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USGS2012

Evidence for a new geomagnetic reversal from lava flows in Idaho: Discussion of short polarity reversals in the Brunhes and late Matuyama polarity chrons

K-Ar ages and paleomagnetic data for basalt samples from a new core hole (site E) at the Idaho National Engineering Laboratory (INEL) indicate that the age of the reversed polarity event recorded in Snake River Plain lavas is older than 465±50 ka (1000 years before present) reported previously by Champion et al. (1981). Nine basalt flows, eight with normal polarity and one with reversed polarity, were recognized in the site E core hole. The flows above and below the reversed flow have ages of 491±80 ka and 580±93 ka, respectively. The inclination of the paleomagnetic field direction of the reversed flow at site E agrees with the inclination of reversed flows elsewhere at INEL which have an age of 565±14 ka. These reversed flows were previously thought to be correlative with the Emperor event. We suggest that this polarity event is an older event which we name the Big Lost Reversed Polarity Subchronozone and Subchron. A review of data documenting short reversal records from volcanic and sedimentary rocks shows that there is evidence for eight polarity subchrons in the Brunhes and two besides the Jaramillo in the late Matuyama. These 10 short subchrons begin to indicate the many short events that Cox (1968) hypothesized must exist if polarity interval lengths have a Poisson distribution. These events are true subchrons, not excursions, and may or may not have low associated paleointensities, although low field strengths might explain why the reversal process aborts. The mean sustained polarity interval length since late Matuyama Chron time is 90,000 years. The similarity of this number with the 10 5 -year period of the Earth's orbital eccentricity suggests anew that linkage between geomagnetic, paleoclimatic, and possible underlying Earth orbital parameters should be evaluated.

Journal of Geophysical Research Solid Earth↗
USGS2012

Paleomagnetic constraints on rotation within Mount Abbot quadrangle, central Sierra Nevada, California

The regional photolineament pattern in Mount Abbot quadrangle in the central Sierra Nevada is sigmoidal in plan view. The fractures in the southwest section of the quadrangle, near Florence Lake, strike N10°–20°E, while those in the central section, along Bear Creek, strike N50°–70°E and those in the northeast strike N25°–40°E. A paleomagnetic investigation was undertaken to determine if this sigmoidal pattern was formed by kinking of originally straight photolineaments after the rock mass cooled below the blocking temperature. If the inner limb of the kink rotated 40°–60° clockwise with respect to the outer limbs, then there should be a similar difference in the paleomagnetic declinations between the Florence Lake and Bear Creek areas. The paleomagnetic results indicate a clockwise difference in declination of 13.4°±7° of the Bear Creek area with respect to Florence Lake. Thus the sigmoidal pattern could not have formed entirely by rotation after cooling below the blocking temperature. It is also possible that the observed declination discrepancy is due to tilting, apparent polar wander, or paleomagnetic dispersion instead of rotation. Sigmoidal patterns, formed by kink folding, occur on the outcrop scale with inner limb rotations of 15°–30°; three of these kinks were sampled to test the paleomagnetic method in an area of demonstrable rotation. The data from these sites indicate that although no difference in declination is evident, a reheating event may have occurred which could have removed any rotation from the paleomagnetic record. Reheating, possibly due to the circulation of hydrothermal fluids, may have been causally related to the outcrop scale kinking, since reheating is not observed away from the outcrop scale kinks. The data further indicate that there may have been continuous deformation of the rock mass during the formation of the kinks.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Curie temperature isotherm analysis and tectonic implications of aeromagnetic data from Nevada

