Page 304 - Global Tectonics

OROGENIC BELTS 289

In the Andes, compressional regimes have been to bending and other mechanical adjustments within
established several times since the early Mesozoic, with the subducting oceanic lithosphere. Farther east thrust-
the most recent phase beginning about 25–30 Ma type solutions are most abundant with some strike-slip
(Allmendinger et al., 1997). The beginning of this latest motion (Gutscher et al., 2000; Siame et al., 2005). In
phase of compression has been interpreted to refl ect general, the axes of maximum compressional stress
two major processes: the trenchward acceleration of are aligned with the plate motion vector, suggesting
the South American plate (Pardo-Casas & Molnar, 1987; that plate boundary stresses are transmitted up to
DeMets et al., 1990; Somoza, 1998) and strong inter- several hundred kilometers into the South American
plate coupling between the subducting oceanic litho- plate.
sphere and the overriding continent (Jordan et al., 1983; The distribution of earthquake hypocenters with
Gutscher et al., 2000; Yáñez & Cembrano, 2004). One depth indicates that the margin is divided into fl at and
of the principal aims of tectonic studies in the Andes is steep subduction segments (Barazangi & Isacks, 1979;
to determine the origin of the highly variable response Jordan et al., 1983). Beneath southern Peru and Bolivia,
of the South American plate to this compression. This the Benioff zone dips about 30° (Fig. 10.3a,b). Beneath
section provides a discussion of the fi rst-order physical north-central Chile, it initially forms an angle of 30° to
characteristics of the central and southern Andes that a depth of ∼100 km and then dips at angles of 0–10° for
allow geoscientists to make inferences about the genesis several hundred kilometers (Fig. 10.3c). To allow sub-
of the mountain range. duction to take place at such different angles, either a
lithospheric tear or a highly distorted down-going plate
must accommodate the transitions between the fl at and
steep segments.
Above zones of flat subduction, shallow seismicity

10.2.2 Seismicity, is more abundant and broadly distributed than over
plate motions, and neighboring steep segments (Barazangi & Isacks, 1979;
Jordan et al., 1983). The seismic energy released in the
subduction geometry upper plate above flat slabs is, on average, three to fi ve

times greater than in steep (>30°) segments between
The general pattern of seismicity in the Andes is in 250 and 800 km from the trench (Gutscher et al., 2000).
accord with the eastward subduction of the Nazca plate These differences suggest that flat slab segments are

beneath South America (Molnar & Chen, 1982). Geo- strongly coupled to the overlying continental plate
detic data suggest that convergence velocities with (Section 9.6). The coupling appears to be controlled by
−1
respect to South America are 66–74 mm a at the trench the presence of a cool slab at shallow depths beneath
(Norabuena et al., 1998; Angermann et al., 1999; Sella the continental lithosphere, which strengthens the
−1
et al., 2002). These rates are slower than the 77–80 mm a upper plate and enables it to transmit stresses over long
predicted by the NUVEL-1A model of plate motions distances.

(Section 5.8) and appear to reflect a deceleration from In addition to influencing mechanical behavior,
−1
a peak of some 150 mm a at 25 Ma (Pardo-Casas & variations in the dip of the subducting plate affect pat-
Molnar, 1987; Somoza, 1998; Norabuena et al., 1999). terns of volcanism. In the central Andes, where the
Currently, relative motion results in a variable compo- slab dips steeply, Neogene volcanism is abundant (Plate
nent of trench-parallel displacement along the margin. 10.1a between pp. 244 and 245). By contrast, above the

In the central Andes, this component is minor and flat slab segments of north-central Peru and Chile (30°S

appears to be accommodated mostly within the sub- latitude), significant Neogene volcanism is absent.
ducted slab itself (Siame et al., 2005). In the southern These volcanic gaps and flat slab segments align with

Andes, a moderate component of trench-parallel the location of partially subducted aseismic ridges.
motion is accommodated by slip along major strike-slip Gutscher et al. (2000) used relocated earthquake hypo-
faults (Cembrano et al., 2000, 2002). centers (Engdahl et al., 1998) below 70 km depth to
Focal mechanism solutions from shallow (≤70 km generate a three-dimensional tomographic image of
depth) earthquakes show that the South American plate the subducted Nazca plate beneath the central and
is currently in compression (Fig. 10.2). Near the Peru– northern Andes (Plate 10.1b between pp. 244 and 245).
Chile Trench, some normal faulting occurs in response The image shows two morphological highs that cor-