Pacific-North American Plate Boundary Research Articles

The northbrae rhyolite of Berkeley (California, USA) constrains motion of the proto-Hayward Fault

ABSTRACT Right-lateral transform motion associated with the Pacific-North American plate boundary in the modern-day San Francisco Bay Area occurs across a series of sub-parallel fault zones. Much of this motion is accommodated east of the San Andreas Fault by the faults of the East Bay fault system. A major tool for reconstructing the spatial and temporal history of fault motion is the correlation of offset Neogene volcanic rocks. These Neogene volcanics within the California Coast Ranges formed in association with the slab gap that grew as the Mendocino Triple Junction migrated northward. Some of the volcanic centres have been variably offset by subsequent strike-slip faulting. A felsic volcanic unit exposed in Berkeley, CA, known as the Northbrae rhyolite has variably been interpreted to be one of these Neogene volcanic units or to be a Mesozoic volcanic unit associated with the Coast Range ophiolite. A new U-Pb zircon date of 11.10 ± 0.09 Ma confirms the Neogene volcanic interpretation. This date is indistinguishable from previously published Ar/Ar dates from the Burdell Mountain volcanics of the North Bay region as well as a new U-Pb zircon date of 11.07 ± 0.10 Ma. In addition to the indistinguishable ages, similarities in bulk lithology, zircon crystallization/dissolution textures, and zircon trace element geochemistry are consistent with these rhyolites being associated with the same volcanic centre. This correlation implies that 40 ± 5 km of right-lateral offset occurred to the west of the modern-day position of the Hayward-Rodgers Creek fault zone. This offset represents ∼ 20% of the total offset along the East Bay fault system. A proto-Hayward Fault with a different geometry than that of the present-day played a significant role in the evolution of the fault system. This result highlights the dynamic spatiotemporal variability of strike-slip faults along transform margins.

Prolonged post-orogenic extension in the southeastern Canadian Cordillera: Miocene reactivation of the Columbia River Fault

Post-orogenic extension in the southeastern Canadian Cordillera manifested in the form of metamorphic core complexes exhumed during the Paleocene–Eocene, but new thermochronology data suggest this interval may be longer. The Columbia River Fault is a brittle-ductile normal fault bounding the eastern margin of the Monashee Complex, southeastern British Columbia. Vertical motions along this fault are investigated here using apatite fission-track and (U-Th)/He thermochronology and samples from two elevation profiles. Apatite fission track ages range from 29.3 to 17.7 Ma, and apatite (U-Th)/He ages range from 26.6 to 7.0 Ma. Inverse thermal history modeling of these profiles suggests the footwall underwent rapid cooling between 18 and 16 Ma, whereas the hanging wall experienced more protracted cooling since 40 Ma, with two small cooling pulses at 20 and 10 Ma. These new data suggest the Columbia River Fault was reactivated in the early Miocene and continued to exhume the Monashee Complex an additional ∼2 km. Combining all thermochronology data from the region suggests three phases of accelerated cooling in the Monashee Complex; 1) Eocene (56–40 Ma); 2) Miocene (20–16 Ma); and 3) Pliocene to recent. Thermal history modeling of thermochronometric data reported from elevation profiles from across the wider region of the southern Canadian Cordillera find early Miocene cooling phases contemporaneous with that seen along the Columbia River Fault. This widespread exhumation coincides with early Miocene plate tectonic reorganization of the Pacific-North American margin which resulted in increased transtension in northwestern North America. We propose that the early Miocene reactivation of the Columbia River Fault is a splay fault associated with dextral strike slip faulting along the Rocky Mountain Trench, driven by an increase in transtensional stresses along the Pacific-North American plate boundary. Building on earlier models, pre-existing gravitational instabilities in the southeastern Canadian Cordillera aided in lateral extension of the upper crust along unstable inherited structures throughout the Shuswap Metamorphic Core Complex

Tectonic stress fields inferred from long-term CMT data ranging over different periods

