Showing papers in "Geological Society of America Bulletin in 1985"

Devonian eustatic fluctuations in Euramerica

Abstract: The Devonian System of Euramerica contains at least 14 transgressive-regressive (T-R) cycles of eustatic origin. These are separated into three groups (or depophases) and from Carboniferous cycles by three prominent regressions. Twelve post-Lochkovian T-R cycles are recognized, and they commonly appear to result from abrupt deepening events followed by prolonged upward shallowing. Deepening events in the western United States (especially Nevada), western Canada, New York, Belgium, and Germany have been dated in the standard conodont zonation and are demonstrably simultaneous in several or all five regions. This synchroneity indicates control by eustatic sea-level fluctuations rather than by local or regional epeirogeny. Facies shifts in shelf sedimentary successions are more reliable indicators of the timing of sea-level fluctuations than are strandline shifts in the cratonic interior, because the latter are more influenced by local epeirogeny. Strandline shifts are most useful in estimating the relative magnitude for sea-level fluctuations. Devonian facies progressions and the three prominent regressions are of a duration and an order of magnitude that could have been caused by episodes of growth and decay of Devonian oceanic ridge systems. The described T-R cycles could have formed in response to mid-plate thermal uplift and submarine volcanism. The latter process may have been a control on small-scale (1–5 m thick), upward-shallowing cycles within the major T-R cycles. Continental glaciation could have been a factor in sea-level fluctuations only in the Famennian and could not have been responsible for the Devonian facies progressions or the numerous T-R cycles. The Frasnian extinctions were apparently cumulative rather than due to a single calamity. Two rapid sea-level rises occurred just before, and one at, the Frasnian-Famennian boundary. It is probable that this series of deepening events reduced the size of shallow-shelf habitats, caused repeated anoxic conditions in basinal areas, and drowned the reef ecosystems that had sustained the immensely diverse Devonian benthos.

Pan-African microplate accretion of the Arabian Shield

Abstract: The late Proterozoic Arabian Shield is composed of at least five geologically distinct terranes (microplates) separated by four ophiolite-bearing suture zones. Three ensimatic island-arc terranes occur in the western shield, whereas the two terranes in the eastern shield have continental affinities. The western two sutures are island-arc-island-arc joins, whereas the eastern two sutures collectively form a major coilisional orogenic belt. Accretion of the five terranes to form an Arabian neocraton occurred from 715 to 630 Ma. After accretion, intracratonic deformation and magmatism related to collision continued and resulted in the formation of molasse, intermediate to silicic volcanic rocks, peralka-line to peraluminous granites (640-570 Ma), and a major left-lateral wrench fault system (∼630-550 Ma) which displaced the northern part of the Arabian neocraton ∼250 km to the northwest. These tectonic events represent the accretion of the Arabian portion of Gondwanaland during the Pan-African event.

Rapid production of continental crust 1.7 to 1.9 b.y. ago: Nd isotopic evidence from the basement of the North American mid-continent

Abstract: Nd isotopic analyses of Precambrian granitic rocks from the central United States were made to determine crust-formation ages and thereby to investigate the structure and evolution of central North America. Samples of anorogenic granites that have 1.4-b.y. crystallization ages yield model TDM ages of 1.77 to 1.98 b.y. that indicate a previously undocumented 0.4- to 0.6-b.y.-old precrystallization crustal history. These and previously published data approximately define the boundaries of a large segment of continental crust that was formed from the mantle during a relatively short interval ∼1.9 b.y. ago. Both 1.4- and 1.8-b.y.-old samples from the Penokean terrene of central Wisconsin yield distinctly older TDM model ages of 2.1 to 2.3 b.y. Approximately 1.1-b.y.-old samples from the Llano uplift in Texas yield crust-formation ages of 1.3 to 1.4 b.y. The data appear to delineate provinces with well-defined, nonoverlapping crust-formation ages that young to the south, indicating episodic formation of the North American continental crust during Proterozoic time. The initial 143Nd/144Nd ratios of the 1.4-b.y.-old granites are varied ( ϵ Nd = +4.8 to −2.0). Most of the samples analyzed probably represent crustal melts, but the existence of some high initial ϵ Nd values indicates an admixture of mantle-derived material. The isotopic patterns suggest that the 1.4-b.y.-old “anorogenic” plutonism may be an inland manifestation of subduction activity related to formation of Llano crust. The data also indicate that the Penokean terrane is not entirely 1.8-b.y.-old, mantle-derived material, nor simply reworked Archean crust. It may represent a mixture of Archean and juvenile mantle material added during the 1.84 Ga Penokean orogeny. Patterns of continental growth deduced from this study emphasize the episodicity of crust formation. Construction of an apparent crustal growth curve for North America indicates that the average age of the continent is ∼2.2 b.y., which is significantly older than the average age of ∼ 1.5 b.y. derived from Rb-Sr studies.

Response of alluvial rivers to slow active tectonic movement

Abstract: Alluvial rivers respond to valley-slope deformation caused by active tectonics in various ways depending on the rate and amount of surficial deformation and on the type of river. On the basis of experimental results and field examples, hypothetical models of river response to anticlinal uplift and synclinal subsidence were developed for different types of alluvial rivers. An experimental braided channel responded to anticlinal uplift across the channel with degradation and terrace formation in the central part of the uplift. With subsidence, aggradation in the central reach was the main response. Transverse bars developed downstream of the subsidence axis. An experimental meandering channel responded to slope steepening with a sinuosity increase. Bank erosion and point-bar growth occurred downstream of the anticlinal axis and upstream of the synclinal axis. Upstream of the uplift axis and downstream of the subsidence axis, where the slope was flattened, water flooded over bars. Local convexity in longitudinal profiles of the middle Rio Grande, New Mexico, is considered to be formed by a domal uplift. Local aggradation and degradation could be explained by the effect of uplift. The San Joaquin River, California, which is now highly controlled, does not show clear adjustment to the rapid subsidence due to ground-water withdrawal. It shows, however, a channel-pattern adjustment to active tectonic subsidence that has been occurring for a long time. The San Antonio and Guadalupe Rivers in Texas both increase their sinuosity significantly where monoclinal movements steepen valley slopes.

