And Physiography

Continental transforms, like their oceanic counterparts (Section 4.2.1), are conservative plate boundaries where lithosphere is neither created nor destroyed and strike-slip deformation results in lateral displacements across the fault zone. Strike-slip faults generally may occur at a variety of scales in virtually any tectonic setting. Only transform faults represent plate boundaries.

In contrast to oceanic fracture zones, which are characterized by a relatively simple linear trough (Section 6.12), continental transforms exhibit a structural complexity that reflects differences in the thickness, composition, and pressure-temperature profile of oceanic and continental lithosphere (Sections 2.7, 2.10.4). In the southwestern United States, for example, relative motion between the Pacific and North American plates is distributed across a zone that ranges from hundreds to a thousand kilometers wide (Fig. 8.1). Similarly, in New Zealand (Fig. 8.2), oblique convergence on the South Island has produced a >100-km-wide zone of deformation on the continental portion of the Pacific plate. These diffuse, commonly asymmetric patterns generally reflect lateral contrasts in lithospheric strength and areas where continental lithosphere is especially weak (Section 8.6.2). In areas where continental lithosphere is relatively cool and strong, transforms tend to display narrow zones of deformation. The Dead Sea Transform is an example of this latter type of system where deformation has localized into a zone that is only 20-40 km wide (Fig. 8.3).

In this chapter, the shallow (Section 8.2) and deep (Section 8.3) structure of continental transforms and major strike-slip faults is illustrated using examples from the southwestern U.S., New Zealand, the Middle East, and elsewhere. Other topics include the evolution of transform continental margins (Section 8.4), the use of velocity fields to describe crustal motion (Section 8.5), and the mechanisms that control the localization and delocalization of strain during strike-slip faulting (Section 8.6). This latter subject, and the overall strength of large strike-slip faults (Section 8.7), are especially important for explaining how continental transforms accomplish large magnitudes of slip.

The following fault styles and physiographic features characterize the surface and upper crust of continental transforms and major continental strike-slip faults:

1 Linear fault scarps and laterally offset surface features. Large continental strike-slip faults typically display linear scarps and troughs that result from the differential erosion of juxtaposed material and the erosion of fault gouge (Allen, 1981). Surface features along active or recently active fault traces may be displaced laterally due to the strike-slip motion. The age and magnitude of these offsets provide an important means of determining slip rates. In New Zealand, for example, the Alpine Fault is marked by a nearly continuous, linear fault trace that extends across the South Island for a distance of ~850 km (Fig. 8.2). Glacial moraines, rivers, valleys, lake shores, and other topographic features are offset laterally across the fault (Fig. 8.4), suggesting late Pleistocene slip rates of 21-24 mm a-1 (Sutherland et al., 2006). Vertical motion between parallel fault segments also is common and may create areas of localized uplift and subsidence that are expressed as pressure ridges and sag ponds, respectively (Sylvester, 1988).

2 Step-overs, push-ups, and pull-apart basins. Most large strike-slip faults are composed of multiple fault segments. Where one active segment terminates in proximity to another sub-parallel segment, motion is transferred across the intervening gap, resulting in zones of localized extension or contraction (Fig. 8.5a). In these step-overs, the initial geometry and sense of slip on the adjacent faults control whether the area separating them is extended or shortened (Dooley & McClay, 1997; McClay & Bonora, 2001). Normal faults and extensional troughs called pull-apart basins characterize step-overs where the intervening region is thrown into tension. Thrust faults, folds, and topographic uplifts known as push-ups form where the intervening region is compressed. In these

North American Plate ' r. Ai if //<' j Basin and Range i fii ' IK Ii/

mmim

San Francisco\\

saf "ifolswl^

transverse jecszn^\

Parkfield

Pacific Ocean

Pacific Plate

Salton Sea

Figure 8.1 Shaded relief map showing major faults and topographic features in California and western Nevada. Fault traces are from Jennings (1994) and Oldow (2003). SAF, San Andreas Fault; HF, Hayward Fault; CF, Calaveras Fault; GF, Garlock Fault; SGF, San Gabriel Fault; EF, Elsinore Fault; SJF, San Jacinto Fault; ECSZ, Eastern California Shear Zone; OVF, Owens Valley Fault; DVF, Death Valley Fault; CNSB, Central Nevada Seismic Belt. SWL, CWL, and NWL are the southern, central, and northern Walker Lane, respectively. Box shows location of Fig. 8.8a. Map was constructed using the same topographic data and methods as in Fig. 7.1.

