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USGS2009

Microbial abundance in the deep subsurface of the Chesapeake Bay impact crater: Relationship to lithology and impact processes

Asteroid and comet impact events are known to cause profound disruption to surface ecosystems. The aseptic collection of samples throughout a 1.76-km-deep set of cores recovered from the deep subsurface of the Chesapeake Bay impact structure has allowed the study of the subsurface biosphere in a region disrupted by an impactor. Microbiological enumerations suggest the presence of three major microbiological zones. The upper zone (127–867 m) is characterized by a logarithmic decline in microbial abundance from the surface through the postimpact section of Miocene to Upper Eocene marine sediments and across the transition into the upper layers of the impact tsunami resurge sediments and sediment megablocks. In the middle zone (867–1397 m) microbial abundances are below detection. This zone is predominantly quartz sand, primarily composed of boulders and blocks, and it may have been mostly sterilized by the thermal pulse delivered during impact. No samples were collected from the large granite block (1096–1371 m). The lowest zone (below 1397 m) of increasing microbial abundance coincides with a region of heavily impact-fractured, hydraulically conductive suevite and fractured schist. These zones correspond to lithologies influenced by impact processes. Our results yield insights into the influence of impacts on the deep subsurface biosphere.

Virginia↗

USGS2009

Contamination assessment in microbiological sampling of the Eyreville core, Chesapeake Bay impact structure

Knowledge of the deep subsurface biosphere is limited due to difficulties in recovering materials. Deep drilling projects provide access to the subsurface; however, contamination introduced during drilling poses a major obstacle in obtaining clean samples. To monitor contamination during the 2005 International Continental Scientific Drilling Program (ICDP)-U.S. Geological Survey (USGS) deep drilling of the Chesapeake Bay impact structure, four methods were utilized. Fluorescent microspheres were used to mimic the ability of contaminant cells to enter samples through fractures in the core material during retrieval. Drilling mud was infused with a chemical tracer (Halon 1211) in order to monitor penetration of mud into cores. Pore water from samples was examined using excitation-emission matrix (EEM) fl uorescence spectroscopy to characterize dissolved organic carbon (DOC) present at various depths. DOC signatures at depth were compared to signatures from drilling mud in order to identify potential contamination. Finally, microbial contaminants present in drilling mud were identified through 16S ribosomal deoxyribonucleic acid (rDNA) clone libraries and compared to species cultured from core samples. Together, these methods allowed us to categorize the recovered core samples according to the likelihood of contamination. Twenty-two of the 47 subcores that were retrieved were free of contamination by all the methods used and were subsequently used for microbiological culture and culture-independent analysis. Our approach provides a comprehensive assessment of both particulate and dissolved contaminants that could be applied to any environment with low biomass. ?? 2009 The Geological Society of America.

Special Paper of the Geological Society of America↗

USGS2015

Association of anatase (TiO2) and microbes: unusual fossilization effect or a potential biosignature?

We combined microbial paleontology and molecular biology methods to study the Eyreville B drill core from the 35.3-Ma-old Chesapeake Bay impact structure,Virginia, USA. The investigated sample is a pyrite vein collected from the 1353.81-1353.89 m depth interval, located within a section of biotite granite. The granite is a pre-impact rock that was disrupted by the impact event. A search for inorganic (mineral) biosignatures revealed the presence of micron-size rod morphologies of anatase (TiO2) embedded in chlorite coatings on pyrite grains. Neither the Acridine Orange microbial probe nor deoxyribonucleic acid (DNA) extraction followed by polymerase chain reaction (PCR) amplifi cation showed the presence of DNA or ribonucleic acid (RNA) at the location of anatase rods, implying the absence of viable cells in the investigated area. A Nile Red microbial probe revealed the presence of lipids in the rods. Because most of the lipids are resistant over geologic time spans, they are good biomarkers, and they are an indicator of biogenicity for these possibly 35-Ma-old microbial fossils. The mineral assemblage suggests that rod morphologies are associated with low-temperature (<100 °C) hydrothermal alteration that involved aqueous fl uids. The temporal constraints on the anatase fossils are still uncertain because pre-impact alteration of the granite and postimpact heating may have provided identical conditions for anatase precipitation and microbial preservation.

Geological Society of America Bulletin↗

USGS2009

Hydrothermal processes above the Yellowstone magma chamber: Large hydrothermal systems and large hydrothermal explosions

