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USGS2024

Manganese-rich sandstones as an indicator of ancient oxic lake water conditions in Gale crater, Mars

Manganese has been observed on Mars by the NASA Curiosity rover in a variety of contexts and is an important indicator of redox processes in hydrologic systems on Earth. Within the Murray formation, an ancient primarily fine-grained lacustrine sedimentary deposit in Gale crater, Mars, have observed up to 45× enrichment in manganese and up to 1.5× enrichment in iron within coarser grained bedrock targets compared to the mean Murray sediment composition. This enrichment in manganese coincides with the transition between two stratigraphic units within the Murray: Sutton Island, interpreted as a lake margin environment, and Blunts Point, interpreted as a lake environment. On Earth, lacustrine environments are common locations of manganese precipitation due to highly oxidizing conditions in the lakes. Here, we explore three mechanisms for ferromanganese oxide precipitation at this location: authigenic precipitation from lake water along a lake shore, authigenic precipitation from reduced groundwater discharging through porous sands along a lake shore, and early diagenetic precipitation from groundwater through porous sands. All three scenarios require highly oxidizing conditions and we discuss oxidants that may be responsible for the oxidation and precipitation of manganese oxides. This work has important implications for the habitability of Mars to microbes that could have used Mn redox reactions, owing to its multiple redox states, as an energy source for metabolism.

JGR Planets↗

USGS2024

A robust quantitative method to distinguish runoff-generated debris flows from floods

Debris flows and floods generated by rainfall runoff occur in rocky mountainous landscapes and burned steeplands. Flow type is commonly identified post-event through interpretation of depositional structures, but these may be poorly preserved or misinterpreted. Prior research indicates that discharge magnitude is commonly amplified in debris flows relative to floods due to volumetric bulking and increased frictional resistance. Here, we use this flow amplification to develop a metric ( Q* ) to separate debris flows from floods based on the ratio of observed peak discharge to the theoretical maximum water discharge from rainfall runoff. We compile 642 observations of floods and debris flows and demonstrate that Q* distinguishes flow type to ∼92% accuracy. Q* allows for accurate identification of debris flows through simple channel cross-section surveys rather than through qualitative interpretation of deposits, and therefore should increase the performance of models and engineered structures that require accurate flow-type observations.

Geophysical Research Letters↗

USGS2025

Paleo-scours within the layered sulfate-bearing unit at Gale crater, Mars: Evidence for intense wind erosion

The surface of modern Mars is largely shaped by wind, but the influence of past wind activity is less well constrained. Sedimentary rocks exposed in the lower foothills of Aeolis Mons, the central mound within Gale crater, record a transition from predominantly lacustrine deposition in the Murray formation to aeolian deposition in the Mirador formation. Here, we report a series of enigmatic decameter-wide, concave-up scour-and-fill structures within the Mirador formation and discuss their formation mechanisms. Using panoramic images of stratigraphy exposed in cliff faces acquired by the Curiosity rover, we map the extent, distribution and orientation of the scour-and-fill structures and document the sedimentary facies within and surrounding these structures. The scours are grouped into two classes: (A) scours with a simple, symmetric morphology and light-toned, draping infill; and (B) scours with lateral pinching and dark-toned infill. We find that the scour-enclosing environment is composed of planar, even-in-thickness laminations with a pin-stripe pattern which we interpret as wind-ripple strata formed within an aeolian sandsheet environment. Class B contains cm-scale cross-bedding and a wing-shaped feature making this scour-and-fill structure consistent with fluvial processes. We interpret scour fill of class A as an aeolian infill due to similarities with the surrounding sandsheet strata. The broad morphologies and distribution of class A are also consistent with the geometry of blowout structures formed by localized, enhanced wind deflation. These paleo-blowout structures occur clustered within the same stratigraphic interval, which may imply that they record an interval of intensified wind activity at Gale crater.

JGR Planets↗

USGS2025

Are equilibrium shoreline models just convolutions?

