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At least 1,657 records · Page 92Linked to original sources
USGS2012

A tomographic glimpse of the upper mantle source of magmas of the Jemez lineament, New Mexico

The 800-km-long Jemez lineament is the most active volcanic feature in the southwestern United States. It is the southeastern tectonic boundary of the Colorado Plateau and crosses the Rio Grande rift at the Jemez Mountains. The primary volcanism of the lineament is basaltic and has occurred in the last 4.5 m.y. To infer spatial distributions of partial melt in the upper mantle source zones for the Rio Grande rift and the Jemez lineament, we investigated the lateral variations of P wave velocity in the upper mantle beneath these features. We used teleseismic P wave delays recorded at a 22-station network to perform a damped least squares, three-dimensional inversion for these lateral variations. Our technique employed velocity interpolation within a three-dimensional grid of points, rather than using blocks of constant P wave velocity. This method allows highly realistic computation of seismic ray paths as well as accurate computation of the matrix elements in our system of equations. Determinations of resolution of results were done in two independent ways, both of which gave consistent estimates of resolution. In our best resolved volume the inversion showed no significant concentration of relative low velocity for P waves beneath the Rio Grande rift. However, directly beneath the Jemez lineament there is a ∼100-km-wide, 1–2% low-velocity feature in the depth range of 50–160 km. Because of the association of the low P wave velocity with the Jemez volcanic lineament but not with the Rio Grande rift, because lowered P wave velocity can be associated with increased partial melt, and because the volume of recent volcanism at the lineament greatly exceeds that at the rift, we infer that a large magmatic source zone exists beneath the Jemez lineament but not beneath the Rio Grande rift. This implies that the volcanic potential of the Jemez lineaments continues to greatly exceed that of the Rio Grande rift. The mantle source zones for volcanics of the Jemez lineament are not overridden by, but rather track, the motion of the North American plate; this implies that these sources are within the lithospheric plate, as is clarified in the discussion. The magmatic source zones of the Jemez lineament are modeled as due to clockwise rotation of the Colorado Plateau about a pole in northeastern Colorado. This rotation caused extension of the lithosphere beneath the Jemez lineament, permitting concentration there of partially melted rock in the upper mantle.

Journal of Geophysical Research Solid Earth↗
USGS2012

Midcontinent rift volcanism in the Lake Superior region: Sr, Nd, and Pb isotopic evidence for a mantle plume origin

Between 1091 and 1098 Ma, most of a 15- to 20-km thickness of dominantly tholeiitic basalt erupted in the Midcontinent Rift System of the Lake Superior region, North America. The Portage Lake Volcanics in Michigan, which are the youngest MRS flood basalts, fall into distinctly high- and low-TiO 2 types having different liquid lines of descent. Incompatible trace elements in both types of tholeiites are enriched compared to depleted or primitive mantle (La/Yb = 4.3–5.3; Th/Ta = 2.12–2.16; Zr/Y = 4.3–4.4), and both basalt types are isotopically indistinguishable. Sr, Nd, and Pb isotopic compositions of the Portage Lake tholeiites have 87 Sr/ 86 Sr i ≈ 0.7038, ε Nd (1095 Ma) ≈ 0±2, and μ 1 ≈ 8.2. Model ages with respect to a depleted mantle source ( T DM ) average about 1950–2100 Ma. Portage Lake rhyolites fall into two groups. Type I rhyolites have Nd and Pb isotopic characteristics (ε Nd (1095 Ma) ≈ 0 to −4.7; μ 1 ≈ 8.2–7.8) consistent with contamination of tholeiitic rocks by 5–10% Archean crust. The one type II rhyolite analyzed has Nd and Pb isotopic compositions (ε Nd (1095 Ma) ≈ −13 to −16; μ 1 ≈ 7.6–7.7) which are consistent with partial melting of Archean crust. Early Proterozoic crust was not a major contaminant of MRS rocks in the Lake Superior region. Most reported Nd and Pb isotopic compositions of MRS tholeiites from the main stage of volcanism in the Lake Superior region and of the Duluth Complex are comparable to the Nd and Pb isotopic data for Portage Lake tholeiites. The isotopic enrichment of the MRS source compared to depleted mantle is striking and must have occurred at least 700 m.y. before 1100 Ma. There are two likely sources for such enriched MRS tholeiitic magmatism: subcontinental lithospheric mantle enriched during the early Proterozoic or enriched mantle derived from an upwelling plume. Subcontinental lithospheric mantle alone as a source cannot be ruled out strictly on isotopic grounds, but melting of enriched lithosphere potentially ranging in age from 3.6 to 1.8 Ga would have been less likely to generate either the homogeneity or the volume of MRS tholeiites. Decompression melting of an upwelling enriched mantle plume in a region of lithosphere thinned by extension could have successfully generated the enormous volume (850×10 3 km 3 ) of relatively homogeneous magma in a restricted time interval.