Migration of glacial centers across Gondwana during Paleozoic Era

Abstract: Continental glaciation, as recorded by sedimentary facies and by pavements scoured into underlying rocks, affected northern Africa at the end of the Proterozoic Era. This glaciation was followed in the Cambrian Period by a long, warmer interval without recorded ice sheets upon the Gondwana supercontinent. Strong glaciation ensued in Late Ordovician time in central northern Africa, and centers moved into then adjoining northern Brazil and on westward into southern Brazil, southern Africa, and Bolivia and into northern Argentina by the Early Silurian. From Middle Silurian time, world-wide and Gondwanan climate ameliorated until Late Devonian (Famennian) time, when glaciation again affected Brazil and perhaps parts of Africa. Glacial centers apparently waned after the late Famennian (latest Devonian) but waxed again in Andean regions and northern Brazil in Early and mid-Early Carboniferous times to begin the strong Late Paleozoic Ice Age. Many ice caps and ice sheets came and went across the wide reaches of Gondwana during the late Paleozoic, beginning on the west, culminating in southern Africa in the Late Carboniferous, continuing strongly in India and Australia in the Permian, and dying out in eastern Australia and in Antarctica in early Late Permian time. The path of ice-center migration closely follows published paleomagnetic wander paths. The record suggests that, among other terrestrial factors which cause ice ages, glaciation flourished when Gondwana lay in south polar regions and that glaciation disappeared when Gondwana glided, so that the pole lay in oceanic or coastal regions.

Structural styles in Mesozoic and Cenozoic mélanges in the western Cordillera of North America

Abstract: The term “melange” is currently used to describe several different kinds of mudstone-rich rocks that are broadly characterized by an obscure stratigraphy, stratal disruption, or a chaotic, “block-in-matrix” fabric. Four types of melange, which can be defined in outcrop on the basis of mesoscopic fabric and lithologic composition, are particularly widespread and distinctive. Type I includes sequences of originally interbedded sandstone and mudstone that record incipient to thorough disruption and fragmentation of strata accomplished largely by layer-parallel extension. Type II consists of similarly deformed, thin layers of green tuff, radiolarian ribbon chert, and minor sandstone originally interbedded with black mudstone. Disruption in both types I and II, which probably occurred while the sediments were incompletely consolidated, has been ascribed to either imbricate faulting in accretionary wedges or gravitationally driven deformation. Type III comprises inclusions of diverse shapes, sizes, and compositions enveloped in a locally scaly, pelitic matrix. The ultimate source of fragments is obscure, because the majority were not derived by either the progressive disruption of interbedded sediments or in situ tectonic plucking and abrasion of adjacent rocks. Although some type III melanges may have originated deep within accretionary prisms, final emplacement as olistostromes (muddy debris-flow deposits) or mud diapirs seems likely. Type III melanges are mechanically analogous to scaly, “sheared” serpentinites; many probably have been tectonically remobilized or even intruded into shallow-level fault zones. Type IV consists of lenticular inclusions bounded by an anastomosing network of subparallel faults. Their fabric records progressive slicing in brittle fault zones. Each of the four types of melange described here could, in theory, have formed in a variety of settings on or within an accretionary wedge at an active convergent margin; none can yet be singled out as a uniquely diagnostic “subduction melange.”

A theoretical model of the fracture of rock during freezing

Abstract: We present a mathematical model for the breakdown of porous rock by the growth of ice within cracks. The model is founded upon well-established principles of fracture mechanics and recent advances in soil physics, iJong with the assumption that progressive crack growth results from water migrating to ice bodies in cracks, much as water migrates to ice lenses in freezing soil. Our model predicts crack-growth rates compatible with empirical data. Calculations for a granite and a marble indicate that sustained freezing is most effective in producing crack growth when temperatures range from —4 °C to -15 °C. At higher temperatures, thermodynamic limitations prevent ice pressure from building up sufficiently to produce significant crack growth; at lower temperatures, the migration of water necessary for sustaining crack growth is strongly inhibited. In hydraulically "open" systems, in which pore-water pressure remains near atmospheric pressure during the freezing process, crackgrowth rates during continuous cooling will generally be greatest at low rates of cooling, less than -0.1-0.5 °C/h. At higher rates of cooling, the influx of water to growing cracks is significantly inhibited. The model delineates clearly the role of material parameters (elastic moduli, fracture-mechanical properties, grain size and shape, and crack size), environmental conditions (temperature, temperature gradient, water pressure), and time in frost damage to rocks. Our calculations, along with recent experimental work on water migration in freezing rocks (Fukuda, 1983), lead us to question the widely accepted importance of two phenomena—freezing of water in sealed cracks and freeze-thaw cycling—in the fracture of rock exposed to natural freezing conditions.

A Cretaceous and Jurassic geochronology

Abstract: An integrated geomagnetic polarity and geologic time-scale for the Jurassic and Cretaceous periods is presented, based on various methods according to the availability of definitive isotopic ages. An age-calibrated sea-floor–spreading model is used to interpolate the ages of the Kimmeridgian to Barremian, and the Campanian to Maestrichtian stages. Numerical age estimates for the Aptian to Santonian stage boundaries follow published isotopic age determinations. The hypothesis of equal duration of ammonite zones is employed as a vernier to apportion time for the Hettangian to Oxfordian stages. The new scale results in ages of 208 Ma for the base of the Jurassic, 144 Ma for the Jurassic/Cretaceous boundary and 66.5 Ma for the top of the Cretaceous. The integrated biostratigraphic, magnetostratigraphic, and geochronometric record serves as a working hypothesis for geologic correlation of Jurassic and Cretaceous strata.

Case for periodic, colossal jökulhlaups from Pleistocene glacial Lake Missoula

Abstract: Two classes of field evidence firmly establish that late Wisconsin glacial Lake Missoula drained periodically as scores of colossal jokulhlaups (glacier-outburst floods). (1) More than 40 successive, flood-laid, sand-to-silt graded rhythmites accumulated in back-flooded valleys in southern Washington. Hiatuses are indicated between flood-laid rhythmites by loess and volcanic ash beds. Disconformities and nonflood sediment between rhythmites are generally scant because precipitation was modest, slopes gentle, and time between floods short. (2) In several newly analyzed deposits of Pleistocene glacial lakes in northern Idaho and Washington, lake beds comprising 20 to 55 varves (average = 30–40) overlie each successive bed of Missoula-flood sediment. These and many other lines of evidence are hostile to the notion that any two successive major rhythmites were deposited by one flood; they dispel the notion that the prodigious floods numbered only a few. The only outlet of the 2,500-km 3 glacial Lake Missoula was through its great ice dam, and so the dam became incipiently buoyant before the lake could rise enough to spill over or around it. Like Grimsvotn, Iceland, Lake Missoula remained sealed as long as any segment of the glacial dam remained grounded; when the lake rose to a critical level ∼600 m in depth, the glacier bed at the seal became buoyant, initiating underflow from the lake. Subglacial tunnels then grew exponentially, leading to catastrophic discharge. Calculations of the water budget for the lake basin (including input from the Cordilleran ice sheet) suggest that the lakes filled every three to seven decades. The hydrostatic prerequisites for a jokulhlaup were thus re-established scores of times during the 2,000- to 2,500-yr episode of last-glacial damming. J Harlen Bretz9s “Spokane flood” outraged geologists six decades ago, partly because it seemed to flaunt catastrophism. The concept that Lake Missoula discharged regularly as jokulhlaups now accords Bretz9s catastrophe with uniformitarian principles.

