Regional Restoration.

Geoscientists use regional features to determine the approximate amount of slip along strike-slip faults. The Great Glen Fault in Scotland displaced a zoned granite batholith an apparent distance of 65 miles (105 km) (Kennedy 1946). In California, the San Andreas Fault offsets rocks to the north of the Salton Sea and those flanking the Soledad Basin by 250 km (Crowell 1962). The Garlock Fault apparently displaces a Mesozoic dike swarm and other geological features over a horizontal distance of 65 km (Fig. 12-7). The Philippine Fault system offsets Oligocene-Miocene intermediate and siliceous igneous rocks, ophiolite belts, and the intervening Central Luzon Valley-Llocos Basins up to 200 km to 300 km (Fig. 12-8). Gravity, isopach, and isochron maps can be subject to similar regional restoration.

Regional restorations of long-wavelength features such as volcanic chains and mountain belts, gravity and magnetic anomalies, unconformity intersections, and isopach thicknesses provide insight into the approximate amount of horizontal slip on strike-slip faults. These approximate restorations, in support of strike-slip motion, are more convincing when supported by the criteria discussed in the section on criteria for strike-slip faulting.

Local Restoration.

Commonly, the best available evidence in support of strike-slip faulting is the presence of the ubiquitous restraining bends and releasing bends (Crowell 1974a, 1974b). Restraining and releasing bends document horizontal displacements, and the restoration of these features, using piercing point and piercing line evidence, can determine the approximate slip along fault surfaces (Hill and Dibblee 1953).

The known strike-slip faults of the world are not perfectly straight faults, but contain small to large en echelon offsets, called stepovers, that form restraining and releasing bends (Aydin and Nur 1985) (Fig. 12-9). Table 12-1, taken from Aydin and Nur (1982), lists constraining and releasing bends from various areas of the world.

Alternatively, restraining and releasing bends form at bends in strike-slip fault surfaces (Crowell 1982). If slip along the linked strike-slip fault system moves material away from the stepover or bend in a fault surface, then the resulting extension forms releasing bends (Fig. 12-9a). If, however, slip along the stepover or bend moves material into the stepover or bend in the fault surface, then the resulting compression causes restraining bends (Fig. 12-9b) (Crowell 1974a, 1974b; McClay and Bonora 2001).

Releasing Bends.

If motion at a stepover or bend in a strike-slip fault creates extension, then a basin bounded by normal faults develops (Fig. 12-9a and b). Two examples of this type of motion are segments of the Hope Fault, New Zealand, and the San Jacinto Fault, California, USA (Suppe 1985) (Fig. 12-10b and c). The San Jacinto Fault is part of the San Andreas Fault system.

Releasing bends form as material within a stepover is subjected to extension. The amount of extension is related to the amount of slip on the strike-slip fault system. According to the releasing bend model, strike-slip faults bound the basin on two parallel sides and normal faults bound the basin on the other two sides. The strike-slip faults form the walls of the basin between the stepover, and normal faults form basin margins at the ends of the stepover (Fig. 12-9a). As the basin extends, material slumps into the void created by the extension parallel to the direction of strike-slip fault motion, and thus the extension records the amount of displacement that formed the basin. To determine the approximate amount of motion that formed the bend, restore the bend by moving the correlatable strata back in a direction that is opposite to the direction of strike-slip displacements.

If the subsidence rate in the basin exceeds the sedimentation rate, then the syntectonic sediments deposited concurrent with strike-slip displacements may contain initial offsets across the normal fault surfaces. Although releasing bends provide direct evidence for horizontal displacements, the restoration of releasing bends may overstate the amount of the strike-slip displacements, if the upthrown block does not contain growth sediments. Sequence stratigraphic evidence, based on high-stand or low-stand evidence, can minimize the amount of error encountered during the restoration process. However, if the stratigraphy easily correlates across fault surfaces, then any error in restoration is likely to be small.

The normal faults that form the margins of the basin contain hanging wall and footwall cutoffs that form piercing lines (Fig. 12-11a and b). These piercing lines represent the hanging wall and footwall cutoffs mapped in three dimensions. The piercing lines form as the blocks at the edge of the extensional basin slump into the basin subnormal to the surface trace of the strike-slip fault. Thus, if we can assume that the direction of strike-slip motion is parallel to the surface trace of the strike-slip fault (i.e., in the strike direction of the mapped fault surface), then we can restore, or close, the basin by moving the strata in the opposite direction of fault displacements and along any number of profiles that parallel the surface trace of the strike-slip fault (Fig. 12-11b and c). The hanging wall cutoffs restore back into the footwall cutoffs at their corresponding piercing points located along the piercing line (Fig. 12-11c).

Restraining Bends.

If material moves into a stepover or bend, compression occurs in the restraining bend (Crowell 1974a, 1974b; Aydin and Nur 1985; Cunningham and Mann 2007). The compression forms reverse faults, thrust faults, and pressure ridges, or pop-ups (Fig. 12-9b). A description of the complex deformation occurring on a pop-up from 3D seismic data is in Durand-Riard et al. (2013). This example is on a clear restraining bend. McClay and Bonora (2001) present several examples of restraining stepovers from Nevada, Wyoming, Chile, and the Netherlands. The Transverse Ranges of California, which exist on the Great Bend, a stepover in the San Andreas Fault, are an example of a large restraining bend (Fig. 12-10d). Contraction also occurs at restraining bends on continuously curved strike-slip fault surfaces. Modern 3D seismic time seismic data image these offset bends (Benesh et al. 2014). We discuss an example along the Loma Prieta bend in the San Andreas Fault in a later section in this chapter.

Let’s restore a small restraining bend to illustrate how the process confirms the presence of strike-slip displacements, restores the initial Pliocene stratigraphic trends, and allows us to estimate the amount of Pliocene displacements. This knowledge will allow interpreters to converge on the geometry and history of the Pliocene structures and the associated petroleum system. The bend is located on the flank of the Long Beach Anticline in the Newport-Inglewood Trend, southern California, USA. The Newport-Inglewood Trend is a classic zone of “transpressional” deformation (Harding 1973). The strike-slip faults along the trend form left-stepping, en echelon offsets, and the Cherry Hill and Northeast Flank Faults are major faults in the Newport-Inglewood Trend. Along the southern flank of the Long Beach Anticline, the Northeast Flank Fault steps over to the Cherry Hill Fault (Wright 1991) (Fig. 12-12a). Trenching indicates that the Cherry Hill Fault dies out to the southeast of the map area. Motion along the Newport-Inglewood Trend is right-lateral and, therefore, the bend should be subject to compression. If we assume that material enters the bend parallel to the surface trace of the Northeast Flank Fault or the Cherry Hill Fault, then the resulting contraction is restorable. This contraction forms the Signal Hill Promontory, or pressure ridge (Fig. 12-12b), and a reverse fault that dips at about 50 deg to the southeast (Taylor 1973) (Fig. 12-13). The method of implied fault strike (Tearpock et al. 1994), when applied to Taylor’s map of the bend in the Northeast Flank Fault (Fig. 12-12a), shows that the fault strikes about N65E beneath Signal Hill. The reverse fault motions cause the hanging wall beds and footwall beds to form piercing lines that strike northeast-southwest beneath Signal Hill.