Yes. Equilibrium shoreline models, which simulate wave-driven cross-shore erosion and accretion, are mathematically equivalent to a discrete convolution (i.e., a weighted, moving average) of a time series of wave-forcing conditions with a parameterized memory-decay kernel function. The direct equivalence between equilibrium shoreline models and convolutions reveals key theoretical aspects of equilibrium behavior. Convolutions (representing quasi-low-pass filter operations) provide an intuitive theoretical description of shoreline erosion and accretion behavior in response to waves: that is, shoreline position often mirrors the weighted moving average of wave time series. Model-convolution equivalence also provides a conceptual basis to interpret, evaluate, and construct data-driven Machine-Learning/Deep-Learning (ML/DL) models that use convolutions to extract features from data and then apply them for prediction (e.g., Convolutional Neural Networks (CNNs)). Finally, our findings provide a methodological pathway (based on Fourier transforms) for future understanding of wave-driven shoreline change, which can be used to interpret the coherence between the frequency spectrum of the processes of waves and shoreline change and construct more computationally efficient and effective shoreline-modeling approaches.

JGR Earth Surface↗

USGS2026

Monazite and xenotime U-Pb geochronology and thermometry of the Blue Ridge and Inner Piedmont of North Carolina: Implications for the thermal-metamorphic evolution of the southern Appalachian metamorphic “core”

The southern Appalachian orogen preserves a complex distribution of metamorphism and deformation varying in timing, magnitude, and spatial extent. These complexities give rise to disparate interpretations for southern Appalachian tectonic evolution, which complicates the testing and interpretation of tectonic models in this system. New monazite (Mnz) and xenotime (Xtm) laser ablation split stream (LASS) analyses alongside Mnz-Xtm thermometry in the orogenic core in the eastern Blue Ridge (EBR), western Inner Piedmont (WIP), and Cat Square terranes (CST) of North Carolina yield new constraints that define distinct pro- and retrograde metamorphic events. The EBR preserves two prograde thermal events: the Taconic (∼470-440 Ma, >660°C) and Neoacadian (∼380-340 Ma, 600–700°C), separated by a period of cooling (exhumation?) and followed by garnet breakdown from 339 to 329 Ma. Evidence of pervasive Neoacadian ductile deformation in the EBR is largely limited to the Brevard fault zone (BFZ), indicating that a major rheological gradient existed across the BFZ during the Neoacadian and early Alleghanian. Southeast of the BFZ, in the WIP and CST, monazite data define a protracted Neoacadian evolution from early mineral growth at ∼405 Ma at ∼450–600°C to >700°C at ∼360 Ma, followed by early Alleghanian retrograde metamorphism and deformation (<345 Ma, 350–500°C). These constraints, together with previously reported thermobarometric data, define a P-T-t evolution for the WIP and CST consistent with Neoacadian crustal flow, while the coeval presence of a thermal-rheological boundary along the BFZ further supports a model of Neoacadian crustal “escape” flow within the orogen.

North Carolina↗

USGS2026

The geologic history of the Chehalis Forearc Basin, Washington State, USA

The Chehalis basin is located between the Cascade arc and the Coast Range in southwest Washington State. It consists of sedimentary and volcanic rocks deposited throughout the Cenozoic and is underlain by the Siletzia terrane, a thick, large igneous province accreted in the Eocene. Here, we constrain evolution of the Chehalis basin depocenter using isochore maps derived from isostatic gravity anomalies, borehole data, and stratigraphy for several time periods: the Eocene (47.6–36.8 Ma), the Oligocene (36.8–20 Ma), and the Neogene (20–0 Ma). Our results suggest that local subsidence is driven by deformation on faults that bound and intersect the basin. We see northward depocenter migration, interpreted as a shift in deformation from the northwest striking Cedar Creek fault to the west striking Doty fault. We interpret these data in terms of the long-term north-south shortening and clockwise rotation of the Cascadia forearc. During the Eocene, the Cedar Creek fault was preferentially aligned with north-south shortening, but became less active when clockwise rotation brought the Doty fault into east-west alignment with the stress field. Sediment accumulation rates decreased from 196 m/Myr in the Paleogene (∼40–20 Ma) to 27 m/Myr in the Neogene (∼20–0 Ma) as Miocene uplift of the Coast Range limited accommodation space and shifted the depositional environment from marine to fluvial. Our results are consistent with the geologic evolution of the Portland and Tualatin basins to the south and reveal a depositional and structural history uniquely shaped by clockwise rotation of the Cascadia forearc.