Sapping processes and the development of theater-headed valley networks on the Colorado Plateau

Abstract: Ground-water sapping is an erosional process that produces landforms with unique characteristics. Sapped drainage systems differ in morphology, pattern, network spatial evolution, rate of erosion, and degree of structural control from their fluvial counterparts. Investigation of deeply entrenched theater-headed valleys in the Glen Canyon region of the Colorado Plateau indicates that ground-water sapping is the predominant mechanism of growth. The canyons occur in the Navajo Sandstone, a highly transmissive aquifer underlain by essentially impermeable rocks. Within this formation, two populations of valleys with markedly different features are identified. The first group exhibits theater heads: longitudinal profiles with high, step-like discontinuities and commonly asymmetric, structurally controlled patterns. The second group is characterized by tapered terminations; a relatively smooth, concave-up profile; and a more arborescent network. Because the valleys have developed under the same lithologic, stratigraphic, and climatic conditions, the differences in form are attributed primarily to structural constraints that determine the relative effectiveness of overland-flow and ground-water (sapping) processes. Of particular importance is the dip direction of the beds relative to that of valley growth, inasmuch as this relationship controls the occurrence and distribution of ground-water seepage at valley walls. Laterally flowing ground water also exploits fractures at depth, so that the drainage pattern of theater-headed valleys reflects that of the regional jointing pattern. Martian valleys exhibit numerous morphologic similarities to canyons formed in the Navajo Sandstone. These include theater-shaped heads, nearly constant width from source to outlet, high and steep sidewalls, hanging outlets, and a large degree of structural control. Although the constituent materials, scale, climate, structure, and ground-water conditions of Mars cannot be replicated in any Earth analog, the striking similarities in form suggest that the gross geomorphic processes may be similar and that sapping processes have operated to create the Martian valleys.

Laacher See Tephra: A widespread isochronous late Quaternary tephra layer in central and northern Europe

Abstract: A late Quaternary tephra layer, widespread in central and northern Europe, resulted from explosive Plinian and phreatomagmatic eruptions of the Laacher See Volcano 11,000 yr B.P. The tephra is distinguished from other late Quaternary andesitic-rhyolitic airfall tuff layers in northern Europe and from basaltic or trachytic tuff deposits in southern Europe by its phonolitic composition and abundance of sanidine, plagioclase, clinopyroxene, amphibole, and sphene. The proximal tephia sequence at Laacher See is divided into three main deposits: the predominantly Plinian deposits of Lower and Middle Laacher See Tephra (LLST and MLST) and phreatomagmatic deposits of the Upper Laacher See Tephra (ULST). The MLST member is further subdivided into beds A, B, and C1, C2, and C3. The chemical composition of the magma is highly differentiated phonolite in the LLST to MLST B sections but mafic phonolite in the MLST C1 to ULST sections. All deposits are considered to be isochronous, the frequency maximum of 16 radiocarbon datings indicating an eruption about 11,000 ±50 yr B.P. Distal ash was deposited in three main fans directed to the northeast (LST traced up to 1,100 km distance), south (LST traced up to 600 km), and southwest (LST traced up to 100 km). Tephrostratigraphic correlation of the distal ash deposits is based on (a) the major-element composition of glass shards, (b) lithology, and (c) heavy-mineral analyses. The northeastern fan consists of deposits from LLST, MLST B, and MLST C1 eruptive phases, the southern fan comprises MLST A, MLST C2, and ULST deposits, and the southwestern fan consists exclusively of ash from the ULST eruptive phase. Northeastern transport of ash during eruptive phases, with high Plinian eruption columns, but southern and southwestern transport of ash along phases of relatively low eruption columns, are interpreted in terms of prevailing southwesterly paleowinds at high altitudes (tropopause level?) but northerly winds dominating in the lower atmosphere. The Laacher See eruption columns were emplaced into an atmosphere vertically zoned with respect to paleowind directions, which also explains the near-vent shifting of LLST, MLST B, and MLST C1 iso-pach axes from east-southeast to northeast within the first 20 km of transport.

Major dextral transcurrent displacements along the Northern Rocky Mountain Trench and related lineaments in north-central British Columbia

Abstract: The Northern Rocky Mountain Trench and a number of other prominent lineaments, along and east of the eastern margin of the Intermontane Belt, mark faults along which dextral transcurrent movements have been dominant. Offsets of shelf to off-shelf facies boundaries in lower Paleozoic rocks indicate a cumulative displacement of at least 750 km, and probably >900 km, within the system of faults related to those in the Northern Rocky Mountain and the Tintina Trenches. Farther west, another system of faults appears to offset plutons and stratigraphic assemblages along the eastern margin of the Intermontane Belt by as much as 300 km. These faults, including the Kutcho and the Pinchi, connect in part with the Teslin Suture Zone in Yukon Territory and probably with the Fraser River-Straight Creek fault zone in southern British Columbia. Although dextral transcurrent faulting may have taken place between the Middle Jurassic and early Cenozoic, the most convincing evidence points to middle Cretaceous and particularly to early Cenozoic (Eocene?) displacements. The Eocene(?) movements were temporally related to plutonism, volcanism, lamprophyre dike emplacement, high heat flow, sedimentation in grabens, and rapid uplift of northwesterly trending elongate ranges. Climactic episodes of granite emplacement, particularly in and near the northern Omineca Crystalline Belt, at ∼100 m.y., 70 m.y., and 50 m.y. ago may have been facilitated by changes from dominantly compressional to dominantly transcurrent and related tensional strain.

