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Using seismically-observed basement structure as an offset marker Cumulative offset of the San Andreas fault in central California: A seismic approach Justin Revenaugh* Colin Reasoner Institute of Tectonics, Earth Sciences, University of California, Santa Cruz, California 95064 ABSTRACT Scattered-wave imaging of upper crustal heterogeneity along nearly 500 km of the San Andreas fault in central California is used to estimate cumulative offset of basement rocks in the fault zone. Optimal cross-fault realignment of scattering patterns is achieved through removal of ~315 km of right-lateral slip. This value agrees with most previous estimates of early Miocene displacement, placing the initiation of movement on the San Andreas no earlier than ca. 23.1 Ma. Scattering along the fault correlates with segment boundaries established on the basis of historic and paleo seismicity, corroborating evidence from southern California that the upper crustal structures responsible for scattering are important in seismogenesis. INTRODUCTION Beginning with its recognition as a major fault in the aftermath of the great earthquake of 1906, study of offset along the San Andreas fault has mirrored the development of modern tectonic theory peppered with innovation, and major revisions of thought and controversy, often of a persistent nature. Is displacement strike slip or dip slip? Is it measured in kilometres, tens of kilometres or hundreds of kilometres? When did motion begin? Has it been steady through the fault s lifetime? How has the fault s geometry changed? That these questions have endured is a testament to the difficulty of measuring fault offset. Marker formations hundreds of kilometres apart must be identified, mapped, and correlated; piercing points must be extrapolated from irregular contacts and uncertain contours; and, ages must be determined. In central California the San Andreas fault is well exposed, linear at the scale of Figure 1A and compared to southern California part of a simple right-lateral strain system. Displacement since the early Miocene is determined to be 300—320 km, on the basis of cross-fault correlation of the Pinnacles and Neenach volcanic formations (Matthews, 1976), fan deposits of the Temblor Formation and the Vaqueros Sandstone of the Santa Cruz Mountains (Graham et al., 1989), and paleoshorelines recorded in the northernmost Gabilan Ranges and San Emigdio Mountains at Pleito Hills (Huffman et al., 1973). an estimate of San Andreas basement offset in central California is still needed to delimit initiation of movement on the San Andreas (Stanley, 1987; Graham et al., 1989), constrain slip on the San Gregorio—Hosgri and Rinconada-Reliz fault systems (e.g., Graham and Dickinson, 1978), and to provide an important boundary condition on total slip in southern California (e.g., Powell, 1993; Matti and Morton, 1993). METHOD Revenaugh (1995a; 1995b) and Revenaugh and Mendoza (1996) document a migration algorithm capable of mapping crustal scattering variability using teleseismic earthquakes recorded by a regional seismic network. Unlike tomography that maps velocity perturbations, the method maps variations in a non-dimensional indicator of scattering intensity. Specifically it estimates the local significance of scattering, or scattering potential. Over the scale length of the migration operator (~60 km), the estimator is uniform, such that closely spaced scatterers have potentials proportional to their relative scattering strengths. Scattering is the product of elastic heterogeneity, in particular, abrupt variation in shear and compressional wave velocity and density. Metre-scale and larger cracks and faults appear to play a prominent role in crustal scattering (Revenaugh, 1995b; Aki, 1995), but other structures, such as intrusive contacts and tight folds, are important also. the scattered-wave image is a vertical average of scattering intensity within the upper ~15 km of crust, the offsets are substantially for basement rocks and should be cumulative. Figure 1B displays teleseismic P to S scattering potential in the San Andreas fault zone of central California. The image was derived from analysis of 8295 seismograms of 215 earthquakes recorded by the Northern California and Southern California Seismic Networks between 1980 and 1994. Along the San Andreas, station coverage is good, but in places the coverage approximates a linear array resulting in some offfault circular blurring of the scattering image. Nonetheless, typical resolution