Australia: The Land Where Time Began
Transform and strike-Slip Faults-Continental
As with oceanic transforms, continental transforms are conservative plate boundaries, lithosphere being neither created nor destroyed, lateral displacements across the fault zone occurring by strike-slip deformation, the strike-slip can occur at different scales and in almost any tectonic setting. Plate boundaries are represented by only transform faults.
Continental transform faults display a more complex structure, reflecting the differences between continental and oceanic lithosphere, such as thickness, composition and pressure-temperature, unlike oceanic transform faults, troughs that are relatively simple. Relative motion between the Pacific and North American plates is distributed across a zone from hundreds to thousands of kilometres wide in the southwestern United States. A zone of deformation on the continental part of the Pacific Plate >100 km wide has been produced on the South Island of New Zealand by oblique convergence. Lateral contrasts of the strength of the lithosphere, and areas of continental lithosphere that are especially weak, is reflected in diffuse patterns that are commonly asymmetric. Transform faults tend to have narrow deformation zones in areas of relatively cool, strong continental lithosphere. An example of the latter type is the Dead Sea Transform in which the zone of deformation has been localised to be 20-40 km wide.
Kearey et al. (2008) have listed 5 styles of fault and physiography. 1. Linear fault scarps and laterally offset surface features, 2 step-overs and push-ups, and pull-apart basins, 3 releasing and restraining bends, 4 strike-slip duplexes, fans and flower structures, 5 strike-slip partitioning in transpression and transtension.
Linear fault scarps and laterally offset surface features. These are typically displayed as linear scarps and troughs, by large continental strike-slip faults, resulting from differential erosion of material that is juxtaposed, and fault gouge erosion (Allen, 1981). The motion associated with strike-slip faulting may displace surface features laterally along active fault traces or faults that were recently active. Slip rates can be determined by the age and magnitude of these offsets. The alpine Fault in New Zealand is marked by a linear fault trace, that is almost continuous, extending for about 850 km across the South Island. Topographic features such as glacial moraines, rivers, valleys, lake shores and others are all laterally offset across the fault, suggesting that during the Late Pleistocene rates of slip were 21-24 mm/year (Southerland et al., 2006). It is also common to have vertical motion between segments of the fault that are parallel, possibly leading to localised uplift and subsidence in some areas, expressed respectively as pressure ridges and sag ponds.
Step-overs and push-ups, and pull-apart basins. Multiple fault segments comprise most large strike-slip faults. Localised zones of expansion and contraction result from transference of motion across the intervening gap in places of termination of an active segment in the proximity of another sub-parallel segment. The initial geometry and sense of slip on adjacent faults determine whether the intervening area expands or contracts in these step-overs (Dooley & McClay, 1997; McClara & Bonora, 2001). Pull-apart basins, comprised of normal faults and extensional troughs, are characteristic of step-overs areas in which the intervening region is thrown into tension. Push-ups, thrust faults, folds and topographic uplifts, occur in places of compression of the intervening area. The combination of strike-slip motion and extension in these settings is 'transtension'. Transpression is the combination of contraction and strike-slip motion.
In extensional step-overs, many structural and physiographic features are found to be common. An example that displays many of these features is the El Salvador Fault Zone of Central America. Convergence between the Cocos and Caribbean Plates, that is oblique, leads to a component of dextral motion within a volcanic arc that is situated above the Middle America Trench (Martinez-Diaz et al., 2004). Several irregular depressions, as well as oblique normal faults, formed between San Vincente and Berlin fault segments, in an extensional step-over, are found in the Rio Lampa pull-apart basin (Corti et al., 2005). Offsets across prominent fault scarps are volcanic edifices, river terraces and alluvial fans, of Pleistocene age.