Washington↗

USGS2012

Imaging the midcontinent rift beneath Lake Superior using large aperture seismic data

We present a detailed velocity model across the 1.1 billion year old Midcontinent Rift System (MRS) in central Lake Superior. The model was derived primarily from onshore-offshore large-aperture seismic and gravity data. High velocities obtained within a highly reflective half-graben that was imaged on coincident seismic reflection data demonstrate the dominantly mafic composition of the graben fill and constrain its total thickness to be at least 30km. Strong wide-angle reflections are observed from the lower crust and Moho, indicating that the crust is thickest (55–60km) beneath the axis of the graben. The total crustal thickness decreases rapidly to about 40 km beneath the south shore of the lake and decreases more gradually to the north. Above the Moho is a high-velocity lower crust interpreted to result from syn-rift basaltic intrusion into and/or underplating beneath the Archean lower crust. The lower crust is thickest beneath the axis of the main rift half-graben. A second region of thick lower crust is found approximately 100km north of the axis of the rift beneath a smaller half graben that is interpreted to reflect an earlier stage of rifting. The crustal model presented here resembles recent models of some passive continental margins and is in marked contrast to many models of both active and extinct Phanerozoic continental rift zones. It demonstrates that the Moho is a dynamic feature, since the pre-rift Moho is probably within or above the high-velocity lower crust, whereas the post-rift Moho is defined as the base of this layer. In the absence of major tectonic activity, however, the Moho is very stable, since the large, abrupt variations in crustal thickness beneath the MRS have been preserved for at least a billion years.

Lake Superior↗

USGS2012

Paleomagnetism of the Late Triassic Hound Island Volcanics: Revisited

The collision and accretion of the Alexander terrane profoundly influenced the geologic history of Alaska and western Canada; however, the terrane's displacement history is only poorly constrained by sparse paleomagnetic studies. We studied the paleomagnetism of the Hound Island Volcanics in order to evaluate the location of the Alexander terrane in Late Triassic time. We collected 618 samples at 102 sites in and near the Keku Strait, Alaska, from the Late Triassic Hound Island Volcanics, the Permian Pybus Formation, and 23-Ma gabbroic intrusions. We found three components of magnetization in the Hound Island Volcanics. The high-temperature component (component A) resides in hematite and magnetite and was found only in highly oxidized lava flows in a geographically restricted area. We think it is primary, or acquired soon after eruption of the lavas, principally because the directions pass a fold test. The paleolatitude indicated by this component (19.2° ± 10.3°) is similar to those determined for various portions of Wrangellia, consistent with the geologic interpretation that the Alexander terrane was with the Wrangellia terrane in Late Triassic time. We found two overprint directions in the Hound Island Volcanics. Component B was acquired 23 m.y. ago due to intrusion of gabbroic dikes and sills. This interpretation is indicated by the similarity of upper-hemisphere directions in the Hound Island Volcanics to those in the gabbro. Component C, found in both the Hound Island Volcanics and the Permian Pybus Formation, is oriented northeast and down, fails a regional fold test, and was acquired after regional deformation around 90 to 100 Ma. This overprint direction yields a paleolatitude similar to, but slightly higher than, slightly older rocks from the Coast Plutonic Complex, suggesting that the Alexander terrane was displaced 17° in early Late Cretaceous time. The occurrence of these two separate overprinting events provides a satisfying explanation of the earlier puzzling results from the Hound Island Volcanics (Hillhouse and Grommé, 1980). Finally, great-circle analysis of the paleomagnetic data from the Pybus Formation suggests the Alexander terrane may have been in the northern hemisphere in Permian time.

