Crustal deformation of the Lhasa terrane, Tibet plateau from Project INDEPTH deep seismic reflection profiles

Abstract

International Deep Profiling of Tibet and the Himalaya (INDEPTH) deep reflection data in the Yangbajain‐Damxung graben of southern Tibet yield evidence of magmatism and deformation beneath the southern Lhasa terrane and Yarlung‐Zangbo suture. Shallow reflections and low‐velocity first arrivals indicate a thin, Quaternary graben fill of generally less than a few hundred meters thickness. Underlying stratified reflections, extending to a maximum of about 11.5 km depth, likely originate from deformed Paleozoic‐Cenozoic supracrustal strata of the Lhasa terrane. A prominent, undulatory band of reflections within the crystalline basement extends beneath the length of the Yangbajain‐Damxung graben (depth ranges from ∼12 km to ∼18 km). This horizon has been interpreted to mark the top of a midcrustal partial‐melt zone underlying southern Tibet. The undulatory character of this horizon additionally suggests that it may be tectonically deformed. A somewhat deeper, subhorizontal, wide‐angle reflection extends southward beneath the outcrop of the Yarlung‐Zangbo suture, indicating that the suture is cut off or superposed by a younger structure at depth. Three gently north dipping reflections at 19 km, 24 km, and 27 km depth beneath the Gangdese batholith are suggestive of a midcrustal duplex and may mark the northward (downdip) extension of the late Oligocene‐early Miocene Gangdese thrust system. A prominent ∼40° north dipping reflection imaged in the deep crust beneath the Gangdese batholith, between 40 and 60 km depth, might mark the downdip expression of the Yarlung‐Zangbo suture or, alternatively, a younger reverse fault in the lower crust (or both). Viewed in aggregate, the reflection data are suggestive of moderate, postcollisional shortening of the upper crust of the Lhasa terrane, accompanied by melting of the middle crust. Although the data are permissive of wholesale underthrusting or fluid injection of Indian continental crust beneath the Lhasa terrane, they show no direct evidence for this having occurred.