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At least 73 records · Page 4Linked to original sources
USGS1979

Recent crustal movements in the Sierra Nevada-Walker lane region of California-Nevada: Part i, rate and style of deformation

This review of geological, seismological, geochronological and paleobotanical data is made to compare historic and geologic rates and styles of deformation of the Sierra Nevada and western Basin and Range Provinces. The main uplift of this region began about 17 m.y. ago, with slow uplift of the central Sierra Nevada summit region at rates estimated at about 0.012 mm/yr and of western Basin and Range Province at about 0.01 mm/yr. Many Mesozoic faults of the Foothills fault system were reactivated with normal slip in mid-Tertiary time and have continued to be active with slow slip rates. Sparse data indicate acceleration of rates of uplift and faulting during the Late Cenozoic. The Basin and Range faulting appears to have extended westward during this period with a reduction in width of the Sierra Nevada. The eastern boundary zone of the Sierra Nevada has an irregular en-echelon pattern of normal and right-oblique faults. The area between the Sierra Nevada and the Walker Lane is a complex zone of irregular patterns of ho??rst and graben blocks and conjugate normal-to right- and left-slip faults of NW and NE trend, respectively. The Walker Lane has at least five main strands near Walker Lake, with total right-slip separation estimated at 48 km. The NE-trending left-slip faults are much shorter than the Walker Lane fault zone and have maximum separations of no more than a few kilometers. Examples include the 1948 and 1966 fault zone northeast of Truckee, California, the Olinghouse fault (Part III) and possibly the almost 200-km-long Carson Lineament. Historic geologic evidence of faulting, seismologic evidence for focal mechanisms, geodetic measurements and strain measurements confirm continued regional uplift and tilting of the Sierra Nevada, with minor internal local faulting and deformation, smaller uplift of the western Basin and Range Province, conjugate focal mechanisms for faults of diverse orientations and types, and a NS to NE-SW compression axis (??1) and an EW to NW-SE extension axis (??3). ?? 1979.

Tectonophysics↗
USGS1979

Strain pattern represented by scarps formed during the earthquakes of October 2, 1915, Pleasant Valley, Nevada

The pattern of scarps developed during the earthquakes of October 2, 1915, in Pleasant Valley, Nevada, may have formed as a result of a modern stress system acting on a set of fractures produced by an earlier stress system which was oriented differently. Four major scarps developed in a right-stepping, en-echelon pattern suggestive of left-lateral slip across the zone and an extension axis oriented approximately S85°W. The trend of the zone is N25°E. However, the orientation of simple dip-slip on most segments trending approximately N20—40° E and a right-lateral component of displacement on several N- and NW-trending segments of the scarps indicate that the axis of regional extension was oriented between N50° and 70° W, normal to the zone. The cumulative length of the scarps is 60 km, average vertical displacement 2 m, and the maximum vertical displacement near the Pearce School site 5.8 m. Almost everywhere the 1915 scarps formed along an older scarp line, and in some places older scarps represent multiple previous events. The most recent displacement event prior to 1915 is interpreted to have occurred more than 6600 years ago, but possibly less than 20,000 years ago. Some faults expressed by older scarps that trend northwest were not reactivated in 1915, possibly because they are oriented at a low angle with respect to the axis of modern regional extension. The 1915 event occurred in an area of overlap of three regional fault trends oriented northwest, north, and northeast and referred to, respectively, as the Oregon—Nevada, Northwest Nevada, and Midas—Battle Moutain trends. Each of these trends may have developed at a different time; the Oregon—Nevada trend was possibly the earliest and developed in Late Miocene time (Stewart et al. 1975). Segments of the 1915 scarps ar

Nevada↗
USGS1979

Fault-crossing P delays, epicentral biasing, and fault behavior in central California

The P delays across the San Andreas fault zone in central California have been determined from travel-time differences at station pairs spanning the fault, using off-fault local earthquake or quarry blast sources. Systematic delays as large as 0.4 sec have been observed for paths crossing the fault at depths of 5-10 km. These delays can account for the apparent deviation of epicenters from the mapped fault trace. The largest delays occur along the San Andreas fault between San Juan Bautista and Bear Valley and Between Bitterwater Valley and Parkfield. Spatial variations in fault behavior correlate with the magnitude of the fault-crossing P delay. The delay decreases to the northwest of San Juan Bautista across the "locked" section of the San Andreas fault and also decreases to the southeast approaching Parkfield. Where the delay is large, seismicity is relatively high and the fault is creeping.

