Gangdese Thrust Research Articles

Cenozoic kinematic histories of the Tidding and Lohit thrusts in the northern Indo-Burma Ranges: Implications for crustal thickening and exhumation of Gangdese lower arc crust along the Indus-Yarlung suture zone

Crustal thickening has been a key process of collision-induced Cenozoic deformation along the Indus-Yarlung suture zone, yet the timing, geometric relationships, and along-strike continuities of major thrusts, such as the Great Counter thrust and Gangdese thrust, remain inadequately understood. In this study, we present findings of geologic mapping and thermo- and geochronologic, geochemical, microstructural, and geothermobarometric analyses from the easternmost Indus-Yarlung suture zone exposed in the northern Indo-Burma Ranges. Specifically, we investigate the Lohit and Tidding thrust shear zones and their respective hanging wall rocks of the Lohit Plutonic Complex and Tidding and Mayodia mélange complexes. Field observations are consistent with ductile deformation concentrated along the top-to-the-south Tidding thrust shear zone, which is in contrast to the top-to-the-north Great Counter thrust at the same structural position to the west. Upper amphibolite-facies metamorphism of mélange rocks at ∼9−10 kbar (∼34−39 km) occurred prior to ca. 36−30 Ma exhumation during slip along the Tidding thrust shear zone. To the north, the ∼5-km-wide Lohit thrust shear zone has a subvertical geometry and north-side-up kinematics. Cretaceous arc granitoids of the Lohit Plutonic Complex were emplaced at ∼32−40 km depth in crust estimated to be ∼38−52 km thick at that time. These rocks cooled from ca. 25 Ma to 10 Ma due to slip along the Lohit thrust shear zone. We demonstrate that the Lohit thrust shear zone, Gangdese thrust, and Yarlung-Tsangpo Canyon thrust have comparable hanging wall and footwall rocks, structural geometries, kinematics, and timing. Based on these similarities, we interpret that these thrusts formed segments of a laterally continuous thrust system, which served as the preeminent crustal thickening structure along the Neotethys-southern Lhasa terrane margin and exhumed Gangdese lower arc crust in Oligocene−Miocene time.

Late Oligocene - Miocene morpho-tectonic evolution of the central Gangdese batholith constrained by low-temperature thermochronology

The morpho-tectonic evolution of the Tibetan Plateau is controlled by complicated interactions between tectonic uplift and surface erosion. The Gangdese batholith in the southern Lhasa terrane is a key orogenic belt for exploring the complicated morpho-tectonic evolution of the Tibetan Plateau. In this contribution, we apply apatite fission track (AFT) thermochronology to constrain the thermo-tectonic evolution of the central segment of the Gangdese batholith. Twenty-four granitoid samples were collected from both river valleys (e.g., the Yarlung and Xiang Rivers) and from the internal batholith areas located farther from river drainage (and/or local faults) networks. All samples exhibit Miocene AFT ages between ∼19.9 and ∼ 6.1 Ma. Inverse thermal history modeling results reveal that the central Gangdese batholith underwent a two-stage accelerated basement cooling in the Miocene. The first cooling stage took place during the late Oligocene to middle Miocene (∼25–15 Ma), this period of moderate to rapid basement cooling coincides with activity along the Gangdese thrust and Great Counter thrust system, and the Oligocene-Miocene delamination of the Lhasa lithosphere and concomitant asthenosphere upwelling. These tectonic processes acted as first-order control on regional basement uplift, denudation and exhumation. Second, a middle-late Miocene (∼14–5 Ma) rapid cooling is widely recognized in the whole Gangdese batholith. We suggest that this middle-late Miocene cooling is due to exhumation in response to tectonic and surface erosion processes such as N-S normal faults and enhanced river incision induced by the intensification of Asian monsoon. Finally, in combination with published low-temperature thermochronological and paleoaltimetry data, it is deduced that the present-day low-relief landscape of the southern Lhasa terrane resulted from a long-term balance between intense regional tectonic activity and surface erosion.

