scispace - formally typeset
Search or ask a question

Showing papers in "Geological Society of America Bulletin in 1970"


Journal ArticleDOI
TL;DR: In this paper, the authors examined the history of plate motions between the American and Pacific plates in the late Cenozoic and found that the two plates were fixed with respect to one another until 5 m.y.
Abstract: Magnetic anomaly patterns in the northeast Pacific Ocean combined with plate theory indicate that a trench existed offshore from western North America during mid-Tertiary time and that the present episode of strike-slip motion in the San Andreas fault system originated after the cessation of subduction, not earlier than 30 m.y. ago. At present, the American and Pacific plates appear to be moving past one another parallel to the San Andreas fault at a rate of 6 cm/yr. Data concerning the late Cenozoic history of motions between these plates is inconclusive, and so 2 probable models are examined. One assumes a constant motion of 6 cm/yr throughout the late Cenozoic, whereas the other assumes that the 2 plates were fixed with respect to one another until 5 m.y. ago, at which time they broke along the San Andreas fault system and began moving at 6 cm/yr. The second model implies that the San Andreas fault took up all the motion at the boundary between the North American and Pacific plates, while the first model suggests the broader view that much of the late Cenozoic tectonic activity of western North America is related to this boundary deformation. The models make testable predictions for the distribution of igneous rocks and for the total amount and timing of deformation expected. Extrapolation of the model of constant motions to the early Cenozoic suggests an era of slightly compressional strike-slip at the edge of North America. A major change in plate motions in late Mesozoic time is suggested.

1,846 citations


Journal ArticleDOI
TL;DR: In this article, an examination of the formation and development of shear zone structures on microscopic scale in the shear box test, intermediate scale in Riedel experiment, and regional scale in earthquake fault is made.
Abstract: An examination is made of the formation and development of shear zone structures on (1) the microscopic scale in the shear box test, (2) an intermediate scale in the Riedel experiment, and (3) the regional scale in the earthquake fault. On the basis of the resistance to shear, three structural stages are chosen for detailed study: the peak structure occurring at peak shearing resistance, the post-peak structure occurring after peak shearing resistance, and the residual structure occurring at residual shearing resistance. Most of the similarities in structure between the different scales at each of these stages are interpreted in terms of the mechanical properties of the material, the Coulomb failure criterion, and the kinematic restraints inherent in the type of deformation. Other similarities which are not as yet understood are described and suggested as topics for future research.

1,042 citations


Journal ArticleDOI
TL;DR: In this article, the steady state deformation of dunite and peridotite was investigated in solid pressure media in the presence and absence of externally released H 2 O. The steady state data were best fit by a power creep equation with the stress exponent, n ≃ 2.4 and 4.8 and the creep activation energy, Q ≃ 80 and 120 Kcal/mole, respectively.
Abstract: Deformation experiments on dunite and peridotite, both in the presence and absence of externally released H 2 O, have been carried out in solid pressure media apparatus in the confining pressure range 5 to 30 kb, temperature interval 300°C to 1400 ° C at constant strain rates ranging from 10 −3 to 10 −8 /sec. At the lower temperatures and higher rates, olivine deforms by plastic flow on the system T = (110), t = [001], the slip systems changing with increasing temperature and decreasing strain rate through {0kl} [100] to (010) [100]. At a strain rate of 10 −3 /sec, polygonization (dislocation climb) is first observed at about 1000 ° C and recrystallization first appears at about 1050 ° C; these temperatures decrease by about 50 ° C with a ten-fold reduction in strain rate. Preliminary mechanical results indicate that steady state deformation takes place only in the temperature-strain rate range in which the diffusion controlled processes of polygonization and recrystallization are important or dominant. The steady state data are best fit by a power creep equation with the stress exponent, n ≃ 2.4 and 4.8 and the creep activation energy, Q ≃ 80 and 120 Kcal/mole for deformation in the presence and absence of externally released H 2 O, respectively. Extrapolation of the equation for the dunite deformed dry to a representative geological strain rate of 10 −14 /sec gives shear stresses in the range 100 bars to 1 bar and effective viscosities from 10 23 to 10 20 poise for the temperature interval 1000 ° C to 2000 ° C. The estimated variation in shear stress over most of the upper mantle, where T/Tm (the ratio of ambient to melting temperature) is greater than 0.75, is about 6 to 15 bars, and in viscosity in the range 10 20 to 10 21 poise. These inferences are in accord with estimates based on other geophysical observations, and our results suggest that flow in the upper mantle may be governed primarily by a non-linear creep law.

565 citations


Journal ArticleDOI
TL;DR: In this paper, a two-stage computer-based approach was used to solve the problem of petrologic mixing using linear programming and a conventional least square calculation, with the analyses represented by non-negative solution values as input to yield an optimum set of solution values.
Abstract: Problems of petrologic mixing have been solved using a two-stage computer-based calculation. First, linear programming is used to obtain an approximate solution and to identify non-negative solution values. Then a conventional least squares calculation is performed using the analyses represented by non-negative solution values as input to yield an optimum set of solution values. The error attached to each solution value is estimated by an empirical procedure. Petrologic application of the program has been demonstrated with three types of calculations: chemical mode, magma mixing, and liquid line of descent.

487 citations


Journal ArticleDOI
TL;DR: In terms of plate tectonic theory, and by analogy with modern continental margins, the Appalachian orogen evolved through a sequence of interrelated sedimentation-deformation deformation-metamorphism patterns within the eastern margin of the North American continent.
Abstract: In terms of plate tectonic theory, and by analogy with modern continental margins, the Appalachian orogen evolved through a sequence of interrelated sedimentation-deformation-metamorphism patterns within a tectonic belt situated along the eastern margin of the North American continent. As exemplified by the northern part of the orogen, Appalachian stratigraphic-tectonic zones and deformation sequences are related to Late Precambrian to Ordovician expansion, followed by Ordovician through Devonian contraction, of a Proto-Atlantic ocean. This oceanic opening and closing was achieved by initial extensional necking of a single North American/African continent and by lithosphere plate accretion, followed by contractional plate loss along a trench, or complex of trenches, marginal to the drifted North American continent. A lithosphere plate model for the evolution of the orogen incorporates spatial and chronologic relations within and between bulk stratigraphic units and tectonic events. Pre-orogenic Appalachian sedimentation patterns were essentially the same as those found along modern continental margins; that is, shelf/slope/rise/abyss. Appalachian tectonic patterns are also analogous with modern tectonic patterns of continental margins, island arcs, and trenches, and involved continent-ward driven thrust sheets and ancillary exogeosynclines.

