A NEW MAP OF CRUSTAL TERRANES IN THE UNITED-STATES FROM COCORP DEEP SEISMIC-REFLECTION PROFILING

A new map has been constructed to summarize major, systematic variations in seismic reflection character observed by over 10 000 km of COCORP profiling in the US. Although seismic coverage is far from comprehensive, and important regions are still unsampled, the existing COCORP profiles, and other deep seismic results in the US, span a diverse range of crustal structures and ages. Most importantly, certain reflection patterns are correlatable over large distances, and some can be related to surficial tectonic expressions. Deep reflection patterns are especially useful in areas where basement relations are obscured by superficial sedimentary cover (e.g. the central US). Among the seismic characteristics represented on this map are: crustal scale detachments, which can be traced as relatively low-angle reflectors over large areas; deeply penetrating, dipping reflection zones, some of which mark major shear zones, perhaps even crustal sutures; Moho reflectivity, which appears to vary from craton to Phanerozoic periphery; seismic transparency which can sometimes be interpreted as geologic 'homogeneity'; seismic layering, prominent in the lower crust in many areas (often associated with a transparent upper crust) and now known to dominate the upper basement in other regions; and unusually strong reflections, or bright spots, at mid-crustal levels that may mark deep crustal fluids. Although some of these features are merely subsurface extensions of structures well known from surficial observations, others represent new characterizations of deep heterogeneity which are mappable even though their specific origins may still be incompletely understood. To a substantial degree, such seismic characteristics of the deep crust are generally independent of technique and thus relatively robust geological discriminants. However, it must be re-emphasized that the spatial density of deep seismic lines in the US is still extremely sparse and important regions yet unsampled. Attempts to map such characteristics, including this one, are now primarily demonstrations of their potential utility and, perhaps more importantly, the need for a more comprehensive strategy of lithospheric exploration than simple isolated transects.