Three‐Dimensional Crustal Architecture Beneath the Sikkim Himalaya and Its Relationship to Active Deformation

Abstract

We study the 3‐D variation of the crustal structure of the Sikkim Himalaya using broadband seismological data acquired from a focused network of seven stations spanning the Lesser, Higher, and Tethyan Himalaya. Common conversion point stacking of receiver functions recorded along an across‐strike profile of the Himalaya reveals first‐order northward dip on the Main Himalayan Thrust (MHT), a midcrustal discontinuity and the Moho, along with higher‐order lateral variations. Three‐dimensional images generated from joint inversion of receiver functions and surface wave dispersions show that the MHT has a ramp‐flat‐ramp geometry. The ramps are located beneath the Lesser Himalaya and the Tethyan Himalaya with dips of ∼7∘ and ∼15∘, respectively, connected by flat segments. The ramp beneath the Lesser Himalaya forms a dome structure, upwarping the thrust sheets associated with the Peling and Main Central Thrust. The erosional surface of this dome forms the arcuate geometry of thrusts observed in the Lesser Himalaya. The thickness of the underthrust Indian crust is 35–42 km and has an average VS of 3.63 km/s, similar to that of the Indian Shield crust. The Moho also has dome‐like structures separated by elongated, deeper sections trending NW‐SE. These are intersected by steeply dipping transverse low‐velocity zones, oblique to the strike of the Himalaya. We conjecture that these low‐velocity zones are the dextral‐strike slip faults known to be active beneath the Sikkim Himalaya. The observed alternate shallow and deep segments of the Moho must be a consequence of several cycles of strike‐slip displacement on these transverse faults.

Signatures of the Existence of Frontal and Lateral Ramp Structures Near the Kishtwar Window of the Jammu and Kashmir Himalaya: Evidence From Microseismicity and Source Mechanisms

“…Joint inversion of receiver function and surface wave dispersion study in this region (Sharma et al, 2018) shows the MHT at 10-14 km for station BADR and 14-16 km in PHAG. Upwarping of the overlying thrusts due to the MHT structure were also reported in Sikkim (Paul & Mitra, 2017). West of our study region, the MHT is found at depths of 8-10 km (SMVD).…”

“…The steep dip of the Kishtwar thrust (as reported in Singh, 2010) compares well with the steep ramp of the MHT beneath the NE cluster of events, while the gentler dip of MCT (Singh, 2010) is similar to the gentle dip of MHT beneath the SW cluster of events. The capability of lateral heterogeneities terminating rupture propagation were discussed in detail for subduction scenarios in Wang and Bilek (2011) and its applicability to the MHT in the Sikkim Himalaya in Paul and Mitra (2017). Bilham et al (2013) and Schiffman et al (2011) reported normal faults in the SE edge of Kashmir Valley (33.56 ∘ N, 75.51 ∘ E) ∼30 km NW of the location of the M L 4.0 normal fault earthquake in this study.…”

“…These two stations are closest to the SW cluster and the NE cluster, respectively. Two relevant observations from the Sikkim Himalaya regarding the dome-shaped (or elevated) structures on the MHT are as follows: (i) they can give rise to a network of fractures and hence microseismicity in the region overlying them, and (ii) they can make the overlying thrust sheets wrap around them resulting in an arcuate surface expression (Paul & Mitra, 2017). Another joint inversion of receiver function and dispersion study (Mir et al, 2017) in the Kashmir Basin (NW of the Kishtwar Window) shows the MHT at ∼15 km.…”

Seismic Attenuation of the Eastern Himalayan and Indo‐Burman Plate Boundary Systems, Northeast India

“…However, the lateral variation in Q 0 perturbation differentiates the Sikkim Himalaya (with low Q 0 ) from the Eastern Himalaya (with intermediate Q 0 ). This variation in attenuation is possibly a result of the upwarping and arcuate shape of the Himalayan thrust sheets beneath the Sikkim Himalaya (Paul & Mitra, ), which has not been reported from the Bhutan Himalaya (Diehl et al, ). The southern Tibetan Plateau is marked by low V s and low Q 0 perturbations due to elevated temperatures and presence of partial melts within the crust (Nelson et al, ; Priestley & McKenzie, ), which results in anelastic attenuation.…”

Rupture of the Indian Slab in the 2011 Mw 6.9 Sikkim Himalaya Earthquake and Its Tectonic Implications

“…However, Sheehan et al () observed low attenuation of seismic shear waves in the uppermost mantle beneath the Higher Himalaya of eastern Nepal, which corresponds to low temperatures and hence brittle conditions. Further, Paul and Mitra () noticed a low velocity layer in the lower crust beneath the Higher Himalaya.…”