It is widely assumed that the boundary layer above the core is the source of intraplate volcanoes such as Hawaii, Samoa, and Yellowstone, and that the sub-plate boundary layer at the top of the mantle is thin and entirely subsolidus. In fact, this upper layer is thicker and has higher expansivity, buoyancy, and insulating power than the lower one, and may have higher potential temperatures. The observed seismic structure of the low-velocity zone (LVZ) including attenuation, anisotropy, sharp boundaries, and a reduction of both compressional and shear moduli can be taken as strong evidence for the ubiquitous presence of melt in the upper mantle. If the LVZ contains as little as 1%–2% melt, then it is the most plausible and accessible source for midplate magmas; deeply rooted active upwellings are unnecessary. The upper boundary layer is also the most plausible source of ancient isotopic signatures of these magmas and their inclusions.

Abstract Large-volume, high-crystallinity, chemically homogeneous ignimbrites, dubbed ‘monotonous intermediates’, provide a unique opportunity to investigate the evolution of crustal magmatic reservoirs. We present the results of hydrothermal experiments on a dacite from Fish Canyon Tuff (FCT) in Colorado (USA), a classic example of a monotonous intermediate deposit, in order to characterize the variations in chemical and physical properties of hydrous dacite magmas as a function of temperature. The experiments (200 MPa, 720–1100°C) span the inferred pre-eruptive conditions of FCT magmas, and are shown to provide the best match to the chemical and physical properties of the erupted magmas at 790±10°C under conditions at or close to water-saturation. The results show the important effect of water content in controlling the chemical and physical evolution of magma, and the contrasted behaviour of water-saturated v. water-undersaturated magmas. In both cases, however, there is a broad interval of temperature (200°C) over which crystal fraction changes little. By recasting this behaviour in terms of enthalpy, rather than temperature, as the independent variable we show that this interval corresponds to a minimum in the change in crystallinity per unit of energy added or subtracted from the system, such that small perturbations to the heat content of the system (e.g. by cooling or new magma injections) results in very little change in magma properties. The crystal content in this interval is 55–65 wt%, which is close to the phenocryst content (40–55 wt%) of monotonous intermediates. We propose that crystal-rich magmas tend to settle in this ‘petrological trap’, changing little in physical and chemical properties over time as the system grows. Petrological trapping enables very large volumes of intermediate magma to accumulate in the shallow crust until such time as the net buoyancy force of these crystal-rich magma is sufficient to overcome the strength of the roof rocks, leading to a potentially very large eruption.

Abstract Arrival of magma from depth into shallow reservoirs has been documented as one of the possible processes leading to eruption. Magma intruding and rising to the surface interacts with the already emplaced, degassed magmas residing at shallower depths, leaving chemical signatures in the erupted products. We performed two-dimensional numerical simulations of the arrival of gas-rich magmas into shallow reservoirs. We solve the fluid dynamics for the two interacting magmas, evaluating the space–time evolution of the physical properties of the mixture. Convection and mingling develop quickly into the chamber and feeding conduit/dyke, leading on longer timescales to a density stratification with the lighter, gas-richer magma, mixed with different proportions of the resident magma, rising to the top of the chamber due to buoyancy. Over timescales of hours, the magmas in the reservoir appear to have mingled throughout, and convective patterns become harder to identify. Our simulations have been performed changing the geometry of the shallow reservoir and the gas content of the initial end-member magmas. Horizontally elongated magma chambers, as well as higher density contrasts between the two magmas, cause faster ascent velocities and also increase the mixing efficiency. Supplementary material: Videos showing the evolution of magma composition with time in the shallow chamber for simulations 1–4 are available at www.geolsoc.org.uk/SUP18819

