The San Andreas fault system comprises an interactive network of right- and left-lateral strike-slip faults and related reverse and normal faults. In southern and central California, right- and left-lateral faults of the San Andreas system transect a crystalline terrane of Proterozoic through Mesozoic igneous and metamorphic rocks overlapped by Upper Cretaceous through Eocene marine sedimentary strata and by Oligocene and lower Miocene terrestrial volcanic and sedimentary strata. Paleogeologic patterns in these rocks define regional terranes and local reference domains, the reassembly of which permits determination of overall displacement on the strike-slip faults that disrupt them. Timing of fault movement is recorded by incremental displacements of upper Cenozoic sedimentary deposits and by the sequence of fault movements required to effect reassembly of the reference domains. Reassembling the pre-late Cenozoic regional paleogeologic framework of southern and central California leads to a balanced palinspastic reconstruction of the San Andreas system that differs from previously published reconstructions, both conceptually and in terms of magnitude of displacement restored on many of the principal strike-slip faults. Four reference domains that constrain the balanced reconstruction consist of paleogeologic patterns reassembled from crystalline and sedimentary rocks now found: (1) in the Transverse Ranges in the Frazier Mountain-Mount Pinos area, the eastern Orocopia Mountains and vicinity, and the Sierra Pelona-northern San Gabriel Mountains area; (2) in the Salinian block in the La Panza Range and in the Transverse Ranges in the Liebre Mountain block and western San Bernardino Mountains; (3) in the Salinian block in the Gabilan Range, in the southern tail of the Sierra Nevada in the San Emigdio and Techachapi Mountains, and in the Portal Ridge area of the northwestern-most Mojave Desert; and (4) in the southern part of the Transverse Ranges west of the San Andreas fault, in the northern Peninsular Ranges, and in the southern Chocolate Mountains. Simultaneous reconstruction of the four paleogeologic reference domains specifies the magnitude and sequence of displacement on the major right- and left-lateral faults of the San Andreas system. The Clemens Well-Fenner-San Francisquito fault is the earliest (and now abandoned) strand of the San Andreas fault system in southern California. It formed a continuous structure with the early San Andreas fault zone of central California, is today cut by the San Andreas fault in the Transverse Ranges, and diverges southeastward from the San Andreas fault east of the Salton trough at least as far as the Little Chuckwalla Mountains. The Clemens Well-Fenner-San Francisquito-early San Andreas fault accumulated a displacement of about 100 or 110 km during the interval between 22 and 13 Ma and probably during the more restricted interval between 18 to 17 and 13 Ma. The disposition of displacement southeastward from the Transverse Ranges is problematic because the Clemens Well-Fenner-San Francisquito fault neither rejoins the modern San Andreas fault in the Salton trough nor extends indefinitely into the continent. Hypothetically, the southeastward extension of the fault is absorbed by coeval sinistral kinking along the southern margins of the Transverse Ranges and Chocolate Mountains and/or by synchronous detachment faulting in southeasternmost California and southwesternmost Arizona. A zone of sinistral deformation trends roughly east-west along the southern boundary of the Transverse Ranges province. The earliest expression of this deformation is sinistral kinking that began to develop after 22 to 20 Ma and prior to 17 Ma. During the interval from 17 to 13 Ma, the zone of sinistral deformation was characterized by widespread volcanism and perhaps by faulting associated with left-oblique extension. Left-lateral displacement of about 40 to 45 km across this zone during this interval is attributed here to sinistral kinking. The zone of sinistral kinking and faulting apparently initiated in a right step between the southeastern terminus of the Clemens Well-Fenner-San Francisquito fault and the northwestern terminus of dextral faults such as the East Santa Cruz basin fault that were active coevally in the continental borderland. Subsequently, the zone served as the southern terminus of right-lateral strands of the San Gabriel fault system. Still later, it served as the northern terminus of the Elsinore and San Jacinto fault zones. In the Transverse Ranges, the San Gabriel fault system, including, from oldest to youngest, the Canton, San Gabriel, and Vasquez Creek faults, began to splay southward from the older Clemens Well-Fenner-San Francisquito-early San Andreas fault as early as 12 to 13 Ma. The San Gabriel fault system has since accumulated a displacement of 42 km, deforming the older Clemens Well-Fenner-San Francisquito-early San Andreas fault in