Compressional Features Research Articles

Orientation and origin of ridges in the Lunae Palus—Coprates region of Mars

Numerous wrinkle ridges similar in appearance to lunar mare ridges occur on volcanic plains units surrounding the Tharsis region of Mars, where they are oriented circumferential to the center of the plateau. By analogy with lunar mare ridges and from morphologic evidence such as the smooth slopes of arches, these features are interpreted to result primarily from compressional folding of the volcanic plains, although the uppermost ridge may represent faulting of the near surface materials. East of the Tharsis region, ridges are best developed in the Lunae Palus and Coprates quadrangles, where they can be used to indicate the degree of compressional stress occurring there. Comparison of ridge orientations in these quadrangles to a Tharsis center at 1°N, 122°W indicates that more than 60% of the total length of ridges is orthogonal to a normal from this center. Non‐orthogonal ridges are seen to have general northerly orientations after removal of the Tharsis orthogonal population. Trends of graben in surrounding terrain indicate that non‐orthogonal ridges are not greatly affected by regional scale structures. Comparison of the frequency of orthogonal and non‐orthogonal ridges further suggests that loading due to the Tharsis plateau was effective in producing compressional features as far as 4000 km from the center of the load; additional sources for compressional stress are necessary to account for ridge systems at greater distances from the Tharsis plateau.

Petroleum Potential of Continental Margin off Southeastern United States: ABSTRACT

The continental margin off the southeastern United States is underlain by two major basins, the Carolina trough off North Carolina and South Carolina and the Blake Plateau Basin off Florida and Georgia, with the latter basin's landward extension, the Southeast Georgia embayment. The embayment, beneath the continental shelf is the only area in which drilling has taken place. Strata of the landward part of the Blake Plateau Basin including the embayment form a wedge of onlapping marine End_Page 563------------------------------ deposits that might provide stratigraphic traps, such as pinch-outs, barrier islands, and channels. Drapes over basement highs might trap petroleum. The seaward part of the Blake Plateau Basin contains a thick (14 km), landward-dipping section, probably composed mainly of carbonate platform deposits. Carbonate banks and an Albian-Aptian rudist reef might provide traps, although the seaward part of the platform has been breached by erosion. In the Carolina trough, flow of Jurassic(?) salt has formed diapirs, and withdrawal of salt has caused a large growth fault complex to form along the landward side of the trough. Because the block of sedimentary rock (12 km thick) above the salt is subsiding almost vertically, structures at the fault may include compressional features. The diapirs and faults may provide traps. A large shelf-edge anticline exists more than 150 km long and with a closure of as much as 500 m. Other possible traps might result from stratigraphic features of the onlapping sedimentary wedge landward of the Carolina trough and the eroded and buried paleoslope on the seaward side of the trough. End_of_Article - Last_Page 564------------

Active faulting in the dead sea rift

Manifestations of Late Quaternary and Holocene faulting were studied in a 500 km long segment of the Dead Sea transform (rift). Most prominent are left-slip faults, whose characteristic physiographic features are recognizable along most of the studied segment. Where these faults bend or are stepped to the left, rhomb-shaped grabens (or pull aparts) are produced, forming depressions. In the reverse situation compressional features such as pressure ridges, domes and folds form positive topographic features. Such structures are combined on a variety of scales ranging from a few hundred meters long to tens of kilometers. Normal faults, sub-parallel to the left slip faults, produce a trough-like valley along much of the Dead Sea transform, but are most prominent along the margins of the large rhomb-grabens, e.g., the Dead Sea trough. They apparently record a small component of transverse extension. Generally, their motion is slow: young slip did not occur along some segments during the last few 10 4 y. Elsewhere throws of 10–20 m at least occurred in this period. The Dead Sea transform is seismically active. The instrumental and historic records indicate a seismic slip rate of 0.15–0.35 cm/y during the last 1000–1500 y, while estimates of the average Pliocene—Pleistocene rate are 0.7–1.0 cm/y. Either much creep takes place, or the slip rate varies over periods of a few 10 3 y.

Rotational displacements in Southeastern Australia and their influence on hydrocarbon occurrence

From Late Jurassic through Middle Eocene times, southeastern Australia experienced a deformation which created the Gippsland, Bass, Otway and West Tasmania Basins. This basin-forming deformation can be ascribed to a “dextral divergent rotation” of Antarctica relative to Australia. Detailed strain and fracture patterns resulting from dextral divergent rotational displacement can be studied in a homogeneous medium such as ice. However, the geometry of southeastern Australian sedimentary basins results from stresses acting on a heterogeneous medium. Thus pre-Late Jurassic zones of basement weakness were rejuvenated and influenced basin geometry. Sinistral divergent rotation of the Lord Howe—New Zealand continental mass relative to Australia occurred, possibly in the Late Jurassic. This separation created the narrow East Tasmania Basin but does not appear to have structurally influenced Gippsland Basin. By latest Eocene, the structural environment in southeastern Australia had changed dramatically, responding to a “dextral convergent rotation”. Late Eocene through Recent compressional features resulted mainly in Gippsland, with minor features in northern Bass Basin and in various parts of northern Otway Basin. Hydrocarbon entrapment in Gippsland Basin is directly related to traps associated with anticlinical growth of Late Eocene through Recent movements.