SUMMARYWe have developed an inversion method for estimating 3-D tectonic stress fields from centroid moment tensor (CMT) data of seismic events. To obtain reliable inversion analysis results, we need to use as much data as possible, ranging over different periods. In this study, we improved the previously developed CMT data inversion method to incorporate preceding inversion analysis results into the present analysis as direct prior information about the stress field. The spatial distribution of seismic events and/or their focal mechanisms in a region would change from one period to another, reflecting temporal stress changes there. In the improved inversion method, in addition to the hyperparameter controlling the roughness of stress fields, we introduced another hyperparameter that controls the relative weight of direct prior information to the observed data to be analysed. The relative weights of direct and indirect prior information to the observed data are appropriately determined by using Akaike's Bayesian Information Criterion. We applied the improved inversion method to the observed CMT data of seismic events of 24 yr (1997−2020) in northeast Japan, where the Tohoku-oki megathrust event (Mw 9.0) occurred on 11 March 2011 at the North American–Pacific Plate boundary. To investigate temporal changes in tectonic stress orientation, we divided the target period into the inter-, pre- and post-seismic periods of the megathrust event: P1 (January 1997−January 2007), P2 (February 2007−February 2011), and P3 (March 2011−December 2020). According to successive inversion analysis results for P1 and P2, we confirmed that the tectonic stress orientation was stable for 14 yr before the megathrust event. On the other hand, successive inversion analysis results for P2 and P3 indicated a possibility that the 2011 megathrust event caused significant changes in tectonic stress orientation in three regions. However, two of them were negative in the quantitative evaluation of the reliability of detected temporal changes. In conclusion, the stress orientation change in the region off Fukushima and Ibaraki, the southern margin of the main rupture area of the megathrust event, seems to be real.

Middle-late Miocene rift-related magmatism in Guaymas, Sonora, Mexico at the eastern margin of the Gulf of California: Petrogenetic implications associated to the Pacific-north American plate boundary

Miocene volcanism in the Guaymas region at the eastern margin of the Gulf of California, in coastal Sonora, NW Mexico, was dominated by arc magmatism. However, the presence of rift-related magmatism is also exposed. Rocks associated with subduction of the Farallon plate beneath North America plate are of Oligocene-Miocene ages, while the magmatic rocks with ages between ca. 12.5 to ca. 6 Ma occur in a post-subduction tectonic setting. This period is sometimes referred to as the proto-Gulf phase. The proto-Gulf magmatic rocks are exposed as part of a volcanic igneous plumbing system, herein named Guaymas Group. The Guaymas Group has an age range between ∼12 and 10 Ma and is defined by two magmatic events: an intermediate compositional pulse (Unit 1) and a felsic pulse (Unit 2).The lithological units from the Guaymas Group have similar geochemical features, exhibiting LILE and LREE enrichment and Nb, Ta and Ti depletion. These rocks show a transitional to tholeiitic composition affinity that differs from the arc magmatism from coastal Sonora. The geochemical data suggest that assimilation and fractional crystallization (AFC) processes played an important role in the geochemical variations observed in volcanic and subvolcanic rocks. The geochemical characteristics of the Guaymas Group rocks also could indicate an origin by decompression melting of a mantle source metasomatized by subduction processes.Finally, considering the geological, geochronological and geochemical study, we suggest that the emplacement of the Guaymas Group rocks is contemporaneous with the oblique rifting environment associated with an incipient Pacific-North American plate boundary.

Clustering Analysis Methods for GNSS Observations: A Data-Driven Approach to Identifying California's Major Faults.

We present a data‐driven approach to clustering or grouping Global Navigation Satellite System (GNSS) stations according to observed velocities, displacements or other selected characteristics. Clustering GNSS stations provides useful scientific information, and is a necessary initial step in other analysis, such as detecting aseismic transient signals (Granat et al., 2013, https://doi.org/10.1785/0220130039). Desired features of the data can be selected for clustering, including some subset of displacement or velocity components, uncertainty estimates, station location, and other relevant information. Based on those selections, the clustering procedure autonomously groups the GNSS stations according to a selected clustering method. We have implemented this approach as a Python application, allowing us to draw upon the full range of open source clustering methods available in Python's scikit‐learn package (Pedregosa et al., 2011, https://doi.org/10.5555/1953048.2078195). The application returns the stations labeled by group as a table and color coded KML file and is designed to work with the GNSS information available from GeoGateway (Donnellan et al., 2021, https://doi.org/10.1007/s12145-020-00561-7; Heflin et al., 2020, https://doi.org/10.1029/2019ea000644) but is easily extensible. We demonstrate the methodology on California and western Nevada. The results show partitions that follow faults or geologic boundaries, including for recent large earthquakes and post‐seismic motion. The San Andreas fault system is most prominent, reflecting Pacific‐North American plate boundary motion. Deformation reflected as class boundaries is distributed north and south of the central California creeping section. For most models a cluster boundary connects the southernmost San Andreas fault with the Eastern California Shear Zone (ECSZ) rather than continuing through the San Gorgonio Pass.