Hydrothermal explosions are violent and dramatic events resulting in the rapid ejection of boiling water, steam, mud, and rock fragments from source craters that range from a few meters up to more than 2 km in diameter; associated breccia can be emplaced as much as 3 to 4 km from the largest craters. Hydrothermal explosions occur where shallow interconnected reservoirs of steam- and liquid-saturated fluids with temperatures at or near the boiling curve underlie thermal fields. Sudden reduction in confining pressure causes fluids to flash to steam, resulting in significant expansion, rock fragmentation, and debris ejection. In Yellowstone, hydrothermal explosions are a potentially significant hazard for visitors and facilities and can damage or even destroy thermal features. The breccia deposits and associated craters formed from hydrothermal explosions are mapped as mostly Holocene (the Mary Bay deposit is older) units throughout Yellowstone National Park (YNP) and are spatially related to within the 0.64-Ma Yellowstone caldera and along the active Norris-Mammoth tectonic corridor. In Yellowstone, at least 20 large (>100 m in diameter) hydrothermal explosion craters have been identified; the scale of the individual associated events dwarfs similar features in geothermal areas elsewhere in the world. Large hydrothermal explosions in Yellowstone have occurred over the past 16 ka averaging ~1 every 700 yr; similar events are likely in the future. Our studies of large hydrothermal explosion events indicate: (1) none are directly associated with eruptive volcanic or shallow intrusive events; (2) several historical explosions have been triggered by seismic events; (3) lithic clasts and comingled matrix material that form hydrothermal explosion deposits are extensively altered, indicating that explosions occur in areas subjected to intense hydrothermal processes; (4) many lithic clasts contained in explosion breccia deposits preserve evidence of repeated fracturing and vein-filling; and (5) areal dimensions of many large hydrothermal explosion craters in Yellowstone are similar to those of its active geyser basins and thermal areas. For Yellowstone, our knowledge of hydrothermal craters and ejecta is generally limited to after the Yellowstone Plateau emerged from beneath a late Pleistocene icecap that was roughly a kilometer thick. Large hydrothermal explosions may have occurred earlier as indicated by multiple episodes of cementation and brecciation commonly observed in hydrothermal ejecta clasts. Critical components for large, explosive hydrothermal systems include a water-saturated system at or near boiling temperatures and an interconnected system of well-developed joints and fractures along which hydrothermal fluids flow. Active deformation of the Yellowstone caldera, active faulting and moderate local seismicity, high heat flow, rapid changes in climate, and regional stresses are factors that have strong influences on the type of hydrothermal system developed. Ascending hydrothermal fluids flow along fractures that have developed in response to active caldera deformation and along edges of low-permeability rhyolitic lava flows. Alteration of the area affected, self-sealing leading to development of a caprock for the hydrothermal system, and dissolution of silica-rich rocks are additional factors that may constrain the distribution and development of hydrothermal fields. A partial low-permeability layer that acts as a cap to the hydrothermal system may produce some over-pressurization, thought to be small in most systems. Any abrupt drop in pressure initiates steam flashing and is rapidly transmitted through interconnected fractures that result in a series of multiple large-scale explosions contributing to the excavation of a larger explosion crater. Similarities between the size and dimensions of large hydrothermal explosion craters and thermal fields in Yellowstone may indicate that catastrophic events which result in large hydrothermal explosions are an end phase in geyser basin evolution. The Mary Bay hydrothermal explosion crater complex is the largest such complex in Yellowstone, and possibly in the world, with a diameter of 2.8 km in length and 2.4 km in width. It is nested in Mary Bay in the northern basin of Yellowstone Lake, an area of high heat flow and active deformation within the Yellowstone caldera. A sedimentary sequence exposed in wave-cut cliffs between Storm Point and Mary Bay gives insight into the geologic history of the Mary Bay hydrothermal explosion event. The Mary Bay explosion breccia deposits overlie sand above varved lake sediments and are separated locally into an upper and lower unit. The sand unit contains numerous small normal faults and is coextensive with the Mary Bay breccia in its northern extent. This sand may represent deposits of an earthquake-generated wave. Seismicity associated with the earthquake may have triggered the hydrothermal explosion responsible for development of the Mary Bay crater complex. Large hydrothermal explosions are rare events on a human time scale; however, the potential for additional future events of the sort in Yellowstone National Park is not insignificant. Based on the occurrence of large hydrothermal explosion events over the past 16,000 yr, an explosion large enough to create a 100-m-wide crater might be expected every 200 yr.

Wyoming↗

USGS2009

Hurricane Rita and the destruction of Holly Beach, Louisiana: Why the chenier plain is vulnerable to storms

Hurricane Rita devastated gulf-front communities along the western Louisiana coast in 2005. LIDAR (light detection and ranging) topographic surveys and aerial photography collected before and after the storm showed the loss of every structure within the community of Holly Beach. Average shoreline change along western Louisiana's 140-km-long impacted shore was −23.3 ± 30.1 m of erosion, although shoreline change in Holly Beach was substantially less, and erosion was not pervasive where the structures were lost. Before the storm, peak elevations of the dunes, or berms in the absence of dunes, along the impacted shore averaged 1.6 m. The storm surge, which reached 3.5 m just east of Holly Beach, completely inundated the beach systems along the impacted western Louisiana shore. The high surge potential and low land elevations make this coast extremely vulnerable to hurricanes. In fact, most of the western Louisiana shore impacted by Rita will be completely inundated by the storm surge of a worst-case Saffir-Simpson category 1 hurricane. All of this shore will be inundated by worst-case category 2–5 storms.

