Eugene D. Humphreys

Post-Laramide removal of the Farallon slab, western United States

I propose that post-Laramide removal of the subhorizontally subducting Farallon slab occurred by buckling downward along an approximately east-northeast–trending axis. This process was accomplished by a tearing or necking separation of the subducted slab near the northern and southern boundaries of the United States and propagation of the separated edges toward the central axis of downwelling, accompanied by aesthenosphic upwelling behind the trailing edges. Initial buckling probably began near 50 Ma, and slab removal was complete by 20 Ma. This model is based primarily on the space-time evolution of the “ignimbrite flare-up” (a major mid-Tertiary igneous event of mantle origin), which involved two propagating fronts of initiation of volcanism that followed the proposed motions of the separated slab edges as they converged on central Nevada from the north and southeast. Post-Laramide uplift, extension, establishment of the Cascadia subduction zone, and active magmatism may be consequences of lithosphere-scale modifications caused by the Laramide removal of the slab and the resulting asthenospheric upwelling.

How Laramide-Age Hydration of North American Lithosphere by the Farallon Slab Controlled Subsequent Activity in the Western United States

Starting with the Laramide orogeny and continuing through the Cenozoic, the U.S. Cordilleran orogen is unusual for its width, nature of uplift, and style of tectonic and magmatic activity. We present teleseismic tomography evidence for a thickness of modified North America lithosphere <200 km beneath Colorado and >100 km beneath New Mexico. Existing explanations for uplift or magmatism cannot accommodate lithosphere this thick. Imaged mantle structure is low in seismic velocity roughly beneath the Rocky Mountains of Colorado and New Mexico, and high in velocity to the east and west, beneath the tectonically intact Great Plains and Colorado Plateau. Structure internal to the low-velocity volume has a NE grain suggestive of influence by inherited Precambrian sutures. We conclude that the high-velocity upper mantle is Precambrian lithosphere, and the lowvelocity volume is partially molten Precambrian North America mantle. We suggest, as others have, that the Farallon slab was in contact with the lithosphere beneath most of the western U.S. during the Laramide orogeny. We further suggest that slab de-watering under the increasingly cool conditions of slab contact with North America hydrated the base of the continental lithosphere, causing a steady regional uplift of the western U.S. during the Laramide orogeny. Imaged low-velocity upper mantle is attributed to hydration-induced lithospheric melting beneath much of the southern Rocky Mountains. Laramide-age magmatic ascent heated and weakened the lithosphere, which in turn allowed horizontal shortening to occur in the mantle beneath the region of Laramide thrusting in the southern Rocky Mountains. Subsequent Farallon slab removal resulted in additional uplift through unloading. It also triggered vigorous magmatism, especially where asthenosphere made contact with the hydrated and relatively thin and fertile lithosphere of what now is the Basin and Range. This mantle now is dry, depleted of basaltic components, hot, buoyant, and weak.

Anatexis and metamorphism in tectonically thickened continental crust exemplified by the Sevier hinterland, western North America

The generation of granitoid magmas by partial melting of crustal rocks during continental thickening events is well documented in many geological provinces throughout the world, including the late Mesozoic Sevier belt of western North America. We present a thermal and petrologic model of anatexis and metamorphism in regions of crustal thickening where the only mantle contribution is the normal conductive supply of heat through the base of the lithosphere (i.e. advection of mass and energy are excluded). We distinguish between formation of migmatites and generation of mobile granitoid magmas and examine the temporal and spatial relationships between these two distinct anatectic processes, between anatexis and regional deformation and between anatexis and metamorphism. A fundamental conclusion is that, if protoliths rich in hydrous minerals are present, regional anatexis is the end-product of classical Barrovian metamorphism in thickened continental crust, even in the absence of a free water-rich fluid phase. Barrovian metamorphic facies series are predicted with thickening ratios (maximum crustal thickness attained/initial crustal thickness) as low as 1.3, but mobile granitoid magmas are not formed if this ratio is less than approximately 1.5. Above these lower bounds, Barrovian metamorphism and anatectic granitoid magmatism occur independently of the magnitude of thickening and of the way in which thickening is accomplished. Both processes are sensitive to a diminished heat supply; lowering either Moho heat flow or crustal radioactive heat production results in blueschist-eclogite metamorphism and inhibits the formation of mobile granitoid magmas. We model anatexis under fluid-absent conditions and show that, with such a constraint, migmatization is always a syn-kinematic process (relative to the crustal thickening event), whereas generation of mobile granitoid magmas is in most cases post-kinematic (relative to crustal thickening) but can be syn-kinematic if thickening takes more than approximately 50 Myr. The typical time intervals for melting are consistent with geological observations; mobile granitoid magmas are predicted by most of our models within approximately 10 Myr of the end of the crustal thickening event. This “incubation period” results primarily from the temperature increase required for the dehydration-melting reactions capable of producing large melt fractions to occur. The energetic requirements of anatexis are relatively minor compared to conductive crustal thermal budgets, as shown by the fact that once the necessary P-T conditions are attained, melting reactions are completed within time intervals on the order of 1 Myr, i.e. 1–2 orders of magnitude smaller than the characteristic time scales of the tectonic processes involved in crustal thickening.

