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At least 793 records · Page 44Linked to original sources
USGS2012

Evidence for a shallow pluton beneath the Goat Rocks Wilderness, Washington, from gravity and magnetic data

A gravity and aeromagnetic study was conducted in and adjacent to the Goat Rocks Wilderness area, Washington. This work was done in conjunction with geologic and geochemical studies which were critically important to our interpretation. The Bouguer gravity anomaly map displays a large central positive gravity anomaly and three superimposed smaller positive anomalies. The main anomaly, about 26 by 11 km with an amplitude of about 16 mGal seems to delineate a dense large pluton or sheeted dike complex intruding less dense, older volcanic and sedimentary rocks lying directly below the volcano. The pluton or sheeted dike complex may have fed the Goat Rocks volcano throughout its 1–2 m.y. history. Three small feeders, mapped on the surface as andesite intrusions, may be the cause of three smaller gravity anomalies superimposed on the main anomaly. The magnetic anomaly maps also exhibit a large central anomaly whose source is the Goat Rocks pluton. The magnetic data suggest that the three shallow intrusive cupolas have undergone a significant amount of hydrothermal alteration. In addition, to some extent the magnetic data reflect the terrain of the area. Topographic highs produce magnetic highs, and topographic lows produce magnetic lows. The deviations from this pattern delineate subsurface geology. Magnetic lows may be of particular interest in locating zones of mineralization where magnetic minerals have been altered and have become significantly less magnetic.

Washington↗
USGS2012

Fracture characterization by means of attenuation and generation of tube waves in fractured crystalline rock at Mirror Lake, New Hampshire

Results are presented from experiments carried out in conjunction with the U. S. Geological Survey at the Hubbard Brook Experimental Forest near Mirror Lake, New Hampshire. The study focuses on our ability to obtain orientation and transmissivity estimates of naturally occurring fractures. The collected data set includes a four-offset hydrophone vertical seismic profile, full waveform acoustic logs at 5, 15, and 34 kHz, borehole televiewer, temperature, resistivity, and self-potential logs, and borehole-to-borehole pump test data. Borehole televiewer and other geophysical logs indicate that permeable fractures intersect the Mirror Lake boreholes at numerous depths, but less than half of these fractures appear to have significant permeability beyond the annulus of drilling disturbance on the basis of acoustic waveform log analysis. The vertical seismic profiling (VSP) data indicate a single major permeable fracture near a depth of 44 m, corresponding to one of the most permeable fractures identified in the acoustic waveform log analysis. VSP data also indicate a somewhat less permeable fracture at 220 m and possible fractures at depths of 103 and 135 m; all correspond to major permeable fractures in the acoustic waveform data set. Pump test data confirm the presence of a hydraulic connection between the Mirror Lake boreholes through a shallow dipping zone of permeability at 44 m in depth. Effective fracture apertures calculated from modeled transmissivities correspond to those estimated for the largest fractures indicated on acoustic waveform logs but are over an order of magnitude larger than effective apertures calculated from tube waves in the VSP data set. This discrepancy is attributed to the effect of fracture stiffness. A new model is presented to account for the mechanical strength of asperities in resisting fracture closure during the passage of seismic waves during the generation of VSPs.

New Hampshire↗
USGS2012

Geology of the Mount St. Helens area: Record of discontinuous volcanic and plutonic activity in the Cascade Arc of southern Washington

