Basin inversion

Many sedimentary basins record a reversal in the sense of motion on dip-slip faults at different stages in their evolution. This reversal is known as inversion. At present there is no universal definition of the process. However, the most common type refers to the compressional reactivation of pre-existing normal faults in sedimentary basins and passive margins that originally formed by extension or transtension (Turner & Williams, 2004). Fault reactivation changes the architecture of the basin and commonly results in the uplift of previously subsided areas and the exhumation of formerly buried rocks. Evidence for this type of inversion occurs at a wide range of scales in many different settings, including in collisional and noncollisional orogens and in regions of strike-slip faulting. At convergent margins the tectonic inversion of extensional backarc and intra-arc basins is an especially important process that accommodates crustal shortening, localizes contractional deformation, and results in an along-strike segmentation of the margin.

In many basins, a common criterion for recognizing fault-controlled inversion is the identification of the null point in vertical profiles or the null line in three dimensions. 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 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 divides the area displaying reverse displacement from that displaying normal displacement. As the magnitude of the inversion increases, the null point will migrate along the fault. The uplift and folding of synrift and postrift sediments also indicate that inversion has occurred by the compressional reactivation of a normal fault.

Basin inversion is caused by a variety of mechanisms. Continent-continent or arc-continent collision can result in compression, uplift, and fault reactivation. Changes in the rate and dip of subduction (Section 10.2.2, Fig. 9.18) also may cause basin inversion at ocean-continent convergent margins. In regions of strike-slip faulting, rapid reversals in the sense of motion on faults commonly occur between releasing bends and restraining bends (Section 8.2, Fig. 8.9). Isostatic, flex-ural, and thermal mechanisms also have been proposed to explain the uplift associated with basin inversion. However, many authors view these latter mechanisms as subordinate to external horizontal stresses that drive the compressional reactivation of faults.

Basin Inversion
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.

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