437 citations


Journal ArticleDOI
TL;DR: The Chile earthquake sequence of May 21-22, 1960, was accompanied by linear zones of tectonic warping, including both uplift and subsidence relative to sea level.
Abstract: The Chilean earthquake sequence of May 21–22, 1960, was accompanied by linear zones of tectonic warping, including both uplift and subsidence relative to sea level. The region involved is more than 200 km wide and about 1000 km long, and lies along the continental margin between latitude 37° and 48° S. Significant horizontal strains accompanied the vertical movements in parts of the subsided zone for which triangulation data are available. Displacements were initiated near the northern end of the deformed region during the opening earthquake of the sequence (M s ≅ 7.5) on May 21 at 10h 02m 50s GMT and were extended over the remainder of the region during the culminating shock (M s ≅ 8.5) on May 22 at 19h llm 17s GMT. During the latter event, sudden uplift of adjacent portions of the continental shelf and much or all of the continental slope apparently generated the destructive tsunami that immediately followed the main shock. Available data suggest that the primary fault or zone of faulting along which displacement occurred probably is a complex thrust fault roughly 1000 km long and at least 60 km wide; it dips eastward at a moderate angle beneath the continental margin and intersects the surface on the continental slope. Dip slip required to satisfy the surface displacements is at least 20 m and perhaps as large as 40 m. There is some evidence that there was a minor component of right-lateral slip on the fault plane.

392 citations


Journal ArticleDOI
TL;DR: Pockmarks are cone-shaped despressions that occur in large numbers across the LaHave clay of the Scotian Shelf as discussed by the authors and normally range in diameter from 15 to 45 m and in depth from 5 to 10 m. They were possibly formed by ascending gas or subsurface water leakage from underlying coastal plain sediments.
Abstract: Pockmarks are cone-shaped despressions that occur in large numbers across the LaHave clay of the Scotian Shelf. They normally range in diameter from 15 to 45 m and in depth from 5 to 10 m. They were possibly formed by ascending gas or subsurface water leakage from underlying coastal plain sediments.

387 citations


Journal ArticleDOI
TL;DR: In this paper, a structural analysis of the fractures formed in the fault zone associated with the Dasht-e Bayaz earthquake of August 31, 1968 is presented, which is compatible with a predominant movement along the east-west lineament, followed by stress readjustments along the WNW-ESE lineament.
Abstract: A structural analysis is presented of the fractures formed in the fault zone associated with the Dasht-e Bayaz earthquake of August 31, 1968. The segment of the fault zone studied in detail here is 25 km long, 2 to 3 km wide, and located in the Quaternary sediments of the Nimbluk Valley. The maximum relative displacements ohserved in the fault zone (up to 450 cm left-lateral and 250 cm vertical) are concentrated in an east-west principal displacement zone 2 to 100 m wide. This zone is in turn, on a larger scale, composed of en echelon shear zones. The fault zone contains also many small fracturesdispersed throughout the area and reflecting, to a certain extent, the dominant trends of the principal displacement zone. Both on the scale of the whole fault zone and on the larger scale of the principal displacement zone, the structures are characteristic of a simple shear type of deformation. Their sense of movement and initial directions can be interpreted in terms of the Coulomb failure criterion applied to a material with an angle of shearing resistance of 35 to 40. The analysis also shows that the Nimbluk Valley contains many fault lineaments that extend into the mountains around the valley- In many places the cast-west principal displacement zone follows very precisely one of these lineaments. Evidence of fault reactivation is also found on another important lineament that crosses the principal displacement zone in a WNW-ESE direction. The earthquake fracture pattern in the Nimbluk Valley is compatible with a predominant movement along the east-west lineament, followed by stress readjustments along the WNW-ESE lineament.

359 citations


Journal ArticleDOI
TL;DR: In this paper, relative abundances of planar cross-stratification and horizontal stratification, as well as bed-relief indices were measured in sandstones and conglomerates of the Lower Silurian Shawangunk Conglomerate, Green Pond Conglerate, and Tuscarora Sandstone in New Jersey and Pennsylvania.
Abstract: Studies of the South Platte-Platte River in Colorado and Nebraska substantiate Ore9s (1964) conclusion that braided patterns in streams are created mainly by accretion of longitudinal bars and dissection of transverse bars. Distribution of bars in the South Platte-Platte River depends on texture of the bed load. Coarse, poorly sorted sediment favors formation of longitudinal bars, and finer grained, better sorted materials form transverse bars. The relative proportion of transverse to longitudinal bars increases downstream, following the river9s tendency to fractionate its load into finer sizes downstream. This is accompanied by an increase in the ratio of planar cross-stratification to horizontal stratification and a decrease in cross-channel topographic relief expressed as a bed-relief index. Relative abundances of planar cross-stratification and horizontal stratification, as well as bed-relief indices were measured in sandstones and conglomerates of the Lower Silurian Shawangunk Conglomerate, Green Pond Conglomerate, and Tuscarora Sandstone in New Jersey and Pennsylvania. These formations display downslope trends similar to those of the South Platte and Platte Rivers, and, combined with paleocurrent, grain-size distribution, and other data, suggest that the coarse eastern facies (Green Pond, Shawangunk) represent proximal braided stream deposits with longitudinal bars that grade westward and northwestward into finer grained distal braided stream sediments (Tuscarora) characterized by transverse bars.

