Micah Jessup | University of Tennessee Knoxville (original) (raw)

Papers by Micah Jessup

Geological Society, London, Special Publications, 2006

The Greater Himalayan Slab (GHS) is composed of a north-dipping anatectic core, bounded above by ... more The Greater Himalayan Slab (GHS) is composed of a north-dipping anatectic core, bounded above by the South Tibetan detachment system (STDS) and below by the Main Central thrust zone (MCTZ). Assuming simultaneous movement on the MCTZ and STDS, the GHS can be modelled as a southward-extruding wedge or channel. New insights into extrusionrelated flow within the GHS emerge from detailed kinematic and vorticity analyses in the Everest region. At the highest structural levels, mean kinematic vorticity number (Wm) estimates of 0.74-0.91 (c. 45-28% pure shear) were obtained from sheared Tethyan limestone and marble from the Yellow Band on Mount Everest. Underlying amphibolite-facies schists and gneisses, exposed in Rongbuk valley, yield Wm estimates of 0.57-0.85 (c. 62-35% pure shear) and associated microstructures indicate that flow occurred at close to peak metamorphic conditions. Vorticity analysis becomes progressively more problematic as deformation temperatures increase towards the anatectic core. Within the MCTZ, rigid elongate garnet grains yield Wm estimates of 0.63-0.77 (c. 58-44% pure shear). We attribute flow partitioning in the GHS to spatial and temporal variations that resulted in the juxtaposition of amphibolite-facies rocks, which record early stages of extrusion, with greenschist to unmetamorphosed samples that record later stages of exhumation.

Journal of the Geological Society, 2008

An inverted metamorphic field gradient associated with a crustal-scale south-vergent thrust fault... more An inverted metamorphic field gradient associated with a crustal-scale south-vergent thrust fault, the Main Central Thrust, has been recognized along the Himalaya for over 100 years. A major problem in Himalayan structural geology is that recent workers have mapped the Main Central Thrust within the Greater Himalayan Sequence high-grade metamorphic sequence along several different structural levels. Some workers map the Main Central Thrust as coinciding with a lithological contact, others as coincident with the kyanite isograd, up to 1-3 km structurally up-section into the Tertiary metamorphic sequence, without supporting structural data. Some workers recognize a Main Central Thrust zone of high ductile strain up to 2-3 km thick, bounded by an upper thrust, MCT-2 (¼ Vaikrita thrust), and a lower thrust, MCT-1 (¼ Munsiari thrust). Some workers define an 'upper Lesser Himalaya' thrust sheet that shows similar P-T conditions to the Greater Himalayan Sequence. Others define the Main Central Thrust either on isotopic (Nd, Sr) differences, differences in detrital zircon ages, or as being coincident with a zone of young (,5 Ma) Th-Pb monazite ages. Very few papers incorporate any structural data in justifying the position of the Main Central Thrust. These studies, combined with recent quantitative strain analyses from the Everest and Annapurna Greater Himalayan Sequence, show that a wide region of high strain characterizes most of the Greater Himalayan Sequence with a concentration along the bounding margins of the South Tibetan Detachment along the top, and the Main Central Thrust along the base. We suggest that the Main Central Thrust has to be defined and mapped on strain criteria, not on stratigraphic, lithological, isotopic or geochronological criteria. The most logical place to map the Main Central Thrust is along the high-strain zone that commonly occurs along the base of the ductile shear zone and inverted metamorphic sequence. Above that horizon, all rocks show some degree of Tertiary Himalayan metamorphism, and most of the Greater Himalayan Sequence metamorphic or migmatitic rocks show some degree of pure shear and simple shear ductile strain that occurs throughout the mid-crustal Greater Himalayan Sequence channel. The Main Central Thrust evolved both in time (earlymiddle Miocene) and space from a deep-level ductile shear zone to a shallow brittle thrust fault.

Geological Society, London, Special Publications, 2006

Recent suggestions that the Greater Himalayan Sequence (GHS) represents a mid-crustal channel of ... more Recent suggestions that the Greater Himalayan Sequence (GHS) represents a mid-crustal channel of low viscosity, partially molten Indian plate crust extruding southward between two major ductile shear zones, the Main Central thrust (MCT) below, and the South Tibetan detachment (STD) normal fault above, are examined, with particular reference to the Everest transect across Nepal-south Tibet. The catalyst for the early kyanite + sillimanite metamorphism (650-6808C, 7-8 kbar, 32-30 Ma) was crustal thickening and regional Barrovian metamorphism. Later sillimanite + cordierite metamorphism (600-6808C, 4 -5 kbar, 23-17 Ma) is attributed to increased heat input and partial melting of the crust. Crustal melting occurred at relatively shallow depths (15-19 km, 4-5 kbar) in the crust. The presence of highly radiogenic Proterozoic black shales (Haimanta-Cheka Groups) at this unique stratigraphic horizon promoted melting due to the high concentration of heat-producing elements, particularly U-bearing minerals. It is suggested that crustal melting triggered channel flow and ductile extrusion of the GHS, and that when the leucogranites cooled rapidly at 17-16 Ma the flow ended, as deformation propagated southward into the Lesser Himalaya. Kinematic indicators record a dominant southvergent simple shear component across the Greater Himalaya. An important component of pure shear is also recorded in flattening and boudinage fabrics within the STD zone, and compressed metamorphic isograds along both the STD and MCT shear zones. These kinematic factors suggest that the ductile GHS channel was subjected to subvertical thinning during southward extrusion. However, dating of the shear zones along the top and base of the channel shows that the deformation propagated outward with time over the period 20-16 Ma, expanding the extruding channel. The last brittle faulting episode occurred along the southern (structurally lower) limits of the MCT shear zone and the northern (structurally higher) limits of the STD normal fault zone. Late-stage breakback thrusting occurred along the MCT and at the back of the orogenic wedge in the Tethyan zone. Our model shows that the Himalayan-south Tibetan crust is rheologically layered, and has several major low-angle detachments that separate layers of crust and upper mantle, each deforming in different ways, at different times.

Journal of Structural Geology, 2007

The use of porphyroclasts rotating in a flowing matrix to estimate mean kinematic vorticity numbe... more The use of porphyroclasts rotating in a flowing matrix to estimate mean kinematic vorticity number (W m ) is important for quantifying the relative contributions of pure and simple shear in penetratively deformed rocks. The most common methods, broadly grouped into those that use tailed and tailless porphyroclasts, have been applied to many different tectonic settings; however, attempts have not been made to unify the various methods. Here, we propose the Rigid Grain Net (RGN) as an alternative graphical method for estimating W m . The RGN contains hyperbolas that are the mathematical equivalents to the hyperbolic net used for the porphyroclast hyperbolic distribution (PHD) method. We use the RGN to unify the most commonly used W m plots by comparing the distribution of theoretical and natural tailless porphyroclasts within a flowing matrix. Test samples from the South Tibetan detachment, Tibet yield indistinguishable results when the RGN is compared with existing methods. Because of its ease of use, ability for comparing natural data sets to theoretical curves, potential to standardize future investigations and ability to limit ambiguity in estimating W m , the RGN makes an important new contribution that advances the current methods for quantifying flow in shear zones.

