Crust restoration for the western Lachlan Orogen using the strain-reversal, area-balancing technique: implications for crustal components and original thicknesses

Abstract

Strain reversal of structural/stratigraphic profiles at different scales in the western Lachlan Orogen provides a perspective on original crustal thickness estimates, the former depositional basin width of the proto-western Lachlan Orogen, the original sedimentary-fan thickness, and the possible length extent of lower crust lost by subduction. Retrodeformation using strain-reversal techniques allows basin reconstruction giving an original width of the western Lachlan Orogen basin receptor of between 800 km (minimum) and ∼1150 km (maximum), depending on the amount of stratal duplication allowed in the turbidites. Crude area balancing of the regional cross-section, adding in sectional volume lost by erosion and assuming strain compatibility between the upper and lower crust, suggests that the predeformation crustal thickness ranges between 15 km and ∼21 km, with a lower crustal thickness (oceanic lithosphere) of ∼9 km and a turbidite fan thickness of ∼6 km (minimum) and ∼12 km (maximum allowable), respectively. Disparity between the calculated fan thickness and that derived from measured stratigraphic sections adjusted for strain (∼6 km) indicates that some form of crustal stacking must be important in structural thickening of the turbidite crustal component. By varying shortening due to fault stacking, mass balance dictates the mismatch of the upper crustal (uc) and lower crustal (lc) retrodeformed lengths, and therefore provides an estimate of lower crustal loss by subduction. End members range from: (i) a 12 km-thick fan without fault duplication, a basin width of ∼800 km where uc = lc giving no lower crustal loss by subduction; to (ii) a ∼6 km fan, requiring duplication by faulting, a basin of ∼1150 km where uc > lc, and ∼360 km of lower crust length (∼30%) lost by subduction. This suggests that the total thickness of underplated igneous material in the western Lachlan Orogen is low, probably < ∼2 km.