the process. The Canton fault, active between 13 and 10 Ma, accumulated a displacement of 15 to 17 km; the San Gabriel fault, active between 10 and 5 Ma, accumulated a displacement of 22 to 23 km; and the Vasquez Creek fault, active between 6 Ma and the present, accumulated a displacement of no more than about 5 km. Displacement on these faults merged northwestward with that of the early San Andreas fault north of the Transverse Ranges, whereas movement ceased on the Clemens Well-Fenner-San Francisquito fault. In the Salinian block, the San Gregorio-Hosgri, Rinconada-Reliz, and Red Hills-Ozena faults developed coevally with the San Gabriel fault and also merged northwestward with the early San Andreas fault. Displacements of 45 km on the Rinconada-Reliz fault and 105 km on the San Gregorio-Hosgri fault south of Monterey Bay merge on the San Gregorio fault north of the bay for total of 150 km. Hypothetically, displacement on the San Gregorio fault is split between 70 km (after 6 to 6.5 Ma) west of the Montara Mountain block and 80 km (prior to 6 to 6.5 Ma) east of that block, so as not to leave the Montara Mountain block dangling north of the restored Salinian block. Displacement on the San Gregorio fault is transferred northward onto the San Andreas fault, thereby increasing its overall displacement to about 440 km north of the junction of the two faults. None of the displacements on the Salinian block faults and no more than 22 or 23 km of slip on the San Gabriel fault have been shown to remerge southeastward with the Clemens-Well-Fenner-San Francisquito-early San Andreas fault. Hypothetically, these displacements stepped west to the continental margin across the western Transverse Ranges, where left-oblique extensional faulting continued through the late Miocene into the early Pliocene. Along the southern boundary of the Transverse Ranges west of the San Gabriel Mountains, the ancestral Malibu Coast-Santa Monica fault system accumulated a sinistral component of displacement of as much as 35 km in addition to the earlier sinistral kinking, whereas to the east, the ancestral Raymond-Cucamonga-Banning fault system accumulated no more than about 10 to 20 km. The modern San Andreas fault emerged about 5 Ma. In central California, it coincides with the pre-5-Ma San Andreas fault, whereas in southern California it diverged from the older Clemens Well-Fenner-San Francisquito fault and actually crosscuts the older fault to merge southeastward with the Salton trough at the north end of the Gulf of California. Displacement on the post-5-Ma San Andreas fault varies along the fault because the crustal blocks adjoining the fault are deformed by coeval strike-slip faults, including right-lateral faults such as the San Jacinto, Calaveras-Hayward-Rodgers Creek-Maacama-Garberville, and San Gregorio-Hosgri faults, and left-lateral faults such as the Garlock fault and the east- to northeast-trending faults of the Transverse Ranges. Displacement restored on the modern San Andreas fault as measured along the present trace ranges from about 160 to 185 km. Simultaneous palinspastic reconstruction of the four reference domains is possible only in conjunction with restoration of slip along a zig-zag system of secondary strike-slip faults of the San Andreas system that distort the crustal blocks adjoining the modern San Andreas fault in southern California. This system includes right-lateral fault s in the Peninsular Ranges, Mojave Desert, and Death Valley area, and left-lateral faults in the Transverse Ranges and between the Sierra Nevada and Mojave Desert. Overall displacements restored on these secondary faults are generally well constrained by offsets of crystalline rocks and overlying Cenozoic strata. In the Peninsular Ranges, dextral displacement restored on the San Jacinto fault is 28 km, and that restored on the Elsinore fault is 5 km. About 10 km of Pliocene and Quaternary left slip is restored on faults along the southern boundary of the Transverse Ranges, where earlier left-oblique extensional faulting was overprinted by reverse and left-oblique reverse faulting by late Pliocene and Quaternary time. In the eastern Transverse Ranges, sinistral displacement restored on the major east-trending faults includes 16 km on the Pinto Mountain fault, 5 km on the Blue Cut fault, 11 km on the Chiriaco fault, and 8 km on the Salton Creek fault. In the Mojave Desert, dextral displacements restored on northwest-trending faults include 3 km on the Helendale fault, 3 km on the Lockhart-Lenwood fault, 4 km on the Harper-Harper Lake-Camp Rock-Emerson fault, 9 km on the Blackwater-Calico-Mesquite Lake fault, and 16 km on the Pisgah-Bullion fault. Although displacement on the central and eastern parts of the Garlock fault is well documented to be about 60 km, displacement at the western end, as limited by reassembly of the Gabilan Range-San Emigdio Mountains-Portal Ridge reference domain, can be no greater than 12 km.

