Kenneth L. Cook

The Problem of the Mantle-Crust Mix: Lateral Inhomogeneity in the Uppermost Part of the Earth's Mantle*†

Publisher Summary The material with compressional wave velocities of 7.4 to 7.7 km/sec, existing beneath many active tectonic belts, is postulated to be a mixture of mantle- and crustal-type rocks. The belts with such velocities include mid-ocean ridges, island arcs, and rift valleys. The uplift and lateral extension, as well as the high heat flow and volcanism generally characteristic of the active belts, indicate expansion and an upwelling of convection currents in the mantle, thus giving lateral inhomogeneities in the uppermost mantle. The mantle-crust mix is postulated to comprise a mixture of eclogite and basalt in the phase-transformation zone, which may be of considerable thickness. This chapter discusses some of the evidence to date for the possible lateral inhomogeneity of the Earths uppermost mantle and gives possible explanations of this inhomogeneity. The topic is speculative and much more data is needed before firm conclusions can be reached. It is hoped, however, that this chapter may stimulate more thought and discussion—and, above all, the gathering of more experimental data —on the subject.

A summary of the geology, geochemistry, and geophysics of the Roosevelt Hot Springs thermal area, Utah

The Roosevelt Hot Springs thermal area is a newly discovered geothermal power prospect in Utah. Seven production wells have been drilled with a maximum per well flow capability averaging 4.5×105kg of combined vapor and liquid per hour at a shut‐in bottom hole temperature near 260°C. The thermal area is located on the western margin of the Mineral Mountains, which consist dominantly of a Tertiary granitic pluton 32 km long by 8 km wide. Rhyolitic tuffs, flows, and domes cover about 25km2 of the crest and west side of the Mineral Mountains within 5 km of the thermal area. The rhyolitic volcanism occurred between 0.8 and 0.5 m.y. ago and constitutes a major Pleistocene thermal event believed to be significant to the evaluation of the Roosevelt Hot Springs thermal area. Thermal waters of the (now) dry spring, a seep, and the deep reservoir are dilute (ionic strength 0.1 to 0.2) sodium chloride brines.

Interpretation of resistivity data over filled sinks

Solutions of Laplace’s equation in prolate and oblate spheroidal coordinates are applied to problems that arise in resistivity surveys over filled hemispheroidal sinks. Comprehensive sets of theoretical curves are presented for both horizontal and vertical profiles in the vicinity of filled sinks. The effectiveness of these theoretical curves is demonstrated not only for the interpretation of resistivity data but also for the planning of proper field techniques in resistivity surveying over such sinks. Excellent correlation between theoretical and observed field resistivity curves is shown over a shale sink in the Tri‐State lead‐zinc mining district, near Joplin, Missouri. It is shown that a filled sink can be approximated in its resistivity edge effects by 1) a vertical dike if the width of the sink is small in comparison with its length and its depth; and 2) a vertical fault if the sink is large in comparison with the electrode separation. A study of the Lee and Wenner configurations indicates that the ...

Active rift system in the basin and range province

Abstract The active rift system along the postulated landward extension of the East Pacific Rise into western North America (Heezen, 1960; Menard, 1960; Cook, 1961) consists of several active rift zones which follow belts of seismicity and probable high heat flow and include: (a)the belt of grabens along the eastern margin of the Sierra Nevada, the Walker Lane, and the Churchill Arc; (b)the system of great trenches extending through western Arizona, central Utah, southeastern Idaho, western Wyoming, western Montana, and British Columbia; and (c)the Rio Grande rift belt. The present paper reviews the following additional recent geophysical evidence that defines in more detail the active rift system in the eastern part of the Basin and Range province: 1. (1) In north central Utah, gravity surveys, together with refraction and/ or reflection seismic data along some profiles, indicate that the Wasatch Trench is a northward-trending belt of grabens about 160 miles long and up to 13 miles wide. 2. (2) In south central and southwestern Utah, gravity surveys indicate grabens adjacent to the northward-trending Sevier and Hurricane fault zones, which lie along the rift belt. 3. (3) In New Mexico, several gravity profiles across the Rio Grande rift belt indicate the width and depth of the grabens. 4. (4) Sparse data tentatively indicate high heat flow in the rift belts. The possible mechanism of the Late Cenozoic vulcanism and diastrophism in the Basin and Range province is explained by a postulated mantle convection current (associated with the formation of the Rise) that rose beneath the eastern part of the Basin and Range province (or perhaps the Colorado Plateau) and flowed westward (Cook, 1961). The active rift system is in the region of tension over the upwelling convection currents. To the west, in the region of translation and compression, it is postulated that the westward-moving mantle convection current dragged along the continental crust in the California region. The crustal drag caused a large continental-type block to move westward into the Pacific Ocean about 50–100 km in the California region between the landward projections of the Mendocino and Murray fracture zones, which were probably major lines of weakness existing before the formation of the East Pacific Rise. On the northern and southern margins of the block, respectively, the movement occurred along transcurrent and/or transform faults in: 1. (1) the Cape Mendocino area (Gorda escarpment). 2. (2) the Garlock fault, Big Pine fault, Channel Islands. Transverse Ranges, and Pinto Mountain fault areas. The westward movement of the block, which occurred along transcurrent and/or transform faults associated with these two fracture zones, probably caused: (a) the folding of the Coast Ranges in California along the frontal (i.e., west) segment of the block and (b) the bending of the San Andreas fault zone in two areas along the southern margin of the block. The right-lateral movement of the San Andreas fault zone is explained by a postulated second, northwestward-trending mantle convection current with a velocity gradient increasing southwest; this movement has apparently moved into the western Basin and Range province recently to cause transcurrent faulting there (Billings, 1960), and a change in the direction of the mantle convection currents there is postulated. The concept that a westward-moving continental-type crustal block has overridden the oceanic crust in the Pacific Ocean and that the driving force probably originated in the upper-mantle spreading beneath the Basin and Range province (or perhaps the Colorado Plateau) is fundamental to the interpretation of the Late Cenozoic tectonics in the southwestern United States. If the United States, along with the North American continent, has drifted westward as a consequence of the sea-floor spreading along the Mid-Atlantic Ridge and overriden the East Pacific Rise, as postulated by Vine (1966), the westward movement of the above-mentioned crustal block would constitute about 50–100 km of horizontal displacement in addition to the “normal” westward drift of the North American continent as a whole.

