The waning stage of a greenstone belt: The Mesoarchaean Mosquito Creek Basin of the East Pilbara, Western Australia

Abstract

The Mosquito Creek Basin stands apart in the Archaean granite–greenstone terrane of the east Pilbara Craton because of the rectangular outcrop but crescent-shaped subcrop, and its composition of low-grade metamorphic mainly turbiditic clastic sedimentary rocks. These sediments belong to the c. 2.9 Ga Mosquito Creek Formation. Tectonic interpretations of the basin have varied from a simple synclinorium, to an accretionary prism, an extensional passive margin, and more recently into a rift basin separating the Kurrana Terrane from the East Pilbara Terrane (EPT). Combined sedimentological and structural–geological investigations show coarse-clastic fan-delta marginal facies with stacked unconformities, rimming an E–W basin centre with predominantly finer-grained mass flow-dominated sediments. A N–S traverse therefore contains a complete cross-section of the Mosquito Creek Basin, shortened to about 50% of its original width by tight folding and thrusting. The basin fill shows one major FU sequence from fan-delta sandstone to distal turbidites, unconformably overlying an older, partially preserved clastic sequence of comparable facies. A large slump unit occupies the basin axis. The unconformities along the margins interfere with folding of the sediments which also influenced basin-centre turbiditic palaeocurrents. The basin is interpreted as a near-symmetrical, underfilled, compressional, intramontane, clastic sedimentary basin unconformably superposed on a volcanic greenstone belt rock assemblage with enclosed low-angle shear zones. Compression started during sedimentation under conditions of net subsidence within a rim syncline between adjacent domes. This occurred after phases of low-angle extensional and compressional deformation of the substrate not related to the present-day geometry of the granitoid domes. The geological setting of the Mosquito Creek Basin does not differ fundamentally from that of other adjacent E–Woriented greenstone belts of the EPT. The basin is inferred to represent the last, stratigraphically uppermost, and because of subsequent erosion rarely preserved, stage of greenstone belt evolution.