Abstract Geomorphic, thermochronologic, geochemical, structural, and geophysical data all lend support to the hypothesis that the Colorado Rocky Mountains are an example of dynamic topography that has responded variably to broad epeirogenic uplift since the late Miocene. Our view is that this epeirogenic uplift is primarily related to mantle buoyancy and to a lesser extent, on isostatic adjustments caused by regional denudation. Neogene uplift components were superimposed on earlier (Laramide and mid-Tertiary) uplift events such that the present-day high topography of the Colorado Rocky Mountains reflects a composite uplift history. Newly recognized gravels of the ancestral Colorado River located beneath lava flows of Grand Mesa suggest that the Colorado River flowed west from the Rocky Mountains out onto the Colorado Plateau by 11 Ma. Radiometric dating of late Miocene basalt flows and thermochronologic data for western Colorado indicate that regional denudation and river incision became much more rapid ca. 9.5–6.0 Ma, prior to integration of the Colorado River through Grand Canyon. Post-10 Ma river incision rates and magnitudes are variable, and we interpret these variations to reflect differential uplift of discrete areas of the Rocky Mountain region during the late Cenozoic. For instance, in areas such as Grand Mesa, the Flat Tops, and the Park Range, incision rates and magnitudes are generally ~100–150 m/m.y. and 1.0–1.5 km, respectively. For comparison, these values are 2–3 times larger than incision rates and magnitudes over the past 5–10 Ma in the upper Green River basin. Differential incision across major structural boundaries indicates that post-10 Ma river incision has been driven by Neogene rock uplift. Additional evidence of post-Laramide deformation includes warping of the Oligocene to early Miocene pre-Browns Park Formation erosion surface. This surface has up to 1.8 km of structural relief that has been produced since the Miocene. For basalt-capped regions such as Grand Mesa and the Flat Tops, Neogene rock uplift of 1.0–1.5 km has probably been accompanied by a commensurate amount of surface uplift, resulting in increased local relief ( England and Molnar, 1990 ). Flexural isostatic modeling suggests that isostatic adjustments account for only 10%–40% of the post-10 Ma rock uplift recorded in western Colorado, and that 850–1500 m of probable post-10 Ma rock uplift cannot be accounted for by the isostatic response to denudation. Areas such as Grand Mesa and the Flat Tops are associated with the largest magnitudes of post-10 Ma rock uplift, and generally overlie areas of anomalously low P-wave velocities (the largest of which is the Aspen Anomaly), which suggests that mantle buoyancy could have driven Neogene uplift of the Colorado Rocky Mountains. 3 He/ 4 He ratios strongly suggest that there are direct mantle-to-surface interactions of neotectonics and surface waters. In summary, we conclude that the Rocky Mountains are continuing to evolve in response to neotectonic events that accelerated in the late Miocene, and which are continuing to drive base-level changes and drainage integration of the modern upper Colorado River system. The combined data are best explained by broad epeirogenic Neogene uplift of the Colorado Rocky Mountains driven by mantle flow and buoyancy.

Flat subduction of the Farallon plate beneath the western United States during the Laramide orogeny was caused by the combined effects of oceanic plateau subduction and unusually great suction in the mantle wedge, the latter of which was caused by the shallowing slab approaching the North American craton root. Once in contact with basal North America, the slab cooled and hydrated the lithosphere. Upon removal, asthenospheric contact with lithosphere resulted in magma production that was especially intense where the basal lithosphere was fertile (in what now is the Basin and Range Province), and this heating weakened the lithosphere and made it convectively unstable. Small-scale convection has since affected many areas. With slab sinking and unloading, the western United States elevated into a broad plateau, and the weak portion gravitationally collapsed. With development of a transform plate boundary, the western part of the weak zone became entrained with the Pacific plate, and deformation there is dominated by shear.

The proposed ages for the collision of the Carnegie Ridge with the South America trench, offshore Ecuador, range from 1 to 15 Ma. In this time frame, many geological features of Ecuador are commonly linked to the subduction of the Carnegie Ridge. (1) After the ridge collided with the trench at ca. 15 Ma, the subsequent interplate coupling produced high exhumation rates of volcanic materials at ca. 9 Ma. (2) The oblique convergence of the Carnegie Ridge would have caused the northward drift of the North Andean block and the opening of the Gulf of Guayaquil. (3) During the late Miocene, the subduction of the Carnegie Ridge would have triggered a regional tectonic inversion along the forearc. (4) Along the collision front of the ridge with the trench, subduction-related erosion is occurring, and the Ecuadorian continental margin is being uplifted in the present day. (5) The chemistry of the active volcanic arc is explained as resulting from the arrival of the Carnegie Ridge into the trench. For instance, the adakitic signal, which appears at 1.5 Ma, is thought to be ridge-induced. (6) The buoyancy of the subducted Carnegie Ridge would explain the flatness of the slab beneath Ecuador. In this paper, we review the geological evolution of the Northern Andes in order to establish which of these geological events may be related to the subduction of the Carnegie Ridge. This review suggests that there is no clear deformation linked with the subduction of the Carnegie Ridge or with its landward prolongation postulated at depth.