The Mesozoic South Atlantic Ocean and evolution of its continental margins

Gravity and magnetic anomalies bordering the continental margins of the southern South Atlantic Ocean are compared, in detail, on conjugate sides of the ridge crest, and a model for the boundary between oceanic and continental basement is given. The area of study includes the predominantly sheared margins of the Agulhas‐Falkland fracture zone and the rifted margins of Argentina and southern Africa south of the Rio Grande Rise and Walvis Ridge, respectively. These margins have associated with them, for the most part, linear magnetic anomalies that can be modeled as edge effect anomalies separating oceanic from continental basement. Coincident with the magnetic anomalies are gradients in the isostatic gravity anomaly. We have taken the location of these geophysical lineaments on the African margin and rotated them clockwise to fit the anomalies on the Argentine margin. This fit, which gives us a new pole of total closing for the South Atlantic Ocean, obviates, for the most part, the gaps and overlaps observed in other reconstructions. The improved fit thereby suggests rigid plate behavior and minimum stretching of continental crust during the early opening of the southern South Atlantic Ocean. A zone of crustal stretching may be present in the southernmost Argentine and Cape basin margins. New poles of early opening for the South Atlantic Ocean have been determined (from 130 to 107 m.y. B.P. and from 107 to 80 m.y. B.P.) utilizing the above reconstruction as well as the strike of the Agulhas‐Falkland fracture zone where it is well determined. The earliest pole, which is located much farther south than previously determined early poles, satisfies not only the geophysical data in the southern regions but allows us to explain a number of outstanding problems north of the Rio Grande Rise‐Walvis Ridge area. These problems include the timing of the onset of sedimentation on the northern Brazil margin, the origin of the compressional features along the Venezuela margin, and the onset of open marine circulation between the North and South Atlantic oceans. Paleoreconstructions using the new early poles also align very well the seaward edge of the salt boundaries off Brazil and West Africa. The age of the salt, as inferred from the paleoreconstruction to its seaward boundaries, is younger than the age of magnetic anomaly M0. Furthermore, our paleoreconstructions show barriers for salt deposition not only across its southern termination (Walvis Ridge area) but also farther north in the equatorial regions. The salt, for the most part, has been deposited on oceanic crust. The new predrift reconstruction and early opening poles, taken together with the new identifications of the Mesozoic and Late Cenozoic sequences of magnetic anomalies, allow us to determine the magnitude and time interval of spreading center migration. In particular, we can demonstrate that ridge crest migrations of ∼1000 km have occurred along the strike of the Falkland escarpment. We demonstrate that the isostatic gravity gradient associated with the boundary between oceanic and continental basement is independent of the location of major sediment accumulations. We have modeled this anomaly as resulting from elevated oceanic basement adjacent to continental crust. This crustal model satisfies the limited upper crustal seismic data available near margins. The oceanic basement elevations are relics of a transient phenomenon associated with young rifted margins such as the East African and Red Sea rifts.

On sheared passive continental margins

Studies in intra-continental and intra-oceanic shear zones reveal structures that may be developed during the formation of a sheared passive continental margin. During the intra-continental shear stage of margin development, rapid vertical movement of the crust may occur resulting in small, tectonically-active basins containing thick sedimentary sequences. At deeper levels in the continental crust, more plastic deformation may lead to a zone of strongly sheared rocks that widens downwards. The tectonic fabric in this zone may exert some control over the subsequent development of the continent-ocean transition under the influence of regional stresses. The thermal event related to asthenosphere upwelling at sheared margins is a transient one and thus of less effect than the event on rifted margins. Nevertheless, following the event the cooling and contraction of oceanic crust against the continent may throw the oceanic crust into tension and lead to normal, block faulting in the oceanic regions analogous to the faulting seen in oceanic fracture zones. The subsidence of oceanic crust as it ages at the margin will either drag down the adjacent continental crust or, more likely, cause the oceanic crust to slip down by normal faulting along the continent-ocean boundary. The kinds of compressional features observed in oceanic fracture zones may also occur at sheared margins.