The Quaternary edifice of Mount St. Helens volcano was built upon a deeply eroded terrane of gently folded and altered volcanic and plutonic rocks that represent the core of the Tertiary Cascade magmatic arc. These rocks constitute an east dipping homoclinal sequence, several kilometers thick, of subaerially erupted mafic to silicic flows and volcaniclastic strata; K‐Ar ages from this section range from about 28 to 23 Ma (late Oligocene and earliest Miocene), which corresponds to an apparent lull in Cascade volcanism to the north of Mount Rainier. Volcanism was essentially continuous during this period of time, and neither a well‐defined base nor top of the section is exposed within the mapped area. Basalt and basaltic andesite dominate the lower part of the mid‐Tertiary section, whereas andesitic and dacitic rocks comprise most of the upper part. This section was intruded by numerous mafic to silicic dikes, sills, and irregular plutonic bodies, most no more than a few million years younger than their host rocks, and subjected to pervasive burial metamorphism and widespread hydrothermal alteration. Large areas of hornfelsed rock surrounding even relatively small intrusions indicate that the proportion of plutonic rock becomes significantly greater at shallow depth beneath the existing erosion surface. A large granitic pluton intruded the mid‐Tertiary section north of Spirit Lake at about 21 Ma. The Earl porphyry copper deposit occurs within the pluton but appears too young (17 Ma) to be genetically related to it. In contrast to the rather continuous and voluminous Oligocene to early Miocene activity, volcanism since then in the Mount St. Helens area has been localized and Volumetrically minor. Products of three younger eruptive periods have been recognized: a sequence of 15 m.y. old pyroxene andesite flows resting unconformably on mid‐Tertiary strata south of Mount St. Helens, widespread shallow dikes and sills of pyroxene andesite between 10 and 8 m.y. old, and compositionally diverse rocks erupted during the past 3 m.y. The Quaternary lavas are more potassic than the Tertiary lavas and typically contain phenocrysts of hornblende and biotite, which are absent from the older rocks. A number of Tertiary structures define a broad NNE trending zone that may reflect a deep‐seated lithospheric flaw that has controlled the locus of Cascade magmatism in southern Washington for the past 25 m.y. Mount St. Helens lies within this zone at the intersection of the NNW striking St. Helens seismic zone (SHZ) and an ENE trending alignment of Pleistocene silicic plug‐domes. No surface breakage has been detected along the SHZ, which is apparently very young. The linear zone of silicic vents is probably controlled by a fault that has been interpreted from seismic records to occur directly beneath the volcano. This zone parallels the directions of regional maximum horizontal compressive stress and North America/Juan de Fuca plate convergence. Mount St. Helens is an example of a low‐volume tectonically controlled magmatic system in an early stage of development.

Washington↗
USGS2012

Tectonics and conductivity structures in the Southern Washington Cascades

The tectonic setting of the southern Washington Cascades has been studied with the aid of magnetotelluric (MT) and other geophysical data. The main feature of interest in the geophysical data is a broad high-conductivity anomaly mapped with MT and geomagnetic variation (GMV) data. This anomaly is located roughly within the triangle formed by the volcanoes Mount Rainier, Mount St. Helens, and Mount Adams but exceeds beyond Mount Rainier to the northwest. We interpret the cause of the anomaly to be conductive rocks with resistivities of 1–4 ohm m and thicknesses possibly greater than 15 km. These conductive rocks are found 2–8 km beneath the overlying less conductive volcanic and sedimentary rocks at the surface. Two aeromagnetic lows follow the trend of the conductivity anomaly, and linear belts of strike-slip seismicity are coincident with both these magnetic lows. One of the aeromagnetic lows is coincident with the western margin of the conductivity anomaly. The geophysical data appear to outline a suture zone of probable Eocene age caused by accretion of a large seamount complex (Siletzia) and that may contain large thicknesses of compressed forearc basin and accretionary prism sedimentary rocks of Cretaceous to Eocene age. Part of the shallower conductive rocks may be associated with carbonaceous continental and transitional marine sedimentary rocks of the Puget Group. The contact between the hypothesized compressed basin and the accreted terranes to the west may localize the release of shear stresses in this region of oblique subduction. Several possible explanations for the conductivity anomaly are considered in addition to the compressed basin hypothesis.