Louisiana↗

USGS2009

Field camp: Using traditional methods to train the next generation of petroleum geologists

The summer field camp experience provides many students with their best opportunity to learn the scientific process by making observations and collecting, recording, evaluating, and interpreting geologic data. Field school projects enhance student professional development by requiring cooperation and interpersonal interaction, report writing to communicate interpretations, and the development of project management skills to achieve a common goal. The field school setting provides students with the opportunity to observe geologic features and their spatial distribution, size, and shape that will impact the student's future careers as geoscientists. The Les Huston Geology Field Camp (a.k.a. Oklahoma Geology Camp) near Ca??on City, Colorado, focuses on time-tested traditional methods of geological mapping and fieldwork to accomplish these goals. The curriculum consists of an introduction to field techniques (pacing, orienteering, measuring strike and dip, and using a Jacob's staff), sketching outcrops, section measuring (one illustrating facies changes), three mapping exercises (of increasing complexity), and a field geophysics project. Accurate rock and contact descriptions are emphasized, and attitudes and contacts are mapped in the field. Mapping is done on topographic maps at 1:12,000 and 1:6000 scales; air photos are provided. Global positioning system (GPS)-assisted mapping is allowed, but we insist that locations be recorded in the field and confirmed using visual observations. The course includes field trips to the Cripple Creek and Leadville mining districts, Floris-sant/Guffey volcano area, Pikes Peak batholith, and the Denver Basin. Each field trip is designed to emphasize aspects of geology that are not stressed in the field exercises. Students are strongly encouraged to accurately describe geologic features and gather evidence to support their interpretations of the geologic history. Concise reports are a part of each major exercise. Students are grouped into teams to (1) introduce the team concept and develop interpersonal skills that are fundamental components of many professions, (2) ensure safety, and (3) mix students with varying academic backgrounds and physical strengths. This approach has advantages and disadvantages. Students with academic strengths in specific areas assist those with less experience, thereby becoming engaged in the teaching process. However, some students contribute less to fi nal map projects than others, and assigning grades to individual team members can be diffi cult. The greatest challenges we face involve group dynamics and student personalities. We continue to believe that traditional fi eld methods, aided by (but not relying upon) new technologies, are the key to constructing and/or interpreting geologic maps. The requirement that students document fi eld evidence using careful observations teaches skills that will be benefi cial throughout their professional careers. ??2009 The Geological Society of America. All rights reserved.

Special Paper of the Geological Society of America↗

USGS2009

The surface of Mars: An unusual laboratory that preserves a record of catastrophic and unusual events

Catastrophic and unusual events on Earth such as bolide impacts, megafloods, supereruptions, flood volcanism, and subice volcanism may have devastating effects when they occur. Although these processes have unique characteristics and form distinctive features and deposits, we have diffi culties identifying them and measuring the magnitude of their effects. Our diffi culties with interpreting these processes and identifying their consequences are understandable considering their infrequency on Earth, combined with the low preservation potential of their deposits in the terrestrial rock record. Although we know these events do happen, they are infrequent enough that the deposits are poorly preserved on the geologically active face of the Earth, where erosion, volcanism, and tectonism constantly change the surface. Unlike the Earth, on Mars catastrophic and unusual features are well preserved because of the slow modifi cation of the surface. Signifi cant precipitation has not occurred on Mars for billions of years and there appears to be no discrete crustal plates to have undergone subduction and destruction. Therefore the ancient surface of Mars preserves geologic features and deposits that result from these extraordinary events. Also, unlike the other planets, Mars is the most similar to our own, having an atmosphere, surface ice, volcanism, and evidence of onceflowing water. So although our understanding of precursors, processes, and possible biological effects of catastrophic and unusual processes is limited on Earth, some of these mysteries may be better understood through investigating the surface of Mars. ?? 2009 The Geological Society of America.

Special Paper of the Geological Society of America↗

USGS2009

Effects of megascale eruptions on Earth and Mars

Volcanic features are common on geologically active earthlike planets. Megascale or "super" eruptions involving >1000 Gt of magma have occurred on both Earth and Mars in the geologically recent past, introducing prodigious volumes of ash and volcanic gases into the atmosphere. Here we discuss felsic (explosive) and mafi c (flood lava) supereruptions and their potential atmospheric and environmental effects on both planets. On Earth, felsic supereruptions recur on average about every 100-200,000 years and our present knowledge of the 73.5 ka Toba eruption implies that such events can have the potential to be catastrophic to human civilization. A future eruption of this type may require an unprecedented response from humankind to assure the continuation of civilization as we know it. Mafi c supereruptions have resulted in atmospheric injection of volcanic gases (especially SO 2 ) and may have played a part in punctuating the history of life on Earth. The contrast between the more sustained effects of flood basalt eruptions (decades to centuries) and the near-instantaneous effects of large impacts (months to years) is worthy of more detailed study than has been completed to date. Products of mafi c supereruptions, signifi cantly larger than known from the geologic record on Earth, are well preserved on Mars. The volatile emissions from these eruptions most likely had global dispersal, but the effects may not have been outside what Mars endures even in the absence of volcanic eruptions. This is testament to the extreme variability of the current Martian atmosphere: situations that would be considered catastrophic on Earth are the norm on Mars. ?? 2009 The Geological Society of America.

