Central California Coast Ranges Research Articles

High-resolution environmental magnetic study of a Holocene sedimentary record from Zaca Lake, California

Mineral magnetic studies of Holocene lake sediments recovered from Zaca Lake, in coastal southern California have yielded a 3000-year high-resolution record of terrestrial hydrologic variability and paleolimnology. Samples for magnetic analysis were obtained from an 8.73 m core recovered from the central region of Zaca Lake. Ages, constrained using a combined paleomagnetic and radiocarbon chronostratigraphy, yield sedimentation rates of 2–10 mm/yr with an average rate of 3 mm/yr over the 3000-year interval. Parameters reflecting decadal-scale variability in magnetic concentration (susceptibility, ARM, SIRM) and grain size (ARM/Chi) were measured every 2 cm. Additional rock magnetic tests on whole rock samples and magnetic separates indicate that magnetite and an iron sulfide are the chief magnetic carriers. Detailed analysis of the rock magnetic record reveals four major temporal intervals: 0–135 cm (~AD 2009–1757) the period of European settlement; 135–270 cm (~AD 1757–1275) the period of Native American, Chumash occupation of the watershed encompassing the bulk of the wet interval known as the little ice age (LIA); 270–600 cm (AD 1275–100) encompassing the drier Medieval Climate Anomaly (MCA); and 600–875 cm (2000–3000 ybp) an era dominated by a long interval of low magnetic mineral intensity and concentration that has been interpreted as a time of epic drought. Beginning with the cessation of drought conditions at Zaca Lake around ~2000 ybp, the magnetic proxy record of rock magnetic cyclicity and hydrologic variability shows a pronounced decadal to centennial-scale periodicity. We interpret these periodicities as oscillations between summer-dominated sedimentary sequences, characterized by whiting events with their resulting carbonate deposits, and winter dominated sedimentary sequences, typified by sulfide-rich clastics, reflecting lacustrine overturn and enhanced advection of detrital flux during winter storm events. The long-term variation of summer versus winter-dominated weather patterns reflects the climate variability of central California coast-ranges.

Geologic evolution of a Cretaceous tectonometamorphic unit in the Franciscan Complex, western California

ABSTRACTThe Franciscan Yolla Bolly terrane of the NE California Coast Ranges consists mainly of quartzose metagreywackes containing sparse high-pressure/low-temperature (HP/LT) neoblastic minerals, including ubiquitous lawsonite. Some Yolla Bolly rocks also contain one or more of the newly grown phases, pumpellyite, aragonite, glaucophane, and/or jadeitic pyroxene. These blueschist-facies metasandstones recrystallized under physical conditions of ~200–300°C and ~8 kbar at subduction-zone depths approaching 30 km. Petrologically similar Franciscan metaclastic-rich map units – Yolla Bolly terrane-like rocks, here designated the ‘YB’ unit – crop out in the central and southern California Coast Ranges. Recently published detrital zircon U‒Pb SIMS and LA-ICPMS data for 19 ‘YB’ metagreywackes indicate maximum ages of formation as follows: ~110–115 Ma (8) in the NE California Coast Ranges; ~95–107 Ma (7) in the San Francisco Bay area + Diablo Range; and ~85–92 Ma (4) in the dextrally offset Nacimiento Block. These fault-bounded ‘YB’ strata do not constitute coeval parts of a single tectonostratigraphic unit. Instead the term tectonometamorphic is proposed for such time-transgressive map units. Based on the current and likely Cretaceous 30° angular divergence between NS-palaeomagnetic stripes of the Farallon oceanic plate and the NNW-trending California convergent margin, I infer that arrival at the arc margin and underflow of a relatively thick segment of oceanic crust and its largely clastic sedimentary blanket may have resulted in progressive southeastward migration of an accreted, subducted, then exhumed HP/LT metagreywacke section. During the ~30 million year interval, ~115–85 Ma, the locus of ‘YB’ accretion, underflow, and tectonic regurgitation evidently moved SE along an ~1000 km stretch of the accretionary margin of western California.

