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At least 343 records · Page 19Linked to original sources
USGS1971

Tectonics of the Mendocino triple junction

Interpretation of reflection profiles and of the magnetic anomaly pattern over the Gorda Basin and Escarpment gives broad agreement with the triple junction model of McKenzie and Morgan (1969). However, the basin has undergone internal deformation, a local departure from rigid plate tectonics, and the escarpment has had a component of underthrusting by the Gorda block. Faults in the Gorda Basin which disturb young turbidites parallel the trends of magnetic anomalies, suggesting deformation of the oceanic crust along lines of primary weakness. The northeast trends of the faults give a constraint on first-motion solutions for earthquakes within the basin and suggest left-lateral slip on the faults. Analysis of the geometry and timing of the Gorda Basin deformation based on the magnetic pattern gives an average gross tectonic strain rate of 10 −14 /sec. These observations give a measure of the mechanics of deformation of oceanic lithosphere very close to a spreading rise crest.

California, Oregon↗
USGS1971

Age of the Mesozoic oceanic crust in the California Coast Ranges

K-Ar ages of approximately 155 m.y. were measured on minerals from ophiolites at two localities in the California Coast Ranges. Upper Jurassic rocks of the Great Valley sequence are in depositional contact with underlying ophiolites, which are interpreted as Mesozoic oceanic crust that formed during differentiation of mantle material at a spreading ridge some 15 m.y. before deposition of the Great Valley sequence began.

California↗
USGS1971

Small plate tectonics in the northeastern Pacific

Lithospheric plate motions in the northeastern Pacific were complicated at about 2.5 m.y. B.P. by the movement along a major northeast-trending fault cutting Cascadia Basin. An estimate of the slip rate along this fault gives critical information on the relative motions of four geometrically interdependent blocks. The fault is presently inactive. Seventy km of slip along this fault during 2 m.y. or less gives an average slip rate of about 3-5 cm/yr or greater, and resulting plate motions suggest a significantly greater rate of net subduction along the continental margin off Oregon than off Washington and Vancouver Island. Subduction rate off Oregon is less sensitive to slip rate along this fault than is subduction off Washington.

British Columbia, Oregon, Washington↗
USGS1971

Tertiary igneous chronology of the Great Basin of western United States — Implications for tectonic models

The chronology of igneous activity in the Great Basin of western United States is used as a time framework for a simple plate model. This chronology suggests that a plate (Farallon plate) became underthrust to sufficient depth by the middle Tertiary to trigger the eruption of volcanic rocks of andesitic to rhyolitic composition in the central part of the Great Basin, 40 m.y. ago. This plate continued to be underthrust until about 19 m.y. ago, at which time it was completely consumed and volcanic activity ceased. When the oceanic ridge reached a certain point under the Great Basin about 16 m.y. ago, this resulted in the widespread eruption of olivine basalt and the main initial phase of Basin and Range faulting.

Arizona, California, Idaho, Nevada, Oregon, Utah↗
USGS1971

Upper Cenozoic basalts with high Sr87/Sr86 and Sr/Rb ratios, southern Great Basin, western United States

Upper Cenozoic basalts from southwestern Nevada and east-central California are unusually rich in both strontium (~ 1,200 ppm) and Sr 87 (initial Sr 87 /Sr 86 ~ 0.707). The average Rb/Sr ratio of these basalts is too low to have generated the observed Sr 87 /Sr 86 ratio during the 4.6 b.y. of the Earth's existence, and the high strontium contents and low Rb/Sr ratios effectively rule out introduction to the basalts of the high Sr 87 /Sr 86 values through contamination by more radiogenic material during ascent through the crust. Instead, the basalts must have been derived from unusual mantle material in which an originally high Rb/Sr ratio was markedly lowered during an earlier phase of magmatic activity.