Washington↗
USGS2012

Evidence from gravity data for an intrusive complex beneath Mount St. Helens

On the basis of gravity data, aided by aeromagnetic, magnetotelluric, side-looking radar, and geologic information, we tentatively identify a large, shallow intrusive complex beneath Mount St. Helens. The complex is roughly 5–6 km thick and has apparently intruded a buried and compressed Mesozoic forearc sedimentary sequence. The lateral extent of the intrusive complex is uncertain, and we give three alternative models varying from about 18 by 10 km to as much as 18 by 22 km. The western boundary of the inferred Mount St. Helens intrusion abuts several of a number of Tertiary and younger plutons that crop out in the area. The Mount St. Helens intrusion apparently is adjacent to sedimentary rock or shallow volcanic rock along most of the remainder of its boundary. A ringlike drainage pattern around Mount St. Helens suggests subsidence caused by removing magma from deep beneath Mount St. Helens and adding this mass to the Mount St. Helens edifice and its underlying intrusion. Our analysis indicates that the average bulk density of the volcanic rock comprising Mount St. Helens is about 2.15 g/cm 3 .

Washington↗
USGS2012

Morphometric variability within the axial zone of the southern Juan de Fuca Ridge: Interpretation from Sea MARC II, Sea MARC I, and deep-sea photography

The morphometric characteristics of the axial regions of oceanic spreading centers are determined by (1) the type of volcanic flows, (2) the relation between primary volcanic relief (on a scale of a few meters to tens of meters) and degree of sediment cover, and (3) the extent of surficial expression and timing of tectonic disruption of the young oceanic crust. Even within a single, continuous, linear spreading-ridge segment with relatively uniform axial valley dimensions over a distance of 50 or more kilometers, such as along the southern Juan de Fuca Ridge, the changes in morphometric characteristics along axis within the youngest crust indicate distinct variation in tectonic and volcanic activity over short distances within short time periods. An integrated analysis of Sea MARC I, Sea MARC II, and photographic data for the southernmost continuous segment of the Juan de Fuca Ridge shows that generalizations about tectonic and volcanic processes at spreading ridges must consider both the temporal scale of processes as well as the physical scales of observations if predictive models are to be successful. Comparison of the morphometric expression within the major hydrothermal vent area and the rest of the southernmost ridge segment suggests that the mapped distribution of hydrothermal vents may reflect the extent of survey effort rather than uniqueness of geologic setting.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Structure of the southern Juan de Fuca Ridge from seismic reflection records

Twenty-four-channel seismic reflection records were obtained from the axial region of the southern Juan de Fuca Ridge. Two profiles are normal to the strike of the spreading center and intersect the ridge at latitude 44°40′N and 45°05′N; a third profile extends south along the ridge axis from latitude 45°20′N and crosses the Blanco Fracture Zone. Processing of the axial portions of the cross-strike lines resolved a weak reflection centered beneath the axis. The reflector is at a depth similar to seismically detected magma chambers on the East Pacific Rise and a Lau Basin spreading center; we suggest that the reflector represents the top of an axial magma chamber. In the migrated sections the top of the probable magma chamber is relatively flat and 1–2 km wide, and the subbottom depth of the chamber is greater where the depth to the ridge axis is greater.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Chemistry of hydrothermal solutions from the southern Juan de Fuca Ridge

Fluids from three vent fields on the southern Juan de Fuca Ridge were sampled in September 1984 using the DSRV Alvin. The fluids are uniquely high in both chloride, which ranges up to twice the seawater value, and in metal content. Simple vapor-liquid phase separation could not have produced both the high chlorinity and gas concentrations observed in these fluids. The cause of the elevated chlorinity can not be uniquely identified but may be the result of either or a combination of two processes: (1) subsurface formation of a degassed brine and subsequent mixing of a small amount of this brine with a hydrothermal seawater which has not undergone a phase separation and (2) dissolution of a chloride-rich phase combined with a possible small loss of gas during sampling procedures. Although measured temperatures were all less than 300°C, quartz geothermometry suggests that the fluids have equilibrated at greater than 340°C. Quartz geobarometry is also in agreement with geophysical estimates of depth to the local magma chamber.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Friction of ice