Distinctive porphyritic bodies of alkalic monzogranite and quartz monzonite of Triassic age that occur in the Mill Creek region of the San Bernardino Mountains and on the opposite side of the San Andreas fault at the northwest end of Liebre Mountain appear to be segments of a formerly continuous pluton that has been severed by the fault and displaced about 160 km. Reassembly of the megaporphyritic bodies by restoration of sequential right-lateral displacements on various strands of the San Andreas and San Gabriel fault zones leads to a palinspastic reconstruction for southern California that reassembles crystalline and sedimentary terranes differently from widely cited reconstructions. Reassembled crystalline rocks establish three coherent patterns: (1) The reunited Liebre Mountain and Mill Creek Triassic megaporphyry bodies form western outliers of a province of Permian and Triassic alkalic granitoid rocks that occurs in the western Mojave Desert and San Bernardino Mountains. (2) Following sequential restorations of 160 km on the San Andreas fault and 44 km on the San Gabriel and Cajon Valley faults, the Table Mountain and Holcomb Ridge basement slices currently positioned along the west edge of the Mojave Desert near Wrightwood and Valyermo area reassembled with the Liebre Mountain and San Bernardino Mountains blocks. This restoration unites terranes of Mesozoic granitoid rock and Paleozoic(?) marble, metaquartzite, and pelitic gneiss that have strong lithologic and compositional similarities, and brings together within a single province quartz diorite and granodiorite that are hosts for three known bedrock occurrences of aluminous dike rocks (“polka-dot” granite) that previously have been used to assemble a different reconstruction for the San Andreas fault. (3) Crystal-line rocks of the San Gorgonio Pass region, including Triassic monzogranite and grano-diorite of the Lowe igneous pluton and Jurassic blastoporphyritic quartz monzonite, are juxtaposed adjacent to the southern Chocolate Mountains where similar rocks have been mapped. The reconstructed crystalline rocks provide a paleogeographic framework for Pliocene and late Miocene sedimentary basins now scattered along the San Gabriel and San Andreas faults. Ridge Basin is juxtaposed adjacent to the southwestern San Bernardino Mountains in an orogenic setting compatible with stratigraphic relations, depositional fabrics, and clast compositions in the Ridge Basin fill. Synorogenic sediments of the upper Miocene Ridge Route and Peace Valley Formations accumulated in this setting at a time (9 to 5 Ma) when the western San Bernardino Mountains were undergoing uplift and erosion. The Pliocene Hungry Valley Formation was deposited toward the end of this orogenic pulse (5 to 4 Ma) when early movements on the San Andreas fault began to displace Ridge Basin away from the San Bernardino Mountains. Other late Miocene basins widely dispersed today are reassembled within an early Salton Trough that included, from northwest to southeast, the Punchbowl and Mill Creek basins, the Coachella Fanglomerate and Hathaway Formation, and the fanglomerate of Bear Canyon in the southern Chocolate Mountains. The new reconstruction allows only about 205 km of displacement on the combined San Andreas and San Gabriel fault zones, which is about 110 km short of the 315 ± 10 km post-early Miocene offsets documented for the San Andreas in central California. This shortfall requires the existence of another Miocene strand of the San Andreas fault system in southern California. The Clemens Well-Fenner-San Francisquito fault system of Powell (this volume) provides this additional fault strand. Together, the Clemens Well, San Gabriel, and San Andreas faults generated about 315 ± 10 km of right slip comparable to displacements documented for the San Andreas fault in central California. The displacements in southern California have occurred in middle Miocene through Holocene time, and extend the history of late Cenozoic right-lateral faultin g farther back into the Miocene than envisioned by traditional reconstructions for the San Andreas fault system in southern California.