Geophysical observations pertaining to solid-state convection in the terrestrial planets

We present a broad-based review of the observational evidence that pertains to or otherwise implies solid-state convection to be occurring (or have occurred) in the interiors of the terrestrial planets. For the Earth, the motion of the plates is prima facie evidence of large-scale mantle convection. Provided we understand upper-mantle thermal conductivity correctly, heat flow beneath the old ocean basins may be too high to be transported conductively from the upper mantle through the base of the lithosphere and therefore convection on a second smaller scale might be operative. The horizontal scale of plate dimensions implies, due to typical cell aspect ratios observed in convection, that the motion extends to the core-mantle boundary. Improved global data coverage and viscoelastic modeling of isostatic rebound due to Pleistocene deglaciation imply a uniform mantle viscosity, and thus indicate that whole-mantle convection could exist. Additionally, there is some seismic evidence of lithospheric penetration to depths deeper than 700 km. We discuss some salient features and assumption boundedness of arguments for convection confined to the upper mantle and for convection which acts throughout the mantle since the vertical length scale has a profound effect upon the relevance of geophysical observations. The horizontal form of mantle convection may be fully three-dimensional with complex planform and, therefore, searching for correlative gravity patterns in the ocean basins may not be useful without additional geophysical constraints. Many long-wavelength gravity anomalies may arise from beneath the lithosphere and must be supported dynamically, although thermal convection is not a unique explanation. Topography is an additional geophysical constraint, but for wavelengths greater than a few hundred kilometers, a general lack of correlation exists between oceanic residual gravity and topography, except at specific locations such as Hawaii. Theoretical calculations predict a complex relationship between these two observational types. Oceanic gravity data alone shows no regular planform and there is no correlation with any small-scale convective pattern predicted by laboratory experiments. All of the observational evidence argues against Martian plate tectonics occurring now or over much of the history of this planet, but lack of plate tectonics is not an argument against interior convection. The Tharsis uplift on Mars may have resulted from convective processes in the mantle, and the present-day gravity anomaly associated with Tharsis must be supported by the finite strength of the lithosphere or by mantle convection. Stresses imparted by the present topographic load would be greater than a kilobar, in excess of long-term finite strength. Observed fracture patterns are probably a direct result of this load, and the key question concerns the level of resultant strain relief. The global topographic and geomorphic dichotomy between the northern and southern hemisphere required a solid-state flow process to create the accompanying center-of-figure to center-of-mass offset. Lunar heat flow values, in analogy with oceanic heat flow on the Earth, strongly imply a convective mechanism of heat transport in the interior which, based on seismic Q values, is limited to the lower mantle. The presence of moonquakes in this region does not preclude solid-state convective processes. Lunar conductivity profiles provide no information on convection because of the difficulty in conductivity modeling, uniqueness of models, and the uncertainty in the conductivity-temperature relationship. The excess oblateness of the lunar figure over the hydrostatic value does not require convective support; in fact, such a mechanism is unlikely. The presence of a dipole magnetic field on Mercury does not provide a constraint on mantle convection unless its existence can be inextricably linked to a molten core. The non-hydrostatic shape of the equatorial figure, required for the observed 3 2 resonance between Mercury's rotational and orbital periods, is most likely related to surface processes, as opposed to convection. The 3n 2 resonance implies escape from a 2 n resonance and, therefore, is related to the question of a molten core. Further dynamical data is needed to constrain interior models. Interpretation of limited radar imagery for the surface of Venus is enigmatic in terms of plate tectonics and therefore interior convection. Linear tensional and possibly compressional features are observed, but there are also crustal regions which appear to show large impact structures and are thus geologically old and may not have been recycled.

Candelaria and other left-oblique slip faults of the Candelaria region, Nevada

Research Article| February 01, 1979 Candelaria and other left-oblique slip faults of the Candelaria region, Nevada R. C. SPEED; R. C. SPEED 1Department of Geological Sciences, Northwestern University, Evanston, Illinois 60201 Search for other works by this author on: GSW Google Scholar ALLEN H. COGBILL ALLEN H. COGBILL 1Department of Geological Sciences, Northwestern University, Evanston, Illinois 60201 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1979) 90 (2): 149–163. https://doi.org/10.1130/0016-7606(1979)90<149:CAOLSF>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation R. C. SPEED, ALLEN H. COGBILL; Candelaria and other left-oblique slip faults of the Candelaria region, Nevada. GSA Bulletin 1979;; 90 (2): 149–163. doi: https://doi.org/10.1130/0016-7606(1979)90<149:CAOLSF>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The region comprising the Candelaria Hills and Excelsior Mountains, Nevada, is structurally anomalous with respect to surrounding tracts of the Basin and Range Province. Its major faults strike east, and some, if not all of them, have undergone significant left slip. Ratios of lateral to vertical displacement components on strands of such faults vary from 2 to 6.5. The region of left-oblique slip faulting seems to constitute the intermediate leg of a gigantic Z pattern of Basin and Range faults.One of these faults, the Candelaria fault system, has been studied closely in pursuit of kinematic details that might characterize the regional faulting. The Candelaria system consists of three en echelon faults of 5 to 10 km trace length joined by connector zones that possess compressional features. The amount of lateral slip transferred from one en echelon fault to another is uncertain; analysis of a complex of faults in one connector zone indicates at least some degree of integrated motion. The same connector zone is the site of a bulge that has an uplift estimated to have been 275 m since 2.8 m.y. B.P. On the basis of fault-plane striae, the extension direction is ∼ N82°W. We propose that the Candelaria fault system and a major asymmetric trough in its northern wall were created in Oligocene time by a regional left-lateral shear system oriented in the extension direction. The connector zones may be kinematic impediments which, at shallow levels at least, have prevented the system from maturing into a single plane fault. The existence of the bulge and the estimated rate of slip on part of the Candelaria fault system suggest that the system is still active. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geological history and significance of a laminated and slumped unit in the Carboniferous Limestone of the Monsal Dale region, Derbyshire