The frictional strength of ice seems to be well below that for all other rocks. Triaxial testing of cylinders of pure water ice containing a 45° inclined sawcut, at temperatures of 77 ≤ T ≤ 115 K and confining pressures 0.1 ≤ P ≤ 250 MPa, reveals the frictional laws τ = 0.20 σ n + 8.3 MPa for P ≥ 10 MPa and τ = 0.55 σ n + 1.0 MPa for P ≤ 5 MPa, where τ and σ n are the shear and normal stresses on the sawcut surface. Friction is independent of T and, over the one order of magnitude tested, of average sliding velocity. The sliding behavior is invariably stick slip. Examination of sliding surfaces shows only minor gouge development. A curious arrangement of densely packed short vertical fractures occurs primarily in samples with anomalously low strength. These results apply to models of near‐surface tectonic activity on Jupiter's three large icy moons, Europa, Callisto, and Ganymede. They indicate, for example, that a global expansion on Ganymede of 3 linear percent will cause extensional movement on preexisting faults at depths to 7±3 km.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Structure of the lower crust beneath the Carolina Trough, U.S. Atlantic continental margin

Data from three large-offset seismic profiles provide information on the crustal structure beneath the Carolina trough. The profiles, obtained by the U.S. Geological Survey, the Naval Oceanographic Research Development Agency, and the Scripps Institution of Oceanography in 1985, were oriented parallel to the trough and were located (1) seaward of the East Coast Magnetic Anomaly (ECMA), which is generally thought to represent the boundary between oceanic and continental crust; (2) along the axis of the trough between the ECMA and the hinge zone, which is thought to reflect the landward limit of highly stretched and altered transitional crust; and (3) along the Carolina platform landward of the basement hinge zone on crust thought to have been thinned only slightly during rifting. These data constrain the velocity structure of the lower crust and provide evidence for a thick lens of high-velocity (>7.1 km/s) lower crustal material that extends beneath the Carolina trough and the adjacent ocean basin. This lens reaches a maximum thickness of about 13 km beneath the deepest part of the trough, thins to about 5 km seaward of the ECMA, and is either very thin or absent landward of the hinge zone. It is interpreted to represent material that was underplated beneath and/or intruded into the crust during the late stage of continental rifting and that led to an anomalously thick plutonic layer during the early seafloor spreading phase. These data thus support the recent conclusions of White et al. (1987b) and Mutter et al. (1988) that the initiation of seafloor spreading is attended in many, if not most, cases by the generation of an anomalously large volume of melt.

Carolina Trough↗
USGS2012

The competition between thermal contraction and differentiation in the stress history of the Moon

The scarcity of both extension and compression features on the Moon strongly constrains the history of the lunar radius—to variations of less than ±1 km over the past 3.8 Gyr. This limit has traditionally been interpreted as requiring a delicate balance between thermal contraction of the near‐surface and expansion of a substantial cold interior region. Recent theories of lunar origin (e.g., giant impact), in contrast, favor a “hot” initial state. We propose that a reconciliation may be possible by taking account of the volume change Δ V / V | d due to differentiation. We calculate STP densities based on simplified normative mineralogies for a suite of estimates of the bulk lunar composition, of primary lunar basalt, and of the residuum left when the maximum amount of the latter is extracted from the former. Typically Δ V / V | d ≃ 2 to 5%—an expansion equivalent to heating by ∼10 3 K. Provided the timing of differentiation is correct, one might offset the cooling of a magma ocean as much as 630 km deep by differentiation of the remainder of the Moon (which need not start much below the solidus temperature). A large but not impossible amount of gabbroic melt production is implied: ∼100 times the volume of mare basalts known to have been extruded. We do not address the detailed genetic relationship of this melt to the basalts observed on the lunar surface but point out that it need not have reached the surface directly or even have entered the crust in order for the expansion to have occurred. To assess the timing of melt formation, we investigate a simple conductive lunar thermal model which takes account of both Δ V / V | d and thermal contraction. Our initial state is characterized by a central temperature T c and a depth Z 0 above which the material (derived from the magma ocean) is already at the solidus and is not suceptible to volume changes upon further differentiation. We find a range of models satisfying the limits on radius increase and decrease. The hottest has T c = 1210 K, Z 0 = 400 km; without Δ V / V | d , we would need a larger or colder (or both) core, e.g., T c ≲ 700 K for Z 0 = 200–400 km, in agreement with previous investigators. Our modeling thus lends credence to the idea that the Moon could have been initially ≳50% molten (with the remainder relatively close to the solidus) and yet experienced little volume change over the last 3.8 Gyr.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Crustal structure of the Chugach Mountains, southern Alaska: A study of peg‐leg multiples from a low‐velocity zone