It is generally accepted that the San Andreas fault formed between 4 and 5 Ma and that rocks west of it are now part of the Pacific plate, moving northwest relative to North America at 5 to 6 cm/yr. This model is inconsistent with the geologic record in the central Transverse Ranges. Right-lateral shear began in the vicinity of the San Andreas fault system in early Miocene time. The San Andreas fault system in the central Transverse Ranges has since evolved through three major phases; this development has led to a generally simpler, more throughgoing main trace. Slip rates on the San Andreas system were about 1 cm/yr in the Miocene, increasing to their current level of 3.5 cm/yr between 4 and 5 Ma. The modern San Andreas fault still only accounts for just over half the current relative plate rate and retains kinematic complexities inherited from its earliest geometry. The Early San Andreas transform system originated during early Miocene time in one of three transtensive zones that lay interior to the continent and east of the locus of transform motion between the Pacific and North American plates. The current three-fold division of motion in the plate boundary between the San Andreas fault, a coastal system, and an eastern California system dates to this time, as does the “anomalous” trend of the San Andreas fault through the Transverse Ranges. Basins and volcanic centers associated with this transtensive zone became dismembered as faults became integrated into a throughgoing system. Early motion led to juxtaposition of different rocks across faults now recognized as part of the Early San Andreas transform system, and to the development of sedimentary provincialism associated with uplift along the fault zone. Middle Miocene basins, including the Caliente, Cajon, Crowder, and Santa Ana basins that had previously received most of their sediments from sources far to the east, began to reflect local Transverse Ranges provenance. At least 100 km of slip is associated with the Early San Andreas transform system during early and middle Miocene time. Slip across the geometrically complex late Miocene San Gabriel transform system—which includes the San Gabriel, Cajon Valley, and early Punchbowl faults—produced uplift in the proto-Transverse Ranges at a postulated restraining bend in the fault system. Compressional structures associated with this restraining bend include the Squaw Peak and Liebre Mountain thrusts, related east-striking late Miocene reverse faults and folds, and, perhaps, northeast-striking left-lateral faults in the San Gabriel Mountains. Narrow fault-controlled basins formed during this period, including the Ridge basin, Devil’s Punchbowl basin, Mill Creek basin, and part of the Santa Ana Sandstone basin. Offset of structures and relief associated with the proto-Transverse Ranges provides the best evidence for late Miocene restorations of the modern San Andreas fault. As much as 60 km of offset is associated with the late Miocene San Gabriel transform system. Between 4 and 5 Ma, the modern San Andreas fault became the dominant member of the plate boundary system, cutting through the proto-Transverse Ranges and connecting more northerly striking traces to the north and south. The slip rate across the San Andreas fault system accelerated from 1 cm/yr to its current slip rate of 3.5 cm/yr prior to 4 Ma. The Pliocene rocks in the central Transverse Ranges do not contain evidence for relief as great as that of late Miocene or Quaternary time. The Pliocene trace of the modern San Andreas fault may have temporarily “solved” the geometric problem that led to late Miocene uplift. About 90 km of right-lateral displacement occurred on the modern San Andreas fault during Pliocene time. During Quaternary time new regions of localized vertical deformation developed in the Transverse Ranges, apparently as the result of new geometric problems within the Pliocene solution to the restraining geometry of the fault system. Left-lateral motion on east-striking faults, probably due to a northward increase in Basin and Range extension, kinked the San Andreas fault at both ends of the Transverse Ranges, producing regions of extreme shortening and uplift. The development of young right-lateral faults through the Peninsular Ranges, including the San Jacinto and Elsinore faults, also contributed to renewed uplift in the Transverse Ranges. Sixty kilometers of right-lateral slip occurred across the San Andreas fault zone during Quaternary time.

The pre-Quaternary geology of the southern Chocolate and Cargo Muchacho mountains correlates with that exposed in San Gorgonio Pass between the Mission Creek and Banning branches of the San Andreas fault. Matching features include: (1) the southernmost exposures of the Triassic Mount Lowe Granodiorite on opposite sides of the San Andreas; (2) the presence in both areas of distinctive melanocratic quartz monzonite, dated as mid-Jurassic in the southern area; (3) the occurrence in both areas of kyanite schist formed by hydrothermal leaching of granitic and gneissic rocks; (4) the correlation of the Bear Canyon fanglomerate and associated late Miocene basalt flows in the southern area with the Coachella Fanglomerate and associated late Miocene andesite and basalt flows in the northern area; and (5) the correlation of the northern limit of the late Miocene to Pliocene marine Bouse Formation in the Yuma area east of the Salton Trough, with the northern limit of the late Miocene to Pliocene marine Imperial Formation adjacent to the Mission Creek fault in San Gorgonio Pass. These correlations