Mendocino Triple Junction Research Articles

Acquiring Real Time Data from the Broadband Ocean Bottom Seismic Observatory at Monterey Bay (MOBB)

Two-thirds of the Earth's surface is covered by oceans, which represents a considerable challenge to investigations of global-scale dynamic processes in the Earth's interior and of tectonic processes at ocean-continent boundaries. Long-term ocean-floor observations are also necessary to better constrain regional tectonics, such as on the western margin of North America where tectonics and seismic activity do not stop at the continental edge. In northern California, for example, the most active seismic zone is near the Mendocino triple junction and is mostly offshore, as are a number of hazardous faults such as the San Gregorio and Hosgri faults and part of the San Andreas fault. Much effort has been expended to deploy networks of seismic stations in the western United States, most recently broadband stations, with the simultaneous goals of monitoring the background seismicity, understanding modes of strain release, documenting seismic hazards, and providing constraints on crustal and upper-mantle structure. However, because there are very few offshore islands in central and northern California, practically all stations are located on the continent. As a consequence, the study of plate-boundary processes, as afforded by regional seismological investigations, is heavily skewed on the continental side of the San Andreas fault (SAF) system. Offshore seismicity is poorly constrained, both in location and in mechanisms, as is crustal structure at the edge of the continent. The need for long-term ocean-floor seismic observatories has been widely recognized, and several national and international efforts have been striving for more than two decades to resolve the technological and logistical issues associated with such deployments and establish such observatories ( e.g. , Le Pichon et al. 1987; Purdy and Dziewonski 1988; Purdy 1995; Forsyth et al. 1995; Montagner and Lancelot 1995; Suyehiro et al. 2002, 2006). In April 2002 we deployed a broadband …

Dislocation models of interseismic deformation in the western United States

The GPS‐derived crustal velocity field of the western United States is used to construct dislocation models in a viscoelastic medium of interseismic crustal deformation. The interseismic velocity field is constrained by 1052 GPS velocity vectors spanning the ∼2500‐km‐long plate boundary zone adjacent to the San Andreas fault and Cascadia subduction zone and extending ∼1000 km into the plate interior. The GPS data set is compiled from U.S. Geological Survey campaign data, Plate Boundary Observatory data, and the Western U.S. Cordillera velocity field of Bennett et al. (1999). In the context of viscoelastic cycle models of postearthquake deformation, the interseismic velocity field is modeled with a combination of earthquake sources on ∼100 known faults plus broadly distributed sources. Models that best explain the observed interseismic velocity field include the contributions of viscoelastic relaxation from faulting near the major plate margins, viscoelastic relaxation from distributed faulting in the plate interior, as well as lateral variations in depth‐averaged rigidity in the elastic lithosphere. Resulting rigidity variations are consistent with reduced effective elastic plate thickness in a zone a few tens of kilometers wide surrounding the San Andreas fault (SAF) system. Primary deformation characteristics are captured along the entire SAF system, Eastern California Shear Zone, Walker Lane, the Mendocino triple junction, the Cascadia margin, and the plate interior up to ∼1000 km from the major plate boundaries.