Special Paper of the Geological Society of America↗

USGS2009

Megascale processes: Natural disasters and human behavior

Megascale geologic processes, such as earthquakes, tsunamis, volcanic eruptions, floods, and meteoritic impacts have occurred intermittently throughout geologic time, and perhaps on several planets. Unlike other catastrophes discussed in this volume, a unique process is unfolding on Earth, one in which humans may be the driving agent of megadisasters. Although local effects on population clusters may have been catastrophic in the past, human societies have never been interconnected globally at the scale that currently exists. We review some megascale processes and their effects in the past, and compare present conditions and possible outcomes. We then propose that human behavior itself is having effects on the planet that are comparable to, or greater than, these natural disasters. Yet, unlike geologic processes, human behavior is potentially under our control. Because the effects of our behavior threaten the stability, or perhaps even existence, of a civilized society, we call for the creation of a body to institute coherent global, credible, scientifi cally based action that is sensitive to political, economic, religious, and cultural values. The goal would be to institute aggressive monitoring, identify and understand trends, predict their consequences, and suggest and evaluate alternative actions to attempt to rescue ourselves and our ecosystems from catastrophe. We provide a template modeled after several existing national and international bodies. ?? 2009 The Geological Society of America.

Special Paper of the Geological Society of America↗

USGS2010

Implications of geophysical analysis on basin geometry and fault offsets in the northern Colorado River extensional corridor and adjoining Lake Mead region, Nevada and Arizona

The northern Colorado River extensional corridor and Lake Mead region are characterized by prominent gravity and magnetic anomalies that provide insight into the geometry of extensional basins, amount of vertical and strike-slip offset on faults that bound these basins, and composition of major basement blocks. Although large-magnitude extension throughout the extensional corridor and major strike-slip faulting north of Lake Mead have highly disrupted many basins, most of the older basins (middle to late Miocene) are not associated with prominent geophysical anomalies. Instead, the most conspicuous anomalies (e.g., gravity lows) generally correspond to the younger (late Miocene to recent), structurally more coherent basins. Most of the geophysically expressed basins lie north of Lake Mead and are bounded by Quaternary normal and/or strike-slip fault zones. Both Quaternary faults and geophysically conspicuous basins are largely absent south of Lake Mead, where the only prominent gravity low corresponds to a structurally intact basin filled primarily with halite along the less extended, eastern margin of the corridor. Relatively continuous northeast-trending magnetic anomalies south of Lake Mead, presumably caused by Proterozoic basement rocks, suggest that strike-slip displacement is negligible on many of the major normal faults. In contrast, magnetic anomalies are smeared along the Lake Mead fault system and Las Vegas Valley shear zone. Offset anomalies suggest left-lateral displacement of 12–20 km for the Hamblin Bay fault zone, 12–15 km for the Lime Ridge fault, and 12 km on the Gold Butte fault. These values are compatible with or lower than published estimates based on geologic mapping.

Arizona, Nevada↗

USGS2010

Erosion of tilted fault blocks and deposition of coarse sediments in half-graben basins during late stages of extension: Gold Butte area, Basin and Range Province

The provenance and stratigraphic architecture of basin-filling Miocene sediments around the Gold Butte area, southern Nevada, and adjacent highlands record the erosion of fault blocks that progressively tilted during extension. This study focuses especially on upper Miocene correlatives of the red sandstone unit and the Muddy Creek Formation that were deposited during waning stages of extension. Upper parts of the underlying middle Miocene Horse Spring Formation are also addressed. The large east-tilted South Virgin–White Hills block, including the Gold Butte block, was the primary source of coarse detritus into the adjacent half-graben basins on both sides. Voluminous, very coarse-grained sediments were shed eastward down the back slope of this tilt block into the Grand Wash Trough. This suggests that there were large middle and late Miocene catchments on that side of the block, possibly inherited from a gentler dip slope early in the tilting history. The block uplifted and tilted during slip on the west-dipping South Virgin–White Hills normal fault that bounds the west side of the block. Its exposed footwall shed coarse-grained debris to the west. While the fault was active, this debris included rock-avalanche megabreccias. Longitudinal transport of coarse-grained sediment also occurred along the axes of basins on both sides of the block. In the late Miocene, fault death at ca. 10 Ma followed rotation of the South Virgin–White Hills fault, and the along-strike Quail Spring fault, from initial dips >55° to dips <30°. This cessation of faulting coincided with and likely caused an eastward shift in locus of faulting to the steeper Wheeler fault system. Coarse sediment shed from the South Virgin–White Hills tilt block gradually declined as deformation waned and limestone-rich sedimentation expanded onto the basin margins against the block. Where the rising sedimentary fills eventually bridged across the block and connected basins on either side, these bridge sites served to focus later integrated regional drainage—the Pliocene Colorado River. Progressive Miocene tilting of the highland block would have broadened its structural footwall on the west and narrowed its east-dipping back slope. Migration of the drainage divide by erosion and piracy, influenced by changing tilt slopes, can explain the modern position of the divide in the Gold Butte block as one that separates drainage roughly equally down the two sides.