Earthquake Magnitudes from Dynamic Strain

ABSTRACTDynamic strains have never played a role in determining local earthquake magnitudes, which are routinely set by displacement waveforms from seismic instrumentation (e.g., ML). We present a magnitude scale for local earthquakes based on broadband dynamic strain waveforms. This scale is derived from the peak root-mean-squared strains (A) in 4589 records of dynamic strain associated with 365 crustal earthquakes and 77 borehole strainmeters along the Pacific-North American plate boundary on the west coast of the United States and Canada. In this data set, catalog moment magnitudes range from 3.5≤Mw≤7.2, and hypocentral distances range from 6≤R≤500 km. The 1D representation of geometrical spreading and attenuation of A common to all strain data is logA0(R)=−0.00072R−1.45log(R). After correcting for instrument gain, site terms, and event terms, the magnitude scale, MDS=logA−logA0(R)−log(3×10−9), scales as ≈0.92Mw with a residual standard deviation of 0.19. This close association with Mw holds for events east of the −124° meridian; west of this boundary, however, a constant correction of 0.41 is needed to adjust for additional along-path attenuation effects. As a check on the accuracy of this magnitude scale, we apply it to dynamic strain records from three strainmeters located in the near field of the 2019 M 6.4 and 7.1 Ridgecrest earthquakes. Results from these six records are in agreement to within 0.5 magnitude units, and five out of six records are in agreement to within 0.34 units.

Quaternary slip rates on the White Mountains fault zone, eastern California: Implications for comparing geologic to geodetic slip rates across the Walker Lane

AbstractThe White Mountains fault zone in eastern California is a major fault system that accommodates right-lateral shear across the southern Walker Lane. We combined field geomorphic mapping and interpretation of high-resolution airborne light detection and ranging (LiDAR) digital elevation models with 10Be cosmogenic nuclide exposure ages to calculate new late Pleistocene and Holocene right-lateral slip rates on the White Mountains fault zone. Alluvial fans were found to have ages of 46.6 + 11.0/–10.0 ka and 7.3 + 4.2/–4.5 ka, with right-lateral displacements of 65 ± 13 m and 14 ± 5 m, respectively, yielding a minimum average slip rate of 1.4 ± 0.3 mm/yr. These new slip rates help to resolve the kinematics of fault slip across this part of the complex Pacific–North American plate boundary. Our results suggest that late Pleistocene slip rates on the White Mountains fault zone were significantly faster than previously reported. These results also help to reconcile a portion of the observed discrepancy between modern geodetic strain rates and known late Pleistocene slip rates in the southern Walker Lane. The total middle to late Pleistocene slip rate from the southern Walker Lane near 37.5°N was 7.9 + 1.3/–0.6 mm/yr, ∼75% of the observed modern geodetic rate.

Slip history and the role of the Agua Blanca fault in the tectonics of the North American–Pacific plate boundary of southern California, USA and Baja California, Mexico

Slip history and the role of the Agua Blanca fault in the tectonics of the North American–Pacific plate boundary of southern California, USA and Baja California, Mexico

Sedimentary signals of recent faulting along an old strand of the San Andreas Fault, USA

Continental transform fault systems are fundamental features in plate tectonics. These complex systems often constitute multiple fault strands with variable spatio-temporal histories. Here, we re-evaluate the complex history of the San Andreas Fault along a restraining bend in southern California (USA). The Mission Creek strand of the San Andreas Fault is a major geologic structure with ~90 km of strike-slip displacement but is currently mapped as inactive. Quaternary deposits record sediment dispersal across the fault from upland catchments and yield key markers of the fault’s displacement history. Our sediment provenance analysis from the Deformed Gravels of Whitewater and the Cabezon Fanglomerate provide detrital geochronologic and lithologic signatures of potential sources within the San Bernardino Mountains and Little San Bernardino Mountains. Statistical analysis shows that the Cabezon Fanglomerate is most compatible with the Mission Creek and Morongo Valley Canyon sources, rather than the Whitewater Canyon as previously suggested. We propose that displacement since deposition ~500–100 ka across the Mission Creek strand has separated these deposits from their original sources. These findings challenge the current paradigm that the Mission Creek strand is inactive and suggest that the fault continues to be a primary structure in accommodating deformation along the Pacific-North American plate boundary.

