Paper summaries 2018

Abstract: Geologic reconstructions of the Main Himalayan Thrust in Nepal show a laterally extensive midcrustal ramp, hypothesized to form the downdip boundary of interseismic locking. Using a recent compilation of interseismic GPS velocities and a simplified model of fault coupling, we estimate the width of coupling across Nepal using a series of two-dimensional transects. We find that the downdip width of fault coupling increases smoothly from 70 to 90 km in eastern Nepal to 100–110 km in central Nepal, then narrows again in western Nepal. The inferred coupling transition is closely aligned with geologic reconstructions of the base of the midcrustal ramp in central and eastern Nepal, but in western Nepal, the data suggest that the location is intermediate between two proposed ramp locations. The result for western Nepal implies either an anomalous coupling transition that occurs along a shallowly dipping portion of the fault or that both ramps may be partially coupled and that a proposed crustal-scale duplexing process may be active during the interseismic period. We also find that the models require a convergence rate of 15.5 ± 2 mm/year throughout Nepal, reducing the geodetic moment accumulation rate by up to 30% compared with earlier models, partially resolving an inferred discrepancy between geodetic and paleoseismic estimates of moment release across the Himalaya.

Highlights

• Locally, MHT décollement can be as shallow as 2 km depth.

• Depth to décollement varies in both trench-parallel and -perpendicular directions.

• Seismic data allow direct measurements of fault and bedding dip at depth.

• Blind fault strands preserve growth strata that may record individual earthquakes.

• Sub-surface evidence of base level changes large enough to form strath terraces.

Plain language summary:

When one tectonic plate dives beneath another, the fault between them is called a megathrust. The shallow part of these faults is not well understood. Generally, it is thought that if this area is pushed, it will freely slip (and will not store energy that would be released as earthquakes). Researchers make models of megathrusts using GPS measurements to determine which parts of it are slipping and which are not (which means they are storing energy that will be released as earthquakes). These models are not well constrained far from the GPS measurements, and in many areas it is difficult to make measurements near the shallow megathrust because they are under the ocean. We use a simple model that considers the forces acting on the fault to show that if the deeper megathrust is not slipping, then it will act as a buffer to prevent the shallow part from moving. We compare our model to megathrusts with many measurements in Japan and Nepal and show that the GPS data cannot tell us if the shallow fault is stuck together by friction or not. This is important because the behavior of the shallow fault affects potential earthquake size and tsunami risk.

Abstract: Eastern Tibet is an important example of oblique convergence and associated strain partitioning, as suggested by recent 2-D and 3-D structural interpretations, yet the nature and evolution of oblique strain partitioning of this region remain poorly known. Here we use seismic reflection profiles, borehole data, and field investigations in the Longmen Shan piedmont to determine the subsurface structural architecture, and we observe several nearly N-S striking thrusts and reactivation of NE striking preexisting faults. We interpret that this behavior is due to a regional principal compressional stress oriented in the E-W direction, oblique to the NE striking Longmen Shan. Using the records of fault activity and related Late Pliocene and Quaternary foreland sediments and growth strata, as well as the coseismic rupture of the 2008 M_w 7.9 Wenchuan earthquake, we demonstrate that the Longmen Shan has experienced E-W crustal shortening and oblique motion since ~5–2 Ma. We present two strain partitioning models arising from oblique thrusting in eastern Tibet and suggest that the eastward extrusion from Tibet is mainly accommodated on the strike-slip Longriba fault and the dip-slip Longmen Shan-Min Shan fault zones. These results enhance our understanding of the tectonic relationship between the Songpan-Ganzi terrane and the Sichuan basin and provide additional constraints for studies of the geodynamic response of eastern Tibet to the ongoing India-Eurasia collision.