Extension In Tibet Research Articles

Fluid-fluxed melting in the Himalayan orogenic belt: Implications for the initiation of E-W extension in southern Tibet

The geochemistry of granite is largely controlled by physical and chemical parameters that are closely linked to tectonic processes in evolving orogenic belts. Therefore, temporal changes in the geochemical compositions of granites could be used to infer critical shifts in tectonic processes. The Himalayan leucogranites are crustal anatexis products, providing a case to formulate petrogenetic models for granites and test tectonic models. From west to east, in the High Himalaya and the Tethyan Himalaya, two groups of leucogranites are derived from fluid-absent melting (Group A) and fluid-fluxed melting of muscovite (Group B), respectively. In the Cona and Mount Everest areas, Group B granites crystallized at 26−10 Ma, and Group A granites formed at 19−13 Ma. Group B granites have higher CaO, Sr, Ba, Zr, Hf, Th, Sr/Y, Zr/Hf, and Th/U, and lower Rb, Nb, Ta, U, Rb/Sr, and 87Sr/86Sr than those in Group A granites. These geochemical differences highlight the role of deep-origin fluids and the dissolution control of the accessory phases on the geochemical compositions in silicic magma systems. Field and microstructural observations show that E-W extension occurred synchronously with the granite intrusion derived from fluid-fluxed melting. Elevated heat flow accompanying the E-W extension could dehydrate hydrous minerals and release fluids from deep-seated crust (e.g., Lesser Himalayan Sequence). Such fluids could flux and melt the metasedimentary rocks within the High Himalaya and produce Group B granites. Together with literature data, from the Lhasa terrane to the Himalayan belt, E-W extensions in Tibet may have initiated as early as 26 Ma.

Decoupling between upper crustal deformation of southern Tibet and underthrusting of Indian lithosphere

AbstractMore accurate normal fault mapping and more recently constrained extension rates in southern and central Tibet allow to better discuss the mechanical processes responsible for the distribution of extension in Tibet. First, we show that the location of the rifts in southern Tibet south of the Karakorum‐Jiali fault zone (KJFZ) does not exactly correspond to that of the rigid Indian lower lithosphere flattening below southern Tibet (underthrusting) inferred by P‐waves global tomography, thus suggesting an absence of mechanical coupling between the two processes. Instead, E‐W extension south of the KJFZ appears primarily due to divergent, orthogonal thrusting along the curved Himalayan arc, as proposed earlier. North of the KJFZ however, lower amplitude extension, distributed on numerous scattered normal faults in the western Qiangtang terrane, absorbs distributed eastward extrusion, while eastern Qiangtang is extruded more rigidly by the Xianshuihe fault, following a slip‐line resulting from in‐plane forces due to the collision/indenter, visible as a major discontinuity in the GPS velocity field.

Pacific subduction control on Asian continental deformation including Tibetan extension and eastward extrusion tectonics

The India-Asia collision has formed the highest mountains on Earth and is thought to account for extensive intraplate deformation in Asia. The prevailing explanation considers the role of the Pacific and Sunda subduction zones as passive during deformation. Here we test the hypothesis that subduction played an active role and present geodynamic experiments of continental deformation that model Indian indentation and active subduction rollback. We show that the synchronous activity and interaction of the collision zone and subduction zones explain Asian deformation, and demonstrate that east-west extension in Tibet, eastward continental extrusion and Asian backarc basin formation are controlled by large-scale Pacific and Sunda slab rollback. The models require 1740 ± 300 km of Indian indentation such that backarc basins form and central East Asian extension conforms estimates. Indentation and rollback produce ~260–360 km of eastward extrusion and large-scale clockwise upper mantle circulation from Tibet towards East Asia and back to India.

Normal faulting and viscous buckling in the Tibetan Plateau induced by a weak lower crust

Flow of weak lower crust has been invoked to reconcile observed topographic gradients, uniform elevations, slow seismic velocity, and high conductivity measured in the Tibetan Plateau, with viscosity estimates of 1016–1021 Pa·s. Here we investigate the dynamic response resulting from a range of lower crust viscosities in a 3-D lithospheric-scale geodynamic simulation of the India–Eurasia collision zone to determine bounds of physically viable lower crustal strengths. We show that thickening of the plateau is accommodated through viscous buckling of the upper crust in response to lower crustal flow for a lower crustal viscosity on the order of 1020 Pa·s. This generates two east–west trending bands of surface subsidence and dilatation consistent with observed normal faulting and estimates of vertical velocity. These results suggest viscous buckling of the upper crust, induced by lower crustal flow from gravitational pressure gradients due to high topography, is responsible for the observed extension in Tibet.

