Plate tectonics and Cenozoic deformation shaped the present-day geology of California
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Multiple peer-reviewed studies document how plate tectonic interactions along the Pacific-North American boundary and Cenozoic deformation events, such as faulting, compression, and basin formation, directly shaped California's present-day geological features.
New geologic mapping and Ar-Ar geochronology of the late Cenozoic volcanic-sedimentary units in central and southern Panamint Valley, California, provide the first known Miocene palinspastic reconstruction vectors for Panamint Valley. Panamint Valley contains active faulting and potentially accommodates a significant percentage of the slip of the Walker Lane at this latitude. Volcanism in Panamint Valley occurred during two time intervals, one ca. 15–13.5 Ma ago and a second ca. 4.5–4 Ma ago. The reconstruction vectors are based on unique relationships of sedimentary source areas and the only known Miocene intrusive zones to determine the displacement across Panamint Valley since ca. 15 Ma ago. The Argus Range was displaced ~17 km to the west-northwest, and the southern Slate Range was displaced 10.5 km to the north-northwest relative to the Panamint Range. Our displacement vector for reconstructing the past ~15 Ma of slip across Panamint Valley is 14 km shorter than previously published reconstruction models. We interpret this smaller slip value to be a function of the previous studies using displacement vectors that included a component of pre–15 Ma ago slip. The Harrisburg fault of the Tucki Mountain detachment system is a likely candidate for an earlier slip, possibly during the regionally observed extension during Late Cretaceous and Eocene. We created a model of the ca. 0–15 Ma ago displacement history of Panamint Valley using our new slip vectors and the slip vector for the Hunter Mountain fault. The Miocene extension begins with or slightly before ca. 15 Ma ago volcanism and may have continued to <~13.5 Ma ago. We interpreted the slip during Miocene extension to have occurred on one master detachment fault. Pliocene and younger extension is oblique to the Miocene extension, and the detachment fault was then cut up into discrete segments, the Emigrant, Panamint, and Slate Range detachment faults. The Panamint detachment was reactivated in an oblique normal sense, while slip on the other two detachment faults ceased; slip now occurs on nearby steeper normal faults. The Panamint detachment ends to the north and south in triple junctions: at the north end, slip is partitioned onto the Hunter Mountain and Towne Pass faults, and at the south end, slip is partitioned onto the Manly Pass and Southern Panamint Valley faults. The southern triple junction has an unstable geometry and it must migrate northward, lengthening the Southern Panamint Valley fault at the expense of the Panamint detachment. The continued slip on the unfavorably oriented low-angle Panamint detachment may be explained by the presence of weak fault gouge along it or by a regional pattern of slip partitioning. Major regional strike-slip faults, the Northern Death Valley and Garlock faults, are proximal to the northern and southern triple junctions. These two large faults may drag the two ends of the Panamint detachment with them, creating the triple junctions. The modern complex geometries and kinematics of Panamint Valley may therefore be a function of older structures being reactivated and interference with nearby faults.
Tectonic activity can drive speciation and sedimentation, potentially causing the fossil and rock records to share common patterns through time. The Basin and Range of western North America arose through widespread extension and collapse of topographic highlands in the Miocene, creating numerous basins with rich mammalian fossil records. We analyzed patterns of mammalian species richness from 36 to 0 million years ago in relation to the history of sediment accumulation to test whether intervals of high species richness corresponded with elevated sediment accumulation and fossil burial in response to tectonic deformation. We found that the sedimentary record of the Basin and Range tracks the tectonic evolution of landscapes, whereas species-richness trends reflect actual increased richness in the Miocene rather than increased fossil burial. The sedimentary record of the region broadly determines the preservation of the fossil record but does not drive the Miocene peak in mammalian species richness.