Nevada↗

USGS2010

Geometry and kinematics of the eastern Lake Mead fault system in the Virgin Mountains, Nevada and Arizona

The Lake Mead fault system is a northeast-striking, 130-km-long zone of left-slip in the southeast Great Basin, active from before 16 Ma to Quaternary time. The northeast end of the Lake Mead fault system in the Virgin Mountains of southeast Nevada and northwest Arizona forms a partitioned strain field comprising kinematically linked northeast-striking left-lateral faults, north-striking normal faults, and northwest-striking right-lateral faults. Major faults bound large structural blocks whose internal strain reflects their position within a left step-over of the left-lateral faults. Two north-striking large-displacement normal faults, the Lakeside Mine segment of the South Virgin–White Hills detachment fault and the Piedmont fault, intersect the left step-over from the southwest and northeast, respectively. The left step-over in the Lake Mead fault system therefore corresponds to a right-step in the regional normal fault system. Within the left step-over, displacement transfer between the left-lateral faults and linked normal faults occurs near their junctions, where the left-lateral faults become oblique and normal fault displacement decreases away from the junction. Southward from the center of the step-over in the Virgin Mountains, down-to-the-west normal faults splay northward from left-lateral faults, whereas north and east of the center, down-to-the-east normal faults splay southward from left-lateral faults. Minimum slip is thus in the central part of the left step-over, between east-directed slip to the north and west-directed slip to the south. Attenuation faults parallel or subparallel to bedding cut Lower Paleozoic rocks and are inferred to be early structures that accommodated footwall uplift during the initial stages of extension. Fault-slip data indicate oblique extensional strain within the left step-over in the South Virgin Mountains, manifested as east-west extension; shortening is partitioned between vertical for extension-dominated structural blocks and south-directed for strike-slip faults. Strike-slip faults are oblique to the extension direction due to structural inheritance from NE-striking fabrics in Proterozoic crystalline basement rocks. We hypothesize that (1) during early phases of deformation oblique extension was partitioned to form east-west–extended domains bounded by left-lateral faults of the Lake Mead fault system, from ca. 16 to 14 Ma. (2) Beginning ca. 13 Ma, increased south-directed shortening impinged on the Virgin Mountains and forced uplift, faulting, and overturning along the north and west side of the Virgin Mountains. (3) By ca. 10 Ma, initiation of the younger Hen Spring to Hamblin Bay fault segment of the Lake Mead fault system accommodated westward tectonic escape, and the focus of south-directed shortening transferred to the western Lake Mead region. The shift from early partitioned oblique extension to south-directed shortening may have resulted from initiation of right-lateral shear of the eastern Walker Lane to the west coupled with left-lateral shear along the eastern margin of the Great Basin.

Arizona, Nevada↗

USGS2011

High geologic slip rates since early Pleistocene Initiation of the San Jacinto and San Felipe fault zones in the San Andreas fault system: southern California, USA