is sufficient to distinctly image scatterer volumes separated by as little as 10 km. We find little evidence of the San Andreas fault in scattering, much as observed for the San Jacinto fault and the San Gorgonio Pass stretch of the San Andreas fault system in southern California (Revenaugh, 1995b). The central creeping zone and adjacent Parkfield segments are associated with slightly elevated mean scattering levels, but are not otherwise apparent. In interpreting this, it must be remembered that the relation of scattering potential to absolute scatterer strength is local. High potential scatterers are locally strong, but in regions of low overall scattering, a high potential scatterer need not be strong in a global sense. A consequence of this behavior is that any long expanse of fault associated with high scattering strength will be marked by scattering potential highs at the ends rather than a continuous high. Thus if scattering is strong throughout the creeping zone it is unlikely that we would image it as an extended zone of high scattering potential. What we image are the shorter wavelength (≤30 km) variations. Profiles of scattering potential along the southwest and northeast sides of the San Andreas fault were obtained by averaging scattering potential within boxes measuring 15 km in the fault-normal Using seismically-observed basement structure as an offset marker Method: observe scattering potential profile along both sides of SAF, then autocorrelate profiles (that is, line them up optimally). • Uses teleseismic P to S wave scattering. • Scattering potential—high if there are many variations of seismic velocity in a small area. • NOT tomography. This is more like migration, for those of you who know/care. Figure 1. A: Physiographic feature map of central California. BP = Big Pine fault; BT = Butano fault; CV = Calaveras fault; GL = Garlock fault; HW = Hayward fault; OR = Ortigalita fault; PC = Pilarcitos fault; PL = Pleito fault; RR = Rinconanda-Reliz fault system; SCO = South Cuyama-Ozena fault system; SG/H = San Gregorio–Hosgri fault system; SJ = San Juan fault; WW = White Wolf fault; ZVG = ZayanteVergeles fault. Triangles are stations of the Southern and Northern California Seismic Networks. B: Depth slice at 10 km of the P to S (teleseismic P scattered into upgoing S) scattered-wave image obtained from ~8300 recordings of 215 teleseisms representing geographic variation of scattering potential in upper crust. Scattering potential near unity (light yellow) implies locally strong scattering within the 0.02° by 0.02° cell, whereas zero (light blue) implies very little or no reradiated energy. White denotes regions of poor azimuthal station coverage or fewer than 200 contributing seismograms. Dashed green line follows center of simplified fault zone used to compute scattering profiles; bars alternate colors at 50 km intervals. Black dots are epicenters of ML ≥ 2 seismicity since 1969. at the southeastern and northwestern ends of the mapped fault zone and linearly interpolating offset between them. We tested right-lateral offsets between 0 and 400 km at 2 km intervals, limiting the increase (or decrease) of offset along the fault to 200 km to avoid excessive profile stretch. The preferred model, i.e., the model yielding the greatest correlation coefficient (r = 0.65), has a mean offset of 315 km; displacement decreases from 320 km at the southernmost extent of the study area to 311 at the northern limit of overlap (Figure 2A). Before bounding the uncertainty of this estimate, we first document its significance since many offset models were tried and the possibility of spurious correlation is real. Although removal of a running mean largely eliminated autocorrelation peaks at large lag distances, the profiles remain autocorrelated at short lags, such that there are fewer degrees of freedom than scattering potential pairs in the correlation. To estimate the former, we divided the length of offset profile overlap by 6 km the variogram range (e.g., Journel and Huijbregts, 1978). By this method, the preferred offset model has 28¡ of freedom and a correlation significance in excess of 99.99% (<1 in 10 000 chance of occurrence in uncorrelated data). By comparison, the secondhighest correlation peak (r = 0.45; mean offset of 279 km) attains 98.8% significance while correlation at zero offset (r = 0.26) falls just shy of 98% (Fig. 2B). Although both are highly significant, they are more than two orders of magnitude more likely to occur by chance than the peak correlation and are entirely in keeping with the number of independent offset models tested. We conclude that if the peak correlation is chance, it is a rare chance. 