Paul H. Wetmore

U-Pb detrital-zircon geochronology of northern Salinian basement and cover rocks

Salinia is an out-of-place granitic terrane in central coastal California whose debated origin is critical to understanding the tectonic history of southwestern North America. Salinian metasedimentary and sedimentary rocks that respectively host and cover its predominant arc rocks should contribute important data about its origin and kinematic history, but pervasive intrusion, high-grade metamorphism and Cenozoic erosion of the Salinian block have inhibited their widespread characterization and correlation. To further address these problems, we report 605 U-Pb detrital-zircon geochronologic ages collected by laser-ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) from seven Salinian metasedimentary framework (Sur Series) and sedimentary cover samples. Samples collected from the Sur Series contain Late Archean (2.5–2.9 Ga), late Paleoproterozoic (1.6–1.9 Ga), Mesoproterozoic (0.9–1.5 Ga), Neoproterozoic (0.65–0.8 Ga), Paleozoic (250–450 Ma), and possibly Mesozoic U-Pb detrital-zircon ages. Samples collected from Upper Cretaceous cover units have various age-peak distributions, which collectively include late Paleoproterozoic (1.6–1.8 Ga), early Mesoproterozoic (1.35– 1.55 Ga), Permo-Triassic (220–290 Ma), and Jurassic-Cretaceous (80–190 Ma) peaks. From these data, several interpretations are made. (1) Maximum depositional ages of the Sur Series and cover intervals are 280– 360 Ma and 78–90 Ma, respectively. (2) The presence of Late Archean, early Paleoproterozoic, and Neoproterozoic zircons in Salinian metasedimentary rocks suggest that uplift and erosion of adjacent basins recycled sediment onto Salinia. (3) The abundant pre-Mesoproterozoic detrital-zircon ages in Sur Series and cover units preclude the possibility that Salinia originated in southern Mexico, as has been previously suggested. (4) Five of six key detrital-zircon age peaks identifi ed in Salinian basement and cover units are nowhere more closely arranged than in the Mojave Desert–Peninsular Ranges region of Baja and southern Alta California. (5) Paleozoic and early Mesozoic detrital zircons in Sur Series and cover units match the ages of several plutonic events that occurred along the western margin of North America—however, Permian ages favor a Mojave Desert origin over other candidates. Collectively, these and other data suggest that Salinia resided in the Mojave Desert–Peninsular Ranges region from the late Paleozoic until the Late Cretaceous, after which it was rapidly exhumed, deposited upon, and then translated outboard and northward to its current position.

Relationship between dike and volcanic conduit distribution in a highly eroded monogenetic volcanic field: San Rafael, Utah, USA

We mapped 63 conduits, ∼2000 dike segments, and 12 sills in the San Rafael subvolcanic field, Utah (United States), where this Pliocene magmatic system is eroded to a depth of ∼0.8 km and is exceptionally well exposed. Although the number of mapped conduits, dikes, and sills might represent minimums, depending on the level of erosion and exposure, mapped dikes are more numerous around the areally extensive sills and interact with sills and conduits in complex ways. We analyze conduit distribution using kernel density methods and compare results with dike and sill distribution. We find that the distribution of conduits matches the major features of dike distribution, including development of clusters and distribution of outliers. These statistical models are then applied to the distributions of volcanoes in several recently active volcanic fields, where intrusion distributions must be inferred from very sparse data, and compared with San Rafael conduit distribution. This comparison supports the use of statistical models in probabilistic hazard assessment for distributed volcanism. Specifically, renewed dike intrusion and potential eruptions in active basaltic systems can be assessed probabilistically from the distribution of older volcanoes in distributed volcanic systems.