! Median

Australian Plate

Tasman Sea

! Median

Australian Plate

Tasman Sea

Pacific Ocean

Pacific Plate

^ Puysegur \ L Trench

Stewart Island

Pacific Ocean

Pacific Plate

^ Puysegur \ L Trench

Stewart Island

West coast j_l_

Southern Alps

Exhumation , Haast schist

Australian plate

Exhumation , Haast schist

Décollement fo0f 5.8 km s"1

Lower crust

Canterbury Plain

East coast

Graywacke

Décollement

fo0f 5.8 km s"1

Lower crust

Pacific plate

280 300

Distance (km)

Figure 8.2 (a) Shaded relief map showing major faults and tectonic features of the Australian-Pacific plate boundary on the South Island of New Zealand. Map was constructed using the same topographic data and methods as in Fig. 7.1. WF, Wairau Fault; AF, Awatere Fault; CF, Clarence Fault; HF, Hope Fault; HFF, Hollyford Fault; FBF, Fiordland Boundary Fault. (b) Seismic velocity profile constructed without vertical exaggeration (modified from Van Avendonk et al., 2004, by permission of the American Geophysical Union. Copyright © 2002 American Geophysical Union).

Jordan Transform Faults Dead Sea

Figure 8.3 (a) Tectonic map and (b) shaded relief map showing major fault segments of the Dead Sea Transform and pull-part basin (images provided by U. ten Brink and modified from Al-Zoubi & ten Brink, 2002, with permission from Elsevier). Digital topography in (b) is from Hall (1993), 1967 coastline of the Dead Sea, showing subsidence of the basin, is from Neev & Hall (1979). Profile A-A' is shown in Fig. 8.11. Folds reflect Mesozoic-Early Cenozoic shortening.

Figure 8.3 (a) Tectonic map and (b) shaded relief map showing major fault segments of the Dead Sea Transform and pull-part basin (images provided by U. ten Brink and modified from Al-Zoubi & ten Brink, 2002, with permission from Elsevier). Digital topography in (b) is from Hall (1993), 1967 coastline of the Dead Sea, showing subsidence of the basin, is from Neev & Hall (1979). Profile A-A' is shown in Fig. 8.11. Folds reflect Mesozoic-Early Cenozoic shortening.

settings, the combination of strike-slip motion and extension is known as transtension. The combination of strike-slip motion and contraction is known as transpression.

The El Salvador Fault Zone in Central America illustrates many of the physiographic and structural features that are common to extensional step-overs. In this region, oblique

Kaino river Lake McKerrow

Cascade river

Jacksons Bay

Kaino river Lake McKerrow

Cascade river

Jacksons Bay

Figure 8.4 (a) Shaded relief map and (b, c) 1: 50,000 topographic maps showing linear scarp and offset surface features along a segment of the Alpine Fault on the South Island of New Zealand (images provided by R. Sutherland and modified from Sutherland et al., 2006, with permission from the Geological Society of America). Maps are derived from 1:50,000 NZMS 260 digital data. Curved bold black lines in (b) and (c) are rivers or creeks. Contour interval is 20 m.

Figure 8.4 (a) Shaded relief map and (b, c) 1: 50,000 topographic maps showing linear scarp and offset surface features along a segment of the Alpine Fault on the South Island of New Zealand (images provided by R. Sutherland and modified from Sutherland et al., 2006, with permission from the Geological Society of America). Maps are derived from 1:50,000 NZMS 260 digital data. Curved bold black lines in (b) and (c) are rivers or creeks. Contour interval is 20 m.

convergence between the Cocos and Caribbean plates (Fig. 8.6a) results in a component of dextral motion within a volcanic arc above the Middle America Trench (Martínez-Díaz et al., 2004). The Río Lempa pull-apart basin is marked by several irregular depressions and oblique normal faults that have formed in an extensional step-over between the San Vicente and Berlin fault segments (Fig. 8.6b) (Corti et al., 2005). Late Pleistocene volcanic edifices, river terraces, and alluvial fans are offset across prominent fault scarps.

