Using geochemical fingerprinting to determine transpressive fault movement history: Application to the New Zealand Alpine Fault

Abstract

Geochemical fingerprinting of conglomerate clasts from depocenters adjacent to a strike‐slip fault, and of source areas on the opposite side of the fault, provides a method of determining fault movement history in instances where potential piercing points have been removed by erosion or buried in pull‐apart or down‐warp basins. Our example demonstrating this method is the Neogene Maruia Basin adjacent to the New Zealand Alpine Fault, the on‐land Australian‐Pacific plate boundary, which shows Permian Pelorus‐Caples terrane offset by 480 km. Twenty‐six sandstone and schist conglomerate clasts from the Maruia Basin Rappahannock Group were analyzed for major elements and 18 trace elements by X‐ray fluorescence and compared to similar analyses from potential regional source areas. Significant source terranes, the Caples Group and the Torlesse terrane were identified, both located southeast of the fault. Dextral movement on the fault provided a conveyor belt of material to the Maruia Basin, first supplying (at ∼11.5 Ma) Caples sandstone, then Caples schist followed by Torlesse schist (from ∼9.5 Ma), which translates to an average Neogene movement rate of 36–37 mm yr−1. The increase in metamorphic grade reflects accelerated exhumation of source rocks as they passed through a restraining bend adjacent to the Maruia Basin and from later regional uplift from a change in the Australian Pacific plate motion vector and more transpressive movement from ∼5 Ma. This robust method of geochemical fingerprinting could be readily applied to determining movement history of transpressive faults of other regions.