Terrane accretion in the southwestern Yilgarn Craton: Evidence from a deep seismic crustal profile

Interpretation of a seismic, gravity and magnetic traverse across the western margin of the Yilgarn Craton, undertaken approximately 110 km north of Perth, together with new Rb-Sr and U-Pb geochronological data from the southwestern portion of the craton, have led to a re-evaluation of the geological relationships and to the development of a terrane accretion model for the region. The geophysical data established the three-layered nature of the crust, with E-dipping reflectors present throughout its thickness of 32-40 km. Reflectors in the upper crust correlate with shear zones mapped at the surface and are interpreted as major thrusts that have transported crustal slices from the east. These 'sole-out' at a depth of about 10 km along a well-defined decollement and indicate 'thin-skinned' tectonism. The shear zones define the boundaries of three terranes which, from west to east are herein defined as the Balingup, Boddington and Lake Grace terranes. The Balingup Terrane is composed of trough facies metasediments, believed to have been deposited at similar to 2.8 Ga and subsequently metamorphosed under medium pressure amphibolite facies conditions. This terrane contains some of the youngest granitoids in the Yilgarn Craton (2612 and 2527 Ma) and these have been extensively sheared by late Archaean movement along the Darling Fault Zone. The boundary with the Boddington Terrane to the east is not precisely defined, partly due to lack of exposure. It is marked in the south by a narrow, 100 km long strip of tectonised quartz monzonite and further north by a change in the Rb-Sr biotite ages of granitoids; from 400-500 Ma in the west to 2300-2600 Ma in the east. The Boddington Terrane contains a sequence of Archaean shelf facies metasediments (part of the Jimperding Metamorphic Belt), believed to have been deposited at similar to 3.1 Ga, and metamorphosed up to amphibolite-granulite transition facies under low-pressure conditions. These sequences occur mainly in flat-lying nappes, which may have been tectonically transported from the east during assembly with the Lake Grace Terrane. The Boddington Terrane also contains two low-grade greenstone belts, formed between 2650 and 2670 Ma and intruded by 2640 Ma granitoids. The eastern boundary of the Boddington Terrane is marked by a major change in gravity, which also corresponds with a series of shear zones at the surface, a change in structural style and a major change in the nature of the granitoids. East of this boundary in the Lake Grace Terrane, granitoids of two ages have been recognised: charnockites formed at 2640 Ma and post-tectonic granodiorites formed at similar to 2580 Ma. Metasedimentary rocks, metamorphosed to granulite facies, occur as rafts and lenses with steep upright folds and are enclosed in leucocratic, garnetiferous granitic gneiss. Greenstones, also metamorphosed to granulite facies and recording ages of around 2790 Ma, are also enclosed within the charnockites and are thus substantially older than the low-grade greenstones in the Boddington Terrane to the west. Assembly of the Balingup and Boddington terranes appears to have been completed prior to the emplacement of the Logue Brook Granite at 2612 Ma. The Boddington and Lake Grace terranes came together after the 2640 Ma granulite: facies metamorphic event but the precise timing of assembly is nor currently constrained. It was certainly completed by 2411 Ma. the time of emplacement of a major set of E-W-trending mafic dykes (the Widgemooltha Dyke Suite) which extend for over 500 km across the Yilgam Craton. Although the Widgemooltha Dyke Suite appears to be absent from the Balingup Terrane, this is possibly related to extensive NW-shearing during the Proterozoic, which also led to extensional re-activation of the decollement that marks the base of the upper crust in the region.