Regional Gravity Survey of the Northern Great Salt Lake Desert and Adjacent Areas in Utah, Nevada, and Idaho

From 1957 to 1961 a regional gravity survey was made over the northern part of the Great Salt Lake Desert and adjacent areas in Utah, eastern Nevada, and southeastern Idaho. A total of 1040 stations were taken over an area of about 7000 square miles. The results were compiled as a Bouguer gravity anomaly map with a contour interval of 2 mgal. The Bouguer values ranged from a high of about —120 mgal over the outcrop areas to a low of about —196 mgal over the alluvium-covered graben areas. The gravity high over the Raft River Mountains apparently corresponds with the Raft River Mountains anticline. A belt of gravity contours, with a total relief of 15–20 mgal, extending for 40 miles between the Wildcat Hills and the southern part of the Grouse Creek Mountains and beyond, is interpreted provisionally as caused partly by abrupt thickening and/or downwarping of the rocks of late Paleozoic age in a general northwestward direction, perhaps to form a foredeep (part of the Butte-Deep Creek trough of Steele, 1960) south of the Raft River Mountains; however, this anomaly could also be partly caused by overthrusting. Many Basin and Range faults, grabens, and horsts are indicated by the gravity data. In the northern part of the surveyed area, Junction Valley, Upper Raft River valley, and Curlew Valley are indicated to be grabens. The Newfoundland Range, in the northeastern part of the Great Salt Lake Desert, is a horst flanked by a graben on each side. The northwestern margin of the Great Salt Lake Desert comprises a complex pattern of Basin and Range fault blocks, large and small, that lie along a generally northward-trending belt or zone. The Silver Island-Pigeon fault block, which comprises the Silver Island Mountains, the Little Pigeon Mountains, and Pigeon Mountain, forms an elongate, arcuate horst that is flanked by a belt of grabens on the west (Pilot Valley, Lucin, and Grouse Creek grabens) and east (Wendover, Crater Island, Little Pigeon, and Pigeon grabens). A major fault zone is indicated along the east margin of the Pilot Range. The Pilot-Grouse Creek rift belt, at least 90 miles long, extends northward between Pilot Valley and the Upper Raft River valley and constitutes a major lineament in the earth9s crust along which the graben blocks were displaced downward relative to the adjacent mountain blocks. In the southern part of the rift belt, the grabens are separated by blocks (the Lemay and Lucin horsts) that were probably downfaulted relative to the large mountain blocks, but became lodged at intermediate height; in the northern part, however, the blocks (for example the Junction Valley graben), apparently merely broke from the main crustal unit along this belt of weakness. The indicated thickness of the valley fill of Cenozoic age in some of the graben areas ranges up to about 6000 feet.