Alaska↗

USGS2012

The geology and distribution of impact craters on Venus: What are they telling us?

Magellan has revealed an ensemble of impact craters on Venus that is unique in many important ways. We have compiled a data base describing the 842 craters on 89% of Venus' surface mapped through orbit 2578. (The craters range in diameter from 1.5 to 280 km.) We have studied the distribution, size‐density, morphology, geology, and associated surface properties of these craters both in the aggregate and, for some craters, in greater detail. We find that (1) the spatial distribution of craters is highly uniform; (2) the size‐density distribution of large craters (diameters ≥35 km) is similar to the young crater populations on other terrestrial planets but at a much lower density that indicates an average age of only about 0.5 Ga (based on the estimated population of Venus‐crossing asteroids); (3) unlike the case on other planets, the density of small craters (diameters ≤35 km) declines rapidly with decreasing diameters because of atmospheric filtering; (4) the spectrum of crater modification differs greatly from that on other planets: 62% of all craters are pristine, only 4% are embayed by lavas, and the remainder are affected by tectonism, but none are severely and progressively depleted (as extrapolated from the size‐density distribution of larger craters); (5) large craters have a progression of morphologies generally similar to those on other planets, but small craters are typically irregular or multiple rather than bowl shaped; (6) diffuse radar‐bright or ‐dark features surround some craters, and 367 similar diffuse “splotches” with no central crater are observed; and (7) other crater features unique to Venus include radar‐bright or ‐dark parabolic arcs opening westward and extensive outflows originating in crater ejecta. The first three of these observations are entirely unexpected. We interpret them as indicating that the planet's cratering record was erased by a global resurfacing event or events, the latest ending about 0.5 Ga, after which volcanic activity declined (but did not cease entirely). Since the last resurfacing event, a maximum of 10% of the planet has been resurfaced and only about 4% of the craters have been obliterated. Convective thermal evolution models support this interpretation (Arkani‐Hamed and Toksoz, 1984). Observations 3–7 confirm quantitatively the expectation that the dense atmosphere of Venus has strongly affected the production of craters. Large impactors have been relatively unaffected, intermediate‐sized ones have been fragmented and have produced overlapping or multiple craters, a narrow size range has produced shock‐induced “splotches” but no craters, and the smallest bodies have had no observable effect on the surface. The number of craters eliminated by the “atmospheric filter” is enormous, about 98% of the craters between 2 and 35 km in diameter that Magellan might have observed on a hypothetical airless Venus. Unique crater‐related features such as parabolas and outflow deposits demonstrate the roles of Venus' high atmospheric density and temperature in modifying the crater formation process. Finally, heavily fractured craters and lava‐embayed craters are found to have higher than average densities along the major fracture belts and rifted uplands connecting Aphrodite Terra and Atla, Beta, Themis, and Phoebe regiones. These craters thus provide physical evidence for recent volcanic and tectonic activity at a low level.

Journal of Geophysical Research E: Planets↗

USGS2010

Mid‐Cretaceous extensional tectonics of the Yukon‐Tanana Terrane, Trans‐Alaska Crustal Transect (TACT), east‐central Alaska