The Point Sal–Point Piedras Blancas correlation and the problem of slip on the San Gregorio–Hosgri fault, central California Coast Ranges

Existing models for large-magnitude, right-lateral slip on the San Gregorio–Hosgri fault system imply much more deformation of the onshore block in the Santa Maria basin than is supported by geologic data. This problem is resolved by a model in which dextral slip on this fault system increases gradually from 0–10 km near Point Arguello to ∼150 km at Cape San Martin, but such a model requires abandoning the cross-fault tie between Point Sal and Point Piedras Blancas, which requires 90–100 km of right-lateral slip on the southern Hosgri fault. We collected stratigraphic and detrital zircon data from Miocene clastic rocks overlying Jurassic basement at both localities to determine if either section contained unique characteristics that could establish how far apart they were in the early Miocene. Our data indicate that these basins formed in the early Miocene during a period of widespread transtensional basin formation in the central Coast Ranges, and they filled with sediment derived from nearby pre-Cenozoic basement rocks. Although detrital zircon data do not indicate a unique source component in either section, they establish the maximum depositional age of the previously undated Point Piedras Blancas section to be 18 Ma. We also show that detrital zircon trace-element data can be used to discriminate between zircons of oceanic crust and arc affinity of the same age, a potentially useful tool in future studies of the California Coast Ranges. Overall, we find no characteristics in the stratigraphy and provenance of the Point Sal and Point Piedras Blancas sections that are sufficiently unique to prove whether they were far apart or close together in the early Miocene, making them of questionable utility as piercing points.

Conversion to drip irrigated agriculture may offset historic anthropogenic and wildfire contributions to sediment production

This study is an investigation into the roles of wildfire and changing agricultural practices in controlling the inter-decadal scale trends of suspended sediment production from semi-arid mountainous rivers. In the test case, a decreasing trend in suspended sediment concentrations was found in the lower Salinas River, California between 1967 and 2011. Event to decadal scale patterns in sediment production in the Salinas River have been found to be largely controlled by antecedent hydrologic conditions. Decreasing suspended sediment concentrations over the last 15years of the record departed from those expected from climatic/hydrologic forcing. Sediment production from the mountainous headwaters of the central California Coast Ranges is known to be dominated by the interaction of wildfire and large rainfall/runoff events, including the Arroyo Seco, an ~700km2 subbasin of the Salinas River. However, the decreasing trend in Salinas River suspended sediment concentrations run contrary to increases in the watershed's effective burn area over time. The sediment source area of the Salinas River is an order of magnitude larger than that of the Arroyo Seco, and includes a more complicated mosaic of land cover and land use. The departure from hydrologic forcings on suspended sediment concentration patterns was found to coincide with a rapid conversion of irrigation practices from sprinkler and furrow to subsurface drip irrigation. Changes in agricultural operations appear to have decreased sediment supply to the Salinas River over the late 20th to early 21st centuries, obscuring the influence of wildfire on suspended sediment production.

Assembling the world’s type shallow subduction complex: Detrital zircon geochronologic constraints on the origin of the Nacimiento block, central California Coast Ranges

Temporal and spatial patterns in the architecture of the Franciscan Complex provide valuable insights into the subduction processes through which such patterns arise. The Nacimiento Franciscan belt is an allochthonous sliver of subduction assemblages in the central California Coast Ranges displaced either: (1) from southern California by >300 km of Neogene dextral slip along the San Andreas fault system or (2) from central California to southern California and back again, by >500 km of Late Cretaceous–Paleocene sinistral slip along the Sur-Nacimiento fault followed by San Andreas–related motion. New U-Pb detrital zircon data from 20 (meta)clastic samples indicate that the Nacimiento Franciscan section was assembled between ca. 95 and 80 Ma. Abundant Cretaceous (particularly Late Cretaceous) and diminishing amounts of Jurassic and Proterozoic zircon grains point to a southern California origin for Nacimiento Franciscan protoliths, precluding significant sinistral strike-slip along the Sur-Nacimiento fault. Furthermore, the suite of detrital zircon ages reported here bears a strong resemblance to new and existing data from subduction complexes in southern California that were emplaced during Laramide shallow subduction (i.e., Sierra de Salinas, Portal Ridge, Quartz Hill, Rand, San Emigdio, and Tehachapi schists). Hence, the Nacimiento Franciscan is distinct from Franciscan rocks in central and northern California and more likely represents an outboard element of the Late Cretaceous southern California low-angle subduction system. Upon restoring the Nacimiento block to its Late Cretaceous position, an inboard-younging trend is apparent in the composite Nacimiento–southern California schist belt, suggesting that progressively younger accretionary materials were underplated farther inboard by tectonic erosion. We posit that arc and forearc elements absent from southern California were removed by a combination of physical and tectonic erosion attending shallow subduction, interleaved in the subduction complex, and recycled into the mantle. Steepening of the Laramide slab was marked by a phase of crustal extension in the overriding plate. During this phase, the Sur-Nacimiento fault likely functioned as a segment of a low-angle normal fault system spanning the southern Sierra Nevada batholith to the Nacimiento accretionary system.

