Shear wave splitting and crustal anisotropy in the Eastern Ladakh-Karakoram zone, northwest Himalaya (original) (raw)
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Journal of Geodynamics, 2012
The Kachchh province of Western India is a major seismic domain in an intraplate set-up. This seismic zone is located in a rift basin, which was developed during the early Jurassic break-up of the Gondwanaland. The crustal strain determined from the GPS velocity data of post-seismic time period following the 2001 Bhuj earthquake indicates a maximum strain rate of ~266 × 10−9 per year along N013°. Focal mechanism solutions of the main event of 26 January 2001 and the aftershocks show that the maximum principal stress axis is close to this high strain direction. Maximum shear strain rate determined from the GPS data of the area has similar orientation. The unusually high strain rate is comparable in magnitude to the continental rift systems. The partitioning of the regional NE–SW horizontal stress (SHmax) by the preexisting EW-striking boundary fault developed the strike–slip components parallel to the regional faults, the normal components perpendicular to the faults, NE-striking conjugate Riedel shear fractures and tension fractures. The partitioned normal component of the stress is considered to be the major cause for compression across the regional EW faults and development of the second-order conjugate shear fractures striking NE–SW and NW–SE. The NE-striking transverse faults parallel to the anti-Riedel shear planes have become critical under these conditions. These anti-Riedel planes are interpreted to be critical for the seismicity of the Kachchh region. The high strain rate in this area of low to moderate surface heat flow is responsible for deeper position of the brittle–ductile transition and development of deep seated seismic events in this intraplate region.
Upper Mantle Anisotropy beneath the Western Segment, NW Indian Himalaya, Using Shear Wave Splitting
Lithosphere
This study investigates the upper mantle deformation pattern beneath the Indo-Eurasia collision zone utilizing the core-refracted (S(K)KS) phases from 167 earthquakes recorded by 20 broadband seismic stations deployed in the Western Himalaya. The 76 new shear wave splitting measurements reveal that the fast polarization azimuths (FPAs) are mainly oriented in the ENE-WSW direction, with the delay times varying between 0.2 and 1.7 s. The FPAs at most of the stations tend to be orthogonal to the major geological boundaries in the Western Himalaya. The average trend of the FPAs at each station indicates that the seismic anisotropy is primarily caused due to strain-induced deformation in the top ~200 km of the upper mantle as a result of the ongoing Indo-Eurasian collision. A contribution from the mantle flow in the direction of the Indian plate motion is possible. The mantle strain revealed in the present study may be due to a combination of basal shear resulting from plate motion and d...
Lateral variations in the crustal structure of the Indo–Eurasian collision zone
Geophysical Journal International, 2018
The processes involved in continental collisions remain contested, yet knowledge of these processes is crucial to improving our understanding of how some of the most dramatic features on the Earth have formed. As the largest and highest orogenic plateau on the Earth today, Tibet is an excellent natural laboratory for investigating collisional processes. To understand the development of the Tibetan Plateau, we need to understand the crustal structure beneath both Tibet and the Indian Plate. Building on previous work, we measure new group velocity dispersion curves using data from regional earthquakes (4424 paths) and ambient noise data (5696 paths), and use these to obtain new fundamental mode Rayleigh wave group velocity maps for periods from 5 to 70 s for a region including Tibet, Pakistan and India. The dense path coverage at the shortest periods, due to the inclusion of ambient noise measurements, allows features of up to 100 km scale to be resolved in some areas of the collision zone, providing one of the highest resolution models of the crust and uppermost mantle across this region. We invert the Rayleigh wave group velocity maps for shear wave velocity structure to 120 km depth and construct a 3-D velocity model for the crust and uppermost mantle of the Indo-Eurasian collision zone. We use this 3-D model to map the lateral variations in the crust and in the nature of the crust-mantle transition (Moho) across the Indo-Eurasian collision zone. The Moho occurs at lower shear velocities below northeastern Tibet than it does beneath western and southern Tibet and below India. The east-west difference across Tibet is particularly apparent in the elevated velocities observed west of 84 • E at depths exceeding 90 km. This suggests that Indian lithosphere underlies the whole of the Plateau in the west, but possibly not in the east. At depths of 20-40 km our crustal model shows the existence of a pervasive mid-crustal low velocity layer (∼10% decrease in velocity, V s < 3.4 km s −1) throughout all of Tibet, as well as beneath the Pamirs, but not below India. The thickness of this layer, the lowest velocity in the layer and the degree of velocity reduction vary across the region. Combining our Rayleigh wave observations with previously published Love wave dispersion measurements, we find that the low velocity layer has a radial anisotropic signature with V sh > V sv. The characteristics of the low velocity layer are supportive of deformation occurring through ductile flow in the mid-crust.