Some textbooks seem to imply that strike-slip and compressional displacements are causatively related, and that transpressional strike-slip displacements commonly generate compressional folds adjacent to strike-slip faults (Fig. 12-1). The Signal Hill pressure ridge formed on the flank of the Long Beach Anticline, but strike-slip displacements need not be the cause of the Long Beach Anticline itself (Wright 1991). In this case, a decoupling of the strike-slip displacements from the compressional displacements may be more appropriate.

Several empirical observations support a decoupling process, which could change the interpretation of the Newport-Inglewood Trend. For example, according to the transpressional explanation shown in Figure 12-1, the axis of the folds would initiate at 30-deg to 45-deg angles to the strike-slip fault. The strains required to rotate a fold axis through a 30-deg to 45-deg angle suggest large amounts of strike-slip displacements. As the axis of the Long Beach Anticline is parallel to the Cherry Hill Fault (Fig. 12-12a), the strike-slip displacement on the Cherry Hill Fault should be substantial. Harding (1973) makes the observation that fold axes are presently offset only by 200 m to 800 m, but some or most of this displacement could be an initial offset of the axes of compartmentalized folds (Chapter 8, Fig. 8-74). The front limb of the Long Beach Anticline is not offset from its crest (Wright 1991) (Figs. 12-13 and 12-14). A second observation is that the structural contours in the hanging wall of the Northeast Flank Fault (Fig. 12-12a) are compatible with the footwall structural contours. This structural compatibility also implies small Plio-Pleistocene displacements (see Chapter 8). Thus, some or all of the displacements on the Northeast Flank Fault appear to be younger than the Long Beach Anticline, which supports Wright’s (1991) observations that the Newport-Inglewood Trend exhibits a complex structural and stratigraphic history not easily reconciled with a simple strike-slip origin.

If you construct a fault surface map for the Northeast Flank Fault along the southeastern flank of the Long Beach Anticline, it shows that the fault surface curves beneath Signal Hill (Taylor 1973), where the fault strikes at about N65E (Fig. 12-12). To the southeast of the anticline, the fault dips at high angles and strikes at about N60W (Fig. 12-12). A seismic or geological profile, such as cross section C-C′ taken in the NW-SE direction (and across the curved portion of the fault surface), will confirm whether the beds are thrust, reverse, or normally faulted. In this case they are reverse faulted (Fig. 12-13) and, referring to Figure 12-9, we can deduce the correct sense of strike-slip motion. That confirms the Signal Hill restraining bend to be a small but obvious restraining bend.

It is obvious from this discussion that in order to locate bends in fault surfaces, it is necessary to construct accurate maps of the fault surfaces, as described in Chapter 7. Typically, a 2D seismic grid is sufficient for constructing general fault surface maps and permits the detection of gentle bends in fault surfaces. Bends or offsets in fault surfaces have important consequences concerning the correct interpretation of data, prospect generation, or the absence of prospects (Tearpock et al. 1994). This is another reason for constructing loop-tied fault surface maps. As curved fault surfaces create releasing and restraining bends along strike-slip faults, maps of these fault surfaces may readily solve difficult structure problems.

The Signal Hill restraining bend is restorable along any number of profiles aligned subparallel to the surface trace of the Cherry Hill or Northeast Flank Faults (Figs. 12-13 and 12-14). The Northeast Flank Fault exhibits about 150 m of vertical separation and about 170 m of horizontal separation in the Pliocene Lower Wilber and Alamitos horizons. These displacements are in general agreement with Harding’s (1973) estimates. The Northeast Flank Fault cuts the southeastern limb of a pre-existing Long Beach anticline (Figs. 12-12 and 12-14) and appears younger than the compressional folding that formed the Long Beach anticline. The strike-slip motion may decouple from the compressional motions that formed the Long Beach Anticline (Shaw and Suppe 1996). Therefore, the compressional and strike-slip motions may not be directly related.

A misconception concerning piercing points is that offset fold axes, once thought to represent a continuous line, are good piercing point evidence because these offset fold axes restore to a single line. However, fold compartmentalization caused by tear faults are known to exist (Chapter 8, Fig. 8-74), and associated folds may form with an initial offset, and not along a single unbroken fold axis. These initial offsets can be large. In addition, folds forming along tear faults may grow during the deformation process, contributing to the offset. Unfortunately, we find that most types of piercing point evidence are rare or absent from most subsurface data sets.

In summary, the releasing and restraining bends, which are common to the known strike-slip faults throughout the world, often provide the best evidence in support of strike-slip faulting (Aydin and Nur 1985). If you are working a suspected strike-slip fault, locate a bend to confirm the strike-slip motions. Partially linked strike-slip fault systems typically contain stepovers. Often, a quick look at a structure map containing a strike-slip fault interpretation can resolve the presence or absence of strike-slip faulting in a matter of minutes. If strike-slip faults are thought to be in the area, examine a structure map for the presence of en echelon stepovers. Next, inquire as to the direction of fault motion. If fault motion moves material into the bend, then a structural high should exist on the map in the area of the restraining bend (Figs. 12-9b and 12-12). If, however, fault motion moves material out of the bend, then a low should exist on the map in the area of the releasing bend (Figs. 12-9a and 12-10b and c). An exception to this quick-look technique is structural inversion, which can also produce high and low areas along en echelon inversion faults. If a regional strike-slip fault interpretation does not contain restraining and releasing bends, perhaps style of faulting is present.

The documentation of strike-slip motion may be no more difficult than the documentation of other types of fault motion, but specific technical work must be done. First, fault surface maps, constructed in support of the deformation, should contain geometries that are consistent with the highs and lows on horizon structure maps. Second, in other tectonic regimes, whether it be compressional, extensional, or salt-related deformation, geoscientists and management require direct evidence of the type and style of the deformation. Strike-slip faults are not two-dimensional, and thus they require a 3D analysis. In many cases, the answer lies in a 4D analysis of the problem (Wright 1991). We presented techniques that can rapidly resolve the interpretation of strike-slip faulting that are no more difficult than techniques required to confirm other types of faulting. These techniques are supported by empirical evidence that establishes 3D structural validity.