Geology, 2013

The Neogene elevation history of the Mount Everest region is key for understanding the tectonic h... more The Neogene elevation history of the Mount Everest region is key for understanding the tectonic history of the world's highest mountain range, the evolution of the Tibetan Plateau, and climate patterns in East and Central Asia. In the absence of fossil surface deposits such as paleosols, volcanic ashes, or lake sediments, we conducted stable isotope paleoaltimetry based on the hydrogen isotope ratios (δD) of hydrous minerals that were deformed in the South Tibetan detachment shear zone during the late Early Miocene. These minerals exchanged isotopically at high temperature with meteoric water (δD water = −156‰ ± 5‰) that originated as high-elevation precipitation and infi ltrated the crustal hydrologic system at the time of detachment activity. When compared to age-equivalent near-sea-level foreland oxygen isotope (δ 18 O) paleosol records (δ 18 O water = −5.8‰ ± 1.0‰), the difference in δ 18 O water is consistent with mean elevations of ≥5000 m for the Mount Everest area. Mean elevations similar to modern suggest that an early Himalayan rain shadow may have infl uenced the late Early Miocene climatic and rainfall history to the north of the Himalayan chain.

The Leo Pargil dome (LPD) in northwest India exposes an interconnected network of pre-, syn-, and... more The Leo Pargil dome (LPD) in northwest India exposes an interconnected network of pre-, syn-, and postkinematic leucogranite dikes and sills that pervasively intrude amphibolite-facies metapelites of the mid-crustal Greater Himalayan sequence. Leucogranite bodies range from thin (5-cm-wide) locally derived sills to thick (2-mwide) crosscutting dikes extending at least 100 m. Threedimensional exposures elucidate crosscutting relations between different phases of melt injection and crystallization. Combined laser ablation inductively coupled plasma mass spectrometry U-Th/Pb geochronology and trace element analysis on well-characterized monazite grains from nineteen representative leucogranites yields a large, internally consistent data set of approximately 700 U-Th/Pb and 400 trace element analyses. Grain-scale variations in age correlate with trace element distributions and indicate semi-continuous crystallization of monazite from 30 to 18 Ma. The youngest U-Th/Pb ages in a given sample are consistent with the outcrop-scale crosscutting relations, whereas older ages within individual samples record inheritance from partially crystallized melt and source metapelites. U-Th/Pb isotopic and trace element data are incorporated into a model of melting within the LPD that involves (1) steadystate equilibrium batch melting of compositionally homogeneous metapelitic sources; (2) pulses of increased melt mobility lasting 1-2 m.y. resulting in segregation of melt from its source and amalgamation into mixed magmas; and (3) rapid emplacement and final crystallization of leucogranite bodies. Melt systems in the LPD evolved from locally derived, in situ melt in migmatitic source rocks into a vast network of dikes and sills in the overlying non-migmatitic host rocks.

Journal of Structural Geology, 2012

Kinematic analysis and field mapping of the Homestake shear zone (HSZ) and Slide Lake shear zone ... more Kinematic analysis and field mapping of the Homestake shear zone (HSZ) and Slide Lake shear zone (SLSZ) in central Colorado may provide insight into the interaction between subvertical and low-angle shear zones in the middle crust. The northeast-striking, steeply dipping HSZ comprises a w10-kmwide set of anastomosing ductile shear zones and pseudotachylyte-bearing faults. Approximately 4 km south of the HSZ, northenortheast-striking, shallowly dipping mylonites of the SLSZ form three 1e10-mthick splays. Oblique stretching lineations and shear sense in both shear zones record components of dipslip (top-up-to-the-northwest and top-down-to-the-southeast) and dextral strike-slip movement during mylonite development. Quartz and feldspar deformation mechanisms and quartz [c] axis lattice preferred orientation (LPO) patterns suggest deformation temperatures ranging from w280e500 C in the HSZ to w280e600 C in the SLSZ. Quartz [c] axis LPOs suggest plane strain general shear across the shear system. Based on the relative timing of fabric development, compatible kinematics and similar deformation temperatures in the SLSZ and the HSZ, we propose that both shear zones formed during strain localization and partitioning within a transpressional shear zone system that involved subvertical shuffling in the mid-crust at 1.4 Ga.

Journal of Metamorphic Geology. DOI: 10.1111/j.1525-1314.2012.00998.x, 2012

The Leo Pargil dome, northwest India, is a 30 km-wide, northeast-trending structure that is cored... more The Leo Pargil dome, northwest India, is a 30 km-wide, northeast-trending structure that is cored by gneiss and mantled by amphibolite facies metamorphic rocks that are intruded by a leucogranite injection complex. Oppositely dipping, normal-sense shear zones that accommodated orogen-parallel extension within a convergent orogen bound the dome. The broadly distributed Leo Pargil shear zone defines the southwest flank of the dome and separates the dome from the metasedimentary and sedimentary rocks in the hanging wall to the west and south. Thermobarometry and in-situ U-Th-Pb monazite geochronology were conducted on metamorphic rocks from within the dome and in the hanging wall. These data were combined with U-Th-Pb monazite geochronology of leucogranites from the injection complex to evaluate the relationship between metamorphism, crustal melting, and the onset of exhumation. Rocks within the dome and in the hanging wall contain garnet, kyanite, and staurolite porphyroblasts that record prograde Barrovian metamorphism during crustal thickening that reached 530-630°C and 7-8 kbar, ending by c. 30 Ma. Cordierite and sillimanite overgrowths on Barrovian assemblages within the dome record dominantly top-down-to-the-west shearing during near-isothermal decompression of the footwall rocks to 4 kbar by 23 Ma during an exhumation rate of 1.3 mm year )1 . Monazite growth accompanied Barrovian metamorphism and decompression. The leucogranite injection complex within the dome initiated at 23 Ma and continued to 18 Ma. These data show that orogen-parallel extension in this part of the Himalaya occurred earlier than previously documented (>16 Ma). Contemporaneous onset of near-isothermal decompression, top-down-to-the-west shearing, and injection of the decompression-driven leucogranite complex suggests that early crustal melting may have created a weakened crust that was proceeded by localization of strain and shear zone development. Exhumation along the shear zone accommodated decompression by 23 Ma in a kinematic setting that favoured orogen-parallel extension.

Petrologic and microstructural/crystal fabric data indicate that isotherms recorded in Greater Hi... more Petrologic and microstructural/crystal fabric data indicate that isotherms recorded in Greater Himalayan Series (GHS) schists and gneisses in the footwall to the South Tibetan Detachment System (STDS) have undergone extreme telescoping during penetrative flow associated with southward extrusion of the GHS. In the Rongbuk Valley, to the north of Mount Everest, we have made three vertical sampling traverses from the STDS down into the GHS and estimated temperatures associated with penetrative deformation using the opening angles of quartz c-axis fabrics measured on dynamically recrystallized grains. From north to south, the deformation temperature data indicate apparent thermal field gradients of 369, 385 and 420 C per km for our three traverses, traced over a maximum vertical sampling distance of 0.5 km. Adopting a differential flow path model, simple geometric analysis using sections drawn parallel to the local transport direction indicates that detachment-parallel transport magnitudes of 25e170 km are needed to explain the extreme telescoping of isotherms in the immediate footwall to the STDS, depending on assumed original geothermal gradient, dip of detachment, etc. These particle transport estimates are similar to those previously calculated from barometry data of GHS rocks in the Everest region and are compatible with channel flow models for extrusion and exhumation of the GHS.

