Towards a tectonic framework for normal faults in Waitematā Group rocks, North Island, Aotearoa-New Zealand

Abstract

Analyses of active and past normal fault behaviour in extensional settings provide key insights into regional-scale tectonic processes driven by plate boundary forces. To better understand past tectonic environments in the Tāmaki Makaurau-Auckland region of northern Aotearoa-New Zealand, we examine normal faults with Miocene to Quaternary movements dissecting the Early Miocene Waitematā Group. Structural data were collected from coastal outcrops, including fault geometry and kinematics, and reveal a dominant NE-trending strike direction, indicating NW-SE extension. Faults dissecting coeval and younger Waitākere Group rocks west of the study site exhibit the same NE-striking fabric, and thus may represent the westward continuation of Waitematā Group extensional structures. Furthermore, a NE-trending normal fault fabric is present within the adjacent Taranaki Basin, suggesting that the observed extension in the Waitematā Group was widespread over northern Aotearoa, with some extension still occurring today in southern parts of the Taranaki Basin. We propose that many of the observed normal faults in the Waitematā Group represent the northern portion of the Taranaki Basin fault system, with the Taranaki faults likely representing the younger continuation of the extension recorded in the Waitematā Basin, resulting from the migration of back-arc extension due to roll-back of the Hikurangi subduction margin.