Evidence has been found recorded in the strike-slip faults in the area of San Francisco Bay indicating topographic uplift and shortening of the crust associated with a series of contractional step-overs. Mt Diablo, the largest push-up in the region, in the east of the bay, is the core of an anticlinorium between the Greenville and Concord Faults (Unruh & Sawyer, 1997). A series of oblique anticlines, thrust faults and surface uplifts forming a stepped, overlapping en echelon pattern resulted from the transfer of about 18 km of dextral strike-slip motion across the step-over in the Late Cainozoic. Evidence from deformed fluvial terraces suggests that uplift has occurred at the rate of about 3 mm/year for the last 10,000 years. This uplift rate is comparable to the slip rates on the adjacent faults (Sawyer, 1999). GPS velocities have been combined with the data from interferometric synthetic aperture radar (InSAR) from a period of 8 years to resolve the vertical crustal motion rates, associated with several contractional step-overs near San Francisco Bay (Bürgmann et al., 2006). The highest rates of uplift were found to occur over the foothills of the southern part of Mt Diablo, as determined from the InSAR residuals, after seasonally varying movements of the ground were allowed for. Between Hayward Fault and Calveras Fault, the Mission Hills step-over was the site of a rapid uplift zones, as well as between faults in the Santa Cruz Mountains. Seismicity in the former area is consistent with slip on the Calveras Fault being transferred onto the northern Hayward Fault by way of the Mission Hills (Waldhauser & Ellsworth, 2002). Non-tectonic displacements, e.g. landslides, subsidence and rebound above aquifers, and unconsolidated sediments along the margins of the bay settling may be the origin of other vertical movements, though this is less certain. A pattern of vertical movement, that is highly localised, is revealed by the data, that is associated with regions where strike-slip faulting is active.
Releasing and restraining bends. The strike of a fault may deviate from a simple linear trend that follows a small circle on the Earth's surface in zones of continuous strike-slip faults, zones of localised shortening and extension being created by the curvature of the fault plane, depending on whether the sides of the bend converge or diverge (Harding 1974; Christie-Black & Biddle, 1985), the zones being similar to those forming in step-overs. In zones of subsidence and deposition, such as pull-apart basins, and normal faults, characterise releasing bends. Thrust faults, push-ups and folds characterise restraining bends.
A large restraining bend in the San Andreas Fault is illustrated by the Transverse Ranges of southern California. A combination of dextral motion and compression across a portion of the fault, striking more westerly than the fault system, uplifted these ranges. It has been found from evidence in wells and seismic reflection profiles that beneath the San Gabriel Mountains these faults dip to the north at 25-35o, intersecting at about 21 km, about mid-crust depth, the San Andreas Fault that is 83o, near vertical (Fuis et al., 2001, 2003). It can be seen from earthquake focal mechanisms showing thrust solutions of fault splays branching upward from the décollement surface. The result of the combination of movement in a zone of transpression and topographic uplift is usually called Big Bend.
Along the southernmost section of the Alpine Fault in southwest New Zealand are examples of releasing bends and strike-slip basins. Dextral strike-slip movement between the Australian and Pacific Plates is accommodated by 3 semicontinuous fault segments near Fiordland. Geophysical surveys have found a pull-apart of Pleistocene age, the Dagg Basin, along the Resolution segment of the plate boundary (Barnes et al., 2001, 2005). On the northwest it is bounded by an active reverse fault beneath a ridge, as indicated by a seismic reflection profile. Beneath the ridge are a number of inactive faults. Oblique extension is accommodated by upward splaying faults at the centre of the basin, to form a graben. Wedge-shaped strata deposited between the development of 2 unconformities indicate that the inactive west-dipping splays were active simultaneously with the east-dipping splays at some time in the past. The pull-apart basin is suggested to have formed in a step-over prior to unconformity DB3 (Barnes et al., 2001, 2005), as indicated by the geometry. The Five Fingers Basin is another pull-apart that formed in a similar step-over 10 km to the south. The releasing bends that are smooth shaped at the present, are believed to have formed later than unconformity DB3, faults joining across the gap between the step-overs having resulted from subsequent strike-slip movement.
Reverse faulting and uplift is evident at the southern end of the Dagg Basin, unlike the extension characteristic of the northern end. The Dagg Ridge, that formed by shortening and strike-slip faulting associated with a restraining bend, was squeezed upward between the main traces of the Alpine Fault on the west, and beneath the eastern margin, a curved oblique-dip fault. The Breaksea Basin, to the south of the ridge, shows evidence that it was previously continuous with the Dagg Ridge, which suggests the reverse faulting occurred following the formation of the pull-apart (Barnes et al.,2005). Some faults were abandoned and others formed linkages cutting across the extensional basins, the results being push-ups and ridges where they formed restraining bends. The rapid evolution of large strike-slip faults is illustrated by these relationships, as well as localised strike-slip basins and uplifts and development on different parts of the fault zone, the timescales being from 10s to 100s of years.
