Page 258 - Global Tectonics

244 CHAPTER 8

of rocks that once resided at mid-crustal depths (Batt Southern Alps. Nevertheless, discrepancies also exist.
et al., 2004). For example, despite the thermal weakening and strain
To investigate how erosion, exhumation, and heat localization caused by exhumation and thermal advec-
advection cause these asymmetries and result in the tion, the retro-shear zone in Fig. 8.23b remains several
localization of strain on a dipping fault plane, research- kilometers thick and does not narrow toward the
ers have developed numerical experiments of plate con- surface. Batt & Braun (1999) speculated that this lack of

vergence and transpression (Koons, 1987; Beaumont fit between the model and observations in New Zealand

et al., 1996; Batt & Braun, 1999; Willet, 1999). In most reflects the absence of strain-induced weakening, high

of these experiments, crustal deformation is driven by fluid pressures, and other processes that affect strain
underthrusting the mantle lithosphere of one plate localization (e.g. Section 7.6.1). Nevertheless, the model
beneath an adjacent, stationary plate (Fig. 8.23b). As explains the prominence of the Alpine Fault as a dis-
mantle lithosphere subducts, the crust accommodates crete, dipping surface that accommodates large amounts
the convergence by deforming. A doubly vergent accre- of slip in the Southern Alps.
tionary wedge develops, whose geometry is determined To determine whether positive strain-softening feed-
by the internal strength of the crust and mantle, the backs allow the Alpine Fault to accommodate oblique-

coefficient of friction on the basal detachment (Dahlen slip along a single dipping fault, Koons et al. (2003)
& Barr, 1989), and patterns of erosion at the surface developed a three-dimensional numerical description of
(Willett, 1992; Naylor et al., 2005). transpression for two end-member cases. In both cases,
Figure 8.23c,d show the results of an experiment a three-layered Pacific plate is dragged along its base

applied to the Southern Alps. In this case, the moving toward an elastic block located on the left side of the
and stationary blocks represent the Pacific and Austra- model (Fig. 8.24a). The elastic block simulates the

lian plates, respectively. Initial conditions include a 30- behavior of the strong, relatively rigid Australian plate;
km-thick crust with a feldspar-dominated rheology and the crustal layers of the Pacific plate accommodate the

a fixed temperature of 500°C at its base (Batt & Braun, majority of the strain. A pressure-dependent Mohr–
1999; Batt et al., 2004). Over a period of 10 Ma, two Coulomb rheology simulates brittle behavior in a strong

ductile shear zones form and define a doubly vergent upper crust. Ductile deformation in a weak lower crust
wedge that becomes progressively more asymmetric is described using a thermally activated plastic rheology.
through time (Fig. 8.23c,d). A retro-shear zone develops As in most other models of this type, a zone of basal
into a major, crustal-scale thrust. A pro-shear zone also shear separates the lower crust from Pacifi c mantle

forms but does not accumulate significant strain. Surface lithosphere. Oblique plate convergence results in veloc-
−1
−1
erosion and crustal exhumation are concentrated ities of 40 mm a parallel to and 10 mm a normal to a
between the two shear zones, reaching maxima at the vertical plate boundary. Maintaining the western slope
retro-shear zone. The effects of these processes are illus- at a constant elevation simulates asymmetric erosion at
trated in Fig. 8.23c by the white arrows, which show the the surface.
exhumation trajectory of a selected particle. The dashed In the first experiment (Figs. 8.24a-f), the Pacifi c

envelope above the model represents the approximate plate exhibits a horizontally layered crust. As deforma-
volume of eroded material. As heat is advected upwards tion proceeds, two well-defined fault zones extend

in response to the exhumation the mechanical behavior down from the plate boundary through the upper
of the deforming region changes. The heat decreases crust, forming a doubly vergent wedge. This wedge
the strength of the retro-shear zone, which brings hot includes a vertical fault that accommodates lateral
material from the base of the crust to the surface, and (strike-slip) movement and an east-dipping convergent
weakens the fault. This preferential weakening of the (thrust) fault along which deep crustal rocks are
retro-shear zone relative to the pro-shear zone increases exhumed (Fig. 8.24f). In the second experiment (Figs.
the localization of strain on the former and enhances 8.24g–l), the Pacific plate exhibits a thermally per-

the asymmetry of the model (Fig. 8.23d). turbed crust in which advection of hot rock has weak-
The results of this experiment explain how erosion, ened the upper crust and elevated the 350°C isotherm
exhumation, and thermal weakening result in a concen- to within the upper 10 km of the crust. In this model,
tration of strain along a dipping thrust surface in the strain is concentrated within the thermally perturbed
upper crust during continental collision. The model region. Through the upper crust, the lateral and con-
predictions match many of the patterns observed in the vergent components of strain occur along the same