Toroidal mantle flow through the western U.S. slab window

Research Article| April 01, 2008 Toroidal mantle flow through the western U.S. slab window G. Zandt; G. Zandt 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar E. Humphreys E. Humphreys 2Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA Search for other works by this author on: GSW Google Scholar Author and Article Information G. Zandt 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA E. Humphreys 2Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA Publisher: Geological Society of America Received: 26 Jul 2007 Revision Received: 05 Dec 2007 Accepted: 07 Dec 2007 First Online: 03 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2008 Geological Society of America Geology (2008) 36 (4): 295–298. https://doi.org/10.1130/G24611A.1 Article history Received: 26 Jul 2007 Revision Received: 05 Dec 2007 Accepted: 07 Dec 2007 First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation G. Zandt, E. Humphreys; Toroidal mantle flow through the western U.S. slab window. Geology 2008;; 36 (4): 295–298. doi: https://doi.org/10.1130/G24611A.1 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 The circular pattern of anisotropic fast-axis orientations of split SKS arrivals observed in the western U.S. cannot be attributed reasonably to either preexisting lithospheric fabric or to asthenospheric strain related to global-scale plate motion. A plume origin for this pattern accounts more successfully for the anisotropy field, but little evidence exists for an active plume beneath central Nevada. We suggest that mantle flow around the edge of the sinking Gorda–Juan de Fuca slab is responsible for creating the observed anisotropy. Seismic images and kinematic reconstructions of Gorda–Juan de Fuca plate subduction have the southern edge of this plate extending from the Mendocino triple junction to beneath central Nevada, and flow models of narrow subducted slabs produce a strong toroidal flow field around the edge of the slab, consistent with the observed pattern of anisotropy. This flow may enhance uplift, extension, and magmatism of the northern Basin and Range while inhibiting extension of the southern Basin and Range. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Predictive model of San Andreas fault system paleogeography, Late Cretaceous to early Miocene, derived from detailed multidisciplinary conglomerate correlations

Paleogeographic reconstruction of the region of the San Andreas fault system in western California, USA, was hampered for more than two decades by the apparent incompatibility of authoritative lithologic correlations. These led to disparate estimates of dextral strike-slip offsets across the San Andreas fault, notably 315 km between Pinnacles and Neenach Volcanics, versus 563 km offset between Anchor Bay and Eagle Rest peak. Furthermore, one section of the San Andreas fault between Pinnacles and Point Reyes had been reported to have six pairs of features showing only ~ 30 km offset, while several younger features in that same area were reported consistent with ~ 315 km offset. Estimates of total dextral slip on the adjoining San Gregorio fault have ranged from 5 km to 185 km. Sixteen Upper Cretaceous and Paleogene conglomerates of the California Coast Ranges, from Anchor Bay to Simi Valley, were included in a multidisciplinary study centered on identification of matching unique clast varieties, rather than on simply counting general clast types. Detailed analysis verified the prior correlation of the Upper Cretaceous strata of Anchor Bay at Anchor Bay with a then-unnamed conglomerate at Highway 92 and Skyline Road (south of San Francisco); and verified that the Paleocene or Eocene Point Reyes Conglomerate at Point Reyes is a tectonically displaced segment of the Carmelo Formation of Point Lobos (near Monterey). The work also led to three new correlations: Point Reyes Conglomerate with granitic source rock at Point Lobos; a magnetic anomaly at Black Point (near Sea Ranch) with a magnetic anomaly near San Gregorio; and strata of Anchor Bay with previously established source rock, the potassium-poor Logan Gabbro of Eagle Rest peak, at a more recently recognized subsurface location just east of the San Gregorio fault, south of San Gregorio. From these correlations, a Late Cretaceous to early Oligocene paleogeography was constructed which was unique in utilizing modern shapes of tectonic blocks in a paleogeographic resolution synthesizing both the proposed 315 and ~ 30 km San Andreas fault offsets, as well as honoring the lithologic correlation of Anchor Bay with Eagle Rest peak. The model has proved predictive. Since its first introduction, in April and June 1998, other authors have reported seven subsequently identified correlative pairs of geological and geophysical features consistent with it. These required both lateral and temporal expansion of the model: The paleogeography now incorporates 58 pairs of correlative features, covers the period from 70 to 21.3 Ma, and extends from Pelona and Orocopia to the Mendocino triple junction. The model supports the view that fault is not the boundary between the Pacific and North American plates is a wide zone encompassing the San Andreas fault system. However, the model suggests the San Andreas fault is a temporary assemblage of separate segments having different motions, and is neither the primary plate boundary, nor the dominant fault of the San Andreas fault system. By improving resolution of complex spatial–temporal distribution of slip along this evolving tectonic margin, the model provides a firmer foundation for resolution of geophysical issues such as slab window and Pacific Plate subduction models.