Modern explorationists place emphasis on the petroleum system and its associated risk factors. To better understand how structures form and how faulting affects structural development and hydrocarbon migration, a good understanding of fault timing and geometry is necessary. We know of no way to address these issues concerning risk without fault surface maps, correctly interpreted from loop-tied data. If fault surface maps do not exist, then interpreters working an area have imprecise knowledge as to how the local structures formed and how the recognized faults may have channeled hydrocarbons. How can geoscientists correctly identify and understand the type and style of faulting, and generate viable prospects, without constructing viable maps based on loop-tied data? Furthermore, these maps of the fault surfaces may contain subtle bends that indicate small restraining and releasing curves, thus generating additional prospective structures. Fault surface maps also may record the strike-slip motions, as we show in a following section.

If accurate fault surface maps are not available, then it is possible to misidentify the structural style. On 2D seismic data, one may factor flower structure fault geometries (Harding 1985) into the risk analysis, where in reality inversion is the correct structural style. The petroleum system model may erroneously contain thick-skinned vertical faults, whereas in reality the faults are thin-skinned, low-angle faults. This can have a major effect on the interpretation of how hydrocarbons migrate and enter structures. Thus, differences in structural style can impact the understanding of the petroleum system, the interpretation model, the determination of risk, and the ultimate success of an exploration or development program.

Strike-slip faulting is sometimes over-interpreted and confused with other structural styles (Harding 1985), and some interpretations may lack positive evidence in support of horizontal displacements. Too often, strike-slip fault interpretation and the related structures are supported by a lack of data, no-seismic-data zones, high bed dips, axial surfaces, misapplied seismic interpretation rules, and so on (Tearpock et al. 1994). Certainly, strike-slip faulting should be subject to the same scientific methods and subsurface mapping standards that apply to other styles of faulting. Your success as a geoscientist depends on high-quality interpretations, maps, and prospects. Only solid scientific work, as described here, can ensure the best chance of success.

Generic Example of Strike-Slip Compressional Folding.

If fault surface maps are obtainable from well log or seismic data, then balanced models of hanging wall geometries are possible. Balanced models of hanging wall geometries lead to viable, low-risk prospects. A generic example is shown in the cross section presented in Figure 12-20. Perhaps your working group was subject to the following type of problem that employed well log and seismic data. Let us assume that two companies find hydrocarbons in Wells No. 2 and 3, in the A, B, and C Horizons adjacent to a vertical fault. Outcrop and seismic data suggest that the near-vertical fault, containing restraining and releasing bends, is located between Wells No. 1 and 2 (Fig. 12-20). The two companies construct cross sections of the new field in order to propose additional development wells. In Figure 12-20, well log data constrain the fault geometry, but what is the limit of the field, and is it necessary to drill additional wells?

We consider two interpretations of the data: a qualitative interpretation presented by Company A and a quantitative interpretation presented by Company B. We discuss the Company A interpretation first. We make the assumption that geoscientists from Company A do not have a solid background in structural geology and therefore have not been trained in volume conservation or structural balancing concepts. They construct a cross section through the field that employs conceptual, but not volume, conservation concepts. On the other hand, geoscientists from Company B construct a balanced cross section of the existing data. How may these two working groups and their interpretations differ? Can the difference affect future exploration and success? We will assume that the 3D seismic data that crosses the area is of reasonable quality but suffers from the usual problems, such as surface statics and the inability to image steeply dipping beds.

After examining the data present in Figure 12-20, the Company A geoscientist interprets a secondary fault along the western flank of the structure. Two independent sources of evidence exist for this fault. The first piece of evidence is the no-data seismic zone on the flank of the structure (Fig. 12-20). The second piece of evidence is the change in thickness of the stratigraphic units above the D Horizon, between Well No. 2 and Well No. 3. The interpretation is shown in Figure 12-21. The proposed secondary fault exhibits normal, strike-slip, and reverse separations, and it explains the bed dip and thickness variations between the B and C Horizons and the C and D Horizons in Wells No. 2 and 3. This fault geometry also accounts for the slip reversal between the B and D Horizons. Strike-slip faults can exhibit normal, reverse, and lateral separations. The interpreted fault turns down and merges with the large, master strike-slip fault at depth.

Lastly, Company A completes the seismic interpretation by correlating the reflections from the crest of structure into the off-structure flank position located to the west of the no-data zone on the seismic data set. In the low dip area to the west of the no-data zone, the seismic images a gently folded structure in which the reflectors turn up beneath the secondary fault (Fig. 12-21). The turned-up beds are interpreted to be caused by fault drag on the secondary fault, as is shown in many structural geology textbooks (Billings 1972).

The Company A team present the cross section of the folded and faulted structure shown in Figure 12-21. Using cross sections and maps of the field, they propose two additional wells to define the western limits of the field. These wells will test the upturned A and B Horizons located beneath the fault.

The Company A interpretation makes three basic assumptions: first, that the no-data zone on the seismic data set results from faulting; second, that faulting causes changes in thickness of stratigraphic intervals as seen in well logs, perhaps associated with repeated or missing section; third, that folds tend to be gently curved, rounded features, as shown in some textbooks on structural-metamorphic geology (see Chapter 10). Does another interpretation of the data exist that applies to the low temperature portions of mobile belts?

The Company B geoscientist team, having knowledge of structural geology concepts and techniques, attempts to balance the data shown in Figure 12-20 using procedures developed in Chapter 10 and in this chapter. They notice that the D Horizon is on the same structural level in Wells No. 2 and 3, and thus this horizon is not subject to possible faulting or folding between these two wells. Procedures outlined in the section on compressional folding along strike-slip faults suggest that the D Horizon may be near a bend in the vertical fault. Using the fault cuts located below the D Horizon in Wells No. 2 and 3, the Company B geoscience team reasons that the cutoff angle (θ) between the D Horizon and the deeper part of the fault is 60 deg (Fig. 12-22). Thus, the difference (ϕ) in dip angle of the fault is 30 deg.

The geoscientists can now complete the cross section shown in Figure 12-22, using the methods outlined in the previous section on compressional folding along strike-slip faults. Strata are deformed above the bend in the fault surface, so an axial surface emanates from this fault bend. They determine the dip of the axial surface by using the method described in the preceding section. Knowing that θ = 60 deg and ϕ = 30 deg, they use Figure 11-34 (right graph) to determine the axial surface angle γ to be 60 deg. As the regional dip is 0 deg, the actual dip of the axial surface is 60 deg. The axial surface projects upward at 60 deg from the bend in the fault (Fig. 12-22). The axial surface bisects the hinges of a fold, so by construction the A, B, and C Horizons dip at 60 deg through the no-seismic-data zone. The axial surface correlates to the western limit of the no-data zone on the seismic, and thus the no-data zone can result from changing bed dips and not from faulting, as assumed in the first interpretation.

