INDEPTH geophysical and geological observations imply that a partially molten midcrustal layer exists beneath southern Tibet. This partially molten layer has been produced by crustal thickening and behaves as a fluid on the time scale of Himalayan deformation. It is confined on the south by the structurally imbricated Indian crust underlying the Tethyan and High Himalaya and is underlain, apparently, by a stiff Indian mantle lid. The results suggest that during Neogene time the underthrusting Indian crust has acted as a plunger, displacing the molten middle crust to the north while at the same time contributing to this layer by melting and ductile flow. Viewed broadly, the Neogene evolution of the Himalaya is essentially a record of the southward extrusion of the partially molten middle crust underlying southern Tibet.

INDEPTH seismic reflection profiling shows that the decollement beneath which Indian lithosphere underthrusts the Himalaya extends at least 225 kilometers north of the Himalayan deformation front to a depth of approximately 50 kilometers. Prominent reflections appear at depths of 15 to 18 kilometers near where the decollement reflector apparently terminates. These reflections extend north of the Zangbo suture to the Damxung graben of the Tibet Plateau. Some of these reflections have locally anomalous amplitudes (bright spots) and coincident negative polarities implying that they are produced by fluids in the crust. The presence of geothermal activity and high heat flow in the regions of these reflections and the tectonic setting suggest that the bright spots mark granitic magmas derived by partial melting of the tectonically thickened crust.

Metamorphic minerals within the shear zones indicate at least amphibolite grade metamorphism with increasing metamorphic grade down section. The seismic data, coupled with the new geologic information and geomorphologic data from Landsat imagery, suggest that the structure of the northern YadongGulu rift is comparable to other well-documented rift systems in which steep range-front normal faults terminate at, or sole into, moderately dipping normal-sense detachments at depth that have undergone substantial footwall uplift and rotation during extension. This geometry implies that the extension of the Tibetan Plateau crust has been accommodated at depth by lateral flow within the middle crust. It also suggests that E-W extension of the plateau may have been substantially greater than has generally been presumed.

Abstract. Project INDEPTH (International DeepProfiling of Tibet and the Himalaya) has collected over 300 km of multichannel, deep seismic reflection data using explosive sources as part of a multidisciplinary effort to image the structure of the crust and uppermost mantle of the Tibetan plateau. The reflection profiles lie within the Yadong-Gulu rift and were acquired in the summers of 1992 and 1994. Data processing utilized typical industry tools, and a new method was used to migrate the data. Both unmigrated and migrated sections are presented here in large format to facilitate further interpretations.

Depth imaging with anisotropic models has been shown to deliver more geologically plausible models and accurate images. Deriving parameters that describe the anisotropic properties of the subsurface requires incorporating well information. There are, however, vast exploration areas around the world with very limited to no well control that do require high-quality anisotropic imaging to allow adequate interpretation of deeper targets below complex structures. We present a general workflow for building tilted transversely isotropic (TTI) models in areas of very limited well control. We incorporate published knowledge of the area and analysis of data anellipticity, with a derivation of Thomsen's δ in wells from adjacent areas. We use single regional Thomsen's parameter trends hung from the water bottom. Finally, we present a case study that applies this workflow to seismic data from the Kwanza basin offshore Angola where TTI models were built over more than 12000 km2 in two adjacent areas. By accounting for TTI anisotropy, we have produced geologically plausible and interpretable images from relatively old narrow-azimuth streamer data with only moderate offsets of 4.8 km.