Arizona, California, Nevada, Utah↗
USGS1971

Deformation of lee-side laminae in eolian dunes

Processes responsible for structures in sand dunes consist of (l) primary deposition by saltation and creep and by settling from suspension, (2) redeposition accompanying avalanching, and (3) penecontemporaneous erosion. Characteristics of dune structures were examined in the field by introducing marker beds of magnetite at times of sand deposition, thus recording original surfaces and making possible the determination of subsequent changes. Similar structures were examined in the laboratory by testing processes and comparing the resulting structural forms with corresponding natural features. Avalanching in sand is of two types: sand flow and slumping. Deformational structures characteristic of each were recorded in the field and were reproduced in the laboratory. Nine varieties of deformational structures are recognized and described. Analysis of these structures suggests criteria for distinguishing compressional types (lower dune slope) from tensional types (upper dune slope). The analysis of deformational structures also serves to distinguish between forms developed in cohesive sand and those in non-cohesive sand. Since the degree of cohesion is largely a function of the amount of moisture in the sand at the time of avalanching, the deformational structures provide a means for recognizing original dry sand, wet sand, sand crusts, and saturated sand surfaces in ancient deposits. A testing of these criteria was made by comparing laboratory samples with those of dry sand at White Sands, New Mexico, and with those of coastal dunes (probably wet sand) in southern Brazil.

New Mexico↗
USGS1971

Thin skin distension in Tertiary rocks of southeastern Nevada

Volcanic rocks of late Tertiary age, aggregating about 17,000 ft, accumulated on a surface of low relief cut on Precambrian rocks in the Basin and Range province south of Lake Mead, in Nevada and Arizona. They consist mostly of lava and flow breccia of intermediate composition with minor ash-flow tuff, bedded tuff, and lava of rhyolitic composition. The last of three main phases of volcanism was accompanied by widespread epizonal plutonism and intense faulting. All or parts of six similarly but separately fault-deformed structural units are recognized in a 92-sq-mi mapped area. The structural units are highly distended by a system of closely spaced north-to northwest-striking shingling normal faults (many of which are low angle) that displace younger over older rocks in a west to west-southwest direction. Cumulative amounts of distension approximate the breadth of the structural units and are as much as 20,000 ft, whereas cumulative vertical displacements are much less and in some places are minimal. The structural units are floored at or near the present level of exposure by complex low-angle zones of detachment or décollement into which the numerous shingling normal faults merge. Where the units abut along their strike, they are separated by complex zones of transcurrent faults that appear to merge with the detachment structures and thus mark the ultimate limits of the structural units. Displacement on the detachment structures has the same sense as, but in some places is much greater than, that of the cumulative offset on the shingling faults, thus indicating low-angle movement of the structural units as platelike or lobate masses. These relationships indicate remarkably thin-skinned, large-scale, fault-related tectonism of a type which is present in a broad belt south of Lake Mead and in numerous other areas in the Basin and Range province. The best exposed structural units exhibit a serial eastward progression from broad areas of steeply dipping strata, low-angle faults, and deep denudation to gently dipping strata, high-angle faults, and little denudation. Reverse-drag flexing, a volume-compensating mechanism for movement on concave-upward faults, is inferred to have produced the gentle to moderate dips of the strata, whereas the nearly vertical dips in the western parts of the units probably resulted from a combination of reverse-drag flexing and rotation related to uplift. Evidence of compression-related folding is absent. The extreme distension is viewed as a surficial feature of a crustal belt that was subjected to a brief episode of tensional rifting. Rifting at subjacent levels along the belt was compensated for by emplacement of plutons. The surficial rocks were stretched and thinned over the plutons.