Composition of Modern Sand from the Sierra Nevada, California, U.S.A.: Implications for Actualistic Petrofacies of Continental-Margin Magmatic Arcs

Abstract The Sierra Nevada of California represents the roots of a long-lived magmatic arc (primarily Cretaceous) that is presently being as the range is uplifted, beginning in the south and progressing northward. This dissection is occurring concurrently with northward migration of the Mendocino triple junction, south of which magmatic-arc activity is absent, and north of which magmatic-arc activity continues. A north-to-south transect along the Sierra Nevada represents transitions of active magmatic arc to arc to and in the southern Sierra Nevada, to uplifted basement. Thus, analysis of composition of modern sand derived from these separate parts of the Sierra Nevada provides a test for actualistic sand(stone) petrofacies of magmatic arcs at all stages of dissection. Rigorous statistical analysis of the composition of modern sand from the Sierra Nevada characterizes stages of magmatic-arc dissection. Discriminant analysis of point-count data from Sierra Nevada modern sand defines four compositional groups and distinguishes clearly between undissected-arc and uplifted-basement end members. Two intermediate compositional groups reflect the complex geologic history of the Sierra Nevada. Volcanic and plutonic rocks dominate one intermediate group; metamorphic and plutonic rocks dominate the other. The former group exhibits characteristics of Dickinson's (1985) transitional arc, whereas the latter is equivalent to dissected arc. No single compositional parameter is diagnostic of the degree of magmatic-arc dissection, although volcanic-lithic content clearly differentiates between undissected and groups. Regional compositional trends are evident when recalculated point-count data are plotted against latitude. Sand composition in the southern Sierra Nevada reflects significant Cenozoic uplift of the batholith, resulting from Basin and Range extension. High concentrations of quartz and feldspar, and extreme lithic depletion are typical at the southernmost Sierra Nevada; this extreme depletion characterizes basement uplift. The primary drainage divide of the Sierra Nevada forms a rough compositional boundary between eastern and western groups. Potassium-feldspar content best distinguishes these groups, with higher values in the east. Rigorous confidence regions in ternary diagrams yield results that are similar, but not identical, to hexagonal fields of compositional variation. Arithmetic means, associated with hexagonal fields, are not identical to geometric means, associated with logratio confidence regions. Definition of sand(stone) provenance groups, in general, depends on both data-acquisition methods (e.g., Gazzi–Dickinson method) and statistical methods (e.g., Weltje method), so that direct comparison of data analyzed using different methods is unwarranted.

Fault locking, block rotation and crustal deformation in the Pacific Northwest

SummaryWe interpret Global Positioning System (GPS) measurements in the northwestern United States and adjacent parts of western Canada to describe relative motions of crustal blocks, locking on faults and permanent deformation associated with convergence between the Juan de Fuca and North American plates. To estimate angular velocities of the oceanic Juan de Fuca and Explorer plates and several continental crustal blocks, we invert the GPS velocities together with seafloor spreading rates, earthquake slip vector azimuths and fault slip azimuths and rates. We also determine the degree to which faults are either creeping aseismically or, alternatively, locked on the block-bounding faults. The Cascadia subduction thrust is locked mainly offshore, except in central Oregon, where locking extends inland. Most of Oregon and southwest Washington rotate clockwise relative to North America at rates of 0.4–1.0 ° Myr−1. No shear or extension along the Cascades volcanic arc has occurred at the mm/yr level during the past decade, suggesting that the shear deformation extending northward from the Walker Lane and eastern California shear zone south of Oregon is largely accommodated by block rotation in Oregon. The general agreement of vertical axis rotation rates derived from GPS velocities with those estimated from palaeomagnetic declination anomalies suggests that the rotations have been relatively steady for 10–15 Ma. Additional permanent dextral shear is indicated within the Oregon Coast Range near the coast. Block rotations in the Pacific Northwest do not result in net westward flux of crustal material—the crust is simply spinning and not escaping. On Vancouver Island, where the convergence obliquity is less than in Oregon and Washington, the contractional strain at the coast is more aligned with Juan de Fuca—North America motion. GPS velocities are fit significantly better when Vancouver Island and the southern Coast Mountains move relative to North America in a block-like fashion. The relative motions of the Oregon, western Washington and Vancouver Island crustal blocks indicate that the rate of permanent shortening, the type that causes upper plate earthquakes, across the Puget Sound region is 4.4 ± 0.3 mm yr−1. This shortening is likely distributed over several faults but GPS data alone cannot determine the partitioning of slip on them. The transition from predominantly shear deformation within the continent south of the Mendocino Triple Junction to predominantly block rotations north of it is similar to changes in tectonic style at other transitions from shear to subduction. This similarity suggests that crustal block rotations are enhanced in the vicinity of subduction zones possibly due to lower resisting stress.

