Crustal transpressional fault geometry influenced by viscous lower crustal flow
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Abstract The San Andreas fault (California, USA) is near vertical at shallow (<10 km) depth. Geophysical surveys along the San Andreas fault reveal that, at depths of 10–20 km, it dips ~50–70° to the southwest near the Western Transverse Ranges and dips northeast in the San Gorgonio region. We investigate the possible origin of along-strike geometric variations of the fault using a three-dimensional thermomechanical model. For two blocks separated by transpressional faults, our model shows that viscous lower crustal material moves from the high-viscosity block into the low-viscosity block. Fault plane-normal flow in the viscous lower crust rotates the fault plane due to the simple shear flow at the brittle-ductile transition depth. This occurs irrespective of initial fault dip direction. Rheological variations used to model the lower crust of Southern California are verified by independent observations. Block extrusion due to lower crustal viscosity variation facilitates the formation of the Garlock Fault and sustains the geometric complexity of the fault.Keywords:
Brittleness
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Normal fault
The Teton normal fault crops out along the eastern base of the Teton Range and relative motion across this fault has both uplifted the Teton Range and down-dropped Jackson Hole. On surface maps the normal fault appears to lie across older Laramide faults at a high angle, thus suggesting that previous structures had little to do with the position of the normal fault. Therefore, this field study was undertaken to test the following question: have preexisting Laramide or basement structures affected the position and/or geometry of the Teton normal fault? This question becomes important when considering the potential for contemporary earthquakes along the Teton normal fault and understanding the geologic environment of these earthquakes.
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In one of the largest oil-gas fields in Daqing, China, the anticlines are important structures that hold natural gas. The origin of the symmetric anticlines, which have bends on both the limbs, remains under debate. This is especially true in the case of the anticline in Xujiaweizi (XJWZ), which has recently been the focus of gas exploration. A compressive force introduced by a ramp/flat fault was suggested as its origin of formation; however, this is inconsistent with the reconstruction of the regional stress fields, which show an extensive environment. An alternative explanation suggests a normal fault-related fold under extensive stress. However, this mechanism has difficulty explaining the very localized, rather than wide-spread, development of the anticline along the proposed controlling normal fault. The well-developed bends on both limbs of the anticline are also very different from the typical roll-over anticline. Here, we conduct an experimental study showing that the very localized development of the bent-on-both-limbs anticline is controlled by the geometry of the underlying fault-plane. A ramp/flat fault plane can introduce an anticline with bends on both limbs, while a smooth fault plane will develop a roll-over anticline with a bend on only one limb.
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Five distinct faults described by various earlier workers were recognized as parts of one great overthrust fault in the field seasons of 1924 and 1925. This fault extends from Kootenay River in Canada to Clark Fork River in Montana, a distance of 118 miles. It has been named the Moyie-Lenia overthrust fault. The fault plane dips west. The vertical stratigraphic interval may be more than 45,000 feet at some places, but it may be as low as 15,000 feet at others. There is no evidence that the overthrust displacement is greater than the vertical displacement. The age of the overthrust was probably post-Jurassic and pre-Eocene, and it it is believed to be part of the great overthrust system known to exist farther south.
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Repeated first‐order leveling surveys conducted by the National Geodetic Survey (NGS) in 1972, 1974, 1976, 1978, and 1981 provide evidence of contemporary relative uplift near the junction of the San Andreas fault and the Brawley Seismic Zone. Uplift, which extends over a distance of about 5 km where crossed by the leveling line, apparently developed progressively between 1972 (possibly before) and 1978. Maximum relative uplift during this period reached 58 ± 4 mm. This spatially and temporally coherent pattern of uplift was interrupted between 1978 and 1981 possibly as a result of the 1979, M6.6 Imperial Valley earthquake. While the cause of the observed uplift is unknown, given the location, one interpretation is that it represents a zone of concentrated strain possibly associated with fault activity.