Tectonic synthesis of the Taconian orogeny in western New England

Abstract: A tectonic synthesis based on stratigraphic and structural analysis of western New England is proposed for the Ordovician Taconian orogeny. It emphasizes arc-continental collision in which ocean-floor, continental-margin, and ensialic-rift rocks were imbricated westward in a repeatedly deformed accretionary wedge. Continued compression displaced segments of the North American sialic crust to the west and deformed the earlier emplaced slices of the Taconic allochthons which were derived from the continental margin. Critical arguments for this synthesis are (1) the west-to-east stratigraphic relations among the basal rift clastic rocks of the Dalton, Pinnacle, and Hoosac Formations of late Precambrian to Early Cambrian age; (2) the stratigraphic and sedimentological similarities between the rocks of the lower Taconic sequence and rocks in the Pinney Hollow and Underhill slices to the east and north of the Green Mountain massif; (3) the environmental similarities between the Cambrian and Lower Ordovician section of the Giddings Brook slice and the age-equivalent section in the St. Albans synclinorium; (4) the presence of carbonate platform rocks as slivers between each of the successively higher and younger premetamorphic slices (groups 1 and 2) of the Taconic allochthons; (5) the presence of synmetamorphic, fault-related structures in the youngest and highest slices (group 3) of the Taconic allochthons; (6) the recognition of extensive thrust zones in the pre-Silurian eugeoclinal sequence east of the middle Proterozoic basement of the Housatonic, Berkshire, Green Mountain, and Lincoln massifs; (7) the location of the Taconic root zone within the pre-Silurian eugeoclinal sequence; (8) the recognition of numerous faults in the serpentinite belt; (9) the similarity between the rocks of the Moretown Formation and modern fore-arc basin sequences; (10) the recognition that the volcanic arc-continental complexes of the Ascot-Weedon and Bronson Hill have been displaced westward over the Moretown and/or Hawley Formations along such faults as the Bristol and Coburn Hill thrusts; (11) the allochthonous and internally imbricated nature of the North American basement in the Berkshire massif; (12) the proposition that the Housatonic, Green Mountain, and Lincoln massifs, as well as the middle Proterozoic cored domes of southeastern Vermont, are also thick sialic slices of North American basement; (13) the recognition of medium-high- to high-pressure metamorphic mineral assemblages in the pre-Silurian eugeoclinal rocks of Vermont; and (14) the recent synthesis of isotopic age data by Sutter and others (1985). On the basis of an analysis of the foregoing arguments and relationships, a chronological sequence of seven structural sections between Albany, New York, and the Bronson Hill anticlinorium in central Massachusetts is used to depict the evolution of the Taconian orogeny. Retrodeformed distances are based on structural overlap and restoration of the Taconic slices to their depositional setting along the ancient North American continental margin. These easterly younging, diverticulated slices formed as a result of horizontal compression rather than gravity sliding. This palinspastic analysis implies the following. (1) Approximately 1,000 km of shortening has occurred during the emplacement of the Taconic allochthons and the subsequent imbrication of North American basement as thick sialic slices. Approximately 330 km of this shortening is attributed to multiple cleavage generations. (2) Repeated movement along such major surfaces as the Cameron9s Line-Whitcomb Summit-Belvidere Mountain thrust zone has buried the Taconic root zone. We suggest that the northern extension of this root zone is exposed to the east of the Lincoln massif in Vermont where the Underhill, Pinney Hollow, and Hazens Notch Formations are exposed. These formations, here considered thrust slices, disappear along the Belvidere Mountain-Whitcomb Summit thrust zone as it is traced southward into western Massachusetts and western Connecticut. (3) Taconian metamorphic rocks, particularly the older medium-high-pressure rocks in northern Vermont, have been transported westward on such reactivated surfaces as the Belvidere Mountain thrust. (4) The anticlinorial form of the middle Proterozoic basement in the Green Mountain and Lincoln massifs may have resulted from fault-bend folding on deep mantle-involved thrusts that developed late in the Taconian orogeny.

Holocene rate of slip and tentative recurrence interval for large earthquakes on the San Andreas fault, Cajon Pass, southern California

Abstract: Detailed mapping of the San Andreas fault zone where it crosses Cajon Creek, southern California, has revealed a number of late Quaternary deposits and geomorphological features offset by the fault. Radiocarbon dates from alluvial and swamp deposits provide a detailed chronology with which to characterize the activity of the San Andreas fault. Four independent determinations of the slip rate on the San Andreas fault yield an average rate of 24.5 ± 3.5 mm/yr for the past 14,400 years. The similarity of the four values, which span different intervals of time from 5900 to 14,400 years ago, suggest that the slip rate has been constant during this period. The slip rate confirms that the San Andreas fault is accumulating slip faster than any other fault in the plate boundary, throughout California. Also, the sum of the slip rates on the San Andreas (24.5 mm/yr) and the San Jacinto (10 mm/yr) faults south of their junction is the same as the San Andreas north of their junction (35 mm/yr). An excavation provided evidence for at least two and up to four earthquakes that caused rupture on the fault between 1290 and 1805 A.D., and tentative evidence for six earthquakes in about the last 1000 years. Both lines of evidence imply an average recurrence interval for large earthquakes of about 1 to 2 centuries. Combined with the historic record, this investigation indicates that the last major earthquake at Cajon Creek was probably at the beginning or middle of the 18th century. While the record at Cajon Creek may suggest that the next major earthquake on the San Andreas fault is overdue, the data can be interpreted to indicate that the recurrence interval for earthquakes on the San Andreas south of the 1857 rupture is 3 to 4 centuries and that the anticipated event should not be expected until the next century.

Esker characteristics in terms of glacier physics, Katahdin esker system, Maine

Abstract: The characteristics of large, subglacially formed eskers, such as the Katahdin system, are closely related to two special peculiarities of water-filled tunnels along the beds of ice sheets: (1) the water pressure approximates the weight of the overlying ice; and (2) in tunnels that descend and those that ascend less steeply than ∼1.7 times the ice-surface gradient, the walls melt, producing a sharply arched tunnel cross section, whereas in those that ascend more steeply, they freeze, producing a wide, low one. The first peculiarity primarily governs the paths of these eskers. It causes the tunnels to follow the paths ordinary rivers would follow were the land tipped downglacier ∼11 times the local ice-surface gradient. The paths therefore trend in the general direction of the former ice flow but tend to deviate so as to follow valleys and to cross divides at the lowest passes, as observed. Ice-surface gradients calculated from path deviations at two localities on the Katahdin esker system indicate relatively thin, sluggish ice the surface of which lay ∼200 m below the summit of Mount Katahdin, in agreement with independent geologic evidence. The second peculiarity primarily governs the form, composition, and structure of these eskers. Strong melting causes a large inflow of basal ice and entrained debris to the tunnel and produces sharp-crested eskers of poorly sorted, poorly bedded sand, gravel, and boulders with lithologies like the adjacent till, whereas weaker melting produces multiple-crested ones of similar composition. Freezing precludes inflow and produces broad-crested eskers of fairly well-sorted, well-bedded, more water-worn, coarse sand with few large clasts. Ice-surface gradients calculated from transitions from the multiple-crested type to small areas of broad-crested type on the Katahdin system agree closely with those computed from the paths at nearby localities. An anomalously low gradient calculated from a transition to an area of broad-crested type approximately twice as wide and long as the probable ice depth apparently confirms that, as expected, the basal ice was supported by water pressure over most, if not all, of the width of the esker.

