Tectonic Mode Switches Recorded at the Northern Edge of the Australian Plate During the Pliocene and Pleistocene

We report new data from medium-high grade metamorphic rocks found at the northern margin of the Lengguru Fold Belt in West Papua. The study involved a systematic analysis of cross-cutting structures to establish the relative timing of deformation, together with isotopic dating to define when these tectono-thermal events occurred. These data show that the region underwent multiple episodes of deformation within the last six million years. Metamorphic mineral growth was associated with the development of ductile shear zones. This episode occurred during a phase of crustal stretching associated with the formation of a metamorphic core complex. Metamorphic zircon growth at 4.9 to 5.3Ma was documented in two of the dated samples. These data are interpreted to postdate the peak pressure and temperature conditions of the phase of regional crustal stretching. The shear fabrics associated with the metamorphic core complex were later overprinted by at least two generations of folds. The change in mode from crustal extension to shortening reflects a tectonic mode switch. A subsequent mode switch is documented by numerous brittle extensional faults that cross-cut the earlier formed ductile fabrics. We interpret ca. 0.75-1.51Ma (U-Th)/He age data to reflect cooling associated with the later stages of crustal shortening (marked by folds) or the later extensional unroofing of the peninsula. This work demonstrates that an orogen can record multiple tectonic mode switches within several million years. These outcomes should be considered in studies of ancient orogens where analytical uncertainties associated with isotopic dating may mask short-lived mode switches. Plain Language Summary How much time is required for a mountain belt to develop? Our knowledge of mountain building is somewhat limited because many of the best-studied mountain belts are quite old and our ability to date tectonic events is less precise the farther back we look through time. The work presented here investigates a mountainous peninsula that developed at the northern margin of the Australian Plate within the last six million years. This region records multiple episodes of deformationwe show that the relatively young rocks have been stretched, then pushed together and then stretched again (much like an accordion). These cycles must occur over a period of one to two million years, which is very fast compared to numerous studies of ancient mountain belts that consider the same processes occur over tens of millions of years.