Simultaneous inversion modeling of gravity and aeromagnetic data applied to a geothermal study in Utah

Aeromagnetic and gravity surveys were conducted in the Black Rock Desert, Utah to assess the geothermal potential of the Meadow‐Hatton Known Geothermal Resource Area (KGRA). The presence of basalt flows less than 1000 yr old and a 400 000 yr old rhyolite dome suggested that a hot intrusive body, which should be detectable in both types of potential field data, may provide the heat source for hot springs in the study area. A simultaneous inversion computer program was developed as part of this study to model these potential field data. The resulting models indicate hydrothermal alteration about the hot springs extending to a depth of approximately 1 km. Normal faults above a low‐angle detachment appear to reach a depth of approximately 4 km and provide a path for the circulation of groundwater in the area. No evidence for a buried igneous body was found in the study area, and it is therefore concluded that the migration of fluids along the deep faults is sufficient to account for the water temperatures est...

Quantitative interpretation of vertical magnetic anomalies over veins

By using ordinary magnetic induction methods of analysis, Haalck, Heiland, and others have developed formulas which express the magnetic anomaly over a vertical or inclined vein of tabular shape as a function of the susceptibility, dimensions, shape, and disposition of the vein, and of the strength and direction of the earth’s magnetic field. On the basis of these fundamental formulas, other formulas for the vertical component of the magnetic field are derived in the present paper for such veins in intermediate northern magnetic latitudes. Special emphasis is given to the orientation of the veins relative to the magnetic north direction. Several families of vertical magnetic intensity curves for veins with different strikes and dips are given. All theoretical curves for veins striking magnetic north are plotted in terms of a parametric unit so that, once plotted, they can be used repeatedly in different districts, provided a proper multiplying factor is chosen for the observed curve. The importance of the...

Electrical properties of synthetic metalliferous ore

Ninety small cores of synthetic metalliferous ore were constructed from solid glass spheres averaging 0.5 mm in diameter, lead spheres 1.0 mm in diameter, and refractory cement. The lead content of the cores varied from zero to 50 percent by frame volume. The effective porosity was controlled by the manufacturing pressure and ranged from 10 to 20 percent. The cores were saturated with NaCl solution. The apparent impedance of the cores was measured with a modified Wheatstone bridge as a function of frequency and current density. The low‐frequency effects of induced polarization were separated from the over‐all decrease of impedance with increase of frequency by taking advantage of the dependence of these effects upon current density. The over‐all decrease of the impedance with frequency and the polarization effects were found dependent upon the effective porosity and the lead content. Both the polarization effects and the over‐all decrease of the impedance with frequency increased with decreasing porosity....

Regional Gravity Survey of the Hurricane Fault Area and Iron Springs District, Utah

During 1963 and early 1964 a regional gravity survey consisting of 660 stations was made along the Hurricane fault area and in the Iron Springs district, Utah, over an area of about 1900 square miles. The complete Bouguer gravity anomaly map (2-mgal contour interval) shows gravity values ranging from about –156 mgal at low elevations near Washington in the south to about –212 mgal at intermediate elevations near Cedar City in the north. The gravity patterns confirm that: (1) the Basin and Range type Hurricane and Washington faults both intersected the north-eastward trending Virgin-Kanarra fold of Laramide age) and (2) the Virgin anticline and the Kanarra fold constituted a single continuous fold prior to transection and dislocation by the Hurricane fault. The gravity anomalies in the Cedar Valley and Escalante Desert areas indicate several other major Basin and Range faults or fault zones that bound grabens and horsts, including the Cedar Valley, Avon, and Lund grabens and the Table Butte horst. An extensive gravity high over the Iron Springs district indicates that the quartz monzonite porphyry forming the three aligned outcropping intrusions (The Three Peaks, Granite Mountain, and Iron Mountain) and the Stoddard Mountain intrusion is probably continuous at relatively shallow depth throughout the district and extends west-southwest from the Stoddard Mountain area. Another gravity high of approximately the same magnitude corresponds with the Antelope Range and may be caused either by an extension at depth of the same quartz monzonite porphyry mass or by a structural block that may exist throughout the Iron Springs district.

SEISMIC VELOCITY STUDY OF SYNTHETIC CORES

The velocity of a longitudinal elastic wave through rock at room temperature and at atmospheric pressure depends upon the nature of the rock frame, the porosity of the rock, and the nature of the pore‐filling fluid. In the present investigation longitudinal elastic wave velocities were measured for sixty synthetic cores. The rock frame consisted of sorted quartz sand grains and cement, the percentage of cement varying from ten to fifty percent. The core porosities varied from 8.8 percent to 22 percent. The velocities were measured for dry air‐filled cores and for cores saturated with various liquids. These pore‐filling liquids were distilled water, ethyl alcohol, benzene, carbon tetrachloride, and chloroform. The measured velocities ranged from 2,360 feet per second to 14,710 feet per second. The wave velocity in liquid‐filled cores of 10 percent porosity was approximately twice the velocity for cores of 20 percent porosity, the same type of cement being used in both instances. For any given core, flooded...