In northern California, dextral strike-slip faults in the San Francisco Bay area record crustal shortening and topographic uplift related to

Releasing step-over

Pull-apart basin

Restraining step-over

Releasing bend

_7 Push-up

Subsidence

Restraining bend

Uplift

Uplift

Extensional strike-slip duplex ■ at releasing bend

Contractional strike - slip duplex at restraining bend

Strike - slip fan

(d) Cross sections

(d) Cross sections

Negative flower structure

Negative flower structure

Positive qY® flower structure

Figure 8.5 Map views of (a) step-overs and (b) bends and associated structures (after McClay & Bonora, 2001, Bull. Am. Assoc. Petroleum Geols. AAPG © 2001, reprinted by permission of the AAPG whose permission is required for further use). (c) Map and (d) cross-sections of strike-slip duplexes, fans and flower structures developed at bends (after Woodcock & Rickards, 2003, with permission from Elsevier).

a series of contractional step-overs. East of the bay, Mt. Diablo (Fig. 8.7a) marks the core of an anticlinorium that has formed between the Greenville and Concord faults (Unruh & Sawyer, 1997). The transfer of about 18 km of dextral strike-slip motion across this stepover during the late Cenozoic has resulted in a series of oblique anticlines, thrust faults, and surface uplifts that form a typical stepped, overlapping en echelon pattern. Mt. Diablo is the largest push-up in the region. Studies of deformed fluvial terraces suggest an uplift rate of 3 mm a-1 over the last 10,000 years, which is comparable to the rates of slip on the adjacent faults (Sawyer, 1999).

Bürgmann et al. (2006) resolved the rates of vertical crustal motion associated with several contractional step-overs near San Francisco Bay by combining GPS velocities with interferometiic synthetic aperture radar (InSAR) data (Section 2.10.5) collected over an 8 year period. After filtering out seasonally varying ground motions, the InSAR residuals (Fig. 8.7b) showed that the highest uplift rates occur over the southern foothills of Mount Diablo. Other zones of rapid uplift occur in the Mission Hills step-over between the Hayward and Calaveras faults, and between faults in the Santa Cruz Mountains. In the former area, seismicity is consistent with the transfer of slip on the Calaveras Fault onto the northern Hayward Fault through the Mission Hills (Waldhauser & Ellsworth, 2002). The origin of other vertical

Aerial 282 Jpg

Figure 8.6 (a) Shaded relief map and (b) interpretation of the Río Lempa pull-apart basin between the San Vicente and Berlin fault segments of the El Salvador Fault Zone (images provided by G. Corti and modified from Corti et al., 2005, with permission from the Geological Society of America). Digital elevation model is derived from SRTM data (http://srtm.usgs.gov/) and Landsat ETM 7 satellite images processed by the University of Maryland, Global Land Cover facility. Inset shows the plate tectonic setting of Central America and plate velocities (mm a~') after DeMets, 2001.

Figure 8.6 (a) Shaded relief map and (b) interpretation of the Río Lempa pull-apart basin between the San Vicente and Berlin fault segments of the El Salvador Fault Zone (images provided by G. Corti and modified from Corti et al., 2005, with permission from the Geological Society of America). Digital elevation model is derived from SRTM data (http://srtm.usgs.gov/) and Landsat ETM 7 satellite images processed by the University of Maryland, Global Land Cover facility. Inset shows the plate tectonic setting of Central America and plate velocities (mm a~') after DeMets, 2001.

movements is more uncertain and may reflect some nontectonic displacements, such as active landslides, subsidence and rebound over aquifers, and the settling of unconsolidated sediments along the bay margins. Nevertheless, the data reveal a pattern of highly localized vertical motion associated with regions of active strike-slip faulting.

3 Releasing and restraining bends. In zones where strike-slip faults are continuous, the strike of the faults may locally depart from a simple linear trend following a small circle on the Earth's surface. In these areas, the curvature of the fault plane creates zones of localized shortening and extension according to whether the two sides of the bend converge or diverge (Harding, 1974; Christie-Blick & Biddle, 1985) (Fig. 8.5b). These zones are similar to those that form in step-overs. Pull-apart basins, zones of subsidence and deposition, and normal faults characterize releasing bends. Restraining bends display thrust faults, folds, and push-ups.