Journal of Geophysical Research Solid Earth↗
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

Gabbroic xenoliths from the northern Gorda Ridge: Implications for magma chamber processes under slow spreading centers

Abundant gabbroic xenoliths in porphyritic pillow basalt were dredged from the northern Gorda Ridge. The host lava is a moderately fractionated, normal mid-ocean ridge basalt with a heterogeneous glass rind (Mg numbers 56-60). Other lavas in the vicinity range from near primary (Mg number 69) to fractionated (Mg number 56). On the basis of textures and mineral compositions, the xenoliths are divided into five types. The xenoliths are not cognate to the host lava, but they are genetically related. Chemistry of mineral phases in conjunction with textural features suggests that the xenoliths formed in different parts of a convecting magma chamber that underwent a period of closed system fractionation. The chamber was filled with a large proportion of crystalline mush when new, more primitive, and less dense magma was injected and mixed incompletely with the contents in the chamber, forming the hybrid host lava. -from Authors

Journal of Geophysical Research Solid Earth↗
USGS2012

Source complexity of the 1987 Whittier Narrows, California, earthquake from the inversion of strong motion records

Strong motion records for the Whittier Narrows earthquake are inverted to obtain the history of slip. Both constant rupture velocity models and variable rupture velocity models are considered. The results show a complex rupture process within a relatively small source volume, with at least four separate concentrations of slip. Two sources are associated with the hypocenter, the larger having a slip of 55–90 cm, depending on the rupture model. These sources have a radius of approximately 2–3 km and are ringed by a region of reduced slip. The aftershocks fall within this low slip annulus. Other sources with slips from 40 to 70 cm each ring the central source region and the aftershock pattern. All the sources are predominantly thrust, although some minor right-lateral strike-slip motion is seen. The overall dimensions of the Whittier earthquake from the strong motion inversions is 10 km long (along the strike) and 6 km wide (down the dip). The preferred dip is 30° and the preferred average rupture velocity is 2.5 km/s. Moment estimates range from 7.4 to 10.0×10 24 dyn cm, depending on the rupture model.

Journal of Geophysical Research Solid Earth↗
USGS2012

High spectral resolution reflectance spectroscopy of minerals

The reflectance spectra of minerals are studied as a function of spectral resolution in the range from 0.2 to 3.0 μm. Selected absorption bands were studied at resolving powers (λ/Δλ) as high as 2240. At resolving powers of approximately 1000, many OH‐bearing minerals show diagnostic sharp absorptions at the resolution limit. At low resolution, some minerals may not be distinguishable, but as the resolution is increased, most can be easily identified. As the resolution is increased, many minerals show fine structure, particularly in the OH‐stretching overtone region near 1.4 μm. The fine structure can enhance the ability to discriminate between minerals, and in some cases the fine structure can be used to determine elemental composition. For example, in amphiboles and talcs, four absorption bands are observed in the samples analyzed in this study that are due to hydroxyl linked to Mg 3 , Mg 2 Fe, MgFe 2 , and Fe 3 sites. The band intensities have been shown by other investigators to give the Fe:Fe+Mg ratio from transmission spectra. This study shows that the same equations can be used to obtain the ratio from reflectance spectra of unprepared samples. High‐resolution reflectance Spectroscopy of minerals may prove to be a very important tool in the laboratory, in the field using field‐portable spectrometers, from aircraft, and from satellites looking at Earth or other planetary surfaces.