California↗
USGS1979

Earthquake recurrence on the Calaveras fault east of San Jose, California

Occurrence of small (3 ⩽ M L < 4) earthquakes on two 10-km segments of the Calaveras fault between Calaveras and Anderson reservoirs follows a simple linear pattern of elastic strain accumulation and release. The centers of these independent patches of earthquake activity are 20 km apart. Each region is characterized by a constant rate of seismic slip as computed from earthquake magnitudes, and is assumed to be an isolated locked patch on a creeping fault surface. By calculating seismic slip rates and the amount of seismic slip since the time of the last significant ( M ⩾ 3) earthquake, it is possible to estimate the most likely date of the next ( M ⩾- 3) event on each patch. The larger the last significant event, the longer the time until the next one. The recurrence time also appears to be increased according to the moment of smaller (2 < M L < 3) events in the interim. The anticipated times of future larger events on each patch, on the basis of preliminary location data through May 1977 and estimates of interim activity, are tabulated below with standard errors. The occurrence time for the southern zone is based on eight recurrent events since 1969, the northern zone on only three. The 95% confidence limits can be estimated as twice the standard error of the projected least-squares line. Events of M ⩾ 3 should not occur in the specified zones at times outside these limits. The central region between the two zones was the locus of two events ( M = 3.6, 3.3) on July 3, 1977. These events occurred prior to a window based on the three point, post-1969 slip-time line for the central region.

California↗
USGS1979

Earthquakes and fault creep on the northern San Andreas fault

At present there is an absence of both fault creep and small earthquakes on the northern San Andreas fault, which had a magnitude 8 earthquake with 5 m of slip in 1906. The fault has apparently been dormant after the 1906 earthquake. One possibility is that the fault is ‘locked’ in some way and only produces great earthquakes. An alternative possibility, presented here, is that the lack of current activity on the northern San Andreas fault is because of a lack of sufficient elastic strain after the 1906 earthquake. This is indicated by geodetic measurements at Fort Ross in 1874, 1906 (post-earthquake), and 1969, which show that the strain accumulation in 1969 (69 · 10 −6 engineering strain) was only about one-third of the strain release (rebound) in the 1906 earthquake (200 · 10 −6 engineering strain). The large difference in seismicity before and after 1906, with many strong local earthquakes from 1836 to 1906, but only a few strong earthquakes from 1906 to 1976, also indicates a difference of elastic strain. The geologic characteristics (serpentine, fault straightness) of most of the northern San Andreas fault are very similar to the characteristics of the fault south of Hollister, where fault creep is occurring. Thus, the current absence of fault creep on the northern fault segment is probably due to a lack of sufficient elastic strain at the present time.

California↗
USGS1979

Aging and strain softening model for episodic faulting

Episodic slip on shallow crustal faults can be qualitatively explained by postulating a fault constitutive law that is the superposition of two limiting material responses: (1) strain softening after peak stress during large strain rates, and (2) strength (peak stress) recovery during aging at small strain rates. A single law permits a variety of seismic and aseismic phenomena to occur over a range of space and time scales. Specific cases are determined by the spatial variation of material constants, recent deformation history, crustal rigidity, and remote forcing.

Tectonophysics↗
USGS1979

Monitoring massive fracture growth at 2-km depths using surface tiltmeter arrays

Tilt due to massive hydraulic fractures induced in sedimentary rocks at depths of up to 2.2 km have been recorded by surface tiltmeters. Injection of fluid volumes up to 4 · 10 5 liters and masses of propping agent up to 5 · 10 5 kg is designed to produce fractures approximately 1 km long, 50–100 m high and about 1 cm wide. The surface tilt data adequately fit a dislocation model of a tensional fault in a half-space. Theoretical and observational results indicate that maximum tilt occurs at a distance off the strike of the fracture equivalent to 0.4 of the depth to the fracture. Azimuth and extent of the fracture deduced from the geometry of the tilt field agree with other kinds of geophysical measurements. Detailed correlation of the tilt signatures with pumping parameters (pressure, rate, volume, mass) have provided details on asymmetry in geometry and growth rate. Whereas amplitude variations in tilt vary inversely with the square of the depth, changes in flow rate or pressure gradient can produce a cubic change in width. These studies offer a large-scale experimental approach to the study of problems involving fracturing, mass transport, and dilatancy processes.

