CRUSTAL STRUCTURE OF GRANITE-GREENSTONE TERRANES IN THE EASTERN GOLDFIELDS, YILGARN-CRATON, AS REVEALED BY SEISMIC-REFLECTION PROFILING

A deep crustal seismic reflection profile across granite-greenstones o f the Eastern Goldfields in the Archaean Yilgarn Craton has revealed n ew constraints on upper crustal geometries. The Ida Fault, which forms the western boundary of the Eastern Goldfields, is a 30 degrees east- dipping normal fault that can be traced to a depth of about 25 km. A m id-crustal boundary is displaced with a throw of some 5 km across this fault. In a broad region below the Ida Fault, the crust-mantle bounda ry (Moho) is gently warped from a depth of c. 33 km in the west to a d epth of c. 38 km to the east. Greenstones of the Eastern Goldfields ar e underlain by a regional detachment outlined by prominent reflectors at depths between 4 and 7 km. Open anticlines of the greenstone succes sion are truncated against this basal detachment or against gently dip ping reflectors that can be correlated with terrane and domain boundar y faults identified during regional mapping. These truncations are not associated with changes in thickness of the successions. These geomet ries suggest several stages of large scale movement, possibly includin g out-of-section (north-south) movements unresolved by the east-west s eismic reflection line. Reflectors representing boundary faults mostly sole out into the basal detachment. However, the west-dipping Bardoc Shear Zone is continuous across the basal detachment, and may have bee n reactivated as a conjugate fault to the late stage Ida Fault. Granit e plutons are imaged as steep-sided and flat-bottomed opaque regions, locally underlain by continuous reflectors indicating greenstone strat igraphy. These geometries, in combinations with the map observations, suggest that the plutons were emplaced as elongate ovoid sheets, presu mably fed through fractures. (C) 1997 Elsevier Science B.V.