Horizontal tectonics pre-dating upper gorge creek group sedimentation pilbara block, western australia

Abstract

Abstract The greenstone belt stratigraphy of the c. 3500 Ma old Pilbara Block has been subdivided into the lower dominantly volcanic Warrawoona Group and the upper mainly-sedimentary Gorge Creek Group. Regional mapping indicated that these units evolved as a near continuous succession with only minor internal unconformities, and that both groups suffered their most intense tectonic imprint together as a result of granite diapirism. However, an upper Gorge Creek conglomerate (a Lalla Rookh correlate), with a basal angular unconformity, contains felsic schist clasts derived from highly strained portions of the Warrawoona Group. The folds in the conglomerate are upright as expected for diapirically generated structures and are as complex as any attributed to this process. The schist clasts were, therefore, probably generated in a pre-doming event. High-strain and high-grade metamorphism are confined to the lower parts of the greenstone stratigraphy where a polyphase deformation history may be clearly defined. The low-strain, low-grade areas are generally regarded as having a simpler tectonic history but some examples contain very tight minor folds that pre-date a widespread subhorizontal cleavage, and both events pre-date the doming. In other low-grade areas thrust structures are truncated by the angular unconformity at the base of the Lalla Rookh correlate and the thrust geometry suggests they formed in already folded rocks.