Nevada↗
USGS1971

Nomenclature and correlation of some upper Precambrian and basal Cambrian sequences in western Utah and southeastern Idaho

Recent stratigraphic studies in three widely separated localities in southeastern Idaho and western Utah have revealed a startling continuity of both individual rock units and of rock sequences over a distance of some 300 mi parallel to the strike of a late Precambrian and Cambrian depositional trough. Between 15,000 and 25,000 ft of beds were deposited in the axis of the trough, whereas only 1000 to 3300 ft of correlative rocks were laid down on the shelf to the east. In several areas a diamictite is present near the base of the sequence; this is underlain locally and overlain generally by argillites containing lenticular limestones and dolomites; these in turn are succeeded by quartzitic rocks containing a thick grayish-red to maroon unit—the Mutual Formation. In each area the sequence includes, at the top, quartzites typical of the basal Cambrian. Deposition in the basin was essentially continuous from late Precambrian into Cambrian time but was interrupted by uplift and erosion on the shelf. The hinge line of the ancient seaway is inferred to have coincided roughly with the present “Wasatch line,” but erosion prior to deposition of the Tintic Quartzite has removed most of the data needed to establish this with certainty. Rocks in each of the three areas described here in detail are regarded as allochthonous and appear to have been thrust eastward during the Sevier orogeny. A precise reconstruction of the sedimentary basin must therefore await not only additional stratigraphic studies in such areas as the Promontory Range of Utah and the Bannock and Malad Ranges of southern Idaho, but also final resolution of the structural events.

Idaho, Utah↗
USGS1971

Thrust and strike-slip faulting in the Plomosa Mountains, southwestern Arizona

Thrust and strike-slip faulting are recognized in the Plomosa Mountains, southwestern Arizona. The distribution of rock types and the geometry of the thrust faults necessitate that the upper plate moved from east to west. The amount of displacement is not known, but is considered to be large. Apparent separations along the strike-slip faults are in a right-lateral sense and are greater than 19,000 ft. Rhyodacite flows, dated by K-Ar methods at 19 to 20 m.y., unconformably overlie the thrust faults and are cut by the strike-slip faults.

Arizona↗
USGS1971

A further revision of the stratigraphic nomenclature of the Wissahickon Formation in Maryland

The Wissahickon Formation, the thickest and most extensive unit of the Glenarm Series, was divided into lithofacies several years ago. We suggest revision of two of these lithofacies and addition of another. We also suggest that the term lithofacies be shortened to facies. The added facies, the quartzite facies, is distinguished by metamorphosed orthoquartzites and protoquartzites. It corresponds in part to the former Peters Creek quartzite.

Maryland↗
USGS1971

Surf-beat origin for pulsating bottom currents in the Rio Balsas submarine canyon, Mexico

A previously unreported process was observed at the head of a tributary to the Rio Balsas submarine canyon system in Mexico. During a period of large surf, river discharge deflected a pulsating longshore current [peaking at over 7 km/hr (2 m/sec)] seaward over the tributary heading in the surf zone. This pulsating flow occasionally entered the river mouth, causing rhythmic fluctuations with amplitudes of at least 30 cm and a period of about 3 minutes within the mouth, as recorded by a partially filtered tide gage. In diving to the bottom of the tributary at a depth of 18 m, we encountered current pulses with estimated velocities of 4 km/hr (more than 1 m/sec) transporting large amounts of suspended sand down an axial slope of 26°. This bottom flow was at least 3 m thick, and was characterized by pulses separated by quiet periods in phase with the surface rip current. The upper 20 cm of the canyon fill during this time consisted of sand smoothly laminated parallel to the bottom, indicating net deposition on the steeply sloping floor. Estimated water budget suggested that the entire water column below the rip current at 18 m was not flowing seaward during the pulses. The bottom flow landward of the dive site probably separated from the surface flow and was propagated downslope as a turbidity current. Its magnitude was sufficient to erode the canyon walls. These observations substantiate that rip currents play a role in the formation of some submarine canyons. Surf-beat induced rhythmic flushing of the river mouth, however, did not cause density currents in the main canyon head. The need for future canyon studies under extreme conditions is pointed out.

