The effective elastic thickness of the India Plate from receiver function imaging, gravity anomalies and thermomechanical modelling

The range and the meaning of the effective elastic thickness (EET) in continental areas have been subject to controversy over the last two decades. Here we take advantage of the new data set from the Hi-CLIMB seismological experiment to re-estimate the EET of the India Plate along a south-north profile extending from the Ganges basin to central Tibet. Receiver functions give a high-resolution image of the base of the foreland basin at similar to 5 km depth and constrain the crustal thickness, which increases northwards from similar to 35 km beneath the indo-gangetic plain to similar to 70 km in southern Tibet. Together with available data sets including seismic profiles, seismological images from both INDEPTH and HIMNT experiments, deep well measurements and Bouguer anomaly profiles, we interpret this new image with 2-D thermomechanical modelling solutions, using different type of crustal and mantle rheologies. We find that (1) the EET of the India Plate decreases northwards from 60-80 to 20-30 km as it is flexed down beneath Himalaya and Tibet, due to thermal and flexural weakening; (2) the only resistant layer of the India Plate beneath southern Tibet is the upper mantle, which serves as a support for the topographic load and (3) the most abrupt drop in the EET, located around 200 km south of the MFT, is associated with a gradual decoupling between the crust and the mantle. We show that our geometrical constraints do not allow to determine if the upper and lower crust are coupled or not. Our results clearly reveal that a rheology with a weak mantle is unable to explain the geometry of the lithosphere in this region, and they are in favour of a rheology in which the mantle is strong

Dislocation modeling of blind thrusts in the eastern Los Angeles basin, California

The East and West Coyote Hills in the eastern Los Angeles Basin are the surface expression of uplift accompanying blind reverse faulting. Folded Quaternary strata indicate that the hills are growing and that the faults underlying them are active. Detailed subsurface mapping in the East Coyote Oil Field shows that a previously mapped, reverse separation fault is predominantly an inactive, left‐lateral, strike‐slip fault that is not responsible for the uplift of the East Coyote Hills. The fault responsible for folding and uplift of the Coyote Hills does not cut wells in either the East or West Coyote Oil Fields. To characterize the geometry of the blind fault responsible for folding, we employ dislocation modeling. The dip and upper fault tip depths obtained from modeling suggest that the thrust fault beneath the Coyote Hills may be an extension of the Puente Hills blind thrust fault that continues westward beneath the Santa Fe Springs Oil Field. Modeling results suggest that the segment of the thrust fault responsible for folding the Coyote Hills would have accumulated 1500 m of reverse displacement over the last 1.2 Myr, yielding an average slip rate of 1.3 ± 0.5 mm/yr. The Santa Fe Springs segment of the fault has a slip rate of 1.5 ± 0.4 mm/yr for the last 1.2 Myr. The estimated moment magnitude for a reverse displacement earthquake on the Puente Hills blind thrust ranges from 6.6 to 7.2, depending on the length of the rupture. The estimated average recurrence interval for these earthquakes is 1700–3200 years.Keywords: Southern California, Blind thrust faults, Earthquake

Lower edge of locked Main Himalayan Thrust unzipped by the 2015 Gorkha earthquake

Large earthquakes are thought to release strain on previously locked faults. However, the details of how earthquakes are initiated, grow and terminate in relation to pre-seismically locked and creeping patches is unclear ^1-4. The 2015 Mw 7.8 Gorkha, Nepal earthquake occurred close to Kathmandu in a region where the prior pattern of fault locking is well documented ^5. Here we analyze this event using seismological records measured at teleseismic distances and Synthetic Aperture Radar imagery. We show that the earthquake originated northwest of Kathmandu within a cluster of background seismicity that fringes the bottom of the locked portion of the Main Himalayan Thrust fault (MHT). The rupture propagated eastwards for about 140 km, unzipping the lower edge of the locked portion of the fault. High-frequency seismic waves radiated continuously as the slip pulse propagated at about 2.8 km s-1 along this zone of presumably high and heterogeneous pre-¬seismic stress at the seismic-aseismic transition. Eastward unzipping of the fault resumed during the Mw 7.3 aftershock on May 12. The transfer of stress to neighbouring regions during the Gorkha earthquake should facilitate future rupture of the areas of the MHT adjacent and up-dip of the Gorkha earthquake rupture.This is the author accepted manuscript. The final version is available from Nature Publishing Group via http://dx.doi.org/10.1038/ngeo251

Structure of the crust and the lithosphere in the Himalaya-Tibet region and implication on the rheology and eclogitization of the India

The Himalayas and the Tibetan Plateau are considered as the classical case of continental collision