The notion of climate-driven strath-terrace production assessed via dissimilar stream-process response to late Quaternary climate

Previous research results from the Gabilan Mesa are combined with new optically stimulated luminescence (OSL) age estimates and sedimentological analyses with the aim of identifying factors that inhibit climate-driven strath-terrace production, and factors that make possible strath-terrace production independent of climate forcing. The factors are revealed by comparing the morphostratigraphy and OSL age estimates of terraces in the adjacent San Lorenzo Creek and Pancho Rico Creek drainage basins of the central California Coast Ranges. OSL age estimates on San Lorenzo Creek fill-terrace alluvium overlying bedrock at two paleofluvial levels range between 50.5 and 41.3ka and between 33.4 and 18.2ka. These OSL age estimates indicate that although the channel of Pancho Rico Creek was degrading at these times, San Lorenzo Creek aggradation was synchronous with previously documented regional, climatically driven aggradation that elsewhere in southern California led to strath production and alluvial deposition. The regional-scale climate forcing events had different effects on San Lorenzo and Pancho Rico Creeks because of the influences of drainage-basin lithology on bedload size and tectonic tilting direction on base-level fall. The Holocene history of channel denudation and strath production of Pancho Rico Creek is also different from that of San Lorenzo Creek, and different from that of many other streams in southern California. After Pancho Rico Creek captured the upper part of the drainage basin of San Lorenzo Creek sometime after 15.5 to 11.7ka, Pancho Rico Creek has been producing unpaired, erosional strath terraces. The weak, clay rich, fine-grained sedimentary rock underlying Pancho Rico Valley is an ideal substrate in which to form straths. The meandering channel of Pancho Rico Creek produces straths, and weathering resistant, relatively hard bedload introduced by stream capture ensures their preservation as strath terraces.

Sediment supply, base level, braiding, and bedrock river terrace formation: Arroyo Seco, California, USA

In many settings, rivers alternate between carving wide valley bottoms (straths) and cutting narrow gorges over time, thereby creating longitudinally continuous paired bedrock strath terraces along valleys. Strath terraces are used ubiquitously in geomorphology and tectonics; however, how and why they form remain poorly understood. Here, we focus on Arroyo Seco in the central California Coast Ranges, where we test hypotheses for strath planation and subsequent strath terrace formation. Several lines of evidence indicate that strath planation is triggered by braiding in bedrock channels. In particular, hydraulic modeling reveals that the width of Arroyo Seco’s most recently formed terrace is comparable to the width of currently braided channel reaches. Additionally, a comparison of currently braided reaches to abandoned bedrock meander cutoffs shows that braided channels have valleys that are several times wider than single-thread meandering bedrock channel reaches. Lastly, in locations where the modern channel is currently braided, terraces are poorly preserved, suggesting that evidence for past episodes of braiding, in the form of paired strath terraces, is apparently largely destroyed by subsequent episodes of braiding. Field observations combined with mapping of terrace levels using an objective light detection and ranging (LiDAR)–based terrace identification algorithm reveal that temporal variation in tectonic uplift rate, sea level, and/or alluvial cover along the river cannot explain strath planation and subsequent terrace formation in Arroyo Seco. Rather, our results provide evidence that aggradation and degradation of alluvial sediments downstream of the Reliz Canyon fault result in impulsive base-level forcing of Arroyo Seco’s bedrock channel. Strath abandonment and terrace formation apparently occur as incision into downstream alluvial sediments propagates upstream into bedrock. Braiding and planation of straths, in contrast, occur during intervals of low vertical incision rate associated with downstream aggradation or immediately following pulses of vertical lowering triggered by downstream incision of alluvial sediments.

Fault geometry and cumulative offsets in the central Coast Ranges, California: Evidence for northward increasing slip along the San Gregorio-San Simeon-Hosgri fault

Estimates of the dip, depth extent, and amount of cumulative displacement along the major faults in the central California Coast Ranges are controversial. We use detailed aeromagnetic data to estimate these parameters for the San Gregorio–San Simeon–Hosgri and other faults. The recently acquired aeromagnetic data provide an areally consistent data set that crosses the onshore-offshore transition without disruption, which is particularly important for the mostly offshore San Gregorio–San Simeon–Hosgri fault. Our modeling, constrained by exposed geology and in some cases, drill-hole and seismic-reflection data, indicates that the San Gregorio–San Simeon–Hosgri and Reliz-Rinconada faults dip steeply throughout the seismogenic crust. Deviations from steep dips may result from local fault interactions, transfer of slip between faults, or overprinting by transpression since the late Miocene. Given that such faults are consistent with predominantly strike-slip displacement, we correlate geophysical anomalies offset by these faults to estimate cumulative displacements. We find a northward increase in right-lateral displacement along the San Gregorio–San Simeon–Hosgri fault that is mimicked by Quaternary slip rates. Although overall slip rates have decreased over the lifetime of the fault, the pattern of slip has not changed. Northward increase in right-lateral displacement is balanced in part by slip added by faults, such as the Reliz-Rinconada, Oceanic–West Huasna, and (speculatively) Santa Ynez River faults to the east.

Superimposed extension and shortening in the southern Salinas Basin and La Panza Range, California: A guide to Neogene deformation in the Salinian block of the central California Coast Ranges

We synthesized data from geologic maps, wells, seismic-reflection profiles, potential-field interpretations, and low-temperature thermochronology to refine our understanding of late Cenozoic extension and shortening in the Salinian block of the central California Coast Ranges. Data from the La Panza Range and southern Salinas Basin document early to middle Miocene extension, followed by Pliocene and younger shortening after a period of little deformation in the late Miocene. Extension took place on high-angle normal faults that accommodated ∼2% strain at the scale of the ∼50-km-wide Salinian block (oriented perpendicular to the San Andreas fault). Shortening was accommodated by new reverse faults, reactivation of older normal faults, and strike-slip faulting that resulted in a map-view change in the width of the Salinian block. The overall magnitude of shortening was ∼10% strain, roughly 4–5 times greater than the amount of extension. The timing and magnitude of deformation in our study area are comparable to that documented in other Salinian block basins, and we suggest that the entire block deformed in a similar manner over a similar time span. The timing and relative magnitude of extension and shortening may be understood in the context of central Coast Range tectonic boundary conditions linked to rotation of the western Transverse Ranges at the south end of the Salinian block. Older models for Coast Range shortening based on balanced fault-bend fold-style cross sections are a poor approximation of Salinian block deformation, and may lead to mechanically improbable fault geometries that overestimate the amount of shortening.

Tomographic Vp and Vs structure of the California Central Coast Ranges, in the vicinity of SAFOD, from controlled-source seismic data

A seismic reflection/refraction survey across the San Andreas fault near Parkfield, California, has refined our knowledge of the upper crustal structure of the central California Coast Ranges at the San Andreas Fault Observatory at Depth (SAFOD). The survey consisted of a 46-km-long line of seismographs (25–50 m spacing) and 63 explosions (25–200 kg; nominal spacing of 500 m, with some gaps). The traveltimes of refracted P and S waves from the explosions constitute independent data sets of relatively high quality that were inverted to produce P- and S-wave velocity models (Vp, Vs) along the profile, extending to as much as 5 km depth. The Vp and Vs models show a prominent lateral drop in velocities a few hundred metres northeast of SAFOD, between the drill hole and the San Andreas fault. The Vp model shows particularly well a southwest-dipping velocity inversion beneath SAFOD, the top of which correlates with a fault penetrated by the drill hole that separates granitic rocks above from sedimentary rocks below. In addition to Vp and Vs models, a Vp/Vs model was derived. A Vp/Vs ratio lower than 1.73 is seen only at depth, in a narrow zone beginning at the target earthquakes for SAFOD and extending downward and northeastward into the North America Plate. Clusters in the parameter space spanned by Vp/Vs ratios and Vp can be identified by two different methods, one more intuitive analytical method and one more abstract method based on neural network techniques. These clusters are correlated to different rock types, based on laboratory and in situ data. These clusters are remapped back into xߞz plane along the profile. Prominent features mapped this way include Salinian granitic rocks beneath and west of SAFOD, and a body of sedimentary rocks faulted beneath these granitic rocks along what we and others interpret to be a branch of the Buzzard Canyon Fault (BCF) system. These sedimentary rocks extend from this fault to the San Andreas fault system. Unfortunately, our cluster analysis shows no significant discontinuity at the San Andreas fault, owing presumably to the fact that the San Andreas fault is located within sedimentary rocks having similar elastic properties. This paper is an attempt to ‘downward’ continue a geological map by geophysical means based on elastic properties of rock samples from the region.

U‐Pb ages of detrital zircons in Pacheco Pass metagraywackes: Sierran‐Klamath source of mid‐Cretaceous and Late Cretaceous Franciscan deposition and underplating

Using laser ablation ICP‐MS techniques, U‐Pb ages are reported for 338 detrital zircons separated from four Franciscan metagraywacke samples from imbricate thrust sheets cropping out in the Pacheco Pass area, east central California Coast Ranges. Structurally higher slabs are clastic quartz ± albite‐bearing rocks typified by neoblastic lawsonite + jadeitic pyroxene; the lowest exposed slab lacks jadeitic pyroxene but contains traces of newly grown pumpellyite and lawsonite in addition to abundant quartz + albite. Studied specimens contain moderate amounts of phengite + titanite ± chlorite, stilpnomelane, carbonate, iron oxides, rock fragments, and carbonaceous matter. These Diablo Range clastic sediments were deposited in mid‐Cretaceous and Late Cretaceous time. The highest allochthon, unit I, was deposited after ∼102 Ma, unit IV was deposited after ∼90–102 Ma, and unit V was deposited after ∼86 Ma. Metagraywacke depositional‐accretionary ages within slab IV young upward, but ages of the slabs decrease progressively downward. In the four‐sample aggregate, most zircons have igneous ages falling in the 135–170 Ma range, with a smaller population at 85–120 Ma. The rocks contain rare zircons of Middle Proterozoic–Late Archean ages. A minor source in northern California and NW Nevada seems likely for the Pacheco Pass metagraywackes, but the Sierran‐Klamath calcalkaline arc provided most of the clastic debris. Judging by the volcanic nature of lithic clasts in these Eastern Belt metagraywackes, and because massive Sierran plutonism occurred at ∼85–120 Ma, the abundant 135–170 Ma zircons probably were derived chiefly from the eroding comagmatic volcanic arc rather than from the less voluminous Jurassic arc plutons.

Tectonic underplating of trench sediments beneath magmatic arcs: the central California example

We summarize the post Late Cretaceous regional tectonic evolution of the central California Coast Ranges, west of the San Andreas fault system. The Monterey terrane of North American origin was laterally transferred to the Pacific plate via the San Andreas fault. The Monterey terrane is an assembly of three tectonic units, Salinia, Nacimiento and Sierra de Salinas blocks, two of which have been previously identified as separate terranes. These blocks are separated by two regionally important thrust faults: the Sur fault as well as the Salinas shear zone. Based on thermobarometric and thermochronologic constraints and the existence of a common younger cover sequence, these blocks were juxtaposed together after the latest Cretaceous. The Salinian assemblage represents a crustal section through the continental interior side of the Mesozoic California arc and formed during the Late Cretaceous, primarily during a regionally significant magmatic flare‐up between 95 and 80 Ma. In the Santa Lucia Range, parts of the arc are exposed to palaeo‐depths in excess of 30 km. The Nacimiento and Sierra de Salinas assemblages comprise basement rocks representing Late Cretaceous variants of the Franciscan Complex and are interpreted to be correlative. They represent the lower plate of a regionally important thrust system; the upper plate is the Salinian assemblage, whereas the Sur and Salinas faults are local exposures of the structure. We concur with previous estimates of 150 to 180 km of shortening during a brief time span (<6 my), at a rate of >3 cm/yr. This fault system corresponds to the megathrust of the Farallon subduction beneath North America during the early stages of the regionally extensive episode of shallow subduction (Laramide orogeny). As a result, trench sediment was thrust under North America and tectonically underplated to the lower crust of North America. The Salinas shear zone, in particular, is a ductile expression of shallow subduction; thermobarometry in the upper plate, lower plate and the shear zone itself indicate that this is the fossil subduction megathrust originating at depths of ∼35 km. The entire system collapsed extensionally soon after the trench sediment was underthrust, possibly because of the lack of strength of the lower plate. Arc magmatism in the upper plate ceased at the onset of underplating. This regional example illustrates the significance of tectonic underplating in shallow subduction systems. Accretion‐related trench sediment was shuffled from the trench to the sub‐arc region of the upper plate, but not recycled into the mantle. This process requires that the subduction megathrust be located solely within the North American crust. This geometry requires a sudden migration of the subduction interface toward the arc and may apply to other regional examples, including the modern shallow subduction of the Cocos plate beneath southern Mexico. The tectonically underplated trench sediment undergoes regional, Barrovian metamorphism, after initially following a high‐pressure/low‐temperature path. Moreover, the shear zone marking the fossil intracrustal megathrust was subject to granulite‐facies metamorphism and limited partial melting.