The lithosphere-asthenosphere boundary beneath the western United States

SUMMARY S receiver functions from 67 broad-band seismic stations in the western United States clearly reveal the existence of a mantle discontinuity with velocity reduction downward, which we interpret as the lithosphere‐asthenosphere boundary (LAB). The average depth of the LAB is ∼70 km. The boundary is relatively sharp with an overall sharpness of less than 20 km. The boundary is more prominent south of the Mendocino Triple Junction, where the Farallon Plate has completely subducted. This may indicate partial melts at the base of the lithosphere caused by the upwelling of the asthenospheric flow through the slab window. A double low velocity zone is observed at base of the lithosphere beneath southern Sierra Nevada, implying a second melting zone at a depth of ∼100 km, well correlated with previous studies of lithospheric delamination in the area.

Abrupt changes in crustal structure beneath the Coast Ranges of northern California - developing new techniques in receiver function analysis

SUMMARY New approaches to receiver function analyses at a suite of broad-band stations in the Coast Ranges of northern California identify rapid changes in crustal thickness accompanied by melt accumulation in the lower crust. These processes are associated with the northward migration of the Mendocino triple junction through the area over the past ∼8 Myr. By simultaneously solving for crustal thickness, Poisson's ratio and Moho strike and dip, we facilitate the application of receiver function analyses in areas of crustal complexity where data sets are sparse. We also introduce a straightforward approach to calculating the Vp/Vs velocity ratio of the seismogenic zone, which is used here to aid in the interpretation of results from the receiver function analysis. Results are consistent with triple junction related crustal modifications occurring in the plate motion direction in a narrow swath of crust between the Maacama and Bartlett Springs Faults. Crustal thicknesses decrease from over 35 km thick in Round Valley, south of the current triple junction location, to 20 km thick just 100 km further south, near Clear Lake. Major crustal thinning is localized to a narrow area of the crust around Redwood Valley, where the Moho dips approximately 17° to the northeast. This thinning may also be restricted to the lower crust rather than being evenly distributed throughout the crustal column. Coincident with these changes, melts are inferred in the lower crust from low velocity zones and elevated values of Poisson's ratio (σ≥ 0.32) at two seismic stations. These melts occur in the same area as melt bodies suggested in other seismological studies. Together these results provide the first detailed description of the crustal modifications—and the spatial extent of that deformation—associated with the passage of the Mendocino triple junction.

Rupture lengths and temporal history of significant earthquakes on the offshore and north coast segments of the Northern San Andreas Fault based on turbidite stratigraphy

74 piston, gravity and jumbo Kasten cores were collected from channel and canyon systems draining the northern California continental margin to investigate the record of periodic Holocene turbidites for possible connection to large magnitude earthquakes on the adjacent Northern San Andreas Fault. Poorly known channel systems were mapped with multibeam sonar to define pathways and channel confluences. Cores sampled all major and many minor channel systems extending from Cape Mendocino to just north of Monterey Bay. Sampling both along and across channels was done and particular attention was paid to channel confluences, as these areas afford opportunities to test for synchronous triggering of turbidity currents. While at sea, all cores were scanned using a GEOTEK multisensor core logger (MSCL), which collects high-resolution photography, P-wave velocity, gamma-ray density, and magnetic susceptibility data from the unsplit cores. Lithology was logged visually, and cores were later imaged with X-radiography.We use 14C ages, relative dating tests at channel confluences, and stratigraphic correlation using physical properties to determine whether turbidites deposited in separate channel systems are correlative, implying they were triggered by a common event. These tests can, in most cases, separate earthquake-triggered turbidity currents from other possible sources. The late Holocene turbidite record off northern California passes these tests for synchronous triggering, and can be correlated with multiple proxies from site to site between Noyo Channel and the latitude of San Francisco. Preliminary comparisons of our event ages with existing and in progress work at onshore coastal sites show good correlation, further circumstantial evidence that the offshore record is primarily earthquake generated. During the last ∼2800 yr, 15 turbidites are recognized, including the great 1906 earthquake. Their chronology establishes an average repeat time of ∼200 yr, similar to the onshore value of ∼230 yr. Along-strike correlation suggests that at least 8 of the youngest 10 of these events ruptured the 320 km distance from the Mendocino Triple Junction to near San Francisco.

Geodetic constraints on areal changes in the Pacific–North America plate boundary zone: What controls Basin and Range extension?

Using ∼1500 geodetic velocities we model the present-day spatial patterns of areal changes inside the Pacific–North America plate boundary zone. From this model we show that between the central Gulf of California and the Queen Charlotte Islands there is no significant net change in surface area. This zero net areal-change result allows us to relate regions of areal growth to areas of equivalent contraction elsewhere within the plate boundary zone. We find that areal growth of the Basin and Range province (BRP) and its eastern margin (∼5.2 ± 0.1 × 10 3 m 2 /yr) is balanced by areal reduction near northwestern California between 38°N and 42°N. The San Andreas fault system south of 38°N and the plate boundary zone north of ∼42°N (including the Juan de Fuca and Gorda Ridge systems) each have no significant net areal change. Our results suggest a kinematic relationship between extension in the BRP and contraction near the northern California Coast Ranges and Klamath Mountains. From these observations we propose that, although BRP extension may be caused by internal forces, the southernmost Cascadia subduction zone provides a “window of escape” that acts as a stress guide to BRP extension as well as northwestward Sierra Nevada motion. Such a dynamic model is consistent with independent findings that (1) the least principal horizontal stress orientations in the BRP are toward northern California, (2) extension directions in the BRP have changed orientation to track the northward migration of the Mendocino triple junction, and (3) the southernmost Cascadia subduction zone is a relatively weak plate boundary.

Late Neogene and Quaternary landscape evolution of the northern California Coast Ranges: Evidence for Mendocino triple junction tectonics

A landscape records the surface response to tectonics at time scales intermediate between short time-scale information derived from seismic imaging and global positioning systems and the long-term geologic record. We link late Neogene and Quaternary deposits and landforms in the northern California Coast Ranges to the tectonics of the Mendocino triple junction. In the northern California Coast Ranges, the Mendocino crustal conveyor geodynamic model describes crustal thickening, thinning, and dynamic topography that produce a pattern of uplift that migrates northward with the Mendocino triple junction. The tectonics are manifest in the drainage system and elevation pattern of the Coast Ranges. At long wavelengths, the elevation pattern closely matches the predicted double-peaked shape of Mendocino crustal conveyor topography, and the high points of uplift control the location of drainage divides. Presently, the divide between the Russian and Eel Rivers and the divide between the Eel and Van Duzen Rivers approximately correspond to the peaks of uplift predicted by the Mendocino crustal conveyor model. As the triple junction migrates northward, the double-humped pattern of uplift and subsidence migrates, and the Coast Ranges emerge. Smaller drainages develop and evolve by stream capture and flow reversal, and the two main divides migrate in concert with the triple junction. In contrast to the systematic development of the small streams, the largest trunk streams can maintain grade through regions of high uplift, and coastal river mouths remain stationary despite the uplift moving north. Before ca. 2 Ma, the majority of the Coast Range drainage flowed to a southern coastal outlet near the present mouth of the Russian River. At 2 Ma, facilitated by headwater stream capture at key locations, the drainage direction reversed, and the majority of Coast Range rivers now drain into the north-flowing Eel River. The major drainage reorganization at 2 Ma highlights the potential for complexity in geomorphic response to tectonics.

