Crustal structure of granite-greenstone terranes in the Eastern Goldfields, Yilgarn Craton, as revealed by seismic reflection profiling

Abstract

A deep crustal seismic reflection profile across granite-greenstones of the Eastern Goldfields in the Archaean Yilgarn Craton has revealed new constraints on upper crustal geometries. The Ida Fault, which forms the western boundary of the Eastern Goldfields, is a 30° east-dipping normal fault that can be traced to a depth of about 25 km. A mid-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 boundary (Moho) is gently warped from a depth of c. 33 km in the west to a depth of c. 38 km to the east. Greenstones of the Eastern Goldfields are underlain by a regional detachment outlined by prominent reflectors at depths between 4 and 7 km. Open anticlines of the greenstone succession are truncated against this basal detachment or against gently dipping reflectors that can be correlated with terrane and domain boundary faults identified during regional mapping. These truncations are not associated with changes in thickness of the successions. These geometries suggest several stages of large scale movement, possibly including out-of-section (north-south) movements unresolved by the east-west seismic 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 been reactivated as a conjugate fault to the late stage Ida Fault. Granite plutons are imaged as steep-sided and flat-bottomed opaque regions, locally underlain by continuous reflectors indicating greenstone stratigraphy. These geometries, in combinations with the map observations, suggest that the plutons were emplaced as elongate ovoid sheets, presumably fed through fractures.