309 citations


Journal ArticleDOI
TL;DR: In this paper, a detailed thermal structure for the Benioff zone and the regions on either side is presented, showing that the cold descending material heats very slowly and should form a thin cold slab extending to depths of more than 500 km.
Abstract: The ocean-floor spreading hypothesis requires that island arc areas be loci of descending mass transport and destruction of oceanic crust. All island arc areas are characterized by volcanic activity and in some, unusually high heat flow values have been measured for hundreds of kilometers behind the arc. Downward mass motion is associated presumably with the sinking of cool and thus relatively denser matter, and high temperature phenomena normally would not be expected where this was occurring. The only feasible means of generating heat in such areas appears to be by frictional dissipation along the seismically active zone of movement (the Benioff zone) between the moving oceanic crust and upper mantle, and the relatively stationary material that it underrides. It has been shown elsewhere that dissipative heating is of the right order of magnitude to explain the observations, but the required shear stresses are rather high; in this paper a detailed thermal structure for the Benioff zone and the regions on either side is presented. The cold descending material heats very slowly and should form a thin cold slab extending to depths of more than 500 km. Within the zone of movement, temperatures are buffered by the melting temperatures of the various components of the oceanic crust that partially fuse to give the main members of the calc-alkaline igneous suite. The thermal structure of the region above the Benioff zone is subject to uncertainty because processes other than lattice conduction (for instance, magmatic activity) are probably involved in the transfer of heat to the surface. Observed rates of magmatic activity in island arcs require a relatively small degree of fusion of the descending material and probably only the oceanic crust is involved. The siliceous and presumably water-bearing sedimentary material of the oceanic crust is the most important source of material added to island arcs; growth occurs either by physical addition of sedimentary material, which, for mechanical reasons, may not be carried down, or by partial fusion of sedimentary material along the fault zone and by rise of the resultant magmas to the surface. Rates of growth suggested by these processes are compatible with the known ages of arcs. Older arcs should have larger crustal cross-sectional areas than younger ones, if both have been continuously active at similar rates.

292 citations


Journal ArticleDOI
TL;DR: The geology of the Uralides accords with the concept that the two subcontinents approached and collided as the intervening oceanic plate slid beneath them along subduction (Benioff) zones as mentioned in this paper.
Abstract: The Uralides—the late Precambrian and Paleozoic orogenic terrane between the Russian and Siberian Platforms—in part are exposed in the Ural Mountains, in the central Soviet Arctic, along the west edge of the Siberian Platform, and in southern Siberia and Kazakhstan, and in part are buried beneath the fill of the West Siberian Lowlands and other basins. Paleomagnetic orientations suggest that the Russian and Siberian Platforms were far apart during the early Paleozoic, converged during the middle Paleozoic, and collided in the Permian or Triassic. The geology of the Uralides accords with the concept that the two subcontinents approached and collided as the intervening oceanic plate slid beneath them along subduction (Benioff) zones. The medial eugeosyncline of the Uralides consists largely of what may be oceanic material scraped off against the edges of the opposed subcontinents. Basalt-and-spilite belts may represent ocean-floor abyssal tholeiite, and the manganiferous cherts and other sediments upon them may be pelagic oozes. Andesite belts may have formed as island arcs within the ocean, swept subsequently against the continents. Fossil subduction zones are recorded by great faults soled by, or containing tectonic injections of, mafic and ultramafic rocks from the lower oceanic crust and upper mantle, and containing high-pressure metamorphic rocks. Granitic and silicic-volcanic rocks may have formed above the subduction zones in the accreted parts of the continental plates. Both these continental-margin magmatic rocks and the island-arc complexes display ratios of potassium to silicon that vary across strike and so indicate the directions of dip of the subduction zones. From the distribution of such indicators of various ages, a history of the continental margins can be deduced. An active subduction zone dipped beneath the Siberian Platform during at least parts of late Precambrian and early, middle, and late Paleozoic time. The late Precambrian and Cambrian history of the Russian side is unclear, but in the Ordovician and Silurian the Russian continental margin was stable, while somewhere offshore an island arc was present whose trench was on the Russian side; the last of the intervening oceanic plate vanished down the subduction zone in about the Early Devonian, and the island arc became part of the continental margin. During the remainder of the Devonian and during the Carboniferous and Early Permian, a subduction zone was present along the margin of the enlarged Russian continent and dipped beneath it. Each subcontinent grew oceanward as oceanic material was accreted against it, and the subduction zones stepped oceanward correspondingly. The continental magmatic zones migrated oceanward behind the accreting edges of the continental plates, so the tectonic and magmatic progression with time at any one place is analogous to the variations present across the entire orogenic belt at any one time. Severe right-lateral deformation of the Uralides, the Russian side having moved northward relative to the Siberian side during Mesozoic and early Cenozoic time, is inferred from structural and magnetic-anomaly patterns. The deformation was accomplished by oroclinal folding, strike-slip faulting, and tensional thinning of the crust. The Uralides may have been continuous in early Mesozoic time with the Ellesmerides of North Greenland and the Canadian Arctic islands. The Cenozoic (and late Mesozoic?) opening of the Arctic Ocean was accomplished by spreading of the Eurasia Basin, and by opening of the Canada Basin behind a counterclockwise-rotating Alaska.

