The Influence of Basement Terranes on Tectonic Deformation: Joint Earthquake Travel‐Time and Ambient Noise Tomography of the Southern South Island, New Zealand

Abstract

Determining how the crustal structure developed in previous tectonic regimes influences current deformation is important for understanding seismogenic behavior. Paleozoic‐Mesozoic convergent‐margin tectonics produced the southeastern South Island crust, which is currently involved in late Cenozoic transpressional collision. We examine crustal velocity structure through joint inversion of earthquake travel times and ambient noise derived group velocities to increase the resolution of Vp (P‐wave velocity) and Vp/Vs across the region. This method improves the details of the shallow velocity structure and highlights lithological properties, uplifted regions, and distributed fracture zones. The Otago schist shows thick lower crust while contrasting higher Vp below 15 km depth characterizes the Matai terrane and Median batholith, both backstops during accretion and deformation. The complex velocity structure, integrated with late Cenozoic faults, structural domains, seismicity patterns, and geodynamic modeling, shows how Cenozoic structures relate to deeper properties. Seismicity within the schist is distributed within transition regions rather than along specific faults, and these zones contain low Vp and low Qp (1/attenuation). The most prominent seismicity zone is along the transition from northern Otago ranges to the Southern Alps, where faulting and brittle deformation corresponds to the eastern edge of ductile crustal thickening above the bending of the strong oceanic lower crust. The thick weak crust rheology of Otago, relative to Canterbury, enables broad deformation, with numerous late Cenozoic faults that may not persist to great depth. Along the southern stronger plutonic terranes, there is a band of sparse deeper (15–35 km) seismicity, consistent with strong elastic brittle crust.

Supplemental Material: Crustal block-controlled contrasts in deformation, uplift, and exhumation in the Santa Cruz Mountains, California, USA, imaged through apatite (U-Th)/He thermochronology and 3-D geological modeling

“…Apatite fission track and zircon (U-Th)/He ages collected by Benowitz et al (2022) from within the weak Alaska Range suture zone delineated by Fitzgerald et al (2014) demonstrate that exhumation rates in this weak parcel of crust are also very high. Along the Alpine fault in the Southern Alps in New Zealand, patterns of uplift and exhumation do not vary between fault-bounded crustal blocks, but rather appear correlated with differences in fault slip rates and fault geometries at depth driven by rheological differences in the lithospheric mantle (Little et al, 2005;Eberhart-Phillips et al, 2022).…”

Late Quaternary Slip Rates for the Hyde and Dunstan Faults, Southern New Zealand: Implications for Strain Migration in a Slowly Deforming Continental Plate Margin

“…This supports a hypothesis of fault interaction and stress migration within the system, at least between neighboring faults, on timescales of 10 kyr (Beanland & Berryman, 1989). A low‐angle detachment fault, or broader ductile shear zone, underlying the province has been proposed to link the reverse faults at depth (Beanland & Berryman, 1989; Eberhart‐Phillips et al., 2022; Upton et al., 2009), and is suggested to connect to the Alpine Fault to the west (Lamb et al., 2015; Warren‐Smith et al., 2016). None of the faults within the system has ruptured within written historical times (since 1840 CE), although microseismicity has been observed at depth on the down‐dip projections of the Dunstan, Pisa, and Nevis‐Cardona faults (Warren‐Smith et al., 2017), and at the north end of the Akatore Fault (Todd et al., 2020).…”