Paleogeothermal gradient and deformation style, Pacific front of the Southern Alps Orogen: Constraints from fission track thermochronology

Abstract

The application of apatite fission track (FT) thermochronology to a suite of samples from different elevations within the exhumed apatite partial annealing zone, in Rakaia valley region of the Southern Alps (New Zealand), establishes a pre-cooling/denudation (latest Miocene-early Pliocene), upper crustal, geothermal gradient value of 20 ± 3°C/km within indurated Triassic sandstone. This is comparable to the equivalent modern value of 23 ± 6°C/km in basement outside the oblique continent-continent collision zone; that is, where isotherms have not been perturbed by late Cenozoic advection resulting from crustal thickening, rock uplift and denudation. The normalisation of the maximum paleotemperature estimates for sample sites at various elevations to those expected at a standard elevation (1000 m), shows that the exhumed late Cenozoic partial annealing zone is disrupted by reverse faults striking parallel (throws of 1000–2400 m) and orthogonal (throw of 500–1000 m) to the orogen, and by an open symmetrical dome (dimensions: 20 km long and 13 km wide) that conforms to the shape of the Mt Hutt Range. An effect of this deformation is repetition of the exhumed partial annealing zone across the eastern margin of the Southern Alps. Interpretation of existing geological maps suggests that the basement folding quantified for the Mt Hutt Range from FT data, is widespread in Mid and South Canterbury. A triangular zone along the eastern range front, which increases in width along the orogen to the southwest, comprises numerous antiformal reverse faulted basement ranges flanked by outliers of mid-Cenozoic marine cover rocks that were formerly continuous across the region. This basement folding and reverse faulting represents propagation of the orogen back across the incoming Pacific plate, as predicted by recent numerical and geodynamical models.