The Hidden Past of the Alai Valley: Understanding the Seismic History and Behavior of the Central Pamir Frontal Thrust System through Paleoseismology.

Magda Patyniak, Angela Landgraf, Ramon Arrowsmith, Atyrgul Dzhumabaeva, Alana M. Williams, Kanatbek Abdrakhmatov, & Manfred Strecker

Published August 15, 2018, SCEC Contribution #8640, 2018 SCEC Annual Meeting Poster #241 (PDF)

The Pamir–Tien Shan convergence zone represents a major tectonic part of the Indian-Eurasian collision zone by accommodating nearly one-third of the total 44 mm/yr total shortening in the greater Pamir region. These high deformation rates across relatively short distances compared to other intracontinental orogenic settings drive strain release with resulting large-magnitude earthquakes of up to M7 or higher. In comparable intraplate settings, deformation patterns are expected to be complex and distributed in space and time, the earthquake recurrence intervals are long, and rupture behavior often associated with reactivation of pre-existing crustal anisotropies.

In the Pamir mountains, however, GPS data show that the highest shortening rates across the greater Pamir-Tien Shan collision zone are observed along the northward-propagating Pamir Frontal Thrust system (PFT), the northern most arc-shaped boundary of the Pamir orogen with an intermontane basin – the Alai Valley - separating it from the Tien Shan mountains to the north. Extensive studies of temporary seismic networks across the PFT underline the recent seismic activity and distribution of shallow and intermediate-depth earthquakes. However, modern seismicity patterns do not fully explain the relation between seismicity and surface rupture behavior, nor the detailed rupture zone geometry and might thus not reflect the long-term deformation history.

Our paleoseismological study presents preliminary data and new slip rate estimates for different late Quaternary timescales from trenches along the central segment that is characterized by the geomorphological youngest thrust zone of the PFT. This fairly straight sector of the mountain front comprises several uplifted fluvial terraces in the hanging wall, and hosts fault scarps with colluvial wedges suggesting multiple seismic events in the recent past. We combined high-resolution structure-from-motion analysis of offset stratigraphic horizons in the trenches with UAV-based DEM analysis and dGPS profiling of displaced landforms. Our earthquake chronology is supported by representative radiocarbon and luminescence dating of distinct event horizons.
We were able to define at least 4 seismic events that offset alluvial-fan strata of the Pamir mountain front. These new data provide new insights into the late Quaternary earthquake history and ultimately help to refine the earthquake recurrence times along the central segment of the PFT.

Key Words
Paleoseismology, Neotectonics, Pamir, Pamir Frontal Thrust System, Intracontinental

Citation
Patyniak, M., Landgraf, A., Arrowsmith, R., Dzhumabaeva , A., Williams, A. M., Abdrakhmatov , K., & Strecker, M. (2018, 08). The Hidden Past of the Alai Valley: Understanding the Seismic History and Behavior of the Central Pamir Frontal Thrust System through Paleoseismology.. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology