Scott B. Smithson

Seismic reflectivity of mylonite zones in the crust

Deep fault zones are characterized by mylonitic rocks with strong preferred orientation of constituent minerals and retrograde mineral assemblages. Sonic velocities are consequently lower (for propagation directions normal to mylonite foliation) than in surrounding unmylonitized rocks. Synthetic seismograms computed for low-velocity, near-horizontal, thick mylonite zones of complex, laminated geometry show multicyclic reflections with amplitudes up to twice those generated by a single interface. Seismic characteristics of mylonite zones are sufficient to produce reflections in crystalline rocks, provided noise levels are relatively low.

Absorbing boundary conditions and surface waves

One of the major problems in numerically simulating waves traveling in the Earth is that an artificial boundary must be introduced to produce unique solutions. To eliminate the spurious reflections introduced by this artificial boundary, we use a damping expression based on analogies to shock absorbers. This method can reduce the amplitude of the reflected wave to any pre‐specified value and is successful for waves at any angle of incidence. The method can eliminate unwanted reflections from the surface, reflections at the corners of the model, and waves reflected off an interface that strike the artificial boundary. Many of the boundary conditions currently used in the numerical solution of waves are approximations to perfectly absorbing boundary conditions and depend upon the angle of incidence of the incoming wave at the artificial boundary. Stability problems often occur with these boundary conditions. The method we use at the artificial boundary allows use of stable Dirichlet or von Neumann condition...

Nature of the Wind River thrust, Wyoming, from COCORP deep-reflection data and from gravity data

Critical data for the interpretation of Laramide structure, a major tectonic problem bearing on the formation of the Rocky Mountains, have been obtained by the Consortium for Continental Reflection Profiling (COCORP) in the form of deep crustal seismic-reflection profiles. The Wind River Mountains in Wyoming have been uplifted by the Wind River thrust, which can be traced on COCORP seismic-reflection profiles to at least 24-km depth with an average dip of 30° to 35°. This Laramide uplift is thus the result of extensive horizontal compression with a minimum horizontal displacement of 21 km and a minimum vertical displacement of 13 km. The crust appears to have deformed essentially as a rigid plate. Gravity anomalies across the uplift can be modeled by a thrust, with the same geometry as indicated by the seismic-reflection profiles.

Compositional variation and the origin of deep crustal reflections

Abstract Identifying the sources of crustal reflections is essential for deriving more geological information from deep crustal reflection profiles. Theoretical and model studies help place constraints on the role of compositional variation in producing deep crustal reflections. Analysis of laboratory-derived velocities and densities of rock types typical for the continental crust indicates that reflection coefficients are generally small, but significantly, 17% of the possible reflection coefficients have magnitudes between 0.1 and 0.2. Comparison between reflection coefficients derived from well logs and reflections observed in associated seismic profiles indicates that constructive interference associated with geological layering is at least as important as the magnitude of the reflection coefficients for producing detectable reflections. Constructive interference can increase reflection amplitude by two to three times but is limited to a relatively narrow range of layer thickness. For a typical 10–40 Hz seismic wavelet and typical crustal velocities of about 6 km/s, constructive interference occurs for layer thickness ranging between about 35 and 80 m. Layers thinner than 35 m interfere destructively. If reflections result from compositional variation, seismic models of hypothetical and observed geologic relations provide analogs for interpreting complex reflection patterns observed in deep crustal reflection profiles. Such models show reflection patterns similar to those observed in the reflection profiles. The models indicate that the reflections could originate in the complexly deformed and intruded terranes that are common in the crystalline crust and it may not be necessary to appeal to unobserved phenomena such as special lamellae or fluid-filled fractures to explain the reflections.