Tectonophysics↗
USGS1979

Late Cenozoic uplift of the southwestern Colorado Plateau and adjacent lower Colorado River region

Rocks deposited near sea level under marine, estuarine, and lacustrine conditions, and located along the course of the lower Colorado River from the mouth of the Grand Canyon as far as the Mexican border, have been displaced to present positions as high as 880 m a.s.l. and as low as 1600 m b.s.l. The rocks include the marine and estuarine Bouse Formation and the lacustrine or marine Hualapai Limestone Member of the Muddy Creek Formation. A profile joining spot elevations that represent the highest erosional remnants of these rocks preserved at any one locality gives an approximation (in most cases a minimum value) for the uplift or downdropping of the region relative to sea level since about 5.5 m.y. ago, the K/Ar age of the most widespread and critical unit. The profile shows that most of the lower Colorado region has risen at least 550 m through broad and rather uniform upwarping and at an average rate of about 100 m/m.y. In addition to these 550 m, the nearby Colorado Plateau has risen by discrete movement along Wheeler fault, which is parallel to and about 8 km west of the plateau's edge, to a total uplift of at least 880 m, at a rate that may be as high as 160 m/m.y. Before warping and faulting, the top of the plateau was about 1100 m above the fill of adjacent basins; the top of this fill probably was at or a little below sea level. The profile shows two major south-facing rises in slope. The bigger one, near Yuma, occurs where the profile intersects the northwest-trending San Andreas-Salton trough system of faults; it is interpreted as rifting resulting from transcurrent movement along the faults. At the Mexican border, the base of the Bouse Formation is 1600 m b.s.l., which corresponds to a rate of subsidence since the beginning of Bouse time that may be as high as 290 m/.m.y. The top of the Bouse is at 1000 m b.s.l., corresponding to a rate of subsidence of about 180 m/m.y. In this area, the “older marine sedimentary rocks” of Olmsted et al., (1973) occur as much as 2100 m b.s.l. These rocks are evidence for marine invasion and deformation that predate the Bouse Formation. The second rise in slope occurs at the intersection with a regional northwest-trending lineament that separates widely different geologic terranes and is associated with extensive Miocene volcanism. This lineament may represent an old transform fault similar in geologic function to the present San Andreas system.

Arizona, California, Colorado, Nevada↗
USGS1979

The petrogenesis and trace-element geochemistry of intermediate lavas from Humphreys Peak, San Francisco volcanic field, Arizona

The San Francisco Mountain lavas exposed in the upper portion of the southeast slope of Humphreys Peak are composed of three petrographically distinct types: (1) a lower series of hornblende pyroxene andesites; (2) a group of hypersthene dacites; and (3) an upper series of olivine andesites. These rocks have been shown to be related by crystallization differentiation through comparison of groundmass and bulk rock-chemical data. The major and trace elements from rocks representative of the volcanic field form continuous trends, an indication of differentiation rather than separate and discrete magma sources. Xenoliths found in the volcanic field are composed of the proper major and trace element contents to allow their extraction from a primary melt to form residual melts of more silicic lavas. The alkali olivine basalts, probably formed by partial melting of the mantle at a depth of 35-60 km, may well have differentiated to the alkali-rich highalumina basalts at depths of 15-35 km (the lower crust of the Colorado Plateau) by removal of olivine and clinopyroxene. Some of both lava types erupted periodically onto the surface while others continued to differentiate. Removal of plagioclase, with more minor amounts of olivine and pyroxenes (of less mafic composition than those above), from the high-alumina basalts eventually may have resulted in magmas similar in composition to the intermediate and more silicic rocks. The intermediate and silicic magmas contain hornblende and probably formed between 5 and 8 km. This places a minimum depth of penetration by the normal faults, such as the Mesa Butte fault, along which many silicic centers are aligned, at a depth of 5-8 km. Because of the continuous linear trends of the chemical data and the repetition over the past 6 m.y. of most of the rock types, the mantle beneath this southwestern margin of the Colorado Plateau apparently has not changed significantly during the past 6 m.y., nor is it likely to be very heterogeneous.