Journal ArticleDOI
TL;DR: In this paper, it was shown that syntectonic recrystallization is an important or dominant mode of flow in the upper mantle at temperatures above about 500°C, and that fabrics in olivine tectonites are similar to those produced experimentally.
Abstract: Olivine in dunite, peridotite, and olivine powder has been recrystallized syntectonically during compression tests at temperatures from 950°C to 1350°C, confining pressures from 5 to 30 kb, and strain rates from 10 −3 to 10 −8 /sec. For intragranular recrystallization of the dunite, the orientations of the new grains are controlled by the host orientation. For inter-granular recrystallization and total recrystallization of the dunite and of olivine powder, the orientations of the grains are controlled dominantly by the stress. The fabric produced over the entire range of physical conditions is a [010] maximum parallel to σ 1 and [001] and [100] girdles in the σ 2 = σ 3 plane. With few exceptions, and allowances for the experimental requirement that σ 2 = σ 3 , fabrics in olivine tectonites are similar to those produced experimentally. Such probable upper mantle source materials as ultramafic nodules in diamond-bearing kimberlites and in under-saturated basalts and “Alpine-type” peridotites show these same preferred orientations. We propose, therefore, that Syntectonic recrystallization is an important or dominant mode of flow in the upper mantle at temperatures above about 500°C. The implications of this conclusion, as they bear on seismicity and seismic anisotropy, are considered in light of the two most seriously regarded hypotheses advanced to explain motions of spreading plates of the lithosphere.

Journal ArticleDOI
TL;DR: The oceanic fractionation of silica from lime through basin-basin exchange of water at different depth levels is due to both supply and preservation patterns as discussed by the authors, and its implications for the geological record are discussed.
Abstract: There are two types of marginal seas: the lagoonal type with deep water outflow, and the estuarine type with deep water inflow. Lagoonal basins are characterized by waters that have low nutrient concentrations and high salinity and are well aerated; their sediments are rich in calcium carbonate. Estuarine basins contain waters with high nutrient concentrations, low salinity, and relatively low oxygen values; their sediments are rich in silica and organic matter and tend toward being anaerobic. The Atlantic Ocean is “lagoonal” in that it has a deep water outflow and collects lime on its floor; the Pacific Ocean is “estuarine” with a deep water inflow and silica-rich sediments. The oceanic fractionation of silica from lime through basin-basin exchange of water at different depth levels is due to both supply and preservation patterns. Supply of biogenous precipitates to the ocean floor exceeds influx of solutes to the ocean, requiring dissolution on the ocean floor to maintain steady state. Supply is greatest in estuarine basins. Dissolution of lime proceeds fastest in estuarine basins, which have CO 2 -rich deep waters, whereas dissolution of silica is greatest in lagoonal basins, where the deep water is farthest from saturation because of low dissolved nutrient concentrations. Trace elements may conceivably be affected also by this fractionation mechanism. Some implications for the geological record are discussed.

Journal ArticleDOI
TL;DR: In this paper, a detailed input-output study of a small forested watershed draining the Wissahickon Formation in the Piedmont of Maryland revealed that chemical solution is five times as effective in removing material as is mechanical erosion, and that on a longterm basis approximately one-half of the erosion of the Pond Branch watershed is caused by chemical solution of the silicate minerals kaolinite, vermiculite, biotite, and oligoclase.
Abstract: A detailed input-output study of a small forested watershed draining the Wissahickon Formation in the Piedmont of Maryland revealed that chemical solution is five times as effective in removing material as is mechanical erosion. Solution weathering removes 16.9 tons/sq mi/yr of material compared with 3.2 tons/sq mi/yr by mechanical erosion. Plant activity during the growing season increased the concentration of silica, bicarbonate, calcium, and potassium, thus increasing total dissolved solids by one-third. Autumn leaf fall also caused a short-term increase of these ions. Rainfall does not simply dilute floodwaters as the concentration of sulfate, potassium, and calcium increases whereas silica and bicarbonate decrease in concentration during a flood cycle. Our data suggest that during the first half of a flood cycle, both the flood water and the dissolved solids in it come from an area in and immediately adjacent to the flood plain. The weathering model derived from our study suggests that on a long-term basis approximately one-half of the erosion of the Pond Branch watershed is caused by chemical solution of the silicate minerals kaolinite, vermiculite, biotite, and oligoclase. This contrasts to short-term ratio of solutional to mechanical weathering of five to one.

Journal ArticleDOI
TL;DR: In this paper, deep-sea piston cores taken in the Guiana Basin off northeast South America between lat 20° N. and lat 10° S. support previous conclusions that an arid to semiarid climate dominated large portions of equatorial South America during the Pleistocene glacial phases.
Abstract: Arkosic sands of latest Wisconsin age from deep-sea piston cores taken in the Guiana Basin off northeast South America between lat 20° N. and lat 10° S. support previous conclusions that an arid to semiarid climate dominated large portions of equatorial South America during the Pleistocene glacial phases in complete contrast to the present-day and Pleistocene interglacial humid tropical climate. Thirty-nine cores showed stratigraphic relations that characterize the transition from Wisconsin to Holocene. Sand beds from 23 of the cores were of latest Wisconsin age and contained 25-60 percent feldspar. In contrast, a sample of Holocene sand taken from the continental shelf northeast of the mouth of the Amazon River contained only 17—20 percent feldspar. The Quaternary history of Brazil appears to have been climatically controlled by a repeated displacement of the South Atlantic high-pressure cell by some 1500 km—northward during the glacial phases and southward during the interglacial phases. It was further influenced during glacial phases by a lowering of the snow line in the Andes by about 1000 m. In consequence, the glacial phase climates of the lower Amazon Basin were marked by cold, dry, southerly winds while the South Atlantic trade winds were deflected off the northeast coast. A semiarid to and climate ensued, coupled with a eustatic drop of sea level that caused degradation of the principal river valleys near the coast. In this way unweathered feldspars, chlorite, and other relatively coarse elastics were transported into the tropical Atlantic in contrast to the usual lateritic clays (gibbsite and kaolimte) of interglacial stages.