Evidence for melt injection in the crust of northern California?

The occurrence of an earthquake sequence near Lake Pillsbury, California during a 6-month period in mid-2000 can be linked to magma transport in the crust; this activity is potentially a pre-cursor to future volcanism. The position of Cenozoic volcanism in the crust of the northern California Coast Ranges is directly linked to the northward migration of the Mendocino triple junction (MTJ) and the formation of a slab window beneath North American crust. The most recent manifestation of this is the Clear Lake volcanics, which began erupting ∼ 2 Ma. With the continued migration of the triple junction, volcanic activity is expected to occur to the north of Clear Lake in the future. These volcanic centers erupt in areas where the crust is undergoing significant modification as a result of slab window processes, and reflect important components in the development of the San Andreas plate boundary system. When precisely relocated, the earthquake sequence defines a streak of seismicity at ∼ 8 km depth. This sequence demonstrates a local time–space migration over a distance of ∼ 10 km in two major pulses of activity in the 6-month time period. Stress modeling allows us to infer that these earthquakes may be the result of several dike injections. In combination with evidence for lower crustal melts in the same region from other seismological studies, we propose that these earthquakes help map the occurrence of melt injection in the mid-crust to depths of ∼ 10 km, and present evidence that MTJ-related melt in the crust of northern California has migrated north of the Clear Lake volcanic center.

GPS-derived strain in northwestern California: Termination of the San Andreas fault system and convergence of the Sierra Nevada–Great Valley block contribute to southern Cascadia forearc contraction

GPS-derived velocities (1993–2002) in northwestern California show that processes other than subduction are in part accountable for observed upper-plate contraction north of the Mendocino triple junction (MTJ) region. After removing the component of elastic strain accumulation due to the Cascadia subduction zone from the station velocities, two additional processes account for accumulated strain in northern California. The first is the westward convergence of the Sierra Nevada–Great Valley (SNGV) block toward the coast and the second is the north–northwest impingement of the San Andreas fault system from the south on the northern California coastal region in the vicinity of Humboldt Bay. Sierra Nevada–Great Valley block motion is northwest toward the coast, convergent with the more northerly, north–northwest San Andreas transform fault-parallel motion. In addition to the westward-converging Sierra Nevada–Great Valley block, San Andreas transform-parallel shortening also occurs in the Humboldt Bay region. Approximately 22 mm/yr of distributed Pacific–SNGV motion is observed inland of Cape Mendocino across the northern projections of the Maacama and Bartlett Springs fault zones but station velocities decrease rapidly north of Cape Mendocino. The resultant 6–10 mm/yr of San Andreas fault-parallel shortening occurs above the southern edge of the subducted Gorda plate and at the latitude of Humboldt Bay. Part of the San Andreas fault-parallel shortening may be due to the viscous coupling of the southern edge of the Gorda plate to overlying North American plate. We conclude that significant portions of the upper-plate contraction observed north of the MTJ region are not solely a result of subduction of the Gorda plate but also a consequence of impingement of the western edge of the Sierra Nevada–Great Valley block and growth of the northernmost segments of the San Andreas fault system.

Multimethod detrital thermochronology of the Great Valley Group near New Idria, California

The simultaneous use of several thermochronological methods on replicate sedimentary rock samples can reveal their pre- and postdepositional history. Single grain U/Pb dating of zircon, zircon and apatite fi ssion track dating, and vitrinite refl ectance measurements were performed on Cretaceous through Miocene sedimentary rocks of the Great Valley Group and the Temblor Formation near Coalinga and New Idria, California. The data show that the Sierra Nevada was exhumed and cooled at ~0.5–1 km/m.y. or ~20 °C/m.y. during the Cretaceous. After deposition in the Great Valley forearc basin, Sierra Nevada erosional products were buried at great depth under low thermal gradients. At ca. 12–14 Ma, northward progression of the Mendocino triple junction triggered folding on the eastern fl anks of the California Coast Ranges and rapid exhumation of the New Idria serpentinite diapir. This middle Miocene event caused the deposition of spectacular deposits of sedimentary serpentinite (Big Blue Formation). The rapid rise of the hot serpentinite body created a thermal pulse that may have provided the enigmatic heat source for oil fi elds in the shallow Vallecitos syncline, a few kilometers north of New Idria.

Late Neogene and Quaternary landscape evolution of the northern California Coast Ranges: Evidence for Mendocino triple junction tectonics

Late Neogene and Quaternary landscape evolution of the northern California Coast Ranges: Evidence for Mendocino triple junction tectonics

Waveform Cross-Correlation-Based Differential Travel-Time Measurements at the Northern California Seismic Network

We processed the complete digital seismogram database for northern California to measure accurate differential travel times for correlated earthquakes observed at common stations. Correlated earthquakes are earthquakes that occur within a few kilometers of one another and have similar focal mechanisms, thus generating similar waveforms, allowing measurements to be made via cross-corre- lation analysis. The waveform database was obtained from the Northern California Earthquake Data Center and includes about 15 million seismograms from 225,000 local earthquakes between 1984 and 2003. A total of 26 billion cross-correlation measurements were performed on a 32-node (64 processor) Linux cluster, using improved analysis tools. All event pairs with separation distances of 5 km or less were processed at all stations that recorded the pair. We computed a total of about 1.7 billion P-wave differential times from pairs of waveforms that had cross- correlation coefficients (CC) of 0.6 or larger. The P-wave differential times are often on the order of a factor of ten to a hundred times more accurate than those obtained from routinely picked phase onsets. 1.2 billion S-wave differential times were mea- sured with CC 0.6, a phase not routinely picked at the Northern California Seismic Network because of the noise level of remaining P coda. We found that approxi- mately 95% of the seismicity includes events that have cross-correlation coefficients of CC 0.7 with at least one other event recorded at four or more stations. At some stations more than 40% of the recorded events are similar at the CC 0.9 level, indicating the potential existence of large numbers of repeating earthquakes. Large numbers of correlated events occur in different tectonic regions, including the San Andreas Fault, Long Valley caldera, Geysers geothermal field and Mendocino triple junction. Future research using these data may substantially improve earthquake lo- cations and add insight into the velocity structure in the crust.

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.

Southward-younging apatite (U–Th)/He ages in the northern California Coast Ranges due to a northward-migrating crustal welt

(U–Th)/He in apatite dating of Franciscan Formation samples from between 39°47′N and 39°08′N in the northern California Coast Ranges (NCCR) reveals a southward- and downward-younging pattern of ages (21.8 ± 0.6 Ma to 7.9 ± 0.9 Ma). These ages are interpreted to represent a northward or northeastward dipping fossil He partial retention zone that was tilted and exhumed by a northward-migrating pulse of crustal thickening that begins ∼150–200 km ahead of the migrating Mendocino Triple Junction (MTJ) [K.P. Furlong, R. Govers, Ephemeral crustal thickening at a triple junction: the Mendocino crustal conveyor, Geology 27 (1999) 127–130). The ∼8 Ma ages from the southern end of the sample transect are interpreted to come from rocks that were immediately below the He partial retention zone when crustal thickening began, ∼5 my before the MTJ passed by; these rocks have now reached the surface, ∼3 my after passage of the MTJ. The older ages from the central and northern portions of the sample transect are interpreted to represent rocks that were within the He partial retention zone prior to crustal thickening ahead of the MTJ, and which therefore record ages transitional from older subduction-related cooling and exhumation to more recent transform-related cooling and exhumation. In addition to providing support for the model of a migrating crustal welt associated with the MTJ, the existence of an essentially intact, gently tilted fossil partial retention zone in the northern California Coast Range raises the possibility of using isochronous surfaces as neotectonic structural markers in this geologically complex and heavily vegetated region.