Estimates of the depth to the Curie temperature isotherm in Nevada are in accordance with other regional geologic and geophysical information and together can be explained in the context of present-day tectonism. A method to estimate the depth extent of magnetic sources from the statistical properties of magnetic anomalies was applied to a statewide compilation of aeromagnetic data from Nevada. Basal depths of magnetic sources show no apparent correlation with the so-called magnetic quiet zone, which trends northerly through the eastern part of the state, or with basin-and-range topography. However, certain correlations with published heat flow measurements are apparent and suggest that undulations in basal depth of magnetic sources are related in part to undulations in the Curie temperature isotherm. For example, an area of shallow basal depth (<10 km) near Battle Mountain corresponds to an area of exceptionally high conductive heat flow and indicates a shallow depth to the Curie temperature isotherm in this region. A narrow zone of shallow basal depth extends south from the Battle Mountain area along the 118°W meridian to at least latitude 38°N, which also is a zone of historic surface offsets and high-magnitude earthquakes. The correspondence along the 118° meridian of shallow basal depth, high heat flow, high lower crustal seismic velocities, attenuated P and S wave arrivals, historic faulting, and large earthquakes suggests that they each are related to an active north trending spreading zone in this part of the Basin and Range province.

Journal of Geophysical Research Solid Earth↗
USGS2012

Analysis of geophysical well logs obtained in the State 2-14 borehole, Salton Sea geothermal area, California

A complete suite of conventional geophysical well logs was obtained in the upper part of a 3220-m- deep borehole drilled into geothermally altered alluvial sediments on the southeastern edge of the Salton Sea. In situ temperatures greater than 300°C and an inability to cool parts of the borehole by circulation limited the suite of logs run below 2000 m in depth to deep induction, spontaneous potential, un-calibrated neutron, natural gamma, and temperature. Bottom-hole temperature trends given by repeat temperature logs were extrapolated to undisturbed temperatures approaching 355°C at a depth of 3220 m. Geophysical logs obtained in the State 2–14 borehole indicate that neutron porosity, gamma-gamma, and deep-induction logs provide useful information on lithologic trends with depth. The natural gamma log contains almost continuous, high-frequency fluctuations that obscure lithologic trends and that may be related to recent radioisotope redistribution and departure from radiometric equilibrium. Acoustic transit time logs give unrealistically low in situ compressional velocities ranging from 1.8 to 3.0 km/s, whereas acoustic waveform logs indicate that sediment compressional velocities range from less than 3.0 km/s shallower than 1000 m in depth to almost 5.0 km/s at depths greater than 2000 m. Analyses indicate that most log values lie between two lithologic end points: an electrically conductive claystone with moderate neutron porosity, but no effective porosity, and an electrically nonconductive, fully cemented siltstone that has small but finite porosity. A limited number of clean sandstones depart from this trend; geophysical logs from these sandstones indicate an effective porosity ranging from 5 to 14%, and saturation with brines having equivalent NaCl concentrations greater than 100,000 mg/L. Depth- averaged trends in neutron porosity and deep-induction logs, along with trends in acoustic velocity determined from acoustic waveform logs, demonstrate that major changes in the properties of alluvial sediments occur within the depth range from 1200 to 1800 m. Although caliper logs were not obtained deeper than 2000 m, resistivity values less than 10 ohm m at those depths probably correspond to borehole enlargements in production zones rather than local increases in effective porosity. The transition in sediment properties indicated by the geophysical logs in the depth interval from 1200 to 2000 m apparently represents a detailed vertical profile of the transition from relatively unaltered clay minerals in alluvial sediments to electrically nonconductive alteration products such as epidote and feldspar.

Journal of Geophysical Research Solid Earth↗
USGS2012

Thermal regime of the State 2-14 well, Salton Sea Scientific Drilling Project

Temperature logs were made repeatedly during breaks in drilling and both during and after flow tests in the Salton Sea Scientific Drilling Project well (State 2–14). The purpose of these logs was to assist in identifying zones of fluid loss or grain and to characterize reservoir temperatures. At the conclusion of the active phase of the project, a series of logs was begun in an attempt to establish the equilibrium temperature profile. Initially, we were able to log to depths below 3 km, but beginning in late May of 1986, it was impossible to log below about 1.8 km owing to casing failure. Our best estimates of formation temperature below 1.8 km are 305° ± 5°C at 1890 m and 355° ± 10°C at 3170 m. For the upper 1.8 km the latest temperature log (October 24, 1986), using a digital “slickline” (heat-shielded downhole recording) device, was within a few degrees Celsius of equilibrium, as confirmed by a more recent log (July 31, 1987) to a depth of ∼ 1 km. As in most other wells in the Salton Sea geothermal field, there is an impermeable, thermally conductive “cap” on the hydrothermal system; this cap extends to a depth of more than 900 m at the State 2–14 well. Thermal conductivities of 19 samples of drill cuttings from this interval were measured at room temperature. The conductivity values were corrected for in situ porosity as determined from geophysical logs and for the effects of elevated temperature. Thermal gradients decrease from about 250 mK m −1 (same as degrees Celsius per kilometer) in the upper few hundred meters to just below 200 mK m −1 near the base of the conductive cap. Using one interpretation, thermal conductivities increase with depth (mainly because of decreasing porosity), resulting in component heat flows that agree reasonably well with the mean of about 450 m W m −2 . This value agrees well with heat flow data from shallow wells within the Salton Sea geothermal field. A second interpretation, in which measured temperature coefficients of quartz- and carbonate-rich rocks are used to correct thermal conductivity, results in lower mean conductivities that are roughly constant with depth and, consequently, systematically decreasing heat flux averaging about 350 mW m −2 below 300 m. This interpretation is consistent with the inference (from fluid inclusion studies) that the rocks in this part of the field were once several tens of degrees Celsius hotter than they are now. The age of this possible disturbance is estimated at a few thousand years.

Journal of Geophysical Research Solid Earth↗
USGS2012

Chemistry and geothermometry of brine produced from the Salton Sea Scientific drill hole, Imperial Valley, California

The December 29–30, 1985, flow test of the State 2–14 well, also known as the Salton Sea Scientific drill hole, produced fluid from a depth of 1865–1877 m at a reservoir temperature of 305° ± 5°C. Another flow test at a depth of 3170 m produced brine contaminated by drilling fluid and diesel oil. Therefore we focus on the first flow test. Samples were collected at five different flashing pressures. The brines are Na-Ca-K-Cl-type waters with very high metal and low SO 4 and HCO 3 contents. Compositions of the flashed brines were normalized relative to the 25°C densities of the solutions, and an ionic charge balance was achieved by adjusting the Na concentration. The composition of the preflashed reservoir fluid was calculated using enthalpy-chloride relations applied to the normalized and charge-balanced brines. The calculated total dissolved solids in the preflashed reservoir fluid ranges from about 24.8 wt %, assuming insignificant thermal losses from the erupting fluid before sampling, to 26.0 wt %, assuming a 10% enthalpy loss by conduction of thermal energy through casing and surface piping. The preferred total dissolved solids of the reservoir fluid is 25.05 wt %. The calculated specific density of the preflashed reservoir fluid at 305°C and 1870 m depth ranges from 0.9980 (no thermal loss prior to sampling) to 1.0107 ± 0.0023 g cm −3 (10% thermal loss). Of the various cation geothermometers that are now in common use, the Na-K-Ca method gives a temperature (310°C) closest to the measured temperature (305°C) in the production horizon. Calculated Na/K geothermometer temperatures, using equations suggested by different investigators, range from 326° to 364°C. The Mg/K 2 method gives a temperature of about 350°C, Mg/Li 2 about 282°, and Na/Li 395°–418°C.

Journal of Geophysical Research Solid Earth↗
USGS2012

Three-dimensional gravity modeling of the geologic structure of Long Valley caldera

A 48-mGal gravity low coincides with Long Valley caldera and is mainly attributed to low-density caldera fill. Gravity measurements by Unocal Geothermal have been integrated with U.S. Geological Survey data, vastly improving gravity station coverage throughout the caldera. A strong regional gravity trend is mainly attributed to isostasy. A “best fitting” (based on regional control of basement densities) Airy-Heiskanen isostatic model was used for the regional correction. A three-dimensional, multiple-unit gravity modeling program with iterative capabilities was developed to model the residual gravity. The density structure of Long Valley caldera and vicinity was modeled with 22 discrete density units, most of which were based on geologic units. Information from drill hole lithologies, surface geology, and structural geology interpretations constrain the model. Some important points revealed by the three-dimensional gravity modeling are that (1) the volume of ejected magma associated with the Bishop Tuff eruption is greater than previously thought, (2) the caldera structure is strongly influenced by precaldera topography and the extensions of major, active faults, (3) the main west ring fracture is coincident with the Inyo Domes-Mono Craters fracture system, (4) a relatively low-density region probably underlies the caldera, and (5) a silicic magma chamber may underlie Devils Postpile.

Journal of Geophysical Research Solid Earth↗
USGS2012

Principal component analysis of geodetically measured deformation in Long Valley caldera, eastern California, 1983-1987

Typical geodetic measurements of deformation consist of repeated surveys of a particular geodetic network. Such deformation data can be interpreted as a consequence of one or more self-coherent sources by means of principal component analysis. A self-coherent source is defined as any source that produces deformation that is time and space separable. Principal component analysis then gives the time and space factors that characterize the deformation attributed to each self-coherent source. Geodetic measurements of deformation at Long Valley caldera provide two examples of the application of principal component analysis. A 40-line trilateration network surrounding the caldera was surveyed in midsummer 1983, 1984, 1985, 1986, and 1987. Principal component analysis indicates that the observed deformation can be represented by a single coherent source. The time dependence for that source displays a rapid rate of deformation in 1983–1984 followed by less rapid but uniform rate in the 1984–1987 interval. The spatial factor seems consistent with expansion of a magma chamber beneath the caldera plus some shallow right-lateral slip on a vertical fault in the south moat of the caldera. An independent principal component analysis of the 1982, 1983, 1984, 1985, 1986, and 1987 leveling across the caldera requires two self-coherent sources to explain the deformation. The deformation pattern produced by the larger of these two sources appears to be roughly consistent with that found from the trilateration data. The deformation due to the second source is a nearly uniform tilt in the uplift profile. Presumably, that tilt is simply an artifact of systematic error in the leveling.

Journal of Geophysical Research Solid Earth↗
USGS2012

The growth of geological structures by repeated earthquakes: 2, Field examples of continental dip-slip faults

A strong test of our understanding of the earthquake cycle is the ability to reproduce extant fault-bounded geological structures, such as basins and ranges, which are built by repeated cycles of deformation. Along strike-slip faults, the coseismic and interseismic deformation can be nearly equal in magnitude and opposite in sign, resulting in little permanent deformation except for the fault offset. For dip-slip faults, portions of the crust are lifted and dropped, and so buoyancy forces are exerted. The seismic and interseismic deformations do not balance, and structures grow and become subject to erosion and deposition. We consider three examples for which the structure and fault geometry are well known: the White Wolf reverse fault in California, site of the 1952 Kern County M=7.3 earthquake, the Lost River normal fault in Idaho, site of the 1983 Borah Peak M=7.0 earthquake, and the Cricket Mountain normal fault in Utah, site of Quaternary slip events. Basin stratigraphy and seismic reflection records are used to profile the structure, and coseismic deformation measured by leveling surveys is used to estimate the fault geometry. To reproduce these structures, we add the deformation associated with the earthquake cycle (the coseismic slip and postseismic relaxation) to the flexure caused by the observed sediment load, treating the crust as a thin elastic plate overlying a fluid substrate. The cumulative deformation is principally dependent on the elastic plate thickness, modestly sensitive to the sediment-substrate density difference, and insensitive to the fluid viscosity for the 4- to 8-Ma structures. We deduce a longterm flexural rigidity of 2–15 × 10 19 Nm; this is equivalent to an elastic plate thickness of 2–4 km for a Young's modulus of 2.5 × 10 10 Nm −2 . This value is found where independent estimates of the elastic thickness from the coherence between surface topography and gravity yield values of about 4 km, but where coseismic fault slip extends to a depth of 10–15 km. Thus much of the seismogenic crust must weaken substantially during the life of active faults, causing the fault-bounded basins to narrow over time.

Journal of Geophysical Research Solid Earth↗
USGS2012

On the state of lithospheric stress in the absence of applied tectonic forces

Numerous published analyses of the nontectonic state of stress are based on Hooke's law and the boundary condition of zero horizontal deformation. This approach has been used to determine the gravitational stress state as well as the effects of processes such as erosion and temperature changes on the state of lithospheric stress. The major disadvantage of these analyses involves the assumption of lateral constraint which seems unrealistic in view of the observational fact that the crust can deform horizontally in response to applied loads. If the same problems are addressed by assuming that the remote stress state is constant, instead of the condition of zero horizontal deformation, then the resulting stress states are entirely different and in good accord with observations. In the absence of applied tectonic forces the only likely gravitational stress states are those for which all three principal stresses are nearly equal. To the contrary, the gravitational stress states developed on the basis of the lateral constraint assumption can be ruled out. The processes of erosion and sedimentation have slight tendencies to increase and decrease, respectively, the state of deviatoric stress. In particular, for initial stress states in the range of slightly extensional to compressional, erosion has the effect of enhancing the ratio of average horizontal to vertical stress, which may explain, at least in part, the common observation of high near-surface horizontal stresses. Temperature changes have only minor effects on the stress state, as averaged over the thickness of the lithosphere.

Journal of Geophysical Research Solid Earth↗
USGS2012

Vapor-dominated zones within hydrothermal systems: Evolution and natural state

Three conceptual models illustrate the range of hydrothermal systems in which vapor-dominated conditions are found. The first model (model I) represents a system with an extensive near-vaporstatic vapor-dominated zone and limited liquid throughflow and is analogous to systems such as The Geysers, California. Such systems can evolve within low-permeability barriers without changes in boundary conditions or rock properties, given an adequate supply of heat. Their scarcity in nature may be due to the need for a long-lived, potent heat source and for a low-permeability aureole that remains intact for significant lengths of time. Models II and III represent systems with significant liquid throughflow and include steam-heated discharge features at higher elevations and high-chloride springs at lower elevations, connected to and fed by a single circulation system at depth. In model II, as in model I, the vapor-dominated zone has a near-vaporstatic vertical pressure gradient and is generally underpressured with respect to local hydrostatic pressure. The vapor-dominated zone in model III is quite different, in that phase separation takes place at pressures close to local hydrostatic and the overall pressure gradient is near hydrostatic. A relatively large number of high-temperature systems in regions of moderate to great topographic relief are similar to either model II or model III; however, in most cases there are insufficient data to establish a single preferred model.