Journal ArticleDOI
TL;DR: In this article, the authors derived possible recurrence intervals between earthquakes of different magnitude along the San Andreas fault by relating long-term offset rates since mid-Tertiary time, displacements, and lengths of breaks recorded for historic earthquakes, and tectonic creep rates.
Abstract: Possible recurrence intervals between earthquakes of different magnitude that may be generated along the San Andreas fault are derived by relating long-term offset rates since mid-Tertiary time, displacements, and lengths of breaks recorded for historic earthquakes, and tectonic creep rates. The recurrence interval for earthquakes of different magnitude at a given point on the fault is believed to follow the relation: ![Formula][1] where: Rx : recurrence interval at a point on the fault, D : displacement accompanying an earthquake of given magnitude (related empirically to Richter magnitude), S : long-term strain rate (from offset of geologic units), C : tectonic creep rate. The recurrence interval for earthquakes of different magnitudes for the total length of the fault is then derived by weighting [equation (1)][2] according to the number of break lengths in the total length as follows: ![Formula][3] where: Rt : recurrence interval for entire fault, L : length of break (related empirically to Richter magnitude), Lt : total length of fault. Tectonic creep is believed to be related to Richter magnitude, for example, small for segments of the fault characterized by earthquakes of large magnitude, and large for segments characterized by small earthquakes; and [equations (1)][2] and [(2)][4] can be weighted according to this relationship. [1]: /embed/graphic-1.gif [2]: #disp-formula-1 [3]: /embed/graphic-2.gif [4]: #disp-formula-2

Journal ArticleDOI
TL;DR: In this paper, the gross petrologic evolution throughout the San Juan remnant of this field was relatively simple, with initial intermediate lavas and breccias, followed closely in time by more silicicic ash-flow tuffs, and ending with a bimodal association of basalt and rhyolite.
Abstract: Volcanic rocks in the San Juan Mountains constitute the largest erosional remnant of a once nearly continuous volcanic field that extended over much of the southern Rocky Mountains and adjacent areas in Oligocene and later time. Recent regional studies have shown that the gross petrologic evolution throughout the San Juan remnant of this field was relatively simple, with initial intermediate lavas and breccias, followed closely in time by more silicic ash-flow tuffs, and ending with a bimodal association of basalt and rhyolite. More limited data from other remnants of the original field indicate a similar evolution. In the San Juan field, voluminous early lavas and breccias—mainly alkali andesite, rhyodacite, and mafic quartz latite—were erupted from numerous scattered central volcanoes onto an eroded tectonically stable terrane. They formed mostly during the interval 35 to 30 m.y. ago, but some probably were erupted earlier and others up to several million years later. About 30 m.y. ago, major volcanic activity changed to explosive ash-flow eruptions of quartz latite and low-silica rhyolite that persisted until about 26 m.y. ago. Source areas for the ash flows are marked by large calderas in the central and western San Juan Mountains. Two groups of lavas and associated rocks of intermediate composition intertongue with the ash-flow sequence: (1) quartz latitic lavas that were erupted in and adjacent to caldera structures and are genetically related to the ash-flow activity; and (2) other, generally more mafic lavas and related rocks that are widely distributed without evident structural relation to the ash-flow eruptive centers. The second group apparently represents a continuation of the early intermediate activity into the period of major ash-flow eruption. In the early Miocene the character of volcanism changed notably. Whereas the Oligocene volcanics are predominantly intermediate lavas and related silicic differentiates, the younger rocks are largely a bimodal association of basalt and high-silica alkali rhyolite. Basalt and minor rhyolite were erupted intermittently through the Miocene and Pliocene, and at one time formed a widespread thin veneer over the older volcanic terrane. The marked contrast between the Oligocene intermediate to low-silica rhyolitic magmas and the later basaltic and rhyolitic magmas implies either different conditions of magma generation or processes of differentiation for the two suites. This petrologic change coincides approximately in time with nearby development of the Rio Grande depression, a major rift that is the local expression of widespread late Cenozoic crustal extension. Whatever the cause of the petrologic change, the progression from predominantly intermediate to bimodal basalt-rhyolite volcanism, approximately concurrent with initiation of late Tertiary crustal extension, appears characteristic of Cenozoic volcanism for much of the western interior United States.

Journal ArticleDOI
TL;DR: In this article, the authors compared the volume of terrigenous deposits overlying the land, the continental margin, and filling the trench with that expected from continental denudation.
Abstract: The hypotheses of sea-floor spreading and plate tectonics require the removal of sediment from oceanic trenches either by crustal underthrusting or by folding against the base of a continental or insular margin. Accordingly, over a period of time the volume of sediment removed by way of spreading must be equal to the difference between the observable volume of undeformed terrigenous deposits in a trench and the volume contributed to it by continental erosion. To assess possible sediment loss from the central Chilean segment (23°–44° S.) of the Peru-Chile Trench, we have compared the volume of terrigenous deposits overlying the land, the continental margin, and filling the trench with that expected from continental denudation. Our data indicate that an episode of sediment removal occurred at the base of the margin and adjacent deep-sea floor in Late Cretaceous and perhaps earlymost Tertiary time and may imply spreading. Nearly 100 × 10 3 km 3 of deposits of Tertiary age, chiefly Eocene to Pliocene, have accumulated on the margin, and perhaps an additional 5 × 10 3 km 3 in the trench. This amount of offshore sediment could be supplied by fairly low rates (3 cm/10 3 yrs) of Tertiary erosion. However, many uncertainties in our denudation-sedimentation budget make it impossible to determine whether or not sediment reaching the base of the margin was removed tectonically in Tertiary time. Between 27° and 44° S., the trench contains nearly 70 × 10 s km 3 of turbidite deposits that we believe accumulated during late Cenozoic periods of glacially lowered sea level. The volume of turbidites in the trench is virtually equal to that expected from continental erosion, which is estimated to have probably been no greater than 5 cm/10 3 yr for the arid region between 27° and 31°, and 50 cm/10 3 yr for the humid and partially glaciated region from 36° to 42°. During this time of rapid erosion and trench filling, magnetic data indicate that convergence of lithospheric plates was taking place below the trench at a rate between 5 and 10 cm/yr. If turbidite deposits were swept from the trench at these rates, then continental denudation must have been exceedingly rapid: 20–40 cm/10 3 yr for the arid zone, and 110–165 cm/10 3 yr for the partially glaciated region. If more conventional estimates of erosion are valid, then either (1) late Cenozoic underthrusting has not taken place (or at a rate much slower than that implied by geophysical data), or (2) underthrusting at the prescribed rates has not involved the removal of a significant volume of sediment from the trench.