Mid‐Cretaceous crustal extension played a fundamental role in the structural evolution of the Yukon‐Tanana terrane (YTT) in the northern Cordilleran interior. In the central portion of the YTT northwest of Delta Junction, Alaska, a mylonitic shear zone juxtaposes greenschist facies rocks in the upper plate against middle to upper amphibolite facies metamorphic rocks in the lower plate, a juxtaposition suggesting elimination of as much as 10 km of crustal section. The mylonites form a partial sheath enveloping a domal footwall structure and kinematic analysis of the mylonite zone yields a uniform transport direction of hanging wall to ESE. These relations suggest analogies to the metamorphic core complexes of the southern Cordillera. However, the YTT structures are entirely ductile, suggesting either a relatively deep erosional level or relatively high geothermal gradients during extension. In the study area remnants of an older preextensional thrusting event are preserved at the highest structural levels at the base of the Seventymile terrane and the leading edge of YTT in the Wickersham terrane. However, most areas display a regional, subhorizontal fabric that is superimposed on older fabrics, and in the study area this latest fabric is subparallel to the mylonitic sheath of the apparent extensional structure. Thus the conventional viewpoint that this latest fabric is related to thrusting needs to be reevaluated and this fabric may be entirely extensional in origin. Further evidence for extension is provided by clear similarities between YTT and characteristic features of other extensional terranes. Thus we suggest that the YTT is a deeply eroded view of highly extended continental crust. The tectonic mechanism for the extensional event and the magnitude of the extension is uncertain because of complications in regional timing relationships and in alternative interpretations of the reconstruction of the crustal section. Three end‐member models based on analogies with Neogene extensional systems are presented as working models to accommodate the alternative interpretations: (1) a Jurassic collision and Cretaceous extension model based on comparisons with the Neogene history of the Mediterranean region; (2) an Early to mid‐Cretaceous syncollisional model analogous to the Carpathian Mountains of eastern Europe; and (3) a syncollisional plateau uplift model with extension driven by gravity spreading.

Alaska↗

USGS2012

Impact crater outflows on Venus: Morphology and emplacement mechanisms

Many of the 932 impact craters discovered by the Magellan spacecraft at Venus are associated with lobate flows that originate at or near the crater rim. They extend for several to several hundred kilometers from the crater, and they commonly have a strong radar backscatter. A morphologic study of all identifiable crater outflows on Venus has revealed that many individual flows each consist of two areas, defined by distinct morphologic features. These two areas appear to represent two stages of deposition for each flow. The part of the flow that is generally deposited closest to the crater tends to be on the downrange side of the crater, flows in the downrange direction, and it is interpreted to be a late-stage ejecta. In many cases, this proximal part of the flow is too thin to completely bury the large blocks in subjacent ejecta deposits. Dendritic channels, present in many proximal flows, appear to have drained liquid from the proximal part in the downhill direction, and they debouch to feed the outer part of the flows. This distal part flows downhill, fills small grabens, and is ponded by ridges, behavior that mimics that of volcanic lava flows. The meandering and dendritic channels and the relation of the distal flows to topography strongly suggest that the distal portion is the result of coalescence and slow drainage of impact melt from the proximal portion. Impact melt forms a lining to the transient crater and mixes turbulently with solid clasts, and part of this mixture may be ejected to form the proximal part of the flow during the excavation stage of crater development. A statistical study of the Venusian craters has revealed that, in general, large craters produced by impacts with relatively low incidence angles to the surface are more likely to produce flows than small craters produced by higher-angle impacts. The greater flow production and downrange focusing of the proximal flows with decreasing incidence angle indicate a strong control of the flows by the impactor flight direction, and a high downrange velocity imparted to the proximal flow material in lower angle impacts. On the Moon, small flows interpreted to be composed of impact melt are observed atop the ejecta of large, fresh craters; on Earth, melt-rich suevite deposits form the uppermost layer of ejecta of some fresh craters. These features, albeit much smaller, may be analogous to the flows on Venus. Numerical models have predicted that larger volumes of impact melt would be produced on Venus than on the cooler terrestrial bodies due to high atmospheric and target temperatures, perhaps 3 times the volume produced on the Moon for a given crater diameter.