Gangdese culmination model: Oligocene–Miocene duplexing along the India-Asia suture zone, Lazi region, southern Tibet

The mechanisms for crustal thickening and exhumation along the Yarlung (India-Asia) suture in southern Tibet are under debate, because the magnitudes, relative timing, and interaction between the two dominant structures—the Great Counter thrust and Gangdese thrust—are largely unconstrained. In this study, we present new geologic mapping results from the Yarlung suture zone in the Lazi region, located ∼350 km west of the city of Lhasa, along with new igneous (5 samples) and detrital (5 samples, 474 ages) U-Pb geochronology data to constrain the crystallization ages of Jurassic–Paleocene Gangdese arc rocks, the provenance of Tethyan Himalayan and Oligocene–Miocene Kailas Formation strata, and the minimum age (ca. 10 Ma) of the Great Counter thrust system. We supplement these data with a compilation of 124 previously published thermochronologic ages from Gangdese batholith, Kailas Formation, and Liuqu Formation rocks, revealing a dominance of 23–15 Ma cooling contemporaneous with slip across the Great Counter thrust system and other potentially linked structures. These data are systematically younger than 98 additional compiled thermochronologic ages from the northern Lhasa terrane, recording mainly Eocene cooling. Structural and thermochronologic data were combined with regional geological constraints, including International Deep Profiling of Tibet and the Himalaya (INDEPTH) seismic reflection data, to develop a new structural model for the Oligocene–Miocene evolution of the Tethyan Himalaya, Yarlung suture zone, and southern Lhasa terrane. We propose that a hinterland-dipping duplex beneath the Gangdese mountains, of which the Gangdese thrust is a component, is kinematically linked with a foreland-dipping passive roof duplex along the Yarlung suture zone, the Great Counter thrust system. The spatial and temporal convergence between the proposed duplex structures along the Yarlung suture zone and the South Tibetan detachment system indicate that they may be kinematically linked, though this relationship is not directly addressed in this study. Our interpretation, referred to as the Gangdese culmination model, explains why the Gangdese thrust system is only locally exposed (at relatively deeper structural levels) and provides a structural explanation for early Miocene crustal thickening along the Yarlung suture zone, relief generation along the modern Gangdese Mountains, early Miocene Yarlung River establishment, and creation of the modern internal drainage boundary along the southern Tibetan Plateau. The progression of deformation along the suture zone is consistent with tectonic models that implicate subduction dynamics as the dominant control on crustal deformation.

Cenozoic thermo-tectonic evolution of the Gangdese batholith constrained by low-temperature thermochronology

Gangdese batholith in the southern Lhasa block is a key location for exploring the Tibetan Plateau uplift and exhumation history. We present the new low-temperature thermochronological data from two north–south traverses in the central Gangdese batholith to reveal their cooling histories and corresponding controls. Zircon fission track ages show prominent clusters ranging from 23.7 to 51.6Ma, apatite fission track ages from 9.4 to 36.9Ma, apatite (U–Th)/He ages between 9.5 and 12.3Ma, and one zircon (U–Th)/He age around 77.8Ma. These new data and thermal modeling, in combination with the regional geological data, suggest that the distinct parts of Gangdese batholith underwent different cooling histories resulted from various dynamic mechanisms. The Late Eocene–Early Oligocene exhumation of northern Gangdese batholith, coeval with the magmatic gap, might be triggered by crust thickening followed by the breakoff of Neotethyan slab, while this stage of exhumation in southern Gangdese batholith cannot be clearly elucidated probably because the most of plutonic rocks with the information of this cooling event were eroded away. Since then, the northern Gangdese batholith experienced a slow and stable exhumation, while the southern Gangdese batholith underwent two more stages of exhumation. The Late Oligocene–Early Miocene rapid cooling might be a response to denudation caused by the Gangdese Thrust or related to the regional uplift and exhumation in extensional background. By the early Miocene, the rapid exhumation was associated with localized river incision or intensification of Asian monsoon, or north–south normal fault.