The San Jacinto right-lateral strike-slip fault zone is crucial for understanding plate-boundary dynamics, regional slip partitioning, and seismic hazards within the San Andreas fault system of southern California, yet its age of initiation and long-term average slip rate are controversial. This synthesis of prior and new detailed studies in the western Salton Trough documents initiation of structural segments of the San Jacinto fault zone at or slightly before the 1.07-Ma base of the Jaramillo subchron. The dextral faults changed again after ca. 0.5–0.6 Ma with creation of new fault segments and folds. There were major and widespread basinal changes in the early Pleistocene when these new faults cut across the older West Salton detachment fault. We mapped and analyzed the complex fault mesh, identified structural segment boundaries along the Clark, Coyote Creek, and San Felipe fault zones, documented linkages between the major dextral faults, identified previously unknown active strands of the Coyote Creek fault 5 and 8 km NE and SW of its central strands, and showed that prior analyses of these fault zones oversimplify their complexity. The Clark fault is a zone of widely distributed faulting and folding SE of the Santa Rosa Mountains and unequivocally continues 20–25 km SE of its previously inferred termination point to the San Felipe Hills. There the Clark fault zone has been deforming basinal deposits at an average dextral slip rate of ≥10.2 +6.9/−3.3 mm/yr for ~0.5–0.6 m.y. Five new estimates of displacement are developed here using offset successions of crystalline rocks, distinctive marker beds in the late Cenozoic basin fill, analysis of strike-slip–related fault-bend folds, quantification of strain in folds at the tips of dextral faults, and gravity, magnetic, and geomorphic data sets. Together these show far greater right slip across the Clark fault than across either the San Felipe or Coyote Creek faults, despite the Clark fault becoming “hidden” in basinal deposits at its SE end as strain disperses onto a myriad of smaller faults, strike-slip ramps and flats, transrotational systems of cross faults with strongly domain patterns, and a variety of fault-fold sets. Together the Clark and Buck Ridge–Santa Rosa faults accumulated ~16.8 +3.7/−6.0 km of right separation in their lifetime near Clark Lake. The Coyote Ridge segment of the Coyote Creek fault accumulated ~3.5 ± 1.3 km since roughly 0.8–0.9 Ma. The San Felipe fault accumulated between 4 and 12.4 km (~6.5 km preferred) of right slip on its central strands in the past 1.1–1.3 Ma at Yaqui and Pinyon ridges. Combining the estimates of displacement with ages of fault initiation indicates a lifetime geologic slip rate of 20.1 +6.4/−9.8 mm/yr across the San Jacinto fault zone (sum of Clark, Buck Ridge, and Coyote Creek faults) and about ~5.4 +5.9/−1.4 mm/yr across the San Felipe fault zone at Yaqui and Pinyon ridges. The NW Coyote Creek fault has a lifetime slip rate of ~4.1 +1.9/−2.1 mm/yr, which is a quarter of that across the Clark fault (16.0 +4.5/−9.8 mm/yr) nearby. The San Felipe fault zone is not generally regarded as an active fault in the region, yet its lifetime slip rate exceeds those of the central and southern Elsinore and the Coyote Creek fault zones. The apparent lower slip rates across the San Felipe fault in the Holocene may reflect the transfer of strain to adjacent faults in order to bypass a contractional bend and step at Yaqui Ridge. The San Felipe, Coyote Creek, and Clark faults all show evidence of major structural adjustments after ca. 0.6–0.5 Ma, and redistribution of strain onto new right- and left-lateral faults and folds far removed from the older central fault strands. Active faults shifted their locus and main central strands by as much as 13 km in the middle Pleistocene. These changes modify the entire upper crust and were not localized in the thin sedimentary basin fill, which is only a few kilometers thick in most of the western Salton Trough. Steep microseismic alignments are well developed beneath most of the larger active faults and penetrate basement to the base of the seismogenic crust at 10–14 km. We hypothesize that the major structural and kinematic adjustments at ca. 0.5–0.6 Ma resulted in major changes in slip rate within the San Jacinto and San Felipe fault zones that are likely to explain the inconsistent slip rates determined from geologic (1–0.5 m.y.; this study), paleoseismic, and geodetic studies over different time intervals. The natural evolution of complex fault zones, cross faults, block rotation, and interactions within their broad damage zones might explain all the documented and implied temporal and spatial variation in slip rates. Co-variation of slip rates among the San Jacinto, San Felipe, and San Andreas faults, while possible, is not required by the available data. Together the San Jacinto and San Felipe fault zones have accommodated ~25.5 mm/yr since their inception in early Pleistocene time, and were therefore slightly faster than the southern San Andreas fault during the same time interval. If the westward transfer of plate motion continues in southern California, the southern San Andreas fault in the Salton Trough may change from being the main plate boundary fault to defining the eastern margin of the growing Sierra Nevada microplate, as implied by other workers.

California↗

USGS2013

Origin of the Blytheville Arch, and long-term displacement on the New Madrid seismic zone, central United States

The southern arm of the New Madrid seismic zone of the central United States coincides with the buried, ~110 km by ~20 km Blytheville Arch antiform within the Cambrian–Ordovician Reelfoot rift graben. The Blytheville Arch has been interpreted at various times as a compressive structure, an igneous intrusion, or a sediment diapir. Reprocessed industry seismic-reflection profiles presented here show a strong similarity between the Blytheville Arch and pop-up structures, or flower structures, within strike-slip fault systems. The Blytheville Arch formed in the Paleozoic, but post–Mid-Cretaceous to Quaternary strata show displacement or folding indicative of faulting. Faults within the graben structure but outside of the Blytheville Arch also appear to displace Upper Cretaceous and perhaps younger strata, indicating that past faulting was not restricted to the Blytheville Arch and New Madrid seismic zone. As much as 10–12.5 km of strike slip can be estimated from apparent shearing of the Reelfoot arm of the New Madrid seismic zone. There also appears to be ~5–5.5 km of shearing of the Reelfoot topographic scarp at the north end of the southern arm of the New Madrid seismic zone and of the southern portion of Crowley's Ridge, which is a north-trending topographic ridge just south of the seismic zone. These observations suggest that there has been substantial strike-slip displacement along the Blytheville Arch and southern arm of the New Madrid seismic zone, that strike-slip extended north and south of the modern seismic zone, and that post–Mid-Cretaceous (post-Eocene?) faulting was not restricted to the Blytheville Arch or to currently active faults within the New Madrid seismic zone.