Journal of Geophysical Research Solid Earth↗
USGS2012

Discontinuities in the shallow Martian crust at Lunae, Syria, and Sinai Plana

Detailed photoclinometric profiles across 125 erosional features and 141 grabens in the western equatorial region of Mars indicate the presence of three discontinuities within the shallow crust. Pits, troughs, and wall valleys (tributary canyons) within Noctis Labyrinthus and Valles Marineris and escarpments within the fretted terrain of Sacra Fossae and Kasei Valles show distinct erosional base levels at depths of 0.3–0.6 km, 1 km, and 2–3 km. The shallowest discontinuity corresponds to thickness estimates for the ridged plains unit in this region, and thus the discontinuity probably is the contact between a sequence of layered rock making up this unit and the underlying megaregolith. The 1-km discontinuity is reflected in the base levels of erosion of all the features studied, and it may correspond to the base of the proposed layer of ground ice. Model calculations using photoclinometric profiles of simple grabens (corrected for the effects of mass wasting) and the 60° dip of bounding faults (measured from the exposed traces of faults on trough walls) show that graben-bounding faults consistently intersect at the mechanical discontinuity at about 1 km depth. This discontinuity may represent an interface between ice-laden and dry regolith, ice-laden and water-laden regolith, or pristine and cemented regolith. A correlation between wall valley head depth and local thickness of the faulted layer suggests that the 1-km discontinuity also controlled the depth of the heads of sapping canyons. There is no apparent relation between the pit and trough depths and the local thickness of the faulted layer, which can be explained if pits and troughs developed by subsidence into underlying tension cracks that disrupted the 1-km discontinuity. The data do suggest, however, that erosion of shallow pits and troughs was influenced by the 1-km discontinuity. The third discontinuity, at a depth of 2–3 km, corresponds to the proposed base of the Martian megaregolith and is probably the interface between overlying, ejected breccia and in situ, fractured basement rocks.

Journal of Geophysical Research Solid Earth↗
USGS2012

The relation of catastrophic flooding of Mangala Valles, Mars, to faulting of Memnonia Fossae and Tharsis volcanism

Detailed stratigraphic relations indicate two coeval periods of catastrophic flooding and Tharsis centered faulting (producing Memnonia Fossae) in the Mangala Valles region of Mars. Major sequences of lava flows of the Tharsis Montes Formation and local, lobate plains flows were erupted during and between these channeling and faulting episodes. First, Late Hesperian channel development overlapped in time the Tharsis-centered faulting that trends north 75° to 90°E. Next, Late Hesperian/Early Amazonian flooding was coeval with faulting that trends north 55° to 70°E. In some reaches, resistant lava flows filled the early channels, resulting in inverted channel topography after the later flooding swept through. Both floods likely originated from the same graben, which probably was activated during each episode of faulting. Faulting broke through groundwater barriers and tapped confined aquifers in higher regions west and east of the point of discharge. The minimum volume of water required to erode Mangala Valles (about 5×10 12 m 3 ) may have been released through two floods that drained a few percent pore volume from a relatively permeable aquifer. The peak discharges of the floods may have lasted from days to weeks. The perched water discharged from the aquifer may have been produced by hydrothermal groundwater circulation induced by Tharsis magmatism, tectonic uplift centered at Tharsis Montes, and compaction of saturated crater ejecta due to loading by lava flows.