Journal of Geophysical Research Solid Earth↗
USGS2012

Tectonic history of the Syria Planum province of Mars

We attribute most of the development of extensive fractures in the Tharsis region to discrete tectonic provinces within the region, rather than to Tharsis as a single entity. One of these provinces is in Syria Planum. Faults and collapse structures in the Syria Planum tectonic province on Mars are grouped into 13 sets based on relative age, areal distribution, and morphology. According to superposition and fault crosscutting relations and crater counts we designate six distinct episodes of tectonic activity in the following chronologic order: stage I is an early to late Noachian deformation forming mostly east-west structures (fault set IA); some large volcanoes also formed. Faults were produced possibly by flexural uplift. Also, arcuate, north trending grabens (set IB) indicate that faulting is transitional to the next stage. Stage II is late Noachian to early Hesperian radial faulting centered in Syria Planum, possibly due to isostatic uplift in late Noachian (set IIA) to early Hesperian (set IIB) time. Stage III is early to late Hesperian faulting tangential to Syria Planum that was related to local centers of uplift (sets III1-III3) on the periphery of Syria Planum. Stage IV is a late Hesperian graben formation that was circumferential to Syria Planum (set IVA), caused either by collapse associated with eruption of magma or by flexure of the lithosphere due to volcanic loading. In association with volcanism, minor faulting occurred, producing an oval pattern of faults in southwest Syria Planum (set IVBl) and a radial pattern south of the planum (set IVB2) that apparently rejuvenated buried stage II faults. Stage V is a late Hesperian to early Amazonian development of grabens and troughs of Noctis Labyrinthus and western Valles Marineris (set V) that was probably instigated by local uplift; exposure of groundwater or ground ice zones may have produced further collapse and trough enlargement. Stage VI is early Amazonian northwest trending faulting in Noctis Fossae (set VIl), perhaps due to Tharsis Monies-centered tectonism, and north northwest normal faulting along the eastern side of the Claritas rise (set VI2) that was due to tectonic subsidence. The duration of tectonic activity in the Syria Planum province was perhaps 2–3 b.y. Photoclinometric topographic profiles across 132 grabens and fault scarps show that Syria Planum grabens have widths (average of 2.5 km, and most range from 1 to 6 km) similar to lunar grabens, but the Martian grabens have slightly higher side walls (average about 132 m) and gentler wall slopes (average of 9° and range of 2°–25°) than lunar grabens (93 m high and 18° slopes). Scarp degradation on Mars has progressed through quakes, impact shaking, and dry slope processes; the lower slopes may be due to Mars’ higher gravity. Estimates of the amount of extension for individual grabens range from 20 to 350 m; most estimates of the thickness of the faulted layer range from 0.5 to 4.5 km (average is 1.5 km). This thickness range corresponds closely to the 0.8-to 3.6-km range in depth for pits, troughs, and canyons in Noctis Labyrinthus and along the walls of Valles Marineris. We propose that the predominant 1-to 1.5-km values obtained for both the thickness of the faulted layer and the depths of the pits, troughs, and theater heads of the canyons reflect the initial depth to the water table in this region, as governed by the depth to the base of ground ice. Maximum depths for these features may indicate lowered groundwater table depths and the base of ejecta material.

Journal of Geophysical Research Solid Earth↗
USGS2012

Geochemistry of some gases in hydrothermal fluids from the southern Juan de Fuca Ridge

Five samples of hydrothermal fluids from two vent areas on the southern Juan de Fuca Ridge were analyzed for dissolved gases. Concentrations in the end-member hydrothermal fluid of H 2 (270–527 μmol/kg), CH 4 (82–118 μmol/kg), and CO 2 (3920–4460 μmol/kg) are well above values in ambient seawater and are similar to concentrations reported for other ridge crest hydrothermal systems. The carbon isotopic ratios of the CH 4 (δ 13 C = −17.8 to −20.8) and CO 2 (δ 13 C = −3.6 to −4.7) suggest that at least some of the CH 4 and CO 2 in the fluids is basalt-derived. The range of δ 13 C values for the basalt-derived CO 2 is −6.8 to −9.7, calculated by assuming conservation of recharge ΣCO 2 during hydrothermal circulation. Apparent temperatures of equilibration between the CH 4 and the basalt-derived CO 2 range from 640°C to 750°C. Small amounts of ethane (C 2 H 6 /CH 4 ≅ 0.9 × 10 −3 −2.2 × 10 −3 ), propane, and butane detected in the samples may also have formed in the basalt. One sample of almost pure (95.5%) hydrothermal fluid contained a significant fraction, up to 63% and 74%, respectively, of the recharge Ar and N 2 . This suggests that the fluid has not undergone extensive vapor-liquid phase separation.