Persistent influence of Proterozoic accretionary boundaries in the tectonic evolution of southwestern North America Interaction of cratonic grain and mantle modification events

Northeast-striking tectonic provinces and boundaries were established during 1.8–1.6-Ga assembly of juvenile continental lithosphere in the southwestern United States. This continental grain repeatedly has influenced subsequent intracratonic tectonism and magmatism. After 200 m.y. of stability, cratonic lithosphere was affected by regional, ∼1.4-Ga, dominantly granitic magmatism and associated tectonism that reactivated older northeast-striking shear zones in the Proterozoic accreted terranes, but not the Archean lithosphere. In contrast, 1.1-Ga, dominantly mafic magmatism and rifting did not reactivate northeast-striking zones, but occurred along new north–south fracture zones (e.g., Rocky Mountain trend) that reflect cracking of Laurentian lithosphere at a high angle to the Grenville collision. By 500 Ma, rifting had thinned the crust and mantle in the western United States creating the north–south Cordilleran miogeocline. East of the Cordilleran hingeline, isopachs in Lower Paleozoic sedimentary rocks follow northeast-trending structures (Cheyenne belt, Transcontinental arch, and Yavapai–Mazatzal province boundary), suggesting that older boundaries influenced isostatic response of the craton during thermal subsidence of the margin. Ancestral Rockies and Laramide uplifts and basins did not strongly reactivate northeast-striking boundaries. However, Ancestral Rockies structures end at the Archean–Proterozoic boundary, and Laramide magmatism (Colorado mineral belt) and metallogenic provinces follow northeast-striking Proterozoic boundaries, both suggesting deep-seated lithospheric influences on tectonism. Present mantle structure and topography in the Rocky Mountain region continue to record an interaction between older crustal structures and younger mantle reorganization. Zones of partially molten mantle underlie northeast-striking Proterozoic boundaries (e.g., Snake River Plain, Saint George lineament, and Jemez lineament) and the north-striking Rio Grande rift, and are inferred to record replacement of lithosphere by asthenosphere preferentially along Archean–Proterozoic, Mojave–Yavapai, Yavapai–Mazatzal, and 1.1-Ga lithospheric anisotropies. Highest topography coincides with areas of low-velocity mantle, suggesting an importance of mantle buoyancy in the isostatic balance. Changes in topographic character across ancient crustal boundaries suggests a continued influence of crustal structures in differential uplift and denudation. Inheritance of the Proterozoic northeast grain involves two basic factors: (1) “volumetric” inheritance, in which density and fertility of lithospheric blocks of differing compositions influence isostatic and magmatic response to tectonism; and (2) “interface” inheritance, in which mechanical boundaries are zones of weakness and mass transport. “Volumetric” inheritance is suggested by the distinctive isotopic signatures of different provinces and by the observation that Archean lithosphere has been consistently less fertile for magmas than Proterozoic lithosphere, due to thicker, colder mantle, and compositional differences. We infer that distinct mantle lithospheres have been attached to their respective crustal provinces (at scales of 100 km) since accretion. “Interface” inheritance controls include mechanical reactivation of northeast-striking province boundaries and shear zones as magma conduits, zones of renewed shearing, and zones accommodating differential uplift.

