Page 321 - Global Tectonics

304 CHAPTER 10

10.3.4 Modes of shortening in lement surface as the wedge thickens or as material
moves over a thrust ramp (Section 9.7). It also may
foreland fold-thrust belts result where an advancing thrust sheet encounters a
buttress made of a strong material, such as the volcanic
A common characteristic of fold and thrust belts is the arc in an accretionary prism (Section 9.7) or the bound-
presence of one or more décollement (or detachment) ary between a rigid plate and a weaker plate (Section
surfaces that underlie shortened sequences of sedimen- 8.6.3). Buttresses also may result from a change in lithol-
tary and volcanic rock (Section 9.7). The geometry of ogy across an old normal or thrust fault, from a thicken-
these surfaces tends to conform to the shape of the ing sequence of sedimentary rock, or from any other
sedimentary and volcanic sections in which they form. mechanism.
In most foreland basins sedimentary sequences thin The lateral (across-strike) growth of thrust wedges
toward the foreland, resulting in décollements that dip (Section 9.7) and the involvement of deep levels in the
toward the hinterland (Figs 10.5b, 10.7). In thin-skinned deformation are controlled by the temperature and
thrust belts (Section 10.2.4, Fig. 10.5b), the lowermost, relative strengths of the shallow and deep crust. If the
or basal, décollement separates a laterally displaced upper crust is strong and the deep crust relatively hot
sedimentary cover from an underlying basement that is and weak, then shortening may localize into narrow
still in its original position. In thick-skinned styles (Fig. zones and thick-skinned styles of deformation result
10.5d), the décollement surface cuts down through and (Ellis et al., 2004; Babeyko & Sobolev, 2005). A weak
involves the crystalline basement. middle and lower crust promotes ductile fl ow and
The development of thin- or thick-skinned styles of inhibits the lateral growth of the thrust wedge. Deep
shortening commonly is controlled by the presence of crustal flow also tends to result in low critical tapers

inherited stratigraphic and structural heterogeneities in (Section 9.7) and a symmetric crustal structure that
the crust. In the central Andean foreland, for example includes both forward- and back-breaking thrusts.
(Section 10.2.3), variations in the thickness and distribu- During basin inversion, more normal faults tend to
tion of sedimentary sequences have been linked to dif- reactivate if the middle or lower crust is weak relative
ferent modes of Neogene shortening (Kley et al., 1999). to the upper crust (Nemcˇok et al., 2005; Panien et al.,
Thin-skinned styles preferentially occur in regions that 2005). By contrast, if the upper crust is weak and the
have accumulated >3 km of sediment, where the low deep crust is cool and strong, then shortening leads to
mechanical strength of the sequences localizes defor- a mechanical failure of upper crustal sequences and the
mation above crystalline basement (Allmendinger & orogen grows laterally by thin-skinned deformation. In
Gubbels, 1996). Thick-skinned styles tend to occur in scenarios involving a strong lower crust, thrust wedges
regions where Mesozoic extensional basins have tend to show high tapers, asymmetric styles (mostly
inverted (Sections 9.10, 10.3.3). As these latter basins forward-breaking thrusts), and rapid lateral growth.
experience the shift from extension to contraction, old A combination of these effects may explain why con-
normal faults involving basement rock reactivate tractional deformation led to the rapid lateral growth
(Turner & Williams, 2004; Saintot et al., 2003; Mora of a foreland fold and thrust belt in the Central Andes
et al., 2006). and not in the Southern Andes (Section 10.2.3). All-
In many fold and thrust belts, and especially in thick- mendinger & Gubbels (1996) recognized that deforma-
skinned varieties, shortening results in some faults that tion in these two regions involved two distinctive modes
dip in a direction opposite to that of the basal décolle- of shortening. In an older pure shear mode of shorten-
ment, creating a doubly vergent thrust wedge composed of ing, deformation of the upper and lower crust occurred
forward-breaking and back-breaking thrusts. These simultaneously in the same vertical column of rock.
doubly vergent wedges may occur at any scale, ranging North of 23°S, this type of deformation was focused
from relatively small basement massifs (Fig. 10.5d) to within the Altiplano. Later, during the Late Miocene the
the scale of an entire collisional orogen (Fig. 8.23b,d). deformation migrated eastward, forming a thin-skinned

Their bivergent geometry reflects a condition where the foreland fold and thrust belt in the sub-Andean ranges
material on the upper part of an advancing thrust sheet while the middle and lower crust of the Altiplano con-
or plate encounters resistance to continued forward tinued to deform. This latter mode of shortening,
motion (Erickson et al., 2001; Ellis et al., 2004). The where deformation in the upper crust and the deep
resistance may originate from friction along the décol- crust are separated laterally, is known as simple shear.