Thermal history of the giant Qulong Cu–Mo deposit, Gangdese metallogenic belt, Tibet: Constraints on magmatic–hydrothermal evolution and exhumation

A complete thermal history for the Qulong porphyry Cu–Mo deposit, Tibet is presented. Zircon U–Pb geochronology indicates that the mineralization at Qulong resulted from brecciation-veining events associated with the emplacement of a series of intermediate-felsic intrusions. Combined with previously published ages, our results reveal a whole intrusive history of the Qulong composite pluton. Causative porphyries were emplaced at ~16.0Ma as revealed by 40Ar–39Ar dating of hydrothermal biotite (15.7±0.2Ma) and sericite (15.7±0.2Ma). Zircon and apatite (U–Th)/He (ZHe and AHe) dating of Qulong revealed that both followed similar, monotonic thermal trajectories from 900°C (U–Pb ages: 17.5–15.9Ma) to 200°C (ZHe: 15.7–14.0Ma), and that the causative porphyries experienced faster cooling at a maximum rate of greater than 200°C/myr. The Qulong deposit was exhumed between 13.6Ma and 12.4Ma (AHe) at an estimated rate of 0.16–0.24mm/y, which is consistent with previous estimates for other Gangdese Miocene porphyry deposits. Our AHe thermochronology results suggest that neither the Gangdese thrust system, nor the Yadong–Gulu graben affected or accelerated exhumation at the Qulong deposit.

Exhumation History of the Gangdese Batholith, Southern Tibetan Plateau: Evidence from Apatite and Zircon (U-Th)/He Thermochronology

To test previously suggested exhumation histories of the Gangdese Batholith in the central part of the Transhimalayan plutonic belt, we conducted paired apatite and zircon (U-Th)/He thermochronological investigations of the Yarlung Zangbo gorge in the central part of the batholith. Age-elevation relationships and multisystem thermochronometers showed three periods of accelerated exhumation (∼46–48, ∼22–18, and ∼11–8 Ma). Combining these data with previously published thermochronological ages and synthesizing these ages with regional geological events provides an entire exhumation history. The Cretaceous–Early Paleogene exhumation of the Gangdese Batholith was probably caused by both the northward subduction of the Neo-Tethys and the collision between the Lhasa and Qiangtang blocks. The Early Miocene rapid exhumation might be a response to shortening caused by the Gangdese Thrust or erosion driven by dynamic uplift following lithospheric delamination. In contrast, the Late Miocene exhumation is coincident with both the proposed capture of the Yarlung Zangbo gorge by a foreland draining catchment and the intensification of the Asian monsoon, as well as normal faulting. Hence, the latest stage of exhumation might be attributed to the incision of the Yarlung Zangbo gorge, the activity of a north-south fault, or both.

Tectonic evolution of the India–Asia suture zone since Middle Eocene time, Lopukangri area, south-central Tibet