ABSTRACT The Neogene stratigraphic and tectonic history of the Mount Diablo area is a consequence of the passage of the Mendocino triple junction by the San Francisco Bay area between 12 and 6 Ma, volcanism above a slab window trailing the Mendocino triple junction, and crustal transpression beginning ca. 8–6 Ma, when the Pacific plate and Sierra Nevada microplate began to converge obliquely. Between ca. 12 and 6 Ma, parts of the Sierra Nevada microplate were displaced by faults splaying from the main trace of the San Andreas fault and incorporated into the Pacific plate. The Mount Diablo anticlinorium was formed by crustal compression within a left-stepping, restraining bend of the eastern San Andreas fault system, with southwest-verging thrusting beneath, and with possible clockwise rotation between faults on its southeast and northwest sides. At ca. 10.5 Ma, a drainage divide formed between the northern Central Valley and the ocean. Regional uplift accelerated at ca. 6 Ma with onset of transpression between the Pacific and North America plates. Marine deposition ceased in the eastern Coast Range basins as a consequence of the regional uplift accompanying passage of the Mendocino triple junction, and trailing slab-window volcanism. From ca. 11 to ca. 5 Ma, andesitic volcanic intrusive rocks and lavas were erupted along the northwest crest of the central to northern Sierra Nevada and deposited on its western slope, providing abundant sediment to the northern Central Valley and the northeastern Coast Ranges. Sediment filled the Central Valley and overtopped the Stockton fault and arch, forming one large, south-draining system that flowed into a marine embayment at its southwestern end, the ancestral San Joaquin Sea. This marine embayment shrunk with time, and by ca. 2.3 Ma, it was eventually cut off from the ocean. Fluvial drainage continued southwest in the Central Valley until it was cut off in turn, probably by some combination of sea-level fluctuations and transpression along the San Andreas fault that uplifted, lengthened, and narrowed the outlet channel. As a consequence, a great lake, Lake Clyde, formed in the Central Valley at ca. 1.4 Ma, occupying all of the ancestral San Joaquin Valley and part of the ancestral Sacramento Valley. The lake rose and fell with global glacial and interglacial cycles. After a long, extreme glacial period, marine oxygen isotope stage (MIS) 16, it overtopped the Carquinez sill at 0.63 Ma and drained via San Francisco valley (now San Francisco Bay) and the Colma gap into the Merced marine embayment of the Pacific Ocean. Later, a new outlet for Central Valley drainage formed between ca. 130 and ca. 75 ka, when the Colma gap closed due to transpression and right-slip motion on the San Andreas fault, and Duxbury Point at the south end of the Point Reyes Peninsula moved sufficiently northwest along the San Andreas fault to unblock a bedrock notch, the feature we now call the Golden Gate.
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 a
The San Andreas transform system joins the Gulf of California divergent plate boundary in the Salton Trough region of Southeastern California. Here several strands of the system end on the southeast at active spreading centers beneath about 6 km of upper Cenozoic sediments, both marine and nonmarine. Neotectonic deformation, earthquakes, geothermal activity, and volcanicity document these spreading centers. Before this pattern of tectonics and sedimentation developed about 4.5 m.y. ago and the present Gulf of California began to open, the San Andreas fault was already active. It apparently originated during oblique plate convergence 8 or 10 m.y. ago along the margins of a proto-gulf. Conglomerates of this age have now been offset laterally about 320 km from their source areas. Before this, beginning abut 22 m.y. ago, basin-range tectonics prevailed. These basins and their surrounding terranes pre-date the San Andreas system in Southeastern California. In early tertiary and late Cretaceous time, plate convergence prevailed and the region lay near the northeastern margin of a forearc basin.
The Mojave block of southern California has undergone significant late Cenozoic north-south contraction. This previously unappreciated deformation may account for part of the discrepancy between neotectonic and plate-tectonic estimates of Pacific-North American plate motion, and for part of the Big Bend in the San Andreas fault. In the eastern Mojave block, contraction is superimposed on early Miocene crustal extension. In the western Mojave block, contractional folds and reverse faults have been mistaken for extensional structures. The three-dimensional complexity of the contractional structures may mean that rigid-block tectonic models of the region based primarily on paleomagnetic data are unreliable.
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