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[1] Rolandone et al. [2008] report estimates of the strain rate adjacent to the creeping segment of the San Andreas Fault in central California. Some of their strain-rate estimates are based on monument velocities inferred from short runs of data (e. g., 2003.4–2004.8) that include coseismic corrections for the 2003.975 San Simeon and 2004.744 Parkfield earthquakes. This comment uses only the longer runs of their data for an alternative analysis that leads to strain rate estimates significantly closer to zero than they propose. In this comment tensor, not engineering, strain is used, extension is reckoned positive, and the uncertainties quoted in the text and table are standard deviations. [3] Other estimates of strain rate on the blocks on either side of the creeping segment of the San Andreas Fault are shown in Table 1. The second entry represents the strain rates deduced (see auxiliary material) from the changes in distances between monuments observed in the 1982.82 EDM (electromagnetic distance measurement) survey of the Benito network [Sauber et al., 1989] and the subsequent 1998.88 GPS survey. Figure S1 (auxiliary material) shows that the EDM strain in Table 1 does not fit the observed changes in distance very well. There are also GPS data over an interval of 5.5 or more years [Rolandone et al., 2008, Table S3] for 6 monuments (CHLN, SHAD, SWTR, 0508, 0510, and 05TG; see Figure 1) on the southwestern (SW) fault block. I have used the velocities at those monuments in (1) to estimate the strain rates on the SW block (fourth entry in Table 1). [4] I have also calculated the strain rates expected solely from steady-state slip (creep) on the San Andreas and Calaveras faults. I used essentially the same slip model (see auxiliary material) as proposed by Rolandone et al. [2008, Table S6] to calculate the velocities predicted by a dislocation model of the slip distribution at each of the monuments used in the GPS solutions (all of the monuments in Figure 1 except BITT and BONT). [5] A comparison of the three essentially independent measurements (GPS and EDM for the Benito network and GPS for the SW block) of strain rate along the San Andreas Fault in Table 1 indicates general agreement among the estimates. There is acceptable agreement between the three independent measurements of each of the strain rate components (ɛxx, ɛxy, and ɛyy) in Table 1, although the agreement between the GPS and EDM measurements of ɛxx in the Benito network is only marginal. Moreover, except for the EDM estimate of ɛxx in the Benito network, the observed values of strain rate are in reasonable agreement with the estimates from the dislocation model. I regard the GPS measurement in the Benito network (first entry in Table 1) as the most reliable estimate of strain accumulation along the creeping section of the San Andreas Fault. Figure S2 (auxiliary material) shows the fit of strain rates ɛxx in Table 1 to the GPS observed values of fault-normal displacement. [6] The azimuth of the maximum contraction rate is directed N05°E ± 16° and N21°E ± 9° for the GPS and EDM measurements in the Benito network and N06°E ± 20° for the GPS measurements on the SW block (Table 1). The azimuth of principal contraction rate is directed ∼N08°E ± 6° for the dislocation models in Table 1. Sauber et al. [1989] found the azimuth of the maximum contraction rate in the Benito network over the 1962–1982 interval was N16°E ± 14°. These data suggest that the axis of maximum contraction rate makes an angle of about 51° ± 4° with the strike (N41°W) of the San Andreas Fault. One might expect that the azimuth of the maximum contraction rate would be the same as the azimuth of maximum compression (i.e., stress and strain rate are coaxial). However, Provost and Houston [2001, Figure 8] report that the axis of principal compression makes an angle of about 82° ± 8° with the strike of the San Andreas Fault near the Benito network. [7] The velocity of the Sierra Nevada-Great Valley microplate relative to the Pacific plate implies a normal convergence rate across the creeping section of the San Andreas Fault of 3.2 ± 0.7 mm/a (profile DD′ in Table 2 of Argus and Gordon [2001]). This convergence presumably is taken up by uplift of the Coast Ranges, which run along side of the fault at this latitude. If the breadth of the Coast Ranges at the latitude of the Benito network is taken as 100 km (profile DD′ in Figure 5a of Argus and Gordon [2001]), the average fault-normal contraction rate across the Coast Range would be 32 ± 7 nstrain/a, a value consistent with the estimates in Table 1. See Argus and