Figure 8.7 (a) Geologic map of the Mount Diablo contractional step-over (after Wakabayashi et al., 2004, with permission from Elsevier). Geologic data are from Wagner et al. (1990) and Unruh & Sawyer (1997). (b) Fault map showing residual permanent scatterer InSAR rates (dots) after removing the contribution of tectonic horizontal motions and all points located on late Pleistocene substrate (image provided by R. Bürgmann and modified from Bürgmann et al., 2006, with permission from the Geological Society of America). Permanent scatterers are stable radar-bright points such as buildings, outcrops, utility poles etc. that are used to identify time-dependent surface motions. Modeled range rates include 115,487permanent scatterers relative to point labeled FIXED for the years 1992-2000. Positive residuals correspond to uplift. MD, Mount Diablo; MH, Mission Hills; SCM, Santa Cruz Mountains. Black arrows show residual (observed minus modeled) GPS horizontal velocities, which provide a measure of how well the model fits the observations (Section 8.5.3). Inset in (b) shows the geometry of folds and thrust faults in a contractional step-over between the Hayward and Calaveras faults (after Aydin & Page, 1984, with permission from the Geological Society of America).

Figure 8.7 (a) Geologic map of the Mount Diablo contractional step-over (after Wakabayashi et al., 2004, with permission from Elsevier). Geologic data are from Wagner et al. (1990) and Unruh & Sawyer (1997). (b) Fault map showing residual permanent scatterer InSAR rates (dots) after removing the contribution of tectonic horizontal motions and all points located on late Pleistocene substrate (image provided by R. Bürgmann and modified from Bürgmann et al., 2006, with permission from the Geological Society of America). Permanent scatterers are stable radar-bright points such as buildings, outcrops, utility poles etc. that are used to identify time-dependent surface motions. Modeled range rates include 115,487permanent scatterers relative to point labeled FIXED for the years 1992-2000. Positive residuals correspond to uplift. MD, Mount Diablo; MH, Mission Hills; SCM, Santa Cruz Mountains. Black arrows show residual (observed minus modeled) GPS horizontal velocities, which provide a measure of how well the model fits the observations (Section 8.5.3). Inset in (b) shows the geometry of folds and thrust faults in a contractional step-over between the Hayward and Calaveras faults (after Aydin & Page, 1984, with permission from the Geological Society of America).

km

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Figure 8.8 (a) Shaded relief map and (b) seismic profile of the Central Transverse Ranges, southern California showing faults (thin black lines, dotted where buried), shotpoints (gray circles), seismographs (thick black line), and epicenters of earthquakes greater than M = 5.8 since 1933 (images provided by G. Fuis and modified from Fuis et al., 2003, with permission from the Geological Society of America). Focal mechanisms with attached magnitudes: 6.7a, Northridge (Hauksson et al., 1995);6.7b, San Fernando (Heaton, 1982);5.9, Whittier Narrows (Hauksson et al., 1988);5.8, Sierra Madre (Hauksson, 1994); 6.3, Long Beach (Hauksson, 1987). HF, Hollywood Fault; MCF, Malibu Coast Fault; MHF, Mission Hills Fault; NHF, Northridge Hills Fault; RF, Raymond Fault; SF, San Fernando surface breaks; SSF, Santa Susana Fault; SmoF, Santa Monica Fault; SMFZ, Sierra Madre Fault Zone; VF, Verdugo Fault. In (b) gray area represents refraction coverage, thin black lines represent velocity contours or boundaries; contour interval 0.5 km s- to 5.5 km sand arbitrary above 5.5 km s~'. Large numbers on either side of the San Andreas Fault are average basement velocities.