Spatio-Temporal Mapping of Plate Boundary Faults in California Using Geodetic Imaging

The Pacific–North American plate boundary in California is composed of a 400-km-wide network of faults and zones of distributed deformation. Earthquakes, even large ones, can occur along individual or combinations of faults within the larger plate boundary system. While research often focuses on the primary and secondary faults, holistic study of the plate boundary is required to answer several fundamental questions. How do plate boundary motions partition across California faults? How do faults within the plate boundary interact during earthquakes? What fraction of strain accumulation is relieved aseismically and does this provide limits on fault rupture propagation? Geodetic imaging, broadly defined as measurement of crustal deformation and topography of the Earth’s surface, enables assessment of topographic characteristics and the spatio-temporal behavior of the Earth’s crust. We focus here on crustal deformation observed with continuous Global Positioning System (GPS) data and Interferometric Synthetic Aperture Radar (InSAR) from NASA’s airborne UAVSAR platform, and on high-resolution topography acquired from lidar and Structure from Motion (SfM) methods. Combined, these measurements are used to identify active structures, past ruptures, transient motions, and distribution of deformation. The observations inform estimates of the mechanical and geometric properties of faults. We discuss five areas in California as examples of different fault behavior, fault maturity and times within the earthquake cycle: the M6.0 2014 South Napa earthquake rupture, the San Jacinto fault, the creeping and locked Carrizo sections of the San Andreas fault, the Landers rupture in the Eastern California Shear Zone, and the convergence of the Eastern California Shear Zone and San Andreas fault in southern California. These examples indicate that distribution of crustal deformation can be measured using interferometric synthetic aperture radar (InSAR), Global Navigation Satellite System (GNSS), and high-resolution topography and can improve our understanding of tectonic deformation and rupture characteristics within the broad plate boundary zone.

Accommodation of missing shear strain in the Central Walker Lane, western North America: Constraints from dense GPS measurements

We present 264 new interseismic GPS velocities from the Mobile Array of GPS for Nevada Transtension (MAGNET) and continuous GPS networks that measure Pacific–North American plate boundary deformation in the Central Walker Lane. Relative to a North America-fixed reference frame, northwestward velocities increase smoothly from ∼4 mm/yr in the Basin and Range province to 12.2 mm/yr in the central Sierra Nevada resulting in a Central Walker Lane deformation budget of ∼8 mm/yr. We use an elastic block model to estimate fault slip and block rotation rates and patterns of deformation from the GPS velocities. Right-lateral shear is distributed throughout the Central Walker Lane with strike-slip rates generally <1.5 mm/yr predicted by the block model, but extension rates are highest near north-striking normal faults found along the Sierra Nevada frontal fault system and in a left-stepping, en-echelon series of asymmetric basins that extend from Walker Lake to Lake Tahoe. Neotectonic studies in the western Central Walker Lane find little evidence of strike-slip or oblique faulting in the asymmetric basins, prompting the suggestion that dextral deformation in this region is accommodated through clockwise block rotations. We test this hypothesis and show that a model relying solely on the combination of clockwise block rotations and normal faulting to accommodate dextral transtensional strain accumulation systematically misfits the GPS data in comparison with our preferred model. This suggests that some component of oblique or partitioned right-lateral fault slip is needed to accommodate shear in the asymmetric basins of the western Central Walker Lane. Present-day clockwise vertical axis rotation rates in the Bodie Hills, Carson Domain, and Mina Deflection are between 1–4°/Myr, lower than published paleomagnetic rotation rates, suggesting that block rotation rates have decreased since the Late to Middle Miocene.

