Benjamin M. Page

Diverse Pliocene-Quaternary tectonics in a transform environment, San Francisco Bay region, California

The San Francisco Bay region occupies part of the diffuse transform boundary zone between the Pacific and North American plates. Although dextral strike-slip faulting is dominant, the plate motion is expressed in a variety of ways. Some strike-slip faults are parallel with the plate boundary, but some are slightly oblique. The major strike-slip faults are zones in which an interplay occurs between strands, some of which are en echelon. This interplay may be responsible for some apparent pull-apart basins and presumed normal faults along the Calaveras fault zone. In extensive areas between strike-slip fault zones, there are many compressional structures—folds and reverse faults—that are Pliocene-Quaternary in age, hence largely coeval with the strike-slip faults. Some of the compressional structures are oblique to, and others are parallel with, the major strike-slip faults. We have divided the San Francisco Bay region into domains on the basis of the types, orientations, and relationships of structures, or, in one case, lack of strong deformation. Each domain has responded in its own way to the regional northwest-southeast dextral shear imposed by the relative plate motion. In an attempt to understand the origin of various structures and the interrelationships among them, we compared established models with our observations. Some observed geometric relationships agree quite well with classical Coulomb-Anderson and simple shear models, but others require further explanation. Models based on fault interaction seem to apply to certain cases. For example, the East Bay Hills domain, which is between the left-stepping Calaveras and Hayward–Rodgers Creek fault zones, is under compression resulting from interaction between the two strike-slip zones.

OVERVIEW: Late Cenozoic tectonics of the central and southern Coast Ranges of California

The central and southern Coast Ranges of California coincide with the broad Pacific‐North American plate boundary. The ranges formed during the transform regime, but show little direct mechanical relation to strike-slip faulting. After late Miocene deformation, two recent generations of range building occurred: (1) folding and thrusting, beginning ca. 3.5 Ma and increasing at 0.4 Ma, and (2) subsequent late Quaternary uplift of the ranges. The ranges rose synchronously along the central California margin and are still rising; their long axes are quasiparallel to the plate boundary and strike-slip faults. The upper crustal internal and marginal structures of the ranges are contractional, dominated by folds and thrusts resulting from the convergent component of plate motion. Newly constructed transects using seismic reflection and refraction, plus gravity and magnetic studies, reveal lower crustal basement(s) at depths of 10‐22 km. The upper surface of the basement and Moho show no effect of the folding and thrusting observed in the upper crust. We conclude that horizontal shortening is accommodated at depth by slip on subhorizontal detachments, and by ductile shear and thickening. The ranges are marked by high heat flow; weak rocks of the Franciscan subduction complex; high fluid pressure; bounding high-angle reverse, strike-slip, or thrust faults; and uplift at a rate of 1 mm/yr beginning about 0.40 Ma. Transverse compression manifested in folding within the Coast Ranges is ascribed in large part to the well-established change in plate motions at about 3.5 Ma.

Sur-Nacimiento Fault Zone of California: Continental Margin Tectonics

The Sur-Nacimiento fault zone extends northwest through the southern and central Coast Ranges of California, and presumably continues offshore on the Continental Shelf. In part, or perhaps in its entirety, it forms the approximate boundary between the Franciscan trench(?) assemblage on the southwest and the granitic and regionally metamorphosed basement rocks of the Salinian block on the northeast. The Sur-Nacimiento fault zone includes the Sur fault zone, the Nacimiento fault, and a number of other faults of various kinds and various ages. There has been a long history of recurrent activity characterized by sequential changes in the types of movement. The Sur fault zone is here considered to have originated at the former margin of the continent during latest Cretaceous or Early Tertiary time as the culmination of processes that operated during the entire Late Mesozoic. Throughout the latest Jurassic, Early Cretaceous, and mid-Cretaceous, Franciscan deposits were probably carried into a trench and pushed against or partly beneath the crust to the east, by ocean-floor spreading. The Franciscan assemblage has a melange-like aspect which is appropriate to this kind of disturbance. Some rocks within the assemblage are about the same age (Early or mid-Cretaceous) as the adjacent Salinian granite, but they are unaffected by the granite. The postulated trench containing the Franciscan assemblage was probably far offshore during the time of granitic intrusion, but westward drift of North America narrowed the intervening distance. Conveyor-belt action of eastward-moving oceanic crust impinging against and beneath the edge of the continent may have ripped off and carried downward blocks of sialic crust. Perhaps this was the mechanism that eventually brought the Franciscan assemblage against the Salinian granite, creating the ancestral Sur fault zone. The Franciscan terrain is locally tectonically covered by allochthonous masses of Great Valley-type Late Mesozoic clastic sedimentary rocks predominantly of two ages: (1) Tithonian-Valanginian, and (2) Campanian-Maestrichtian(?). It is tentatively suggested that these allochthonous rocks were deposited between the continent and the trench in which the Franciscan accumulated, and that they were underthrust by the Franciscan during the same action that produced the Sur fault, in latest Cretaceous or Early Tertiary time. In the Tertiary, normal faulting occurred along the Sur-Nacimiento zone, following the annihilation of the active trench system. There are indications that strike-slip faulting and reverse faulting ensued at various times.