Suture zones often archive complex geologic histories underscored by episodes of varying style of deformation associated with intercontinental collision. In the Lopukangri area of south-central Tibet (29°54′N, 84°24′E) field relationships between tectonic units juxtaposed by the India–Asia suture are well exposed, including Indian passive margin rocks (Tethyan Sedimentary Sequence), forearc deposits (Xigaze Group), magmatic arc rocks (Gangdese batholith and Linzizong Formation) and syncollision deposits (Eocene–Miocene conglomerates). To better understand the structural history of this area, we integrated geologic mapping with biotite 40Ar/39Ar thermochronology and zircon U–Pb geochronology. The first-order structure is a system of north-directed thrusts which are part of the Great Counter thrust (GCT) that places Indian passive margin rocks and forearc deposits on top of magmatic arc rocks and syn-tectonic conglomerates. We infer the south-directed Late Oligocene Gangdese Thrust (GT) exists at unexposed structural levels based on field mapping, cross sections, and regional correlations as it has been documented immediately to the east. A granite in the footwall has a U–Pb zircon age of 38.4 ± 0.4 Ma, interpreted to be the age of emplacement of the granite, and a younger 40Ar/39Ar biotite age of 19.7±0.1Ma. As the granite sample is situated immediately below a nonconformity with low grade greenschist facies rocks, we interpret the younger age to reflect Miocene resetting of the biotite Ar system. Syn-tectonic deposits in the Lopukangri area consist of three conglomerate units with a total thickness of ∼1.5km. The lower two units consist of cobble gravel pebble conglomerates rich in volcanic and plutonic clasts, transitioning to conglomerates with only sedimentary clasts in the upper unit. We correlate the syncollision deposits to the Eocene–Oligocene Qiuwu Formation based on field relationships, stratigraphy and petrology. Petrology and clast composition suggest the lower two units of the Qiuwu Formation had a northern provenance (Lhasa block and magmatic arc) and the upper unit had a southern provenance (Tethyan Sedimentary Sequence). Our observations are consistent with paleocurrent data from other studies which suggest a predominant south-directed paleoflow for this formation. We propose a model in which: (1) granites intrude at 38.4±0.4Ma; (2) are exhumed by erosion; (3) and buried due to regional subsidence and initial deposition of a conglomerate unit; (4) exposed by the GT at ∼27–24Ma to provide detritus; (5) buried a second time by hanging wall-derived sedimentary deposits and the GCT, then (6) exposed from a depth of ∼12–10km by a blind thrust at ∼19Ma. An alternate model describes: (1) intrusion of the granites at 38.4±0.4Ma, followed by (2) exhumation of the granites via normal faulting to provide detritus; (3) then burial by the GCT at ∼24Ma, followed by (4) exhumation via regional erosional denudation at ∼19Ma. Exposure of the GT west of Xigaze has not been confirmed. We suggest that shallower structural levels of the India-Asia suture zone are exposed to the west of the study area, compared to the east, where the GT has been previously documented. The GCT in the area is short-lived, as it is cut and offset by a Middle Miocene ∼N-striking W-dipping oblique normal fault system.

U-Pb zircon constraints on the tectonic evolution of southeastern Tibet, Namche Barwa Area

The eastern syntaxis of the Himalayas is expressed in the crust as a pronounced southward bend in the orogen. The change in strike of geologic features coincides with the high topography of the Namche Barwa region, the exposure of granulite-grade metamorphic rocks, and a 180-degree bend in the Yalu Tsangpo. We have conducted a geochronologic and geochemical investigation of several suites of granitoids collected from the Namche Barwa massif and subjacent terranes of southeastern Tibet, ranging from cm-scale dikes and sills to larger, outcrop-scale intrusions. U-Pb SHRIMP-RG zircon ages establish at least five magmatic episodes: ∼400 to 500 Ma, ∼120 Ma, 40 to 70 Ma, 18 to 25 Ma, and 3 to 10 Ma. These episodes broadly correlate to spatial patterns in sample localities, as follows: 400 to 500 Ma ages occur in zircon cores collected from within the massif proper; ∼120 Ma granites, related to early Gangdese arc plutonism, are primarily located northeast of Namche Barwa; later (40 - 70 Ma) Gangdese activity is expressed in granites west of Namche Barwa. 18 to 25 Ma granites occur both along the suture zone west of Gyala Peri, and directly north of Namche Barwa along the area of the Jiali fault zone, and are attributed both to shearing within the Jiali fault zone and to an early Miocene Gangdese Thrust event. Exceptionally young (<10 Ma) zircon ages are clustered near the core of the massif, along the Yalu Tsangpo gorge. Trace-element geochemical data indicates the presence of both fluid-present and fluid absent melts, with a fluid-absent (decompression) melting regime dominating near the core of Namche Barwa.