Journal ArticleDOI
TL;DR: In this article, the authors used phase diagrams, mineralogical and chemical data, and textural evidence to show that ultramafic and mafic nodules found in basaltic rocks at Kilbourne Hole, New Mexico were formed by the interaction of partial fusion-partial crystallization processes.
Abstract: Consideration of phase diagrams, mineralogical and chemical data, and textural evidence suggests that ultramafic and mafic nodules found in basaltic rocks at Kilbourne Hole, New Mexico, were formed by the interaction of partial fusion-partial crystallization processes. The olivine compositions of the nodules vary from 7 to 32 mol percent fayalite. Very few nodules were found with olivine compositions in the Fa 12 percent to Fa 14 percent interval, and significant mineralogical and chemical differences are found on either side of this compositional range. The model proposes that the original undepleted upper mantle composition falls in this range, and that it may be calculated from the modal and chemical data. Comparison of the chemical composition of the Earth9s mantle by models based on extraterrestrial data to the Earth9s unaltered upper mantle by Ringwood9s pyrolite model and Nicholl9s model, and to the partial fusion-partial crystallization model reveals convergence toward a similar over-all composition for the Earth9s undepleted upper mantle. The averaged undepleted upper mantle mineralogical composition for the major phases in volume percent under Kilbourne Hole, New Mexico, is: olivine, 50.9 ± 10.9; orthopyroxene, 21.7 ± 6.2; clinopyroxene, 23.0 ± 6.7; and spinel, 4.3 ± 1.2. The averaged undepleted, vapor-free, upper mantle chemical composition in weight percent under Kilbourne Hole, New Mexico, is: SiO 2 , 42.86 ± 0.32; TiO 2 , 0.33 ± 0.07; A1 2 O 3 , 6.99 ± 0.28; Cr 2 O 3 , 0.18 ± 0.04; total iron as Fe, 7.22 ± 0.17; MnO, 0.14 ± 0.01; NiO, 0.20 ± 0.01; MgO, 35.07 ± 0.24; CaO, 4.37 ± 0.11; Na 2 O, 0.45 ± 0.08; K 2 O, 0.003 ± 0.002; CoO, 0.014 ± 0.001; CuO, 0.0012 ± 0.0005; and ZnO, 0.0087 ± 0.0008.

Journal ArticleDOI
TL;DR: In this article, the shape and orientation of deformed pebbles and oolites are studied as ellipses on plane sections, and a shape factor grid enables accurate calculation of the magnitude and orientations of the strain ellipse.
Abstract: To determine finite strain from material such as deformed pebbles and oolites, it is essential to consider the initial shape and orientation of the particles. The ellipsoidal objects are studied as ellipses on plane sections. The distribution of axial ratios and orientations of undeformed particles is used as the basis for a classification of initial distributions. Simple and general equations relating the initial and deformed shape and orientation of the particles are derived by considering the matrix algebra. A shape factor grid enables accurate calculation of the magnitude and orientation of the strain ellipse. A variety of important initial distributions can be determined and used. The sudden onset of continuous cleavage and oriented textures observed in some orogenic areas is partially due to the effect of the initial shape of crystals, grains, and clastic particles.

Journal ArticleDOI
TL;DR: The till pellets are especially significant in identifying an ancient glacial setting because they originate through a process unique to glaciers, the flow-and recrystallization-induced segregation of originally disseminated fine sediment as mentioned in this paper.
Abstract: Observations of icebergs in a modern glacial marine environment indicate that ancient rocks that received iceberg-rafted material should contain: (1) local concentrations of stones that originated when icebergs overturned, and (2) small pellets of till that were originally sediment filling the spaces between clear ice crystals. The till pellets are especially significant in identifying an ancient glacial setting because they originate through a process unique to glaciers—the flow-and recrystallization-induced segregation of originally disseminated fine sediment. Thus when freed by melting and deposited by iceberg rafting, the pellets would reliably indicate the presence of glacial ice in an ancient environment. In the Gowganda Formation, a Precambrian glacial deposit, strata that contain outsized, presumably iceberg-rafted stones also contain abundant small flattened clasts of unsorted graywacke interpreted as the lithified counterparts of the till pellets observed on modern icebergs.

Journal ArticleDOI
TL;DR: The present lack of volcanic activity in the Western Aleutian Arc correlates with the lack of underthrusting at the western Aleutians trench predictable by plate tectonic theory.
Abstract: The present lack of volcanic activity in the Western Aleutian Arc correlates with the lack of underthrusting at the western Aleutian trench predictable by plate tectonic theory. A simple plate model allows one to predict that another plate (Kula plate) and a spreading ridge (Kula ridge) lay south of the arc in early Tertiary time and that the Aleutian trench consumed the ridge in the middle Tertiary. Early Tertiary volcanism in the Western Aleutians may be related to underthrusting by the Kula plate. A mid-Tertiary orogeny in the Central Aleutians and a late Tertiary orogeny in Alaska may be related to the subduction of the Kula ridge.