Morphology of a delta prograding by bulk sediment transport

Abstract: A theoretical geomorphic model is developed for the progradation of a river delta in which the patterns of deposition and movement of sediment on the delta front slope are dominated by bulk-transport processes, such as creep and landsliding. The model predicts an exponential delta front profile, in which the maximum slope is controlled by the sediment supply to the delta, the rate of sediment transport on the delta front slope, and the depth of the receiving basin. Applications of the model to the Fraser and Mississippi River Deltas and to the Rhine River Delta in Lake Constance give good agreement with the progradation and morphology of these deltas. A difference in transport coefficient between the Mississippi River and the other deltas is attributed to differences in sediment composition and/or rate of sediment supply between this and the other deltas. The derived sediment-transport coefficients are found to be much higher than those for subaerial hillslopes. Our results suggest that deltas prograding by the processes considered in this paper will adjust quickly to changes in sediment supply.

Initiation and flow behavior of the 1980 Pine Creek and Muddy River lahars, Mount St. Helens, Washington

Abstract: Two large, high-velocity lahars (volcanic debris flows) were triggered by a pyroclastic surge during the first few minutes of the May 18, 1980, eruption of Mount St. Helens. The initial surge cloud evolved progressively by gravity segregation from a gas-mobilized, highly inflated density flow to a dense, water-mobilized, basal debris flow (lahar) and accompanying ash cloud as it flowed down the east flank of the volcano. The main source of the water for the lahars was probably from eroded snow and ice incorporated into the flow by turbulent mixing, but ground water, expelled together with the rock debris by the initiating volcanic explosions, also may have contributed. Peak lahar discharge from the Pine-Muddy fan, upper Smith Creek, and Ape Canyon probably exceeded 250,000 m 3 /s initially but decreased exponentially in the downstream direction. Total volume of the lahars was in excess of 1.4 × 10 7 m 3 . Initial peak-flow velocities in excess of 30 m/s also decreased markedly downstream. Where flow was not impeded, velocity was strongly related to the depth-slope term (R 2/3 S 1/2 ) from the Manning uniform-flow equation as a power-law function. During much of the route traveled, lahar flow appears to have been supercritical. Deposits left in channels were generally thin relative to flow depth (0 to 2.5 m). Particles up to small boulder size were randomly distributed in the poorly sorted, nonstratified matrix, indicating complete suspension in a fully developed debris-flow slurry; however, much larger clasts were transported as “bed-load.” Computed sediment concentrations of matrix slurry samples ranged from 84% to 91% solids by weight and were similar for the two lahars. Two indirect methods for computing peak-flow velocity, previously only tentatively applied to debris flows, were tested for accuracy by comparing computed lahar arrival times with recorded arrival times at Swift Reservoir. The computed velocities appear to be ∼15% slower than the recorded velocities, which is consistent with the restriction that the velocity formulas produce only minimum values.

Late Cenozoic landscape evolution on lava flow surfaces of the Cima volcanic field, Mojave Desert, California

Abstract: Landscape evolution in the eastern Mojave Desert is recorded by systematic changes in Pliocene to latest Pleistocene volcanic land-forms that show discrete periods of eolian deposition, surface stabilization, drainage-network expansion, and erosion on basaltic lava flows. These processes are documented by K-Ar dating in conjunction with morphometric, sedimentologic, pedologic, and geophysical studies. Lava-flow surfaces are composed of constructional bedrock highs and accretionary eolian mantles with overlying stone pavements. The stratigraphy of these mantles records episodic, climatically induced influxes of eolian fines derived from playa floors and distal piedmont regions. The relative proportions of mantle and exposed bedrock vary with flow age, and flows between 0.25 and 0.75 m.y. old support the most extensive eolian mantle and pavement reflecting landscape stability. Drainage networks evolve on flows by (1) rapid initial extension, (2) maximum extension and elaboration, and (3) abstraction of drainage. Increases in bedrock exposures and erosion of the eolian mantle on flows >0.70 m.y. old coincide with maximum drainage extension and significant changes in soil and hydrologic properties within this mantle. Increasing the content of pedogenic clay and CaCO 3 causes the accretionary mantle9s permeability to decrease; decreased mantle permeability promotes increased runoff, surface erosion, and drainage development. In the late Cenozoic landscape evolution of lava flows, four major stages reflect variations in landscape stability that are controlled by the impact of episodic influxes of eolian fines and increasing soil-profile development on infiltration-runoff properties of the flow surfaces.

U-Pb ages from the Nipigon plate and northern Lake Superior

Abstract: U-Pb ages have been measured for six lithologies that are relevant to the Proterozoic igneous activity around Lake Nipigon and the northern part of Lake Superior. The data show the presence of previously unrecognized pre-Keweenawan felsic magmatic events and provide a framework for relating events in the Nipigon plate to the Keweenawan rift. Tonalite gneiss from the Archean basement underlying the Nipigon plate has a minimum age of 2716 Ma. An anorogenic granite pluton at English Bay, Lake Nipigon, that is observed to grade into rhyolite is dated at 1536.7 + 10/−2.3 Ma. The rhyolites are intercalated with quartz arenites, and this may date the initiation of Sibley Group sedimentation. Samples (two) of the Logan diabase sills from different parts of Lake Nipigon have been dated using zircon and baddeleyite. The zircon fractions define an age of 1108.8 + 4/−2 Ma. A rhyolite from the Osier volcanics at Agate Point on the north shore of Lake Superior gives an age of 1097.6 ± 3.7 Ma. This is from the magnetically reversed sequence and provides a maximum age for the magnetic reversal. The zircons show evidence of inheritance and define a mixing line with an upper intercept of 2635 +143/−125 Ma. A porphyry, probably a flow, from the base of the Osier Group, dated at 1107.5 + 4/−2 Ma, is similar in age to the Logan sills. The presence of inherited zircons in the felsic rocks of the Osier Group indicates partial melting of Aichean crust during emplacement of basaltic magma. A variety of zircon ages from a conglomerate containing felsic porphyry clasts at the base of the Osler Group suggests that pre-Keweenawan felsic magmatism took place at about 1730 Ma and 1600 Ma in the Black Bay Peninsula area.

Structure and deformation history of the Bitlis suture near Lake Hazar, southeastern Turkey

Abstract: Detailed mapping of the Bitlis suture zone near Lake Hazar shows that it is composed of three tectonostratigraphic units which crop out as distinct north-dipping thrust slices. From south to north, these are the Puturge Metamorphic Complex, the Maden Melange, and the Elazig Igneous Complex. The Puturge Metamorphic Complex consists of pre-Tertiary, continental-margin sediments metamorphosed to the greenschist facies during the Campanian-Maastrichtian and deformed by four generations of structures. These are, in sequence (1) isoclinal folds and a transposition foliation, (2) open folds, crenulation cleavage, and north-dipping thrust faults, (3) kink bands, and (4) small-displacement faults. The Maden Melange represents middle Eocene, back-arc-basin sediments and volcanics metamorphosed to the greenschist facies and deformed by three generations of structures: (1) a north-dipping cleavage, (2) kink bands, and (3) small-displacement faults. The Elazig Igneous Complex comprises an imbricated Maastrichtian-early Eocene island arc and young marginal-basin terrain. Thrust faults between units are north-dipping, listric, and form a thin-skinned system. These features suggest the following deformational and tectonic history. The Puturge Metamorphic Complex was generated by isoclinal folding and metamorphism of sediments composing the Arabian continental margin during Campanian-Maastrichtian ophiolite obduction to the south. The Elazig arc developed on the deformed margin because of subsequent southward subduction of oceanic lithosphere. The arc migrated to the north, opening a back-arc basin. By the early Eocene, the back-arc basin was filling with volcaniclastics of the Maden Melange, and the Elazig arc collided with a continent to the north. During middle to late Eocene, convergence caused thrust stacking of the Elazig, Maden, and Puturge Complexes; second-generation deformation in the Puturge Complex; and first-generation deformation in the Maden Melange. Continued convergence with the Arabian plate in the late Miocene caused kink banding in the Puturge and Maden Complexes. Since the late Miocene, this convergence has been accommodated by shortening and thickening along numerous internal faults.

Origin and tectonic evolution of the Maclaren and Wrangellia terranes, eastern Alaska Range, Alaska

Abstract: Major portions of the eastern Alaska Range, south of the Denali fault, in the McCarthy, Nabesna, Mount Hayes, and eastern Healy quadrangles, consist predominantly of the Maclaren and Wrangellia tectono-stratigraphic terranes. The Maclaren terrane consists of the Maclaren Glacier metamorphic belt and the regionally deformed and metamorphosed East Susitna batholith. The Maclaren Glacier metamorphic belt is composed of argillite, metagraywacke, and sparse andesite flows that are progressively regionally metamorphosed from lower greenschist facies to middle amphibolite facies near the East Susitna batholith. The East Susitna batholith is composed of gabbro, quartz diorite, granodiorite, and sparse quartz monzonite. Isotopic ages are as old as a K-Ar hornblende age of 87.5 m.y., possibly reset, and a U-Pb zircon age of 70 m.y. The batholith is intensely deformed and regionally metamorphosed under conditions of the middle amphibolite facies. The Wrangellia terrane is divided into two subterranes: (1) the Slana River subterrane, composed of late Paleozoic andesite to dacite flows, tuff, limestone, and argillite, unconformably overlying massive basalt flows of the Triassic Nikolai Greenstone, Late Triassic limestone, and younger Mesozoic flysch; and (2) the Tangle subterrane, a deeper-water equivalent of the Slana River subterrane, composed of late Paleozoic and Early Triassic aquagene tuff, chert, minor andesite tuff and flows, limestone, unconformably overlying pillow basalt and massive basalt flows of the Triassic Nikolai Greenstone, and Late Triassic limestone. Both subterranes are intruded by locally extensive gabbro and diabase dikes and by cumulate mafic and ultramafic sills. Less extensive terranes (two) are the Clearwater terrane, a sequence of intensely deformed chlorite schist, muscovite schist, marble, and greenstone of Late Triassic age; and an unnamed terrane of ultramafic and associated rocks of presumable Paleozoic or Mesozoic age. Each terrane or subterrane generally has (1) a distinctive time-stratigraphic sequence reflecting a unique geologic history; (2) a missing provenance for bedded sedimentary or volcanic rocks; and (3) bounding thrust or strike-slip faults, interpreted as accretionary sutures. The Maclaren and Wrangellia terranes are juxtaposed along the Broxson Gulch thrust, which consists of an imbricate series of north-dipping thrust faults. Paralleling the Broxson Gulch thrust, a few kilometres to the south, is the north-dipping Eureka Creek thrust, along which are juxtaposed the Slana River and Tangle subterranes. The Maclaren terrane is correlated with the Kluane Schist and the Ruby Range batholith in the southern Yukon Territory, which represent the northward extension of the Taku and Tracy Arm terranes. If correct, this correlation defines a minimum displacement of the Maclaren terrane along the Denali fault of ∼400 km. The Maclaren terrane is interpreted to have formed in a synorogenic Andean-type arc setting on the west margin of Mesozoic North America in the middle to late Mesozoic and early Cenozoic. The Wrangellia terrane is interpreted to have initially formed in an island-arc setting during the late Paleozoic. Subsequently in the Late Triassic, the Wrangellia terrane underwent rifting near the paleoequator, with formation of the Nikolai Greenstone and associated mafic and ultra-mafic igneous rocks. In the middle and late Mesozoic, Wrangellia migrated toward, and was accreted during, the middle Cretaceous to the Maclaren terrane along the Broxson Gulch thrust. Subsequent dispersion of both the Maclaren and Wrangellia terranes along the Denali fault and the Broxson Gulch thrust commenced during the early Tertiary and continues through the present.

K-Ar and 40Ar/39Ar dating of the hominid-bearing Pliocene-Pleistocene sequence at Koobi Fora, Lake Turkana, northern Kenya

Abstract: In the Koobi Fora region, east of Lake Turkana, northern Kenya, there occurs a sequence ∼500 m thick of lacustrine, fluvial, and deltaic sediments that contains abundant vertebrate fossils, including hominids, as well as stone tools. Rhyolitic tuffs within the sedimentary sequence have facilitated stratigraphic mapping. Some of the tuffs contain pumice clasts, from which anorthoclase phenocrysts have been separated, providing ideal material for K-Ar and 40 Ar/ 39 Ar dating. Seven tuffs have been dated, generally yielding concordant ages on multiple samples from each tuff. Results are consistent with the stratigraphic sequence. The 40 Ar/ 39 Ar age spectra give nearly ideal flat patterns, indicating that the feldspars have remained undisturbed since crystallization and cooling, with no evidence of thermal overprinting. As the pumice clasts are considered to have been deposited very soon after their eruption, the measured ages provide a close approximation to that of deposition of the tuffs. The Moiti Tuff, ∼30 m above the base of the sequence, has a maximum age of 4.10 ± 0.07 Ma. The Toroto Tuff, some 70 m higher in the sequence, is very securely dated at 3.32 ± 0.02 Ma. Stratigraphically higher tuffs and their ages include the Ninikaa Tuff, 3.06 ± 0.03 Ma; the KBS Tuff, 1.88 ± 0.02 Ma; the Malbe Tuff, 1.86 ± 0.02 Ma; the Chari Tuff, 1.39 ± 0.02 Ma, and the Silbo Tuff, 0.74 ± 0.01 Ma, near the top of the sequence. The geochronology and geological data indicate at least three hiatuses in the sequence, each on the order of 0.5 to 0.7 Ma duration. Conventional K-Ar age measurements on basalts from the basin margin suggest that deposition of the Koobi Fora Formation began no earlier than ∼4.3 Ma ago in the early Pliocene. These results provide a well-documented numerical time framework for the sedimentary sequence in the Koobi Fora region and for the hominids and other fossils contained therein. Most of the hominid fossils, including forms assigned to the Homo lineage and a coexisting Australopithecus lineage, occur within sediments deposited in the basin over the interval from ∼2.0 to 1.4 Ma ago in the late Pliocene and early Pleistocene.