require 185 ± 20 km of right slip on the Mission Creek-Coachella Valley segment of the San Andreas fault since late Miocene. An additional right slip of about 30 km has occurred in San Gorgonio Pass along the Banning-Coachella Valley segment of the San Andreas fault, as indicated by the displacement of the northern limit of the Imperial Formation. A total right slip of 90 ± 20 km is needed on the Banning fault to be consistent with other data presented by us, but the displacement cannot be demonstrated because basement terrane north of the Banning fault is unrelated to that south of the Banning fault. The Salton Creek fault, located east of the San Andreas fault between the Orocopia Mountains and the Chocolate Mountains, correlates with an unnamed fault in Soledad Pass, located west of the San Andreas at the west end of the San Gabriel Mountains. In both areas the correlative faults have volcanic necks along them, and separate Precambrian syenite and related rocks on the northwest from Triassic Mount Lowe Granodiorite overlain by late Oligocene to early Miocene volcanic rocks on the southeast. The displacement of these correlative faults is younger than about 10 Ma, since it postdates deposition of conglomerate within the middle to late Miocene Mint Canyon Formation in Soledad basin. The conglomerate contains clasts of unusual volcanic rocks whose only known source is in the northern Chocolate Mountains. This correlation requires 240 km of right slip on the San Andreas fault, including displacement contributed by the San Jacinto fault. The San Gabriel fault is an abandoned branch of the San Andreas fault located southwest of Soledad Pass. It probably connected to the Banning fault prior to development of the Coachella Valley segment of the San Andreas fault. The San Gabriel fault displaces middle Miocene and older rocks 60 km to the right. The addition of this displacement to the Soledad Pass-Salton Creek offset yields a total right slip of 300 km on the San Andreas fault in southern California since middle Miocene time. Our determination of the total offset on the San Andreas fault is consistent with that obtained by Matthews (1976) from the correlation of the Neenach Volcanic Formation, located east of the San Andreas fault in the central Transverse Ranges, with the Pinnacles Volcanic Formation, located west of the fault in the central Coast Ranges. However, there is a conflict between the evidence presented here for 240 km of right slip on the San Andreas fault between Salton Creek and Soledad Pass and the evidence presented by Frizzell and others (1986) and Matti (this volume) for 160 km of right slip on the San Andreas between the central San Bernardino Mountains and Liebre Mountain, based on their correlation of distinctive Triassic monzogranite in the two areas. This conflict needs to be resolved. Our data are consistent with the hypothesized origin of the San Andreas as a transform fault caused by crustal extension in the Gulf of California. It requires displacement to have started by about 10 Ma. Only about 60 km of right slip needs to have occurred prior to about 5 Ma; 240 Ma of right slip has probably occurred since then.

A variety of extensional and contractional structures is produced by strike slip faulting. The variety and extent of the structures are directly related to the kind and extent of geometric complexities of the fault zone or system. The area of convergence of the San Andreas fault zone and the much younger San Jacinto fault zone in the eastern Transverse Ranges is exquisitely complex. We propose that the San Jacinto fault zone formed in response to a structural knot in San Gorgonio Pass probably within the past 1.5 Ma. In the area of their convergence we propose that slip is transferred both east and west from the San Jacinto fault zone northward to the San Andreas fault zone over a 60-to 70-km band that extends northwestward from the south end of the San Bernardino basin to the east end of the San Gabriel Mountains. We further propose several structural adjustments as a consequence of onset or acceleration of lateral movement on the San Jacinto fault zone: accelerated uplift of the eastern San Gabriel Mountains, development or accentuation of an arcuate schuppen-like structure in the eastern San Gabriel Mountains, inception of the San Bernardino basin, cessation of deposition in the present-day San Timoteo badlands area, inception of the San Jacinto basin, and an increase in compression and uplift in the San Gorgonio Pass area. We interpret the uplift and compression in San Gorgonio Pass to result from two formerly disparate structural blocks—the eastern San Bernardino and San Jacinto blocks—becoming a relatively coherent block, and the San Gorgonio Pass area constituting a left step between the San Andreas fault zone in the Coachella Valley area and the San Jacinto fault zone in the San Jacinto Valley area. The compression and uplift led to the formation of the San Gorgonio Pass thrust faults and disruption of any through-going San Andreas strands, at least at the surface. In partitioning slip between the San Andreas and San Jacinto fault zones, consideration should be given to the bandwidth over which horizontal strain has accumulated. The average slip rate of the northern part of the San Jacinto fault zone during the past 1.5 m.y. may have been about 20 mm/yr and about 15 mm/yr on the San Andreas. South of the San Bernardino basin, current strain accumulation based on repeated geodetic surveys is nearly equally divided between the San Jacinto and San Andreas fault zones.