Arizona↗
USGS1979

On the mechanical interaction between a fluid-filled fracture and the earth's surface

The mechanical interaction between a fluid-filled fracture (e.g., hydraulic fracture joint, or igneous dike) and the earth's surface is analyzed using a two-dimensional elastic solution for a slit of arbitrary inclination buried beneath a horizontal free surface and subjected to an arbitrary pressure distribution. The solution is obtained by iteratively superimposing two fundamental sets of analytical solutions. For uniform internal pressure the slit behaves essentially as if it were in an infinite region if the depth-to-center is three times greater than the half-length. For shallower slits interaction with the free surface is pronounced: stresses and displacements near the slit differ by more than 10% from values for the deeply buried slit. The following changes are noted as the depth-to-center decreases: 1. (1) the mode I stress intensity factor increases for both ends of the slit, but more rapidly at the upper end; 2. (2) the mode II stress-intensity factor is significantly different from zero (except for vertical slits) suggesting propagation out of the original plane of the slit; 3. (3) displacements of the slit wall are asymmetric such that the slit gaps open more widely near the upper end. Similar changes are noted if fluid density creates a linear pressure gradient that is smaller than the lithostatic gradient. Under such conditions natural fractures should propagate preferentially upward toward the earth's surface requiring less pressure as they grow in length. If deformation near the surface is of interest, the model should account explicitly for the free surface. Stresses and displacements at the free surface are not approximated very well by values calculated along a line in an infinite region, even when the slit is far from the line. As depth-to-center of a shallow pressurized slit decreases, the following changes are noted: 1. (1) displacements of the free surface increase to the same order of magnitude as the displacements of the slit walls, 2. (2) tensile stresses of magnitude greater than the pressure in the slit are concentrated along the free surface. The relative surface displacements over a shallow vertical slit are downward over the slit and upward to both sides of this area. The tensile stress acting parallel to the free surface over a shallow vertical slit is concentrated in two maxima adjacent to a point of very low stress immediately over the slit. The solution is used to estimate the length-to-depth ratio at which igneous sills have gained sufficient leverage on overlying strata to bend these strata upward and form a laccolith. The pronounced mode II stress intensity associated with shallow horizontal slits explains the tendency for some sills to climb to higher stratigraphie horizons as they grow in length. The bimodal tensile stress concentration over shallow vertical slits correlates qualitatively with the distribution of cracks and normal faults which flank fissure eruptions on volcanoes. The solution may be used to analyze surface displacements and tilts over massive hydraulic fractures in oil fields and to understand the behavior of hydraulic fractures in granite quarries.

Tectonophysics↗
USGS1980

Folding and faulting of strain-hardening sedimentary rocks

The question of whether single- or multi-layers of sedimentary rocks will fault or fold when subjected to layer-parallel shortening is investigated by means of the theory of elastic-plastic, strain-hardening materials, which should closely describe the properties of sedimentary rocks at high levels in the Earth's crust. The most attractive feature of the theory is that folding and faulting, intimately related in nature, are different responses of the same idealized material to different conditions. When single-layers of sedimentary rock behave much as strain-hardening materials they are unlikely to fold, rather they tend to fault, because contrasts in elasticity and strength properties of sedimentary rocks are low. Amplifications of folds in such materials are negligible whether contacts between layer and media are bonded or free to slip for single layers of dolomite, limestone, sandstone, or siltstone in media of shale. Multilayers of these same rocks fault rather than fold if contacts are bonded, but they fold readily if contacts between layers are frictionless, or have low yield strengths, for example due to high pore-water pressure. Faults may accompany the folds, occurring where compression is increased in cores of folds. Where there is predominant reverse faulting in sedimentary sequences, there probably were few structural units.