Journal ArticleDOI
TL;DR: A study of 193 chemical analyses of plutonic rocks from 132 localities in the central Sierra Nevada showed convincingly that K2O decreases systematically westward and suggests that Fe2O3 and TiO2 may also decrease westward.
Abstract: A study of 193 chemical analyses of plutonic rocks from 132 localities in the central Sierra Nevada shows convincingly that K2O decreases systematically westward and suggests that Fe2O3 and TiO2 may also decrease westward and that FeO, MgO, and CaO may increase. The ratio K2O/SiO2 obviously decreases westward across six of eight provisionally established sequences of granitic rocks. Plots of analyses of rocks from each sequence form discrete fields that are strongly elongate toward zero K2O at 40 to 45 percent SiO2. The boundaries between fields on these plots and between fields on plots of normative minerals on triangular diagrams are sharp. Compositional trends within sequences are different than the compositional changes that take place across the batholith—rocks in the western Sierra Nevada probably are not compositionally identical with rocks that are present at depth beneath the eastern Sierra Nevada. Progressive decrease of K2O in the Paleozoic and Mesozoic country rocks westward across the batholith is consistent with the anatectic model for its origin. However, it also is consistent with the hypothesis developed to explain chemical patterns in volcanic island arcs—that K2O increases toward continental land masses because of increasing depth of magma generation along landward-dipping seismic (Benioff) zones. The seismic-zone hypothesis encounters several difficulties, but it cannot be ruled out.

Journal ArticleDOI
TL;DR: In this paper, a relative Sr depletion (compared to K, Rb, Ca, and Ba) in graywackes of the lower Fig Tree Group (Sheba Formation) is interpreted in terms of an abundance of Srdepleted igneous rocks in their source area.
Abstract: Graywackes of the Fig Tree Group in South Africa were derived from a diverse source area composed principally of chert, volcanics, granitic-metamorphic rocks, and some ultramafics. Chert and quarts pseudomorphs after shard-like fragments and well-formed plagioclase and K-feldspar crystals in some graywackes indicate that some detritus was derived from contemporary volcanism. Immature textures indicate mild source-area weathering and rapid erosion, deposition, and burial. An unusually large amount of dolomite in the graywackes was probably deposited by carbonate-rich pore fluids during diagenesis. A relative Sr depletion (compared to K, Rb, Ca, and Ba) in graywackes of the lower Fig Tree Group (Sheba Formation) is interpreted in terms of an abundance of Sr-depleted igneous rocks in their source area. Large amounts of Ni in all Fig Tree graywackes and shales appear to have been derived from ultramafic source rocks. A progressive stratigraphic increase in granitic components in the graywackes suggests progressive unroofing of a granitic metamorphic terrane which was initially covered by a thick sequence of Onverwacht or Onverwacht-like volcanic rocks. This source appears to have been located in central Swaziland. The relatively abundant granitic detritus in the graywackes indirectly records one or more pre-Fig Tree (3.4-4.0 b.y.) granite-forming events in southern Africa.

Journal ArticleDOI
TL;DR: In this article, a new genus Rhyssochonetes is proposed based on the new subspecies R. aurora solox, which is the first genus of chonetids to appear in the western United States.
Abstract: Marine onlap of major proportions generally has been attributed to the Late Devonian in the United States west of the Mississippi River, but this onlap ensued early during the Taghanic Stage and thus began as a late Middle Devonian event. Pre-Taghanic Givetian marine areas of eastern and western North America were not connected across the southern United States and were connected only briefly between western Canada and the north-central United States (Indiana and Michigan). Pre-Taghanic Middle Devonian brachiopod faunas of eastern and western North America belong to different faunal provinces, that is, to the Appalachian and Old World provinces respectively. The Taghanic onlap of the continental backbone in the southwestern United States provided shallow-water marine areas for dispersal of bethonic animals and the resulting intermigration brought an end to brachiopod provinciality that had prevailed since the Early Devonian. By analogy, provincial shifts in established faunal successions should provide dates for other sedimentary-tetonic events. Because Appalachian Province fossils are known to range as far as Colombia and Venezuela in Emsian-Eifelian time, but not at the same time to the American west, a large land barrier is postulated for times of provinciality, that is, during the intervals Ludlow-early Siegenian, and Emsian-mid-Givetian. Other intervals during the Silurian and Devonian were times of breaching of the land barrier by marine seas. Taghanic brachiopod faunas of Givetian age in western North America are represented by two faunas, a lower one with chonetids of the aurora type and Leiorhynchus of the mesacostale type, and an upper one with Leiorhynchus hippocastanea, Hadrorhynchia sandersoni, and Warrenella occidentalis. Elements of the latter fauna have been reported lower in the Givetian due to confusion with the fauna that includes Leiorhynchus castanea. The Leiorhynchus hippocastanea fauna occurs with hermanni-cristatus zone conodonts above the “C.” aurora fauna and marks the top of the Givetian, above the range of Stringocephalus in North America. The aurora group of chonetids is proposed as a new genus Rhyssochonetes, based on the new subspecies R. aurora solox.

Journal ArticleDOI
TL;DR: In this paper, the authors constructed composite velocity profiles for each rock in the sequence under study, and then spliced these curves proportional to the thickness of each succeeding stratum from the bottom up.
Abstract: Gravity sliding, major thrusting, and the displacement of large crustal plates are accomplished because strata with the lowest viscosity in a rock sequence readily deform by simple (rectilinear) shear flow when subjected to loading. These deforming strata are called decollement zones. Strata above these zones are transported without being deformed, provided their viscosity is one or more orders of magnitude larger than that of the decollement zone. Shear stresses acting at the base of these strata, that is, “basal drag,” are commonly much lower than stresses accompanying friction across a sole fault ( compare with Hubbert and Rubey, 1959). Thus, shear flow in decollement zones most likely represents the operative mechanism in natural settings. The speed of gravity sliding and the ease of tectonic transport increase with increasing decollement zone thickness, dip, and depth of burial, and decrease with increasing decollement zone viscosity. End effects, including (in two dimensions) updip attachment of gravity slides and frontal buttressing of both slides and tectonic plates, are as important as basal drag in determining the speed of slides and the resistance to transport of crustal plates. Structures in the strata above a decollement zone develop independently of those within the zone. In the upper plate, pull-apart grabens form to overcome updip attachment; step thrusts and folds form to overcome frontal buttressing. Structures within decollement zones are proposed to include intensely developed passive disharmonic folds, bedding plane faults at contacts between high and low viscosity strata and through high viscosity masses within the zone, chaotic zones comprised of blocks of high viscosity strata, infolds of adjacent high viscosity strata, and penetrative cleavage, phylonitization, and other features of deformation-induced recrystallization. Velocity profiles for simple shear portray the general mode of deformation and permit study of transport in specific geologic settings. Composite velocity profiles are constructed by first drawing individual profiles for each rock in the sequence under study. Segments from these curves proportional to the thickness of each succeeding stratum are then spliced together from the bottom up.