Aluminous Xenoliths in Miocene Andesite, Central California Coast Ranges: Magma-Crust Interaction in a Subduction-Transform Transitional Setting

Non-foliated, Al-rich inclusions in Miocene andesite dikes and plugs cropping out in the Dowdy Ranch area, central Diablo Range, record two stages of high-temperature metamorphism, as well as complex chemical interactions between the inclusions and their mafic magma host. Both xenoliths and andesitic host show marked compositional zoning and partial resorption of minerals. The first stage of subsolidus contact metamorphism produced andalusite + alkali-feldspar + Na-plagioclase + muscovite + biotite + quartz (± actinolite or epidote?) in xenoliths of the originally quartzofeldspathic wall rock. P-T conditions attending this event were approximately 600°C, ˜2 kbar, with Pfluid approaching Ptotal. Capture of fragments of metamorphosed wall rock by ascending mafic magma resulted in partial fusion of the xenoliths, loss of silica and alkalis to the liquid, and calcium-enrichment in the restitic, increasingly aluminous inclusions. Second-stage neoblastic assemblages are sillimanite + hercynite-rich spinel + corundum + Ca-plagioclase ± Mg-rich biotite or phlogopite ± orthopyroxene. Physical conditions in the xenoliths accompanying decompression partial melting were approximately 700-750°C and ˜1 kbar (with Pfluid < Ptotal). The presence of corundum and ZnO-poor spinel is consistent with a restitic residue produced by dehydration melting under high-temperature, silica-deficient conditions. The parageneses developed in the Al-rich inclusions were an upper crustal thermal response to Miocene passage of the Mendocino triple junction at this latitude.

Late Cenozoic denudation and uplift rates in the Santa Lucia Mountains, California

Apatite (U-Th)/He ages from a vertical transect through the Santa Lucia Mountains, central California Coast Ranges, are used to reconstruct the history of exhumation and of bedrock and surface uplift in this region since ca. 6 Ma. We find a direct correlation between (U-Th)/He ages and elevation, which we interpret to correspond to denudation rates of ;0.35 mm/yr between 6 and 2 Ma. The onset of bedrock uplift and exhumation ca. 6 Ma followed a change in plate motion ca. 8 Ma. After 2 Ma, denudation rates increased substantially (;0.9 mm/yr). This is a rare instance in which long-term average bedrock (;0.85 mm/yr) and surface (;0.20 mm/yr) uplift can be calculated from denudation rates and stratigraphic data. The post‐2 Ma denudation rate is about one order of magnitude higher than independently determined river erosion rates in the area. We suggest that this discrepancy indicates that exhumation of the steep western slopes of this segment of the Coast Ranges has been dominated by mass wasting via landslides, rather than fluvial erosion, at least since ca. 2 Ma. We also show that the bedrock uplift is predominantly tectonic, not isostatic.

Present tectonic motion across the Coast Ranges and San Andreas fault system in central California

Research Article| December 01, 2001 Present tectonic motion across the Coast Ranges and San Andreas fault system in central California Donald F. Argus; Donald F. Argus 1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA Search for other works by this author on: GSW Google Scholar Richard G. Gordon Richard G. Gordon 2Department of Earth Science, Rice University, Houston, Texas 77005, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2001) 113 (12): 1580–1592. https://doi.org/10.1130/0016-7606(2001)113<1580:PTMATC>2.0.CO;2 Article history received: 28 May 1999 rev-recd: 22 Jan 2000 accepted: 01 May 2001 first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Donald F. Argus, Richard G. Gordon; Present tectonic motion across the Coast Ranges and San Andreas fault system in central California. GSA Bulletin 2001;; 113 (12): 1580–1592. doi: https://doi.org/10.1130/0016-7606(2001)113<1580:PTMATC>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Geodetic results from very long baseline interferometry (VLBI), satellite laser ranging (SLR), and the Global Positioning System (GPS) are used to estimate angular velocities between the Sierran microplate, Pacific plate, and North American plate. The Sierra-Pacific pole of rotation lies nearer to the San Andreas fault than does the Pacific–North America pole of rotation and leads to different tectonic implications than if the latter is used. The angular velocities show that the San Andreas fault system and central California Coast Ranges accommodate motion of 39 ± 2 mm/yr, mainly by strike-slip faulting. (All confidence limits following ± signs in this paper are 95% confidence limits.) Fault-normal motion is small, is mainly convergent (at rates up to 3.3 ± 1.0 mm/yr), and varies along the coast, but is divergent (at 2.6 ± 1.2 mm/yr) across San Pablo Bay and associated topographic lows across which the Sierran and Central Valley watershed drains to the Pacific Ocean. The mountain ranges tend to be larger where the fault-normal convergence rates are larger. The low convergence rate (0.5 ± 1.8 mm/yr) normal to the San Andreas fault in the Carrizo Plain differs sharply from that previously inferred (8.2 ± 1.2 mm/yr and 4.9 ± 1.6 mm/yr) by Feigl et al. (1993). The difference is due to differences between their and our elastic strain accumulation models and between how their and our Pacific plate reference frames are defined.The ranges in most places require a minimum of 4 +2/–1 m.y. of fault-normal convergence at the present rate to attain their present cross-sectional area if erosion is neglected, more if it is not. The amount of convergence previously estimated from a balanced cross section across the Diablo Range in central California requires 10 +8/–3 m.y. of convergence at the present rate. The former is consistent with widely held views about the onset of the Coast Range orogeny, but the latter is not. Both are consistent, however, with the recent plate reconstructions by S. Cande, J. Stock, and colleagues, which indicate that Pacific plate motion relative to North America changed to a more convergent direction, 20°–25° clockwise of its prior direction, at ca. 8 to 6 Ma and not at 3.5 Ma, as had been previously inferred. The inferred change in direction of plate motion is large compared with the present angle of convergence across the straight and narrow segment of the San Andreas fault of 0.7°–4.7°, from which we infer that the Sierran microplate changed motion relative to North America at the same time (ca. 8 to 6 Ma) as did the Pacific plate. We further infer that the motion accommodated across the Great Basin must also have changed at the same time.We also examine the hypothesis that stable sliding occurs along the San Andreas fault and other northwest-striking strike- slip faults in central California where the fault-normal convergence rate is low or negative, and that these faults are unstable where the fault-normal convergence rate is high. Such a relationship appears to hold in general, but fails in detail. In particular, there are substantial sections of fault with small inferred rates of fault-normal convergence across which the San Andreas fault is locked. Moreover, the creeping section of the San Andreas fault (i.e., the section between Parkfield and the Calaveras junction) is the locus of greater fault-normal convergence (3.2 ± 1.4 mm/yr) than is the locked part of the fault (0.5 ± 1.8 mm/yr) south of Parkfield. Thus, this hypothesis is at best a partial explanation for the observed distribution of locked and nonlocked sections of the fault. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Synthesis of Crustal Seismic Structure and Implications for the Concept of a Slab Gap beneath Coastal California

