STRUCTURAL CONSTRAINTS IN NEOTECTONIC STUDIES OF THRUST FAULTS FROM THE MAGNETOTELLURIC METHOD, KOCHKOR BASIN, KYRGYZ REPUBLIC

Abstract

A continuous high-resolution magnetotelluric (MT) profile 5.4 km long reveals a 1-to 2-km-thick section of Neogene strata thrust beneath a crystalline range front along a gently dipping décollement in the Kyrgyz Tien Shan, central Asia. The imaged décollement links the intrabasin Akchop Hills fault, which currently accommodates $15% of the total shortening across the central Tien Shan, to a crustal ramp that dips $45° beneath the range. Bounds from the MT data on the base of the sediments and on the dip of the crustal ramp suggest >2.5–7.5 km of total shortening across the basin margin, of which at least 2.5–4.8 km clearly involves the décollement and the recently developed Akchop Hills fault. Thus a significant fraction of the total shortening across the margin has been accommodated by this young fault system. Combined with preliminary studies that suggest a mid-Miocene age for the base of the stratigraphic section, results drawn from the MT section indicate that shortening rates have increased since the onset of deformation, perhaps within the past 1–2 million years., Structural constraints in neotectonic studies of thrust faults from the magnetotelluric method, Kochkor Basin, Kyrgyz Republic