This chapter presents a synthesis of data pertaining to post-early Miocene slip on the San Andreas fault in central California and suggests a three-phase evolition of the San Andreas system. The cricial evidence that supports the three phases of evolution conies from the reversed positions of two exotic rock fragments in the vicinity of Parkfield, California. The three-phase evolution of the San Andreas is also supported by the correlation of other exotic fragments, the basement rocks on which they lie, overlying Tertiary stratigraphic sequences, and distinctive Miocene strata derived from these fragments during their transport along the fault. The 40-km-long section of the San Andreas fault near Parkfield is characterized by exotic blocks composed of Cretaceous hornblende quartz gabbro at Gold Hill and lower Miocene volcanic rocks in Lang Canyon. The gabbro is correlated petrographically with similar rocks near Eagle Rest Peak, 145 km to the southeast, and near Logan, 165 km to the northwest. The lower Miocene volcanic rocks, informally termed the volcanic rocks of Lang Canyon, are correlated with the Neenach Volcanics 220 km to the southeast and the Pinnacles Volcanics 95 km to the northwest. All three fragments of volcanic rocks are unconformably overlain by similar successions of Tertiary sedimentary rocks. The original positions of the bodies of gabbro and volcanic bodies and their overlying sedimentary cover may be reconstructed from these exotic fragments that now lie along the San Andreas fault between San Juan Bautista and the northwestern Mojave Desert. The original undeformed gabbroic body was composed of the hornblende quartz gabbro of Eagle Rest Peak, Gold Hill, and Logan. In its initial prefaulted position, the original gabbroic body lay about 55 km northwest of the early Miocene volcanic assemblage. The undeformed volcanic assemblage was composed of the Neenach Volcanics, Pinnacles Volcanics, and volcanic rocks of Lang Canyon. The original spatial relationship between the undeformed gabbro and volcanic assemblage and their sedimentary cover is preserved in the present position of the gabbro of Logan and the Pinnacles Volcanics. However, in the Parkfield segment of the San Andreas, the gabbro of Gold Hill lies east of the main trace of the San Andreas fault, and the volcanic rocks of Lang Canyon lie 2 km west of the fault. The reversed relative positions of the gabbro of Gold Hill and the volcanic rocks of Lang Canyon suggest a complex history of movement on the San Andreas fault. Consequently, plainspastic reconstruction of these bodies and their overlying sedimentary cover is constrained by the unusual distribution of exotic blocks near Parkfield. The resulting proposed history of movement is divided into three stages that begins with the eruption of the early Miocene volcanic rocks about 24 Ma. The Neenach-Pinnacles Volcanics, erupted after passage of the Mendocino triple junction, were soon cut by the growing San Andreas transform system. During the first phase of movement the Salinian block, which contains the Pinnacles and Logan godies, was detached from the Mojave and Sierran blocks. The Pinnacles and Logan bodies were transported about 95 km northwest from the Neenach Volcanics and the gabbro of Eagle Rest Peak. At the end of the first phase, the Logan and Pinnacles fragments lay adjacent to the west side of what is now the San Joaquin Valley. Concurrently, fan-deltas deposited debris that was derived from the Gabilan Range, the fan-deltas spread across the San Andreas fault into the middle Miocene sea in the San Joaquin trough. During the second phase of movement, the San Andreas—at least locally—stepped eastward and detached a second fragment from the Neenach Volcanics. This fragment consists of the volcanic rocks of Lang Canyon. Slip was transferred to the new trace of the San Andreas fault, and the older trace became completely or largely inactive. After transferral of slip to the new trace of the San Andreas fault, the volcanic rocks of Lang Canyon and the Pinnacles Volcanics remained about 95 km apart on the Salinian Block west of the San Andreas fault. During the third phase, the Gold Hill fragement was slivered off the Logan fragment and was tectonically emplaced on the east side of the San Andreas fault when the Logan fragment lay at the latitude of Gold Hill. The process of slivering off of the Gold Hill fragment was accomplished by deformation of the San Andreas in an eastward bend along what is now the Jack Ranch fault. Bending of the fault was stimulated by the presence of highly sheared Franciscan rocks that crop out near the San Andreas and extend to great depth. Eventually the San Andreas bent to such a degree that slip could not be conducted around the bend, and a new, stable, straight segment was formed. The straightening of the fault resulted in slivering of the Gold Hill fragment from the Logan fragment. After detachment of the Gold Hill fragment, the Salinian block containing the gabbro of Logan, the Pinnacles Volcanics, and the volcanic rocks of Lang Canyon was transported an additional 160 km northwest to its present position. This reconstruction honors the current positions of all the related exotic fragments of gabbro, volcanics, and sedimentary rocks. The timing of the sequence of movements required to reconstruct the original bodies suggests that the three phases of evolution of the San Andreas fault in central California are characterized by increasing slip rates. The rate for the first phase probably averaged about 10 mm/yr over a period of about 8 m.y. The rate for the second phase averaged about 8 mm/yr over a period of about 7 m.y. The rate rate for the third phase averaged about 33 mm/yr over a period of about 5 m.y.