Thrusting of the central California margin over the edge of the Pacific plate during the transform regime

Wide-angle seismic reflection and refraction data, mapped geologic structures, and marine magnetic anomalies provide evidence that oceanic crust underlies the coastal margin of California from Morro Bay northward beyond San Francisco. We interpret the presence of this oceanic slab to be at least partly the result of thrusting of North America over the Pacific plate during Pliocene to Quaternary time, well after the principal subduction regime ended. Likely causes of this thrusting are the change in plate motions at about 3.5 Ma and extension in the Basin and Range province.

Recycled Franciscan Material in Franciscan Mélange West of Paso Robles, California

Recycled Franciscan detritus has been confirmed in the Franciscan Complex west of the Salinian block in the California Coast Ranges. A mass of Late Cretaceous sandstone and conglomerate-breccia, the Las Tablas unit, locally contains blocky clasts of Franciscan greenstone, chert, ultramafic rocks, glaucophane-lawsonite schist, and graywacke, together with larger amounts of non-Franciscan material. The Las Tablas unit is enveloped in a Franciscan tectonic melange, and it contains metamorphic pumpellyite, suggesting the probable onset of blueschist-facies metamorphism. The Franciscan clasts in the Las Tablas unit represent former components of an early melange related to subduction. The heterogeneous melange accumulated as an elongate pile of “scrapings” along the inner wall of a trench. During Late Cretaceous time, submarine debris flows of Franciscan melange material were shed from the pile into the trench, arc-trench gap, or smaller intervening basins to form the Las Tablas conglomerate-breccia. Eventually the Las Tablas unit was “cannibalized” by continued subduction. The unit was technically engulfed in a Late Cretaceous or early Tertiary neo-Franciscan melange and was carried to a depth sufficient to induce the development of pumpellyite.

Franciscan melanges compared with olistostromes of Taiwan and Italy

Abstract Details of origin of Franciscan melanges are unknown, although subduction is accepted as the controlling process. Some melanges near plate boundaries in Taiwan and Italy are evidently olistostromes. How do Franciscan melanges compare with these? The Lichi melange and units of “argille scagliose” type in the Northern Apennines rest upon normal marine sediments. The time of accumulation was brief, as shown by limiting time brackets. These key types of evidence for olistostrome origin are rare or absent in the Franciscan, but the pervasive shearing would probably have obliterated such evidence. Similarities between the above-cited olistostromes and Franciscan melanges include the following: argillaceous matrix; large and small blocks of sedimentary rocks and ophiolites; phacoidal and joint-block shapes; soft-sediment deformation in some sandstone; rotation of blocks; extreme dispersal of distinctive rocks; reappearance of older rocks at younger levels. Collectively, these similarities suggest that Franciscan melanges were originally assembled by olistostrome accumulation. Differences between presumed olistostromes and the Franciscan include the following, in addition to stratigraphic relations mentioned above. The Lichi melange shows faint original gross layering where shearing is minimal. Franciscan melanges show various compositional units, but shearing allows tectonic explanations. Blueschist metamorphism is rare or absent in olistostromes of Taiwan and the Northern Apennines. It occurs in the Franciscan not only in random blocks, but also as extensive units of schist and phyllite near the structural top of the complex, toward the “hanging wall” (Great Valley sequence). In the structurally lowest levels, only zeolite facies metamorphism is prevalent. Similar generalities apply to ages of rocks at highest and lowest structural levels. The age distribution would be just the opposite if the entire Franciscan were simply an east-dipping pile of olistostromes. It is concluded that neither subduction alone nor olistostromes alone could have produced all the features of Franciscan melanges, but both played an important role. Critical original features of olistostromes have been modified or destroyed by recurrent underthrusting.