Strike-slip duplexes, fans and flower structures. 'A strike-slip duplex is an imbricate array of two or more fault-bounded blocks that occur between two or more large bounding faults' (Woodcock & Fischer, 1986). While being analogous to the duplexes forming on the ramps of dip-slip faults, these structures differ by vertical movements not being constrained at the upper, ground, surface. Usually lens-shaped basins of the duplex are bounded by faults. When the faults converge the individual blocks, defined by strike-slip faults, are shortened and uplifted and where the faults diverge they are stretched and downthrown. In plane view, a characteristic braided pattern is formed by the tendency of strike-slip faults to converge or diverge. The predominant faults are those that follow most closely the direction of movement of the plate, grow longer and dip at near vertical angles. When faults are at an angle to the overall plate movement direction they may rotate even further out of alignment, developing dips that are significantly less than vertical, the fault involving a component of dip-slip movement. A normal oblique-slip faults develops if it carried to an extension region by the curvature of the fault. A reversed oblique slip-fault forms if it is carried to a compression region. Another common occurrence is significant rotation about vertical or near vertical axes. Displacements may be dissipated along along arrays of curved faults, linking to the main fault, forming fans or horsetail splays, at the ends of large strike-slip faults. The geometry of the curvature and motion sense on the main fault, either contractional or extensional deformation, being recorded by these structures.
A flower structure can be produced when there is convergence downwards at depth of the various splays of a strike-slip fault zone to produce a characteristic geometry in profile (Harding, 1985Christie-Black & Biddle, 1985). Synform or surface depression above upward-branching faults of mostly normal offsets form negative flower structures. Positive flower structures are formed where mostly reverse offsets are displayed by upward-branching faults beneath an antiform or surface culmination. The southern Dagg Basin illustrates fault geometry forming a positive flower structure.
Strike-slip partitioning in transpression and transtension. Displacements between boundaries of blocks and plates that converge or diverge obliquely can be distributed in several ways. Simultaneous movement on strike-slip and contractional or extensional structures, that are separate, is a common way of displacement distribution. The component of oblique convergence or divergence paralleling the plate boundary is accommodated by strike-slip faults, and the structures that are contractional or extensional accommodate the component orthogonally oriented to the plate boundary. Strike-slip partitioned systems are those in which strike-slip and dip-slip movement occur on separate structures and in different paces. In other cases both components of the deformation, strike-slip and margin-perpendicular, may occur on the same structure, as is illustrated on the central oblique-slip section of the Alpine Fault in New Zealand, or distribution of both may be more or less uniform across a zone. Further classification of deformation, strike-slip and margin-perpendicular, into those dominated by strike-slip and systems that are dominated by thrust or normal faults.
A transpression of strike-slip partitioned style is seen in the southern segment of the Alpine Fault. The fault is at a low angle, 11-25o, to the azimuth of the motion of the Australian-Pacific Plates (Barnes et al., 2005). Along the active trace of the Alpine Fault this results in strike-slip movement that is almost pure, being almost vertical in this area. Structures to the west and east of the Alpine Fault accommodate the contractional deformation component resulting from oblique plate convergence.
The San Andreas Fault is at about 35o to the relative motion direction between the pacific plate and the North American Plate in the San Gabriel Mountains. Reverse faulting and folding to the north of the Los Angeles Basin, in the mountains, accommodates the contraction component resulting from the oblique angle. The San Andreas Fault, as well as a series of other steep west-northwest-trending faults accommodate the oblique strike-slip movement resulting from the oblique angle.
Along the central segment of the Alpine Faults on the New Zealand South Island there is an example of transpressional deformation of a very weakly partitioned or non-partitioned style. At this point the alpine Fault strikes to the northeast, 55o, dipping moderately to the southeast. It has been shown that the slip on the central segment is oblique, which differs from the Fiordland segment, approximately paralleling the interplate vector. Reverse faults that are approximately parallel to the Alpine Fault are present at the eastern and western deformation limits on the South Island, for which relatively low rates and minor strike-slip movement components are inferred (Norris & Cooper, 2001Sutherland et al., 2006). Based on these characteristics it is suggested that the central segment of the Alpine Fault may be weakly partitioned, but appears to be non-partitioned in some parts.
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