The approach to steady-state thermochronological distribution following orogenic development in the Southern Alps of New Zealand

Abstract

ABSTRACT. A diachronous sequence of isotopic ages along the Southern Alps of New Zealand illustrates details of the development of the modern tectonic regime of this orogen at about 5 Ma. Coupled with the rapid cooling rates experienced in the Southern Alps, which ensure negligible residence time at temperatures allowing partial radiogenic product retention during exhumation (and thus effectively instantaneous thermochronological closure), this record presents important general insight into the transient physical and thermal effects of changes in tectonic conditions. The tectonothermal response of the Southern Alps to the change in dynamical conditions at 5 Ma is resolved into two evolutionary stages that are observed with progressive exhumation. The removal by erosion of material that had cooled below the relevant closure temperature prior to the change in dynamics at 5 Ma results in an initial decrease in age. This is followed by a sharp drop to younger ages as material subjected to thermal re-equilibration associated with the modern orogenic regime is exposed, culminating in the eventual exposure of time-invariant ages reflecting the new steady-state dynamics of the region. This two-step response is a direct result of the relationship between cooling and exhumation and illustrates the care needed in reconstructing physical histories from isotopic ages and cooling rates in tectonically active regions.

A frost “buzzsaw” mechanism for erosion of the eastern Southern Alps, New Zealand

“…Modern erosion in the western Southern Alps occurs via fluvial incision (Griffiths, 1979) and shallow and deep-seated landsliding (Hovius et al, 1997;Korup, 2005a), whereas, the eastern Southern Alps erode by a different suite of hillslope processes, including rockfall, debris flows, debris and snow avalanches, and fluvial incision (Pierson, 1980;Whitehouse and McSaveney, 1983;Whitehouse, 1988;Hales and Roering, 2005). The result is that high denudation rates (a maximum of 12 mm year − 1 , Hovius et al, 1997) are typical in the western Southern Alps, consistent with long-term exhumation rates estimated using thermochronology Kamp, 1993, 1995;Batt, 2001;Little et al, 2005). More modest denudation rates (b1 mm year − 1 , Griffiths, 1981;Hales and Roering, 2005) have been estimated in the areally extensive eastern Southern Alps.…”

Asymmetric growth of the Pyrenees revealed through measurement and modeling of orogenic fluxes

“…wedge (Kamp and Tippett, 1993;Batt, 2001;Willett and Brandon, 2002). The pattern of erosion over mountain belts, incorporating asymmetry in precipitation has been investigated using finite element models .…”

Persistent drainage migration in a numerical landscape evolution model

“…Three‐dimensional numerical modeling has become an important technique for understanding the coupled tectono‐geomorphic evolution of mountain belts. One of the most successful ideas to originate from numerical modeling is the concept of steady‐state mountain belts: rapidly‐uplifting mountains may increase in relief until the topography and exhumation become time‐independent [ Stuwe et al , 1994; Willett , 1999; Willett et al , 2001; Batt , 2001]. At the scale of an entire mountain belt, a balance between uplift and erosion in rapidly‐converging orogens is strongly supported by multiple lines of evidence [ Willett and Brandon , 2002].…”