The rivers of Southern California deliver episodic pulses of water, sediment, nutrients, and pollutants to the region's coastal waters. Although river-sediment dispersal is observed in positively buoyant (hypopycnal) turbid plumes extending tens of kilometers from river mouths, very little of the river sediment is found in these plumes. Rather, river sediment settles quickly from hypopycnal plumes to the seabed, where transport is controlled by bottom-boundary layer processes, presumably including fluid-mud (hyperpycnal) gravity currents. Here we investigate the geographical patterns of river-sediment dispersal processes by examining suspended-sediment concentrations and loads and the continental shelf morphology offshore river mouths. Throughout Southern California, river sediment is discharged at concentrations adequately high to induce enhanced sediment settling, including negative buoyancy. The rivers draining the Western Transverse Range produce suspended-sediment concentrations that are orders of magnitude greater than those in the urbanized region and Peninsular Range to the south, largely due to differences in sediment yield. The majority of sediment discharge from the Santa Clara River and Calleguas Creek occurs above the theoretical negative buoyancy concentration (>40 g/l). These rivers also produce event sediment loading as great as the Eel River, where fluid-mud gravity currents are observed. The continental shelf of Southern California has variable morphology, which influences the ability to transport via gravity currents. Over half of the rivers examined are adjacent to shelf slopes greater than 0.01, which are adequately steep to sustain auto-suspending gravity currents across the shelf, and have little (<10 m) Holocene sediment accumulation. Shelf settings of the Ventura, Santa Clara, and Tijuana Rivers are very broad and low sloped (less than 0.004), which suggests that fluid-mud gravity currents could transport across these shelves, albeit slowly (~10 cm/s) and only with adequate wave-generated shear stress and sediment loading. Calleguas Creek is unique in that it discharges directly into a steep-sloped canyon (greater than 0.1) that should allow for violent auto-suspending gravity currents. In light of this, only one shelf setting—the Santa Clara and Ventura—has considerable Holocene sediment accumulation (exceeding 60 m), and here we show that the morphology of this shelf is very similar to an equilibrium shape predicted by gravity-current sediment transport. Thus, we conclude that a wide distribution of river-shelf settings occur in the Southern California Bight, which will directly influence sediment dispersal processes—both dilute suspended and gravity-current transport—and sediment-accumulation patterns.

Elasmosaurid plesiosaur remains containing gastroliths have been recently recovered from the Late Cretaceous Cape Lamb Member of the Snow Hill Island Formation, Antarctic Peninsula, as well as from the Sharon Springs Formation of the Pierre Shale Group, South Dakota. The significance of the Antarctic specimens lies in the large number and relatively small size of the stones, whereas those from South Dakota and elsewhere have larger stones but relatively small numbers. All specimens compared are of relatively large elasmosaurids, and rib diameters indicate similar overall size. One testable hypothesis was whether or not the weight of the gastroliths might be similar among similarly sized individuals. However, comparisons between the gastroliths recovered from an Antarctic specimen and those recovered from the Pierre Shale show great differences in both the size and number of gastroliths. The total mass of the stones collected in the specimen from Antarctica was 3.0 kg, whereas those recovered from the South Dakota specimens totaled 2.2 kg and 8.3 kg, respectively. Perhaps not all stones originally within each plesiosaur were found, but efforts were designed to recover every stone. The number of stones recovered from the Antarctic plesiosaur was exceptionally large (2626) and appears to represent the most gastroliths recovered from a single plesiosaur. Those recovered from South Dakota elasmosaurids totaled only 124 and 253, respectively. Therefore, neither weight nor number of stones corresponds among these large individuals, perhaps mirroring the temporal or behavioral differences. Lack of correspondence among individuals furthers questions concerning the utilization of gastroliths for neutral buoyancy, ballast, or as an aid in digestion. Many parameters remain unexplored, and questions arise not only as to the utilization of the gastroliths but also as to whether physiology of the Antarctic plesiosaurs differs from those at lower latitudes.

New and recycled intermediate to felsic crust in island arcs and continental margins is generated by magmatism surmounting convergent plate junctions. Paired Pacific-type orogens develop adjacent sites of long-lived subduction of oceanic lithosphere. They consist of (1) an outboard accretionary prism deposited in and continentward from the oceanic trench, the filling of which is derived mainly from (2) an inboard calc-alkaline volcanic-plutonic arc. The trench assemblage includes arc-sourced clastic mélanges, minor but widespread deepwater cherts and carbonates, and tectonically disaggregated ophiolites. These relatively incompetent sections recrystallize under high-pressure (HP) conditions and decouple from the descending oceanic plate at depths of ∼15–50 km. A massive, slightly older to coeval andesitic-granodioritic arc dominates the landward belt where new sialic crust is added from I-type magmas, and preexisting continental materials are recycled as S-type melts; high temperature (HT) characterizes local-regional metamorphism of the wall rocks. In contrast, Alpine-type orogens result from the underflow of an oceanic plate that transports island arcs, microcontinents, and/or continental salients into the subduction zone. Reflecting lithospheric coherence, the sialic crust may be carried down as much as 90–140 km, becomes thermally softened, and decouples from the sinking plate. Metamorphism of deeply subducted parts of Alpine belts ranges from high pressure to ultrahigh pressure (UHP), and is not paired with a HT calc-alkaline arc. In both types of subduction complex, outboard thrust faults dip landward and fold vergence is seaward, reflecting the polarity of underflow (and rollback) of the oceanic lithosphere. Antithetic thrusting typifies some contractional continental realms. Propelled by buoyancy, allochthonous nappes conduct relatively low-density HP and UHP sections to midcrustal levels. At convergent syntaxes (plate-margin cusps) of overthickened arcs, tectonic aneurysms may produce domical uplifts, further exhuming UHP units surfaceward. The regional crustal thickness of an active mountain belt partly reflects climate as well as the main convergent-transform plate processes producing the orogen. The volcanogenic Chilean Cordillera parallels the seaward convergent plate junction and the eastward-sinking Nazca plate. The highest ranges and thickest continental crust (∼70 km) occur in the compressed north-central Andes at ∼25°S, a region of extremely low rain- and snowfall. Aridity and low erosion rates help account for the high-standing calc-alkaline volcanic-plutonic contractional arc and the elevated, internally drained Altiplano downwind. Thus erosional degradation is weakly developed. A very different climate typifies the orogen at ∼45°S, where abundant moisture-laden westerlies bring abundant precipitation to the Chilean margin. The rugged mountain belt is of lower elevation compared with the north-central Andes and is supported by a crust of only moderate thickness (35–40 km). Vigorous erosion supplies voluminous detritus to the offshore Chile-Peru Trench as well as eastward, so a plateau is lacking in the lee of the arc. The Himalayas and Tibetan Plateau, the Sierra Nevada and Colorado Plateau, the Japanese island arc, and New Zealand exhibit somewhat similar relationships between crustal thickness and precipitation-linked erosion.