This study combines microstructural observations with Raman spectroscopy on carbonaceous material... more This study combines microstructural observations with Raman spectroscopy on carbonaceous material (RSCM), phase equilibria modelling and U-Pb dating of titanite to delineate the metamorphic history of a well-exposed section through the South Tibetan Detachment System (STDS) in the Dzakaa Chu valley of Southern Tibet. In the hanging wall of the STDS, undeformed Tibetan Sedimentary Series rocks consistently record peak metamorphic temperatures of 340°C. Temperatures increase down-section, reaching 650°C at the base of the shear zone, defining an apparent metamorphic field gradient of 310°C km )1 across the entire structure. U-Th-Pb geochronological data indicate that metamorphism and deformation at high temperatures occurred over a protracted period from at least 20 to 13 Ma. Deformation within this 1-km-thick zone of distributed top-down-to-the-northeast ductile shear included a strong component of vertical shortening and was responsible for significant condensing of palaeo-isotherms along the upper margin of the Greater Himalayan Series (GHS). We interpret the preservation of such a high metamorphic gradient to be the result of a progressive up-section migration in the locus of deformation within the zone. This segment of the STDS provides a detailed thermal and kinematic record of the exhumation of footwall GHS rocks from beneath the southern margin of the Tibetan plateau.

Journal of Geology, Jan 1, 2010

In the South Tibetan Himalaya, major detachment systems exhumed midcrustal rocks from different s... more In the South Tibetan Himalaya, major detachment systems exhumed midcrustal rocks from different structural levels that are exposed in the Ama Drime and Mount Everest massifs. The South Tibetan detachment system (STDS) accommodated exhumation of the Greater Himalayan Series (GHS) until the Middle Miocene. Field-and laboratorybased structural data presented here indicate that early melt-present deformation in the footwall of the STDS accommodated large-scale flow of a low-viscosity middle crust. Decompression-related anatexis and emplacement of leucogranites in the structurally highest positions of the GHS (∼16-18 Ma) mark the final stages of south-directed extrusion within a relatively narrow solid-state mylonite zone that progressed into a brittle detachment. Estimates of mean kinematic vorticity and deformation temperatures record a progression in deformation conditions, from ultramylonitic leucogranite (∼400Њ-500ЊC; 51%-67% pure shear) to mylonitic marble (1300ЊC; 41%-55% pure shear) to moderately foliated marble ∼200Њ-300ЊC (46%-59% pure shear). Previous investigations demonstrated that the cessation of movement on the STDS at ∼16-13 Ma was followed by anatexis in the lower portion of the middle crust (∼12-13 Ma; Ama Drime Massif). The Ama Drime and Nyö nno Ri detachments (∼8-12 Ma; Dinggyê graben) exhumed rocks from the deepest structural position in the central Himalaya during orogen-parallel extension. Brittle faulting dissected the upper crust (!4 Ma; Tingri-Kung Co graben) in a setting that was kinematically linked to extension in the interior of the Tibetan plateau.

Tectonics, Jan 1, 2009

[1] The Ama Drime Massif (ADM) is an elongate north-south trending antiformal feature that extend... more [1] The Ama Drime Massif (ADM) is an elongate north-south trending antiformal feature that extends ∼70 km north across the crest of the South Tibetan Himalaya and offsets the position of the South Tibetan Detachment system. A detailed U(-Th)-Pb geochronologic study of granulitized mafic eclogites and associated rocks from the footwall of the ADM yields important insights into the middle to late Miocene tectonic evolution of the Himalayan orogen. The mafic igneous precursor to the granulitized eclogites is 986.6 ± 1.8 Ma and was intruded into the paleoproterozoic (1799 ± 9 Ma) Ama Drime orthogneiss, the latter being similar in age to rocks previously assigned to the Lesser Himalayan Series in the Himalayan foreland. The original eclogite-facies mineral assemblage in the mafic rocks has been strongly overprinted by granulite facies metamorphism at 750°C and 0.7–0.8 GPa. In the host Ama Drime orthogneiss, the granulite event is correlated with synkinematic sillimanite-grade metamorphism and muscovite dehydration melting. Monazite and xenotime ages indicate that the granulite metamorphism and associated anatexis occurred at <13.2 ± 1.4 Ma. High-grade metamorphism was followed by postkinematic leucogranite dyke emplacement at 11.6 ± 0.4 Ma. This integrated data set indicates that high-temperature metamorphism, decompression, and exhumation of the ADM postdates mid-Miocene south directed midcrustal extrusion and is kinematically linked to orogen-parallel extension.

Journal of Metamorphic Geology, Jan 1, 2008

U(–Th)–Pb geochronology, geothermobarometric estimates and macro- and micro-structural analysis, ... more U(–Th)–Pb geochronology, geothermobarometric estimates and macro- and micro-structural analysis, quantify the pressure–temperature–time–deformation (P–T–t–D) history of Everest Series schist and calcsilicate preserved in the highest structural levels of the Everest region. Pristine staurolite schist from the Everest Series contains garnet with prograde compositional zoning and yields a P–T estimate of 649 ± 21 °C, 6.2 ± 0.7 kbar. Other samples of the Everest Series contain garnet with prograde zoning and staurolite with cordierite overgrowths that yield a P–T estimate of 607 ± 25 °C, 2.9 ± 0.6 kbar. The Lhotse detachment (LD) marks the base of the Everest Series. Structurally beneath the LD, within the Greater Himalayan Sequence (GHS), garnet zoning is homogenized, contains resorption rinds and yields peak temperature estimates of ∼650 ± 50 °C. P–T estimates record a decrease in pressure from ∼6 to 3 kbar and equivalent temperatures from structurally higher positions in the overlying Everest Series, through the LD and into GHS. This transition is interpreted to result from the juxtaposition of the Everest Series in the hangingwall with the GHS footwall rocks during southward extrusion and decompression along the LD system. An age constraint for movement on the LD is provided by the crystallization age of the Nuptse granite (23.6 ± 0.7 Ma), a body that was emplaced syn- to post-solid-state fabric development. Microstructural evidence suggests that deformation in the LD progressed from a distributed ductile shear zone into the structurally higher Qomolangma detachment during the final stages of exhumation. When combined with existing geochronological, thermobarometric and structural data from the GHS and Main Central thrust zone, these results form the basis for a more complete model for the P–T–t–D evolution of rocks exposed in the Mount Everest region.