Physical state of the western U.S. upper mantle

Using observed P wave images of the western U.S. upper mantle, which show lateral variations of up to 8%, and existing scaling relations, we infer that the low-velocity mantle is hot and partially molten to depths of 100–200 km, and that the high-velocity upper mantle is subsolidus. Most the high-velocity upper mantle within a few hundred kilometers of the coastline appears to be relatively dense, suggesting that it is relatively cool (i.e., a thermal lithosphere). This is expected for features associated with the subducting Juan de Fuca and Gorda slabs, and the high velocity upper mantle beneath the Transverse Ranges has been attributed to the sinking of negatively buoyant mantle lithosphere. Other high-velocity mantle structures near the continental margin are consistent with this interpretation. In contrast, the generally high elevations of the continental interior imply a buoyant upper mantle there, an inference that holds for both the high- and the low-velocity upper mantle. The only reasonable way to produce the high-velocity low-density upper mantle is through basalt depletion, thereby creating mantle of increased solidus temperature and decreased density. We distinguish a marginal domain, within ∼250 km of the Pacific coast, from an interior domain. This is based on the inferred upper mantle compositional difference and regional associations: beneath the marginal domain, upper mantle structures trend parallel to the surface physiography and young tectonic structures, whereas upper mantle structures beneath the continental interior trend northeasterly. This northeast orientation is discordant with the young tectonic structures, but aligns with young volcanic activity. The high lateral gradients in observed upper mantle seismic structure found throughout the western United States imply high lateral gradients in the: associated temperature or partial melt fields. Because these fields diffuse on time scales of less than a few tens of millions of years, the imaged upper mantle structure is young. The following upper mantle processes are hypothesized to account for these findings and inferences. Away from the plate margin, small-scale upper mantle convection driven by partial melt-induced buoyancy of hot upper mantle leads to the production and segregation of melt and the creation of compositional variations. The heterogeneous upper mantle P wave structure of the elevated continental interior is largely a consequence of partial meltvariations that are modulated by the compositional variations, and throughout this region we infer high temperatures and low densities. Near the plate margin, relative plate motions force upper mantle flow, although upper mantle flow driven by the positive buoyancy of melt and the negative buoyancy lithosphere is important locally.

Toroidal mantle flow through the western U.S. slab window

The circular pattern of anisotropic fast-axis orientations of split SKS arrivals observed in the western U.S. cannot be attributed reasonably to either preexisting lithospheric fabric or to asthenospheric strain related to global-scale plate motion. A plume origin for this pattern accounts more successfully for the anisotropy field, but little evidence exists for an active plume beneath central Nevada. We suggest that mantle flow around the edge of the sinking Gorda–Juan de Fuca slab is responsible for creating the observed anisotropy. Seismic images and kinematic reconstructions of Gorda–Juan de Fuca plate subduction have the southern edge of this plate extending from the Mendocino triple junction to beneath central Nevada, and flow models of narrow subducted slabs produce a strong toroidal flow field around the edge of the slab, consistent with the observed pattern of anisotropy. This flow may enhance uplift, extension, and magmatism of the northern Basin and Range while inhibiting extension of the southern Basin and Range.

Western U.S. upper mantle structure

Regional three-dimensional inversions of upper mande P wave velocity structure are created from teleseismic P wave travel time residuals recorded by many of the high-density high-frequency regional arrays operated within the western United States. These inversions are adjusted to a global (International Seismic Centre) reference and merged to obtain an image of the upper mantle beneath western United States. The P wave velocities in the upper mantle are slow on average, and the structure is very heterogeneous. Where resolution is good, coherent upper mantle structures are imaged that extend as deep as ∼200 km (the Juan de Fuca and Gorda slabs, which penetrate to greater depths, are exceptions) and deviate from the average velocity (at a given depth) by as much as ±4%. Lateral resolution of these structures usually is very good, although the magnitude of the actual seismic variations is probably greater than that imaged. The long wavelength part of the imaged mantle structure defines coherent elongate features with wavelengths of 200–500 km. Within ∼250 km of the Pacific Coast, these structures have a wavelength of ∼250 km and trend parallel to the surface physiography and young tectonic structures. Beneath the continental interior, where use is made of additional seismic studies to infer average structure in regions of poor teleseismic data coverage, structures have a wavelength of ∼500 km and trend northeasterly. This northeast orientation is discordant with young tectonic structures but aligns with young volcanic activity.