Seismic Rupture on an Oceanic-Continental Plate Boundary: Strike-Slip Earthquakes along the Queen Charlotte-Fairweather Fault

Abstract We investigate the 2013 M w 7.5 Craig, Alaska, earthquake and nearby seismicity to understand better how temperature and composition may control the depth of seismic rupture along a strike‐slip fault offsetting contrasting lithosphere types. The Queen Charlotte–Fairweather (QCF) fault lies between the oceanic lithosphere of the Pacific plate and the accreted Insular superterrane of the North American continent. We use point‐source and finite‐fault modeling of teleseismic body waves to characterize the focal mechanism and the depth extent of seismic rupture of five M w 5.9–7.5 earthquakes. Four of the five earthquakes are consistent with rupture on the QCF fault. We find that these four earthquakes have centroid depths between 11 and 18 km (±3 km) and aftershocks with an order of magnitude less than typical continental earthquakes. Finite‐fault modeling of the 2013 Craig earthquake favors bilateral rupture along a 150 km fault with a depth range of slip between 5 and 25 km, with faster rupture (4–5 km/s) to the north than the south (1 km/s). These results suggest that the transition of brittle to ductile deformation along this section of the Pacific–North American plate boundary is thermally controlled by a more mafic rheology than average continental crust, exhibiting behavior consistent with that of an oceanic strike‐slip fault. Online Material: Figures of best‐fitting point‐source focal mechanism and waveform fits, dip sensitivity, directivity tests, aftershock distributions, and finite‐fault results.

Introduction to the Special Issue on the 2012 Haida Gwaii and 2013 Craig Earthquakes at the Pacific-North America Plate Boundary (British Columbia and Alaska)

The 27 October 2012 M w 7.8 Haida Gwaii thrust earthquake and the 5 January 2013 M w 7.5 Craig strike‐slip earthquake are the focus of this special issue. They occurred along the transform boundary between the Pacific and North American plates (Fig. 1). The most identifiable feature of the plate boundary, the strike‐slip Queen Charlotte fault, might be viewed as typical of continent–ocean transform faults because it separates the continental crust of the North American plate from oceanic crust of the Pacific plate for most of its length. However, the current relative plate motion of about 5 cm/yr is highly oblique to the Queen Charlotte fault, causing a transpressive plate boundary in the region. Figure 1. Location map for the 2012 M w 7.8 Haida Gwaii and 2013 M w 7.5 Craig earthquakes, which occurred on the Pacific–North American plate boundary. Approximate rupture zones for the two events are shown in yellow, whereas other significant events on the plate boundary are depicted by their aftershock zones (red, 1949 Queen Charlotte fault; orange, 1970 Cape St. James; blue, 1972 Sitka). The epicentral positions are given by stars, and the focal mechanisms are plotted in a lower‐hemisphere projection. The black arrows indicate Pacific–North American relative motion. The Queen Charlotte terrace is located west of the Queen Charlotte fault and is interpreted to be a narrow accretionary prism. (Inset) The earthquakes occurred offshore the western margin of northern North America. Barbs indicate subduction of the Juan de Fuca (JDF) plate and smaller plates beneath North America at the Cascadia subduction zone (CSZ). Transform plate boundaries between continental and oceanic crust are often depicted as both simple and common. In reality, such continent–ocean transform boundaries are neither. For example, the San Andreas fault in California carries continental crustal fragments along with it on the seaward side …

Rupture Process of the 2012 Mw 7.8 Haida Gwaii Earthquake from an Empirical Green's Function Method

Abstract This article examines rupture processes of the 28 October 2012 M w 7.8 Haida Gwaii earthquake off the coast of British Columbia, Canada, using an empirical Green’s function (EGF) technique. The Haida Gwaii earthquake was the largest event along the Canadian portion of the Pacific–North American plate boundary since the M s 8.1 Queen Charlotte earthquake of 1949. It occurred along a potentially blind thrust fault dipping gently to the northeast rather than the main, subvertical Queen Charlotte fault. Surface waveforms from a 2001 M w 6.3 event, located only 15 km from the 2012 epicenter and with similar mechanism, are used as an EGF and deconvolved from those of the 2012 mainshock. The resulting source time functions contain minimal path effects, focal mechanism effects, and instrument response, so the waveforms display only properties of the 2012 mainshock rupture itself. By examining azimuthal variations in these source time functions, we constrain parameters such as average rupture velocity, extent, and directivity. In addition, information is obtained about the possible existence of major subevents and their relative locations. Results indicate two subevents within this rupture, the first 12 km south and updip of the epicenter and the second approximately 28 km from the first along a heading parallel to the Queen Charlotte terrace (∼323°). Overall, the rupture front propagated roughly 50 km at an azimuth of 308.5°. This evidence for directivity to the northwest is important, given that earthquakes with strong directivity, such as the 2002 M w 7.9 Denali earthquake, have been shown to be capable of triggering earthquakes thousands of kilometers away. In this case, we suggest that northwest directivity of this earthquake is responsible for amplification of surface waves observed at seismic stations in Alaska (Gomberg, 2013) and may provide a potential link between this 2012 event and the 2013 Craig, Alaska, earthquake. Online Material: Figure of all relative source time functions used in directivity analysis.