Geochemistry Geophysics Geosystems, Jan 1, 2008

Springs issuing from different faults and shear zones along the crest of the Himalayas tap three ... more Springs issuing from different faults and shear zones along the crest of the Himalayas tap three different levels of crust beneath the Tibetan Plateau. From structurally highest to lowest these are the Tingri Graben, the South Tibetan Detachment System (STDS), and the Ama Drime massif (ADM). The aqueous chemistry reflects water-rock interactions along faults and is consistent with mapped rock types. Major ion chemistry and calculated temperatures indicate that spring waters have circulated to greater depths along the N-S trending faults that bound the Tingri Graben and Ama Drime detachment (ADD) compared to the STDS, suggesting that these structures penetrate to greater depths. Springs have excess CO2, N2, He, and CH4 compared to meteoric water values, implying addition from crustal sources. The 3He/4He ratios range from 0.018 to 0.063 RA and are consistent with a crustal source for He. The δ13C values of dissolved inorganic carbon (DIC) and CO2 gas range from −5.5 to +3.8‰ and −13.1 to −0.3‰ versus Peedee belemnite, respectively. Sources of carbon are evaluated by calculating isotopic trajectories associated with near-surface effervescence of CO2. Positive δ13C values of the Tingri graben and STDS springs are consistent with decarbonation of marine carbonates as the source of CO2. Negative values for the ADD springs overlap with mantle values but are best explained by metamorphic devolatilization of reduced sedimentary carbon. The δ15N values of N2 range from −2.2 to +2.1‰ (versus AIR) and are explained by mixtures of air-derived nitrogen, metamorphic devolatilization of sedimentary nitrogen, and nitrogen from near-surface biogenic processes. CO2 flux is estimated by scaling from individual springs (∼105 mol a−1 per spring) to extensional structures across the southern limit of the Tibetan Plateau and likely contributes between 108 and 1011 mol a−1 (up to 10%) to the global carbon budget.

Geology, Jan 1, 2008

Focused denudation and mid-crustal fl ow are coupled in many active tectonic settings, including ... more Focused denudation and mid-crustal fl ow are coupled in many active tectonic settings, including the Himalaya, where exhumation of mid-crustal rocks accommodated by thrust faults and low-angle detachment systems during crustal shortening is well documented. New structural and (U-Th)/He apatite data from the Mount Everest region demonstrate that the trans-Himalayan Ama Drime Massif has been exhumed at a minimum rate of ~1 mm/yr between 1.5 and 3.0 Ma during orogen-parallel extension. The Ama Drime Massif offsets the South Tibetan detachment system, and therefore the South Tibetan detachment system is no longer capable of accommodating south-directed mid-crustal fl ow or coupling it with focused denudation. Previous investigations interpreted the NNE-SSW-striking shear zone on the west side of the Ama Drime Massif as the Main Central thrust zone; however, our data show that the Ama Drime Massif is bounded on either side by 100-300-m-thick normal-sense shear zone and detachment systems that are kinematically linked to young brittle faults that offset Quaternary deposits and record active orogen-parallel extension. When combined with existing data, these results suggest that the Ama Drime Massif was exhumed during orogenparallel extension that was enhanced by, or potentially coupled with, denudation in the trans-Himalayan Arun River gorge. This model provides important insights into the mechanisms by which orogen-parallel extension, which has dominated the southern margin of the Tibetan Plateau since the middle Miocene, has exhumed trans-Himalayan antiformal structures.

Journal of Structural Geology, Jan 1, 2010

The Ama Drime Massif is a north–south trending antiformal structure located on the southern margi... more The Ama Drime Massif is a north–south trending antiformal structure located on the southern margin of the Tibetan Plateau that is bound by the Ama Drime and Nyönno Ri detachments on the western and eastern sides, respectively. Detailed kinematic and vorticity analyses were combined with deformation temperature estimates on rocks from the Ama Drime detachment to document spatial and temporal patterns of deformation. Deformation temperatures estimated from quartz and feldspar microstructures, quartz [c] axis fabrics, and two-feldspar geothermometry of asymmetric strain-induced myrmekite range between ∼400 and 650 °C. Micro- and macro-kinematic indicators suggest west-directed displacement dominated over this temperature range. Mean kinematic vorticity estimates record early pure shear dominated flow (49–66% pure shear) overprinted by later simple shear (1–57% pure shear), high-strain (36–50% shortening and 57–99% down-dip extension) dominated flow during the later increments of ductile deformation. Exhumation of the massif was accommodated by at least ∼21–42 km of displacement on the Ama Drime detachment. Samples from the Nyönno Ri detachment were exhumed from similar depths. We propose that exhumation on the Nyönno Ri detachment during initiation of orogen-parallel extension (11–13 Ma) resulted in a west-dipping structural weakness in the footwall that reactivated as the Ama Drime detachment.

Journal of The Geological Society, Jan 1, 2008

Journal of Structural Geology, Jan 1, 2007

Structural transects through the South Tibetan Detachment system (STDS) in the Dzakaa Chu valley,... more Structural transects through the South Tibetan Detachment system (STDS) in the Dzakaa Chu valley, Tibet reveal a ∼1000-m thick, low-angle (<35°) zone of distributed ductile shear that displaces Paleozoic sediments over amphibolite facies gneisses, calc-mylonites and leucogranites of the Greater Himalayan Series (GHS). Within the shear zone, grain-size reduction with dynamic recrystallisation of quartz and growth of secondary phyllosilicates accommodated ductile deformation at elevated temperatures. Small-scale brittle normal faults and extensional shear veins overprint ductile features recording deformation at lower temperatures. Our structural data indicate that the Dzakaa Chu STDS records a progression from ductile- to brittle-deformation without development of a discrete detachment fault(s) that is common to many STDS sections. U(–Th)–Pb dating of post-kinematic leucogranites suggest that, in the lower part of the shear zone, mylonitic fabric development occurred prior to ∼20 Ma. By integrating structural and geochronological evidences we propose that the Dzakaa Chu STDS represents a deeper structural position than elsewhere in the Himalaya and provides important insight into the early ductile exhumation of the GHS that was dominated by movement along a 1-km wide shear zone without discrete brittle detachments. These findings are an important step towards understanding the development of low-angle detachment fault systems active during continental collision.

Journal of Structural Geology, Jan 1, 2007

The use of porphyroclasts rotating in a flowing matrix to estimate mean kinematic vorticity numbe... more The use of porphyroclasts rotating in a flowing matrix to estimate mean kinematic vorticity number (Wm) is important for quantifying the relative contributions of pure and simple shear in penetratively deformed rocks. The most common methods, broadly grouped into those that use tailed and tailless porphyroclasts, have been applied to many different tectonic settings; however, attempts have not been made to unify the various methods. Here, we propose the Rigid Grain Net (RGN) as an alternative graphical method for estimating Wm. The RGN contains hyperbolas that are the mathematical equivalents to the hyperbolic net used for the porphyroclast hyperbolic distribution (PHD) method. We use the RGN to unify the most commonly used Wm plots by comparing the distribution of theoretical and natural tailless porphyroclasts within a flowing matrix. Test samples from the South Tibetan detachment, Tibet yield indistinguishable results when the RGN is compared with existing methods. Because of its ease of use, ability for comparing natural data sets to theoretical curves, potential to standardize future investigations and ability to limit ambiguity in estimating Wm, the RGN makes an important new contribution that advances the current methods for quantifying flow in shear zones.