Compilation of seismic transects across the central and northern California Coast Ranges provides evidence for the widespread tectonic emplacement beneath the margin of a slab of partially subducted oceanic lithosphere. The oceanic crust of this lithosphere can be traced landward from the former convergent margin (fossil trench), beneath the Coast Ranges, to at least as far east as the Coast Range/Great Valley boundary. Comparison of measured shear and compressional wave velocities in the middle crust beneath the Hayward fault with laboratory measurements suggests that the middle crust is a diabase (oceanic crust). Both of these observations are consistent with recent models of the high heat flow and age progression of Neogene volcanism along the Coast Ranges based on tectonic emplacement (stalling) of young, hot oceanic lithosphere beneath the margin, but appear to contradict the major predictions of the slab-gap or asthenospheric-window model. Finally, the Neogene volcanism and major strike-slip faults in the Coast Ranges occur within the thickest regions (>14 km thick) of the forearc, suggesting that the locations of Cenozoic volcanism and faulting along the margin are structurally controlled by the forearc thickness rather than being determined by the location of a broad slab gap.

Deformation across the western United States: A local estimate of Pacific‐North America transform deformation

We obtain a locally based estimate of Pacific‐North America relative motion and an uncertainty in this estimate by integrating deformation rate along three different paths leading west across southwestern North America from east of the Rio Grande Rift to near the continental escarpment. Data are primarily Quaternary geologic slip rate estimates, and resulting deformation determinations therefore are “instantaneous” in a geologic sense but “long term” with respect to earthquake cycles. We deduce a rate of motion of the Pacific plate relative to North America that is 48±2 mm/yr, a rate indistinguishable from that predicted by the global kinematic models RM2 and NUVEL‐1; however, we obtain an orientation that is 5–9° counterclockwise of these models. A more westerly motion of the Pacific plate, with respect to North America, is calculated from all three paths. The relatively westerly motion of the Pacific plate is accommodated by deformation in the North American continent that includes slip on relatively counterclockwise‐oriented strike‐slip faults (including the San Andreas fault), which is especially relevant in and south of the Transverse Ranges, and a margin‐normal component of net extension across the continent, which is especially relevant north of the Transverse Ranges. Deformation of the SW United States occurs in regionally coherent domains within which the style of deformation is approximately uniform. In the vicinity of the Transverse Ranges, two important shear systems splay from the San Andreas fault: the eastern California shear zone trending NNW from the eastern Transverse Ranges and the trans‐Peninsular faults trending SSE from the western and central Transverse Ranges. Within the Transverse Ranges the right‐lateral San Andreas fault steps left, seemingly requiring large amounts of convergence there. However, most of this convergence is avoided through a “funneling flow” of the crust toward the western Transverse Ranges and into the relatively narrow central California Coast Ranges and the northern motion of the Mojave. The former process involves an alternation of rotation direction from counterclockwise (in and south of the central Transverse Ranges) to clockwise (in the western Transverse Ranges).