Gordon [2001] for a more detailed discussion of the relation of this convergence to uplift. [8] Whereas Rolandone et al. [2008] solved for strain accumulation rates along the entire length of the 170-km-long creeping section of the San Andreas Fault, I have considered only the central 60-km-long segment (Benito network) of the creeping section. In this way I have avoided the more complicated deformation at the ends (near Parkfield on the south and San Juan Bautista on the north) of the creeping section and taken advantage of the better data available in the Benito network. The best estimate of strain and rotation rates within the Benito network is given by the first entry in Table 1. For the Benito network the right-lateral, shear strain rate across vertical planes parallel to the San Andreas Fault was 21 ± 12 nstrain/a whereas for the larger area Rolandone et al. [2008] found that the right-lateral shear strain rates were <83 ± 10 nstrain/a. For the Benito network the fault-normal, extension rate ɛxx was −15 ± 20 nstrain/yr. If the extension rate ɛxx (3 ± 4 nstrain/a) predicted by the dislocation model is subtracted from that extension rate, the resulting, residual, fault-normal contraction rate is 18 ± 20 nstrain/a, which can be compared to the residual, fault-normal contraction rate (85 ± 13 nstrain/a) found by Rolandone et al. [2008, Figure S3] near the center of the creeping section of the San Andreas Fault (i.e., within the San Benito network). Notice that Rolandone et al. [2008, Figure S3] find a much lower value (17 ± 12 nstrain/a) for the residual, fault-normal contraction rate on the SW fault block than on the NW fault block. 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Abstract The Galaxidi earthquake that occurred in the Gulf of Corinth on 18 November 1992 was not followed by a noticeable aftershock sequence, a fact that was also observed for the 1965 Eratini event in the same area. The temporary network of 35 stations that we installed 5 days after the mainshock did not help to identify a cluster of activity related to the mainshock. In a section across the epicentral zone, the focal mechanism of the mainshock and the distribution of a few aftershocks define a plane dipping north, consistent with the nearby Helike fault. We propose that the Galaxidi earthquake was related to an asperity located between the Helike and Xilokastro faults.
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Gold mineralisation in the Eastern Goldfields Province of the Yilgarn Block, Western Australia, is associated with shear zones within the greenstone supracrustal succession. Regional shear zones are imaged in seismic reflection sections as bands of strong reflections. Although individual wavelets within the bands of reflections can only be correlated over small distances, the bands of reflections can be correlated over tens of kilometres. The Bardoc Shear, adjacent to which considerable mineralisation has been found, dips west and links with the Ida Fault, which forms the boundary between the Eastern Goldfields Province and the Southern Cross Province farther west. The Ida Fault dips east at about 40°, and can be traced from the surface to about 27 km depth. Bands of reflections within the upper and middle crust have a similar seismic signature to the Ida Fault, Bardoc Shear Zone and the basal detachment of the greenstones, and are therefore Interpreted as shear zones. Interpreted shear zones in the upper crust under the greenstones mostly dip west. Shear zones in the lower crust dip east. The upper crust east of the Ida Fault and below the Bardoc Shear is an exception. There, east-dipping shear zones, including the Ida Fault, are interpreted to extend from the lower crust into the upper crust, thereby providing a simple plumbing system for mineralising fluids migrating from the lower crust, into the Bardoc Shear, and then to high levels in the greenstones.
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The April to June 1992 Landers earthquake sequence in southern California modified the state of stress along nearby segments of the San Andreas fault, causing a 50-kilometer segment of the fault to move significantly closer to failure where it passes through a compressional bend near San Gorgonio Pass. The decrease in compressive normal stress may also have reduced fluid pressures along that fault segment. As pressures are reequilibrated by diffusion, that fault segment should move closer to failure with time. That fault segment and another to the southeast probably have not ruptured in a great earthquake in about 300 years.
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