M

10 11 12 13 14 15

Antarctic Plate

Nazca Plate

Figure 10.8 (a) Trench topography for 15 profiles of the Andes between 3°N and 56°S showing dip of the Benioffzone at 1:1 scale (left). Bathymetric profiles at right show a vertical exaggeration of 10. (b) Viscous model of trench topography showing mesh grid and boundary conditions (B.C.). Dynamic equilibrium of the trench is controlled by competing buoyancy (top-right) and tectonic (lower-right) forces. Velocity fields represented with arrows are for reference only. (c) Model results showing slab dip under asthenospheric wedge, near-trench slab dip angle, age of subducting slab at the trench, convergence velocity, slip layer viscosity for a layer of 10 km thickness (all images modified from Yáñez & Cembrano, 2004, by permission of the American Geophysical Union. Copyright © 2004 American Geophysical Union). T.J., triple junction; J.F.R., Juan Fernández Ridge.

Crust

Mantle

Taitao T.J

Cembrano (2004) inferred the strength of the interface by finding the smallest displacement field for different ranges of slip parameters.

Figure 10.8c summarizes the results of the modeling. In the plots, the viscosity of the slip interface controls its strength. The slab dip, convergence rate, and the age of the subducting plate also are shown for comparison. The results indicate that the strongest inter-plate coupling occurs in the central Andes near latitude 21°S (Fig. 10.8a) where inner trench slopes are steepest and the age of subducted crust is oldest. Weak coupling occurs in the southern Andes south of 35°S where the age of ocean crust is significantly younger and trench slopes are gentle. For a constant convergent rate, the subduction of young oceanic crust and aseismic ridges results in weak coupling because the higher temperature of the oceanic lithosphere in these zones results in a thermal resetting of the ocean-continent interface.

In the backarc-foreland domain, deformation is controlled by the absolute velocity of the continental plate, its rheology, and the strength of inter-plate coupling at the trench (Yanez & Cembrano, 2004). Strong coupling results in large amounts of compression in the backarc, which increases crustal shortening and thickening. Very weak coupling prevents backarc shortening. The rheology of the continental plate is governed by the strength of the mantle lithosphere and the temperature at the Moho. By varying the strength of coupling at the slip zone and incorporating a temperature- and strain ratesensitive power-law rheology (Section 2.10.3), these authors reproduced several major features of the central and southern Andes. These include variations in the average topographic relief of the Andes, the observed shortening rate and crustal thickness in the Altiplano region, and block rotations (Section 10.2.3). The rotations are induced by differences in buoyancy forces caused by crustal thickness variations and in the strength of inter-plate coupling north and south of the Altiplano. Variations in the strength of inter-plate coupling also may explain differences in the degree of subduction erosion (Section 9.6) along the margin, although alternative models (e.g. von Huene et al., 2004) have been proposed. In addition to the rate and age of subducting lithosphere, another factor that may control the strength of inter-plate coupling along the Peru-Chile Trench is the amount of surface erosion and deposition. Lamb & Davis (2003) postulated that the cold water current that flows along the coast of Chile and Peru inhibits water evaporation, resulting in little rainfall, small amounts of erosion, and minimal sediment transport into the trench. A dry, sediment-starved trench may result in a high degree of friction along the Nazca-South American plate interface, increasing shear stress, and leading to increased compression and uplift in the central Andes. By contrast, in the southern Andes where the flow of westerly winds, abundant rainfall, and the effects of glaciation result in high erosion rates, the Peru-Chile Trench is filled with sediment. The presence of large quantities of weak sediment in this region may reduce friction along the plate interface, effectively reducing the amount of shear stress and resulting in less topographic uplift and less intra-plate deformation.

2 The structure and rheology of the continental plate. Variations in the initial structure and rheology of the continental plate also can explain several first-order differences in the evolution of the central and southern Andes. Among these differences are the underthrusting of the Brazilian Shield beneath the Altiplano-Puna and major lithospheric thinning in the central Andes, and the absence of these features in the southern Andes.

Sobolev & Babeyko (2005) conducted a series of two-dimensional thermomechanical models (Fig. 10.9) that simulated deformation in the central and southern Andes using two different initial structures. The central Andes involve a thick felsic upper crust, a thin gabbroic lower crust, and a total thickness of 40-45 km. This configuration presumes that the crust already had been shortened prior to the start of deformation at 30-35 Ma

Dynamic subduction channel Gabbro / Felsic upper crust Paleozoic sediment

Trench roll-back (b) 17 Myr 0

-100 200 300 400

-100 200 300 400

Brazilian shield

Delaminating lithosphere

400 600 800 1000 1200

Brazilian shield

Delaminating lithosphere

200 400 600 800 Distance (km)

1000 1200

Figure 10.9 (a-c) Time snapshots showing the evolution of shortening for a mechanical model of the Central Andes (modified from Sobolev & Babeyko, 2005, with permission from the Geological Society of America).

