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OROGENIC BELTS 297

presence of thick, strong, cold lithosphere of the Brazil- 1 Interplate coupling at the trench. Yáñez &
ian Shield (Polet et al., 2000). Bouguer gravity anomalies Cembrano (2004) used a continuum mechanics

show that lithospheric flexure (Sections 2.11.4, 10.3.2) approach to examine the effects of variable
supports part of the Eastern Cordillera and the sub- amounts of inter-plate coupling at the trench
Andean zone (Watts et al., 1995). Relationships between on upper plate deformation. These authors
surface elevations and crustal thickness (Yuan et al., noted that patterns of seismicity in the Andes
2002) indicate a lithospheric thickness of 130–150 km suggest that fl at subduction controls some areas
beneath the sub-Andean belt and much thicker mantle of strong inter-plate coupling (Section 10.2.2).
lithosphere farther east (Fig. 10.7). However, the largest seismic energy release
above fl at segments occurs up to several
hundred kilometers inland from the trench
10.2.5 Mechanisms of (Gutscher et al., 2000). By contrast, seismicity
at the Peru–Chile Trench is approximately
noncollisional orogenesis equivalent in both fl at and steep slab segments.
This observation, and the lack of correlation
Orogenesis at ocean–continent convergent margins ini- between the amount of intra-plate shortening
tiates where two conditions are met (Dewey & Bird, and the fl at slab segments, suggests that the
1970): (i) the upper continental plate is thrown into degree of inter-plate coupling at the trench may
compression and (ii) the converging plates are suffi - be equally or more important in controlling
ciently coupled to allow compressional stresses to be deformation of the upper plate.
transmitted into the interior of the upper plate. To test this idea, Yáñez & Cembrano (2004)
Studies of subduction zones in general suggest that divided the South American plate into two
the stress regime in the overriding plate is infl uenced tectonic domains that are characterized by
by the rate and age of subducting oceanic lithosphere different force balances: the forearc and the
(Uyeda & Kanamori, 1979; Jarrard, 1986). High con- backarc-foreland. In the forearc, the age of
vergence rates and the underthrusting of young, thick, the ocean crust and the convergent velocity
and/or buoyant lithosphere tend to induce compres- control the strength of coupling across the
sion, decrease slab dips, and enhance the transfer of ocean–continent interface. The strength of
compressional stresses (Section 10.2.2). However, the coupling regulates the amount of
although these factors may explain general differences deformation. The authors derived an
between Chilean-type and Mariana-type subduction empirical relationship between trench
zones (Section 9.6), they do not explain the along- topography and the degree of coupling
strike differences in the structure and evolution of the across the slipping interface using along-
Andean orogen (Sections 10.2.3, 10.2.4). The Andean strike variations in the shape of the inner
example shows that neither flat subduction nor the trench slope (Fig. 10.8a). This approach is

underthrusting of young and/or buoyant oceanic based on the work of Wdowinski (1992) who
lithosphere control areas of maximum shortening and suggested that after an equilibration period
crustal thickening (Yáñez & Cembrano, 2004). From of 5–10 Ma, trench topography refl ects the
the Altiplano region northward and southward, there balance between the tectonic and buoyancy
is a decrease in the total amount of crustal shorten- forces associated with subduction. Buoyancy
ing and thickening with no direct correspondence to forces associated with continental crust
either the slab age (Jordan et al., 1983; McQuarrie, dominate the force balance if the strength of
2002) or the convergence rate (Jordan et al., 2001). the plate interface is low, resulting in an
These observations indicate that factors other than upward movement of the forearc (Fig. 10.8b).
the geometry, rate, and age of subducting lithosphere Tectonic forces associated with the sinking of
control the response of the upper plate to compres- oceanic lithosphere dominate if the strength
sion. Among the most important of these other factors of the plate interface is high, causing
are: (i) the strength of inter-plate coupling at the trench downward movement of the forearc. By
and (ii) the internal structure and rheology of the assuming the trench topography is in
continental plate. equilibrium with these forces, Yáñez &