Geological Society, London, Special Publications, 2006

The Greater Himalayan Slab (GHS) is composed of a north-dipping anatectic core, bounded above by ... more The Greater Himalayan Slab (GHS) is composed of a north-dipping anatectic core, bounded above by the South Tibetan detachment system (STDS) and below by the Main Central thrust zone (MCTZ). Assuming simultaneous movement on the MCTZ and STDS, the GHS can be modelled as a southward-extruding wedge or channel. New insights into extrusionrelated flow within the GHS emerge from detailed kinematic and vorticity analyses in the Everest region. At the highest structural levels, mean kinematic vorticity number (Wm) estimates of 0.74-0.91 (c. 45-28% pure shear) were obtained from sheared Tethyan limestone and marble from the Yellow Band on Mount Everest. Underlying amphibolite-facies schists and gneisses, exposed in Rongbuk valley, yield Wm estimates of 0.57-0.85 (c. 62-35% pure shear) and associated microstructures indicate that flow occurred at close to peak metamorphic conditions. Vorticity analysis becomes progressively more problematic as deformation temperatures increase towards the anatectic core. Within the MCTZ, rigid elongate garnet grains yield Wm estimates of 0.63-0.77 (c. 58-44% pure shear). We attribute flow partitioning in the GHS to spatial and temporal variations that resulted in the juxtaposition of amphibolite-facies rocks, which record early stages of extrusion, with greenschist to unmetamorphosed samples that record later stages of exhumation.

Journal of the Geological Society, 2008

An inverted metamorphic field gradient associated with a crustal-scale south-vergent thrust fault... more An inverted metamorphic field gradient associated with a crustal-scale south-vergent thrust fault, the Main Central Thrust, has been recognized along the Himalaya for over 100 years. A major problem in Himalayan structural geology is that recent workers have mapped the Main Central Thrust within the Greater Himalayan Sequence high-grade metamorphic sequence along several different structural levels. Some workers map the Main Central Thrust as coinciding with a lithological contact, others as coincident with the kyanite isograd, up to 1-3 km structurally up-section into the Tertiary metamorphic sequence, without supporting structural data. Some workers recognize a Main Central Thrust zone of high ductile strain up to 2-3 km thick, bounded by an upper thrust, MCT-2 (¼ Vaikrita thrust), and a lower thrust, MCT-1 (¼ Munsiari thrust). Some workers define an 'upper Lesser Himalaya' thrust sheet that shows similar P-T conditions to the Greater Himalayan Sequence. Others define the Main Central Thrust either on isotopic (Nd, Sr) differences, differences in detrital zircon ages, or as being coincident with a zone of young (,5 Ma) Th-Pb monazite ages. Very few papers incorporate any structural data in justifying the position of the Main Central Thrust. These studies, combined with recent quantitative strain analyses from the Everest and Annapurna Greater Himalayan Sequence, show that a wide region of high strain characterizes most of the Greater Himalayan Sequence with a concentration along the bounding margins of the South Tibetan Detachment along the top, and the Main Central Thrust along the base. We suggest that the Main Central Thrust has to be defined and mapped on strain criteria, not on stratigraphic, lithological, isotopic or geochronological criteria. The most logical place to map the Main Central Thrust is along the high-strain zone that commonly occurs along the base of the ductile shear zone and inverted metamorphic sequence. Above that horizon, all rocks show some degree of Tertiary Himalayan metamorphism, and most of the Greater Himalayan Sequence metamorphic or migmatitic rocks show some degree of pure shear and simple shear ductile strain that occurs throughout the mid-crustal Greater Himalayan Sequence channel. The Main Central Thrust evolved both in time (earlymiddle Miocene) and space from a deep-level ductile shear zone to a shallow brittle thrust fault.

Geological Society, London, Special Publications, 2006

Recent suggestions that the Greater Himalayan Sequence (GHS) represents a mid-crustal channel of ... more Recent suggestions that the Greater Himalayan Sequence (GHS) represents a mid-crustal channel of low viscosity, partially molten Indian plate crust extruding southward between two major ductile shear zones, the Main Central thrust (MCT) below, and the South Tibetan detachment (STD) normal fault above, are examined, with particular reference to the Everest transect across Nepal-south Tibet. The catalyst for the early kyanite + sillimanite metamorphism (650-6808C, 7-8 kbar, 32-30 Ma) was crustal thickening and regional Barrovian metamorphism. Later sillimanite + cordierite metamorphism (600-6808C, 4 -5 kbar, 23-17 Ma) is attributed to increased heat input and partial melting of the crust. Crustal melting occurred at relatively shallow depths (15-19 km, 4-5 kbar) in the crust. The presence of highly radiogenic Proterozoic black shales (Haimanta-Cheka Groups) at this unique stratigraphic horizon promoted melting due to the high concentration of heat-producing elements, particularly U-bearing minerals. It is suggested that crustal melting triggered channel flow and ductile extrusion of the GHS, and that when the leucogranites cooled rapidly at 17-16 Ma the flow ended, as deformation propagated southward into the Lesser Himalaya. Kinematic indicators record a dominant southvergent simple shear component across the Greater Himalaya. An important component of pure shear is also recorded in flattening and boudinage fabrics within the STD zone, and compressed metamorphic isograds along both the STD and MCT shear zones. These kinematic factors suggest that the ductile GHS channel was subjected to subvertical thinning during southward extrusion. However, dating of the shear zones along the top and base of the channel shows that the deformation propagated outward with time over the period 20-16 Ma, expanding the extruding channel. The last brittle faulting episode occurred along the southern (structurally lower) limits of the MCT shear zone and the northern (structurally higher) limits of the STD normal fault zone. Late-stage breakback thrusting occurred along the MCT and at the back of the orogenic wedge in the Tethyan zone. Our model shows that the Himalayan-south Tibetan crust is rheologically layered, and has several major low-angle detachments that separate layers of crust and upper mantle, each deforming in different ways, at different times.

Journal of Structural Geology, 2007

The use of porphyroclasts rotating in a flowing matrix to estimate mean kinematic vorticity numbe... more The use of porphyroclasts rotating in a flowing matrix to estimate mean kinematic vorticity number (W m ) is important for quantifying the relative contributions of pure and simple shear in penetratively deformed rocks. The most common methods, broadly grouped into those that use tailed and tailless porphyroclasts, have been applied to many different tectonic settings; however, attempts have not been made to unify the various methods. Here, we propose the Rigid Grain Net (RGN) as an alternative graphical method for estimating W m . The RGN contains hyperbolas that are the mathematical equivalents to the hyperbolic net used for the porphyroclast hyperbolic distribution (PHD) method. We use the RGN to unify the most commonly used W m plots by comparing the distribution of theoretical and natural tailless porphyroclasts within a flowing matrix. Test samples from the South Tibetan detachment, Tibet yield indistinguishable results when the RGN is compared with existing methods. Because of its ease of use, ability for comparing natural data sets to theoretical curves, potential to standardize future investigations and ability to limit ambiguity in estimating W m , the RGN makes an important new contribution that advances the current methods for quantifying flow in shear zones.