Mesozoic and Cenozoic tectonic evolution of the Shiquanhe area of western Tibet

In the Shiquanhe area of far‐western Tibet, mid‐Cretaceous strata lie unconformable on ophiolitic melange and Jurassic flysch associated with the Bangong‐Nujiang suture zone. On the basis of our mapping and geochronologic studies, we suggest that these Cretaceous strata were shortened by &gt;57% over a north south distance of 50 km during Late Cretaceous‐early Tertiary time. The Late Cretaceous Narangjiapo thrust placed Permian strata &gt;20 km over ophiolitic melange and Cretaceous strata. North of the Narangjiapo thrust, &gt;40 km of shortening was accommodated by the Late Cretaceous‐early Tertiary south directed Jaggang thrust system that involves Jurassic flysch and Cretaceous strata, and roots into a decollement within ophiolitic melange. The most recent shortening was accommodated to the south of the Narangjiapo thrust, along the north dipping Shiquanhe thrust. The Shiquanhe thrust cuts flat‐lying 22.6 ± 0.3 Ma volcanic rocks and underlying folded, Tertiary nonmarine strata in its footwall and was likely active during slip along the Oligocene Gangdese thrust system of southern Tibet. Ophiolitic melange and structurally overlying Jurassic flysch near Shiquanhe are interpreted to represent remnants of a subduction‐accretion complex and forearc basin, respectively, that were obducted southward onto the margin of the Lhasa terrane during Late Jurassic‐Early Cretaceous closure of the Bangong‐Nujiang Ocean. Subsequent imbrication of the obducted sheet could have produced the two east‐west trending belts of ophiolitic melanges, separated by ∼100 km, in western Tibet. Late Cretaceous‐early Tertiary thin‐skinned shortening may have been accommodated in the deeper crust by northward underthrusting and duplexing of Lhasa terrane rocks beneath the obducted ophiolitic melange and the Qiangtang terrane to the north.

The Gangdese thrust: a phantom structure that did not raise Tibet

ABSTRACTDetailed field investigations do not support the existence of a ‘Gangdese thrust’ along the Yarlung Tsangpo suture zone in southern Tibet. A relationship where Lhasa terrane rocks are thrust southwards over components of this zone was not observed over 2000 km of the suture. On the contrary, at the type locality of this ‘Gangdese thrust’, Miocene conglomerates unconformably overlie an eroded surface of Lhasa terrane rocks. Interpretations that invoke Late Oligocene – Early Miocene south‐directed thrusting on a ‘Gangdese thrust’ as a mechanism for uplift of the Tibetan Plateau must therefore be reassessed.

New insights into the evolution of the Yarlung Tsangpo suture zone, Xizang (Tibet), China

Detailed investigations along the Yarlung Tsangpo suture zone, Tibet result in the following conclusions. (1) Arc, forearc, and subduction complex elements of at least one intra-oceanic island arc which once lay within Tethys and accreted to India prior to its collision with Asia are preserved within the suture. (2) Collision-related tectonic melange is widespread. (3) Temporallydistinct conglomeratic molasse units were developed; they are related to different collision events. (4) Improved radiolarian fossil data place constraints on timing of the development of intra-oceanic terranes and melange formation. (5) A simple unconformable contact occurs between Upper Oligocene–Lower Miocene conglomerates and the southern Lhasa terrane; the socalled south-directed Gangdese thrust does not exist. These new results suggest the former existence of an intra-oceanic subduction system within Tethys and indicate that considerable revision of existing models for the evolution of the Yarlung Tsangpo suture zone is required.

The Zedong Window: A record of superposed Tertiary convergence in southeastern Tibet