Using similar reasoning with respect to Horizons A and B, the up-dip limit of the no-data zone can define a second axial surface. The axial surfaces are positioned as shown in Figure 12-22. As described earlier, seismic character correlation suggests the structural position of Horizons A and B are to the west (Fig. 12-20).

The Company B team assumes that the structure can be modeled using the Kink Method described in Chapter 10, and thus their cross section has more angular folds than the Company A cross section shown in Figure 12-21. Petroleum-scale structures, exhibiting a kink fold structural style, are common to the low temperature portions of fold and thrust belts (Suppe 1985; Boyer 1986). Furthermore, the Company B team are suspicious of the upturned beds imaged below the no-data seismic zone. The folding elevates high density and high velocity strata to a structural level that is higher than the equivalent beds located to the west of the no-seismic-data zone. Thus, the area below the region of high bed dips may be exhibiting velocity pull-up. In addition, as Company B concluded that the no-data zones result from folding and not faulting, there is no reason to postulate a fault-drag effect. They model the beds to the west of the no-data zone to be nearly flat surfaces.

The balanced interpretation presented by Company B also makes three assumptions. First, the no-data zone on the seismic data set results from high bed dips caused by folding. Second, folding causes changes in bed dips as well as in the different well log thicknesses. Third, folds in the low-temperature portions of mobile belts commonly exhibit kink geometry. The interpretation presented in Figure 12-22 has properties similar to lift-off or box folds (Chapter 10). The two interpretations contrast fundamentally. The interpretation presented by Company A emphasizes faulting, whereas the interpretation presented by Company B stresses folding.

The interpretation by Company B may not require any additional wells, depending on the areal extent of the reservoir and the drive mechanisms. However, to maximize development potential, one well is proposed to define the western limit of the accumulation in Horizon A. No wells are proposed to test the footwall plays in Horizons A and B (as presented by Company A), since the interpretation set has no closure in the footwall.

These two different interpretations will have a major impact on the exploration and development program. The interpretation shown in Figure 12-21 requires more wells and higher costs. It is based on negative evidence, limited structural knowledge, and a misunderstanding of the fold deformation in the footwall of the proposed secondary fault. Referring back to Chapter 1, we emphasize that a strong background in structural geology, not to be confused with an understanding of seismic interpretation, is a major component of a successful exploration or development program. This is particularly true in complex structural areas such as those discussed in this section.

The differences in the two interpretations result primarily from philosophical differences as to the approach used to resolve no-data zone problems and as to what represents real data. Rather than repeat many of the structural principles discussed in Tearpock et al. (1994) and this and other texts, we propose the following approach to evaluating conflicting or questionable structural interpretations. This approach is suggested by our experience in reviewing the results of many wells drilled in similar structural settings around the world.

1. Begin by examining interpretations to determine whether they honor all the data. Then look for obvious problems. Tearpock et al. (1994) present many quick-look techniques that aid in this analysis.

2. Certainly, geology can be complex, but where alternative solutions exist that honor all the data, the probable solution is commonly the less complex solution.

3. Valid interpretations of geological structures should be based on positive empirical evidence that confirms the presence of observed features. This requires an understanding of valid structural principles. The fault in Figure 12-21 may exist, but this interpretation is of high risk and is less likely. What constitutes negative evidence? A no-data zone on seismic, if used to support an argument, can qualify as an argument based on negative evidence. Thus, if faulting is proposed that is based on a no-data zone, then this argument is based on negative evidence. Again, this does not mean that the fault is not present; it means that this fault is a high-risk fault. Another way of approaching this problem is to consider what feature(s) on the data set will lower risk.

   If an argument is based on negative evidence, ask yourself what positive evidence is required to support the argument. Throughout the world, most normal faults that dip at 70 deg image on seismic sections. Does a proposed fault in a no-data zone have a surface reflection or exhibit discontinuous reflections across the surface, and if not, why not? If the fault surface images, can a viable map of the fault surface be constructed that supports the faulting? Other observable features, such as large amounts of missing or repeated section in well log data, are direct evidence for faulting. These features lower fault risk. However, a change in dip on a dipmeter log can result from either faulting or the well crossing an axial surface.

4. Can another model better explain the observed features? This is where experience and training are important. Perhaps the no-data zone in our example results from high bed dips caused by folding and not by faulting. Geoscientists who work fold-thrust belts know that high bed dips are common to fold belts. Folding creates high bed dips that do not image on many seismic data sets. Furthermore, our experience shows that if faulting is present, then dip domain analysis (Chapter 10) typically locates direct evidence for the faulting in the form of bed dip discontinuities. If another explanation is as probable, or more probable, then caution is prudent. Risk the uncertainty accordingly. Good scientific work should yield better interpretations, resulting in more success at lower costs.

   A structure similar to Figure 12-22 is shown in Figure 12-23 from Pecos Country, New Mexico, USA (Kelley 1971). The geological map of the region shows several long, arcuate northeast-southwest trending faults attributed to strike-slip displacements. This structure, called the Y-O Buckle, is bound by a near-vertical fault that contains a bend. A monoclinal fold forms above the bend in the fault, and below the bend the beds are not folded. The folded structures along these faults are subparallel to the surface trace of the faults, and they tend to exist at gentle bends in the faults. Thus, some of the folds appear to form at restraining bends. Field mapping demonstrates that the faults contain vertical separations that commonly change from normal to reverse separations along strike (Kelley 1971). The strata are deformed adjacent to the fault surfaces, often as disharmonic folds with near-vertical limbs (Figs. 12-22 and 12-23).

   

Extensional Folding along Strike-Slip Faults

The balancing of releasing bends is similar to the balancing of restraining bends, except that the displacements are in the opposite direction. Rather than employing the techniques used in Chapter 10 on compressional structures, we use the techniques previously discussed for extensional structures (Chapter 11). We apply these ideas to a possible releasing bend in the classic Ridge Basin, southern California, that was carefully mapped by Crowell and his colleagues and students (Crowell 1950, 1982, 2003a, 2003b, 2003c). Crowell (2003c) and Link (2003) describe the detailed structure and stratigraphy of the basin along with the geological complications. In this section, we attempt to integrate surface dip data, seismic data, well log data, sedimentary data, and structural data into a model for Ridge Basin.

Ridge Basin Geology.

Ridge Basin formed along the northeastern boundary of the large right lateral San Gabriel strike-slip fault (Fig. 12-24), which was active during the late Miocene. The San Gabriel Fault is an inclined fault surface that strikes at about N40W and dips 50 deg to 70 deg to the northeast (Crowell 2003c). Piercing point evidence, consisting of offset belts of pre–Late Miocene rocks and structures, suggests that the San Gabriel Fault experienced about 60 to 80 km of strike-slip displacement (Crowell 1982, 2003c).