The Eagle Rest Peak igneous complex in the San Emigdio Mountains, and Logan gabbro, near San Juan Bautista, California, crop out on opposite sides of the San Andreas fault. They have identical U-Pb zircon ages of 161 Ma and similar Sr initial isotopic ratios. These data support previous correlations of these rocks (Ross, 1970) and require 305 km of post-Jurassic slip on the northern San Andreas fault, a figure equal to the total slip on the southern segment. The Eagle Rest Peak complex has previously been proposed as the source for gabbro clasts in the Upper Cretaceous Gualala Formation near Point Arena (Ross, 1970; Ross and others, 1973). This implies >440 km of post-Late Cretaceous slip on the San Andreas fault and the existence of a proto-San Andreas fault. Also, granitic to quartz dioritic clasts in the Gualala Formation have been interpreted as detritus derived from the Salinian block in the Cretaceous. However, new U-Pb zircon data from Gualala Formation gabbroic clasts indicate minimum ages of 163 and 165 Ma, slightly older than the 161-Ma age of the Eagle Rest Peak complex and Logan gabbro. Published K-Ar ages also suggest the Gualala cobbles are older than the Eagle Rest Peak complex. These data and the presence of alternate sources for the Gualala cobbles indicate that the Gualala-Eagle Rest Peak tie is not suitable for determining slip on the San Andreas fault. Despite the differences between specific areas, the Eagle Rest Peak complex, Gold Hill and Logan gabbros, and the Gualala gabbro clasts are similar in age and lithology to mafic-ultramafic complexes that form a widespread part of the Jurassic Sierran-Klamath arc. Although Gualala gabbro clasts cannot be uniquely matched to the Eagle Rest Peak complex and Logan rocks, they probably are derived from similar rocks cropping out in the Sierra foothills or buried in the Great Valley. U-Pb-age, Sr, and Pb isotopic data from a single Gualala Formation granodiorite clast do not support a Salinian provenance. The clast is older than 154 Ma, older than the 80- to 120-Ma Salinian granites. The clast also has less radiogenic Pb and Sr isotopic ratios than plutonic rocks from the Salinian block. A reevaluation of paleocurrent data and clast types from the Gualala Formation also suggests a non-Salinian source. The Gualala area is apparently not part of the Salinian block.

Potassium-argon determinations (n = 19) and whole-rock trace-element analyses (n = 9) on volcanic rocks from the Plush Ranch, Vasquez, and Diligencia Formations located along the San Andreas fault system in southern California support earlier correlations that suggest palinspastic proximity of the transtensional basins in which the lavas erupted. Volcanic rocks of the Plush Ranch and Vasquez Formations, now located southwest of the San Andreas fault, were extruded contemporaneously at about 23.1 to 26.5 Ma and 23.6 to 25.6 Ma, respectively, whereas those of the Diligencia Formation, now located northeast of the fault, erupted about 20.6 to 23.6 Ma. The rocks yield trace-element and isotopic ratios that define a petrologic suite unique in southern California. These subalkaline and calc-alkaline rocks range from medium-potassium basalt to high-potassium dacite. Vasquez samples are generally higher in SiO 2 than are those from either the Plush Ranch or Diligencia Formations. Similar rare-earth-element patterns exhibit moderate light-REE and flat heavy-REE enrichment. Spider diagrams for selected incompatible major and trace elements have relatively tight and featureless patterns. Initial whole-rock 87 Sr/ 86 Sr ratios range from 0.7048 to 0.7062 and plagioclase δ 18 O ranges from 6.2 to 7.7; both values correlate positively with SiO 2 . These data indicate that the volcanic rocks were derived from similar magmas, probably incorporating different amounts of crustal component. Our data indicate that the volcanic rocks formed at only slightly different times, had similar petrogenetic histories, and have been separated along the San Andreas fault system. We cannot distinguish, however, either the magnitude or timing of movement on the various strands of the system in southern California that resulted in their disruption.

Well-lithified Tertiary sedimentary rocks crop out within the San Andreas fault zone south of the San Bernardino Mountains. At the east end of the outcrop is the pre-Pliocene Mill Creek Formation. The diverse compositional facies of the Mill Creek Formation can be explained in terms of a strike-slip basin model. The northern and southern flanks of the basin are characterized by sediments of quite different provevance: garnet- and muscovite-bearing granitoids from the north and Pelona grayschists from the south. Sediments transported into the basin from the southeast are characterized by clasts of volcanic rocks. Conglomeratic sandstone with hornblende- and biotite-bearing granitoid and gneiss clasts entered the basin from the northwest and dominate the axis of the basin. All of the Tertiary outcrop east of the Mill Creek Formation is assigned to the Potato Sandstone, which has much less compositional variety. The two units are separated by a fault that is probably a major strand of the San Andreas fault, the Wilson Creek strand. The composition and paleocurrents of the Potato Sandstone do resemble the axial deposits in the Mill Creek basin that were derived from the northwest, but the rapid facies changes in strike-slip basins make lithostratigraphic correlations rather unreliable. In order to account for the garnet- and muscovite-bearing granitoids on the northern flank of the Mill Creek Basin, we suggest that the basin formed in the active Clemens Well-Fenner-San Francisquito fault zone. This is consistent with the pre-Pliocene age of the basin. The Clemens Well fault formed the southern margin. The fault on the northern margin may have been a very early strand in the San Andreas fault zone. The basement clasts in the Potato Sandstone have affinities with the Little San Bernardino Mountains. This suggests that the Potato Sandstone was deposited to the northwest of the Mill Creek basin, perhaps at a later time.