Topographic lineament swarms - clues to their origin from domain analysis of italy

Abstract: Regional-scale, subparallel linear topographic features characterize almost all planetary surfaces. This experiment concerns their tectonic meaning by focusing on map limits or domains through which individual swarms of these linear features are developed. These domain limits are contrasted with map boundaries of better-known structural features in a tectonically active region (Italy) to seek clues to stress environments and times of origin of the lineaments. The study uses regional raised relief maps to filter the lineament data to a very simple subset: those strictly topographic features of a length and prominence capable of retaining detectability through generalizations required by relief-map production. A total of 5,372 lineaments were drawn using 4 differently lighted images of 1:1,000,000 scale relief maps of Italy. Seven different tests were used for reliability and reproducibility of the data. Rose diagrams were prepared for 86 subareas by computer fitting of gaussians to azimuth-frequency histograms. Individual azimuthal “petals” of these roses were then correlated to delimit the general area over which a given azimuthal swarm is developed. The precise swarm boundaries were then located by computer contouring the population density of lines of each swarm on the original data set. In this way, 48 local swarms were mapped. Boundaries of these swarms correlate poorly with traditional litho-tectonic provinces. Instead, they seem to be associated with basin axes, broad arches, coastal flexures, areas of normal fault swarms, and projections of structural grain from adjacent sea floors. The 48 domains may be grouped into 8 noncontiguous but azimuthally compatible super-swarms covering much of Italy. The most prominent of the super-swarms are greatly expanded versions of regional structural grains: trend of the Po Basin, axis of the upper Adriatic Basin, zone of south Alpine underthrusting, and landward extensions of structural grain of the Tyrrhenian Sea. From these relationships, a model for lineament-swarm origins is proposed, involving very minor regional stretching of the thin, brittle carapace of extremely large, sometimes subtle, structures deforming by ductile mechanisms at depth.

Large-scale, thin-skinned thrusting in the southern Alps: Kinematic models

Abstract: The tectonics of the southern Alps are dominated by thrusting on extensive ramp-flat systems and by folding, some of which is ramp-folding. Basement slivers are involved that, frontally, are not thicker than 5 km. The most unequivocal section is that of the Grigna Mountains east of Lake Como, for which kinematic modeling based on material balance considerations results in a shortening of ∼40 km. It is separated from the Grona section in the west and the Valtorta section in the east by probably pre-existing transverse zones that cut through the important decollement horizons and define blocks of different depths to basement. Local tectonics across these transverse zones change abruptly, while over-all shortening remains constant. In particular, both east and west of the Grigna section, basement ramp folds were wedged into the very incompetent Raibl beds, provoking back-thrusting of the thick Hauptdolomit and higher sediments with the substitution of the original cover of the frontal limb of the basement fold. This new kinematic interpretation is compatible with the information, facilitates the construction of balanced sections, and is required for lateral constancy of shortening.

Bed microtopography and entrainment thresholds in gravel-bed rivers

Abstract: Although increasingly acknowledged as an important factor in bedload transport, few data on the effects of bed conditions on entrainment in gravel-bed rivers exist. This reflects to some degree the substantial difficulties involved in obtaining accurate threshold measurements in such channels. Using a sensor which allows initial movement of labeled particles to be detected during flood flow, the entrainment thresholds of clasts incorporated into the bed microtopography have been compared with those in exposed, open plane-bed positions. The threshold velocity of clasts from cluster bedforms, the most prevalent type of bed microtopography, is found to be above that of more exposed particles of like size and shape. Measurements indicate that the initial movement of ∼70% of particles in gravel-bed rivers will be directly influenced by the presence of neighboring grains. Bed microtopography plays an important role in delaying incipient motion. The influence of microtopography may be invoked to account for phenomena such as discontinuous particle movement and variations in the composition of bedload during discharge events.

Neogene paleostress changes in the Basin and Range: A case study at Hoover Dam, Nevada-Arizona

Abstract: At Hoover Dam, 40 km southeast of Las Vegas, Nevada, well-exposed, highly faulted Miocene rocks provide an excellent opportunity to study the paleostress history of a very small area within a region where previous geologic studies indicate clockwise rotation of paleostress and a nearness to a major strike-slip fault-zone boundary. Within <0.5 km3 of rock, the sense of slip was determined on almost 500 separate faults. The fault-slip data show internal consistency with respect to lithology, size of faults, and location within the small area. With respect to fault slip, however, the data represent an in-homogeneous mixture of primarily strike-slip and dip-slip motions. From this mixture, it is possible to resolve two distinct stress fields with directions of extension that differ by ∼60°. Each stress field corresponds to a mixture of strike-slip and dip-slip faults, and thus the orientations of σ 1 and σ 2 are not tightly constrained. If, however, the data are first separated into strike-slip and dip-slip faulting modes and subsequently searched for sub-populations that correspond to contrasting paleostress orientations, the computations yield tightly constrained tensors that illustrate two distinct stress fields with subhorizontal σ 3 axes that trend N50°E and N75°W. The σ 1 and σ 2 axes permutate in two vertical planes that strike N40°W and N15°E, because σ 1 and σ 2 are close in value, relative to σ 3. These relationships suggest that strike-slip and dip-slip faulting belong to the same tectonic regime. Qualitative observations of polyphase slip, fault-fault offsets, and fault-bedding geometric relationships, when evaluated in the context of changes in σ 3 orientation and permutations of σ 1 and σ 2, provide a basis for a two-stage, late Cenozoic structural evolution at Hoover Dam. These stages are (1) strike-slip faulting (partly pre-tilt) and dip-slip faulting and associated stratal tilting and (2) mostly post-tilt, complexly interrelated strike-slip, oblique-slip, and dip-slip faulting. The qualitative evaluations indicate that strike-slip and dip-slip faulting alternated in time during the second stage of deformation and may have done so during the first. These mixed-mode movements probably represent stress oscillations in time and space rather than discrete stress reorganizations. In contrast, the two different orientations of σ 3 either represent a major clockwise rotation of the stress field or a major counterclockwise rotation of the rocks during the faulting history. Each alternative is consistent with regional geologic relationships, and the choice of which is correct cannot be made within the small area that was studied.