Seismic evidence of mylonite reflectivity and deep structure in the Kettle dome metamorphic core complex, Washington

The first direct test of the seismic reflectivity of mylonites is provided by a seismic reflection profile over a mylonite zone exposed in the Kettle dome metamorphic core complex, northeastern Washington. The mylonite zone developed at mid-crustal levels and has undergone a minimum of retrogression and cataclasis during uplift. Downdip projection of the exposed mylonite zone into the seismic profile indicates that the compositionally layered mylonite zone is reflective and traceable to at least 5-km depth as a complex zone of multicyclic reflections. The seismic data also show that the crust beneath the Kettle dome is characterized by abundant, strongly reflective, moderately dipping layers down to at least 18 km. These layers can be interpreted in terms of compressional or extensional orogenic events or a combination of both. Models for the structure and genesis of metamorphic core complexes must take these data into account.

A model for lower continental crust

Abstract This proposed model is based on geological, geophysical and geochemical data. Previous models suggested for the lower continental crust consisted of basalt, gabbro, or charnockitic rocks; however, experimental and field petrological data indicate that the bulk of crustal rocks are metamorphic. A lower crust of heterogeneous metamorphic rocks also agrees with seismic reflection results which show numerous reflections from “layering”. Geothermal conditions favor a “dry” charnockitic or gabbroic lower crust rather than an amphibolitic lower crust because heat production data imply that wet amphibolitic rocks would have a higher heat production than their dry metamorphic equivalents. Relatively high velocities from field and laboratory measurements in such low-density rocks as granite, syenite, anorthosite and granulitic rocks in general imply that the composition of the lower crust is more felsic than gabbro. Variation in seismic velocity and depths from crustal refraction studies and numerous seismic reflections all indicate a highly heterogeneous lower crust. The lower crust, which has traditionally been described as gabbroic or mafic, may consist of such diverse rocks as granite gneiss, syenite gneiss, anorthosite, pyroxene granulite, and amphibolite, interlayered on a small scale, deformed, and intruded by granite and gabbro. Interlayering of these rocks explains the presence and character of seismic reflections. Abrupt changes in dip, tight folding, disruption of layers, intrusion, and changes in layer thickness explain the characteristic discontinuity of deep reflections. Igneous intrusions may be floored by metamorphic rocks. The lower crust consists of a complex series of igneous and metamorphic rock of approximate intermediate composition.

A continental crustal model and its geothermal implications

Abstract The following crustal model based on realistic estimates of metamorphic rock volumes and H 2 O content is proposed as a basis for geothermal calculations: (1) a surface zone of intermediate metamorphic rocks containing granitic intrusions and grading downward into (2) a more felsic migmatite zone, (3) a lower crustal zone of approximately andesitic composition crystallized in granulite or possibly amphibolite facies. Heat production values and thickness for the three zones are 3 HGU, 5 HGU, 0.5–1.5 HGU and 8, 8, and 18 km respectively. If the surface heat flow is 1.2 HFU, the model predicts a temperature of only 407°C at the Moho and an upper mantle heat flow of 0.3–0.5 HFU. The low temperatures resulting from this model rule out a seismic low-velocity zone in the crust produced by thermal effects.

Heterogeneity of the uppermost mantle beneath Russian Eurasia from the ultra-long-range profile QUARTZ

The 3850-km long Deep Seismic Sounding profile QUARTZ crosses six major geologic provinces in Eurasia and is sourced by 3 nuclear and 48 chemical explosions. We present the first interpretation of the entire data set, using two dimensional (2-D) ray tracing and inversion, resolution analysis, and 1-D amplitude modeling. Our interpretation shows a 42-km-thick, high-velocity crust under the Baltic Shield, a 29-km-thick crust and high-velocity upper mantle under the Mezenskaya depression, 52-km-thick crust with high-velocity lower crust and uppermost mantle under the Urals, and 40-km-thick crust under the West Siberian basin deepening to 45 km under the Altay-Sayan fold belt. High-velocity (8.4 km/s) uppermost mantle is found under the Mezenskaya depression and under the east flank of the Urals. One almost continuous upper mantle boundary occurs at 65- to 80-km depth, and another with an approximately 40-km-thick LVZ occurs at 120- to 140-km depth. The shallow upper mantle blocks and the two extensive interfaces indicate strong upper mantle heterogeneity. Resolution analysis based on direct multivariate model perturbations, artificial neural network and principal component analysis, indicate the depth uncertainty of the 410-km discontinuity within ±6 km, and also its trade-off with dip and velocities above and below the discontinuity. Decreased near-critical amplitudes of reflections from the 410-km and 660-km discontinuities indicate that these boundaries are most likely represented by gradient zones about 15-20 km thick. Lithosphere thins, asthenospheric velocity decreases, and the 410-km discontinuity dips to the SE approaching the Himalayan orogenic belt.

