Seismic investigations of the Earth's crust: Velocity structure and tectonics, Yukon-Tanana Terrane, Alaska, and, Near surface effect on wave propagation, Ross Ice Shelf, Antarctica.

摘要

Two Trans-Alaska Crustal Transect (TACT) seismic refraction studies, sampling the crust beneath the Yukon-Tanana terrane (YTT), Alaska and the terranes along its northwestern boundary, are interpreted using both forward and inverse modeling techniques. These studies reveal a thin (c. 30 km), reflective middle to lower crust beneath the southern YTT and the terranes to its northwest. The lower crust beneath the central YTT is interpreted to be quartz-rich and to result from tectonic underplating. Across the northwestern boundary of the YTT, the refraction data reveal a crustal velocity structure that is divided into three upper crustal and two or three middle to lower crustal domains. The upper crustal domains are delineated by two vertical, low-velocity zones that contain or are bounded by interpreted strands of the Tintina fault. This interpretation suggests that after initial thin-skinned amalgamation of the various terranes caught between the Yukon-Tanana and Ruby terranes, this region experienced thick-skinned tectonic reorganization. Lower crustal reflectivity beneath the YTT is argued to be the result of ductile deformation; strain developed to accommodate differential movement between the upper crust and the upper mantle during margin parallel translation.; A coupled reflection/refraction experiment, imaging the crust beneath the Ross Ice Shelf, Antarctica, is used to study phenomena associated with the unique acquisition environment of Antarctica's floating ice shelves. The near surface firn layer influences the data character by amplifying and frequency modulating the incoming wavefield. In addition, the ice-water column introduces pervasive, high energy seafloor, intra-ice, and intra-water multiples that have moveout velocities similar to the expected sub-seafloor primary velocities. Successful removal of these high energy multiples relies on predictive deconvolution, inverse velocity stack filtering, and frequency filtering. Removal of the multiples reveals a faulted, sedimentary wedge which is truncated at or near the seafloor and a crust that is c. 21-km-thick. These results provide seismic evidence that the extensional features observed in the Ross Sea region of the Ross Embayment extend beneath the Ross Ice Shelf.