124 GEOLOGY, February 1997 Using seismically-observed basement structure as an offset marker Figure 2. A: Comparison of southwest and northeast faultblock scattering potential profiles following removal of ~315 km of right-lateral offset, corresponding to peak correlation model. Profiles are obtained by averaging scattering potential in 15-km-wide by 2-km-long boxes on either side of fault. Offset increases towards the southeast (dotted line; right side axis), varying by ~10 km over a distance of 170 km. Along-fault distance is appropriate for northeastern fault block (southwestern block is shifted). B: Correlation significance for all offset models tested. Models within 2σ of peak significance are shaded at one-half standard-deviation intervals. Offset is made to vary linearly along fault zone by interpolating between values specified at the southeast and northwest ends of sampling. Contour lines indicate mean offset (km) within overlapping region. They curve because overlap is shorter than sampled fault length. Peak correlation is obtained for 294 and 320 km offset at northwest and southeast ends, respectively (black dot; model in A). Offset differentials >200 km require unrealistic extension or contraction of fault blocks and are not shown. Results: • Possible mean offset of 306 km to 319 km • The correlation has less than 1/10,000 chance of occuring in random data. Offset Cretaceous mafic rocks Gabbroic rocks at Eagle Rest Peak, Gold Hill, and Logan are similar. 124 o o 120 o 42 K/Ar dates (state-of-the-art methods for 1970) Cape Mendocino Logan Gold Hill Eagle Rest Peak 156± 8, 172± ? Ma 144± 7 Ma 134± 4, 165± 4, 207± 10 Ma Offsets LOGAN Point Reyes 38 o Logan-Gold Hill Logan-ERP n Sa s ea dr An ul Fa GOLD HILL EAGLE REST PEAK t a G ault kF c rlo 34 Los Angeles N 100 km o 160 km 320 km Provenance of Cretaceous mafic clasts Conglomerate and sandstone at Gualala appears to be composed of clasts of Logan/Gold Hill/Eagle Rest Peak rock. Implication: 560 km of offset 124 o o 120 o 42 Cape Mendocino GUALALA LOGAN Point Reyes 38 o n Sa s ea dr An t ul Fa GOLD HILL EAGLE REST PEAK a G ault kF c rlo 34 Los Angeles N 100 km o Pinnacles-Neenach • The most famous, most convincing offset marker. • Early Miocene (23.5 Ma) volcanic sequence. • Ten rock types found in the sequence. 124 o o 120 o 42 Cape Mendocino Field, petrographic, and geochemical characteristics are identical at the two sites. Point Reyes 38 o PINNACLES n Sa s ea dr An o ul Fa 116 t a G lt Fau ck o rl SAN JOAQUIN BASIN NEENACH Los Angeles N 100 km 34 o Pinnacles-Neenach Cross section sandstones, shales, and interbedded basalts andesite, rhyolite, dacite Rock type La Honda basin: Santa Cruz Mountains San Joaquin basin: Temblor Range Neenach: Los Angeles Co. near Gorman Pinnacles: Pinnacles National Monument, Highway 146, San Benito Co. General location of outcrops quartz-rich gabbro, anorthosite 23.5 Ma Age Gualala conglomerate (located between Fort Ross and Point Area) contains clasts that may be derived from the Eagle Rest Peak - Gold Hill - Logan Gabbro; if this is true, it records ~560 km of displacement. ~150 Ma ~320 km ~210-280 km ~315-320 km ~315 km Displacement Ross, 1970 Ross et al., 1973 Crowell, 1962 Haxel and Dillon, 1978 Jacobsen et al., 2002 Jacobsen and Sorensen, 1986 Graham et al., 1989 Critelli and Nilsen, 2000 Matthews, 1976 Selected references The Pelona - Orocopia Schist is correlated to the Rand Schist and Catalina Schist and other rocks located in California and Arizona. A total of 15 schist/graywacke/gabbro bodies believed to be derived from the same source are displaced by faults in California. Paleocene or late Cretaceous metamorphism; uplifted and exhumed by 23-22 Ma Oligocene - Miocene Extra goodies Logan: Coast Ranges, near Vergeles fault Gold Hill: Coast Ranges near the town of Cholane Eagle Rest Peak: San Emigdio Mountains Butano - Point of Rocks Sandstone is a similar unit (the same?) as the displaced sub-marine fans in the San Joaquin and La Honda Basins and records ~ 300 km of displacement. Eagle Rest Peak - Gold Hill - Logan Gabbro Pelona Schist: San Gabriel Mountains Pelona - Orocopia Schist pelitic schist, metagraywacke and gneiss at greenschist Orocopia Schist: Mecca Hills (state to amphibolite facies highway 195), Gavilan Hills San Joaquin - La Honda basin sub-marine fans Pinnacles - Neenach volcanics Name Selected geologic markers of offset on the San Andreas fault