Compressional uplift in the central California Coast Ranges

Research Article| June 01, 1993 Compressional uplift in the central California Coast Ranges David R. Montgomery David R. Montgomery 1Department of Geological Sciences and Quaternary Research Center, University of Washington, Seattle, Washington 98195 Search for other works by this author on: GSW Google Scholar Author and Article Information David R. Montgomery 1Department of Geological Sciences and Quaternary Research Center, University of Washington, Seattle, Washington 98195 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1993) 21 (6): 543–546. https://doi.org/10.1130/0091-7613(1993)021<0543:CUITCC>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation David R. Montgomery; Compressional uplift in the central California Coast Ranges. Geology 1993;; 21 (6): 543–546. doi: https://doi.org/10.1130/0091-7613(1993)021<0543:CUITCC>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Controversy over the relative importance of transverse and compressional deformation surrounds continuing debate over uplift in the California Coast Ranges. A simple model for compressional deformation in the central California Coast Ranges predicts a significant part of reconstructed post-3 Ma surface uplift. This result suggests that, in addition to passage of the Mendocino riple junction and fault interactions in a dominantly strike-slip setting, late Cenozoic surface uplift in the California Coast Ranges reflects a small compressive component of deformation across the Pacific-North American plate boundary. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Late Quaternary history of colluvial deposition and erosion in hollows, central California Coast Ranges

Colluvial deposits in hollows are the predominant source of debris flows in many mountainous areas, and the depositional history of hollows can provide insight into land-slide frequency and long-term hillslope processes. Detailed study of colluvial deposits in 20 hollows in the northern San Francisco Bay area, California, reveals diverse histories of erosion and deposition. Basal radiocarbon ages range from 1 to 29 ka and document deposition of colluvium throughout this period at different sites. Ages from multiple stratigraphic levels confirm that the deposits are cumulative and have thickened through the Holocene. Radiocarbon dates and stratigraphic observations document unconformities in hollows, reflecting incomplete evacuation of colluvium during both Pleistocene and Holocene events and providing evidence for a cycle of alternating accumulation and evacuation of colluvium. This cycle has apparently been affected by major climatic changes, and the common occurrence of basal ages at ca. 9 to 14 ka may record widespread slope instability during the Pleistocene-Holocene transition. By analogy with modern landsliding, the increased erosion may have been caused by an increased frequency of high-intensity storms during extended periods of meridional flow in the upper atmosphere. The accelerated discharge of colluvium from hillslopes may have contributed to stream aggradation in diverse parts of California.

Geodetic measurement of tectonic deformation in the Santa Maria Fold and Thrust Belt, California

We have analyzed geodetic observations to resolve tectonic deformation across the Santa Maria Fold and Thrust Belt, northwest of Santa Barbara, California. The geodetic network forms a braced quadrilateral with ∼40 km sides whose southwest corner is the Vandenberg very long baseline interferometry station. The data include (1) historical triangulation from the 1880s, 1920s and 1950s; (2) electronic distance measurement trilateration from 1971 and 1985; and (3) Global Positioning System observations from 1986 and 1987. We combine the three different types of data to estimate two‐dimensional station positions and strain rate parameters simultaneously. Using a model which constrains the relative velocity field to be linear in space and constant in time, we find significant strain rates. The maximum compressive strain is oriented N17°E±5°E, and the compressive strain rate in this direction is 0.13±0.03 μstrain/yr. Under the assumption that the unresolved rotational component of the velocity field is zero, we estimate that the integrated rate of deformation across the basin is 7±1 mm/yr oriented at N03°E±13°. This vector can be decomposed into 6±2 mm/yr of crustal shortening on the general structural trend of N30°E and 3±1 mm/yr of right‐lateral shear across this axis. On the basis of these values and earthquake focal mechanisms in the area, we infer that the deformation occurs on northwest trending folds and thrusts within the belt. These results are consistent with the rate and direction of deformation across the central California Coast Ranges inferred by balancing Pacific‐North America plate motion against San Andreas slip and Basin and Range extension. They imply that the Santa Maria Fold and Thrust Belt is the primary active element in transforming motion from the Coast Ranges to the western Transverse Ranges and the Santa Barbara Channel.