Coulomb Stress Changes Following the 2012 Mw 7.8 Haida Gwaii, Canada, Earthquake: Implications for Seismic Hazard

This article examines spatial changes to the local stress field resulting from the 28 October 2012, M w 7.8 Haida Gwaii earthquake, off the west coast of Moresby Island, British Columbia. This event occurred on a northeast‐dipping, potentially blind‐thrust fault rather than on the subvertical Queen Charlotte fault (QCF) that represents the Pacific–North American plate boundary. This was the largest earthquake along the Canadian portion of this plate boundary since the 1949 M s 8.1 Queen Charlotte earthquake. The U.S. Geological Survey Coulomb software is used to quantitatively estimate the effect of the mainshock on the background stress field, the known aftershock nodal planes, and the nearby QCF. We use two different mainshock finite‐fault models, both of which are seismologically derived (by Lay et al. , 2013, and Hayes, 2013, separately) and subsequently adapted by K. Wang to account for the motion detected at four nearby Global Positioning System stations (see Nykolaishen et al. , 2015, for more information). We also use the best‐located set of aftershocks with information provided by a temporary array of ocean‐bottom seismometers. Results indicate an apparent clustering of aftershocks slightly seaward of the main thrust, which is consistent with the modeled zone of promoted normal failure, likely related to extension in the footwall. Using existing models, we found a high number of aftershocks to be consistent with triggering by the mainshock, suggesting that static stress is a dominant control in the months following a large earthquake in this area. Further, we find loading greater than the triggering threshold on the QCF in an area interpreted as a seismic gap. This work improves understanding of the evolving seismic hazard along the Queen Charlotte margin and tests the usefulness of Coulomb modeling in this complex tectonic environment. Online Material: Figures of focal mechanisms and maximum Coulomb stress change, and table of aftershock moment tensor parameters.

Latest Pleistocene and Holocene slip rates on the Lone Mountain fault: Evidence for accelerating slip in the Silver Peak-Lone Mountain extensional complex

Determining the constancy of fault slip rates over time is critical in characterizing strain distribution across plate boundaries such as the Pacific-North American plate boundary in the western U.S. We present results from the Lone Mountain fault, a normal fault within the southern Walker Lane, that suggest slip rates there may have increased approximately twofold since the late Pleistocene. We combine detailed field surficial mapping, topographic surveying, and 10Be cosmogenic nuclide exposure ages to calculate new late Pleistocene and Holocene slip rates on the Lone Mountain fault. Alluvial fans with ages of 14.6 ± 1.4 ka and 8.0 ± 0.9 ka are vertically offset 10.2 ± 0.6 m and 4.7 ± 0.6 m, respectively, yielding vertical slip rates of 0.7 ± 0.1 mm/yr and 0.6 ± 0.1 mm/yr. These slip rates are faster than the rates of 0.1 to 0.4 mm/yr from earlier in the Pleistocene, defining a pattern of accelerating slip on the Lone Mountain fault over a timescale of 104 years. The possibility of accelerating slip rates in parts of the Walker Lane partially reconciles the observed discrepancy between long- and short-term slip rates in this region and elucidates the distribution of strain across an evolving plate boundary.