Oceanic Crust and Mantle Fragment in Subduction Complex near San Luis Obispo, California

A remnant of Mesozoic oceanic crust and mantle overlain by marine sediments occurs northwest of San Luis Obispo, California. The remnant is composed of four principal units. The first (lowermost) unit is serpentinite derived in part from harzburgite, and is more than 1 km thick. The serpentinite, which is separated from the next preserved unit by faults, contains diabase dikes that presumably fed overlying intrusive and extrusive masses. The second unit is largely made up of basaltic breccias and pillow lavas, but the lower part contains countless dikes and sills of diabase, gabbro, trondhjemite, and small amounts of quartz albitite. The collective thickness of the second unit exceeds 1,200 m. The third unit is bedded radiolarian chert 5 to 130 m thick, in depositional contact with the basalt. The chert is overlain concordantly by the fourth unit, consisting of more than 660 m of shale—the Toro Formation of Fairbanks (1904). The lower part of the shale contains latest Jurassic species of Buchia, and the remainder contains Early Cretaceous species of this genus. The oceanic crust and overlying Mesozoic sediments comprise a coherent synclinal remnant which is tectonically underlain by chaotic Franciscan rocks and tectonically overlain by Upper Cretaceous Great Valley-type sandstone. The remnant probably “floated” like a raft while the Franciscan melange passed under it in the upper part of a Subduction zone and dragged it beneath the Upper Cretaceous sandstone.

A theory of concentric, kink and sinusoidal folding and of monoclinal flexuring of compressible, elastic multilayers: VII. development of folds within Huasna Syncline, San Luis Obispo County, California

Folds in the Huasna area of the southern Coast Ranges of California provide an opportunity to study different fold forms and to estimate dimensional and relative rheological properties of rocks at the time of folding. Plunging, concentric-like and chevron-like folds with wavelengths ranging from about 0.1 to 1 km are clearly visible in natural exposures at the south end of the Huasna syncline, which has a wavelength of 12–16 km. Examination of two fresh roadcut exposures in the Miocene Monterey Formation suggests that folding within part of the Monterey was accommodated primarily by layer-parallel slip between structural layers with thicknesses ranging from 30 to 43 m, even though lithologic layers range from a few mm to a few dm in thickness. This part of the Monterey is folded into a series of concentric-like folds, with chevron-like folds at their cores and with a ratio of wavelength to total thickness of layers of about LTT= 0.42. Theoretical analysis of multilayers, comprised of identical, elastic or elastic—plastic layers with frictionless contacts, indicates that the effective, or weighted-average thickness of structural layers corresponding with an LTTratio of 0.42 is about 41 m. Thus, the theoretical predictions are roughly in agreement with available data concerning these folds. Thicknesses of structural units in other folds of this area are inadequately known to closely check theoretical predictions, but most of the data are consistent with predictions. An exception is the Huasna syncline which has a larger wavelength than we would predict. There are several likely explanations for this discrepancy. Layers in the underlying Franciscan complex may have taken part in the folding, making our estimates of total thickness too small. The basement rocks may have been much softer, relative to the overlying sedimentary rocks, than we assumed. The Huasna syncline could be partly a result of gravitational instability of relatively low density, Miocene siliceous and porcelaneous shales, overlain by relatively high density, Pliocene sandstones. The Huasna syncline and some of the smaller folds in the Miocene rocks are doubly in the northwest—southeast direction. Further, the maximum compression was approximately normal to the traces of the large faults in this part of California.

Chaotic Structure and Décollement in Cenozoic Rocks near Stanford University, California

Chaotic structure and possible decollement relations, previously little known in Cenozoic rocks of the Coast Ranges, have recently been exposed 25 miles southeast of San Francisco, California. Orderly Eocene sandstone-mudstone sequences are interrupted by chaotic zones consisting of disordered mudstone with scattered and rotated bodies of sandstone. It is believed that the chaotic structure resulted in part from Eocene submarine sliding. After the Eocene rocks were moderately folded, Miocene strata were deposited unconformably upon them. Pliocene-Pleistocene thrusting and folding moved the Miocene strata northeastward relative to the underlying Eocene rocks, apparently producing a surface of decollement. Continued folding involved the decollement surface itself, increased the structural complexity of the chaotic zones, and locally overturned intact strata.

Time of Completion of Underthrusting of Franciscan beneath Great Valley Rocks West of Salinian Block, California

Underthrusting of the Franciscan assemblage beneath Great Valley rocks seems to have been completed by the end of the Oligocene in the region west of the Salinian block. Both the Franciscan and Great Valley assemblages are latest Jurassic-Cretaceous in age. Clasts derived from both first appear together in the Vaqueros Formation (Upper Oligocene?-Lower Miocene). The Vaqueros is markedly transgressive and is generally the oldest formation in depositional contact with the Franciscan in the region under discussion. The Vaqueros locally rests on both Franciscan and Great Valley rocks, and extends across a fault separating the two. If Underthrusting ceased before the end of the Oligocene, the timing is compatible with the termination of northeast-southwest relative motion at the continental margin, and the beginning of northwest-southeast relative motion when North America encountered the Main Pacific plate. Much or all activity on the San Andreas fault followed this event. The fault offset the zone of Underthrusting.