The Cenozoic Santa Ana basin lies between the San Gorgonio massif and the northern plateau of the San Bernardino Mountains. Cenozoic sediments are considerably thinner on these two upland areas, which are obvious sources only for the Quaternary portion of the basin fill. The Tertiary fill is the Santa Ana Sandstone—an alluvial to lacustrine, preorogenic deposit that includes at least four conglomerate facies with different provenance. Only two of these facies can simply be derived from local basement terrain, and of these only one is compatible with the modern relief. A third facies requires a source of garnet-bearing Pelona Schist, Pelona green-schists and grayschists, greenstones, arkose, and “polka-dot granite” clasts. The distribution of clast sizes suggests a source that lay about 5 km to the south, just across the San Andreas fault. Such a source could have been provided by the Sierra Pelona of the northern San Gabriel Mountains, prior to major offset on the Punchbowl fault zone. The clast suite of the fourth facies was also transported northward, but bears superficial resemblance to the San Gorgonio basement rocks. The reconstruction of the Santa Ana basin requires that the rocks of the San Gabriel Mountains drew alongside before the uplift of the San Bernardino Mountains. At that time the San Gabriel area was relatively high and stood close to the present position of San Gorgonio Mountain. The modern configuration of the Santa Ana basin was acquired during compression of the Santa Ana basin and thrust faulting of its local sources over the northern margin. The fault at the southern margin is obscured by landsliding and superficial deposits, but may deserve inclusion with the San Andreas fault system. The Pelona Schist-bearing facies in the Santa Ana basin is now about 120 km from its inferred sources, separated by the San Andreas fault zone. Unfortunately, the age of that facies is poorly constrained. It is certainly older than the uplift of the northern plateau of the San Bernardino Mountains. In some areas beyond the Santa Ana basin, the uplift appears to have begun by 4.2 Ma; in others it is still undetected by 2.5 Ma. The Pelona Schist-bearing facies is apparently younger than the 15-Ma sediments near the base of the basin fill, and may be younger than 6.2-Ma basalts. This remaining range of age includes possibilities that do not fit well with published reconstructions of the San Andreas fault history.

Abel Mountain, 36 Abel Mountain-Sierra Pelona match-up, 38 accretion, 6 agglomerate, 240 aggregates, 204 Agua Dulce Canyon, 13 Agua Dulce fault, 68 Alamo Mountain, 11, 35 Alamo Mountain block, 79 Alamo Mountain-Frazier Mountain area, 34 Alamo Mountain-Frazier Mountain- Mount Pinos block, 80 Algodones fault, 202, 209 alluvial fan deposits, 45, 57, 115, 144, 146, 216, 218, 241, 296, 298 alluvial wedge, 61 alluvium, 57, 58, 59, 60, 61, 115, 116, 165, 206, 207, 218, 223 Almond Mountain Volcanics, 71, 72 American Girl Canyon, 205 amphibolite, 35, 80, 82, 203 facies, 110 Anacapa Islands, 283 Anchor Bay Member, 259, 260, 266 Anchor Bay sediments, 260 andesite, 13, 15, 32, 65, 207, 209, 234, 236, 237, 240, 241, 276, 277, 282, 295, 296, 311 clasts, 294 Angelus Oak, 317 anomalies gravity, 267 magnetic, 267 anorthosite, 15, 27, 34, 35, 41, 43, 80, 81, 117, 134, 200, 209, 211, 313 anorthosite-syenite complex, 11, 19, 69, 81, 110 anticlines, 54 antiform, 38, 77 Anza area, 224 apatite, 262 arcs magmatic, 80 plutonic, 84 arenite, 240, 241, 295 argillite, 83 Argus range, 72 arkose, 177, 179, 182, 259, 268, 269 Arroyo Seco, 33 Ascension fault, 53 ash, volcanic, 54 ash bed, 60, 208 Baja California, ix Baldwin Gneiss, 82, 83, 313, 314 Banning block, 18, 19, 26, 66, 68, 74, 76, 85 Banning branch, 202, 213 Banning fault, xiii, xiv, 27, 40, 55, 76, 108, 111, 115, 116, 119, 121,122, 125, 126, 127, 139, 150, 151, 201, 208, 212, 222 ancestral, 37, 122 segments, 122