AbstractA laminated and slumped limestone bed from the Carboniferous Limestone of Derbyshire, known as the Rosewood Marble, is described in detail. The laminations comprise alternations of micrite and dolomite, the dolomite laminae originally being fine grainstones. The bed was deposited on a slope and slipping and sliding gave rise to tensional features such as pull‐apart structures and compressional features such as slump folds. Evidence suggests that the palaeoslope was inclined towards the south‐southeast and caused by synsedimentary movements of the Longstone Edge monocline and/or the Priestcliffe syncline. Diagenetic features described include cementation, dolomitization, calcite pseudomorphs after replacement anhydrite and silicification.

Tertiary Stratigraphy and Tectonism in Svalbard and Continental Drift

An arm of the North Atlantic seaway reached Svalbard during the Paleocene, either in advance of, or in response to, early drift of Greenland away from the Eurasian continental block. Initial Tertiary sediments in Svalbard were marine, littoral, and paludal clastics and coals formed during an oscillating northeastward transgression of the sea. Second and third depositional cycles followed consisting of marine clay units that grade upward into fine or, locally, coarse regressive sandstone. Through the early stages of the third depositional cycle (mid? Eocene) movements were epeirogenic, and transgressions were toward a land area in northeastern Svalbard. Although Greenland had been drifting away from Norway along the Spitsbergen fracture zone (dextral transform fault), ther is no evidence of contemporaneous tectonic interaction between the Greenland and Barents blocks. The third depositional cycle was brought to a close by an influx of clastic material from the west in response to rapid uplift either within or along the western margin of the basin. Continued uplift in the west provided a flood of detritus that eventually filled the basin with plant-bearing alluvial, fluviatile, and paludal sediments. Uplift culminated in upthrusting, folding, and local overthrusting of the central or western part of the Tertiary basin; only the northeastern part of the basin is preserved now in the strongly asymmetrical Spitsbergen trough. Tectonism was the result of Greenland pressing against the Barents block while moving parallel with it (transpression), and the resulting structures show dextral shear as well as compressional features. Subsequent parting of the G eenland and Barents blocks resulted in tensional stress with dextral shear (transtension) and the formation of large grabens filled with locally derived, coarse-clastic sediments within the uplifted western margin of the trough. Separation is thought to have been completed by latest Oligocene or possibly early Miocene time.

Tectonic evolution of the Middle East and the Levantine basin (easternmost Mediterranean)

Research Article| December 01, 1975 Tectonic evolution of the Middle East and the Levantine basin (easternmost Mediterranean) David Neev David Neev 1Geological Survey of Israel, 30 Malkhei Israel Street, Jerusalem, Israel Search for other works by this author on: GSW Google Scholar Geology (1975) 3 (12): 683–686. https://doi.org/10.1130/0091-7613(1975)3<683:TEOTME>2.0.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation David Neev; Tectonic evolution of the Middle East and the Levantine basin (easternmost Mediterranean). Geology 1975;; 3 (12): 683–686. doi: https://doi.org/10.1130/0091-7613(1975)3<683:TEOTME>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract A “Central plate,” bounded in the west by the “Pelusium line, ” which stretches in the easternmost Mediterranean Sea some 60 km off Israel, and by the Zagros main fault in the east, is moving northward at a faster rate than the adjacent Northwest African and Iranian plates. The wedging in of the “Central plate, ” which has been maintained since late Paleozoic time, generates lateral compression that is responsible for the simultaneous development of the northeast-trending compressional features and the northwest tensional features in the Levant. An analogous mechanism is suggested for the development of the Mesopotamian and Persian Gulf fold system. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Structural Evolution of Mesozoic and Cenozoic Basins in Western North America: ABSTRACT