River-evolution and tectonic implications of a major Pliocene aggradation on the lower Colorado River: The Bullhead Alluvium

The ∼200-m-thick riverlaid Bullhead Alluvium along the lower Colorado River downstream of Grand Canyon records massive early Pliocene sediment aggradation following the integration of the upper and lower Colorado River basins. The distribution and extent of the aggraded sediments record (1) evolving longitudinal profiles of the river valley with implications for changing positions of the river’s mouth and delta; (2) a pulse of rapid early drainage-basin erosion and sediment supply; and (3) constraints on regional and local deformation. The Bullhead Alluvium is inset into the Hualapai and Bouse Formations along a basal erosional unconformity. Its base defines a longitudinal profile interpreted as the incised end result after the Colorado River integrated through lake basins. Subsequent Bullhead aggradation, at ca. 4.5–3.5 Ma, built up braid plains as wide as 50 km as it raised the Colorado River’s grade. We interpret the aggradation to record a spike in sediment supply when river integration and base-level fall destabilized and eroded relict landscapes and Tertiary bedrock in the Colorado River’s huge catchment. Longitudinal profiles of the Bullhead Alluvium suggest ≥200 m post-Bullhead relative fault uplifts in the upper Lake Mead area, >100 m local subsidence in the Blythe Basin, and deeper subsidence of correlative deltaic sequences in the Salton Trough along the Pacific–North American plate boundary. However, regionally, for >500 km along the river corridor from Yuma, Arizona, to Lake Mead, Arizona and Nevada, the top of the Bullhead Alluvium appears to be neither uplifted nor tilted, sloping 0.5–0.6 m/km downstream like the gradient of a smaller late Pleistocene aggradation sequence. Perched outcrops tentatively assigned to the Bullhead Alluvium near the San Andreas fault system project toward a Pliocene seashore or bayline twice as distant (300–450 km) as either the modern river’s mouth or a tectonically restored 4.25 Ma paleoshore. We conclude that Bullhead aggradation peaked after 4.25 Ma, having lengthened the delta plain seaward by outpacing both 2 mm/yr delta subsidence and 43–45 mm/yr transform-fault offset of the delta. Post-Bullhead degradation started before 3.3 Ma and implies that the river profile lowered and shortened because sediment supply declined, and progradation was unable to keep up with subsidence and plate motion in the delta.

A summary of the late Cenozoic stratigraphic and tectonic history of the Santa Clara Valley, California

The late Cenozoic stratigraphic and tectonic history of the Santa Clara Valley illustrates the dynamic nature of the North American–Pacific plate boundary and its effect on basin and landscape development. Prior to early Miocene time, the area that became Santa Clara Valley consisted of eroding Franciscan complex basement structurally interleaved in places with Coast Range ophiolite and Mesozoic Great Valley sequence, and locally overlapped by Paleogene strata. During early to middle Miocene time, this landscape was flooded by the sea and was deformed locally into deeper depressions such as the Cupertino Basin in the southwestern part of the valley. Marine deposition during the middle and late Miocene laid down thin deposits in shallow water and thick deeper-water deposits in the Cupertino Basin. During this sedimentation, the San Andreas fault system encroached into the valley, with most offset partitioned onto the San Andreas fault southwest of the valley and the southern Calaveras–Silver Creek–Hayward fault system in the northeastern part of the valley. A 6-km-wide right step between the Hayward and Silver Creek faults formed the 40-km-long Evergreen pull-apart basin along the northeastern margin of the valley, leaving a basement ridge between it and the Cupertino Basin. The Silver Creek fault was largely abandoned ca. 2.5 Ma in favor of a compressional left step between the Calaveras and Hayward fault, although some slip continued to at least mid-Quaternary time. Gravity, seismic, stratigraphic, and interferometric synthetic aperture radar (InSAR) data indicate no other major San Andreas system faults within the central block between the present-day range-front faults bounding the valley and the Silver Creek fault. Sometime between 9 and 4 Ma (9 and 1 Ma for the central block), the area rose above sea level, and a regional surface of erosion was carved into the Mesozoic and Tertiary rocks. Alluvial gravels were deposited on this surface along the margins of the valley beginning ca. 4 Ma, but they may not have prograded onto the central block until ca. 1 Ma, because no older equivalents of the Pliocene–Quaternary Santa Clara gravels have been found there. Thus, either the central block was high enough relative to the surrounding areas that Santa Clara gravels were never deposited on it, or any Santa Clara gravels deposited there were stripped away before ca. 1 Ma. Analysis of alluvium on the central block implies a remarkably uniform, piston-like, subsidence of the valley of ∼0.4 mm/yr since ca. 0.8 Ma, possibly extending north to northern San Francisco Bay. Today, the central block continues to subside, the range-front reverse faults are active, and the major active faults of the San Andreas system are mostly outside the valley.

