Papers by Eugene Humphreys
Geophysical Research Letters, 2022
• Crustal seismic anisotropy reveals the coupling between the crust and the mantle • Mantle verti... more • Crustal seismic anisotropy reveals the coupling between the crust and the mantle • Mantle vertical loads drive crustal flow in much of the western US 10 • Mantle loads can depress the Moho, thicken the crust, and cause isostatic com-11 pensation without any topographic expression 12
The Gulf and Peninsular Province of the Californias, 1991
Objective: This study sought to preliminarily investigate the inhibitory effect of metabolites of... more Objective: This study sought to preliminarily investigate the inhibitory effect of metabolites of Aspergillus chevalieri and Trichoderma harzianum on a number of pathogenic bacteria. Methods: The agar well diffusion method was employed to determine the antimicrobial activity of the fungal metabolites. The test microorganisms were Enterococcus faecalis, methicillin-resistant Staphylococcus aureus (MRSA), Salmonella typhi, Escherichia coli and Pseudomonas aeruginosa. Results: Both metabolites had broad-spectrum antibacterial activity. All the test organisms were susceptible to the A. chevalieri metabolites except for S. typhi. Both S. typhi and E. faecalis were however not susceptible to T. harzianum metabolites. P. aeruginosa was highly susceptible to both metabolites with the highest zone of inhibition of 26 mm for the stock metabolite. This activity was comparable to the standard, 10 µg/ml of ciprofloxacin. Conclusion: Metabolites of A. chevalieri and T. harzianum exhibited broad-spectrum activity, and this can be exploited as a source for novel antibiotics.
Upper Mantle Heterogeneities from Active and Passive Seismology, 1997
<p&amp... more <p>Azimuthal anisotropy in the NW U.S. crust is derived using 3-17 s Rayleigh waves derived using ambient noise from about 300 broadband stations. Velocity is resolved between all station pairs in close proximity, and velocity as a function of azimuth is determined for each station. Azimuthal anisotropy orientations point strongly toward tomographically-imaged high-velocity structures in the underlying mantle, but show no relation to the underlying mantle anisotropy field. We suggest that the crustal anisotropy is decoupled from lateral tectonic forces and is created by upper mantle vertical loading, which in turn generates lateral pressure gradients that drive channelized flow in the ductile mid and lower crust. This idea is tested with geodynamic modeling. Using reasonable values for crustal viscosity and mantle buoyancy structure, we find that the local buoyancy sources within the upper mantle will drive the viscous crustal flow in a manner that reproduces well the imaged crustal anisotropy. We conclude that mantle vertical loading, acting independently from mantle flow, can actively control crustal deformation on a scale of several hundred kilometers.</p>
Geological Society of America Abstracts with Programs, 2019
Journal of Geophysical Research, 1992
Slip on an undulatory strike-slip fault induces predictable residual stresses in the adjacent cru... more Slip on an undulatory strike-slip fault induces predictable residual stresses in the adjacent crust. Elastic analytic and finite element models are developed to quantify these stresses for arbitrary fault geometries. Across fault-parallel planes, domains of reverse, normal, right-...
Geophysical Research Letters, 1995
Earthquake recurrence data from the Pallett Creek and Wrightwood paleoseismic sites on the San An... more Earthquake recurrence data from the Pallett Creek and Wrightwood paleoseismic sites on the San Andreas fault appear to show temporal variations in repeat interval. These sites are located near Cajon Pass, southern California, where detailed mapping has revealed geomorphically and structurally expressed domains of alternating extension and contraction respectively associated with releasing and restraining bends of the San Andreas fault. We investigate the interaction between strike-slip faults and auxiliary reverse and normal faults as a physical mechanism capable of producing such variations. Under the assumption that fault strength is a function of fault-normal stress (e.g. Byedee's Law), failure of an auxiliary fault modifies the strength of the strike-slip fault,. thereby modulating the recurrence interval for earthquakes. In our finite element model, auxiliary faults are driven by stress accumulation near restraining and releasing bends of a strike-slip fault. Earthquakes occur when fault strength is exceeded and are incorporated as a stress drop which is dependent on fault-normal stress. The model is driven by a velocity boundary condition over many earthquake cycles. Resulting synthetic strike-slip earthquake recurrence data display temporal variations similar to observed paleoseismic data within time windows surrounding auxiliary fault failures. Although observed recurrence data for the two paleoseismic sites are too short to be definitive about the temporal variations or the physical mechanism responsible for it, our simple model supports the idea that interaction between a strike-slip fault and auxiliary reverse or normal faults can modulate the recurrence interval of events on the strike-slip fault, possibly producing short term variations in earthquake recurrence interval.