The North American segment of the Circum-Pacific east of the Aleutian Trench is distinguished from adjoining segments by the absence of an oceanic trench and the lack of recorded seismicity with focal depths exceeding 70 km. The continental margin seaward of the North American cordillera incorporates a series of irregularly distributed sedimentary basins of late Mesozoic and Cenozoic age. The structural development of these basins was determined by progressive, episodic subsidence accompanied by interbasin uplift. The tectonic history, structural style, and distributions of the basins demonstrate the presence of a composite system of mobile vertical stress fields which has endured over the past 140 m.y. A superimposed horizontal stress field of uncertain duration is expressed tectonically by strike-slip faults and related structures. These data are incompatible with hypotheses of overriding lithospheric plates and their corollaries as currently proposed. Basin development was initiated in late Mesozoic time, with the differentiation of the continental shelf into discrete basins of localized subsidence. Progressive deepening, migration, and reorientation of the basin axes, and local reversals of vertical movement combined to produce the present composite basins. Commonly structural relief of 20-30 km is developed on the steeper flanks and most basins contain sedimentary thicknesses in excess of 10 km. Internal structures, including basin-margin compressional features, reflect the dominant vertical stress system. Second-order drag folds are well developed in association with strike-slip fault systems. The primary vertical tectonic forces which created these basins are still active, as evidenced by earthquake, geodetic, and geomorphic data. A total of 21 billion bbl of oil and 37 trillion cu ft of gas has been discovered in seven of the 36 basins described. A practical understanding of the structural style and framework of these prime target areas is critical to the realization of the energy potential of the relatively untested basins of this province. End_of_Article - Last_Page 1453------------

Geothermal resources in the imperial valley of california

The Imperial Valley is a major rift valley characterized by unusually high heat flow and large quantities of water in storage in the thick fill of alluvium provided by the sediments of the delta of the Colorado River. The inventory of hot water appears to be sufliciently large that if used for water desalination it might add several million acreleet of new water to the resources of the lower Colorado River basin. This distilled water would serve to lower river salinity and provide extra water to help meet the U. S. — Mexico treaty commitments. A major fraction of water desalination costs lie in the cost of energy and are related to desalination technology which is directly related to water chemistry. The discovery of low salinity geothermal waters in the Imperial Valley opened th possibility for a major breakthrough in lowered water desalination costs. We have tried to develop a broad understanding of the origins of the waters of the Imperial Valley and how natural recharge occurs. The chemical composition of the waters of the central portion of Imperial Valley basin waters, while not that of present surface flow of the rivers, nevertheless does have a close affinity to Colorado River water. No sea water seems to be present in the valley although marine sediments appear to occur on basement on West Mesa and on basement to the east in Arizona south of Yuma. Low salinity waters dominate the basin hydrology and waters as saline or more saline than sea water appear to be restricted to the immediate area of the Salton Sea. The isotope work of T. Coplen makes it possible to determine the relative contribution of precipitation runoff from California watersheds and from Colorado River water. Both sources are significant. The Colorado River water in aquifers from 100–400 m appears to have been entrapped from a relatively homogeneous basin which was subject to substantial evaporation. Its original source was snow melt water from the Colorado River. Five types of waters, none of them sea water, were recognized by their salt geochemistry. Bromide/chloride data are particularly effective in resolving different types of water masses. The bromide/chloride data agree with the isotope data and identify rainfall and precipitation runoff from the high mountains to the west. Modern Colorado River water is easily recognized by its salts and two types of ancient Colorado River waters from previous lake stage are proposed on the basis of the bromide/chloride data. One old lake occurring during the pluvial stage associated with the last Ice Age is proposed to account for much of the water in artesian aquifers. Another younger lake stage, possibly with Lake Cahuilla affinities is also suggested. Mountain runoff waters can be distinguished in the subsurface by their relatively lower salinity and high bicarbonate concentration, and their heavy isotopic composition. Revised fluid reserve calculations based on additional porosity data continue to show that the low salinity water resources of the Imperial Valley may exceed two billion acre-feet. The oceanic plate tectonic model is modified in the Imperial Valley by the evidence of a series of complex blocks with the generation of both tensional and compressional features in the valley. Major strike slip faults dominate the tectonic fabric but conjugate features increase complexity by a large degree and a major amount of work will be needed before any geologically sound structural models can be generated. Xenoliths within the obsidians at the volcanoes at the south end of the Salton Sea provide samples of the basement under the Imperial Valley. These xenoliths include partially remelted granitic rocks, fragments of basalt, greenschist, and baked shale and sandstone. This is taken as evidence that the basement in the valley consists in part of partially remelted granite. This would render basement plastic and readily deformed. The source of the heat is suggested to be derived from basalt that comes into the basement and deeper sediments from below. This upward movement of basalt along a spreading zone is the continental equivalent to a sea floor spreading area. In the continental case the insulating blanket of wet sediment retains the heat and appears to produce a major geothermal resource. The geothermal resources of the Imperial Valley are the aggregate of the thermal energy of the large inventory of subsurface water heated by the complex mix of intrusive phenomena. The net result is to generate a polygenetic geothermal resource of very large dimensions.

Plate convergence between the Philippines and the Ryukyu islands

The region between the Philippine and Ryukyu Island arc systems, including the island of Taiwan, is one of the most complex parts of the consuming Philippine-Asian plate boundary. The marine geology of the area east and north of Luzon indicates that the Luzon arc extends northward beyond the south end of Taiwan and has collided with the latter in Pliocene time. Collision later occurred between the northern end of the Luzon arc system and the Ryukyu system. These collisions caused trench and trench slope rocks of the Luzon arc system to be uplifted as the Coastal Range of Taiwan and the arc system to be shortened in a north-south direction by strike-slip faulting. Continued plate consumption was partly absorbed by shallow east-dipping thrust faults in central and western Taiwan. Seismological and marine geophysical evidence suggest that arc polarity reversal, probably in response to collision, is now occurring along the Luzon arc system. A new trench is forming along the east side of Luzon but compressional features have not yet developed along the entire east flank of the Luzon system. Before collision, the Ryukyu and Luzon arc systems were separated by several hundred kilometers and joined by a trench-trench transform fault, which is now destroyed. In Early Tertiary time, there was probably an east-facing arc system along the Luzon trend, which may have been joined by another transform fault to the south end of Taiwan. Behind this Late Cretaceous-Early Tertiary east-facing arc system, the China Basin is thought to have opened as an inter-arc basin.

Ironside Mountain, Oregon: A Late Tertiary Volcanic and Structural Enigma

Note: This paper is dedicated to Aaron and Elizabeth Waters on the occasion of Dr. Waters' retirement. Ironside Mountain is a 6- by 10-km block of folded rhyolite, andesite, and basalt flows of the Strawberry Volcanics bounded by a horse-shoe-shaped reverse fault and surrounded by Mesozoic rocks. The toe of the horseshoe points northeast; the fault dips outward at angles of 45° to 90°, has a minimum displacement of 600 m at the toe, and dies out or is buried at the open southwest end. Flows are folded to vertical in a crosswise anticline at the northeast end, and in a synclinal structure along the southwest edge. Local unconformities and thickening of flows in a small basin show that some deformation accompanied volcanism, which has been dated as late Miocene and early Pliocene. No centers of eruption have been identified within the Horseshoe fault. The Ironside Mountain block seems to be a unique expression of cross folding in the region. The northwest-striking Mine Ridge–Ironside anticline follows the main regional trend, and is faulted where two large down-warps strike into it from the northeast. The Ironside Mountain structure is interpreted as being a transverse compressional feature in which opposing reverse faults took the place of anticlinal folding, directly on strike with a syncline to the northeast. A horst of pre-Tertiary rocks, which has been raised at least 600 m, crosses the axes of the abutting structures; the nose of the Horseshoe fault bounds one side of it, and a range-border fault bounds the other. Cross faults, of which we found only one, are believed to have facilitated arching and longitudinal shortening within the horst.

Focal mechanism of a shock at the northwestern boundary of the pacific plate: Extensional feature of the oceanic lithosphere and compressional feature of the continental lithosphere

First motions of P-wave and S-wave polarization angles for a shallow shock that occurred on July 25, 1965, at about 70 km oceanward from the Kuril trench indicate a double couple dip-slip source with a horizontal tension axis in the direction perpendicular to the trench. This suggests that the Pacific plate in this region is being extended in this direction. This event cannot be attributed to be triggered by any adjacent great earthquakes. On the contrary, the adjacent area is now being considered a prime candidate for a future great earthquake. Triangulation surveys, tide gauge observations, and levelling surveys performed in the eastern Hokkaido facing the shock across the trench indicate that the continental crust is contracting in the direction normal to the trench axis and tilting towards the Pacific Ocean. These tectonic features are conformable to the idea that the continental plate is being compressed and dragged down into the asthenosphere by the sinking oceanic plate. The extensional feature of the oceanic plate in the region oceanward from the trench suggests that a gravitational pull acting the cold part of the sinking plate dominates the tectonic process now going on in this region.

Western Boundary of the North American Continental Plate in Asia

The mid-Atlantic Ridge and its northern extension across the Arctic, the Gakkel Ridge, form the eastern boundary of the North American continental plate. The position of the western boundary separating North America from Asia is less certain and has been sought in the Bering Sea region and in Yakutia, Northeast USSR. Geologic similarities between Alaska and easternmost USSR indicate that these two continental areas have been connected since the Paleozoic and probably since the Precambrian, and that the western plate boundary does not pass through the Bering Strait. The trend of earthquake epicenters aligned along the Gakkel Ridge continues inland along the Cherskiy system of mountain ranges and depressions. High-angle faulting and volcanism in the Moma-Zyryansk Basin along the east side of the Cherskiy region seem to be a landward extension of the seafloor-spreading tectonics on the Gakkel Ridge. Modern seismic activity, faulting, and volcanism in Yakutia may represent incipient rifting of the Eurasian continent, a situation similar to that in the Gulf of California. In Yakutia earlier compressional features of late Paleozoic and especially Mesozoic age on trend with the Arctic Ocean opening contrast strongly with the modern tensional movements. Mesozoic rocks of the Verkhoyansk geosyncline on the west are faulted against a Paleozoic geosyncline in the Cherskiy Mountains that passes eastward into platform deposits of the Kolymski massif. Most of the faults are large thrusts that not only bring Paleozoic rocks against Mesozoic rocks, including synorogenic granite masses, but also displace sedimentary facies in the Paleozoic rocks. High-pressure metamorphic minerals are developed along these faults in wide zones of breccia and mylonite. The geologic history of Yakutia indicates that it was the site of a major seaway that separated the Siberian platform from the Kolymski massif during the early Paleozoic and then again in the early Mesozoic. Deposits of geosynclinal proportions developed along the margins of these subcontinents and were deformed into foldbelts as the seaways closed between the leading edges of the Eurasian and North American continental plates. The boundary between the Cherskiy and Verkhoyansk foldbelts marks the suture of the collision of these plates in Early Cretaceous time.

Seismo-tectonics of the Persian plateau, eastern Turkey, Caucasus, and Hindu-Kush regions

abstract Relocations of nearly 1,500 earthquakes that occurred between 1950 and 1965 in the area extending from 30° to 75°E and 20° to 45°N delineate the major seismically active tectonic zones in this region. The northern boundary of the Arabian plate is marked by a nearly continuous line of epicenters along the Zagros-Taurus mountains and the Levant fracture zone. The entire series of elongated foothills of the Zagros is a region in which the crustal rocks are contracted by folding, faulting, or a combination of both. The seismic and geological evidence can be explained most simply by a relative northeast motion for the Arabian plate with respect to Eurasia; this motion produces the compressional features along the Zagros thrust zone. In Persia, the Zagros thrust zone separates the presently more continuous seismic regime of the southwest from the episodic seismic regime of the median area. In the Caucasus, a deviation between the alignment of epicenters and geological trends may indicate current changes in the tectonic regime. In the Hindu Kush region, the seismic zone appears as a contorted slab-like structure with a width of 20 to 30 km and consists of a clear east-west alignment of epicenters with focal depths of about 200 km. This zone is connected to another broader zone with a trend of N45°E and focal depths of about 100 km. The trend of the deeper seismic zone does not correspond to that of any surface features, although the shallower seismic zone has the general trend of the Hindu Kush mountains. The focal depths in other areas are generally less than 50 km, with occasional exceptions in front of the Zagros-Taurus thrust zone, the tectonic zones of the Elburz and Caucasus mountains, and the western shore of the Caspian Sea.

Alpine Deformation and the Oceanic Areas of the Tethys, Mediterranean, and Atlantic

A preliminary least-squares fit of the 1,000-m contour of the continental fragments around the Mediterranean is presented. It extends the Atlantic reassembly of Bullard and others to the Balearic Islands, Corsica, Sardinia, Italy, Yugoslavia, Greece, and Turkey. Geological and geophysical data suggest that the reconstruction applies to Permo-Triassic time. The large gap between southern Eurasia and the northern and eastern parts of the reassembled fragments shows the probable shape of the Tethyan ocean of this period. During Permo-Triassic time the Mediterranean is thought not to have existed, and therefore it should not be regarded as a remnant of the Tethys. As in several other syntheses, the formation of the Mediterranean, together with that of the Alpine orogenic belts around it, are attributed principally to the relative movements between Africa and Eurasia. Unlike these syntheses, the nature of this motion has been estimated much more precisely by constructing a simple sea-floor spreading model of the central and northern Atlantic Ocean. The model permits the relative movement hypothesis to be tested more rigorously than previously. Available spreading data show that at least three phases of relative movement must have taken place. Relative to Eurasia, the opening model shows the earliest phase consists of an eastward movement of Africa in Lower Jurassic to early Upper Cretaceous time, followed by a westward movement in later Upper Cretaceous to late Eocene time, finally terminating with latest Eocene to present-day northward movement. Anomalous Early Permian and Cretaceous paleomagnetic data, the contrasting age differences between ophiolite-chert sequences in the eastern and western Mediterranean region, and the positions of the Alpine orogenic belts are all shown to be consistent with general features of the reassembly and the opening model. The reassembly and the model also permit a self-consistent plate tectonic interpretation to be made of Lower Jurassic geology, which suggests the existence of a single, irregular plate margin between Africa and Eurasia, whose rotation pole was close to the best-fit pole of Africa and North America on the Atlantic reassembly. It is believed to link such diverse features as central Atlantic opening, sea-floor spreading in Yugoslavia and Albania, transcurrent faulting in the eastern Alps, and compressional features in the Crimea and the Great Caucasus. Although Lower Jurassic geology is consistent with a single irregular plate margin, it is shown that, for the reassembly and opening model adopted, all the larger fragments (Spain, Corsica-Sardinia, Italy, and Yugoslavia-Greece-Turkey) must at times have belonged to a plate or plates independent of Africa and Eurasia. Because the Atlantic spreading pattern has been grossly simplified, and also because verifiable reassemblies of later times cannot yet be made, plate tectonic interpretations of such periods are premature, although partial interpretations are still possible. A possible interpretation is given of a phase of movement of the south Atlas fault and of the relationship of the ophiolite-chert sequences in the Oman Mountains and the Zagros Range of Iran to Atlantic sea-floor spreading. A number of fundamental problems emerge, which include the necessity to postulate different mechanisms for eliminating oceans at compressive plate margins, and the conclusion that present interpretations of structures within orogenic belts provide poor estimates of the relative displacement vectors that have produced them.