Upper mantle P wave velocity structure of the eastern Snake River Plain and its relationship to geodynamic models of the region

Tomographic inversions of ∼5000 teleseismic P wave travel time residuals image a narrow, deep, low-velocity region centered beneath the eastern Snake River Plain, Idaho. Aligned in aie direction of North American plate motion, the eastern Snake River Plain is the locus of time-progressive volcanism leading to the Yellowstone hotspot. The low-velocity anomaly extends to depths of at least 200 km and is flanked by high-velocity mantle to either side. These results are inconsistent with standard mantle plume models which predict a shallow, wide, low-velocity anomaly. By considering the effects of composition, partial melt, and temperature on both P wave velocities and upper mantle densities (assuming local isostasy), we conclude that the low-velocity region is partially molten peridotite and the high-velocity regions on either side are depleted residuum. Swell relief at hotspots has generally been attributed to thermal effects and is commonly used to estimate plume thermal buoyancy fluxes. However, we demonstrate that the compositional effects of mantle melting and depletion can account for at least one fifth and possibly all of the swell relief at Yellowstone. These results suggest that melt generation and segregation are dominant processes within the upper mantle of Yellowstone and allow for the possibility that the fundamental origin of the Yellowstone hotspot is something other than a deep-seated mantle plume.

A lithospheric instability origin for Columbia River flood basalts and Wallowa Mountains uplift in northeast Oregon

Flood basalts appear to form during the initiation of hotspot magmatism. The Columbia River basalts (CRB) represent the largest volume of flood basalts associated with the Yellowstone hotspot, yet their source appears to be in the vicinity of the Wallowa Mountains, about 500 km north of the projected hotspot track. These mountains are composed of a large granitic pluton intruded into a region of oceanic lithosphere affinity. The elevation of the interface between Columbia River basalts and other geological formations indicates that mild pre-eruptive subsidence took place in the Wallowa Mountains, followed by syn-eruptive uplift of several hundred metres and a long-term uplift of about 2 km. The mapped surface uplift mimics regional topography, with the Wallowa Mountains in the centre of a ‘bulls eye’ pattern of valleys and low-elevation mountains. Here we present the seismic velocity structure of the mantle underlying this region and erosion-corrected elevation maps of lava flows, and show that an area of reduced mantle melt content coincides with the 200-km-wide topographic uplift. We conclude that convective downwelling and detachment of a compositionally dense plutonic root can explain the timing and magnitude of Columbia River basalt magmatism, as well as the surface uplift and existence of the observed melt-depleted mantle.

Upper mantle seismic wave attenuation: Effects of realistic partial melt distribution

Frequency dependence of seismic velocity and attenuation resulting from viscoelastic relaxation of partially molten mantle is estimated. We consider the contribution of the melt squirt mechanism, through which pressure differences between disk-shaped inclusions are equalized by melt passing through connecting tubes. The pressure differences arise as a result of shear strain compressing disk-shaped pores differently on the basis of disk orientation with respect to the applied shear. The frequencies over which the transition from the unrelaxed to the relaxed states occurs are determined by representing the melt as a network of tubes connecting oblate ellipsoidal pores. The pressure equalization process is modeled by a system of first-order linear differential equations, whose eigenvalues are the characteristic frequencies for melt squirt relaxation. It is shown that in this framework the set of frequencies is invariant to the absolute scale of the system but is sensitive to melt bulk modulus and viscosity, as well as distribution of melt inside pores and conduits. Use of realistic solid and melt physical properties and pore and conduit geometries demonstrates that it is the relaxed modulus that is most likely excited in the seismic band and that melt mobility has little effect on seismic attenuation. Some conceivable melt distributions, however, would result in detectable attenuation in the seismic band. In all cases investigated, attenuation increases with frequency, indicating that melt squirt is not responsible for global upper mantle Q observations.