Sedimentological constraints on the origins of Precambrian iron-formations

Abstract: A reconnaissance study of six Precambrian cherty iron-formations revealed two dominant patterns of sedimentation. Three of the iron-formations consist largely or entirely of chemical mud (lutite), whereas chemical sands (arenite) are dominant in the other three. The sedimentary structures of the lutitic iron-formations (= classical banded iron-formation) indicate that they were deposited in basinal environments, whereas the arenitic iron-formations have sedimentary structures indicative of platformal sedimentation. Where contacts are conformable, the lutitic iron-formations are sandwiched between other basinal deposits, predominantly flysch sequences. In contrast, arenitic iron-formations are overlain by flysch-type units but underlain by high-energy shelf sandstones. When combined with mineralogical and textural data, these interpretations place certain constraints on the origins of iron-formation, including the following. 1. Iron-formations were deposited in marine environments that were highly variable but exclusively or largely subtidal. 2. Surface waters were sufficiently oxidizing to preclude their transporting any ferrous iron in true solution. 3. The sources of the chemical sediments that ultimately became iron-formation were located to basinward of the depositional environments. 4. Iron-formation deposition was superimposed on existing physical environments. Most iron-formations described in the literature appear to have characteristics that are compatible with these constraints, although some of the iron-formations of the Transvaal Basin of South Africa are notable exceptions. The model for the origin of iron-formations that is most easily reconciled with the sedimentological constraints proposed here is that iron-formations are primarily exhalative or hydrothermal in origin. This fact, combined with several independent lines of evidence, indicates that an exhalative origin far the large, early Proterozoic iron-formations is a hypothesis that deserves critical re-examination.

Uranium-series dating of fossil corals from marine sediments of southeastern United States Atlantic Coastal Plain

Abstract: Extensive low-lying marine deposits border the southeastern United States Atlantic Coastal Plain. Some units are fossiliferous and contain corals as isolated fragments in sediments of a detrital character. These corals are subject to alteration processes such that suites of related samples must be examined to determine the suitability of these coral samples for reliable uranium-series dating. With the exception of those from one location, most samples appear to have remained closed systems with respect to the isotopes of uranium and thorium throughout their geologic history. Extraneous 230Th has been detected in some of the corals due to incorporation of some detrital materials into their skeletons. For these samples, different methods are applied to correct for the initial 230Th contamination. Continued sampling and analyses have resulted in 55 individual uranium-thorium determinations. The average 230Th ages of samples from the Norfolk Formation, and from later- and earlier-deposited sediments of the Wando Formation are ∼71,000, 87,000, and 129,000 yr, and they appear to correlate with oxygen isotope substages 5a, 5c, and 5e, respectively. The average 230Th age of samples from beds of the Rappahannock River, Ponzer, and Ten Mile Hill localities is ∼212,000 yr, and they correlate with oxygen isotope stage 7. The sediment of the Canepatch Formation is ∼460,000, yr old, and it is tentatively correlated with oxygen isotope stage 11. There is general agreement between uranium-series and uranium-trend dates and between the quantitative trends of the amino acid data and uranium-series dates. The amino acid values, however, ure unacceptably high in at least two groups of samples, those from localities near Charleston, South Carolina, and from central Virginia.

Nontransform offsets of the Pacific-Cocos plate boundary and their traces on the rise flank

Abstract: A multi-beam bathymetric reconnaissance of the fast-spreading crest of the East Pacific Rise between 7°N and 2.6°N found the axial ridge to be interrupted by four 4–13 km non-transform offsets. They have probably persisted since initiation by a small change in spreading direction 3.5 Ma. At each offset, the volcanic rift zones veer 15° toward each other and overlap for 5–25 km around deep basins. Intervening lateral displacements of the rift zones are too short (<2.5 km) to break the linear axial volcanoes but cause local bends in their plan shapes; some examples have a morphology that mimics the larger intervolcano offsets. Interpretation of a thorough bathymetric and magnetic survey around the 9 km, 5.5°N offset indicates that the mainly one-sided spreading from the overlapped sections of rift zone occurs by a combination of highly asymmetric dike accretion and episodic small-scale jumps of the eruptive zone. The northern rift zone is elongating (propagating) at the expense of the southern one. The resulting along-axis migration of this and other examples, at speeds comparable to spreading rates, causes the rise-flank scars of the nontransform offsets (equivalent to the fracture zones of transform faults) to be oblique to the spreading direction. These traces are recognizable within the highly lineated fault-block terrain of the rise flanks because of the distinctive morphology of the 5–25-km-wide bands that accreted at overlapped sections of the rift zones. Here, lineation is less marked and oblique to the spreading direction, and volcanic relief is more important than relief caused by faulting. The west-flank trace of the 5.5°N offset is occupied by a line of volcanoes and shallow volcanic ridges. Several other volcanic chains, previously identified as hot-spot traces, seem to have grown in similar tectonic settings.

Implications of the northwestwardly younger age of the volcanic rocks of west-central California

Abstract: Erosional remnants of volcanic fields in west-central California form a linear northwest-trending belt growing younger in age to the northwest. Major fields within the belt are represented by the Neenach Volcanics, Pinnacles Volcanic Formation, Quien Sabe Volcanics, volcanic rocks in the Berkeley Hills, Tolay Volcanics, Sonoma Volcanics, and Clear Lake Volcanics. Dispersion in the age-distance relation is reduced by restoration of inferred offsets on transecting right-lateral fault systems. The offsets include 115 km on the San Gregorio–Hosgri fault, 314 km on the San Andreas fault, 43 km on the Hayward-Rodgers Creek fault, and 28 km on the Carneros-Franklin-Sunol-Calaveras fault. On the basis of the age and restored position of the volcanic rocks, we judge that the locus of initial active volcanism migrated northwestward ∼3.75 cm/yr from 25 to 12 Ma, and ∼1.35 cm/yr from 12 Ma to the present. The volcanic rocks apparently formed south of the northwardly retreating edge of the subducted part of the Juan de Fuca plate, corroborating one corollary of a published model of an expanding hole in the subducted Farallon-Juan de Fuca-Cocos plate. The present position of the locus of melting at Clear Lake, California, requires substantial overthrusting of the Juan de Fuca plate by the Pacific plate, as was postulated on the basis of foreshortening of magnetic anomalies in the Gorda basin. The change in rate of northwestward migration ∼12 Ma reflects a change in spreading direction of the Juan de Fuca plate vis-a-vis the Pacific plate, previously recognized from changes in orientation of oceanic magnetic anomalies. From the migration rates, it can be inferred that the relative movement between the Pacific plate and the westernmost fringe of the North American plate averaged ∼3.5 cm/yr from 27 m.y. ago to the present.