Journal of Geophysical Research Solid Earth↗
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↗
USGS2012

Three-dimensional P and S velocity structure in the Coalinga Region, California

The Coalinga earthquake sequence of 1983 provided a unique opportunity to perform a three-dimensional velocity and hypocenter inversion in an area of complex three-dimensional structure dominated by folding and blind thrusts. Additionally, since other varied geological and geophysical studies have been completed in this area, the three-dimensional inversion solution could be compared to other interpretations. Inversion of 7696 P and 1511 S first arrivals from earthquakes and 696 P first arrivals from refraction shots produced a three-dimensional velocity model with grid spacing of 1–2 km in the hypocentral area. The overall shape and location of velocity features correspond well to the mapped surface geology. The three-dimensional inversion yields details of folds where the resolution is good and the general shape where resolution is lower. The amounts of structural relief inferred for the local folds are similar to values inferred from geologic data. The three-dimensional velocity solution has several distinctive features. There is a linear high-velocity body (6.1–6.5 km/s), about 25 km long, from 6 to 8 km depth, that may represent a fragment of Coast Range ophiolite. A shallow low-velocity zone (LVZ), which extends for 20 km along the fold axis at about 6 km depth and correlates with LVZs observed in both the refraction and the reflection data, may indicate high pore pressure caused by lateral compressive strain. Deeper LVZs occur within inferred Franciscan material and are characterized by horizontal or southwest dipping zones of varied thickness, 4–8 km wide and 5–10 km long. These LVZs may represent multiple thrust faults. Their locations and geometry are consistent with thrust faults inferred with seismic reflection data. The three-dimensional velocity solution compares well to prior two-dimensional seismic reflection and refraction models and observed gravity. The shape of the inferred sedimentary section agrees well with the reflections from the Cenozoic strata. Compared to the refraction model, the three-dimensional solution has similar velocities and similar locations of velocity features but is more detailed in the hypocentral zone where it uses more data. The gravity computed from the three-dimensional velocities is similar to the observed gravity in both shape and amplitude. Both a simple one-dimensional initial model and a complex initial model derived from the refraction interpretation were tried. A simple starting model gave the best results. The S velocity solution has different resolution than the P velocity solution because it uses a different set of stations, and it has lower resolution because it uses fewer arrival times. While the general patterns of velocity variation are similar for both V p and V s , the V s solution tends to have more smearing of velocity features and can have somewhat different locations of velocity features.

Journal of Geophysical Research Solid Earth↗
USGS2012

Some additional observations on inclusions in the granitic rocks of the Sierra Nevada

Microgranular quartz diorite and diorite inclusions are widespread in central Sierra Nevada granitoid rocks and are almost exclusively restricted to hornblende-bearing rocks, most commonly felsic tonalites and mafic granodiorites. Strontium 87/strontium 86 values of the inclusions and host granitoids from individual plutons generally plot on single isochrons that agree closely with previously determined ages. Neodymium 143/neodymium 144 values of five inclusions ranged from 0.51229 to 0.51248 and yield ε Nd values that correlate closely with values of their respective hosts. The Nd-Sm and Rb-Sr systematics indicate that most inclusions were in isotopic equilibrium with enclosing materials at the time of formation. Silica contents of inclusions and granitoids are contiguous, but inclusions generally contain less than, and granitoids more than, 60% SiO 2 . Linear continuity of the femic oxide and H 2 O + trends relative to SiO 2 suggests many inclusions formed as concentrations of hydrous mafic minerals. Variation of other major element oxides and trace elements support this inference. The aforementioned data are generally inconsistent with a direct genetic link between the mafic inclusions and the mantle-derived basalt. However, many initial Sierra Nevada magmas were produced by mixing of mafic, mantle-derived melts and felsic crustal materials, resulting in isotopically varied tonalitic and granodioritic magmas. Most inclusions thus represent fragmented crystal accumulations of early-formed, near-liquidus minerals generated from these previously mixed magmas.