Geology, 1989
Namson and Davis (1988) presented a balanced cross section across the western Transverse Ranges a... more Namson and Davis (1988) presented a balanced cross section across the western Transverse Ranges and the Big Bend of the San Andreas fault and made a number of inferences about the style and rate of shortening. A good estimate of the deformation across this area would provide needed constraint on regional tectonics and seismic hazard, and as such their work could be very useful. We have problems with three major aspects of their model and feel that these problems, if unresolved, are substantial enough to invalidate many of their conclusions. The first issue is the "subduction" of the lower crust. The proposed 53 km of shortening is "balanced" by an equivalent amount of convergence and downward motion of the lower crust and mantle lithosphere. Namson and Davis did not discuss the details of this process; however, they cited the work of Bird and Rosenstock (1984), Humphreys et al. (1984), and Sheffels and McNutt (1986) as supportive of their model of balancing upper crustal shortening with lower crustal subduction. Bird and Rosenstock (1984, p. 946, 955, 956) were careful to state that only the upper mantle is going down under the Transverse Ranges. They detached the mantle from the crust at the Moho, not at the-15 km "brittle-ductile" transition proposed by Namson and Davis, for the obvious reason that they had no evidence for crustal subduction. Bird and Rosenstock (1984) stated that the solution to resolving the issue is mapping out the depth to the Moho boundary, which was done by Hearn (1984). Hearn found that there are only a few kilometres of Moho depression under the Transverse Ranges relative to "typical" continental southern California. Humphreys et al. (1984) imaged the upper mantle, proposed smallscale convection to explain the velocity anomaly under the Transverse Ranges, and called upon upper mantle flow to support the relief. Sheffels and McNutt (1986) proposed flexural support for the Transverse Ranges, caused by "continental subduction." Whereas Sheffels and McNutt's (1986) paper leads one to infer major crustal subduction, the values they are proposing are trivial; they stated (p. 6428) that the thickening of the crust under the western Transverse Ranges is less than 2 km. Namson and Davis also failed to cite a correction (Sheffels and McNutt, 1987) that brings Sheffels and McNutt's work into agreement with Humphreys et al. (1984) on the density of the velocity anomaly under the Transverse Ranges, further diminishing the need for forces other than those derived from small-scale convection and crustal strength to support the Transverse Ranges. 770 GEOLOGY, August 1989 770 in folding, which has been ignored or only partially considered in previous tectonic models. With the exception of the San Andreas fault, the fundamental late Pliocene and Quaternary structures of the western Transverse Ranges are folds and thrust faults, not strike-slip faults.
Geophysical Journal International, 2021
SUMMARY Evidence from seismology, geology and geodynamic studies suggests that regional-scale low... more SUMMARY Evidence from seismology, geology and geodynamic studies suggests that regional-scale lower crustal flow occurs in many tectonic settings. Pressure gradients caused by mantle processes and crustal density heterogeneity can provide driving force for lower crustal flow. Numerically modelling such flow can be computationally expensive. However, by exploiting symmetry in the physical system, it is possible to represent the vertical component of flow in terms of its lateral components, thereby reducing the problem’s spatial dimension by one. Here, we present a mathematical formulation for flow in a viscous channel below an elastic upper plate, which is optimized for solution by common numerical methods. Our formulation drastically reduces the computational load required to simulate lower crustal flow over large areas and long timescales. We apply this model to two example problems, with and without an elastic upper plate, identifying combinations of parameters that are capable of...
Science Advances
Buoyancy anomalies within Earth’s mantle create large convective currents that are thought to con... more Buoyancy anomalies within Earth’s mantle create large convective currents that are thought to control the evolution of the lithosphere. While tectonic plate motions provide evidence for this relation, the mechanism by which mantle processes influence near-surface tectonics remains elusive. Here, we present an azimuthal anisotropy model for the Pacific Northwest crust that strongly correlates with high-velocity structures in the underlying mantle but shows no association with the regional mantle flow field. We suggest that the crustal anisotropy is decoupled from horizontal basal tractions and, instead, created by upper mantle vertical loading, which generates pressure gradients that drive channelized flow in the mid-lower crust. We then demonstrate the interplay between mantle heterogeneities and lithosphere dynamics by predicting the viscous crustal flow that is driven by local buoyancy sources within the upper mantle. Our findings reveal how mantle vertical load distribution can a...