Geology, 2013

The Neogene elevation history of the Mount Everest region is key for understanding the tectonic h... more The Neogene elevation history of the Mount Everest region is key for understanding the tectonic history of the world's highest mountain range, the evolution of the Tibetan Plateau, and climate patterns in East and Central Asia. In the absence of fossil surface deposits such as paleosols, volcanic ashes, or lake sediments, we conducted stable isotope paleoaltimetry based on the hydrogen isotope ratios (δD) of hydrous minerals that were deformed in the South Tibetan detachment shear zone during the late Early Miocene. These minerals exchanged isotopically at high temperature with meteoric water (δD water = −156‰ ± 5‰) that originated as high-elevation precipitation and infi ltrated the crustal hydrologic system at the time of detachment activity. When compared to age-equivalent near-sea-level foreland oxygen isotope (δ 18 O) paleosol records (δ 18 O water = −5.8‰ ± 1.0‰), the difference in δ 18 O water is consistent with mean elevations of ≥5000 m for the Mount Everest area. Mean elevations similar to modern suggest that an early Himalayan rain shadow may have infl uenced the late Early Miocene climatic and rainfall history to the north of the Himalayan chain.

The Leo Pargil dome (LPD) in northwest India exposes an interconnected network of pre-, syn-, and... more The Leo Pargil dome (LPD) in northwest India exposes an interconnected network of pre-, syn-, and postkinematic leucogranite dikes and sills that pervasively intrude amphibolite-facies metapelites of the mid-crustal Greater Himalayan sequence. Leucogranite bodies range from thin (5-cm-wide) locally derived sills to thick (2-mwide) crosscutting dikes extending at least 100 m. Threedimensional exposures elucidate crosscutting relations between different phases of melt injection and crystallization. Combined laser ablation inductively coupled plasma mass spectrometry U-Th/Pb geochronology and trace element analysis on well-characterized monazite grains from nineteen representative leucogranites yields a large, internally consistent data set of approximately 700 U-Th/Pb and 400 trace element analyses. Grain-scale variations in age correlate with trace element distributions and indicate semi-continuous crystallization of monazite from 30 to 18 Ma. The youngest U-Th/Pb ages in a given sample are consistent with the outcrop-scale crosscutting relations, whereas older ages within individual samples record inheritance from partially crystallized melt and source metapelites. U-Th/Pb isotopic and trace element data are incorporated into a model of melting within the LPD that involves (1) steadystate equilibrium batch melting of compositionally homogeneous metapelitic sources; (2) pulses of increased melt mobility lasting 1-2 m.y. resulting in segregation of melt from its source and amalgamation into mixed magmas; and (3) rapid emplacement and final crystallization of leucogranite bodies. Melt systems in the LPD evolved from locally derived, in situ melt in migmatitic source rocks into a vast network of dikes and sills in the overlying non-migmatitic host rocks.

Journal of Structural Geology, 2012

Kinematic analysis and field mapping of the Homestake shear zone (HSZ) and Slide Lake shear zone ... more Kinematic analysis and field mapping of the Homestake shear zone (HSZ) and Slide Lake shear zone (SLSZ) in central Colorado may provide insight into the interaction between subvertical and low-angle shear zones in the middle crust. The northeast-striking, steeply dipping HSZ comprises a w10-kmwide set of anastomosing ductile shear zones and pseudotachylyte-bearing faults. Approximately 4 km south of the HSZ, northenortheast-striking, shallowly dipping mylonites of the SLSZ form three 1e10-mthick splays. Oblique stretching lineations and shear sense in both shear zones record components of dipslip (top-up-to-the-northwest and top-down-to-the-southeast) and dextral strike-slip movement during mylonite development. Quartz and feldspar deformation mechanisms and quartz [c] axis lattice preferred orientation (LPO) patterns suggest deformation temperatures ranging from w280e500 C in the HSZ to w280e600 C in the SLSZ. Quartz [c] axis LPOs suggest plane strain general shear across the shear system. Based on the relative timing of fabric development, compatible kinematics and similar deformation temperatures in the SLSZ and the HSZ, we propose that both shear zones formed during strain localization and partitioning within a transpressional shear zone system that involved subvertical shuffling in the mid-crust at 1.4 Ga.

Journal of Metamorphic Geology. DOI: 10.1111/j.1525-1314.2012.00998.x, 2012

The Leo Pargil dome, northwest India, is a 30 km-wide, northeast-trending structure that is cored... more The Leo Pargil dome, northwest India, is a 30 km-wide, northeast-trending structure that is cored by gneiss and mantled by amphibolite facies metamorphic rocks that are intruded by a leucogranite injection complex. Oppositely dipping, normal-sense shear zones that accommodated orogen-parallel extension within a convergent orogen bound the dome. The broadly distributed Leo Pargil shear zone defines the southwest flank of the dome and separates the dome from the metasedimentary and sedimentary rocks in the hanging wall to the west and south. Thermobarometry and in-situ U-Th-Pb monazite geochronology were conducted on metamorphic rocks from within the dome and in the hanging wall. These data were combined with U-Th-Pb monazite geochronology of leucogranites from the injection complex to evaluate the relationship between metamorphism, crustal melting, and the onset of exhumation. Rocks within the dome and in the hanging wall contain garnet, kyanite, and staurolite porphyroblasts that record prograde Barrovian metamorphism during crustal thickening that reached 530-630°C and 7-8 kbar, ending by c. 30 Ma. Cordierite and sillimanite overgrowths on Barrovian assemblages within the dome record dominantly top-down-to-the-west shearing during near-isothermal decompression of the footwall rocks to 4 kbar by 23 Ma during an exhumation rate of 1.3 mm year )1 . Monazite growth accompanied Barrovian metamorphism and decompression. The leucogranite injection complex within the dome initiated at 23 Ma and continued to 18 Ma. These data show that orogen-parallel extension in this part of the Himalaya occurred earlier than previously documented (>16 Ma). Contemporaneous onset of near-isothermal decompression, top-down-to-the-west shearing, and injection of the decompression-driven leucogranite complex suggests that early crustal melting may have created a weakened crust that was proceeded by localization of strain and shear zone development. Exhumation along the shear zone accommodated decompression by 23 Ma in a kinematic setting that favoured orogen-parallel extension.

Petrologic and microstructural/crystal fabric data indicate that isotherms recorded in Greater Hi... more Petrologic and microstructural/crystal fabric data indicate that isotherms recorded in Greater Himalayan Series (GHS) schists and gneisses in the footwall to the South Tibetan Detachment System (STDS) have undergone extreme telescoping during penetrative flow associated with southward extrusion of the GHS. In the Rongbuk Valley, to the north of Mount Everest, we have made three vertical sampling traverses from the STDS down into the GHS and estimated temperatures associated with penetrative deformation using the opening angles of quartz c-axis fabrics measured on dynamically recrystallized grains. From north to south, the deformation temperature data indicate apparent thermal field gradients of 369, 385 and 420 C per km for our three traverses, traced over a maximum vertical sampling distance of 0.5 km. Adopting a differential flow path model, simple geometric analysis using sections drawn parallel to the local transport direction indicates that detachment-parallel transport magnitudes of 25e170 km are needed to explain the extreme telescoping of isotherms in the immediate footwall to the STDS, depending on assumed original geothermal gradient, dip of detachment, etc. These particle transport estimates are similar to those previously calculated from barometry data of GHS rocks in the Everest region and are compatible with channel flow models for extrusion and exhumation of the GHS.