Geochemical evidence of a near-surface history for source rocks of the central Coast Mountains Batholith, British Columbia

Major and trace elemental concentrations as well as Sr and Pb isotopic data, obtained for 41 plutonic samples from the Coast Mountains Batholith ranging in age from ∼108 to ∼50 Ma, indicate that the source regions for these rocks were relatively uniform and typical of Cordilleran arcs. The studied rocks are mineralogically and chemically metaluminous to weakly peraluminous and are mainly calc-alkaline. Initial whole-rock 87Sr/86Sr ratios range from 0.7035 up to 0.7053, whereas lead isotopic data range from 18.586 to 19.078 for 206Pb/204Pb, 15.545 to 15.634 for 207Pb/204Pb, and 37.115 to 38.661 for 208Pb/204Pb. In contrast to these relatively primitive isotopic data, δ 18O values for quartz separates determined for 19 of the samples range from 6.8 up to 10.0‰. These δ 18O values preclude the possibility that these melts were exclusively generated from the Mesozoic mantle wedge of this continental arc, just as the Sr and Pb data preclude significant involvement of an old (Precambrian) crustal/mantle lithospheric source. We interpret the high δ 18O component to represent materials that had a multi-stage crustal evolution. They were originally mafic rocks derived from a circum-Pacific juvenile mantle wedge that experienced a period of near-surface residence after initial crystallization. During this interval, these primitive rocks interacted with meteoric waters at low temperatures, as indicated by the high δ 18O values. Subsequently, these materials were buried to lower crustal depths where they remelted to form the high δ 18O component of the Coast Mountains Batholith. This component makes up at least 40% (mass) of the Cretaceous through Eocene batholith in the studied area. The remainder of the source materials comprising the Coast Mountains Batholith had to be new additions from the mantle wedge. A prolonged period of contractional deformation beginning with the Early Cretaceous collisional accretion of the Insular superterrane is inferred to have been responsible for underthrusting the high δ 18O component into the lower crust. We suggest that mafic rocks of the Insular superterrane (e.g. Alexander–Wrangellia) are of appropriate composition, and were accreted to and overthrust by what would become the Coast Mountains Batholith just prior to initiation of magmatism in the region.

Role of sills in the development of volcanic fields: Insights from lidar mapping surveys of the San Rafael Swell, Utah

Analysis of airborne and terrestrial lidar data demonstrates that >0.4 km 3 of magma cooled in sills at shallow (<1 km) depth in the now-eroded Pliocene San Rafael Swell distributed volcanic field, Utah (USA). The volumes of each of seven sills are estimated from three-dimensional (3-D) models of the lidar data and range from 10 –4 to 10 –1 km 3 . Directions of magma flow during emplacement are interpreted from precise sill thickness measurements and measurements of linear vertical offsets within the sills, helping to identify feeder conduits and dikes; 3-D map relationships derived from lidar data demonstrate that magma flowed into and out of sills from these active dikes and eruptive conduits. Mapped sill volumes account for >92% of intrusive material within the 50 km 2 study area. We conclude that sills played a significant role in modifying eruption dynamics during activity in San Rafael, and suggest that monitoring of sill inflation and deflation in active distributed volcanic fields may provide key information about unrest and potential eruption dynamics.

Neotectonic faulting and forearc sliver motion along the Atirro–Río Sucio fault system, Costa Rica, Central America

The Atirro–Rio Sucio fault system forms a major northwest-trending strike-slip fault zone in east-central Costa Rica. We examined the kinematics and temporal evolution of this fault system through geomorphic, structural, and seismologic analysis. This 150-km-long strike-slip fault zone traverses the northern flank of the paleovolcanic Cordillera de Talamanca and extends northwestward into the active Cordillera Volcanica Central. Historical seismicity includes frequent minor swarms and occasional moderate-magnitude (M 5.0–6.5) damaging earthquakes. Field geomorphic evidence, fault kinematic data, and earthquake focal mechanisms are consistent in showing dextral slip along the mapped traces of northwest-striking faults. Continuity with other transcurrent faults in northwest Costa Rica indicates that the Atirro–Rio Sucio fault system may form the southeastern end of a regional network of northwest-trending dextral faults that accommodate margin-parallel displacement of the Central American forearc sliver. The Atirro–Rio Sucio fault system originates within the Central Costa Rica Deformed Belt inboard of the indenting Cocos Ridge. We infer that ridge collision drives lateral escape of crustal fragments northwestward along an array of dextral Central Costa Rica Deformed Belt faults including the major structures of the Atirro–Rio Sucio fault system. This zone of arc-parallel extrusion thus represents the root of the Central American forearc sliver. Consistent with recent geodynamic models, we propose that northwestward sliver escape along the Atirro–Rio Sucio faults is driven by rigid indentation of the aseismic Cocos Ridge into southern Costa Rica.