The Laramide orogeny: Evidence from cocorp deep crustal seismic profiles in the wind river mountains, wyoming

Abstract Deep crustal reflection data that are critical for the interpretation of Laramide structure have been obtained by the Consortium for Continental Reflection Profiling (COCORP). The Laramide orogeny, which occurred from the late Cretaceous to early Eocene, is characterized in Wyoming by large uplifts of Precambrian basement, commonly flanked by reverse faults. The attitude of these faults at depth has been a major tectonic problem and is very important for deciding whether horizontal or vertical crustal movements were primarily responsible for the basement uplifts. COCORP has run 158 km of deep seismic reflection profiles (recording to 20-sec two-way travel time) across the southeastern end of the Wind River Mountains, the largest of these Laramide uplifts. Reflections from the thrust fault flanking the Wind River uplift can be clearly traced on the profiles to at least 24-km depth and possibly as deep as about 36 km with a fairly uniform apparent dip of 30°–35°. Other reflection events subparallel to the main Wind River thrust are present in the seismic profiles and may represent other faults. There is at least 21 km of crustal shortening along the thrust. There is no evidence in the reflection profiles for large-scale folding of the basement; the Wind River Mountains were formed predominantly by thrust movements. Gravity anomalies in the Wind River Mountains can be modeled by a thrust that displaces dense material in the lower crust. If the thrust ever cut the Moho, the effect is not observed in the gravity today. A proposed model for the presence of uplifted basement in Wyoming invokes a shallowly dipping, subducted Farallon plate beneath the North American continent; drag between the two plates localized compressional stresses in an area over 800 km into the North American plate causing large thrusts to develop. The earths crust seems to have fractured as a fairly rigid plate

Gravity Investigations of Subsurface Shape and Mass Distributions of Granite Batholiths

Because granitic plutons usually show negative density contrasts with country rocks, they cause moderate to large negative gravity anomalies whose interpretation yields quantitative estimates of subsurface shape. Gravity anomalies suggest that many postkinematic granites extend to depths of about 10 km, which is one third the average continental crustal thickness, whereas some are much thinner. Granite contacts characteristically slope outward, and some granites show strong internal variations in density. Some granites, such as the Mull and Skye granites of Great Britain, are associated with large mafic plutons. Observable structural accommodation appears to be too small to explain emplacement of many large, thick postkinematic granites primarily by forcible intrusion. Stoping may, therefore, be the effective mechanism. Gravity profiles observed across granite batholiths are compared with computed profiles across a large number of hypothetical mass distributions. Negative gravity anomalies show that mass has been removed during the emplacement of most granitic plutons and indicate that the displaced mass of country rocks is neither immediately beneath the pluton nor around its margins. The gravitational effect of displaced country rocks is not apparent in observed gravity profiles. This creates a serious problem for emplacement by magmatic sloping or by granitization which may be overcome: (1) if the displaced material has sunk to great depths, (2) if it is dispersed during emplacement, or (3) if its gravitational effect is negated by a low-density root beneath the granites. Magma could originate: (1) in a granitic layer in the crust, (2) by partial fusion of the lower crust, or (3) in the mantle. Gravity profiles are computed for mass distributions which might represent each of these hypotheses, and the hypotheses are evaluated on the basis of the gravity profile. Gravity interpretation demonstrates that some areally large granitic plutons are thick (Dartmoor Granite), that some large plutons are thin (Fla Granite), and that some plutons are associated with large volumes of mafic rock (Skye granites). Gravity interpretation is a necessary approach to petrologic studies and can lead to a quantitative petrologic interpretation.