Journal of Geophysical Research Solid Earth↗
USGS2012

Effect of far-field slope on morphologic dating of scarplike landforms

The principal finding of this paper is that the far-field slope has a first-order effect on model age determinations of scarplike landforms in weakly consolidated terrains. Observationally, this can be demonstrated in two ways using the Lake Bonneville and Lahontan shoreline scarps as separate and combined data sets. Use of the reduced scarp slope, tan θ s - b (where θ s is the maximum scarp angle and b is the far-field or fan slope), instead of tan θ s alone as the measure of scarp slope measurably reduces separation between the two data sets induced by different average fan slopes for the two data sets and significantly reduces scatter in the slope-offset plot for both the separate and combined data sets. Theoretically, the argument can be put even more strongly, at least within the range of linear and nonlinear diffusion models that we consider here together with a mathematical transformation of the empirical approach of R. C. Bucknam and R. E. Anderson: When one correctly takes into account the far-field slope, one will basically get the same age determination no matter which of these models one uses; conversely, without accounting properly for the effect of far-field slope, one is virtually guaranteed to get an erroneous age determination, no matter which model is used.

Journal of Geophysical Research Solid Earth↗
USGS2012

Heat flow and thermotectonic problems of the central Ventura Basin, southern California

The Ventura Basin, southern California, is located near the Big Bend area of the San Andreas fault system, within the Transverse Ranges physiographic province. Continuous equilibrium temperature logs were measured in 12 idle oil wells located within the onshore Ventura Avenue, San Miguelito, Filmore, Oxnard, and West Montalvo fields to an average depth of about 3100 m (10,200 feet). Thermal conductivities were measured on all available samples. Heat flows were calculated with the aid of a thermostratigraphic scheme based on correlative gradient intervals and average thermal conductivity for the appropriate units. Negative curvature of the Ventura Avenue temperature profiles may be explained by an increase in thermal conductivity associated with tectonic compaction of the underlying Pliocene clastic sequence. Temperature profiles at Fillmore are enigmatic but suggest highly unusual geotectonic conditions. Basinwide, heat flow averages about 48 mW/m 2 , a value which is low relative to most of southern California. As heat flow does not vary systematically to the maximum measured depth of about 4 km, this anomaly is not easily explained in terms of hydrologic effects or recent uplift and erosion. However, a diminution of heat flow is an expectable consequence of the accumulation of cold sediments (up to 12 km) since Eocene time. If 70 mW/m 2 is accepted as the background heat flow, then the sedimentation effect is probably sufficient to explain the anomaly.

Journal of Geophysical Research Solid Earth↗
USGS2012

The timing of uplift, volcanism, and rifting peripheral to the Red Sea: A case for passive rifting?

Prior to the formation of the Red Sea the northeastern Afro/Arabian continent had low relief and was largely below sea level from the Late Cretaceous to the early Oligocene. The events leading to the formation of the Red Sea followed the sequence (1) alkaline volcanism and rifting beginning about 30–32 Ma affecting a narrow linear zone in the continent, (2) rotational block faulting and detachment faulting, well underway by 25 Ma, (3) gabbro and diorite magmatism, andesite to rhyolite volcanism, and fine-grained nonmarine sedimentation in the rift between 20 and 25 Ma, (4) fine-grained marine sedimentation in the rift as the early shelves started to subside in the middle Miocene, and (5) uplift of the adjacent continents (about 3 km) and subsidence of the shelves (about 4 km) between 13.8 and 5 Ma. The youth of the uplift is suggested by 44 fission track dates on apatites from rocks of the Proterozoic Arabian Shield that range in age from 13.8 to 568 Ma. The youngest of these ages, coupled with the present high relief along the Arabian escarpment and published heat flow measurements, indicate that 2.5–4 km uplift has occurred in the last 13.8 m.y. The sequence volcanism/rifting followed by uplift leads to our adoption of a passive mantle model for rift origin. Models that require uplift to create the rift are rejected, because of the late uplift. We advocate a model of lithospheric extension caused by two-dimensional plate stress over those requiring tractional drag at the base of the lithosphere caused by vigorous flow in the asthenosphere. It is acknowledged that traction models could explain the observed data, but they imply a rigid, static lithosphere and seem to require a link between the direction of flow in the asthenosphere and plate motions. Neither requirement is necessary in the extension model. The rift starts with mechanical extension in a narrow zone of lithosphere between 25–32 Ma in our model. The thinned lithosphere is replaced by upwelling asthenosphere and by rocks from the adjacent deep continental lithosphere which flow into the rift. Ductile flow of the deep continental lithosphere is accelerated by partial melting as rocks flow upward toward the rift axis. Once partially melted, rocks formerly part of the continental lithosphere join the upwelling asthenosphere, resulting in a rapid erosion of the lithospheric mantle beneath the continent near the rift edge. The resulting density decrease explains the uplift. We think that the Red Sea began as a consequence of changing plate geometries resulting from the collision of India and Eurasia. After the collision, the segment of the Owens fracture zone north of the Carlsberg Ridge became locked, forcing the northeast corner of Afro/Arabia to rotate with the Indian plate away from the rest of Africa.