Cañón de la Necesidad, Rio Balsas submarine canyon↗
USGS1971

Sierra Nevada plutonic cycle: Part II, tidal energy and a hypothesis for orogenic-epeirogenic periodicities

The dissipative power of the solid earth tides is the order of 10 19 ergs/sec, or a few percent of terrestrial heat flow. It is proposed that this energy is concentrated along oceanic ridge systems and in the asthenosphere by mechanisms of viscous dissipation involving shear melting. Tidal energy localizes and sustains sources of sea-floor spreading through the melting mechanism, convection and magmatic transfer. Components of this energy enter the continent as magmatic heat either where ridge type sources and continents interact or where lateral motions induce shear zones and viscous dissipation within the continent. Temporal maxima of igneous intrusion into continental crust and related epeirogenic oscillations, spaced at intervals of about 30 m.y., are explained in terms of periodic thermal instabilities in the process of shear melting in the mantle. That is, maxima in rates of magma production in the mantle are relieved by vertical magmatic transfer. This process is coupled with lateral motions of the continent in a way analogous to episodic creep episodes of much shorter period in motions of active fault systems. Calculated periodicities are found to be simultaneously compatible with (l) the Sierra Nevada intrusive epochs of Part I, (2) oscillations in the eustatic curve during the Mesozoic Era, (3) concepts of sea-floor spreading, and (4) the magnitude of tidal power. More profound epeirogenic oscillations, having periods of about 200 m.y., are induced by variations in proportioning of tidal energy dissipation between the solid earth and the epicontinental seas. Thus, the tidal deformations of the earth provide information that leads to a general dynamic theory where magmatism, orogency, epeirogeny, sea-floor spreading and continent migration are systematically interrelated.

California↗
USGS1971

Petrologic and geophysical nature of serpentinites

Mineralogically, serpentinites consist predominantly of lizardite, clinochrysotile, and antigorite. Recent work has shown that these minerals are not polymorphs. Chrysotile is the only mineral recognized as a synthetic product in experimental studies of the system MgO-SiO 2 -H 2 O. Antigorite seems to be stable at higher temperatures than lizardite or chrysotile. The density of individual serpentine species is dependent on their morphology; the low-density serpentinites (<2.55g/cc) consist predominantly of clino-chrysotile. Seismic velocities and magnetic susceptibilities of serpentinites are related to the degree of serpentinization. The transition of massive serpentinites from ductile to brittle behavior in laboratory experiments at high confining pressures and temperatures above 300°C has been related to dehydration which may provide a mechanism for developing deep-focus earthquakes along Benioff zones. Serpentinite is formed by direct hydration of ultramafic protolith in the crust. The most common ultramafic protoliths are harzburgite, dunite, and Iherzolite. The assemblage generally developed from these is lizardite + chrysotile + brucite + magnetite. In areas of high-grade metamorphism, antigorite is the predominant serpentine mineral. The common, large, alpine-type serpentinized ultramafic masses contain brucite and have MgO/SiO 2 ratios similar to those of their protolith, resulting in volume increase during serpentinization. Metamorphic serpentinites and some highly sheared alpine-type serpentinites have lower MgO/SiO 2 ratios than their protolith, lack brucite, and appear t o have been formed by volume-for-volume replacement with concomitant loss of magnesium or addition of silica. Many large, young masses of peridotite appear to be slabs of oceanic mantle over-thrust onto continental edges. Subsequent sedimentation, serpentinization, and tectonism have greatly modified these original slabs so that their recognition in older orogenic zones is equivocal. The concept of the tectonic evolution of ultramafic rocks from oceanic crust-mantle slabs invading continental margins and being incrementally serpentinized and moved by later tectonic events provides a working hypothesis that allows a better explanation of the many peculiar and varied occurrences of serpentinite. The evidence does not support Hess' suggestion that the third layer of the oceanic crust consists of partly serpentinized mantle peridotite.

