts and (etch)-anneal-etch experiments indicate that v V increases during the initial etching stages. The (etch)-anneal-etch results show that no pre-etch is in fact required, implying that surface tracks are more resistant to annealing than tracks in the bulk of the mineral. Track-size measurements confirm that v H is also not constant but decreases with increasing etch-pit size. The results show that no recoil-track etch pit in phlogopite etched in 40% HF grows larger than ca. 7 μm and that the track-size distribution becomes quasi-invariant at N 6 min etching. This signifies that there exist accessible etching conditions at which the etch-pit size distribution becomes a fixed, distorted reflection of the size distribution of latent recoil tracks. It is not improbable that track addition and loss also cancel each other out in this equilibrium state, in which case both the etch-pit size distributions and densities ρ RT become independent of etch time.
Lithos, 2014
Greenschist to amphibolite grade Haimanta metasediments of the NW Himalaya preserve much of the p... more Greenschist to amphibolite grade Haimanta metasediments of the NW Himalaya preserve much of the prograde metamorphic history of Eohimalayan crustal thickening, which has been erased by Oligo-/Miocene migmatization elsewhere in the Himalaya. Our zircon and monazite U/Th-Pb data unravel a multi-stage prograde metamorphic evolution. The earliest evidence of prograde Barrovian metamorphic monazite growth is~41 Ma. Peak metamorphic conditions (~8-8.5 kbar,~600-700°C) were attained at 37-36 Ma and followed by a prolonged evolution at high temperatures with at least three distinct episodes of monazite growth, which may be related to the formation of the northern Himalayan nappes (e.g., Shikar Beh nappe, Nyimaling nappe). Rapid exhumation of the crystalline started at~26 Ma and resulted in cooling through the muscovite 40 Ar/ 39 Ar closure temperature by 21.8 Ma. Although a local continuation of the South Tibetan detachment is not unambiguously identified in central Himachal Pradesh extrusion was likely facilitated by a system of several minor late Oligocene/early Miocene top-to-theN to NE shear zones. In contrast to the crystalline of Zanskar and eastern Himachal Pradesh, extrusion was not accompanied by widespread decompression melting.
ABSTRACT Understanding extrusion of the Greater Himalayan crystalline (GHC) is a key to the ongoi... more ABSTRACT Understanding extrusion of the Greater Himalayan crystalline (GHC) is a key to the ongoing debate about mechanisms of middle to lower crustal deformation in the India-Asia and other collision zones worldwide. In the central and eastern Himalaya exposure of deep crustal levels of the GHC has removed the rocks preserving record on the prograde evolution of the crystalline during crustal thickening and early stages of extrusion. Shallower levels of the orogen&#39;s metamorphic core are exposed in Himachal Pradesh, NW Himalaya, arguably providing the opportunity to study the early tectonometamorphic evolution of the GHC and the onset of its extrusion. In Himachal Pradesh, the GHC forms a recumbent fold, the Phojal nappe. Like in the rest of the Himalayas the exhumation of the GHC is explained by SW-vergent extrusion between the basal Main Central thrust and the South Tibetan detachment as the roof; structural position and geometry of the South Tibetan detachment are, however, debated in the NW Himalaya. Our new monazite U/Th-Pb data from six metapelite samples of different structural levels of the GHC document the Eocene/Oligocene prograde metamorphic evolution with distinct periods of monazite growth at ~42, ~36, and ~27 Ma. The youngest monazite U/Th-Pb ages (20 ± 2 Ma) are indistinguishable from our biotite and white mica 40Ar/39Ar cooling ages (22-20 Ma) and pinpoint the onset of rapid cooling due to SW-ward extrusion of the GHC. Onset of extrusion is thus 8-10 m.yr. earlier compared to the eastern Himalaya. The samples were collected above and below the structural level that has been proposed as the South Tibetan detachment by some authors, while others dispute its very existence in the area. The respective age patterns are the same within error and thus inconsistent with the existence of a major shear zone at that level. We propose that the South Tibetan detachment pinches out in central Himachal Pradesh and reapers as the Zanskar shear zone further west, while in the study area it is manifested by distributed top-to-the-N shear at the upper structural levels of the Phojal nappe.
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Papers by Konstanze Stübner