New Madrid seismic zone↗

USGS2013

Late Miocene-Pleistocene evolution of a Rio Grande rift subbasin, Sunshine Valley-Costilla Plain, San Luis Basin, New Mexico and Colorado

The Sunshine Valley-Costilla Plain, a structural subbasin of the greater San Luis Basin of the northern Rio Grande rift, is bounded to the north and south by the San Luis Hills and the Red River fault zone, respectively. Surficial mapping, neotectonic investigations, geochronology, and geophysics demonstrate that the structural, volcanic, and geomorphic evolution of the basin involves the intermingling of climatic cycles and spatially and temporally varying tectonic activity of the Rio Grande rift system. Tectonic activity has transferred between range-bounding and intrabasin faults creating relict landforms of higher tectonic-activity rates along the mountain-piedmont junction. Pliocene–Pleistocene average long-term slip rates along the southern Sangre de Cristo fault zone range between 0.1 and 0.2 mm/year with late Pleistocene slip rates approximately half (0.06 mm/year) of the longer Quaternary slip rate. During the late Pleistocene, climatic influences have been dominant over tectonic influences on mountain-front geomorphic processes. Geomorphic evidence suggests that this once-closed subbasin was integrated into the Rio Grande prior to the integration of the once-closed northern San Luis Basin, north of the San Luis Hills, Colorado; however, deep canyon incision, north of the Red River and south of the San Luis Hills, initiated relatively coeval to the integration of the northern San Luis Basin. Long-term projections of slip rates applied to a 1.6 km basin depth defined from geophysical modeling suggests that rifting initiated within this subbasin between 20 and 10 Ma. Geologic mapping and geophysical interpretations reveal a complex network of northwest-, northeast-, and north-south–trending faults. Northwest- and northeast-trending faults show dual polarity and are crosscut by north-south– trending faults. This structural model possibly provides an analog for how some intracontinental rift structures evolve through time.

Colorado, New Mexico↗

USGS2013

Geophysical constraints on Rio Grande rift structure in the central San Luis Basin, Colorado and New Mexico

Interpretation of gravity, aeromagnetic, and magnetotelluric (MT) data reveals patterns of rifting, rift-sediment thicknesses, distribution of pre-rift volcanic and sedimentary rocks, and distribution of syn-rift volcanic rocks in the central San Luis Basin, one of the northernmost major basins that make up the Rio Grande rift. Rift-sediment thicknesses for the central San Luis Basin determined from a three-dimensional gravity inversion indicate that syn-rift Santa Fe Group sediments have a maximum thickness of ~2 km in the Sanchez graben near the eastern margin of the basin along the central Sangre de Cristo fault zone, and reach nearly 1 km within the Monte Vista graben near the western basin margin along the San Juan Mountains. In between, Santa Fe Group thickness is negligible under the San Luis Hills and estimated to reach ~1.1 km under the Costilla Plains (although no independent thickness constraints exist, and a range of thicknesses of 600 m to 2 km is geophysically reasonable). From combined geophysical and geologic considerations, pre-rift, dominantly sedimentary rocks appear to increase in thickness from none in the Sanchez graben on the east to perhaps 800 m under the San Luis Hills on the west. The pre-rift rocks are most likely early Tertiary in age, but the presence of Mesozoic and Paleozoic sedimentary rocks cannot be ruled out. Geophysical data provide new evidence that an isolated exposure of Proterozoic rocks on San Pedro Mesa is rooted in the Precambrian basement. This narrow, north-south–trending basement high has ~2 km of positive relief with respect to the base of the Sanchez graben, and separates the graben from the structural depression beneath the Costilla Plains. A structural high composed of pre-rift rocks, long inferred to extend from under the San Luis Hills to the Taos Plateau, is confirmed and found to be denser than previously believed, with little or no overlying Santa Fe Group sediments. Major faults in the study area are delineated by geophysical data and models; these faults include significant vertical offsets (≥1 km) of Precambrian rocks along the central and southern zones of the Sangre de Cristo fault system. Other faults with similarly large offsets of the Santa Fe Group include a fault bounding the western margin of San Pedro Mesa, and other faults that bound the Monte Vista graben in an area previously assumed to be a simple hinge zone at the western edge of the San Luis Basin. A major north-south–trending structure with expression in gravity and MT data occurs at the boundary between the Costilla Plains and the San Luis Hills structural high. Although it has been interpreted as a down-to-the-east normal fault or fault zone, our modeling suggests that it also is likely related to pre-rift tectonics. Aeromagnetic anomalies over much of the area are interpreted to mainly reflect variations of remanent magnetic polarity and burial depth of the 5.3–3.7 Ma Servilleta Basalt of the Taos Plateau volcanic field. Magnetic-source depth estimates are interpreted to indicate patterns of subsidence following eruption of the basalt, with maximum subsidence in the Sanchez graben.

