Print this pagePredicting the Brittle-Ductile (B-D) Transition in Continental Crust Through Deep, Long Offset, Prestack Depth Migrated (PSDM), 2D Seismic Data
Since 2001, GX Technology has acquired a number of long-offset (10 km), long-record (18-sec), 2D marine seismic surveys (the “SPAN” surveys) on various passive margins and in some marginal seas, for the purpose of imaging the full crustal architecture. The surveys are processed to PSDM (Prestack Depth Migration, at least to 25 km but many to 40 km) using potential field (gravity) modeling to derive stacking velocities in the crystalline basement. When displayed in “average energy” mode in standard PC-based interpretation software, a good deal of the small-scale noise in wiggle- trace displays is suppressed. Many well-defined faults that cut the sedimentary column and the sediment-basement interface can be traced into the underlying crust along discontinuities in the character of the data. Some discontinuities that terminate at the top of basement are interpreted to be older, un-reactivated faults in the basement. Almost all of these features are listric in form and sole out at the top of an acoustically transparent zone usually below 25 km or deeper. Above that zone the seismic character shows a variety of features assumed to relate to structure, lithology and fracture-related qualities of the crustal lithologies. Moho is invariably below this transparent zone, although sometimes another zone of seismic events lies between the Moho and the transparent zone. The top of these transparent zones is interpreted to be the brittle-ductile transition (B-D) in the continental crust at the time of faulting for two reasons: (1) insofar as the brittle-ductile transition should be a level within or below which extension occurs by horizontal stretching, crustal-scale faults nucleate at low angles of dip and propagate upward at steeper angles in accordance with the ambient stress field; (2) the onset of ductility corresponds to the activation of crystal-plastic processes and consequent ductile flow in the lower part of the crust. The transparency would stem from the healing of fracture-related defects and consequent homogenization of the velocity structure. The B-D is, by nature a thermally activated horizon, and consequently also an isotherm for the rock compositions at a given location. Thus the SPAN surveys can give basin-modeling studies a much-needed calibration of the crustal structure at the time of basin formation, either by directly constraining the temperature structure or, along with the depth to Moho, by providing constraints on stretching models.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil