Showing papers in "Geology in 1985"
TL;DR: In this article, a simple elastic model suggests that these normal faults may have formed during gravitational collapse of the Miocene topographic front between India and Tibet, and that the southward motion of a wedge of crustal rocks bounded above by gently north-dipping normal faults and below by thrust faults that probably dip north.
Abstract: Recent work by Burg et al. (1984) indicates the presence of east-west-striking, gently north-dipping normal faults in the Higher Himalayas and southern Tibet that formed during postcollisional convergence of India and Tibet. These faults extend for at least 800 km along strike. We interpret these normal faults as probable late(?) Miocene extensional features with perhaps several tens of kilometres of down-to-the-north displacement. A simple elastic model suggests that these normal faults may have formed during gravitational collapse of the Miocene topographic front between India and Tibet. In this interpretation, gravitational collapse occurred by southward motion, relative to India and Tibet, of a wedge of crustal rocks bounded above by gently north-dipping normal faults and below by thrust faults that probably dip north. North-south extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere.
510 citations
TL;DR: Oceanic basalts can be subdivided into five distinct groups on the basis of their Sr, Nd, Hf, and Pb isotope composition, represented by mid-ocean ridge basalts (MORB) and St. Helena, Kerguelen, the Society Islands, and the Hawaiian Islands as mentioned in this paper.
Abstract: Oceanic basalts can be subdivided into five distinct groupings on the basis of their Sr, Nd, and Pb isotope composition. These classes, represented by mid-ocean ridge basalts (MORB) and St. Helena, Kerguelen, the Society Islands, and the Hawaiian Islands, may represent different but internally heterogeneous mantle reservoirs or merely distinct groups within which chemical evolution has proceeded in a similar manner. Little systematic geographic distribution of volcanism tapping these sources is apparent. Depletion has been most important in the evolution of the MORB-type reservoirs, whereas crustal recycling has dominated the evolution of sources of the Kerguelen and Society types. Primitive mantle is identifiable in the Sr, Nd, Hf, and Pb isotope characteristics of the Hawaiian source only. The evolution of St. Helena-type sources remains enigmatic.
460 citations
TL;DR: In this paper, it was shown that recrystallization-accommodated dislocation creep may be responsible for the grain-size reduction and strain softening that lead to the formation of many mylonites and ductile shear zones.
Abstract: Feldspar aggregates experimentally deformed in the dislocation creep regime undergo dynamic recrystallization because recovery is difficult due to the limited climb of dislocations. Recrystallizing aggregates have a lower strength because of the cyclic production of small, strain-free grains, and they develop a strong preferred orientation, consistent with that observed in mylonites. Thus, recrystallization-accommodated dislocation creep may be responsible for the grain-size reduction and strain softening that lead to the formation of many mylonites and ductile shear zones.
397 citations
TL;DR: The southern Tongue of the Ocean is a basin in the Bahamas that receives large amounts of sediment from the carbonate platforms surrounding it on three sides as mentioned in this paper, and sediment composition indicates that increased interglacial rates are due to higher accumulation of platform-derived material.
Abstract: The southern Tongue of the Ocean is a 1300-m-deep, flat-floored basin in the Bahamas that receives large amounts of sediment from the carbonate platforms surrounding it on three sides. We have examined five 8–13-m-long piston cores and determined bulk sedimentation rates, turbidite frequency, and turbidite accumulation rates for the past two glacial and interglacial periods. The mean of bulk sedimentation rates is four to six times higher in interglacial periods; average accumulation rates of recognizable turbidites are higher by a factor of 21 to 45, and interglacial turbidite frequency is higher by a factor of 6 to 14. Sediment composition indicates that increased interglacial rates are due to higher accumulation of platform-derived material. Additional data from other Bahamian basins as well as published material from the Caribbean strongly suggest that highstand shedding is a general trend in pure carbonate depositional systems. Carbonate platforms without a siliciclastic component export more material during highstands of sea level when the platform tops are flooded and produce sediment. The response of carbonate platforms to Quaternary sea-level cycles is opposed to that of siliciclastic ocean margins, where sediment is stored on the inner shelf during highstands and passed on to continental rises and abyssal plains during lowstands of sea level.
291 citations
TL;DR: The recognition of Precambrian ophiolite suites and their dismembered remnants in association with intraoceanic island-arc volcanic and plutonic terrains across much of the Arabian-Nubian Shield of eastern Egypt, Sudan, Ethiopia, Yemen, western Saudi Arabia, and Sinai has been used by many authors to support the hypothesis of crustal accretion during late Proterozoic time (∼950−550 Ma) as mentioned in this paper.
Abstract: The recognition of Precambrian ophiolite suites and their dismembered remnants in association with intraoceanic island-arc volcanic and plutonic terrains across much of the Arabian-Nubian Shield of eastern Egypt, Sudan, Ethiopia, Yemen, western Saudi Arabia, and Sinai has been used by many authors to support the hypothesis of crustal accretion during late Proterozoic time (∼950−550 Ma). Reassembly of the various fragments provides a mosaic of Proterozoic microplates in a regular pattern in which at least five oceanic terrains, bounded by the remains of ophiolite belts, lie between remobilized continental plates to east and west.
252 citations
TL;DR: A major, ongoing episode of continental rifting has formed a system of three intersecting grabens in the western Mexican volcanic belt as discussed by the authors, which may mark the initial stage of an eastward spreading-ridge jump beneath continental crust.
Abstract: A major, ongoing episode of continental rifting has formed a system of three intersecting grabens in the western Mexican volcanic belt. These structures may mark the initial stage of an eastward spreading-ridge jump beneath continental crust. Eastward ridge-segment jumps have been propagating northward along this part of the East Pacific Rise for the past 12 m.y., but this is the first plate-boundary reorganization event to involve continental crust since the separation of Baja California in the Miocene. Since Pliocene time, small volumes of unusual alkaline magmas have erupted in the graben areas in close association with the more abundant, subduction-related calc-alkaline magmas erupted from andesitic stratovolcanoes. The alkaline magmas are varieties found in zones of active rifting elsewhere in the world.
240 citations
TL;DR: This paper showed that a mid-Miocene to early Pliocene slope and shelf sedimentary sequence was deposited directly on abyssal-plain turbidites without any detectable stratigraphic or structural discordance.
Abstract: Field relations in the emergent part of the Makran accretionary prism show that a mid-Miocene to early Pliocene slope and shelf sedimentary sequence was deposited directly on abyssal-plain turbidites without any detectable stratigraphic or structural discordance. Sedimentological evidence for rapid shoaling, however, indicates that the underlying sediment column was tectonically thickened by a factor of between two and three during this period. This can be explained by large-scale underthrusting and underplating of sediment—a mode of accretion that is also favored by mass-balance considerations. The visible deformation in the coastal Makran occurred when the region was 70–100 km north of the contemporary prism front.
203 citations
TL;DR: More than fifty transgressive-regressive sequences are present in Carboniferous and Permian shallow marine successions on stable cratonic shelves worldwide as mentioned in this paper, averaging about 2 m.y.
Abstract: More than fifty transgressive-regressive depositional sequences are present in Carboniferous and Permian shallow marine successions on stable cratonic shelves worldwide. These were synchronous depositional events resulting from eustatic sea-level changes that generally ranged from 100 to 200 m. Each transgressive-regressive sequence is correlatable using current fossil knowledge. They average about 2 m.y. and range from 1.2 to 4 m.y. in length.
The presence within these strata of numerous, synchronous unconformities of considerable duration and worldwide extent suggests that the fossil record is very incomplete and that we are studying a punctuated fossil record and not a punctuated evolution based on a highly irregular mutation rate. These late Paleozoic transgressive-regressive depositional sequences facilitate correlations because depositional histories of a rock succession can support interpretations of faunal assemblages and faunal similarities in evaluating age relationships.
201 citations
TL;DR: In this paper, a new class of seamounts consisting of horsts and diapirs of metamorphosed forearc material was identified. But the authors only considered the outer half of the Mariana forearc.
Abstract: Large seamounts occur on the outer half of the Mariana forearc. They represent a new class of seamounts consisting of horsts and diapirs of metamorphosed forearc material. The degree of metamorphism in this material depends on the amount of water available and the pressure-temperature regime of the forearc wedge. The major source for the water involved in the metamorphism is most likely the descending slab. Theoretical models for thermal regimes in convergent margins suggest that the lower grade metamorphic facies will be restricted to the outermost part of a forearc. Zeolite and chlorite facies rocks predominate in dredge hauls from horsts on the outer 50 km of the Mariana forearc. Thermal models indicate that higher grade greenschist facies should occur farther from the trench. Seamounts that were probably formed in response to diapiric emplacement of serpentinite predominate from 50 to 120 km from the trench. Uplift of the horsts and emplacement of the serpentinite diapirs were probably facilitated by vertical tectonic movement in response to subduction of plate seamounts and by fracturing of the Mariana forearc.
199 citations
TL;DR: In this article, both calc-alkaline and K-rich magmas were generated within a mantle heterogenously enriched in LIL elements, by adding material, probably sediments, dragged down by the undergoing slab.
Abstract: Geochemical data on mafic volcanics show that important affinities exist between the Roman and the calc-alkaline rocks from the Aeolian arc (south Tyrrhenian Sea). These affinities, together with the close association of calc-alkaline and K-rich volcanics in the Aeolian arc and in the Naples area, the continuity in the variation of abundances of incompatible elements from calc-alkaline to potassic suites, and the similarity in terms of major-element geochemistry, support a genetic relationship of the Roman magmatism and the subduction processes that affected the Apennines in Tertiary time and are still active under the Aeolian arc. In the genetic model presented here, both calc-alkaline and K-rich magmas were generated within a mantle heterogenously enriched in LIL elements. Composition of the mantle was modified by addition of material, probably sediments, dragged down by the undergoing slab. The geochemical and petrological differences displayed by the calc-alkaline and K-rich volcanics are accounted for by the different conditions of melting as well as by chemical and isotopic heterogeneities of the source. 26 references, 3 figures, 1 table.
195 citations
TL;DR: The 11km-long Inyo volcanic chain consists of 6 magmatic and more than 15 phreatic eruptive centers alined along a north-trending fracture as mentioned in this paper.
Abstract: The 11-km-long Inyo volcanic chain consists of 6 magmatic and more than 15 phreatic eruptive centers alined along a north-trending fracture. About 0.8 km 3 of rhyolitic-rhyodacitic magma erupted along the chain during at least 3 eruptive episodes during the past several thousand years. Pyroclastk deposits make up about 40% of this volume. The chemical similarities, timing of eruptions, and alinement of magmatic and phreatic vents indicate that the third eruptive episode, about 650-550 yr ago, occurred when a vertical, tabular, silicic magma body was emplaced at a shallow depth. A similar dikelike intrusion may explain an increase in seismicity and deformation in the south moat of Long Valley caldera. Events about 650-550 yr ago at the Inyo chair provide a model for possible events during future eruptions in the south moat of the caldera.
TL;DR: In this article, the authors suggest that the fossil axial valleys produced by successive eastward jumps of an approximately north-trending (present coordinates), slow-spreading ridge crest.
Abstract: Orientations of dikes within the sheeted dike complex of the Troodos ophiolite reveal primary spreading structure produced at a complex ridge/transform intersection. Three structural grabens are defined by listric and planar normal faults and rotated dikes that dip symmetrically toward graben axes. Faults flatten at depth into a detachment within the upper parts of the plutonic complex. Large exhalative massive sulfide deposits occur within the pillow sections of two of the grabens and appear to be associated with underlying altered and mineralized normal fault zones that channeled hydrothermal fluids. We suggest that the grabens are fossil axial valleys produced by successive eastward jumps of an approximately north-trending (present coordinates), slow-spreading ridge crest. A simple model for ridge migration indicates eastward jumps of 8–13 km; changes in ridge orientation are suggested by changes in trends of dikes and graben axes. Comparison of the pattern of dikes near the Arakapas fault zone with the structure of active ridge/transform intersections suggests that the fault is a right-offset (sinistral) transform, in contrast to earlier models in which a ridge to the west of the exposed Troodos complex was proposed.
TL;DR: In this paper, the authors show that the volcanic arc south of the Jocotan and Motagua faults may be explained by interaction and rotation of crustal blocks in the overriding Caribbean plate above the magma production zone along the downgoing Cocos slab.
Abstract: Sinistral displacements across the North American–Caribbean plate boundary in northern Central America are distributed among major arcuate faults that have been active in Neogene time. South of the Jocotan and Motagua faults in Guatemala, extensional tectonics has accompanied rotation of the trailing edge of the Caribbean plate around these faults. Segmentation of the volcanic arc in northern Central America, hitherto attributed to transverse breaks in the subducting Cocos plate, may instead be a result of this block rotation. Accompanying changes along the arc are observed in seismicity, gravity anomaly patterns, volume and composition of volcanic products, and topography. Therefore, the complex volcano-tectonic geology south of the main boundary faults may be explained by interaction and rotation of crustal blocks in the overriding Caribbean plate above the magma production zone along the downgoing Cocos slab.
TL;DR: The upper and lower crust of the central oceanic Aleutian arc has been formed by intrusion and extrusion of subduction-related magmas into and onto pre-existing oceanic crust over at least the past 50 m.y as mentioned in this paper.
Abstract: The crust in the central oceanic Aleutian arc has been formed by intrusion and extrusion of subduction-related magmas into and onto pre-existing oceanic crust over at least the past 50 m.y. Upper crustal rocks that have differentiated from a parental magma of olivine tholeiite composition, and residual crystals complementary to them, along with the pre-existing oceanic crust and some early extrusive lavas, form the lower crust. The volumes of upper and lower crust calculated from crystal fractionation models are consistent with Aleutian crustal structure, velocity, and density. Xenoliths of ultramafic to intermediate composition igneous cumulate rocks and garnet-free basaltic pyroxene granulites in Aleutian volcanic rocks confirm the existence of the predicted rock types in lower crust. Residues complementary to the Fe-rich basalts from tholeiitic volcanoes that have differentiated at low pressure should form part of the upper to middle crust. The model implies that continental crust formed by the accretion of island-arc terrains, which comprise most new Phanerozoic crust, has a basaltic lower crust.
TL;DR: The Siwalik biostratigraphic record correlates closely to climatic, oceanographic, and tectonic events, which probably controlled immigration into southern Asia.
Abstract: The fluvial Neogene Siwalik formations of northern Pakistan span long time intervals with only minor hiatuses and, being highly fossiliferous, are uniquely suited for studies of change in mammalian faunas. Magnetostratigraphic correlations of a critical stratigraphic section give dates for 45 middle and late Miocene biostratigraphic events. These mark either first appearances or extinctions in the mammal fauna and show that in the Siwaliks there were major fauna turnovers at between 20 and 16 Ma and at 9.5 and 7.4 Ma. Two minor faunal events are dated at 13.2 and about 12 Ma. Many species making their first appearance were immigrants from Europe or Africa and indicate when connections to those regions existed. Immigration and extinction were the dominant modes of faunal change; in situ evolution was much less important. The Siwalik biostratigraphic record correlates closely to climatic, oceanographic, and tectonic events, which probably controlled immigration into southern Asia. Abiotic events were therefore important factors affecting evolution of the mammal communities.
TL;DR: Geologic and biological data from box cores, gravity cores, and bottom photographs across the modern coastal upwelling system of central California reveal that biogeochemical activity increases along the edges of the oxygen-minimum zone (OMZ; <0.5 ml/1 O2).
Abstract: Geologic and biological data from box cores, gravity cores, and bottom photographs across the modern coastal upwelling system of central California reveal that biogeochemical activity increases along the edges of the oxygen-minimum zone (OMZ; <0.5 ml/1 O2). Macrofauna and benthic foraminifers, as well as relative concentrations of calcium carbonate, glauconite, and fecal pellets, all display edge effects. The major controls appear to be a combination of dissolved oxygen concentration of near-bottom waters, bottom currents, and possibly bacterially mediated nutrient recycling, as well as food supplies. OMZ edge effects should be recognizable in the rock record and may prove to be powerful paleoenvironmental/paleoecological indicators.
TL;DR: In this paper, a balanced cross section through the imbricated upper crust of the Indian plate in the footwall to the Main Mantle thrust of the western Himalaya, restoring to a minimum width of 730 km, the implicit shortening being in excess of 470 km.
Abstract: The systematic restoration of thrust systems provides a powerful constraint on the deep structure of orogenic belts. A balanced cross section, through the imbricated upper crust of the Indian plate in the footwall to the Main Mantle thrust of the western Himalaya, restores to a minimum width of 730 km, the implicit shortening being in excess of 470 km. This requires the Himalaya and southern Tibetan region to be underlain by a wedge of Indian crust. However, a full appraisal of gross crustal structure awaits deep seismic control.
TL;DR: In this paper, a residual weathering profile in the Virginia Piedmont has been analyzed for cosmogenic 10Be and a minimum age for the regolith was calculated based on the delivery rate of 10B and its decay rate.
Abstract: Samples from a residual weathering profile in the Virginia Piedmont have been analyzed for cosmogenic 10Be. Concentrations are highest in clay-rich soil and decrease exponentially to a depth of about 15 m. Despite uncertainties about the processes by which 10Be may be intercepted before entering the solum and eroded after incorporation, a minimum age may be calculated for the regolith. This calculation is based on the delivery rate of 10Be and its decay rate and suggests that this residual profile developed during a period no shorter than 8 × 105 yr. The calculated minimum age may be within a factor of 2 of maximum-age estimates based on surface lowering by erosion and on the rate of rock weathering to saprolite. The vertical distribution of 10Be in the profile could result from a steady-state balance of deposition, weathering, radioactive decay, and erosion.
TL;DR: In this paper, COCORP profiling in the southeastern United States has revealed a broad zone of dipping reflections that extends downward through the crust beneath the coastal plain in western Georgia.
Abstract: New COCORP profiling in the southeastern United States has revealed a broad zone of dipping reflections that extends downward through the crust beneath the coastal plain in western Georgia. The zone is over 50 km wide, and most of the reflections dip moderately steeply toward the south. Regional considerations suggest that this feature marks the late Paleozoic suture between North America and Africa. Where crossed by the COCORP survey, the suture occurs beneath the north flank of the Triassic-Early Jurassic south Georgia basin. The main depocenter of the south Georgia basin occurs about 90 km to the south and is formed by a large half graben containing more than 5 km of rift basin fill. Farther south, the Paleozoic Suwannee basin sequence beneath northern Florida is poorly imaged on the COCORP profiles. However, weak reflections suggest that these strata (including basal felsic volcanics) may have an aggregate thickness of about 6 km in north-central Florida. At the northwest end of the COCORP traverse, a prominent horizon imaged in the upper crust beneath the inner Piedmont probably marks the southern Appalachian detachment. The detachment appears to be cut off by the Towaliga fault, implying that the Towaliga fault is in part a down-to-the-north normal fault. Intermittent Moho reflections occur at 11–12-s two-way time along the length of the COCORP survey, indicating that the crust in the region has a roughly uniform thickness of about 33–36 km (assuming an average crustal velocity of 6 km/s).
TL;DR: In this paper, Berggren et al. showed that Zones NN8 partim and NN9 and associated Epoch 11 correlate with Magnetic Anomaly 5 (= Chron CSn) and require an upward adjustment of 1.5-2.0 m.y.
Abstract: Magnetostratigraphic records from western North Atlantic Deep Sea Drilling Project (DSDP) Sites 563 and 558 are correlated with the geomagnetic polarity time scale (GPTS; Berggren et aI., 1984a, 1984b) using marine magnetic anomalies and selected biostratigraphic datum levels. The magnetochronology established is used to make direct magnetobiostratigraphic correlations that agree with previous Oligocene-early Miocene studies. However, we show that Zones NN8 partim and NN9 and associated Epoch 11 correlate with Magnetic Anomaly 5 (= Chron CSn). This contrasts with previous indirect correlations of Epoch 11 with Anomaly SA and requires an upward adjustment of 1.5-2.0 m.y. for middle-late Miocene calcareous nannofossil zones. We correlate the middle/late Miocene boundary with Zone NN8 and earliest Chron CSn (10.4 Ma).
TL;DR: The evolution of the northern Snake Range decollement (NSRD) and its relation to the present basins and ranges is studied in this article. But the authors focus on the most recent Oligocene time and assume that the NSRD originated as a flat, 20 km wide, 7 km deep zone of decoupling between localized ductile stretching below and more widely distributed, high-angle normal faulting above.
Abstract: New geologic mapping and high-resolution seismic data from east-central Nevada shed light on the evolution of the northern Snake Range decollement (NSRD) and its relation to the present basins and ranges. We suggest that the NSRD originated in Oligocene time as a flat, 20-km-wide, 7-km-deep zone of decoupling between localized ductile stretching below and more widely distributed, high-angle normal faulting above. Younger east-dipping normal faults cut the western part of the NSRD, forming the tilted fault blocks and highly asymmetric grabens that constitute the Duck Creek Range-Duck Creek Valley and Schell Creek Range-Spring Valley, and folded the NSRD to its present antiformal geometry. These youngest faults flatten into pervasive subhorizontal reflectors at a depth of 10 to 15 km that we interpret as the youngest ductile-brittle transition.
TL;DR: The Kap Kabenhavn Formation at lat 82°30′ N, Perry Land, North Greenland, contains an abundance of well-preserved remains of terrestrial vegetation and invertebrate fauna indicative of a mosaic of forest tundra environments having similarities to present conditions in Labrador.
Abstract: Shallow-water marine sediments of the Kap Kabenhavn Formation at lat 82°30′ N, Perry Land, North Greenland, contain an abundance of well-preserved remains of terrestrial vegetation and invertebrate fauna indicative of a mosaic of forest tundra environments having similarities to present conditions in Labrador. The presence of foraminifera faunas, marine molluscs, and one mammal species (supported by studies of paleomagnetism and amino acid diagenesis) suggests an age of ∼2 Ma, at the Pliocene-Pleistocene transition. At this time, the arctic tree line was located 2500 km to the north of its present position in the northwest Atlantic region, and forest tundra vegetation existed in lowland areas bordering on an Arctic Ocean without perennial sea-ice cover.
TL;DR: The recent discovery of ALHA81005, a small meteorite that undoubtedly came from the moon, has raised questions about how an intact rock fragment could survive acceleration to planetary escape velocities as discussed by the authors.
Abstract: The recent discovery of ALHA81005, a small meteorite that undoubtedly came from the moon, has raised questions about how an intact rock fragment could survive acceleration to planetary escape velocities. This acceleration could only have been produced by a large impact on the lunar surface. A small amount of material (0.005 to 0.02 projectile volume) may be ejected from an impact crater at speeds exceeding lunar escape velocity without suffering petrographically detectable shock damage. The ejected material is protected by stress-wave interference close to the free surface. The existence and size of this zone depend upon parameters such as the rise time of the stress pulse produced by an impact. The fragment size is a function of ejection velocity. The lunar meteorite was ejected during an impact event that produced a crater at least 3.6 km in diameter. Other meteorites may have originated in a similar way.
TL;DR: The Upper Pennsylvanian and Lower Permian stratigraphic sequence in Canyonlands National Park contains about 40 flat-topped eolian sand bodies as mentioned in this paper, which are diastems and represent much more geologic time than the rocks themselves.
Abstract: The Upper Pennsylvanian and Lower Permian stratigraphic sequence in Canyonlands National Park contains about 40 flat-topped eolian sand bodies. Rhizoliths, burrows, and traces of former evaporites abound at the tops of these tabular genetic units. Upper surfaces of units were created by eolian deflation to the level of the groundwater table. Analysis of cross-stratification reveals that sands accumulated via migration and climb of relatively small dunes. Brief episodes of sand accumulation were triggered by regression and were terminated when upwind sand supplies were depleted. Upper surfaces of genetic units are diastems and represent much more geologic time than the rocks themselves.
TL;DR: The Socompa volcano in northern Chile experienced a catastrophic collapse of a 70 deg sector of the original cone, causing a debris avalanche that descended nearly 3000 m vertically and traveled more than 35 km from the volcano as discussed by the authors.
Abstract: Between 10,000 and 500 yr ago the Socompa volcano in northern Chile experienced a catastrophic collapse of a 70 deg sector of the original cone, causing a debris avalanche that descended nearly 3000 m vertically and traveled more than 35 km from the volcano. The deposits cover some 490 sq km and have a minimum volume of 15 cu km. Parts of the original cone slumped in a nearly coherent form and are now preserved as large blocks more than 400 m high. The primary avalanche traveled northwestward over sloping ground before coming to rest transiently, forming a prominent marginal ridge, and then slid away northeastward to form a secondary flow, overriding much of the primary avalanche deposit. Abundant, prismatic, jointed dacite blocks within the debris avalanche deposit and a thin, fine-grained pumiceous deposit beneath it suggest that the collapse was triggered by magmatic activity and may have been accompanied by a violent lateral blast. Collapse was followed by eruption of pumiceous pyroclastic flows and extrusion of voluminous dacite domes.
TL;DR: In this article, the authors hypothesize that these valley types originated through the interaction of ground ice and hot springs located along the permeable fringes of slowly cooling impact melts, and that these valleys grew by a combination of headward sapping and down-valley water and ice fluid flow.
Abstract: At least two classes of small valleys (subparallel slope ravines and flat-floored branching valleys) shown on Viking imagery of Mars exhibit spatial relationships to impact structures. Cessation of their formation at the terminus of the heavy bombardment period, about 3.8 Ga, also implies a causal relationship between valley formation and cratering. We hypothesize that these valley types originated through the interaction of ground ice and hot springs located along the permeable fringes of slowly cooling impact melts. The valleys grew by a combination of headward sapping and down-valley water and ice fluid flow. The probable widespread operation of impact-related hydrothermal systems early in Mars history indicates that it is not necessary to infer atmospheric changes to explain valley origin.
TL;DR: Two modes of back-arc spreading, single-rift type and multirift type, are introduced from the comparative study of the Bonin arc and the Japan Sea in this paper, which shows that divergent tectonics in an island arc above a steeply dipping slab will cause a singlerift system in a narrow volcanic zone but will cause an extensive volcanic zone, which is related to a shallow-dipping angle of subduction.
Abstract: Back-arc spreading is initiated with rifting along the volcanic zone of an island arc under extensional tectonics. The width of the volcanic zone of the arc, which is closely related to the dip angle of the subducting slab, should cause a variation in the style of initial rifting and subsequent back-arc spreading. Two modes of back-arc spreading, single-rift type and multirift type, are introduced from the comparative study of the Bonin arc and the Japan Sea. Divergent tectonics in an island arc above a steeply dipping slab will cause a single-rift system in a narrow volcanic zone but will cause a multirift system in a broad volcanic zone, which is related to a shallow-dipping angle of subduction.
TL;DR: The Coast Range ophiolite (CRO) of California contains two geochemically distinct volcanic rock associations that formed in different tectonic settings as mentioned in this paper, and these rocks are associated with Tithonian radiolarian cherts and may be part of the Franciscan Complex.
Abstract: The Middle to Late Jurassic age Coast Range ophiolite (CRO) of California contains two geochemically distinct volcanic rock associations that formed in different tectonic settings. Volcanic rocks from the southern CRO (Point Sal, Cuesta Ridge, Stanley Mountain, Llanada, Quinto Creek, and Del Puerto) and parts of the northern CRO (Healdsburg, Elder Creek) are similar to low-K tholeiites and calc-alkaline rocks of the island-arc suite. The thin volcanic sections of these ophiolite remnants suggest formation by intra-arc rifting. In contrast, volcanic rocks from Stonyford seamount and Paskenta in the northern CRO are transitional subalkaline metabasalts with geochemical characteristics similar to enriched mid-ocean ridge basalts or ocean-island tholeiites. These rocks are associated with Tithonian radiolarian cherts and may be part of the Franciscan Complex. Alternatively, they may represent a change in tectonic setting within the CRO during the Late Jurassic. Regardless, the CRO as currently conceived cannot be considered a single terrane with one mode of origin.
TL;DR: In this paper, the authors interpret duplex development under the Costa Rica forearc, a zone widely considered to be a prime example of nonaccretion because of the apparent absence of an imbricate fan of thrusts at the toe of the wedge, and suggest that emplacement of high P / T metamorphic rocks are better explained in a collisional setting where known uplift rates are high than by flow within noncollisional accretionary wedges.
Abstract: Uplift within accretionary foreland wedges has been explained by the development of duplexes, which provides a mechanism for “underplating” in submarine accretionary wedges with little deformation in the upper part of the wedge. We interpret duplex development beneath the Costa Rica forearc, a zone widely considered to be a prime example of nonaccretion because of the apparent absence of an imbricate fan of thrusts at the toe of the wedge. Large-scale structural geometry of accretionary wedges is compatible with low-angle, trenchward-dipping backstops in centrist to reverse-angle backstops used commonly in models of return flow within wedges, which have been invoked to explain the presence of high- P / T metamorphic rocks. We suggest that emplacement of high- P / T metamorphic rocks are better explained in a collisional setting where known uplift rates are high than by flow within noncollisional accretionary wedges.
TL;DR: In this paper, it is hypothesized that paleo-trade winds transported much of the missing silt and clay to the southern and western United States and southwestern Canada, while the remainder diffused radially off the craton under the influence of variably directed storm winds.
Abstract: Prolonged eolian action on the barren (prevegetation) craton provides a better explanation for the absence of shale in the Cambrian and Lower Ordovician orthoquartzite-carbonate suite of east-central and Arctic North America than does marine bypassing. It is hypothesized that paleo-trade winds transported much of the missing silt and clay to the southern and western United States and southwestern Canada, while the remainder diffused radially off the craton under the influence of variably directed storm winds. The distribution of shale in the Sauk sequence supports this and indicates that the orthoquartzite-carbonate suite of upwind regions is replaced by a shale-carbonate suite in downwind areas.