California↗
USGS1980

Local magnetic field measurements and fault creep observations on the San Andreas fault

Simultaneous creep and magnetic field records have been obtained for more than 60 episodic creep events since early 1974, no clear magnetic transients or offsets, as suggested by Breiner and Kovach (1968), are observed at or up to several days before the occurrence times of these events. Although some patterns of creep onset times at adjacent stations over periods of weeks to months appear to correspond to some periods of longer term change in local magnetic field, these changes do not always occur and other groups of creep events have no corresponding changes in local magnetic field. Changes in stress related to the surface expression of episodic fault creep on the San Andreas fault can be estimated from dislocation models fit to observations of simultaneous strains and tilts at points near the fault. These stress values are generally less than 1 bar. For these stress levels and with the apparent limited extent of surface failure, tectonomagnetic models of creep events indicate that simultaneous observations of related magnetic field variations at detectable levels of a gamma or so are unlikely. Slip at greater depth may occur more smoothly and would load the near-surface material to failure. These data also argue against large-scale dilatant cracking occurring along the region of the fault presently monitored.

California↗
USGS1980

First-order analysis of deformation of a thrust sheet moving over a ramp

John L. Rich introduced the revolutionary concept that many folds in the Appalachian Mountains can be explained as superficial structures formed by passive translation of thrust blocks over ramps in detachment surfaces. The amount of layer-parallel shortening can be negligible in the formation of these folds. Rich primarily was concerned with an explanation for the Powell Valley anticline, in the southern Appalachians, but the essential kinematic features of his model of folding have been verified in other folds in the Appalachians, in the Canadian Rockies, in the Idaho-Wyoming thrust belt, and in the Pyrenees. In this paper we solve the boundary-value problem for an idealized thrust block moving over a detachment surface and ramp with zero drag, and produce theoretical fold forms in the thrust block that closely resemble those in Rich's idealized model. The anticline is narrow and rounded if the translation is small, and broad and flat-topped if the translation is large. The limbs of the anticline are symmetric. We also incorporate drag along the ramp part of the detachment surface in order to derive a possible explanation for the asymmetry of dips of the two limbs of the Powell Valley anticline. We show that drag can explain the asymmetry, particularly if drag between relatively competent rocks in opposition at the ramp caused an initial anticline to form as the thrust block began to move, and then drag reduced markedly as relatively soft shales at the base of the block were thrust over competent rocks in the ramp. The existence of the initial anticline should be reflected in asymmetry of the two limbs and in a bulge at the distal edge of the broad anticline.

eastern United States, southern Appalachian Mounta↗
USGS1981

Contemporary doming of the Adirondack Mountains: Further evidence from releveling

The Adirondack Mountains constitute an anomalously large, domical uplift on the Appalachian foreland. The dome has a NNE—SSW axis about 190 km long, and an east—west dimension of about 140 km. It has a structural relief of at least 1600 m, and a local topographic relief of up to 1200 m. First-order leveling in 1955, and again in 1973 along a north—south line at the eastern margin of the Adirondack shows an uplift rate of 2.2 mm/yr at the latitude of the center of the dome and a subsidence rate of 2.8 mm/yr at the northern end of the line near the Canadian border. The net amount of arching along this releveled line is 9 cm ± 2 cm (Isachsen, 1975). To test the idea that this arching represented an “edge effect” of contemporary doming of the Adirondacks as a whole, the National Geodetic Survey was encouraged to relevel a 1931 north—south line between Utica and Fort Covington (near the Canadian border) which crosses the center of the dome. The releveling showed that the mountain mass is undergoing contemporary domical uplift at a rate which reaches 3.7 mm/yr near the center of the dome (compare with 1 mm/yr for the Swiss Alps). Three other releveled lines in the area support this conclusion.