Crowell (1974a, 1974b, 2003c) proposed that Ridge Basin formed as a result of a restraining bend located to the northwest of Ridge Basin and beneath the Frazier Mountain and Dry Creek Thrusts, that overthrust the northwestern part of Ridge Basin (Fig. 12-24). In his more recent model (Crowell 2003c) removed the extensional deformation from the basement that floors Ridge Basin and proposes squeezing and uplift at the stepover. As the evidence supports that 60 to 80 km of material moved through this restraining bend, a question arises as to why the active east-west-trending compressional Transverse Ranges terminate at the San Gabriel fault (Crowell 2003a) (Fig. 12-24); or why this east-west thrust faulting trend in the Transverse Ranges does not exist within domains 1 and 2 of Ridge Basin (Crowell, 1982, 2003c) (Fig 12-24; consult Restraining Bends section). An alternative interpretation of the regional data is that the right lateral San Gabriel Fault was originally part of the San Andreas Fault, and that modern strand of the San Andreas Fault has about 240 km of displacements (Ehlert 2003).

A seismic line presented by May et al. (1993) trends across the northwestern portions of Ridge Basin (Figs. 12-24 and 12-25). This line contains reflections that diverge toward an apparent listric normal fault that branches (splays) off the San Gabriel Fault. We call this proposed branch fault the Hungry Valley Fault. The deepest coherent reflections at about −10,000 feet on this depth-corrected seismic line dip at about 30 degrees, similar to the surface dips on the Geologic Map of Ridge Basin (Crowell 1982, 2003a, 2003b, 2003c). Most important, the line appears to image a rollover structure (May et al. 1993) (Fig. 12-26; also see Figs. 11-16 and 11-17). Typically, divergent dipping reflections dip toward listric normal faults and dip at about right angles to the normal fault. Therefore, the rolled-over reflections may dip toward a north-striking, branch normal fault that splays off the San Gabriel Fault (Link 2003).

This structural configuration favors an extensional releasing bend (Figs.12-9a and 12-11), subjecting Ridge Basin to a component of both dip-slip and strike-slip deformation. Furthermore, the Hungry Valley Fault, as shown in Figure 12-25, has normal separations that create accommodation space for the accumulation of the Ridge Basin sediments. This depocenter beneath Hungry Valley trends about north-south (Link 2003), so the Hungry Valley Fault also trends in a north-south direction (Link 2003). The Hungry Valley Fault was apparently responsible for the accumulation, rotation, and shingling of a section of Ridge Basin syntectonic sediments that are 14 km thick. The basin, however, was only about 5 km to 6 km deep (Crowell 2003c; Link 2003, 2003). Some deformational process was responsible for the rotation of these strata to dip at about 30 deg to the west (Crowell 2003a, 2003b, 2003c, Geologic Map of Ridge Basin and Geologic Cross-Section of Ridge Basin, profile A).

That this rotation is tectonic in nature is supported by the type of sedimentation in Ridge Basin, which involves the Peace Valley formation, and is about 9.0 km thick (Crowell, Geologic Cross-Section of Ridge Basin, profile A; Link 2003). Within the clastic units of the Peace Valley Formation the paleocurrent directions are to the southwest (Link 2003). According to Link et al. (1978) and Smyth (1982) the Peace Valley formation sediments are largely lacustrine, and 40% to 50% of the Violin Breccia consists of lacustrine fan deltas (Link 2003). These lacustrine deposits within the Peace Valley formations start with the shallow, freshwater Osito Canyon and Cereza Peak members and end with the Alamos Canyon and Posey Canyon members, which consist of brackish, deep-water lake sediments (Wood 1981; Link et al. 1978; Smith 1982). According to Smith (1982), the shallow fresh water deposits contain “wave-ripples, mudcrack casts, mammal tracks and arthropod burrows” and that these features are common. On Crowell’s Cross-Section A, these sediments extend for 15 km along the axis of the basin and downlap a thin veneer of Violin Breccia at high angles directly above horizontal basement. Thus, any model for Ridge Basin should account for these 30-deg dips through a 9.0-km thick lacustrine section.

A geological map of the basin published by Crowell (1982, 2003a, 2003b, 2003c) shows the following major features (Fig. 12-24). The geology adjacent to the San Gabriel Fault is dominated by the syntectonic Violin Breccia, deposited throughout the Late Miocene (Crowell 1982). Paralleling the breccia is the Ridge Basin syncline. The close association of the Violin Breccia with the Ridge Basin syncline suggests that the two features are related and that Ridge Basin syncline is a syntectonic feature. As Ridge Basin Group sediments onlap the flanks of the syncline, the stratigraphy supports the interpretation that the syncline is syntectonic (Link 2003). In the vicinity of Hungry Valley, the Pliocene sediments dip at gentle angles of 5 deg to 10 deg. These gently warped sediments were subject to late-stage Pliocene to Holocene folding that postdated the structure in areas north and northwest of dip domain 1 (May et al. 1993). Thus, the area surrounding Hungry Valley represents a gently dipping domain that was subsequently folded (Fig. 12-24).

Surface bed dips measured by Crowell define two large dip domains that dominate the structure to the southeast of Hungry Valley (dip domains 1 and 2 in Fig. 12-24). The Ridge Basin syncline partitions these domains. The largest of the domains (domain 1) dips to the west and lies about 1 to 3 km to the northeast of the San Gabriel Fault (Wood 1981; Crowell 1982, 2003c). The beds in domain 1 strike from 45 deg to 90 deg relative to the surface trace of the San Gabriel Fault (Fig, 12-24). We interpret this domain to dip toward the Hungry Valley Fault. The average bed dips, partitioned into smaller subdomains, are shown on Figure 12-24.

Dip domain 2 lies adjacent to the San Gabriel Fault and is syncline-separated from domain 1 (Fig. 12-25). In the northwest, this domain extends about 3 km to the northeast of the San Gabriel Fault, but to the southeast of Fisher Spring, the domain narrows to within 1 km of the fault zone. In domain 2 and in map view, the beds generally dip at high angles to the north and uniformly dip away from the San Gabriel Fault at oblique angles to the fault zone. Bed dips in this domain average 34 deg to the north. We emphasize, on typical rollover structures, from different areas of the world, beds dip toward normal faults at about right angles, not away from the fault surface at oblique angles. Thus, dip domains 1 and 2 form the Ridge Basin syncline. The oblique seismic line extends near the northwest portions of dip domain 2, located adjacent to the San Gabriel Fault, before entering dip domain 1, located about 4 km from the fault zone to the seismic line (Fig. 12-25).