Tectonophysics
Construction histories of Archean cratons remain poorly understood; their destruction is even les... more Construction histories of Archean cratons remain poorly understood; their destruction is even less clear because of its rarity, but metasomatic weakening is an essential precursor. By assembling geophysical and geochemical data in 3-D lithosphere models, a clearer understanding of the geometry of major structures within the Rae, Slave and Wyoming cratons of central North America is now possible. Little evidence exists of subducted slab-like geometries similar to modern oceanic lithosphere in these construction histories. Underthrusting and wedging of proto-continental lithosphere is inferred from multiple dipping discontinuities, emphasizing the role of lateral accretion. Archean continental building blocks may resemble the modern lithosphere of oceanic plateau, but they better match the sort of refractory crust expected to have formed at Archean ocean spreading centres. Radiometric dating of mantle xenoliths provides estimates of rock types and ages at depth beneath sparse kimberlite occurrences, and these ages can be correlated to surface rocks. The 3.6-2.6 Ga Rae, Slave and Wyoming cratons stabilized during a granitic bloom at 2.61-2.55 Ga. This stabilization probably represents the final differentiation of early crust into a relatively homogeneous, uniformly thin (35-42 km), tonalite-trondhjemite-granodiorite crust with pyroxenite layers near the Moho atop depleted lithospheric mantle. Peak thermo-tectonic events at 1.86-1.7 Ga broadly metasomatized, mineralized and recrystallized mantle and lower crustal rocks, apparently making mantle peridotite more 'fertile' and more conductive by introducing or concentrating sulfides or graphite at 80-120 km depths. This metasomatism may have also weakened the lithosphere or made it more susceptible to tectonic or chemical erosion. Late Cretaceous flattening of Farallon lithosphere that included the Shatsky Rise conjugate appears to have weakened, eroded and displaced the base of the Wyoming craton below 140-160 km. This process replaced the old refertilized continental mantle with relatively young depleted oceanic mantle.
Rocky Mountain Geology
Northeast-striking tectonic provinces and boundaries were established during 1.8-1.6-Ga assembly ... more 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
Agu Fall Meeting Abstracts, Dec 1, 2002
From a geophysical point of view, the creation and segregation of mantle basaltic melt is importa... more From a geophysical point of view, the creation and segregation of mantle basaltic melt is important seismically and geodynamically. Seismically, small amounts of partial melt reduces P and (especially) S velocities by great amounts (relative to common belief). Thus imaged velocity variations at depths of 100-200 km may be caused by small (<1%) melt fractions. Beneath the Yellowstone swell, we have seismic evidence for partial melt to about 200 km depth under the hotspot track, and basalt-depleted residuum under the swell away from the track. We also have seismic evidence for large volumes of basalt emplaced at the base of the crust and in the mid crust beneath the hotspot track. Beneath the southern Rocky Mountains we have seismic evidence for partial melt extending to about 200 km depth within what is thought to be Precambrian North America lithosphere modified during the Laramide orogeny. Geodynamically, the residuum created by basalt removal can have important geodynamic effects. In particular, because the residuum is relatively buoyant and viscous, it has the long-term effect of making and stabilizing the lithosphere, causing uplift, and controlling asthenosphere flow (e.g., the location of upwelling and hence melting). All these effects are thought to be represented in Yellowstone swell structure and behavior.
We assembled teleseismic data for the Yellowstone hotspot representing a &amp;amp;amp;amp;gt;... more We assembled teleseismic data for the Yellowstone hotspot representing a &amp;amp;amp;amp;gt;1,000 km aperture array. These data combined with finite-frequency (&amp;amp;amp;quot;banana-doughnut&amp;amp;amp;quot;) tomography allowed resolution to the transition zone. The PASSCAL Yellowstone data was augmented with temporary deployments and regional arrays in Utah, Colorado, Montana, Idaho and Utah. In net, these data represent more than 200 seismometers running approximately one year. The
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Papers by Eugene Humphreys