New York↗
USGS1981

Gravitational potential as a source of earthquake energy

Some degree of tectonic stress within the earth originates from gravity acting upon density structures. The work performed by this "gravitational tectonics stress" must have formerly existed as gravitational potential energy contained in the stress-causing density structure. According to the elastic rebound theory (Reid, 1910), the energy of earthquakes comes from an elastic strain field built up by fairly continuous elastic deformation in the period between events. For earthquakes resulting from gravitational tectonic stress, the elastic rebound theory requires the transfer of energy from the gravitational potential of the density structures into an elastic strain field prior to the event. An alternate theory involves partial gravitational collapse of the stress-causing density structures. The earthquake energy comes directly from a net decrease in gravitational potential energy. The gravitational potential energy released at the time of the earthquake is split between the energy released by the earthquake, including work done in the fault zone and an increase in stored elastic strain energy. The stress associated with this elastic strain field should oppose further fault slip.

Alaska↗
USGS1981

Monoclinal bending of strata over laccolithic intrusions

Sedimentary strata on top of some laccolithic intrusions are nearly horizontal and little deformed, but are bent into steeply dipping monoclinal flexures over the peripheries of these intrusions. This form of bending is not explained by previous theories of laccolithic intrusion, which predict either horizontal undeformed strata over the center and faulted strata around the periphery, or strata bent continuously into a dome. However, a slight generalization of these theories accomodates the observed form and contains the previous forms as special cases. A critical assumption is that the strength of contacts within a multilayered overburden is overcome locally by layer-parallel shear. If this strength is less than the strength of the layers themselves, then layers over the center remain bonded together and display negligible bending, whereas layers over the periphery slip over one another and are readily bent into a monoclinal flexure.

Montana, Utah↗
USGS1981

Composition of the earth's upper mantle-I. Siderophile trace elements in ultramafic nodules

Seven siderophile elements (Au, Ge, Ir, Ni, Pd, Os, Re) were determined by radiochemical neutron activation analysis in 19 ultramafic rocks, which are spinel lherzollites-xenoliths from North and Central America, Hawaii and Australia, and garnet Iherzolitexenoliths from Lesotho. Abundances of the platinum metals are very uniform in spinel lherzolites averaging 3.4 ± 1.2 ppb Os, 3.7 ± 1.1 ppb Ir, and 4.6 ± 2.0 ppb Pd. Sheared garnet lherzolite PHN 1611 has similar abundances of these elements, but in 4 granulated garnet lherzolites, abundances are more variable. In all samples, the Pt metals retain cosmic ( Cl-chondrite) ratios. Abundances of Au and Re vary more than those of Pt metals, but the Au/Re ratio remains close to the cosmic value. The fact that higher values of Au and Re approach cosmic proportions with respect to the Pt metals, suggests that Au and Re have been depleted in some ultramafic rocks from an initially chondrite-like pattern equivalent to about 0.01 of Cl chondrite abundances. The relative enrichment of Au and Re in crustal rocks is apparently the result of crust—mantle fractionation and does not require a special circumstance of core—mantle partitioning. Abundances of moderately volatile elements Ni, Co and Ge are very uniform in all rocks, and are much higher than those of the highly siderophile elements Au, Ir, Pd, Os and Re. When normalized to Cl chondrites, abundances of Ni and Co are nearly identical, averaging 0.20 ± 0.02 and 0.22 ± 0.02, respectively; but Ge is only 0.027 ± 0.004. The low abundance of Ge relative to Ni and Co is apparently a reflection of the general depletion of volatile elements in the Earth. The moderately siderophile elements cannot be derived from the same source as the highly siderophile elements because of the marked difference in Cl chondrite-normalized abundances and patterns. We suggest that most of the Ni, Co and Ge were enriched in the silicate by the partial oxidation of pre-existing volatile-poor Fe-Ni, whereas the corresponding highly siderophile elements remained sequestered by the surviving metal. The highly siderophile elements may have been introduced by a population of ~10 3 large (~10 22 g) planetisimals, similar to those forming the lunar mare basins.

Tectonophysics↗
USGS1981

Plumbotectonics-the model

Plumbotectonics is an attempt to model the geochemical behaviour of U, Th and Pb, among major terrestrial reservoirs in agreement with observational data. By recycling rock through the orogenic environment, a dynamically communicating upper crust, lower crust, and mantle can produce the required patterns of lead-isotope evolution.

Tectonophysics↗