Tectonic geomorphology of the eastern extent of the Kashmir Basin Fault (KBF) zone
2013
The Kashmir Basin Fault (KBF) is exposed as a train of discontinuous active fault traces for a strike length of ~120 km (Shah, 2013), in Kashmir, Himalayas. However, its eastern extent was not mapped previously and therefore, this study demonstrates that the active fault trace extends further east, where the geomorphic expression of active faulting is clear for a distance of ~43 km. The fault shows a very prominent dextral strike-slip motion with little to no dip-slip component associated with it, particularly, on the easternmost portion. Further west it mainly shows dip-slip motion with a slight indication of dextral strike-slip. This new active fault trace extends the total strike length of the KBF zone to ~163 km, which has implications for seismic hazard and the distribution of deformation along the NW portion of the Himalayas.
Confrontation of mantle seismic anisotropy with two extreme models of strain, in central Asia
Geophysical Research Letters, 1998
Although most authors agree that convergence is accommodated by a large shortening in Asia, two radically different modes of shortening have been put forward. In one approach, the mechanism of shortening is considered to be homogeneous, producing diffuse deformation in the crust and the mantle. In the second one, motions are localized along faults which are supposed to extend into deep lithospheric shear zones, inducing a heterogeneous deformation pattern. Seismic anisotropy is one observable manifestation of large scale deformation. Large scale seismic anisotropy field are inferred from the two extreme competing models. 3Dmapping of anisotropy, inferred from surface waves, reveals a better coherence on average with filtered heterogeneous strain models between 100 and 200 km depths. It suggests a strong coupling between surface and deep motions down to at least 200 km. At larger depths, the coherence with homogeneous strain models suggests that mantle flow becomes more homogeneous. at the surface corresponding to lithosl)heric shear zones. Motions observed on shallow faults are thus transmitted at depth. These two classes of models are conceptually and dynamically very different. To discriminate between them, these two models are interpreted in terms of predicted seismic anisotropy in the deforming mantle. These predicted anisotropy models are then quantitatively and statistically compared with the azimuthal anisotropy inferred from surface waves, in order to emphasize some correlations dete•xnining the major trends of this region. A recent study on the same issue, [Davis et al., 1997], made comparisons between finite strain from the India-Asia collision zone and shear wave splitting. However, these seismic observations are so geographically localized to provide convincing conclusions in the whole zone, whereas the use of surface waves allows to infer seismic anisotropy at the scale of this wide-ranging collision.
Journal of Seismology, 2009
This paper deals with the data obtained from local networks in northern Pakistan for 251 earthquakes of magnitude ≥4.0 for October 8, 2005 to December 31, 2006 period. The study presents focal mechanism solutions (FMS) of 12 pre- (1904–2005) and 17 post- (October 8, 2005–December, 2005) Muzaffarabad Earthquake, their detailed tectonic interpretation, and correlation with surface evidence of co-seismic rupture with published synthetic aperture radar data. Distribution of landslides obtained from National Engineering Services of Pakistan and the earthquake damages are also discussed. Aftershock distribution, which is more prominent in the crystalline zone (northwest of Muzaffarabad), defines a 50-km-wide NW–SE trending zone that extends for 200 km from the main mantle thrust to the center of the Hazara–Kashmir Syntaxis. The FMS of the main shock and 16 aftershocks having magnitude ≥4.0 indicate thrusting to be the dominant mechanism with rupture planes having NW–SE trend and NE dip. In addition, 12 FMS of pre-Muzaffarabad Earthquake (1904–2004) from the same area have been determined and results are compared. This leads to the conclusion that the wedge-shaped NW–SE trending blind zone, referred to by earlier workers as the Indus Kohistan Seismic Zone (IKSZ), has been activated during the Muzaffarabad earthquake. The right-lateral component in all FMS, supported by the surface evidences, suggests the involvement of Balakot–Bagh Fault (BBF). We propose that the IKSZ is the source of the October 8, 2005 Muzaffarabad earthquake that reactivated the BBF. Furthermore, the IKSZ does not end at the nose of the syntaxis but extends further southeast of it. Tectonic complexity seems to be due to a variety of factors. Also, thrust and reverse solutions near the northern collisional boundary (main mantle thrust) have mostly NE/SW-directed P-axis orientations. From the detailed FMS analysis, three conclusions have been drawn: (1) Shallow events (depth ≤10 km) with prominent strike slip solutions (earlier earthquakes) are associated with the surface strike slip faults (e.g., Muzaffarabad Fault) and/or the Besham domal structure; (2) moderate depth events (depth 10–25 km) with thrust/reverse solutions but having minor right-lateral strike slip component (all Muzaffarabad earthquakes and two earlier) are associated with the IKSZ; (3) deeper earthquakes (depth below IKSZ) with pure thrust/reverse solutions may be related to the under-thrusting of the Indian plate beneath the IKSZ, which represents a major thrust zone. Imbricate thrusting and breaking and thickening of the crust are considered to be caused by steep bending of the under-thrusting plate at the collisional boundary.