Miocene initiation and acceleration of extension in the South Lunggar rift, western Tibet: Evolution of an active detachment system from structural mapping and (U‐Th)/He thermochronology

Ongoing extension in Tibet may have begun in the middle to late Miocene, but there are few robust estimates of the rates, timing, or magnitude of Neogene deformation within the Tibetan plateau. We present a comprehensive study of the seismically active South Lunggar rift in southwestern Tibet incorporating mapping, U‐Pb geochronology and zircon (U‐Th)/He thermochronology. The South Lunggar rift is the southern continuation of the North Lunggar rift and comprises a ~50 km N‐S central horst bound by two major normal faults, the west‐dipping South Lunggar detachment and the east‐dipping Palung Co fault. The SLD dips at the rangefront ~20°W and exhumes a well‐developed mylonite zone in its footwall displaying fabrics indicative of normal‐sense shear. The range is composed of felsic orthogneiss, mafic amphibolite, and leucogranite intrusions dated at ~16 and 63 Ma. Zircon (U‐Th)/He cooling ages are Oligocene through late Pliocene, with the youngest ages observed in the footwall of the SLD. We tested ~25,000 unique thermokinematic forward models in Pecube against the structural and (U‐Th)/He data to fully bracket the allowable ranges in fault initiations, accelerations, and slip rates. We find that normal faulting in the SLR began in the middle Miocene with horizontal extension rates of ~1 mm a−1, and in the north accelerated at 8 Ma to 2.5–3.0 mm a−1 as faulting commenced on the SLD. Cumulative horizontal extension across the SLR ranges from <10 km in the south to 19–21 km in the north.

E–W extension at 19 Ma in the Kung Co area, S. Tibet: Evidence for contemporaneous E–W and N–S extension in the Himalayan orogen

Active faulting in southern Tibet consists of N–S trending extensional faults and linked strike-slip faults, which are an expression of regional E–W extension. A second type of extensional deformation associated with N–S movement is also recognized. This extension is expressed as a series of shear zones and normal faults in the High Himalayas – the Southern Tibetan Detachment System – and mid-crustal rocks exposed in metamorphic domes. Reported constraints on the timing of movements associated with these two phases of extension indicate that N–S extension predates the onset of E–W extension. However, only a few studies have provided clear constraints on the timing of E–W extension and the extent to which the two kinematically distinct domains of extension were contemporaneous is unclear.The Kung Co fault in southern Tibet is a major N–S trending normal fault. The associated E–W extension is locally expressed as high-strain ductile deformation. Both field and microstructural observations show that this deformation occurred synchronously with granite intrusion. Previously reported U–Pb zircon dating shows granite crystallization took place at around 19Ma, implying that ductile E–W extension in the Kung Co area was also active at around 19Ma. This is the oldest documented example of E–W extension in Tibet and shows that E–W extension was at least locally contemporaneous with N–S extension to the south at shallower crustal levels. Simultaneous mid-crustal N–S extension and upper crustal E–W extension may be explained by southward flow of Tibetan crust with a divergent radial component.

Rifting and strike-slip shear in central Tibet and the geometry, age and kinematics of upper crustal extension in Tibet