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Distance (km)

Figure 8.8 (a) Shaded relief map and (b) seismic profile of the Central Transverse Ranges, southern California showing faults (thin black lines, dotted where buried), shotpoints (gray circles), seismographs (thick black line), and epicenters of earthquakes greater than M = 5.8 since 1933 (images provided by G. Fuis and modified from Fuis et al., 2003, with permission from the Geological Society of America). Focal mechanisms with attached magnitudes: 6.7a, Northridge (Hauksson et al., 1995);6.7b, San Fernando (Heaton, 1982);5.9, Whittier Narrows (Hauksson et al., 1988);5.8, Sierra Madre (Hauksson, 1994); 6.3, Long Beach (Hauksson, 1987). HF, Hollywood Fault; MCF, Malibu Coast Fault; MHF, Mission Hills Fault; NHF, Northridge Hills Fault; RF, Raymond Fault; SF, San Fernando surface breaks; SSF, Santa Susana Fault; SmoF, Santa Monica Fault; SMFZ, Sierra Madre Fault Zone; VF, Verdugo Fault. In (b) gray area represents refraction coverage, thin black lines represent velocity contours or boundaries; contour interval 0.5 km s- to 5.5 km sand arbitrary above 5.5 km s~'. Large numbers on either side of the San Andreas Fault are average basement velocities.

The Transverse Ranges in southern California (Figs 8.1, 8.8a) illustrate the characteristics of a large restraining bend in the San Andreas Fault. These ranges have been uplifted in response to a combination of dextral motion and compression across a portion of the fault that strikes more westerly than the general strike of the fault system. Seismic reflection profiles and information from wells indicate that thrust faults dip northward at 25-35° beneath the San Gabriel Mountains and intersect the near vertical (83°) San Andreas Fault at mid-crustal depths of ~21 km (Fuis et al., 2001, 2003). Earthquake focal mechanisms show thrust solutions on fault splays that branch upward off the dipping décollement surface (Fig. 8.8b). This combination of motion has resulted in a zone of transpression and topographic uplift commonly referred to as the Big Bend.

Examples of active releasing bends and strike-slip basins occur along the southernmost part of the Alpine Fault in southwest New Zealand. Near Fiordland, three semicontinuous fault segments accommodate dextral strike-slip motion between the Australian and Pacific plates. Along the Resolution segment of the plate boundary (Fig. 8.9a), geophysical surveys have revealed the presence of a Pleistocene pull-apart called the Dagg Basin (Barnes et al., 2001, 2005). A seismic reflection profile across the northern part of the basin (Fig. 8.9b) shows that it is bounded on the northwest by a ridge above an active reverse fault. Inactive faults are buried beneath the ridge. At the center of the basin, upward splaying faults accommodate oblique extension, forming a graben. Some west-dipping splays (labeled IA in Fig. 8.9b) presently are inactive, although the deposition of wedge-shaped strata between the development of two unconformities (surfaces DB3 and DB4) indicates that they once were active simultaneously with the east-dipping splays. This geometry suggests that the pull-apart basin initially formed in an extensional stepover prior to unconformity DB3 (Barnes et al., 2001, 2005). Another pull-apart, called the Five Fingers Basin, formed in a similar step-

over 10 km farther south (Fig. 8.9a). The current smooth shape of the releasing bends formed later, after unconformity DB3, as subsequent strike-slip motion formed faults that joined across the gap between the stepovers (Fig. 8.10a,b). In contrast to the extension that characterizes the northern Dagg Basin, the southern end of the basin shows evidence of reverse faulting and uplift. A combination of shortening and strike-slip faulting associated with a restraining bend in this region formed the Dagg Ridge, which has been squeezed upward between the main trace of the Alpine Fault on the west and a curved oblique-slip fault beneath its eastern margin (Fig. 8.9c). South of the ridge, the Breaksea Basin preserves features that indicates it was once continuous with the Dagg Basin, suggesting that the reverse faulting occurred after the pull apart had formed (Barnes et al., 2005). As the total plate motion and amount of slip increased, some faults were abandoned and others formed linkages that cut through the extensional basins, resulting in localized pushups and ridges where they formed restraining bends (Fig. 8.10c). These relationships illustrate how large strike-slip faults typically evolve very rapidly and that localized strike-slip basins and uplifts develop along different parts of the fault zone (Fig. 8.10c) on timescales of tens to hundreds of thousands of years.

