Page 317 - Global Tectonics

300 CHAPTER 10

Dynamic subduction channel
Gabbro Felsic upper crust Paleozoic sediment
(a) 0 Myr 0 V 1
Depleted peridotite
100 Depleted peridotite Brazilian shield
Depth (km) 200 Fertile
peridotite
300
400
Trench roll-back 0 200 400 600 V 2 800 1000 1200
(b) 17 Myr 0

100
Depth (km) 200 Delaminating
lithosphere
300
400
0 200 400 600 800 1000 1200
(c) 35 Myr 0 South American drift
100 Thin-skinned
Depth (km) 200 Underthrusting
deformation
shield
300
400 Delaminating
0 200 400 600 800 1000 1200 lithosphere
Distance (km)

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., The numerical experiment that best replicated
2005). The southern Andes consist of an the structure of the central Andes is shown
upper and lower crust of equal thickness and in Fig. 10.9. In this model 58% of the
a total crustal thickness of 35–40 km. In all westward drift of South America over a
models, subduction initially occurs at a low- 35 Myr period is accommodated by roll-back
angle below a 100- to 130-km-thick (Section 9.10) of the Nazca plate at the
continental lithosphere (Fig. 10.9a) and is free trench, with the rest accommodated by intra-
to move as subduction proceeds. The upper plate shortening (37%) and subduction
plate is pushed to the left (V 1 ), simulating the erosion (5%). During shortening the crustal
western drift of South American plate thickness doubles while the lower crust and
(Somoza, 1998). The slab is pulled from mantle lithosphere become thinner by
below at velocities (V 2 ) that conform to delamination (Fig. 10.9b). The delamination
observations. A thin subduction channel is driven by the transformation from gabbro
simulates the plate interface where a to eclogite in the lower crust, which
frictional (brittle) rheology controls increases its density and allows it to peel off
deformation at shallow depths and viscous and sink into the mantle. Another possible
fl ow occurs at deep levels. The depth of this mechanism for reducing lithospheric
change in rheology and the strength of the thickness is tectonic erosion driven by
slip zone are regulated using a frictional convective fl ow in the mantle (Babeyko et al.,
coeffi cient. 2002). These processes lead to an increase in