Journal of Geophysical Research Solid Earth↗
USGS2012

Basal slip and mechanical anisotropy of biotite

The basal slip systems of biotite and their mechanical expressions have been investigated by shortening single crystals oriented to maximize and minimize shear stresses on (001). Samples loaded at 45° to (001) exhibit gentle external rotations associated with dislocation glide. High‐angle kink bands in these samples, unlike those developed in micas loaded parallel to (001), are limited to sample corners. Samples shortened perpendicular to (001) show no evidence of nonbasal slip and fail by fracture over all conditions tested. The mechanical response of biotite shortened at 45° to (001) is nearly perfectly elastic‐plastic; stress‐strain curves are characterized by a steep elastic slope, a sharply defined yield point, and continued deformation at low (mostly 1%. Stresses measured beyond the yield point are insensitive to confining pressure over the range 200 to 500 MPa and exhibit weak dependencies upon strain rate and temperature. Assuming an exponential relationship between differential stress σ and strain rate of the form , the data collected over strain rates and temperatures of 10−7 to 10−4 s−1 and 20° to 400°C, respectively, are best fit by an exponential constant α of 0.41±0.08 MPa‐1 and an activation energy of 82±13 kJ/mol. A power law fits the data equally well with = 18±4 and = 51±9 kJ/mol. Samples oriented favorably for slip in directions [100] and [110] are measurably weaker than those shortened at 45° to [010] and [310], consistent with the reported Burgers vectors 〈100〉, 1/2 〈110〉, and 1/2 〈110〉. The anisotropy of biotite is further revealed by contrasting these plastic strengths with results of samples deformed parallel and perpendicular to (001). Previous studies have shown that biotite loaded in the (001) plane is strong prior to the nucleation of kink bands. The strength of biotite shortened perpendicular to (001) exceeds that measured parallel to (001) and is pressure dependent. Application of the results to deformation within the continental crust suggests that biotite oriented favorably for slip is much weaker than most other silicates over a wide range of geologic conditions. Its presence within foliated rocks and shear zones may limit locally the stresses that can be supported.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Paleomagnetic rotations and the Cenozoic tectonics of the Cascade Arc, Washington, Oregon, and California

Paleomagnetic results from Cenozoic (62–12 Ma) volcanic rocks of the Cascade arc and adjacent areas indicate that moderate to large clockwise rotations are an important component of the tectonic history of the arc. Two mechanisms of rotation are suggested by the regional pattern of paleomagnetic rotations. The progressive increase in rotation toward the coast in arc and forearc rocks results from distributed dextral shear, which is likely driven by oblique subduction of oceanic plates to the west. Simple shear rotation is accommodated in the upper crust by strike-slip faulting. The right-lateral Mount St. Helens seismic zone may be an active manifestation of this process. Dextral shear probably obscures a subequal contribution to arc and forearc rotation that is driven by intraarc or backarc extension. This rotation is suggested by the average southward increase in continental margin rotations into the region outboard of the Basin and Range. The southward increase in rotation parallels a change in the arc tectonic regime from largely compressional in northern Washington to extensional in Oregon. Concomitant with this change is a southward increase in the volume of eruptive rocks and the number of basaltic vents in the arc. A progressive eastward shift of the arc volcanic front with time in the rotated arc terrane is the result of the westward pivoting of the arc block in front of a zone of extension since Eocene time. Westward migration of bimodal Basin and Range volcanism since at least 16 Ma is tracking westward rotation of the frontal arc block and growth of the Basin and Range in its wake.

Journal of Geophysical Research Solid Earth↗
USGS2012

Deep crustal structure of the Cascade Range and surrounding regions from seismic refraction and magnetotelluric data

Several regional seismic refraction and magnetotelluric (MT) profiles have been completed across the Cascade Range and surrounding geologic provinces in California, Oregon, and Washington. Analysis of three MT and two seismic refraction profiles in Oregon and a coincident MT and refraction profile in northern California show a high degree of correlation between resistivity and velocity models. The main feature that is evident in both data sets is a highly conductive (2-20 ohm m) zone that occurs at depths of 6-20 km and largely within a midcrustal velocity layer of 6.4-6.6 km/s, overlying a lower crust with velocities of 7.0-7.4 km/s. Accretionary structures in the southern Washington Cascades have been shown to be related to stress release in the area of Mount St. Helens. In order to explain the similar structures in the MT and refraction models for Oregon and California, a model is proposed involving the effects of metamorphic zonation to produce the velocity structure, combined with metamorphically produced fluids and partial melt to produce the deep conductor.