A seismic refraction profile recorded along the geologic strike of the Chugach Mountains in southern Alaska shows three upper crustal high‐velocity layers (6.9, 7.2, and 7.6 km/s) and a unique pattern of strongly focussed echelon arrivals to a distance of 225 km. The group velocity of the ensemble of echelon arrivals is 6.4 km/s. Modeling of this profile with the reflectivity method reveals that the echelon pattern is due to peg‐leg multiples generated from within a low‐velocity zone between the second and third upper crustal high‐velocity layers. The third high‐velocity layer (7.6 km/s) is underlain at 18 km depth by a pronounced low‐velocity zone that produces a seismic shadow zone wherein peg‐leg multiples are seen as echelon arrivals. The interpretation of these echelon arrivals as multiples supersedes an earlier interpretation which attributed them to successive primary reflections arising from alternating high‐ and low‐velocity layers. Synthetic seismogram modeling indicates that a low‐velocity zone with transitional upper and lower boundaries generates peg‐leg multiples as effectively as one with sharp boundaries. No PmP or Pn arrivals from the subducting oceanic Moho at 30 km depth beneath the western part of the line are observed on the long‐offset (90–225 km) data. This may be due to a lower crustal waveguide whose top is the high‐velocity (7.6 km/s) layer and whose base is the Moho. A deep (∼54 km) reflector is not affected by the waveguide and has been identified in the data. Although peg‐leg multiples have been interpreted on some long‐range refraction profiles that sound to upper mantle depths, the Chugach Mountains profile is one of the few crustal refraction profiles where peg‐leg multiples are clearly observed. This study indicates that multiple and converted phases may be more important in seismic refraction/wide‐angle reflection profiles than previously recognized.

Alaska↗
USGS2012

The crustal structure of the Wrangellia Terrane along the East Glenn Highway, eastern‐southern Alaska

Recently acquired seismic refraction data from eastern‐southern Alaska provide new information on the structure and composition of the Wrangellia and adjacent terranes. The data comprise a 160‐km‐long refraction profile along the East Glenn (Tok‐Cutoff) Highway that was collected as part of the U.S. Geological Survey's multidisciplinary Trans‐Alaska Crustal Transect program. The upper 3 km of the Wrangellia terrane and associated rocks is characterized by low compressional wave velocities (V p = 1.9, 3.3, 4.6, 5.6 km s −1 ) and high‐velocity gradients common to most onshore seismic refraction profiles. There is also clear seismic expression of the West Fork fault system as a steep, down‐to‐the‐southwest fault that separates the Peninsular terrane to the southwest and the metamorphic complex of Gulkana River to the northeast. In contrast, no seismic expression occurs for the Paxson Lake fault, which separates the Wrangellia terrane from the metamorphic complex of Gulkana River. Adjacent to the Denali fault, within the Wrangellia terrane, two high‐velocity bodies (V p = 6.6 km s −1 ) occur in the upper crust. One of these extends to ∼10‐km depth and correlates with a late Paleozoic dioritic complex, suggesting that the Wrangellia terrane is at least 10 km thick in this part of Alaska. From 5 to 23 km depth, the crust appears seismically homogeneous, with velocity increasing from V p = 6.2 to V p = 6.6 km s −1 . Beneath this level, the crust is less well resolved, although evidence exists for a low‐velocity zone between 23 and 26 km and a possible southwest dipping interface at 35 km. No identifiable mantle refraction or reflection is observed, possibly indicating a crust as thick as 55 km. The relatively low seismic velocities in the upper 23 km of the crust compare favorably with laboratory‐measured velocities on pelitic schists and intermediate‐composition plutonic rocks (granites and granodiorites), both of which are recognized in Wrangellia. We interpret the seismic velocities to indicate that silicic‐to‐intermediate‐composition rocks are important constituents of the basement of this part of Wrangellia. Geologic evidence indicates that the Alaskan part of the Wrangellia terrane is a Paleozoic and Mesozoic island arc: our seismic evidence indicates it may have been built mostly on continental crust as opposed to the fragment of Wrangellia from Vancouver Island which was probably built on oceanic crust.