4 Strike-slip duplexes, fans, and flower structures. A strike-slip duplex is an imbricate array of two or more fault-bounded blocks and basins that occur between two or more large bounding faults (Woodcock & Fischer, 1986). These structures are analogous to the duplexes that form on the ramps of dip-slip faults but differ in that vertical movements are not constrained at the upper (ground) surface. The fault-bounded basins that characterize the duplex typically are lens-shaped. The individual blocks defined by the strike-slip faults are shortened and uplifted when the faults converge and stretched and downthrown where the faults diverge (Fig. 8.5 c). This tendency for strike-slip faults to diverge and converge creates a characteristic

Extensional Strike Slip

166°30'E 0 2.5 5 10 Kilometers

-Active strike-slip fault -j--- Active thrust fault

Inactive strike-slip fault -V»- Inactive thrust fault

Subsiding basin (transtensional) Uplifted ridge (transpressional)

166°30'E 0 2.5 5 10 Kilometers

-Active strike-slip fault -j--- Active thrust fault

Inactive strike-slip fault -V»- Inactive thrust fault

Subsiding basin (transtensional) Uplifted ridge (transpressional)

Figure 8.9 (a) Map of the south-central coast of Fiordland and (b,c) line drawings of seismic reflection profiles showing active faults, strike-slip basins, and other physiographic features along the southern segment of the Alpine Fault (images provided by P. Barnes and modified from Barnes et al., 2005, with permission from the Geological Society of America). Map location shown in Fig. 8.2a. Profile (b) shows the subsiding part of the Dagg Basin, profile (c) the uplifting part. Profile locations are shown in (a). Solid faults are active, dashed faults are inactive. Labeled reflections are unconformities.

Figure 8.9 (a) Map of the south-central coast of Fiordland and (b,c) line drawings of seismic reflection profiles showing active faults, strike-slip basins, and other physiographic features along the southern segment of the Alpine Fault (images provided by P. Barnes and modified from Barnes et al., 2005, with permission from the Geological Society of America). Map location shown in Fig. 8.2a. Profile (b) shows the subsiding part of the Dagg Basin, profile (c) the uplifting part. Profile locations are shown in (a). Solid faults are active, dashed faults are inactive. Labeled reflections are unconformities.

Five Fingers Basin

Dagg Basin

Five Fingers Basin

Dagg Ridge

Breaksea Basin

Five Fingers Basin

Dagg Ridge

Breaksea Basin

Dagg Basin

Flower Structure Transpressional Basin

Figure 8.10 Sketches showing the progressive evolution of the Resolution segment of the Alpine Fault near Fiordland at (a) ~1 Ma when a series of pull-apart basins formed between extensional step-overs and (b) presently when linkages among faults have cut through the Dagg Basin forming the Dagg Ridge (shaded) (after Barnes et al., 2005, with permission from the Geological Society of America). (c) Schematic block diagram showing the three-dimensional geometry of adjacent releasing and restraining bends (image provided by P. Barnes and modified from Barnes et al., 2001, with permission from Elsevier). Southern end of basin displays a positive flower structure, push-up, and transpression. Northern end of basin displays pull-apart basin, subsidence, and transtension.

Figure 8.10 Sketches showing the progressive evolution of the Resolution segment of the Alpine Fault near Fiordland at (a) ~1 Ma when a series of pull-apart basins formed between extensional step-overs and (b) presently when linkages among faults have cut through the Dagg Basin forming the Dagg Ridge (shaded) (after Barnes et al., 2005, with permission from the Geological Society of America). (c) Schematic block diagram showing the three-dimensional geometry of adjacent releasing and restraining bends (image provided by P. Barnes and modified from Barnes et al., 2001, with permission from Elsevier). Southern end of basin displays a positive flower structure, push-up, and transpression. Northern end of basin displays pull-apart basin, subsidence, and transtension.

braided pattern in plan view. The faults that most closely follow the direction of plate movements predominate, grow longer, and assume near vertical dips. Other faults at an angle to the overall direction of movement may then rotate farther out of alignment and develop dips significantly less than vertical, so that the fault involves a component of dip-slip motion. If the fault's curvature carries it to a region of extension, a normal oblique-slip fault develops; if to a region of compression, a reversed oblique-slip fault forms. Significant rotations about vertical or near vertical axes also commonly occur (Section 8.5). At the ends of large strike-slip faults, displacements may be dissipated along arrays of curved faults that link to the main fault forming fans or horsetail splays (Fig. 8.5c). These structures may record either contractional or extensional deformation according to the geometry of the curvature and the sense of motion on the main fault.