Journal of Geophysical Research E: Planets↗

USGS2010

Earthquake refraction profiles of the root of the Sierra Nevada

We examine the seismic structure of the Sierra Nevada using records of nine earthquakes and one explosion in and near the Sierra, recorded on stations in the Sierra. We first interpret travel times from these paths, which are confined to a single tectonic block, in terms of one‐dimensional structures. The most nearly reversed pair of earthquakes, the 1966 Truckee and 1983 Durrwood Meadows earthquakes, share refracted ( P n ) arrival times (corrected to surface focus) along a line t =8.75±0.25+Δ/8.0, suggesting that a nearly flat layer of 8.0 km/s mantle material lies at depths of 46–48 km. First arrivals from these events do not constrain velocities from ≈30 to 45 km depth. Secondary arrivals and some first arrivals from other earthquakes suggest that velocities in part of this region range between 6.9 and 7.8 km/s. The presence of this “7.x‐km/s” layer can help to explain previous contradictory observations. The 7.x‐km/s layer could be interpreted as either the mafic bottom of a silicic, Mesozoic magmatic arc or as accreted mafic underplating or rejuvenated mantle related to Cenozoic arc volcanism or Basin and Range spreading. Arrivals at stations in the foothills and the crest of the Sierra cannot be fit with a single longitudinal structure, indicating a lateral variation of velocity structure. These variations support previously inferred variations of lithospheric structure, with higher‐velocity, thinner crust to the west beneath the Sierran foothills and slower‐velocity crust (or possibly upper mantle) beneath the high mountains in the eastern Sierra. Rapid changes in arrival times between stations separated by short distances in the eastern Sierra suggest that a sharp boundary exists between the Sierra and the Basin and Range at Moho depths. We also present fresh evidence of the asymmetry of the root of the Sierra, wherein arrivals from earthquakes on the west of the Sierra are delayed within the Sierra and return to original values in the Basin and Range, while arrivals from earthquakes and explosions from the Sierra into the Great Valley. We suggest that if the 7.x‐km/s material occurs in a wedge above the Moho, then the asymmetry can be explained by arrivals from the west being delayed by the dipping 8.0‐km/s Moho, while those from the east may be entering the root along a 7.x‐km/s layer that is near the depth of the Basin and Range Moho.

New Hampshire↗

USGS2008

Introduction to the special section northeast Japan: A case history of subduction

Subduction-related tectonic events such as strike-slip faulting, uplift of high-pressure metamorphic rocks in the forearc, back arc spreading, arc magmatism, and continental collisions have shaped northeast Japan, making it a case history for subduction. Much of the record of these events is preserved and can be used to reconstruct the tectonic history of the region. Although many geological, geophysical, and geochemical data have been collected in Japan for the last 30 years, interpretation of these data into a plate tectonic context did not really begin until about 10 years ago. Much of the data and interpretation on northeast Japan have been published in Japanese or as individual local studies. The primary goal of this special section is to present new investigations covering a wide range of topics and scales and place them in a tectonic framework. The papers in this special section focus on three main subjects: Mesozoic aspects of northeast Japan; the relation of Japan Sea opening with the tectonics and geochemistry of northeast Japan in the Tertiary; and the geophysical setting. The purpose of this overview is to link these subjects and provide a tectonic framework for north- east Japan and eastern Asia since the Mesozoic.

Journal of Geophysical Research B: Solid Earth↗

USGS1996

Three-dimensional P and S wave velocity structure of Redoubt Volcano, Alaska

The three‐dimensional P and S wave structure of Redoubt Volcano, Alaska, and the underlying crust to depths of 7–8 km is determined from 6219 P wave and 4008 S wave first‐arrival times recorded by a 30‐station seismograph network deployed on and around the volcano. First‐arrival times are calculated using a finite‐difference technique, which allows for flexible parameterization of the slowness model and easy inclusion of topography and source‐receiver geometry. The three‐dimensional P wave velocity structure and hypocenters are determined simultaneously, while the three‐dimensional S wave velocity model is determined using the relocated seismicity and an initial S wave velocity model derived from the P wave velocity model assuming an average Vp / Vs ratio of 1.78. Convergence is steady with approximately 73% and 52% reduction in P and S wave arrival time RMS, respectively, after 10 iterations. The most prominent feature observed in the three‐dimensional velocity models derived for both P and S waves is a relative low‐velocity, near‐vertical, pipelike structure approximately 1 km in diameter that extends from 1 to 6 km beneath sea level. This feature aligns axially with the bulk of seismicity and is interpreted as a highly fractured and altered zone encompassing a magma conduit. The velocity structure beneath the north flank of the volcano between depths of 1 and 6 km is characterized by large lateral velocity variations. High velocities within this region are interpreted as remnant dikes and sills and low velocities as regions along which magma migrates. No large low‐velocity body suggestive of a magma chamber is resolved in the upper 7–8 km of the crust.