Determining the timing, magnitude, and location of deformation due to the Indo‐Asian collision is widely acknowledged as an important step in understanding how the lithosphere responds during continental collision. A puzzling result of geological investigations of the Lhasa Block over the past 2 decades has been the apparent lack of significant Tertiary deformation there. Perhaps the most important structural feature of the Lhasa Block is the south directed Gangdese Thrust System, which developed along its southern edge. The thrust system, which separates the Andean‐type batholith of southern Asia from rocks of Indian affinity, is obscured at most locations across southeastern Tibet by back thrusts of the younger, north directed Renbu Zedong Thrust System. The best documented site where both thrusts are exposed occurs near Zedong (Zedong Window). Systematic geochronologic analyses were conducted in this area. U‐Pb zircon dating of three samples of a synkinematically to postkinematically deformed hanging wall granitoid (the Yaja granodiorite) cut by the Gangdese Thrust indicates a crystallization age of 30.4±0.4 Ma (2σ), thus placing an upper bound on the initiation of the thrust. U‐Pb zircon dating of granitoid samples structurally higher in the Gangdese hanging wall yields emplacement ages of 42.5±1.0 Ma and 48.9±0.8 Ma (2σ), similar to other magmatic complexes within the Gangdese arc. Geochemical results are consistent with these plutons forming in the same environment as precollisional intrusions within the Gangdese batholith, suggesting a significant post‐collisional input of juvenile heat. The40Ar/39Ar thermochronology of samples from a vertical section through the hanging wall within the Yaja granodiorite, coupled with results of a numerical thermal model, indicate an average slip rate along the Gangdese Thrust of 7 mm/yr between 30 and 23 Ma and a minimum displacement of ∼50 km. Farther east in the Zedong Window, thermal effects produced by later north directed thrust sheets of the Renbu Zedong system appear to have obscured thermal history signatures in the Gangdese hanging wall related to earlier south directed thrusting. Thermochronological results from this region indicate that thermal overprinting related to the north directed thrusting occurred between 25 and 10 Ma, consistent with previous estimates.

Tertiary deformation history of southeastern and southwestern Tibet during the Indo-Asian collision

Geologic mapping and geochronological analysis in southwest (Kailas area) and southeast (Zedong area) Tibet reveal two major episodes of Tertiary crustal shortening along the classic Indus-Tsangpo suture in the Yalu River valley. The older event occurred between ca. 30 and 24 Ma during movement along the north-dipping Gangdese thrust. The development of this thrust caused extensive denudation of the Gangdese batholith in its hanging wall and underthrusting of the Xigaze forearc strata in its footwall. Examination of timing of major tectonic events in central Asia suggests that the initiation of the Gangdese thrust was approximately coeval with the late Oligocene initiation and development of north-south shortening in the eastern Kunlun Shan of northern Tibet, the Nan Shan at the northeastern end of the Altyn Tagh fault, the western Kunlun Shan at the southwestern end of the Altyn Tagh fault, and finally the Tian Shan (north of the Tarim basin). Such regionally synchronous initiation of crustal shortening in and around the plateau may have been related to changes in convergence rate and direction between the Eurasian plate and the Indian and Pacific plates. The younger thrusting event along the Yalu River valley occurred between 19 and 10 Ma along the south-dipping Great Counter thrust system, equivalent to the locally named Renbu-Zedong thrust in southeastern Tibet, the Backthrust system in south- central Tibet, and the South Kailas thrust in southwest Tibet. The coeval development of the Great Counter thrust and the North Himalayan granite-gneiss dome belt is consistent with their development being related to thermal weakening of the north Himalayan and south Tibetan crust, due perhaps to thermal relaxation of an already thickened crust created by the early phase of collision between India and Asia or frictional heating along major thrusts, such as the Main Central thrust, beneath the Himalaya.

Crustal deformation of the Lhasa terrane, Tibet plateau from Project INDEPTH deep seismic reflection profiles

International Deep Profiling of Tibet and the Himalaya (INDEPTH) deep reflection data in the Yangbajain‐Damxung graben of southern Tibet yield evidence of magmatism and deformation beneath the southern Lhasa terrane and Yarlung‐Zangbo suture. Shallow reflections and low‐velocity first arrivals indicate a thin, Quaternary graben fill of generally less than a few hundred meters thickness. Underlying stratified reflections, extending to a maximum of about 11.5 km depth, likely originate from deformed Paleozoic‐Cenozoic supracrustal strata of the Lhasa terrane. A prominent, undulatory band of reflections within the crystalline basement extends beneath the length of the Yangbajain‐Damxung graben (depth ranges from ∼12 km to ∼18 km). This horizon has been interpreted to mark the top of a midcrustal partial‐melt zone underlying southern Tibet. The undulatory character of this horizon additionally suggests that it may be tectonically deformed. A somewhat deeper, subhorizontal, wide‐angle reflection extends southward beneath the outcrop of the Yarlung‐Zangbo suture, indicating that the suture is cut off or superposed by a younger structure at depth. Three gently north dipping reflections at 19 km, 24 km, and 27 km depth beneath the Gangdese batholith are suggestive of a midcrustal duplex and may mark the northward (downdip) extension of the late Oligocene‐early Miocene Gangdese thrust system. A prominent ∼40° north dipping reflection imaged in the deep crust beneath the Gangdese batholith, between 40 and 60 km depth, might mark the downdip expression of the Yarlung‐Zangbo suture or, alternatively, a younger reverse fault in the lower crust (or both). Viewed in aggregate, the reflection data are suggestive of moderate, postcollisional shortening of the upper crust of the Lhasa terrane, accompanied by melting of the middle crust. Although the data are permissive of wholesale underthrusting or fluid injection of Indian continental crust beneath the Lhasa terrane, they show no direct evidence for this having occurred.