Any model of the gross structural features in Ridge Basin should qualitatively and quantitatively explain the following structural features: (1) the approximate bed dips and the direction of the dips within the two domains; (2) the close association of the syntectonic Ridge Basin syncline with the syntectonic Violin Breccia and the San Gabriel Fault; (3) the processes that cause the strike of the beds to rotate up to 90 deg near the San Gabriel Fault zone, forming the Ridge Basin synclinal kink fold, and how these processes operate; and (4) how Ridge Basin was filled with at least 14 km of sediments, 7.5 km of which are lacustrine that presently dip at 30 deg to the west (the model also must be compatible with the stratigraphy of Ridge Basin [Link 2003] and with Goguel’s law of volume conservation [Chapter 10]); and (5) the interpretation should be restorable in that the cover should be restorable along with the basement (Chapter 10). Thus, one problem is how to restore the lacustrine sediments to the horizontal along with the basement without leaving a hole (see Crowell 2003a, Plate 1, Geologic Cross-Section of Ridge Basin, profile A). The dip domains and the syncline mapped by Crowell provide insight into the major tectonic processes associated with the San Gabriel Fault and the formation of Ridge Basin. As the subsurface geology is not well-constrained, we consider a solution to the Ridge Basin geology that assumes an extensional origin. This model will test the viability of an extensional origin for Ridge Basin based primarily on surface data. Other explanations may be possible, especially if more seismic data are acquired (consult Link 2003 for a review of the existing proposed models).

Geometry of Strike-Slip Extensional Folding.

The deformation of the hanging wall beds along a releasing bend differs from the deformation along a normal fault, as is seen in the perspective view diagram presented in Figure 12-27. An inclined strike-slip fault and a branch normal fault are shown. The dip of the branch normal fault surface decreases with depth. For purposes of convenience only, in Figure 12-27 we show this deep fault surface to be virtually horizontal. Displacement of the hanging wall block over the footwall causes the beds to be downthrown to the branch normal fault. These displacements also create accommodation space for growth sediments. According to the model, the beds adjacent to the strike-slip fault rotate downward at high and acute angles toward the branch fault. The beds forming to the right of the synclinal axis and located above the deep branch fault surface rotate toward the branch normal fault at right angles. The process generates a synclinally folded structure downthrown to the branch normal fault, with a synclinal axis that subparallels the strike of the strike-slip fault, mimicking the Ridge Basin geologic map (compare Fig. 12-27 to Crowell 2003a, Geologic Map of Ridge Basin, or Fig. 12-25).

The detailed kinematics of the process are best shown in Figure 12-28, which is drawn such that the left-hand portion of the block is in the same vertical plane as the synclinal axis GH in Figure 12-27 (or plane GCDEH in Fig. 12-28b). In Figure 12-28a, slip on the strike-slip fault forms an inclined strike-slip fault surface, ABCD, and forms a nascent right-stepping releasing bend, fault surface ADF (Fig 12-28b) (also consult Figs. 11-2 and 12-30). The hanging wall in Figure 12-28 exhibits normal separations. In other words, the hanging wall block slips a small distance S_H over the inclined strike-slip fault from point A to B, parallel to the strike of the strike-slip fault. This motion opens a small void beneath the hanging wall, a void which extends to the hidden, dashed line BC (Fig. 12-28a). The line BC was originally coincident with the line of bifurcation, AD, of the two fault surfaces (Fig. 12-28a). Thus, this small void exists above the surface ABCDF. As fault ADF is a normal fault subject to oblique-slip, this void is instantaneously filled by gravitational collapse (along Coulomb failure surface ψ_a in Figs. 12-28a and 12-30) of the hanging wall material onto the surface ABCDF (Fig. 11-2).

In Figures 12-27 and 12-28b, line GH defines the plunge of the synclinal axial surface. Volume conservation therefore dictates that point D′ collapses onto the footwall in the plane containing line GH. As points D and D′ lie in the plane defined by the apparent Coulomb collapse angle ψ_a, point D′ collapses onto point D, forming a rollover. The collapse of the hanging wall onto the footwall causes the hanging wall beds to rotate and displace toward point E in Figure 12-28b, by bending along the active axial surface trace line BG. The active axial surface trace line BG is the surface expression of the inactive Coulomb collapse surface, BGC. The strike of the beds adjacent to the strike-slip fault define this deformation, which in the case of Ridge Basin is dip domain 2 in Figure 12-25. This deformation causes the beds to rotate away from the inclined strike-slip fault surface, forming one-half of a synclinal fold.

The remaining half of the synclinal fold is shown in Figure 12-27 as the surface above the fault flat that dips toward the branch normal fault. For clarity, that surface was not shown in Figure 12-28. Collapse along Coulomb shear surfaces causes the surface to the right of the synclinal axis to dip toward the branch fault. In the case of Ridge Basin, this surface is in dip domain 1 (Fig. 12-25).

If a dipping strike-slip fault and branch fault form a fault flat, as shown in Figure 12-27, then two dip domains form as the hanging wall block slides over the footwall. The beds adjacent to the strike-slip fault dip away from the strike-slip fault, and the beds above the fault flat dip toward the branch normal fault. This deformation results in a synclinal structure with an axial surface that trends subparallel to the surface trace of the strike-slip fault (Fig. 12-27). The Ridge Basin syncline exhibits this type of geometry (Fig. 12-25).

Geometry of the Hungry Valley Fault.

After determining the strike and dip of the proposed Hungry Valley Fault, which branches off the San Gabriel Fault, we model the dip of the deep normal fault beneath Ridge Basin. The modeling uses stereographic projection methods and extensional inverse balancing techniques, described in Chapter 11. As stated previously, near Hungry Valley, the San Gabriel Fault strikes at about N40W and dips to the northeast (Crowell 2003c) (Fig. 12-25). In addition, the trend of the Hungry Valley depocenter of about N10E (Link 2003), and the measured average N10E strike of the beds in domain 1, constrain the strike of the proposed Hungry Valley branch normal fault.

Notice on Figure 12-25 or on Crowell’s Geologic Map of Ridge Basin, that the synclinal axial surface near the seismic line subparallels the surface trace of the San Gabriel Fault. This allows the kinematics of strike-slip extensional folding to simply construct a balanced model for Ridge Basin (Fig.12-28b). Notice on Figure 12-28b that any vertical profile, oriented parallel to the surface trace of a dipping master strike-slip fault, contains a zero-deg (α_a = 0.0) apparent dip for the deep fault surface (Fig. 12-30).