Knowledge of the distribution of strain at depth within continental strike-slip fault zones is a significant factor in understanding the seismicity and evolution of such faults and their role in continental deformation. In active fault zones the strain distribution in the ductile component is difficult to constrain. However, information on the strain distribution, kinematics and conditions of deformation at depth can be derived from analysis of crystallographic preferred orientations (CPO) within exhumed ductile fault rocks.
Journal of Geological Research, 2016
The aim of the present research is to provide the base line details of the NNW-SSE trending Raintoli fault (RF) which is running parallel to the North Almora Thrust (NAT) along the Saryu valley from Seraghat-Naichun to Seri in the central sector of the Uttarakhand Himalaya, India. The RF is characterized as dextral strike slip fault and behaves as a ductile shear zone within the zone of NAT. The dextral sense of shear movement of RF is delineated by the fabric of the shear zone rocks including microscopically observed indicators such as sigma and delta porphyroclasts, quartz c-axis, and the field structural data. Additionally, in the quaternary period the dextral strike slip fault is reactivated with oblique slip component as characterized by various geomorphic indicators, for example, triangular facets, abandoned river channels, unpaired fluvial terraces, and V-shaped valleys with recurrent seismicity. Further, the morphometric parameters including Valley Floor Width to Valley Heig...
2006
The Kachchh region of Western India is a pericratonic basin experiencing periodic high magnitude earthquakes events. In 2001 a catastrophic seismic event occurred at Bhuj measuring M w = 7.7. The epicenters of both the 1956 and 2001 earthquakes were along the Kachchh Mainland Fault (KMF), proximal to the eastern end of the Northern Hill Range (NHR). The latter is a topographic expression of an active fault related fold on the hanging wall, and is controlled by a south dipping blind thrust. The present study deals with the eastern sector of NHR and uses optical dating to reconstruct the chronology of tectonically caused incisions. Along the backlimb of the NHR, incision ages on, channel fills and valley fill terraces progressively decrease from ∼12 ka to 4.3 ka. This age progression along with geomorphic evidences (decrease in topographic relief, drainage capture and drainage migration across the fold nose) suggests an active vertical and lateral fold growth along the KMF. Optical ages suggest that during the Late Holocene, the average uplift rate along the eastern NHR was 10 ± 1 mm/a. Recent GPS based estimates on crustal shortening are ∼12 mm/a. The KMF and the South Wagad Fault (SWF) represent the bounding faults of a transtensional basin that formed during the initial rifting. This basin is termed as the Samakhiali basin. The compressive stresses on account of structural inversion from normal to reverse phase resulted in lobate-shaped anticlines along KMF and SWF zone. These anticlines subsequently coalesced and formed linked and overlap segments. The present study suggests that eastward lateral deformation across the eastern portion of KMF has continued and has now resulted in its interaction with a left step over transfer fault called the South Wagad Master Fault (SWMF). This implies an increasing transpersional deformation of the Samakhiali basin. We therefore, suggest that the eastward NHR ridge propagation along KMF resulted in the thrust faulting on the south dipping SWMF resulting in the Bhuj 2001 event. The increasing strain on this basin may cause enhanced seismicity in the future along the eastern KMF and Wagad region.