400 600 800 1000 1200

South American drift

Thin-skinned deformation

Underthrusting shield

200 400 600 800 Distance (km)

1000 1200

Delaminating lithosphere

Figure 10.9 (a-c) Time snapshots showing the evolution of shortening for a mechanical model of the Central Andes (modified from Sobolev & Babeyko, 2005, with permission from the Geological Society of America).

(Allmendinger et al., 1997; McQuarrie et al., 2005). The southern Andes consist of an upper and lower crust of equal thickness and a total crustal thickness of 35-40 km. In all models, subduction initially occurs at a low-angle below a 100- to 130-km-thick continental lithosphere (Fig. 10.9a) and is free to move as subduction proceeds. The upper plate is pushed to the left (Vj), simulating the western drift of South American plate (Somoza, 1998). The slab is pulled from below at velocities (V2) that conform to observations. A thin subduction channel simulates the plate interface where a frictional (brittle) rheology controls deformation at shallow depths and viscous flow occurs at deep levels. The depth of this change in rheology and the strength of the slip zone are regulated using a frictional coefficient.

The numerical experiment that best replicated the structure of the central Andes is shown in Fig. 10.9. In this model 58% of the westward drift of South America over a 35 Myr period is accommodated by roll-back (Section 9.10) of the Nazca plate at the trench, with the rest accommodated by intraplate shortening (37%) and subduction erosion (5%). During shortening the crustal thickness doubles while the lower crust and mantle lithosphere become thinner by delamination (Fig. 10.9b). The delamination is driven by the transformation from gabbro to eclogite in the lower crust, which increases its density and allows it to peel off and sink into the mantle. Another possible mechanism for reducing lithospheric thickness is tectonic erosion driven by convective flow in the mantle (Babeyko et al., 2002). These processes lead to an increase in temperature at the Moho, which weakens the crust and allows its lower part to flow.

After 20-25 Myr in model time, tectonic shortening generates high topography between the magmatic arc and the Brazilian Shield (Fig. 10.10a). The large topographic gradients initiate flow in a weak middle and lower crust that evens out crustal thickness and the surface topography, forming a 4-km-high orogenic plateau after 30-35 Myr. The model also predicts mechanical failure of the wedge of Paleozoic sediments by thin-skinned thrust faulting in the foreland (Section 10.3.4) at 25 Myr model time, followed by underthrusting of the Brazilian Shield under the plateau (Fig. 10.9c). The failure of the foreland sediments marks a change in the mode of shortening from pure shear to simple shear deformation (e.g. Section 10.3.4, Fig. 10.12). Shortening reaches 300-350 km by 30-35 Myr, as indicated by the curve of filled circles in Fig. 10.10b.

These and other models allow the dominant process controlling tectonic shortening in the Andes to be the accelerating westward drift of the South American plate. However, to explain the major tectonic features of the central Andes high convergent velocities and strong inter-plate coupling must be accompanied by an initially thick, weak continental crust (Sobolev & Babeyko, 2005; McQuarrie et al., 2005). Geologic evidence

25 Myr

15 Myr__

\

5 Myr

y

J \

35 Myr

1000

500 Distance (km)

1000

Central Andes

• (0.015, 20-30 mm a-1) • o 0 □ (0 05, 20 mm a-1)

§0 Southern Andes

/ \ \ Fast convergence Delamination Subandean thrusting

Time (Myr)

Figure 10.10 (a) Evolution of surface topography and (b) calculated shortening versus time for models using configuration shown in Fig. 10.9 (modified from Sobolev & Babeyko, 2005, with permission from the Geological Society of America). Note formation of high topography and then plateau during last 10 Ma in (a). Numbers near model results in (b) indicate subduction channel friction coefficient (first number) and western drift velocity of the South American plate (second group of numbers).

suggests that these conditions probably were only achieved in the central Andes, possibly as a consequence of high convergent rates, flat subduction, and/or the underthrusting of thick, buoyant oceanic crust. In addition, the mechanical failure of thick piles of sedimentary rock, continental underthrusting, and lithospheric thinning internally weaken the continental plate and influence its behavior as orogenesis proceeds.

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