Journal of Geophysical Research Solid Earth↗
USGS2012

Paleomagnetism of the Oligocene Kalamazoo Tuff: implications for middle Tertiary extension in east central Nevada

The Oligocene Kalamazoo Tuff (≃35 Ma) was sampled for paleomagnetic analysis across a 100-km-wide zone of highly extended crust in east central Nevada to estimate between-site vertical axis rotations and thus the relative importance of strike-slip faulting to the mechanism of extension. Subordinate sampling was also undertaken in a younger hornblende dacite intrusion (≃35 Ma) and the overlying tuff of North Creek (≃35 Ma). In general, ash flow tuffs cool and are magnetized within a short period of time relative to geomagnetic secular variation and their eutaxitic structures typically indicate the paleohorizontal. These attributes of ash flow tuffs, in addition to their generally widespread occurrence, make them excellent candidates for combined paleomagnetic and structural studies of regional deformation. However, in this study a number of problems associated with the paleomagnetism of the Kalamazoo Tuff are documented; they include topography-related variations of the eutaxitic structures, secular variation during slow cooling, postblocking rheomorphic flow, and local remagnetization of the tuff by younger igneous activity. The tilt-corrected data, with these sources of error reduced or eliminated, exhibit a 28° ± 12° clockwise rotation of the Schell Creek Range relative to the Kern Mountains region. This rotation implies differential extension accommodated by strike-slip faulting or N-S shortening. The paleomagnetic results also suggest that large changes in strike of layered units near faults with presumed strike-slip movement need not be the result of oroclinal bending, but could result from superimposed sets of orthogonal normal faults.

Journal of Geophysical Research Solid Earth↗
USGS2012

Tectonic history of the north portion of the San Andreas fault system, California, inferred from gravity and magnetic anomalies

Geologic and geophysical data for the San Andreas fault system north of San Francisco suggest that the eastern boundary of the Pacific plate migrated eastward from its presumed original position at the base of the continental slope to its present position along the San Andreas transform fault by means of a series of eastward jumps of the Mendocino triple junction. These eastward jumps total a distance of about 150 km since 29 Ma. Correlation of right-laterally displaced gravity and magnetic anomalies that now have components at San Francisco and on the shelf north of Point Arena indicates that the presently active strand of the San Andreas fault north of the San Francisco peninsula formed recently at about 5 Ma when the triple junction jumped eastward a minimum of 100 km to its present location at the north end of the San Andreas fault. Prior to 5 Ma the triple junction was located at the north end of a proposed northwesterly extension of the Pilarcitos fault. This jump took place at what is now about latitude 38°20′N on the North American plate and may have occurred during part of the time when the relative motion between the North American and Pacific plates rotated 20° clockwise, resulting in transpression along the earlier transcurrent San Andreas fault system. The proposed 150-km eastward movement of the triple junction explains the submarine topography near Cape Mendocino where the continental shelf south of the Mendocino fault extends about 130 km farther west than does the shelf directly north of the fault.

Journal of Geophysical Research Solid Earth↗