Journal of Geophysical Research B: Solid Earth↗

USGS1996

Instability of bottom-water redox conditions during accumulation of Quaternary sediment in the Japan Sea

The concentrations of Cd, Cr, Cu, Mo, Ni, Sb, U, V, and Zn were measured in early Quaternary sediment (1.32 to 1.08 Ma) from the Oki Ridge in the Japan Sea. The elements were partitioned between a detrital fraction, composed of terrigenous and volcaniclastic aluminosilicate debris, and a marine fraction, composed of biogenic and hydrogenous debris derived from seawater. The most important factors controlling minor-element accumulation rates in the marine fraction were (1) primary productivity in the photic zone, which largely controlled the flux of particulate organic-matter-bound minor elements settling through the water column and onto the seafloor, and (2) bottom-water redox, which determined the suite of elements that accumulated directly from seawater. This marine fraction of minor elements on Oki Ridge recorded six periods of high minor-element abundance. Assuming a constant bulk sediment accumulation rate, each period lasted roughly 5,000 to 10,000 years with a 41,000-year cycle. Accumulation rates of individual elements such as Cd, Mo, and U suggest sulfate-reducing conditions were established in the bottom water during the 10,000-year periods; accumulation rates of Cr and V during the intervening periods are indicative of less reducing, denitrifying conditions. Interelement ratios, for example, Cu:Mo, V:Cr, and Sb:Mo, further reflect bottom-water instability, such that bottom-water redox actually varied from sulfate reducing to denitrifying during the periods of highest minor-element accumulation rates; it varied from denitrifying to oxidizing during the intervening periods. Sediment lithology supports these interpretations of the minor-element distributions; the sediment is finely laminated for several of the periods represented by Cd, Mo, and U maxima and weakly laminated to bioturbated for the intervening periods. The geochemistry of this sediment demonstrates the unambiguous signal of Mo, principally, but of several other minor elements as well in recording sulfate-reducing conditions in bottom water. The forcing function that altered their accumulation, that is, that altered primary productivity and bottom water redox conditions, is problematic. Currently held opinion suggests that O 2 depletion was most strongly developed during glacial advances. Low sea level during such times is interpreted to have enhanced primary productivity and restricted bottom-water advection.

Paleoceanography and Paleoclimatology↗

USGS2012

Crustal magnetic anomalies

This Quadrennial Report departs in format from its predecessors. Earlier reports provided brief descriptions of the entire range of topics on “crustal magnetic anomalies.” Following guidelines established by the AGU, we instead focus our discussion on only a few selected topics. Our abbreviated list of references reflects this restricted focus. A more comprehensive bibliography can be obtained from the authors as noted subsequently. The past quadrennium was distinguished by a significant change in the scale and scope of crustal magnetic anomaly research. Advances in instrumentation, navigation, and data processing techniques resulted in increasing use of two-dimensional anomaly maps rather than profile data. Although maps are commonly used in aeromagnetic studies of continental areas, their use in the ocean basins has been limited until recently. The other important change has been a trend towards increased integration of magnetic anomaly data with gravity, seismic, geochemical, remote sensing, and geologic data to enhance the interpretation and reduce the ambiguity of magnetic field data.