In other words, Figure 12-28b contains a vertical surface GCDEH that intersects the dipping master fault along dipping surface ABCD. The line of intersection between the two dipping surfaces GCDEH and ABCD, define the angle (α_a). This apparent dip angle (α _a) is independent of the dip on the dipping portion ABCD of the major strike-slip fault surface. This means that a general solution to the problem of strike-slip extensional folding is independent of the dip on the San Gabriel Fault surface. Also notice that from a comparison of Figure 12-28b to 12-30, the surface GHE (Fig 12-28b) allows a calculation of five apparent angles: (1) angle (α_a), (2) angle (θ_a) = the apparent dip of the Ridge Basin syncline, (3) angle (ψ_a1) = the apparent dip of the Coulomb collapse angle, (4) angle (β_a) = the apparent dip of the proposed Hungry Valley Fault, and angle, (5) (ϕ_a) = the apparent change in dip in the apparent fault surface. Also from Fig. 12-25, measurements exists for the calculation of (1) the strike of the San Gabriel Fault trending at N40W, (2) the strike of the branch fault from the averaged strikes of dip domain 1 located to the west of Peace Valley, and (3) the averaged dip and strike of domain 2 adjacent to the San Gabriel fault.

Lastly, the Coulomb collapse angle typically varies from 60 deg to 70 deg (Xiao and Suppe 1992; Bischke and Tearpock 1999). Here we assume a 65-deg Coulomb collapse angle. Next, we propose two balanced extensional folding models for Ridge Basin, one for dip domain 1, which traverses the deeper portions of the basin, and the second for dip domain 2, which parallels and cuts the San Gabriel Fault. Both domains are portrayed on a stereographic projection as thin lines (Fig. 12-29). We start with dip domain 2. These procedures may apply to other strike-slip faults (Fig. 12-33).

Model for Dip Domain 2.

We start with the best constrained data, the strike of the San Gabriel Fault and the Ridge Basin syncline, which is N40W on Figures 12-25 and 12-29. Next, the kinematics of pure strike-slip folding shown in Figures 12-27 and 12-28 permit a balanced and retrodeformable model of Ridge Basin along a vertical plane defined by line GCD (Fig. 12-28a). A profile in dip domain 2, constructed parallel to the surface trace of the San Gabriel Fault, can resolve the rollover structural geometry (Fig. 12-30). The problem is solvable if methods exist to measure (1) the apparent collapse angle ψ_a1 (Bischke and Tearpock 1999), (2) the apparent bed dip (θ_a), and (3) the apparent dip (α_a), of the fault at depth. It then follows that the apparent fault bend (ϕ_a) in the fault surface is obtainable using either forward or inverse modeling techniques described in Chapter 11, in this case, the angles are measurable as ϕ_a = β_a.

Notice in Figure 12-28b that line GHE lies in the plane that parallels the direction of fault slip and the Ridge Basin synclinal plunge GH (Fig. 12-27). Furthermore, material does not cross this plane. Also, notice that the active Coulomb collapse plane BGC intersects line GHE at an acute angle BG, located at the top of the hanging wall (Fig. 12-28a). This means that (1) not only will a balanced profile lie in the plane defined by the direction of fault slip, but (2) the Coulomb collapse angle (ψ_a), must be corrected for apparent dip (along with the other appropriate angles, such as apparent bed dip θ_a, β_a, ψ_a1 (Fig. 12-30).

Assuming a Coulomb collapse angle of 65 deg, we first calculate the apparent collapse angle (ψ_a1), from the strike direction of the Coulomb collapse surface. We obtain this angle by measuring the strike direction of the beds in domain 2 adjacent to the San Gabriel Fault (Fig. 12-25). In the model in Figure 12-28, the calculated strike of the beds in domain 2 is parallel to the active axial surface trace line BG. The measured strike of the beds on dip domain 2 from Figure 12-25 averages N75W and is labeled on stereoplot in Figure 12-29. When θ_a is measured, then the bed dips in dip domain 1 and 2 can be predicted from the stereograph. Accordingly, and using stereographic projection methods, the true Coulomb collapse plane ψ dips at 65 deg and dips perpendicular to the active axial surface line BG, striking at N75W. However, on plane GCDEH (Fig. 12-28b), the apparent Coulomb collapse angle measurement is ψ_a1 = 51 deg and lies in a profile trending at N40W (Figs. 12-29 and 12-30). Next, using the same procedures to determine ψ_a1 (Fig. 12-30), we measure the apparent plunge angle θ_a of the Ridge Basin syncline, and the apparent dip of the branch fault, β_a.

The plunge angle of the Ridge Basin syncline on plane GCDEH of Figure 12-28b is θ_a = 24 deg toward N40W (Fig. 12-30b). This value constrains the model as the great circles of dip domain 1, with dip domain 2, define the Ridge Basin syncline (Figs. 12-27 and 12-29). This 24-deg dip results in two conclusions. First, the great circle defining dip domain 2 determines the maximum dip for dip domain 2, which is about 38 deg to the north (Fig 12-29). Measured values from Figure 12-25 are 34N, striking at N85W, and thus calculated values from Figure 12-29 are close to measured values for dip domain 2. Second, in this solution, the branch fault dips at an apparent dip of 55 deg. SE (β_a on Figs. 12-29 and 12-30).

In generating the model, we will later describe that the strike of the beds in dip domain 1 will be consistent with the strike of the Hungry Valley Branch Fault at about N10E. As stated in the previous paragraph, the apparent bed dip θ_a at the intersection of domains 1 and 2 defines the plunge angle of the Ridge Basin syncline (Figs. 12-29 and 12-30), which means that this 24-deg dip trending at N40W dip in domain 2 must be similar to the apparent dip in domain 1. This is required in order to maintain continuity at and across the synclinal axial surface. This apparent dip lies in the same vertical profile as the strike direction of the strike-slip fault and the Ridge Basin syncline, which in this case is the San Gabriel Fault treading at N40W (Figs. 12-25 and 12-29). If a profile lies in the strike direction of an inclined strike-slip fault, then the apparent fault dip α_a = 0.0 deg (Fig 12-30a), and the apparent bend ϕ_a in the fault surface is equal to β_a, the apparent dip of the Branch Fault (Fig. 12-30). Thus, the value for the apparent dip of the Hungry Valley Branch Fault can also be plotted on the stereographic projection with an apparent dip β_a = 55 deg southeast (Fig. 12-29), and trending at S140E. We discuss in the next section that the trend of the branch fault is about N10E. Thus, the great circle through the apparent dip β_a = 55 deg southeast defines the maximum dip of the branch fault at 59E/N10E (Fig. 12-29).

Inverse Modeling of Dip Domain 1 and the Branch Normal Fault.

Lastly, using the extension inverse theory (Chapter 11), and briefly described in this section, we first justify the shallow geometry of the Hungry Valley Fault from the geological data and then determine the dip of the deeper portion of the Hungry Valley Branch Fault, which forms the basal portions of Ridge Basin. We briefly describe the sedimentary evidence first.

