Page 319 - Global Tectonics

302 CHAPTER 10

suggests that these conditions probably were the internal zone of the orogen where the mountains
only achieved in the central Andes, possibly are highest and rocks the most intensely deformed.
as a consequence of high convergent rates, Foreland basins form where crustal thickening and
fl at subduction, and/or the underthrusting topographic uplift create a mass of crust that is large
of thick, buoyant oceanic crust. In enough to cause flexure (Section 2.11.4) of the conti-

addition, the mechanical failure of thick nental craton. This flexure creates a depression that

piles of sedimentary rock, continental extends much farther into the surrounding craton than
underthrusting, and lithospheric thinning the margin of the thickened crust. It is bounded on one
internally weaken the continental plate and side by the advancing thrust front and on the other by

infl uence its behavior as orogenesis proceeds. a small flexural uplift called a forebulge (e.g. Fig. 10.18).
The basin collects sedimentary material (molasse) that
pours off the uplifting mountains as they experience
erosion and as thrust sheets transport material onto the
10.3 COMPRESSIONAL craton. Its stratigraphy provides an important record of
the timing, paleogeography, and progressive evolution
SEDIMENTARY BASINS of orogenic events.
The shape of a foreland basin is controlled by the
strength and rheology of the lithosphere. A low fl exural
rigidity, which characterizes young, hot and weak litho-
10.3.1 Introduction sphere, results in a narrow, deep basin. A high fl exural
rigidity, which characterizes old, cool and strong litho-
Sedimentary basins that either form or evolve in sphere, produces a wide basin with a better-developed
response to regional compression are common in oro- forebulge (Flemings & Jordan, 1990; Jordan & Watts,
genic belts. Among the most recurrent types are foreland 2005). Variations in the strength and temperature of the
basins (Section 10.3.2), which form as a direct result of lithosphere can thus cause the character of the foreland
the crustal thickening and topographic uplift that basin to change along the strike of the orogen. Other
accompany orogenesis, and basins that initially form factors such as inherited stratigraphic and structural
during a period of extension or transtension and later inhomogeneities also influence basin geometry. In the

evolve during a period of subsequent compression. This Andes, an along-strike segmentation of the foreland
latter process, called basin inversion (Section 10.3.3), also partly coincides with variations in these properties and
occurs in association with strike-slip faulting (Fig. 8.10) with the segmented geometry of the subducted Nazca
and is the mechanism by which old passive margin plate (Section 10.2.3).
sequences deform during continental collision (Section As a result of lithospheric flexure, the sediment thick-

10.4.6). Any sedimentary basin in compression may ness in a foreland basin decreases away from the moun-
develop a fold and thrust belt (Section 10.3.4) whose tain front to a feather edge on the forebulge (Flemings
characteristics reflect the strength of the continental & Jordan, 1990; Gómez et al., 2005). Close to the moun-

lithosphere and the effects of pre-existing stratigraphic tain range the sediments are coarse grained and depos-
and structural heterogeneities. ited in a shallow water or continental environment; at
the feather edge they are fine grained and often turbid-

itic. The sediments thus form a characteristic wedge-

shaped sequence in profile whose stratigraphy refl ects
10.3.2 Foreland basins the subsidence history of the basin as it grows and
migrates outwards during convergence. The stratigra-
In addition to topographic uplift, orogenesis commonly phy is thus characterized by units that thin laterally, over-
results in a region of subsidence called a foreland basin step older members, or may be truncated by erosion.
or foredeep (Dickinson, 1974). The foreland lies at the Belts of deformed sedimentary rock in which the
external edge of the orogen toward the undeformed layers are folded and duplicated by thrust faults are
continental interior (e.g. Fig. 10.7). If a volcanic arc common in foreland basins. Like their counterparts in
is present, it coincides with the backarc region of the accretionary prisms (Section 9.7, Fig. 9.20) and in zones
margin. Its counterpart, the hinterland, corresponds to of transpression (Section 8.2, Fig. 8.8b), foreland fold and