California, Oregon, Washington↗
USGS2012

Volcanism, isostatic residual gravity and regional tectonic setting of the Cascade volcanic province

A technique to locate automatically boundaries between crustal blocks of disparate densities was applied to upward continued isostatic residual gravity data. The boundary analysis delineates a narrow gravitational trough that extends the length of the Pliocene and Quaternary volcanic arc from Mount Baker in northern Washington to Lassen Peak in California. Gravitational highs interrupt the trough at two localities: a northwest trending high in southern Washington and a northeast trending high between Mount Shasta and Lassen Peak. The latter anomaly is one of a set of northeast trending anomalies that, within the Quaternary arc, appear related to volcanic segmentation proposed previously on the basis of spatial and compositional distributions of volcanoes. These northeast trending anomalies extend hundreds of kilometers northeast of the arc, are caused by sources in the upper crust, and in some cases are related to exposed pre-Tertiary rocks. Segmentation models invoke geometric characteristics of the subducting plate as the primary factor controlling location and chemistry of volcanism, and these northeast trending gravity sources also may be a product of disturbance of the upper crust by the subduction process. More likely, the gravity sources may reflect upper crustal structures older than the High Cascades, possibly relicts from earlier accretionary events or more recent crustal deformation, that have actively influenced the spatial location of more recent volcanism. Much of the Pliocene and Quaternary volcanism of the Cascade arc has concentrated on or near contacts between crustal blocks of disparate density. These contacts may promote the ascension of magma to the Earth's surface.

Journal of Geophysical Research Solid Earth↗
USGS2012

Late Cenozoic volcanism, subduction, and extension in the Lassen region of California, southern Cascade Range

Some 537 volcanic vents younger than 7 Ma are identified and these are classified into five age intervals and five compositional categories based on SiO2 content. Maps of vents by age and composition illustrate regionally representative volcanic trends. By 2 Ma, the eastern limit of volcanism had contracted westward toward the late Quaternary arc. Late Quaternary volcanism is concentrated around and north of the silicic Lassen volcanic center. The belt of most recent volcanism (25-0 ka) has been active since at least 2 Ma. Most mafic volcanism is calcalkaline basalt and basaltic andesite. Normal faults and linear groups of vents are evidence of widespread crustal extension throughout most of the Lassen region. NNW orientation of maximum horizontal stress is indicated. The Lassen volcanic region is thought to occur above the subducting Gorda North plate but also within a broad zone of distributed extension that occurs in the North American lithosphere east and southeast of the present Cascadia subduction zone. The scarcity of volcanic rocks older than 7 Ma suggests that a more compressive lithospheric stress regime prior to the late Miocene extensional episode may have suppressed volcanism. -from Authors

Journal of Geophysical Research Solid Earth↗
USGS2012

Quaternary extrusion rates of the Cascade Range, northwestern United States and southern British Columbia

Quaternary (2–0 Ma) extrusion rates change significantly along the Cascade Range volcanic arc. The extrusion rate north of Mount Rainier is about 0.21 km 3 km −1 m.y. −1 ; the rate in southern Washington and northern Oregon south to Mount Hood is about 1.6 km 3 km −1 m.y. −1 ; in central Oregon the rate is 3–6 km 3 km −1 m.y. −1 ; and in northern California, the rate is 3.2 km 3 km −1 m.y. −1 . Eruption style also changes along the arc but at latitudes different from rate changes. At the ends of the arc, volcanism is focused at isolated intermediate to silicic composite volcanoes. The composite volcanoes represent ∼30% of the total volume of the arc. Mafic volcanic fields partly ring some composite volcanoes, especially in the south. In contrast, volcanism is diffused in the middle of the arc, where numerous overlapping mafic shields and a few composite volcanoes have built a broad ridge. Contrasting eruption style may signify diffuse versus focused heat sources or may reflect changes in permeability to ascending magma along the arc.

Journal of Geophysical Research Solid Earth↗