The Bear Mountain intrusive complex, Klamath Mountains, California, is a multiphase, ultramafic to silicic plutonic suite emplaced into the Rattlesnake Creek terrane during the Late Jurassic (ca. 151–147 Ma). The intrusive complex includes five plutonic units: (1) elongated, flanking bodies of ultramafic to gabbroic rocks (Blue Ridge, Clear Creek, and Cedar Creek intrusions); (2) biotite + two-pyroxene diorite/monzodiorite of the Buck Lake plutonic unit; (3) biotite-bearing hornblende ± pyroxene gabbro/diorite of the Punchbowl plutonic unit; (4) biotite + hornblende ± pyroxene (± quartz) diorite of the Doe Flat plutonic unit; and (5) minor biotite ± hornblende quartz diorite to tonalite/granodiorite of the Wilderness Falls plutonic unit. Units 2, 3, and 4 constitute the Bear Mountain pluton. The petrogenesis of these units involved emplacement of distinct batches of magma into the same magmatic center during a complex, multistage intrusion history. Inclusions of older units are found within younger units, and dikes related to younger units intrude older units. Mineral assemblages and Al-in-hornblende barometry indicate early crystallization of pyroxene at >1100 °C and late crystallization of hornblende and biotite at >700 °C between 3 and 5 kb. A dynamothermal aureole extends for ∼600–1000 m from the external intrusive contact of the Bear Mountain pluton. Metabasic rocks within this dynamo-thermal aureole have been strongly deformed and recrystallized to well-foliated and lineated hornblende schist or fine-grained amphibolite and exhibit a “pluton-down” sense of shear (i.e., the Bear Mountain pluton). Adjacent to the intrusive complex, contact-metamorphic conditions were hornblende-hornfels facies and locally reached pyroxene-hornfels facies. At some localities in the inner aureole, anatexis occurred, forming migmatitic amphibolite gneiss. The effects of thermal metamorphism apparently extend >2 km from the intrusive contact and are evidenced by various mineral assemblages in metaserpentinite. A concentric, margin-parallel magmatic foliation is found throughout the main body of the Bear Mountain pluton. Map-scale evidence suggests emplacement of the Bear Mountain pluton as a series of sheetlike intrusive bodies that subsequently sagged into an oval-shaped pluton due to negative buoyancy and overall isostatic downward displacement. The three-dimensional shape of the complex is poorly known, but it may be funnel-shaped with a dense root of ultramafic rocks. The ultramafic rocks of the Bear Mountain intrusive complex are primarily cumulates from H 2 O- and MgO-rich basaltic parental magmas. Differentiation of these magmas gave rise to H 2 O- and Al 2 O 3 -rich basalt/basaltic andesite from which rocks of the Punchbowl unit accumulated. Further differentiation of this magma type resulted in the development of the parental magma for the Doe Flat unit. Magmas parental to the Buck Lake unit were similar to the Punchbowl parent, except that Buck Lake magmas were H 2 O poor. All of these magmas have trace-element, Sr, Nd, and oxygen isotope values characteristic of a supra-subduction zone tectonic setting. The late-stage tonalite/granodiorite magmas have geochemical signatures consistent with an origin by partial melting of metabasic crustal rocks with residual garnet; therefore, they cannot be explained by local, in situ anatexis of host rock metabasites.