Alaska↗
USGS2012

Tertiary basin development and tectonic implications, Whipple Detachment System, Colorado River Extensional Corridor, California and Arizona

Colorado River extensional corridor. In the Mohave Mountains and Aubrey Hills of Arizona and the eastern Whipple Mountains of California near Parker Dam, these deposits comprise four unconformity-bounded sequences composed of locally derived epiclastic and volcanic rocks and the Peach Springs Tuff. The three older sequences represent syntectonic units that were deposited coeval with detachment faulting, and the fourth is interpreted to be postextensional. The sequences are correlated between four fault-bounded regions, which are the remnants of four different depositional basins. Similar sequences can be correlated over broad areas of the extensional corridor despite the general lack of widespread units. The basins developed in about the same positions, relative to each other and to volcanic sources, as they occupy at present. This is shown by gradational changes of pre-Tertiary rock types between regions, systematic variations in the abundance of magmatic units, and correlative volcanic units that occur in two adjacent regions. The basins formed in the early Miocene from segmentation of the upper crust into blocks bounded by high-angle faults that trended both parallel and perpendicular to the direction of extension and which were terminated at middle crustal depths by a low-angle detachment fault. Extreme rotation of one large crustal block, which constitutes the central Mohave Mountains, is recorded by a major unconformity in the lower Miocene section of one basin. Because coeval sections that formed in adjoining basins do not record this rotation, the underlying crustal blocks must have been separated by transfer faults that allowed them to rotate independently. These proposed transfer faults are represented at present by major faults with trends that parallel the direction of extension on the Whipple detachment system.

Arizona, California↗
USGS2012

Lithology and evolution of the crust-mantle boundary region in the southwestern Basin and Range Province

Mantle and crustal xenoliths from volcanic rocks in the southwestern Basin and Range province and Colorado Plateau Transition Zone reveal histories of episodic magmatism and deformation that have profoundly influenced the crustal structure of this region. Seismic transects in this area show a strongly reflective Moho of generally low relief, which, in the area of modern transects, consists of a thin zone (<2 km thick) of short reflectors. The upper mantle is transparent and has a P n of 7.8–8.0 km/s similar to much of the western United States. A lower crustal zone, 2–13 km thick, has variable internal reflectivity and a relatively low velocity of 6.6–6.8 km/s. Upper mantle peridotite xenoliths show both ductile and brittle deformational features and have structures and compositions affected by magmatic intrusion; intrusions form complex dike systems and extensive zones of grain boundary infiltration in peridotite xenoliths. Whereas melt infiltration preceded and followed ductile deformation, brittle deformation, represented by closely spaced joint systems and faults, followed ductile deformation and is related to the youngest magmatic episodes. These structural characteristics and high uppermost mantle temperature (∼1000°C) may combine to explain the relatively low P n . Alternating layers of ductily deformed and undeformed peridotites, with or without igneous intrusions, may contribute to the reflectivity of the Moho. Lower crustal xenoliths are dominantly igneous-textured pyroxenites and mafic to intermediate gabbros identical to the dikes in peridotite xenoliths. The crustal xenoliths also commonly are jointed, and in addition many show partial melting and have abundant cavities that probably were filled with CO 2 -rich fluids. These rocks are interpreted as products of underplated magmas that were fed through the mantle dike systems and may represent the lowest crustal unit identified in the seismic records. The mafic compositions and high densities of the crustal xenoliths indicate that the low velocity of the lower crust may be caused in part by fracture systems, partial melts, and high temperatures. Garnet granulite xenoliths from a locality with no mantle peridotite xenoliths probably represent crust of the region before late Miocene extension. Felsic granulite xenoliths from two localities have velocities like those of the two lower crustal units identified seismically and could be present in the modern crust as unequilibrated remnants of old crust. The preferred model for the evolution of the lower lithosphere is one in which extension affects the upper mantle as well as the crust and is overlapped in time by multiple magmatic episodes. The earliest magmatic events preceded extension, and later events accompanied and followed extension.