Reviews of Geophysics↗

USGS1996

Three-dimensional models of deformation near strike-slip faults

We use three-dimensional elastic models to help guide the kinematic interpretation of crustal deformation associated with strike-slip faults. Deformation of the brittle upper crust in the vicinity of strike-slip fault systems is modeled with the assumption that upper crustal deformation is driven by the relative plate motion in the upper mantle. The driving motion is represented by displacement that is specified on the bottom of a 15-km-thick elastic upper crust everywhere except in a zone of finite width in the vicinity of the faults, which we term the “shear zone.” Stress-free basal boundary conditions are specified within the shear zone. The basal driving displacement is either pure strike slip or strike slip with a small oblique component, and the geometry of the fault system includes a single fault, several parallel faults, and overlapping en echelon faults. We examine the variations in deformation due to changes in the width of the shear zone and due to changes in the shear strength of the faults. In models with weak faults the width of the shear zone has a considerable effect on the surficial extent and amplitude of the vertical and horizontal deformation and on the amount of rotation around horizontal and vertical axes. Strong fault models have more localized deformation at the tip of the faults, and the deformation is partly distributed outside the fault zone. The dimensions of large basins along strike-slip faults, such as the Rukwa and Dead Sea basins, and the absence of uplift around pull-apart basins fit models with weak faults better than models with strong faults. Our models also suggest that the length-to-width ratio of pull-apart basins depends on the width of the shear zone and the shear strength of the faults and is not constant as previously suggested. We show that pure strike-slip motion can produce tectonic features, such as elongate half grabens along a single fault, rotated blocks at the ends of parallel faults, or extension perpendicular to overlapping en echelon faults, which can be misinterpreted to indicate a regional component of extension. Zones of subsidence or uplift can become wider than expected for transform plate boundaries when a minor component of oblique motion is added to a system of parallel strike-slip faults.

Journal of Geophysical Research B: Solid Earth↗

USGS1997

Sedimentary history and mass flow structures of Chryse and Acidalia Planitiae, Mars

Geologic mapping and crater counting in Chryse and Acidalia Planitiae (GAP) reveal five major sedimentary deposits of Hesperian to Early Amazonian age, including (1) a mass flow deposited during the Early Hesperian near Deuteronilus Mensae (northeast of the map region) that may have resulted from the carving of Kasei Valles, >3000 km southwest of the exposed part of the deposit; (2) knobby plains material consisting of channel (likely; from Simud and Tiu Valles and possibly Ares and Shalbatana Valles) and mass-wasting deposits in central and eastern CAP; (3) material largely from Maja and Ares Valles emplaced in at least western and southern CAP (outcrops in southern Chryse Planitia developed thermokarst); (4) a thin mass flow covering much of southern Chryse Planitia that emanated from Simud and Tiu Valles; and (5) a thick, extensive (perhaps >3500 km across) mass flow deposit in central and northern CAP derived from accumulation and backflow of the preceding thin mass flow or perhaps melting of polar deposits. Other possible deposits may not be recognizable owing to burial by younger materials or a lack of morphologic signature. Various associated landforms appear to be consistent with the mass flow interpretations, including lobate and linear scarps along deposit edges, fractures related to desiccation of thick sediments, troughs, and ridges near the edges of the deposit indicative of secondary mass movement and deformation, pitted domes and fissure-fed flows possibly formed by sedimentary (mud) eruptions, and longitudinal channel grooves perhaps formed by roller vortices. No convincing evidence for paleoshorelines or stagnant ice sheets is found in CAP. These findings suggest that mass flow and hyperconcentrated flooding may have been the predominant processes of outflow-channel dissection in CAP. Elsewhere in the northern plains, similar landforms are prevalent. The mass flow interpretation does not require either multiple episodes of extraordinarily high water-discharge rates achieved by freeing huge volumes of water from the crust, repetitive recycling of immense volumes of water into highland aquifers at the heads of Chryse channels, or profound climate change. Mars Pathfinder will most likely land on and inspect the surface of the thin mass flow that originated from the canyons of Simud and Tiu Valles.

Journal of Geophysical Research E: Planets↗

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