Abstract The youngest deformation structures on the Tibet Plateau are about NNE-trending grabens. We first combine remote-sensing structural and geomorphological studies with structural field observations and literature seismological data to study the Muga Purou rift that stretches at c . 86°E across central Tibet and highlight a complex deformation field. ENE-striking faults are dominated by sinistral strike–slip motion; NNE-striking faults have normal kinematics and outline a right-stepping en-echelon array of grabens, also suggesting sinistral strike–slip; along NW-striking fault sets, the arrangement of grabens may indicate a dextral strike–slip component. Thus, in central Tibet, rifts comprise mostly grabens connected to strike–slip fault zones or are arranged en-echelon to accommodate sinistral wrenching; overall strain geometry is constrictional, in which NNE–SSW and subvertical shortening is balanced by WNW–ESE extension. The overwhelmingly shallow earthquakes only locally outline active faults; clusters seem to trace linkage or propagation zones of know structures. The earthquake pattern, the neotectonic mapping, and the local fault–slip analyses emphasize a distributed, heterogeneous pattern of deformation within a developing regional structure and indicate that strain concentration is weak in the uppermost crust of central Tibet. Thus, the geometry of neotectonic deformation is different from that in southern Tibet. Next, we use structural and palaeomagnetic data along the Zagaya section of southern central Tibet to outline significant block rotation and sinistral strike–slip SE of the Muga Purou rift. Our analysis supports earlier interpretations of reactivation of the Bangong–Nujiang suture as a neotectonic strike–slip belt. Then, we review the existing and provide new geochronology on the onset of neotectonic deformation in Tibet and suggest that the currently active neotectonic deformation started c . 5 Ma ago. It was preceded by c . north–south shortening and c . east–west lengthening within a regime that comprises strike–slip and low-angle normal faults; these were active at c . 18–7 Ma. The c . east-striking, sinistral Damxung shear zone and the c . NE-trending Nyainqentanghla sinistral-normal detachment allow speculations about the nature of this deformation: the ductile, low-angle detachments may be part of or connect to a mid-crustal décollement layer in which the strike–slip zones root; they may be unrelated to crustal extension. Finally, we propose a kinematic model that traces neotectonic particle flow across Tibet and speculate on the origin of structural differences in southern and central Tibet. Particles accelerate and move eastwards from western Tibet. Flow lines first diverge as the plateau is widening. At c . 92°E, the flow lines start to converge and particles accelerate; this area is characterized by the appearance of the major though-going strike–slip faults of eastern-central Tibet. The flow lines turn southeastward and converge most between the Assam–Namche Barwa and Gongha syntaxes; here the particles reach their highest velocity. The flow lines diverge south of the cord between the syntaxes. This neotectonic kinematic pattern correlates well with the decade-long velocity field derived from GPS-geodesy. The difference between the structural geometries of the rifts in central and southern Tibet may be an effect of the basal shear associated with the subduction of the Indian plate. The boundary between the nearly pure extensional province of the southern Tibet and the strike–slip and normal faulting one of central Tibet runs obliquely across the Lhasa block. Published P-wave tomographic imaging showed that the distance over which Indian lithosphere has thrust under Tibet decreases from west to east; this suggests that the distinct spatial variation in the mantle structure along the collision zone is responsible for the surface distribution of rift structures in Tibet. Supplementary material: Containing supporting data is available at http://www.geolsoc.org.uk/SUP18446 .

Seismotectonics in Northeast India: a stress analysis of focal mechanism solutions of earthquakes and its kinematic implications

SUMMARY In Northeast India, three major plates interact along two convergent boundaries: the Himalayas and the Indo–Burma Ranges, which meet at the Assam Syntaxis. To clarify this tectonic interaction and the underlying dynamics, we determine the regional seismotectonic stress from the stress inversion of 285 double couple focal mechanism solutions of earthquakes with an average magnitude of 5. We then compare the reconstructed stress regimes with the available information about geodetically determined relative displacements. North–south compression, in a direction consistent with India–Eurasia convergence, prevails in the whole area from the Eastern Himalayas to the Bengal Basin, through the Shillong–Mikir Massif and the Upper Assam Valley. E–W extension in Tibet is related to this N–S India– Eurasia convergence. Not only does the major N–S compression affect the outer segments of the Indo–Burma Ranges, it also extends into the descending slab of Indian lithosphere below these ranges, although stresses at depth are controlled by bending of the slab beneath the Burmese arc. The existence of widespread N–S compression in the Bengal Basin, far away from the Himalayan front, is compatible with the previously proposed convergence between a Shillong– Mikir–Assam Valley block and the Indian craton. E–W compression inside this block supports the hypothesis of a component of eastward extrusion. Stress inversion of focal mechanism solutions in the Indo–Burma Ranges reveals a complex stress pattern. The Burmese arc and its underlying lithosphere experience nearly arcperpendicular extension with ESE–WNW trends in the northernmost, NE-trending segment and ENE–WSW trends in the main N–S arc segment. Such extensional stress, documented from many arcs, is likely a response to pull from and bending of the subducting plate. At the same time, the Indo–Burma Ranges are under compression as a result of oblique convergence between the Sunda and Indian plates. The maximum compressive stress rotates from NE–SW across the inner and northern arc to E–W near the Bengal Basin. This rotation is consistent with the deformation partitioning reflected in the rotation of relative displacement vectors, from a SSW-directed Sunda–Burma motion to a WSW-directed Burma–India motion. As a consequence of this partitioning, the major belt-parallel fault zones show a variety of movements across the main N–S arc segment, from right-lateral slip in the inner ranges to oblique reverse-dextral slip in the outer ranges and pure thrusting in the westernmost foreland belt.