Bulletin of the Geological Society of America↗
USGS1971

Sedimentary and gravity-slide emplacement of serpentinite

Large deposits of serpentinite in alpine-type orogenic areas have been formed by sedimentary processes ranging from the detrital accumulation of bedded serpentinite sandstone and shale to the emplacement of chaotic breccias (olistostromes) and gigantic slide blocks. Known occurrences of sedimentary serpentinite are listed, and eight deposits from the circum-Pacific, Caribbean, and Mediterranean areas are described in detail. Sedimentary serpentinites range in age from early Paleozoic to Quaternary, although most are Cretaceous or Tertiary. Most were deposited in eugeosynclinal environments, early in the geosynclinal cycle. Individual deposits range in thickness from a few centimeters to nearly 3 km, and several extend laterally for tens of kilometers. Graded bedding is common, and many deposits contain marine fossils. Serpentinite is the dominant rock constituent, and clasts foreign to the alpine ultramafic assemblage are rare. Chemical analyses often detrital serpentinites show that these rocks contain slightly more silica and alumina than do nondetrital serpentinites, due to contamination by aluminosilicate minerals and quartz during deposition. This and nine other criteria are potentially useful in the recognition of sedimentary serpentinites. Several features suggest that most sedimentary serpentinites were deposited very rapidly by submarine landslides, mudflows, or turbidity currents. The sources of this serpentinite debris are postulated to be upward-migrating serpentinite protrusions which penetrate the seafloor or Earth's surface upslope from eventual depositional sites. Sedimentary serpentinites are much more abundant in alpine-type orogenic areas than is commonly thought, and many ultramafic masses presently regarded as igneous intrusions or tectonic protrusions may in fact be coeval with, instead of younger than, their enclosing sedimentary or metasedimentary rocks. In eugeosynclinal sequences such as the Franciscan Formation, some elongate bodies now regarded as serpentinite sills may be beds of ultramafic detritus whose sedimentary features have been masked by post-depositional shearing; isolated masses may be exotic slide blocks. A sedimentary origin can explain some of the most persistent and perplexing characteristics of many alpine serpentinites: their conformity with enclosing sedimentary rocks, their grossly planar shapes, and the absence of metamorphism along their contacts.

California↗
USGS1995

Stratigraphic and structural synthesis of a Miocene extensional terrane, southeast California and west-central Arizona

Detailed stratigraphy and isotopic dating of stratigraphic sections in the Colorado River extensional corridor support a regional correlation of highly faulted Tertiary stratigraphic sequences and provide a chronologic framework for interpreting the evolution of low-angle normal (detachment) faults. On the basis of this correlation, we define six tilting domains in the upper plate of the Whipple, Chemehuevi, and Rawhide detachment faults and identify three discrete episodes of detachment faulting that began in the early Miocene and ended in middle Miocene time. Episodes of rapid detachment faulting are indicated by extreme tilting of upper-plate fault blocks and overlying Miocene sequences, fanning dips of basinal deposits, and angular unconformities that represent short time gaps in the accumulation of syntectonic sequences. During the first episode of detachment faulting at about 20 Ma, the upper plate segmented to form the domains. Basin subsidence and extreme tilting of upper-plate fault blocks and syntectonic deposits characterized the eastern Topock, Crossman, Aubrey, Parker Dam, and Buckskin-Rawhide domains, whereas the western Mopah domain was the site of abundant volcanic activity but no basins or tilting. A second episode of extension at about 18 Ma produced extreme tilts in the Buckskin-Rawhide domain but upper-plate blocks in the Mopah domain tilted moderately. A third regionwide faulting episode between 14 and 12 Ma was due to localized uplift of middle and lower crust and eventual exposure of the detachment faults and their footwalls. The upper-plate fault blocks responded passively to localized slip on the detachment faults. Rapid extension began on the Whipple-Chemehuevi detachment fault at 20 Ma and had shifted southward to the Buckskin-Rawhide detachment fault by 18 Ma; volcanic activity also shifted southward to the Buckskin-Rawhide domain at this time. The southward shift of rapid extension and volcanism probably represents buildup and release of strain at localized sites in the lower plate. Otherwise, stratigraphic and structural relations indicate that the locations of upper-plate basins, faulting and tilting of upper-plate blocks, and position of the breakaway zone remained stable throughout the major phases of extension.

Arizona, California↗