Journal ArticleDOI
TL;DR: Abrasion and its relationship to weathering was studied by running fresh and weathered specimens of Colorado River granitic gravel in a Kuenen-type abrasion tank at the University of Texas.
Abstract: Precambnan outcrops in the Llano Uplift of central Texas furnish the Colorado River with a distinctive granitic load which changes systematically as it moves downstream. In 160 miles between Austin and the Columbus-Eagle Lake area, coarse granitic gravel diminishes in size by about 50 percent and changes from a dommantly granite-gneissaplite assemblage to a dominantly pegmatite-graphic granite assemblage. Coarse quartz and chert gravel diminish in size by about 30 percent and 20 percent, respectively. In this reach, the Colorado flows on a flood plain whose alluvial width and depth stay fairly constant, a situation unfavorable for significant down-valley sorting (Mackin, 1963). Field observations generally lead to the same conclusion. Abrasion and its relationship to weathering was studied by running fresh and weathered specimens of Colorado River granitic gravel in a Kuenen-type abrasion tank at the University of Texas. Degree of weathering was slight to moderate, with biotite and feldspar the primary and secondary targets. Abrasion of fresh granitic gravel during 160 miles of travel produced only a 10 percent reduction in size. Abrasion of weathered gravel produced a reduction of more than 50 percent and was clearly related to lithology; the biotite-bearing rocks (granite, gneiss, and some aplite) were the least durable, the biotite-free rocks (pegmatite and graphic granite) most durable. Abrasion of quartz and chert produced a reduction in size of less than 10 percent. Changes in size and lithology of coarse granitic gravel along the lower Colorado River are best explained by abrasion of particles tha t weather slightly during periods of temporary alluvial storage. Reduction in size of quartz and chert are not satisfactorily explained for the reach between Austin and Columbus-Eagle Lake, but downstream from Eagle Lake, sorting appears to be in control.

Journal ArticleDOI
TL;DR: In this article, the results of analyses for condensates from 132 fumaroles of the Central American volcanoes Santiaguito, Fuego, Pacaya, Izalco, Cerro Negro, and Arenal are given.
Abstract: Results of analyses for Cl, F, SO 4 , Ca, Mg, Na, and K in 132 condensates from fumaroles of the Central American volcanoes Santiaguito, Fuego, Pacaya, Izalco, Cerro Negro, and Arenal are given. Each fumarole is of high temperature, greater than 150°C, and is located within 500 m of an active volcanic vent. Condensates were collected over a 5-yr period, August 1964-July 1969. The variation in chemistry of the condensates is shown to depend upon the fumarole location and the time of collection relative to the eruptive and cooling history of the volcano. Ground-water dilution also affects concentration because it is variable and cumulative. The data show that Cl and F in condensates decrease as the magma source is depleted and as the ground-water component in the fumarole increases. F is believed to be depleted by reaction with wallrock, as well. The alkalis, Na and K, are shown to come in part from the wallrock, and probably in part from the magma. Wallrock is suspected to be the chief source of Ca and Mg, as the Ca/Mg ratio in condensates is that of the wallrock. Fumaroles at main active vents are shown to be SO 4 rich, with comparatively low CI/SO 4 ratios. Fumaroles on flows are Cl-rich. The ratio CI/SO 4 decreases at fumaroles in or near a main vent at the beginning of an eruptive period. Regular monitoring of the condensate at such fumaroles should enable prediction of eruptive periods months in advance, although the exact timing would vary with location. The practicality of the method depends upon the existence of accessible fumaroles.

Journal ArticleDOI
TL;DR: In this paper, a regional compression axis, inferred from a laminar flow model for the mylonite zone, is compatible with a compression axis derived from slickensided fractures in granitic rocks north of the mylanite zone.
Abstract: Mylonitization in the southern part of the Cobequid Mountains resulted from deformation of igneous and sedimentary rocks during the Devonian Acadian orogeny. Detailed analyses of mineral lineation, penetrative s-surfaces, and mineral grain fabrics suggest that uniform tectonic flow within the mylonite zone produced rock strains with extension parallel to the mineral lineation. Quartz fabrics originated when large quartz crystals deformed into elongate granular aggregates. A regional compression axis, inferred from a laminar flow model for the mylonite zone, is compatible with a compression axis derived from slickensided fractures in granitic rocks north of the mylonite zone. The subhorizontal, southeasterly trending compression axes are perpendicular to the regional trend of the Appalachians mountain chain in Nova Scotia.

Journal ArticleDOI
TL;DR: Theoretical bursting pressures were calculated thcrmo-dynamically from vapor pressures of the hydrates and from humidities of the air for several air temperatures.
Abstract: Salt bursts can cause extensive spalling in stone structures. Water-soluble salts and their hydrates Na2CO3 H2O (Na2CO3-7H2O, Na2CO3 10H2O), Na2SO4 (Na2SO4 10H2O), and MgSO4-H2O (MgSO4 7H2O) are trapped in pores of stone and concrete by both infiltration upward from the ground and by the reaction of carbonate and silicate rocks with sulfuric and carbonic acids from polluted urban atmospheres. The crystallization of the salts and their recrystalhzation from a lower to a higher hydrate within the range of mineral stability may develop stresses of high magnitude. Theoretical bursting pressures were calculated thcrmo-dynamically from vapor pressures of the hydrates and from humidities of the air for several air temperatures.