Postseismic Deformation Following the 2010 $$M = 7.2$$ M = 7.2 El Mayor-Cucapah Earthquake: Observations, Kinematic Inversions, and Dynamic Models

Due to its location on a transtensional section of the Pacific-North American plate boundary, the Salton Trough is a region featuring large strike-slip earthquakes within a regime of shallow asthenosphere, high heat flow, and complex faulting, and so postseismic deformation there may feature enhanced viscoelastic relaxation and afterslip that is particularly detectable at the surface. The 2010 M=7.2 El Mayor-Cucapah earthquake was the largest shock in the Salton Trough since 1892 and occurred close to the US-Mexico border, and so the postseismic deformation recorded by the continuous GPS network of southern California provides an opportunity to study the rheology of this region. Three-year postseismic transients extracted from GPS displacement time-series show four key features: (1) 1–2 cm of cumulative uplift in the Imperial Valley and ∼ 1 cm of subsidence in the Peninsular Ranges, (2) relatively large cumulative horizontal displacements > 150 km from the rupture in the Peninsular Ranges, (3) rapidly decaying horizontal displacement rates in the first few months after the earthquake in the Imperial Valley, and (4) sustained horizontal velocities, following the rapid early motions, that were still visibly ongoing 3 years after the earthquake. Kinematic inversions show that the cumulative 3-year postseismic displacement field can be well fit by afterslip on and below the coseismic rupture, though these solutions require afterslip with a total moment equivalent to at least a M=7.2 earthquake and higher slip magnitudes than those predicted by coseismic stress changes. Forward modeling shows that stress-driven afterslip and viscoelastic relaxation in various configurations within the lithosphere can reproduce the early and later horizontal velocities in the Imperial Valley, while Newtonian viscoelastic relaxation in the asthenosphere can reproduce the uplift in the Imperial Valley and the subsidence and large westward displacements in the Peninsular Ranges. We present two forward models of dynamically coupled deformation mechanisms that fit the postseismic transient well: a model combining afterslip in the lower crust, Newtonian viscoelastic relaxation in a localized zone in the lower crust beneath areas of high heat flow and geothermal activity, and Newtonian viscoelastic relaxation in the asthenosphere; and a second model that replaces the afterslip in the first model with viscoelastic relaxation with a stress-dependent viscosity in the mantle. The rheology of this high-heat-flow, high-strain-rate region may incorporate elements of both these models and may well be more complex than either of them.

Structural Geometry of a Sector of the Colorado River Delta, Baja California, Mexico, Based on Seismic Reflections

A structural study in the SW section of the Colorado River delta using seismic reflection data is presented. The study area is located along the Cerro Prieto transform fault, which extends from the northern Gulf of California through the Mexicali Valley and is an active fault within the Pacific-North American plate boundary zone. The research was supported by a database of five seismic profiles with a total length of 215 km, collected in the early 80’s by Petroleos Mexicanos. The results show a high density of faults, most of which are buried by sediments. Within the Cerro Prieto fault zone, several faults were identified, such as: Palmas, Mesa, and Pangas Viejas, until now unknown. In addition, even though the Indiviso fault was investigated and superficially identify prior to this work, herein mapped at depth. West of the Cerro Prieto fault zone lies the Las Tinajas basin, bound by the Dunas and Saldana faults and by the Montague basin to the southeast. The deformation zone along the plate boundary is 18-km-wide, stretching from the Cerro Prieto fault in the east to the Pangas Viejas fault in the west. The orientations of the faults are NW–SE, and if projected from the southern side of the Sierra Cucapah southward, the faults tend to join the Cerro Prieto fault. In the Las Tinajas basin, the acoustic basement is deeper than 5,000 m. Some of the largest vertical displacements generated by the 2010 7.2-Mw El Mayor-Cucapah earthquake occurred southeast of the epicenter and coincided with the location of the Pangas Viejas Fault, which is buried by sediments. Before this event, seismic activity was very low, and no structures were known in the area. In this paper, we demonstrate that there are at least seven major faults that may now pose a high seismic hazard.