According to the extensional model, the branch fault forms the western flank of the Hungry Valley depocenter. This assumption seems reasonable, as the stratigraphy of the basin (Link and Osborne 1982; Link 2003) predicts that a depocenter exists in the area of Hungry Valley (Figs. 12-25 and 12-26). A normal fault would form a depocenter. In addition, the direction of sedimentary transport into the Hungry Valley depocenter is from the north at about N10E (Link 2003). If we assume that the shallow portions of the proposed Hungry Valley Fault parallels the strike direction of dip domain 1, then the Hungry Valley Branch Fault may subparallel the present trend of the Hungry Valley basin. Using the observation that dipping beds, related to normal faulting, typically dipping at about a right angles to the strike of the normal fault (typical of rollover structures), measurements of strike of domain 1 (Fig. 12-25) predict a strike for the proposed Hungry Valley Fault is N10E, which is consistent with the sedimentary source direction (Link 2003).

Next, we estimate the strike of the branch fault using (1) the plunge angle of the Ridge Basin syncline of θ_a = deg toward the proposed Hungry Valley Fault, (2) the stereographic plot of the measured domain 1 bed dips, and (3) inverse modeling (Chapter 11). Inverse modeling, which is the reverse of forward modeling, employs surface bed dips or depth-corrected seismic data to predict deep subsurface fault geometry. The consequence of decoupling known surface data from the unknown deep fault geometry introduces uncertainty into the subsurface depth predictions. This uncertainty results primarily from measurement errors that are inherent in all inversion methods, including seismic and potential field methods. Although inversion methods contain error, they have the advantage of being able to make predictions.

The inversion of the surface dip data and the 55-deg southeast apparent dip on the Hungry Valley normal fault predicts the gross crustal structure beneath Ridge Basin. This exercise results in a generic and retrodeformable model of Ridge Basin (Chapter 11). Additional subsurface data, such as seismic profiles, are required to confirm these predictions.

Inverse theory requires a number of assumptions that include knowledge of the Coulomb collapse angle (typically between 60 deg and 70 deg), taken here to be 65 deg. This inversion uses a profile constructed parallel to the strike of the San Gabriel Fault using domain 1 dips in the direction of the San Gabriel Fault slip, which is N40W.

Retrodeformable Figure 12-31 represents any profile that trends in a direction of N40W across dip domain 1. This profile can trend adjacent to the Ridge Basin syncline or across Hungry and Peace Valleys in the northern portions of Ridge Basin (Fig. 12-25). As the profile parallels the slip direction of the San Gabriel Fault, the bed dip angle θ_a and deep normal fault dip angle α_a are apparent dip angles. Domain 1 has an average bed dip θ of about 25 deg west, and it is separated from the gently dipping and folded Hungry Valley domain by an axial surface (Fig. 12-25). Correcting the bed dips in domain 1 to the profile direction of N40W results in an apparent dip θ_a =20 deg. This profile intersects the Hungry Valley Fault at an apparent dip of β_a = 55 deg (Figs. 12-29 and 12-31).

To determine the apparent dip of the deeper part of the fault α^a, use the methods described in the section Procedures for Projecting Large Normal Faults to Depth, in Chapter 11. As described in that section, simply project the distance D₁ (Fig. 12-31) from the active to the inactive axial surface at the projection of the shallow fault surface. This procedure establishes the position and dip of the deeper part of the branch fault α_a. In this case, the dip angle of the deep fault α_a = 12 deg (Fig. 12-31). Using the stereonet (Fig. 12-29), this 12-deg apparent dip of the deeper fault, in a S140E direction, establishes the plane or great circle representing the deep normal fault. The true dip of the fault is 16 deg in a S100E direction (Fig. 12-29).

A normal fault dip of 16 deg projects to the east to intersect the brittle-ductile transition at a depth of about 20 km beneath the Soledad Basin that contains Tertiary volcanic rocks (Ehlert, 2003). This prior volcanic activity may have weakened the crust, facilitating the crustal extension. A generalized fault surface map for the Ridge Basin basement is shown in Figure 12-32. According to the model, hanging wall beds moving over the nonplanar fault surfaces deform at the bends in the fault surfaces. This motion replicates the main structural features and bed dips observed in Ridge Basin. Thus, the fault surface map represents a 3D fault surface model for the Ridge Basin structure (Fig. 12-25). The faults are tied to cross sections constructed by Crowell (2003c) and to the Sun Schmidt Well (Fig. 12-32). On Figure 12-32, all the fault surfaces are planar and the lines of fault intersection are determined with stereoplots. Where well control and seismic data are sparse, this procedure allows for the projection of fault surfaces over long distances and provides strong constraints on an interpretation surface dip data (Suppe 1983). Methods involving curved line projection techniques require dense data control and are not applicable to underconstrained data sets.

The generalized fault surface map for the basement shown in Figure 12-32 is consistent with the major structural features and the bed dips observed in the Ridge Basin surface dips (Fig. 12-25). The generalized model shows that the Ridge Basin syncline emanates from the branch point (BP) of the San Gabriel Fault with the Hungry Valley Branch Fault. As the San Gabriel Fault dips at a higher angle in the southeast area of the map, the intersection of the deep normal fault with the San Gabriel Fault migrates to the south. This map may explain why domain 2 narrows to the southeast near Fisher Spring. The geometry of the San Gabriel Fault controls the position of the Ridge Basin syncline.

Tests of Model.

Measurements and stereographic projection can determine the true bed dip θ for dip domain 2, and measurements on profiles can determine apparent dip of the beds θ_a parallel to the San Gabriel Fault. This apparent dip angle θ_a lies in the same plane, defined by the great circle, as the true bed dip (Fig. 12-29). Near Hungry Valley, the strike of the beds in dip domain 2 is about N75W (Figs. 12-25 and 12-30). As the apparent dip of the beds (θ_a = 24 deg) lies in a profile that trends at N40W and in a plane that strikes at N75W, the maximum bed dip from the stereogram is determined to be 38 deg in a N15E strike direction (Fig. 12-29). This calculated value of 38N/N75W for the dip and strike of the beds in domain 2 is close to the observed average dips of 34N/N85W in dip domain 2 (Fig. 12-25).

Plotted on Figure 12-29 is dip domain 1, which dips and strikes at 25W/N10E. This plane intersects dip domain 2 to form the Ridge Basin syncline, which subparallels the surface trace of the San Gabriel Fault (compare Figs. 12-25 and 12-29). In Figure 12-29, the plane representing domain 1 (25W/N10E) intersects the plane representing dip domain 2 (36N/N75W) in a direction that is subparallel to the surface trace of the San Gabriel Fault and the Ridge Basin syncline at a direction of N43W. Thus, there is a good correspondence between the model and the observed surface bed dip data (Fig. 12-25 and Fig, 12-29).