Thermal evolution of the Gangdese batholith, southern Tibet: A history of episodic unroofing

The Gangdese batholith, southern Tibet, was part of an Andean‐type arc at the southern margin of Asia prior to the collision of India and Asia at approximately 50 to 40 Ma. Fission‐track and 40Ar/39Ar analyses of 28 rocks from 10 Gangdese granitoid plutons along an ∼250 km length of the batholith in the Lhasa region provide a detailed understanding of the age and the postcrystallization erosional and tectonic history of these rocks. These data suggest a range of ages for these plutons of 94 to 42 Ma, with the majority being of Tertiary age. The postcrystallization cooling histories of all of these plutons are characterized by marked discontinuities. We conclude that most of these discontinuities, and all of them after 40 Ma, reflect tectonic changes that produced brief pulses of rapid erosion which were distributed in both space and time. In addition to the initial cooling of hot magma against cold country rock, all of the rocks we studied showed evidence for at least one subsequent episode of rapid cooling, dropping many tens of degrees in a few million years. Conversely, these plutons all experienced intervals during which they cooled very slowly or not at all; these slow‐cooling intervals lasted from 5 to 50 million years. Our data indicate that since the collision between India and Asia began, response to continued convergence has been quite variable in even this relatively small area. The data reported here are consistent with a recently proposed model of Oligo‐Miocene crustal shortening along the Gangdese Thrust system in this area.

Tertiary structural evolution of the Gangdese Thrust System, southeastern Tibet

Structural and thermochronological investigations of southern Tibet (Xizang) suggest that intracontinental thrusting has been the dominant cause for formation of thickened crust in the southernmost Tibetan plateau since late Oligocene. Two thrust systems are documented in this study: the north dipping Gangdese system (GTS) and the younger south dipping Renbu‐Zedong system (RZT). West of Lhasa, the Gangdese thrust juxtaposes the Late Cretaceous forearc basin deposits of the Lhasa Block (the Xigaze Group) over the Tethyan sedimentary rocks of the Indian plate, whereas east of Lhasa, the fault juxtaposes the Late Cretaceous‐Eocene, Andean‐type arc (the Gangdese batholith) over Tethyan sedimentary rocks. Near Zedong, 150 km southeast of Lhasa, the Gangdese thrust is marked by a &gt;200‐m‐thick mylonitic shear zone that consists of deformed granite and metasedimentary rocks. A major south dipping backthrust in the hanging wall of the Gangdese thrust puts the Xigaze Group over Tertiary conglomerates and the Gangdese plutonics north of Xigaze and west of Lhasa. A lower age bound for the Gangdese thrust of 18.3±0.5 Ma is given by crosscutting relationships. The timing of slip on the Gangdese thrust is estimated to be 27–23 Ma from 40Ar/39Ar thermochronology, and a displacement of at least 46±9 km is indicated near Zedong. The age of the Gangdese thrust (GT) is consistent with an upper age limit of ∼24 Ma for the initiation of movement on the Main Central thrust. In places, the younger Renbu‐Zedong fault is thrust over the trace of the GT, obscuring its exposure. The RZT appears to have been active at circa 18 Ma but had ceased movement by 8±1 Ma. The suture between India and Asia has been complexly modified by development of the GTS, RZT, and, locally, strike‐slip and normal fault systems.