Page 316 - Global Tectonics

OROGENIC BELTS 299

Cembrano (2004) inferred the strength of the explain differences in the degree of
interface by finding the smallest displacement subduction erosion (Section 9.6) along the

field for different ranges of slip parameters. margin, although alternative models (e.g.

Figure 10.8c summarizes the results of the von Huene et al., 2004) have been proposed.
modeling. In the plots, the viscosity of the In addition to the rate and age of subducting
slip interface controls its strength. The slab lithosphere, another factor that may control
dip, convergence rate, and the age of the the strength of inter-plate coupling along the
subducting plate also are shown for Peru–Chile Trench is the amount of surface
comparison. The results indicate that the erosion and deposition. Lamb & Davis (2003)
strongest inter-plate coupling occurs in the postulated that the cold water current that
central Andes near latitude 21°S (Fig. 10.8a) fl ows along the coast of Chile and Peru
where inner trench slopes are steepest and inhibits water evaporation, resulting in little
the age of subducted crust is oldest. Weak rainfall, small amounts of erosion, and
coupling occurs in the southern Andes south minimal sediment transport into the trench.
of 35°S where the age of ocean crust is A dry, sediment-starved trench may result in

significantly younger and trench slopes are a high degree of friction along the Nazca–
gentle. For a constant convergent rate, the South American plate interface, increasing
subduction of young oceanic crust and shear stress, and leading to increased
aseismic ridges results in weak coupling compression and uplift in the central Andes.
because the higher temperature of the By contrast, in the southern Andes where the
oceanic lithosphere in these zones results in a fl ow of westerly winds, abundant rainfall,
thermal resetting of the ocean–continent and the effects of glaciation result in high
interface. erosion rates, the Peru–Chile Trench is fi lled
In the backarc-foreland domain, deformation is with sediment. The presence of large
controlled by the absolute velocity of the quantities of weak sediment in this region
continental plate, its rheology, and the may reduce friction along the plate interface,
strength of inter-plate coupling at the trench effectively reducing the amount of shear
(Yáñez & Cembrano, 2004). Strong coupling stress and resulting in less topographic uplift
results in large amounts of compression in and less intra-plate deformation.
the backarc, which increases crustal
shortening and thickening. Very weak 2 The structure and rheology of the continental plate.
coupling prevents backarc shortening. The Variations in the initial structure and rheology
rheology of the continental plate is governed of the continental plate also can explain several

by the strength of the mantle lithosphere and first-order differences in the evolution of the
the temperature at the Moho. By varying the central and southern Andes. Among these
strength of coupling at the slip zone and differences are the underthrusting of the
incorporating a temperature- and strain rate- Brazilian Shield beneath the Altiplano-Puna and
sensitive power-law rheology (Section 2.10.3), major lithospheric thinning in the central
these authors reproduced several major Andes, and the absence of these features in the
features of the central and southern Andes. southern Andes.
These include variations in the average Sobolev & Babeyko (2005) conducted a series
topographic relief of the Andes, the observed of two-dimensional thermomechanical
shortening rate and crustal thickness in the models (Fig. 10.9) that simulated deformation
Altiplano region, and block rotations (Section in the central and southern Andes using two
10.2.3). The rotations are induced by different initial structures. The central Andes
differences in buoyancy forces caused by involve a thick felsic upper crust, a thin
crustal thickness variations and in the gabbroic lower crust, and a total thickness of

strength of inter-plate coupling north and 40–45 km. This configuration presumes that
south of the Altiplano. Variations in the the crust already had been shortened prior to
strength of inter-plate coupling also may the start of deformation at 30–35 Ma