Late Cretaceous Eugeosynclinal Sedimentation, Gravity Tectonics, and Ophiolite Emplacement in Oman Mountains, Southeast Arabia

The Oman Mountains border the Gulf of Oman from the Arabian Sea to the mouth of the Persian Gulf. The Jebel Akhdar anticline forms the central core of the mountains and exposes a section of massive limestones which range in age from Permian (transgressive on a Precambrian nucleus) to late Tertiary; the latter fringe the uplift. Intercalated in this sequence, between well-dated latest Cretaceous sediments, is a thick contorted assemblage of rocks composed, from base to top, of turbidite limestone and radiolarian chert (Hawasina Group), massive exotic blocks of Permo-Triassic limestone, and a thick sheet of serpentinic igneous rock (Semail igneous series). This association of rocks, which is comparable to that known from many complex Laramide orogenic belts, can be studied with advantage in Oman where the sequence has been little disturbed since its emplacement during the latest Cre aceous (Campanian-Maestrichtian). The absence of recognizable latest Cretaceous fossils in the Hawasina, the common occurrence of well-preserved Permian to middle Cretaceous species, and the contorted features of the strata have led some geologists to postulate that these sediments were deposited outside of their present location during a prolonged pre-Late Cretaceous interval and then tectonically emplaced during the latest Cretaceous. However, it is concluded from field data presented herein that the Hawasina was deposited in its present location during latest Cretaceous time. Regional stratigraphic correlations show that northeast Oman was situated far out on the Arabian platform where quiet carbonate sedimentation persisted from Permian to middle Cretaceous time during prolonged regional tectonic quiescence. Sedimentary and tectonic quiescence ended during latest Cretaceous time when the thick Aruma pelagic shale, originating from a northerly source, was deposited across northeastern Oman concomitant with major normal faulting. Hawasina facies, north of the Aruma province, give evidence in the form of basal boulder beds, turbidites, and massive terminal gravity slides of sharp contemporaneous tectonism. Lower Hawasina deep-water turbidite sediments have yielded an inverted faunal sequence of shallow-water fossils, whereas the overlying cherty facies contains only pelagic Radiolaria. Thus the whole sequence cannot be older than the reworked youngest middle Cretaceous fossils present in the basal beds. Distribution, grading, and constitution of the carbonate clastic material in the Hawasina, and the alignment of exotic limestone blocks indicate that the sediment source area was a northwest-southeast-trending uplift of Permian to middle Cretaceous carbonate rocks north of the Oman Mountains. The absence of terrigenous clastic material and terminal submarine volcanism suggest that the source area was a submerged seamount. Erosion from this high is believed to have been by means of turbidity currents activated by repetitive and at times strong block-fault movement. Hawasina sediments compare with present deep-water sediments in the Puerto Rico Trench where faunally barren siliceous oozes of abyssal facies are interbedded with calcareous turbidites rich in reworked older and contemporaneous shallow-water fauna. The absence of contemporaneous (latest Cretaceous) shelf fauna in the Hawasina is attributed to the seamount source area being deeper than neritic, whereas the absence of contemporaneous pelagic calcareous fauna was the result of dissolution during descent to abyssal depths. The latest Cretaceous Aruma Shale is compared with Holocene pelagic globigerinal sediment occurring above 4,500-m depth. The deep trough which received Hawasina sediments appears to have been bounded by a steep block-faulted northeastern limb whereas the southwestern limb became shallower gradually through the neritic Aruma belt to the Arabian carbonate platform. At the close of the time of Hawasina deposition, volcanism and catastrophic tension-relief faulting dislodged the remnants of Permo-Triassic limestone sedimentary rocks from the seamount, and these descended into the trough as huge gravity slides (e.g., Jebel Kawr, 250 sq mi), leaving the seamount as a denuded basement uplift. Regional tension relief was accomplished finally by crustal separation and flood eruption of Semail ultrabasic magma which blanketed the abyssal landscape and cooled slowly, under great hydrostatic pressure, along the axi of the trough. According to this interpretation, the sedimentation of Hawasina beds is considered to have been contemporaneous with the deposition of the latest Cretaceous Aruma Shale, and major block-fault tectonism in the orogenic belt is correlated with known latest Cretaceous tension faulting on the southwest basin slope. Compressional features in the Hawasina are attributed to contemporaneous gravity slumping of the beds on the steep northeast limb of the trough, combined with the subsequent End_Page 626------------------------------ bulldozing effect which massive slides of exotic blocks and the thick Semail eruptives had on the incompetent sediments. Oil prospects in the Hawasina belt are difficult to explore and assess for several reasons, not the least of which is the masking effect which contorted chert has on seismic and gravity exploration for more simple structures in the competent sub-Hawasina limestone succession, which is hydrocarbon-bearing below Aruma Shale at Fahud, Natih, and Yibal oil fields.