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Using paleomagnetism to test rolling hinge behaviour of an active continental low angle normal fault, Papua New Guinea

Abstract:

Metamorphic core complexes (MCCs) have been attributed to slip on long-lived detachment faults in extensional environments. At the surface, such faults are typically shallow dipping (<30°). This is inconsistent with Andersonian faulting theory, which requires that normal faults should nucleate and slip at steeper dips. One possible solution to this mechanical problem is a “rolling hinge” evolution for the fault during its slip history. In this model, faults initiate at steep dips (e.g. 60°) and back-tilt to shallower dips due to flexure and isostatic uplift in response to unloading of the footwall during slip. The Mai'iu fault is an active, rapidly slipping (∼1 cm/yr) low-angle (dip 20-22° at the surface) normal fault. Sampling of the footwall Goropu Metabasalt revealed a consistent normal-polarity component of magnetisation with moderate inclination that we interpret to have been acquired during uplift and cooling of the footwall rocks during the Brunhes chron. Comparison of the direction of the normal component with the expected average (geocentric axial dipole) direction at the site latitude indicates 23.9° ± 2.6° (1σ) of back-tilting of the footwall about the strike of the Mai'iu fault, consistent with a rolling hinge style of rotation. This indicates an original fault dip of 41.3-48.5°, as is consistent with independent estimates of original fault dip from fault-bedding cut-off angles and microseismicity at depth. This study is the first of its kind to demonstrate, using paleomagnetism, large-scale horizontal-axis rotations consistent with a rolling hinge evolution for a continental MCC.