Controls on orogenesis along an ocean-continent margin transition in the Jura-Cretaceous Peninsular Ranges batholith

The Jura-Cretaceous Peninsular Ranges batholith (PRb) of southern and Baja California is a remarkable example of a zoned batholith containing distinct oceanic (western) and continental (eastern) basements. The transition between these basements is marked by a crustal-scale boundary along which distinct volcanosedimentary, structural, and metamorphic histories evolved during Mesozoic orogenesis. Our work across this boundary in the Sierra San Pedro Martir of Baja California, Mexico indicates that it controlled a number of processes in the PRb including magmatism, the location of forearc or intraarc basins, and the locus and extent of contractional deformation and denudation. However, our work farther north indicates that notable differences occur along strike in the character of the western arc and transition zone, and these differences are most pronounced across the modern Agua Blanca fault. This fault was also active in the Mesozoic, at which time it separated the western zone into northern and southern arc segments. In the northern half of the batholith, the western arc (Santiago Peak Volcanics) lie in depositional contact with Triassic(?)-Jurassic sediments that mostly received detritus of North American origin and show a long history of contractional deformation and late extensional overprint. We concur that this part of the batholith evolved in Jura-Cretaceous time across an inherited ocean-continent crustal join. In contrast, south of the Agua Blanca fault, the boundary between the western arc (Alisitos) and transitional zones is marked by reverse mylonite shear zones that typically correspond with inverted metamorphic gradients and sharp steps in maximum pressures and cooling histories. Here the transition zone contains basins that collected mostly volcanogenic detritus during the Late Jurassic-Early Cretaceous and preserve a history of long-lived contractional deformation including shear zone development during collision. Thus, although both northern and southern segments of the transition zone share similarities in their plutonic, structural, metamorphic, and denuda

Detrital zircon ages in Palaeozoic and Mesozoic basement assemblages of the Peninsular Ranges batholith, Baja California, Mexico: constraints for depositional ages and provenance

The origin and continuity of Phanerozoic lithostratigraphic terranes in southern and Baja California remain an unsolved issue in Cordilleran tectonics. We present data from eight detrital zircon samples collected across the southern extent of the Peninsular Ranges that help constrain the provenance of detritus and the depositional ages of these basement units. Detrital zircon signatures from units in the eastern Peninsular Ranges correlate with Palaeozoic passive margin assemblages in the southwestern North American Cordillera. Units in the central belt, which consists of Triassic–Jurassic metasedimentary turbidite assemblages that probably deformed in an accretionary prism setting, and Cretaceous metasedimentary and metavolcanic units that represent the remnants of a continental margin arc, were derived from both proximal and more distal sources. The westernmost units, which are locally structurally interleaved with the Triassic through Cretaceous units of the central belt, are Cretaceous deposits that represent a series of collapsed basin complexes located within and flanking the Cretaceous Alisitos volcanic island arc. Cretaceous intra-arc units show little influx of cratonal material until approximately 110 Ma, whereas coeval sediments on the northern and eastern flanks of the Alisitos arc contain abundant cratonal detritus. Intra-arc strata younger than approximately 110 Ma contain large amounts of Proterozoic and older detrital zircons. These data suggest that basins associated with the Alisitos arc were either too distant or somehow shielded from North American detritus before 110 Ma. In the case of the former, increased influx of continental detritus after 110 Ma would support a tectonic model in which the arc was separated from North America by an ocean basin and, as the arc approached the continent, associated depositional centres were close enough to receive input from continental sources.