In profile, the various splays of a strike-slip fault zone may converge downward at depth to produce a characteristic geometry in profile known as a flower structure (Fig. 8.5d) (Harding, 1985; Christie-Blick & Biddle, 1985). Negative flower structures are those where the upward-branching faults display mostly normal offsets beneath a synform or surface depression (e.g. Fig. 8.9b). Positive flower structures are those where the upward-branching faults display mostly reverse offsets beneath an antiform or surface culmination. A positive flower structure is illustrated by the geometry of faults in southern Dagg Basin (Fig. 8.10).

5 Strike-slip partitioning in transpression and transtension. There are several ways in which displacements may be distributed between the boundaries of obliquely converging or diverging blocks and plates. One common way is by simultaneous motion on separate strike-slip and contractional or extensional structures. In this scenario, strike-slip faults accommodate the component of oblique convergence/divergence that parallels the plate boundary and the contractional or extensional structures accommodate the component oriented orthogonal to the plate boundary. Such systems, where strike-slip and dip-slip motion occur in different places and on separate structures, are strike-slip partitioned. Alternatively, both strike-slip and margin-perpendicular components of the deformation may occur either on the same structure, such as occurs presently on the central oblique-slip section of the Alpine Fault in New Zealand (Section 8.3.3), or both components may be distributed more or less uniformly across a zone. The relative contributions of strike-slip and margin-perpendicular deformation allow further classification into strike-slip-dominated and thrust- (or normal-) dominated systems. The southern segment of the Alpine Fault illustrates a strike-slip partitioned style of transpression. Near the Fiordland margin (Fig. 8.9a), the fault lies at a low angle (1125°) to the azimuth of Pacific-Australian plate motion (Barnes et al., 2005). This low angle results in almost pure strike-slip motion along the active trace of the Alpine Fault, which in this area is nearly vertical. The contractional component of deformation that arises from the oblique plate convergence is accommodated by structures located both west and east of the Alpine Fault. On the western side, a 25-km-wide thrust wedge is composed of a series of active thrust, reverse, and oblique-slip faults that steepen downward toward the Alpine Fault. This fault segment illustrates how shortening occurs simultaneously with dextral strike-slip motion in different places along the plate boundary.

A similar strike-slip partitioned system occurs in the "Big Bend" region of southern California where thrust faults accommodate contraction simultaneously with dextral strike-slip motion. Within the San Gabriel Mountains (Fig. 8.8a), the San Andreas Fault lies at about 35° to the direction of relative motion between the Pacific and North American plates. This oblique angle results in a component of contraction that is accommodated by reverse faulting and folding within the mountains north of the Los Angeles Basin. The oblique angle also results in strike-slip motion, which is accommodated by a series of steep west-northwest-trending faults includes the San Andreas Fault itself (Fuis et al., 2003).

An example of a very weakly or nonpartitioned style of transpressional deformation occurs along the central segment of the Alpine Fault on the South Island of New Zealand. Here, the Alpine Fault strikes to the northeast (55°) and dips moderately to the southeast (Fig. 8.2b, Section 8.3.3). Norris & Cooper (2001) showed that, unlike the Fiordland segment, slip on the central segment of the fault is oblique and approximately parallels the interplate vector (Section 8.3.3). At the eastern and western limits of deformation on the South Island, reverse faults approximately parallel the Alpine fault but are inferred to have relatively low rates and minor components of strike-slip motion (Norris & Cooper, 2001; Sutherland et al., 2006). These characteristics indicate that the central segment of the Alpine Fault system is at best weakly partitioned and appears to be nonpartitioned in some areas.

8.3 THE DEEP STRUCTURE OF CONTINENTAL TRANSFORMS

Was this article helpful?

0 0
Boating Secrets Uncovered

Boating Secrets Uncovered

If you're wanting to learn about boating. Then this may be the most important letter you'll ever read! You Are Going To Get An In-Depth Look At One Of The Most Remarkable Boating Guides There Is Available On The Market Today. It doesn't matter if you are just for the first time looking into going boating, this boating guide will get you on the right track to a fun filled experience.

Get My Free Ebook


Post a comment