Migration of the Mendocino Triple Junction and the Origin of Titanium-Rich Mineral Suites at New Idria, California

Titanium-rich minerals in peculiar mineral suites are associated with the New Idria serpentinite, California. Previously some of these Ti-rich minerals have been described as metasomatic in origin. This paper presents new data on the compositions and parageneses of these mineral suites, and presents a petrologic model for their formation by essentially isochemical metamorphism from precursor assemblages. All of these mineral suites are manifestations of tectonic uplift, M2 metamorphism, and magmatism associated with the passage of the Mendocino Triple Junction underneath New Idria at about 12 Ma. Ti-bearing garnets associated with chlorite and diopside occur in metamorphosed pyroxenite veins that have been tectonically dismembered during emplacement of the serpentinite. These veins have been metamorphosed under greenschist or blueschist (M1) and lower greenschist (M2) conditions. Accessory minerals in these rocks include, among others, perovskite, titanite, zircon, Ti-REEvesuvianite, zirconolite, pyrophanite, and copper sulfides. Benitoite, the state gemstone of California, is formed by lower greenschist M2 metamorphism of tectonic blocks of Franciscan greenstone entrained within the serpentinite. Benitoite is found in association with neptunite, joaquinite, natrolite, actinolite and clinozoisite. Kaersutite crystals up to 30 cm long, containing up to 4 wt% TiO2 occur with albite, apatite, and other accessory minerals in the New Idria syenite, a small group of stocks associated with the serpentinite. This syenite is shown to have an intrusive origin and an age of 12.4 ± 0.8 Ma.

Four-dimensional transform fault processes: progressive evolution of step-overs and bends

Many bends or step-overs along strike–slip faults may evolve by propagation of the strike–slip fault on one side of the structure and progressive shut-off of the strike–slip fault on the other side. In such a process, new transverse structures form, and the bend or step-over region migrates with respect to materials that were once affected by it. This process is the progressive asymmetric development of a strike–slip duplex. Consequences of this type of step-over evolution include: (1) the amount of structural relief in the restraining step-over or bend region is less than expected; (2) pull-apart basin deposits are left outside of the active basin; and (3) local tectonic inversion occurs that is not linked to regional plate boundary kinematic changes. This type of evolution of step-overs and bends may be common along the dextral San Andreas fault system of California; we present evidence at different scales for the evolution of bends and step-overs along this fault system. Examples of pull-apart basin deposits related to migrating releasing (right) bends or step-overs are the Plio-Pleistocene Merced Formation (tens of km along strike), the Pleistocene Olema Creek Formation (several km along strike) along the San Andreas fault in the San Francisco Bay area, and an inverted colluvial graben exposed in a paleoseismic trench across the Miller Creek fault (meters to tens of meters along strike) in the eastern San Francisco Bay area. Examples of migrating restraining bends or step-overs include the transfer of slip from the Calaveras to Hayward fault, and the Greenville to the Concord fault (ten km or more along strike), the offshore San Gregorio fold and thrust belt (40 km along strike), and the progressive transfer of slip from the eastern faults of the San Andreas system to the migrating Mendocino triple junction (over 150 km along strike). Similar 4D evolution may characterize the evolution of other regions in the world, including the Dead Sea pull-apart, the Gulf of Paria pull-apart basin of northern Venezuela, and the Hanmer and Dagg basins of New Zealand.