Page 320 - Global Tectonics

OROGENIC BELTS 303

thrust belts form as the crust is shortened in a regime of In many basins, a common criterion for recognizing

compression (Fig. 10.5). During shortening, small sedi- fault-controlled inversion is the identification of the null

mentary basins called piggyback basins may form on the point in vertical profiles or the null line in three dimen-
top of moving thrust sheets. sions. Figure 10.11 shows a cross-section illustrating the
geometry of an inverted half graben in Indonesia (Turner

& Williams, 2004). The profile shows a reactivated fault
10.3.3 Basin inversion along which the net displacement changes from normal
at its base to reverse near its top. The null point occurs
where the net displacement along the fault is zero and
Many sedimentary basins record a reversal in the sense divides the area displaying reverse displacement from
of motion on dip-slip faults at different stages in their that displaying normal displacement. As the magnitude
evolution. This reversal is known as inversion. At present of the inversion increases, the null point will migrate
there is no universal definition of the process. However, along the fault. The uplift and folding of synrift and

the most common type refers to the compressional postrift sediments also indicate that inversion has occurred
reactivation of pre-existing normal faults in sedimen- by the compressional reactivation of a normal fault.
tary basins and passive margins that originally formed Basin inversion is caused by a variety of mechanisms.
by extension or transtension (Turner & Williams, 2004). Continent–continent or arc–continent collision can
Fault reactivation changes the architecture of the basin result in compression, uplift, and fault reactivation.
and commonly results in the uplift of previously sub- Changes in the rate and dip of subduction (Section
sided areas and the exhumation of formerly buried 10.2.2, Fig. 9.18) also may cause basin inversion at
rocks. Evidence for this type of inversion occurs at a ocean–continent convergent margins. In regions of
wide range of scales in many different settings, includ- strike-slip faulting, rapid reversals in the sense of motion
ing in collisional and noncollisional orogens and in on faults commonly occur between releasing bends and
regions of strike-slip faulting. At convergent margins restraining bends (Section 8.2, Fig. 8.9). Isostatic, fl ex-
the tectonic inversion of extensional backarc and intra- ural, and thermal mechanisms also have been proposed
arc basins is an especially important process that accom- to explain the uplift associated with basin inversion.
modates crustal shortening, localizes contractional However, many authors view these latter mechanisms
deformation, and results in an along-strike segmenta- as subordinate to external horizontal stresses that drive
tion of the margin. the compressional reactivation of faults.

Water
Post-inversion sequences

Syn-inversion
sequences Postrift
Sequences
0
Synrift
TWT (secs.) 1

0 2
Prerift
2 Sequences km

Figure 10.11 Cross-section derived from a seismic reflection profile showing an inverted half graben from the East

Java Sea Basin, Indonesia (redrawn from Turner & Williams, 2004, with permission from Elsevier). TWT is two-way-travel

time of seismic reflections. White dot indicates null point.