Colorado, New Mexico↗

USGS2013

Deformational and erosional history for the Abiquiu and contiguous area, north-central New Mexico: Implications for formation of the Abiquiu embayment and a discussion of new geochronological and geochemical analysis

Geologic mapping, age determinations, and geochemistry of rocks exposed in the Abiquiu area of the Abiquiu embayment of the Rio Grande rift, north-central New Mexico, provide data to determine fault-slip and incision rates. Vertical-slip rates for faults in the area range from 16 m/m.y. to 42 m/m.y., and generally appear to decrease from the eastern edge of the Colorado Plateau to the Abiquiu embayment. Incision rates calculated for the period ca. 10 to ca. 3 Ma indicate rapid incision with rates that range from 139 m/m.y. on the eastern edge of the Colorado Plateau to 41 m/m.y. on the western part of the Abiquiu embayment. The Abiquiu area is located along the margin of the Colorado Plateau–Rio Grande rift and lies within the Abiquiu embayment, a shallow, early extensional basin of the Rio Grande rift. Cenozoic rocks include the Eocene El Rito Formation, Oligocene Ritito Conglomerate, Oligocene–Miocene Abiquiu Formation, and Miocene Chama–El Rito and Ojo Caliente Sandstone Members of the Tesuque Formation (Santa Fe Group). Volcanic rocks include the Lobato Basalt (Miocene; ca. 15–8 Ma), El Alto Basalt (Pliocene; ca. 3 Ma), and dacite of the Tschicoma Formation (Pliocene; ca. 2 Ma). Quaternary deposits consist of inset axial and side-stream deposits of the ancestral Rio Chama (Pleistocene in age), landslide and pediment alluvium and colluvium, and Holocene main and side-stream channel and floodplain deposits of the modern Rio Chama. The predominant faults are Tertiary normal high-angle faults that displace rocks basinward. A low-angle fault, referred to as the Abiquiu fault, locally separates an upper plate composed of the transitional zone of the Ojo Caliente Sandstone and Chama–El Rito Members from a lower plate consisting of the Abiquiu Formation or the Ritito Conglomerate. The upper plate is distended into blocks that range from about 0.1 km to 3.5 km long that may represent a larger sheet that has been broken up and partly eroded. Geochronology ( 40 Ar/ 39 Ar) from fifteen volcanic and intrusive rocks resolves discrete volcanic episodes in the Abiquiu area: (1) emplacement of Early and Late Miocene basaltic dikes at 20 Ma and ca. 10 Ma; (2) extensive Late Miocene–age lava flows at 9.5 Ma, 7.9 Ma, and 5.6 Ma; and (3) extensive basaltic eruptions during the early Pliocene at 2.9 Ma and 2.4 Ma. Clasts of biotite- and hornblende-rich trachyandesites and trachydacites from the base of the Abiquiu Formation are dated at ca. 27 Ma, possibly derived from the Latir volcanic field. The most-mafic magmas are interpreted to be generated from a similar lithospheric mantle during rifting, but variations in composition are correlated with partial melting at different depths, which is correlated with thinning of the crust due to extensional processes.

GSA Special Papers↗

USGS2013

Oblique transfer of extensional strain between basins of the middle Rio Grande rift, New Mexico: Fault kinematic and paleostress constraints

The structural geometry of transfer and accommodation zones that relay strain between extensional domains in rifted crust has been addressed in many studies over the past 30 years. However, details of the kinematics of deformation and related stress changes within these zones have received relatively little attention. In this study we conduct the first-ever systematic, multi-basin fault-slip measurement campaign within the late Cenozoic Rio Grande rift of northern New Mexico to address the mechanisms and causes of extensional strain transfer associated with a broad accommodation zone. Numerous (562) kinematic measurements were collected at fault exposures within and adjacent to the NE-trending Santo Domingo Basin accommodation zone, or relay, which structurally links the N-trending, right-stepping en echelon Albuquerque and Española rift basins. The following observations are made based on these fault measurements and paleostresses computed from them. (1) Compared to the typical northerly striking normal to normal-oblique faults in the rift basins to the north and south, normal-oblique faults are broadly distributed within two merging, NE-trending zones on the northwest and southeast sides of the Santo Domingo Basin. (2) Faults in these zones have greater dispersion of rake values and fault strikes, greater dextral strike-slip components over a wide northerly strike range, and small to moderate clockwise deflections of their tips. (3) Relative-age relations among fault surfaces and slickenlines used to compute reduced stress tensors suggest that far-field, ~E-W–trending σ 3 stress trajectories were perturbed 45° to 90° clockwise into NW to N trends within the Santo Domingo zones. (4) Fault-stratigraphic age relations constrain the stress perturbations to the later stages of rifting, possibly as late as 2.7–1.1 Ma. Our fault observations and previous paleomagnetic evidence of post–2.7 Ma counterclockwise vertical-axis rotations are consistent with increased bulk sinistral-normal oblique shear along the Santo Domingo rift segment in Pliocene and later time. Regional geologic evidence suggests that the width of active rift faulting became increasingly confined to the Santo Domingo Basin and axial parts of the adjoining basins beginning in the late Miocene. We infer that the Santo Domingo clockwise stress perturbations developed coevally with the oblique rift segment mainly due to mechanical interactions of large faults propagating toward each other from the adjoining basins as the rift narrowed. Our results suggest that negligible bulk strike-slip displacement has been accommodated along the north-trending rift during much of its development, but uncertainties in the maximum ages of fault slip do not allow us to fully evaluate and discriminate between earlier models that invoked northward or southward rotation and translation of the Colorado Plateau during early (Miocene) rifting.

New Mexico↗