Neotectonics of the Thakkhola graben and implications for recent activity on the South Tibetan fault system in the central Nepal Himalaya

The Thakkhola graben is one of many north-trending rifts that define the Neogene structural pattern of the southern Tibetan Plateau. Lying at the southern margin of the plateau and extending to the crest of the Himalaya, the graben provides an opportunity to evaluate the kinematic relationships between east-west extensional strain in southern Tibet and north-south extensional strain in the Himalaya. Neotectonic and structural mapping of the Dangardzong fault along the western margin of the graben reveals a southward-decreasing component of normal slip coupled with a southward-increasing component of right-lateral slip that affects Pleistocene basin-fill sediments. We present 14C ages for river terraces in the Thakkhola graben that provide a ca. 17.2 ka minimum age on the latest stage of Dangardzong fault movement. Near the southern termination of the graben, the Dangardzong fault apparently offsets the Annapurna detachment, an early (Miocene) strand of the east-striking South Tibetan fault system. However, the Dangardzong fault itself terminates against a young (i.e., younger than ca. 17.2 ka) strand of the South Tibetan fault system, the Dhumpu detachment. Structural relationships among the Dangardzong, Annapurna, and Dhumpu faults suggest that the Dangardzong structure is a tear fault in the South Tibetan allochthon that accommodates differential amounts and rates of displacement along the South Tibetan fault system. Thus, although the South Tibetan fault system first developed as part of the structural architecture of the Himalaya in Miocene time, at least some strands have been active as recently as the Pleistocene. In a regional context, the South Tibetan fault system serves to accommodate the strain gradient between extension in Tibet and shortening in the Himalaya.

Mode of Cenozoic east‐west extension in Tibet suggesting a common origin of rifts in Asia during the Indo‐Asian collision

Cenozoic rifts in Tibet were traditionally interpreted as a result of topographic collapse of the Tibetan Plateau, reaching the maximum elevation that can be supported by its mechanical strength. Recent studies have emphasized possible similarities between rifting in Tibet and extension in the Basin and Range of the western United States. However, when examined in detail, one finds that spacing of long (>100 km) rifts in Tibet (∼100–300 km) is significantly greater than that in the Basin and Range (∼20–40 km). From south to north, rift spacing decreases systematically: from 191±67 km in the Himalaya south of the Indus‐Yalu suture to 146±34 km in south Tibet between the Indus‐Yalu and Bangong‐Nujiang sutures and farther north to 101±31 km in central Tibet between the Bangong‐Nujiang and Jinsha sutures. Instability analysis suggests that the mantle lithosphere must have been involved in east‐west Tibetan extension. Specifically, the widely spaced rifts in The Himalaya and Tibet may have been related to the presence of a relatively light crust (density <∼2.90 g cm−3) and a strong mantle lithosphere (∼40 km thick and a factor of 5 stronger than the upper crust). The observed systematic decrease in rift spacing can be explained by the known decrease in the crustal thickness in Tibet, from ∼70–80 km in The Himalaya in the south to ∼50–55 km in central Tibet in the north. A regional comparison of rifts in east Asia suggests that both the involvement of the mantle lithosphere and the age of rift initiation are similar for Tibetan rifts, the Baikal rift, and the Shanxi graben. This implies that topographic collapse or a convective event in the mantle cannot be the sole cause for the development of the Tibetan rifts. A regional boundary condition applied throughout east Asia must be required.

Miocene dextral shearing between Himalaya and Tibet

The Main Central thrust zone is the major structural feature of the Himalayas. It was active during Miocene time and accommodated at least 100 km, and possibly as much as 300 km, of convergence between India and Tibet. We present evidence that late in the tectonic history, another large shear zone, located north of the High Himalayas (the North Himalayan shear zone), underwent a phase of dextral strike-slip motion. The most plausible explanation for this phase of motion is that it reflects the onset of extension in Tibet. It is usually thought that extension began about 2.5 Ma. The phase of dextral shear we report occurred between 25 and 15 Ma. If our explanation for this movement is correct, it places the onset of extension at least 12 m.y. earlier than previously thought.