Geophysical structure of the Southern Alps Orogen, South Island, New Zealand

Abstract

The central part of the South Island of New Zealand is a product of the transpressive continental collision of the Pacific and Australian plates during the past 5 million years, prior to which the plate boundary was largely transcurrent for over 10 My.Subduction occurs at the north (west dipping) and south (east dipping) of South Island. The deformation is largely accommodated by the ramping up of the Pacific plate over the Australian plate and near-symmetric mantle shortening. The initial asymmetric crustal deformation may be the result of an initial difference in lithospheric strength or an inherited suture resulting from earlier plate motions.Delamination of the Pacific plate occurs resulting in the uplift and exposure of midcrustal rocks at the plate boundary fault (Alpine fault) to form a foreland mountain chain. In addition, an asymmetric crustal root (additional 8 -17 km) is formed, with an underlying mantle downwarp. The crustal root, which thickens southwards, comprises the delaminated lower crust and a thickened overlying middle crust. Lower crust is variable in thickness along the orogen, which may arise from convergence in and lower lithosphere extrusion along the orogen. Low velocity zones in the crust We first review the range of geophysical data available for the South Island.These data are then used to define the main features of the three-dimensional structure of the orogen and provide the context for the detailed structure and development of the plate boundary zone as defined by the Alpine Fault. The chapter closely complements the previous chapter on the geological data base for the South Island, and uses those and other data to constrain its inferences. The conclusions of the In the south, the mountains are also lower, reaching maximum elevations of about 2500 m, and broader, covering most of the southern South Island (Figure 3c). The morphology of the region is less two-dimensional and reflects the grain of the geology that swings around from a NNE to a ESE trend. The Alpine Fault is a more subdued morphological feature, but it is still remarkably linear until it runs offshore at MilfordSound.

Middle to Late Miocene Age for the End of Amphibolite‐Facies Mylonitization of the Alpine Schist, New Zealand: Implications for Onset of Transpression Across the Alpine Fault

“…This estimate is based on the few available P‐T estimates in exposed Alpine Schist (Grapes & Watanabe, , ; Grapes, ; Vry et al, ) and seismic data, both of which indicate that the hanging wall passes through depths of 35–40 km. This depth lies above the seismically imaged, ~45 km thick crustal root of the Southern Alps (Davey et al, ) (Figure c). Kinematic modeling of low‐temperature thermochronologic data predicts that a listric décollement might sole out at ~35 km depth (Herman et al, ) (Figure c).…”

Potentially active faults in the rapidly eroding landscape adjacent to the Alpine Fault, central Southern Alps, New Zealand

“…Major faults in the central Southern Alps have predominantly dip‐slip motion yet they strike NNE around 30° oblique to the plate boundary (BBF, HRF, MF and LF in Figure 11). Their strike is more closely oriented to the axis of anomalous gravity and seismic velocity, which is 10–20° oblique to the plate boundary, hinting that faulting may be in part controlled by, or responsible for, thickening of Pacific plate material and/or potentially mantle lithosphere deformation (Figure 1e) [ Davey et al , 2007; Ellis et al , 2006; Gerbault et al , 2002, 2003]. The fault motion matches shortening within a duplex structure in semischist rocks (DUP in Figures 6a, 7, and 11) [ Cox et al , 1997].…”

“…Maps of the South Island of New Zealand showing (a) digital terrain model of topography; (b) mean annual rainfall, modeled for the period 1971–2000 [ Tait et al , 2006]; (c) glacial geology and preserved Cretaceous‐Tertiary erosion surface [ Cox and Sutherland , 2007] with extent of ice at the last glacial maximum (white) [ Barrell , 2011] and active faults with known evidence for rupture in the past 120,000 years (GNS Science, Active faults database 2011, from http://data.gns.cri.nz/af/); (d) erosion, calculated as a mean ground lowering from a suspended sediment yield model [ Hicks et al , 1996], assuming an average crustal density of 2.65 t/m 3 ; (e) shallow (<40 km) seismicity for 1964–2011, overlain on Bouguer gravity and showing area of anomalously high seismic velocity (Vp = 6.5 km/s contour) [ Davey et al , 2007]; and (f) maximum rates of contemporary shear strain derived by GPS surveys between 1996 and 2008 (updated from Beavan et al [2007]) together with Pacific‐Australian plate vectors calculated at Fox Glacier [ Beavan et al , 2007; DeMets et al , 1994, 2010] and central Alpine Fault late Quaternary slip rates [ Norris and Cooper , 2001]. …”

Continent‐scale strike‐slip on a low‐angle fault beneath New Zealand's Southern Alps: Implications for crustal thickening in oblique collision zones

“…The crustal structure beneath the Southern Alps, particularly along the SIGHT 1 and 2 lines (see location in Figure a), is well resolved from seismic studies, including tomography and gravity (Figure a) [ Scherwath et al ., , Van Avendonk et al ., , Davey et al ., ; Stern et al ., ; Eberhart‐Philips et al ., ; Boese et al ., ].…”

“…Paleoseimicity studies show that the entire length of the Alpine Fault is capable of rupturing during M7.6 to 8+ events [ Sutherland et al ., ], but the locked zone, capable of earthquake rupture, has been interpreted to extend to a maximum depth of only ∼10 km, with aseismic free slip below this. This is because the heat flow is high near the fault due to the high‐erosion rate, reaching 190 mWm −2 near Fox Glacier, with a geothermal gradient >60°C/km [ Davey et al ., ; Beavan et al ., ; Toy et al ., ; Sutherland et al ., 2013], and there is a ∼10 km cutoff of microseismicity (Figure a) [ Boese et al ., ].…”