ALBERT

Description: [1]  We present new seismicity images based on a two-year seismic deployment in the Pamir and SW Tien Shan. 9,532 earthquakes were detected, located and rigorously assessed in a multistage automatic procedure utilizing state-of-the-art picking algorithms, waveform cross-correlation and multi-event relocation. The obtained catalog provides new information on crustal seismicity and reveals the geometry and internal structure of the Pamir-Hindu Kush intermediate-depth seismic zone with improved detail and resolution. The relocated seismicity clearly defines at least two distinct planes, one beneath the Pamir, the other beneath the Hindu Kush, separated by a gap across which strike and dip directions change abruptly. The Pamir seismic zone forms a thin (ca.10 km width), curviplanar arc that strikes east–west and dips south at its eastern end, then progressively turns by 90 degrees to reach a north–south strike and a due eastward dip at its southwestern termination. Pamir deep seismicity outlines several streaks at depths between 70 and 240 km, with the deepest events occurring at its southwestern end. Intermediate-depth earthquakes are clearly separated from shallow crustal seismicity, which is confined to the uppermost 20–25 km. The Hindu Kush seismic zone extends from 40 to 250 km depth and generally strikes east–west, yet bends northeast, towards the Pamir, at its eastern end. It may be divided vertically into an upper and lower part separated by a gap at approx. 150 km depth. In the upper part, events form a plane that is 15–25 km thick in cross-section and dips sub-vertically north to northwest. Seismic activity is more virile in the lower part, where several distinct clusters form a complex pattern of sub-parallel planes. The observed geometry could be reconciled either with a model of two-sided subduction of Eurasian and previously underthrusted Indian continental lithosphere or by a purely Eurasian origin of both Pamir and Hindu Kush seismic zones, which necessitatesa contortion and oversteepening of the latter.

Description: Results obtained from S and P receiver functions produced a clear image of the top and bottom of the subducting Nazca lithosphere beneath northern Chile. Using data from the teleseismic events recorded at 15 permanent Integrated Plate Boundary Observatory Chile (IPOC) stations, we obtained new constraints on the geometry and thickness of the descending Nazca lithosphere. We observed the subducted crust of the Nazca plate at depths ranging from 50 km beneath the Coastal Cordillera down to 110 km beneath the Western Cordillera. We found significant along-strike variations in the geometry of the Nazca plate beneath northern Chile. On closer inspection, it appears that the oceanic Nazca plate is divided into two distinct segments as it descends beneath the continental South American plate. The transition from the relatively steeper (∼23°) and deeper slab to the north of 21°S to the flatter southern segment (∼19°) is shown reasonably clearly by our data. This feature could well be associated with variations in the curvature of the plate margin and the geometry of the Chile trench, which is mainly curved to the north of 21°S. We have also mapped the continental Moho of the South American plate at depths ranging between 60 and 70 km to the east of the Longitudinal Valley. Beneath the Coastal Cordillera, this boundary becomes invisible, probably due to the serpentinization of the forearc mantle wedge that reduces the velocity in the uppermost mantle. The base of the subducted Nazca plate was clearly identified as a sharp boundary in the results obtained from the P and S receiver functions. The thickness of the subducted oceanic Nazca plate, which has an age of ∼50 My, is estimated to be ∼50 km. Although this thickness is consistent with that predicted by thermal gradients, the explanation of the sharpness of the lithosphere-asthenosphere boundary may require another mechanism such as hydration or melting.