Chapter ................................ 17 Extensional regimes Traditionally, extensional structures have received less attention than their contractional counterparts. However, the tide turned in the 1980s when it was realized that many faults and shear zones traditionally thought to represent thrusts carried evidence of being low-angle extensional structures. First recognized in the Basin and Range province in the western USA, it is now clear that extensional faults and shear zones are widespread in most orogenic belts. Most would agree that the study of extensional structures has significantly changed our understanding of orogens and orogenic cycles. The current interest in extensional faults is also related to the fact that many of the world’s offshore hydrocarbon resources are located in rift settings, and many hydrocarbon traps are controlled by normal faults. Also, the development of most hydrocarbon reservoirs requires a sound understanding of extensional faults and their properties and complexities. 334 Extensional regimes 17.1 Extensional faults the two points. So, we have to evaluate extension perpen- dicular to the strike of a fault to evaluate if it is a true Extensional faults cause extension of the crust or of some extension fault. This is the principal extension direction reference layering in deformed rocks. An extensional fault across an extensional dip-slip fault (Figure 17.1b), just as is illustrated in Figure 17.1b, affecting horizontal layers. it is the principal shortening direction for contractional That fault shows a displacement that is close to the thick- dip-slip faults. A perfect strike-slip fault shows no change ness of the layering, so that sense and amount of displace- in length perpendicular to the fault. ment can be easily identified. Other extensional faults For smaller faults, the term extensional fault can be have accumulated up to a hundred kilometers of offset, used for faults that extend a given reference layer, regard- not quite as much as the many hundreds of kilometers less of the orientation of the layering. In this sense, estimated on some thrust faults and strike-slip faults, but reverse faults can be extensional faults as long as the still quite considerable. At this scale the crust itself is our reference layer is extended by the faults. An example of natural choice of reference. If the distance between two an extensional reverse fault is shown in Figure 17.2, points on the surface of the Earth, one on each side of the together with an interpretation of how it formed by fault, is increased during deformation, then there is rotation. It is therefore useful to specify a reference extension in that direction. But this can also occur across surface, for example by using the terms crustal extension a strike-slip fault, depending on the relative positions of and layer-parallel extension. (a) (a) (b) A A BB CC (b) A (c) (d) A (c) B A B C Figure 17.2 (a) Reverse faults and normal faults coexisting in a Figure 17.1 Using the surface of the Earth as a reference, folded layer along a fault. Both the reverse and normal faults are extensional faults (b) are a spectrum of normal faults between extensional faults because they extend the layers. The reverse vertical (a) and horizontal (c). Vertical and horizontal faults faults probably formed as normal faults (b) before being rotated are neither extensional nor contractional. during the folding (c, d). San Rafael Desert, Utah. 17.2 Fault systems 335 A reverse fault can also be an extensional fault if a 17.2 Fault systems tectonic or sedimentary layering is used for reference. The domino model Fault dip is also important. As indicated in Figure 17.1, Sections through a rifted portion of the upper crust extensional faults cover a spectrum of fault dips between typically show a series of rotated fault blocks arranged vertical (Figure 17.1a) and horizontal (Figure 17.1c). more or less like domino bricks or overturned books in a Vertical faults imply neither extension nor shortening of partly filled bookshelf (Figure 17.3a). This analogy has the crust, only vertical motions (Figure 17.1a). We can given rise to the name bookshelf tectonics or the (rigid) think of vertical block faulting as a large-scale analog domino model: to playing the piano keyboard – keys move vertically but the keyboard retains its length. Such vertical tecton- The rigid domino model describes a series of rigid fault ics dominated much of the Colorado Plateau in the blocks that rotate simultaneously in a uniform sense. western USA during the Cretaceous Laramide phase. In general, faults that are oriented perpendicular to a (a) layer do not stretch or shorten the layer. Horizontal 30° (or layer-parallel) faults represent the other end-member: they neither shorten nor extend horizontal (or fault- parallel) layers (Figure 17.1c). Layer-parallel faults occur (b) as flat portions (flats) of curved extensional as well as 60° contractional faults. Extensional faults are commonly thought to initiate with dips around 60, based on the Coulomb fracture Figure 17.3 (a) Schematic illustration of rigid domino-style criterion and Anderson’s theory of faulting as discussed fault blocks. (b) Such fault blocks can be restored by rigid in Section 7.3 (Figure 7.13). Field mapping and seismic rotation until layering is horizontal. In this case we have interpretation show that both high- and low-angle applied 30 rotation and displacement removal. extensional faults are common. In fact, they coexist in many extensional settings. How can we explain this finding? BOX 17.1 THE RIGID DOMINO MODEL The easiest explanation is that most or all rocks have an anisotropy inherited from earlier phases of deform- No block-internal strain. ation. Thus, a simple explanation for very steep faults is Faults and layers rotate simultaneously and at that they represent reactivated joints or strike-slip faults. the same rate. Recall that joints are close to vertical because they form perpendicular to s3, which tends to be horizontal in the Faults end up with the same offset, which is upper crust. constant along the faults. Arguing along the same lines, low-angle normal faults All faults have the same dip (parallel). can form by reactivation of thrust faults, and many low-angle extension faults have indeed been interpreted Faults have equal offset. as such. At the same time, experiments and field Layers and faults are planar. observations indicate that some high- and low-angle All blocks rotate at the same time and rate. extensional faults formed under a single phase of exten- sion, without the use of preexisting weak structures. In particular, some low-angle normal faults must have rotated from initial high-angle faults to low-angle struc- tures, while other low-angle faults are thought to have formed directly without much rotation. We will start exploring these and related observations by means of a simple model of fault rotation known as the domino model. 336 Extensional regimes BOX 17.2 THE GULLFAKS DOMINO SYSTEM The North Sea Gullfaks oil field consists of a domino system limited by an eastern horst complex. The domino system consists of four to six main blocks, each subdivided by smaller faults. Well data show that a large number of subseismic faults and fault-related structures exists, including a large population of deformation bands. Furthermore, the dipping layering shows a systematic westward decrease in dip within each block. Therefore, a perfect rigid domino model does not work. If the blocks are back-rotated rigidly so that the layering becomes horizontal on average, the faults only obtain a 45 dip. This is lower than the 60 fault dip expected from the mechanical considerations discussed in Chapter 6. The discrepancy can be explained by internal fault block strain. W Domino system Horst complex E Brent Grp. 2 km Dunlin Grp. Upper Jurassic 3 km Statfjord Fm. 4 km Triassic ? Triassic and older 1 km 5 km (a) Figure 17.4 Schematic illustration of the development of a domino system. (a) The transition to the undeformed (b) footwall is accommodated by a listric fault. (b) A new set of faults develops at high extension. (c) The resulting fault (c) pattern can become quite complex. See Nur (1986) for a more detailed description. The rigid domino model is easy to handle, and Figure shown in Figure 17.4. A listric fault can also be placed 17.3 shows how rigid rotation of blocks and faults on the other side, accompanied by an oppositely dipping can restore such systems. However, the characteristics set of domino fault blocks. A graben will then be needed and constraints of this model (see Box 17.1) impose in the middle to connect the two sets. geometric challenges when applied to geologically realis- The second compatibility problem exists between the tic situations such as the example portrayed in Box 17.2. base of the blocks and the substrate. This problem can As always when applying models we have to consider be solved by introducing a mobile medium at the base boundary conditions and compatibility with the sur- of the rotating fault blocks, such as clay, salt or intruding roundings. In particular, open voids and overlaps are magma. Where the blocks are large enough to reach the unacceptable. The first challenge is what is going on at brittle–plastic transition in the crust, the basal space each end of the domino system. This is quite elegantly problem can be eliminated by plastic flow of the subsur- resolved by introducing an appropriate listric fault, as face rocks. 17.2 Fault systems 337 We could also resolve the basal space problem by penetrative deformation of the basal parts of the domino blocks.