Arizona, California, Nevada↗
USGS2012

Keweenaw hot spot: Geophysical evidence for a 1.1 Ga mantle plume beneath the Midcontinent Rift System

The Proterozoic Midcontinent Rift System of North America is remarkably similar to Phanerozoic rifted continental margins and flood basalt provinces. Like the younger analogues, the volcanism within this older rift can be explained by decompression melting and rapid extrusion of igneous material during lithospheric extension above a broad, asthenospheric, thermal anomaly which we call the Keweenaw hot spot. Great Lakes International Multidisciplinary Program on Crustal Evolution seismic reflection profiles constrain end-member models of melt thickness and stretching factors, which yield an inferred mantle potential temperature of 1500°–1570°C during rifting. Combined gravity modeling and subsidence calculations are consistent with stretching factors that reached 3 or 4 before rifting ceased, and much of the lower crust beneath the rift consists of relatively high density intruded or underplated synrift igneous material. The isotopic signature of Keweenawan volcanic rocks, presented in a companion paper by Nicholson and Shirey (this issue), is consistent with our model of passive rifting above an asthenospheric mantle plume.

Midcontinent Rift System↗
USGS2012

Flank tectonics of Martian volcanoes

On the flanks of Olympus Mons is a series of terraces, concentrically distributed around the caldera. Their morphology and location suggest that they could be thrust faults caused by compressional failure of the cone. In an attempt to understand the mechanism of faulting and the possible influences of the interior structure of Olympus Mons, we have constructed a numerical model for elastic stresses within a Martian volcano. In the absence of internal pressurization, the middle slopes of the cone are subjected to compressional stress, appropriate to the formation of thrust faults. These stresses for Olympus Mons are ∼250 MPa. If a vacant magma chamber is contained within the cone, the region of maximum compressional stress is extended toward the base of the cone. If the magma chamber is pressurized, extensional stresses occur at the summit and on the upper slopes of the cone. For a filled but unpressurized magma chamber, the observed positions of the faults agree well with the calculated region of high compressional stress. Three other volcanoes on Mars, Ascraeus Mons, Arsia Mons, and Pavonis Mons, possess similar terraces. Extending our analysis to other Martian volcanoes, we find that only these three and Olympus Mons have flank stresses that exceed the compressional failure strength of basalt, lending support to the view that the terraces on all four are thrust faults.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Dark materials in Valles Marineris: Indications of the style of volcanism and magmatism on Mars

Rifting on the equatorial canyon system of Valles Marineris provides a unique view of the interior of the Martian crust to depths reaching 7 km, exposing several in situ bedrock units which testify to past volcanic and magmatic processes on Mars. A thick, regionally extensive deposit observed in Coprates and Juventae chasmata is interpreted on the basis of spectral reflectance, erosional morphology, and tendency for eolian mobilization to be composed of mafic glass, possibly an ancient Martian analogue of the lunar terra mantling deposits. Spectral mapping suggests that the dark floor‐covering materials in the lower canyons are derived from this unit. A series of cliffs in the Ophir Chasma wall rock is interpreted to be exposures of resistant bedrock; the spectral signature of this massive and uniform unit most closely resembles that of terrestrial mafic rocks altered to or coated by crystalline hematite. Application of computer mapping techniques to probable young volcanic materials in the central troughs yields an inferred distribution of volcanic activity consistent with the interpretation of extrusion along faults near the margins of the canyon floors. This result supports the hypothesis that the valles originated through tectonic extension.

Journal of Geophysical Research B: Solid Earth↗