A new geological slip rate estimate for the Calico Fault, eastern California: implications for geodetic versus geologic rate estimates in the Eastern California Shear Zone

ABSTRACT Accurate estimation of fault slip rate is fundamental to seismic hazard assessment. Previous work suggested a discrepancy between short-term geodetic and long-term geologic slip rates in the Mojave Desert section of the Eastern California Shear Zone (ECSZ). Understanding the origin of this discrepancy can improve understanding of earthquake hazard and fault evolution. We measured offsets in alluvial fans along the Calico Fault near Newberry Springs, California, and used several techniques to date the offset landforms and determine a slip rate. Our preferred slip rate estimate is 3.2 ± 0.4 mm/yr, representing an average over the last few hundred thousand years, faster than previous estimates. Seismic hazard associated with this fault may therefore be higher than previously assumed. We discuss possible biases in the various slip rate estimates and discuss possible reasons for the rate discrepancy. We suggest that the ECSZ discrepancy is an artefact of limited data, and represents a combination of faster slip on the Calico Fault, off-fault deformation, unmapped fault strands, and uncertainties in the geologic rates that have been underestimated. Assuming our new rate estimate is correct and a fair amount (40%) of off-fault deformation occurs on major ECSZ faults, the summed geologic rate estimate across the Mojave section of the ECSZ is 10.5 ± 3.1 mm/yr, which is equivalent within uncertainties to the geodetic rate estimate.

A new approach to probabilistic lava flow hazard assessments, applied to the Idaho National Laboratory, eastern Snake River Plain, Idaho, USA

We present a new probabilistic lava flow hazard assessment for the U.S. Department of Energy’s Idaho National Laboratory (INL) nuclear facility that (1) explores the way eruptions are defined and modeled, (2) stochastically samples lava flow parameters from observed values for use in MOLASSES, a lava flow simulator, (3) calculates the likelihood of a new vent opening within the boundaries of INL, (4) determines probabilities of lava flow inundation for INL through Monte Carlo simulation, and (5) couples inundation probabilities with recurrence rates to determine the annual likelihood of lava flow inundation for INL. Results show a 30% probability of partial inundation of the INL given an effusive eruption on the eastern Snake River Plain, with an annual inundation probability of 8.4 × 10−5 to 1.8 × 10−4. An annual probability of 6.2 × 10−5 to 1.2 × 10−4 is estimated for the opening of a new eruptive center within INL boundaries.

A Geophysical Model for the Origin of Volcano Vent Clusters in a Colorado Plateau Volcanic Field

Variation in spatial density of Quaternary volcanic vents, and the occurrence of vent clusters, correlate with boundaries in Proterozoic crust in the Springerville volcanic field (SVF), Arizona, USA. Inverse modeling using 538 gravity measurements shows that vent clusters correlate with gradients in the gravity field due to lateral variation in crustal density. These lateral discontinuities in the crustal density can be explained by boundaries in the North American crust formed during Proterozoic accretion. Spatial density of volcanic vents is low in regions of high-density Proterozoic crust, high in areas of relatively low-density Proterozoic crust, and is greatest adjacent to crustal boundaries. Vent alignments parallel these boundaries. We have developed 2D and 3D numerical models of magma ascent through the crust to simulate long-term, average magma migration that led to the development of vent clusters in the SVF, assuming that a viscous fluid flow through a porous media is statistically equivalent to magma migration averaged over geological time in the full field scale. The location and flux from the uniform magma source region are boundary conditions of the model. Changes in model diffusivity, associated with changes in the bulk properties of the lithosphere, can simulate preferential magma migration paths and alter estimated magma flux at the surface, implying that large-scale crustal structures, such as inherited tectonic block boundaries, influence magma ascent and clustering of volcanic vents. Probabilistic models of volcanic hazard for distributed volcanic fields can be improved by identifying crustal structures and assessing their impact on volcano distribution with the use of numerical models.