STRUCTURAL VARIATIONS IN CORDILLERAN LITHOSPHERE FROM DEEP SEISMIC REFLECTION PROFILING

The Cordillera of North America have now been probed by a series of deep seismic reflection profiles in Alaska (e.g. TALI), Canada (e.g. LITHOPROBE, ACCRETE), and the co-terminous US (e.g. COCORP, CALCRUST, CD-ROM). While most of Central and South America remain largely unexplored with this technology, a few deep seismic profiles have focused attention on crust-mantle interactions beneath the Andes (e.g. ANCORP, TIPTEC). This sampling is far from comprehensive, so any generalizations are speculative at best. However, certain reflector patterns appear to have more than local significance. Among these are:

Moho morphology and reflectivity: Gross differences in crustal thickness are clearly important to understanding processes of crustal growth (and destruction). Variations in detailed Moho character seem to be equally significant. Extensive regions of minimal Moho topography suggest substantial mobility (ductility) of both lower crustal and upper mantle lithosphere. The contrast in Moho reflectivity (and, more recently, convertivity) between distinct elements of the Cordillera mosaic (e.g. Rocky Mountains vs. Colorado Plateau) is probably recording contrasts in the tectono-thermal history of these areas.

Lower crustal laying and seismic “bright spots”: Contrasts in reflectivity (e.g. upper vs lower crust) seem to be associated in part with magmatism related to mantle processes (e.g. delamination, rifting). Such “fabrics” are thus guides to both material transfer from mantle to crust as well as lateral transfer within the crust.

Mantle reflectors: Although relatively rare among current Cordillera reflection surveys, their preservation in adjacent regions (e.g. Slave craton) serve as constraints on the recycling of sub-cratonic lithosphere during Cordilleran orogenesis.

Detachments: Subhorizontal and listric reflectors on a number of deep seismic lines have been cited to imply structural detachment within the crust, between the crust and upper mantle, and within the mantle. They draw attention to the importance of differential lateral translation at depth in the lithosphere during Cordilleran events.

Future progress in understanding the Cordilleran lithosphere is dependent upon strategic integration of deep reflection with passive imaging techniques.