This study combines microstructural observations with Raman spectroscopy on carbonaceous material... more This study combines microstructural observations with Raman spectroscopy on carbonaceous material (RSCM), phase equilibria modelling and U-Pb dating of titanite to delineate the metamorphic history of a well-exposed section through the South Tibetan Detachment System (STDS) in the Dzakaa Chu valley of Southern Tibet. In the hanging wall of the STDS, undeformed Tibetan Sedimentary Series rocks consistently record peak metamorphic temperatures of 340°C. Temperatures increase down-section, reaching 650°C at the base of the shear zone, defining an apparent metamorphic field gradient of 310°C km )1 across the entire structure. U-Th-Pb geochronological data indicate that metamorphism and deformation at high temperatures occurred over a protracted period from at least 20 to 13 Ma. Deformation within this 1-km-thick zone of distributed top-down-to-the-northeast ductile shear included a strong component of vertical shortening and was responsible for significant condensing of palaeo-isotherms along the upper margin of the Greater Himalayan Series (GHS). We interpret the preservation of such a high metamorphic gradient to be the result of a progressive up-section migration in the locus of deformation within the zone. This segment of the STDS provides a detailed thermal and kinematic record of the exhumation of footwall GHS rocks from beneath the southern margin of the Tibetan plateau.

Journal of Geology, Jan 1, 2010

In the South Tibetan Himalaya, major detachment systems exhumed midcrustal rocks from different s... more In the South Tibetan Himalaya, major detachment systems exhumed midcrustal rocks from different structural levels that are exposed in the Ama Drime and Mount Everest massifs. The South Tibetan detachment system (STDS) accommodated exhumation of the Greater Himalayan Series (GHS) until the Middle Miocene. Field-and laboratorybased structural data presented here indicate that early melt-present deformation in the footwall of the STDS accommodated large-scale flow of a low-viscosity middle crust. Decompression-related anatexis and emplacement of leucogranites in the structurally highest positions of the GHS (∼16-18 Ma) mark the final stages of south-directed extrusion within a relatively narrow solid-state mylonite zone that progressed into a brittle detachment. Estimates of mean kinematic vorticity and deformation temperatures record a progression in deformation conditions, from ultramylonitic leucogranite (∼400Њ-500ЊC; 51%-67% pure shear) to mylonitic marble (1300ЊC; 41%-55% pure shear) to moderately foliated marble ∼200Њ-300ЊC (46%-59% pure shear). Previous investigations demonstrated that the cessation of movement on the STDS at ∼16-13 Ma was followed by anatexis in the lower portion of the middle crust (∼12-13 Ma; Ama Drime Massif). The Ama Drime and Nyö nno Ri detachments (∼8-12 Ma; Dinggyê graben) exhumed rocks from the deepest structural position in the central Himalaya during orogen-parallel extension. Brittle faulting dissected the upper crust (!4 Ma; Tingri-Kung Co graben) in a setting that was kinematically linked to extension in the interior of the Tibetan plateau.

Tectonics, Jan 1, 2009

[1] The Ama Drime Massif (ADM) is an elongate north-south trending antiformal feature that extend... more [1] The Ama Drime Massif (ADM) is an elongate north-south trending antiformal feature that extends ∼70 km north across the crest of the South Tibetan Himalaya and offsets the position of the South Tibetan Detachment system. A detailed U(-Th)-Pb geochronologic study of granulitized mafic eclogites and associated rocks from the footwall of the ADM yields important insights into the middle to late Miocene tectonic evolution of the Himalayan orogen. The mafic igneous precursor to the granulitized eclogites is 986.6 ± 1.8 Ma and was intruded into the paleoproterozoic (1799 ± 9 Ma) Ama Drime orthogneiss, the latter being similar in age to rocks previously assigned to the Lesser Himalayan Series in the Himalayan foreland. The original eclogite-facies mineral assemblage in the mafic rocks has been strongly overprinted by granulite facies metamorphism at 750°C and 0.7–0.8 GPa. In the host Ama Drime orthogneiss, the granulite event is correlated with synkinematic sillimanite-grade metamorphism and muscovite dehydration melting. Monazite and xenotime ages indicate that the granulite metamorphism and associated anatexis occurred at <13.2 ± 1.4 Ma. High-grade metamorphism was followed by postkinematic leucogranite dyke emplacement at 11.6 ± 0.4 Ma. This integrated data set indicates that high-temperature metamorphism, decompression, and exhumation of the ADM postdates mid-Miocene south directed midcrustal extrusion and is kinematically linked to orogen-parallel extension.

Journal of Metamorphic Geology, Jan 1, 2008

U(–Th)–Pb geochronology, geothermobarometric estimates and macro- and micro-structural analysis, ... more U(–Th)–Pb geochronology, geothermobarometric estimates and macro- and micro-structural analysis, quantify the pressure–temperature–time–deformation (P–T–t–D) history of Everest Series schist and calcsilicate preserved in the highest structural levels of the Everest region. Pristine staurolite schist from the Everest Series contains garnet with prograde compositional zoning and yields a P–T estimate of 649 ± 21 °C, 6.2 ± 0.7 kbar. Other samples of the Everest Series contain garnet with prograde zoning and staurolite with cordierite overgrowths that yield a P–T estimate of 607 ± 25 °C, 2.9 ± 0.6 kbar. The Lhotse detachment (LD) marks the base of the Everest Series. Structurally beneath the LD, within the Greater Himalayan Sequence (GHS), garnet zoning is homogenized, contains resorption rinds and yields peak temperature estimates of ∼650 ± 50 °C. P–T estimates record a decrease in pressure from ∼6 to 3 kbar and equivalent temperatures from structurally higher positions in the overlying Everest Series, through the LD and into GHS. This transition is interpreted to result from the juxtaposition of the Everest Series in the hangingwall with the GHS footwall rocks during southward extrusion and decompression along the LD system. An age constraint for movement on the LD is provided by the crystallization age of the Nuptse granite (23.6 ± 0.7 Ma), a body that was emplaced syn- to post-solid-state fabric development. Microstructural evidence suggests that deformation in the LD progressed from a distributed ductile shear zone into the structurally higher Qomolangma detachment during the final stages of exhumation. When combined with existing geochronological, thermobarometric and structural data from the GHS and Main Central thrust zone, these results form the basis for a more complete model for the P–T–t–D evolution of rocks exposed in the Mount Everest region.

Geochemistry Geophysics Geosystems, Jan 1, 2008

Springs issuing from different faults and shear zones along the crest of the Himalayas tap three ... more Springs issuing from different faults and shear zones along the crest of the Himalayas tap three different levels of crust beneath the Tibetan Plateau. From structurally highest to lowest these are the Tingri Graben, the South Tibetan Detachment System (STDS), and the Ama Drime massif (ADM). The aqueous chemistry reflects water-rock interactions along faults and is consistent with mapped rock types. Major ion chemistry and calculated temperatures indicate that spring waters have circulated to greater depths along the N-S trending faults that bound the Tingri Graben and Ama Drime detachment (ADD) compared to the STDS, suggesting that these structures penetrate to greater depths. Springs have excess CO2, N2, He, and CH4 compared to meteoric water values, implying addition from crustal sources. The 3He/4He ratios range from 0.018 to 0.063 RA and are consistent with a crustal source for He. The δ13C values of dissolved inorganic carbon (DIC) and CO2 gas range from −5.5 to +3.8‰ and −13.1 to −0.3‰ versus Peedee belemnite, respectively. Sources of carbon are evaluated by calculating isotopic trajectories associated with near-surface effervescence of CO2. Positive δ13C values of the Tingri graben and STDS springs are consistent with decarbonation of marine carbonates as the source of CO2. Negative values for the ADD springs overlap with mantle values but are best explained by metamorphic devolatilization of reduced sedimentary carbon. The δ15N values of N2 range from −2.2 to +2.1‰ (versus AIR) and are explained by mixtures of air-derived nitrogen, metamorphic devolatilization of sedimentary nitrogen, and nitrogen from near-surface biogenic processes. CO2 flux is estimated by scaling from individual springs (∼105 mol a−1 per spring) to extensional structures across the southern limit of the Tibetan Plateau and likely contributes between 108 and 1011 mol a−1 (up to 10%) to the global carbon budget.

Geology, Jan 1, 2008

Focused denudation and mid-crustal fl ow are coupled in many active tectonic settings, including ... more Focused denudation and mid-crustal fl ow are coupled in many active tectonic settings, including the Himalaya, where exhumation of mid-crustal rocks accommodated by thrust faults and low-angle detachment systems during crustal shortening is well documented. New structural and (U-Th)/He apatite data from the Mount Everest region demonstrate that the trans-Himalayan Ama Drime Massif has been exhumed at a minimum rate of ~1 mm/yr between 1.5 and 3.0 Ma during orogen-parallel extension. The Ama Drime Massif offsets the South Tibetan detachment system, and therefore the South Tibetan detachment system is no longer capable of accommodating south-directed mid-crustal fl ow or coupling it with focused denudation. Previous investigations interpreted the NNE-SSW-striking shear zone on the west side of the Ama Drime Massif as the Main Central thrust zone; however, our data show that the Ama Drime Massif is bounded on either side by 100-300-m-thick normal-sense shear zone and detachment systems that are kinematically linked to young brittle faults that offset Quaternary deposits and record active orogen-parallel extension. When combined with existing data, these results suggest that the Ama Drime Massif was exhumed during orogenparallel extension that was enhanced by, or potentially coupled with, denudation in the trans-Himalayan Arun River gorge. This model provides important insights into the mechanisms by which orogen-parallel extension, which has dominated the southern margin of the Tibetan Plateau since the middle Miocene, has exhumed trans-Himalayan antiformal structures.

Journal of Structural Geology, Jan 1, 2010

The Ama Drime Massif is a north–south trending antiformal structure located on the southern margi... more The Ama Drime Massif is a north–south trending antiformal structure located on the southern margin of the Tibetan Plateau that is bound by the Ama Drime and Nyönno Ri detachments on the western and eastern sides, respectively. Detailed kinematic and vorticity analyses were combined with deformation temperature estimates on rocks from the Ama Drime detachment to document spatial and temporal patterns of deformation. Deformation temperatures estimated from quartz and feldspar microstructures, quartz [c] axis fabrics, and two-feldspar geothermometry of asymmetric strain-induced myrmekite range between ∼400 and 650 °C. Micro- and macro-kinematic indicators suggest west-directed displacement dominated over this temperature range. Mean kinematic vorticity estimates record early pure shear dominated flow (49–66% pure shear) overprinted by later simple shear (1–57% pure shear), high-strain (36–50% shortening and 57–99% down-dip extension) dominated flow during the later increments of ductile deformation. Exhumation of the massif was accommodated by at least ∼21–42 km of displacement on the Ama Drime detachment. Samples from the Nyönno Ri detachment were exhumed from similar depths. We propose that exhumation on the Nyönno Ri detachment during initiation of orogen-parallel extension (11–13 Ma) resulted in a west-dipping structural weakness in the footwall that reactivated as the Ama Drime detachment.

Journal of The Geological Society, Jan 1, 2008

Journal of Structural Geology, Jan 1, 2007

Structural transects through the South Tibetan Detachment system (STDS) in the Dzakaa Chu valley,... more Structural transects through the South Tibetan Detachment system (STDS) in the Dzakaa Chu valley, Tibet reveal a ∼1000-m thick, low-angle (<35°) zone of distributed ductile shear that displaces Paleozoic sediments over amphibolite facies gneisses, calc-mylonites and leucogranites of the Greater Himalayan Series (GHS). Within the shear zone, grain-size reduction with dynamic recrystallisation of quartz and growth of secondary phyllosilicates accommodated ductile deformation at elevated temperatures. Small-scale brittle normal faults and extensional shear veins overprint ductile features recording deformation at lower temperatures. Our structural data indicate that the Dzakaa Chu STDS records a progression from ductile- to brittle-deformation without development of a discrete detachment fault(s) that is common to many STDS sections. U(–Th)–Pb dating of post-kinematic leucogranites suggest that, in the lower part of the shear zone, mylonitic fabric development occurred prior to ∼20 Ma. By integrating structural and geochronological evidences we propose that the Dzakaa Chu STDS represents a deeper structural position than elsewhere in the Himalaya and provides important insight into the early ductile exhumation of the GHS that was dominated by movement along a 1-km wide shear zone without discrete brittle detachments. These findings are an important step towards understanding the development of low-angle detachment fault systems active during continental collision.

Journal of Structural Geology, Jan 1, 2007

The use of porphyroclasts rotating in a flowing matrix to estimate mean kinematic vorticity numbe... more The use of porphyroclasts rotating in a flowing matrix to estimate mean kinematic vorticity number (Wm) is important for quantifying the relative contributions of pure and simple shear in penetratively deformed rocks. The most common methods, broadly grouped into those that use tailed and tailless porphyroclasts, have been applied to many different tectonic settings; however, attempts have not been made to unify the various methods. Here, we propose the Rigid Grain Net (RGN) as an alternative graphical method for estimating Wm. The RGN contains hyperbolas that are the mathematical equivalents to the hyperbolic net used for the porphyroclast hyperbolic distribution (PHD) method. We use the RGN to unify the most commonly used Wm plots by comparing the distribution of theoretical and natural tailless porphyroclasts within a flowing matrix. Test samples from the South Tibetan detachment, Tibet yield indistinguishable results when the RGN is compared with existing methods. Because of its ease of use, ability for comparing natural data sets to theoretical curves, potential to standardize future investigations and ability to limit ambiguity in estimating Wm, the RGN makes an important new contribution that advances the current methods for quantifying flow in shear zones.