Tensile and compressive growth structures: relationships between sedimentation, deformation and granite intrusion in the Archaean Coppin Gap greenstone belt, Eastern Pilbara, Western Australia

Abstract

The Coppin Gap greenstone belt is composed of an east–west striking asymmetric synclinorium between the Muccan and Mount Edgar batholiths. Along the contact with the Muccan batholith, the vertical to overturned northern limb is intensely sheared, folded and tectonically reduced. The southern limb is relatively undisturbed and irregularly intruded by the Mount Edgar batholith. Progressive unconformities and sedimentary infilling of growth structures in the Gorge Creek Group record the deformation history. The unconformities constrain which of the now steeply dipping shear zones have to be rotated with their cover sequence before they can be interpreted kinematically. Four mid-Archaean clastic systems have been recognized: the first and third are transgressive littoral quartzose and feldspathic arenites; the second and fourth comprise conglomeratic and ferruginous alluvial fan-related deposystems. East-verging thrust tectonics affected the belt after unconformable deposition of the first deposystem on the early Archaean Warrawoona Group. In the western extremity of the belt, primary tectonic relief was filled with the second deposystem. Thrust folds were overprinted at right angles by folding sub-parallel to the developing belt. After truncation, the belt was covered by the third and fourth systems. In the eastern part of the belt, relief created by belt-parallel folds affecting the first deposystem was filled by conglomerate overlain by basalts; a sequence also assigned to the Whim Creek Group. We relate systematic depocentre shifts, as observed in the Bamboo Creek Syncline, to differential doming of the adjacent batholiths. Buried thrusts and shear zones may have caused km-scale repetition of the underlying sequences. The observed relationships place uncertainty on the validity of the existing stratigraphic correlations, which now need geochronologic work to resolve. Part of the thrusting appears to have taken place by inversion on pre-existing detachment shear zones in the underlying Warrawoona basalts and ultramafic rocks. The shear zones descend from a high stratigraphic level in the east towards deeper levels in the west. The first motion has been west-block down, listric normal growth faulting, in places accompanied by synsedimentary, explosive intrusion of black chert veins, mega-brecciation and back tilt of fault blocks. Geometrically, these early structures cannot be related to the geometry of batholith doming and are interpreted as the result of either regional tensile stress, local caldera collapse or both mechanisms acting synchronously. The mid-Archaean influx of siliciclastic sediments in the Coppin Gap Belt coincides with this change from a tensile to a complex compressive deformation, the latter caused by regional thrusting and interbatholith folding. Late deformation of the belt includes further steepening of the strata and eastward oblique dextral and 060° sinistral strike–slip.