Major Structural Trends Research Articles

Reconnaissance paleomagnetic results from western Cuba

A paleomagnetic study of Mesozoic rocks from the Sierra de Los Organos and Sierra del Rosario fold belts of western Cuba revealed postfolding magnetisation in diabases of the Late Jurassic El Sábalo Formation and carbonates of the middle Cretaceous Pons and the Late Cretaceous Carmita and Moreno formations. Steep components with inclinations of about 70° were isolated from all three formations; at the same time, postfolding shallow components were also found in a few samples of the Pons limestones. We rule out a possibility to account for these results by either horizontal movements or non-dipole field anomaly. Neither very appealing is a hypothesis of a post-remagnetization tilt of the entire region. All the components appear to be confined to a plane perpendicular to the main structural trends; we hypothesize that the remanences might have been distorted or re-aligned during deformation; this assumption, however, is far from being proven. In contrast, well-defined characteristic components were isolated from basalts of the Aptian-Albian Encrucijada ( D I = 247° 23° , K = 14, a 95 = 9.0°) and the Late Cretaceous Orozco ( D I = 228° 22° , K = 110, a 95 = 4.7) formations from the Bahia Honda zone in the north of western Cuba; the remanence in the Encrucijada Formation is shown to predate deformation. Mean inclinations in both formations match those in Cretaceous volcanics from central Cuba, and all the results show lower latitudes than expected from the reference data for the North American plate thus implying that volcanic domains of Cuba were displaced northward by about 1000 km prior to the Middle Eocene. Cretaceous declinations in western and central Cuba differ by about the same amount as the major structural trends of these two areas suggesting oroclinal bending of Cuba. At the same time, both areas are rotated counterclockwise with respect to North America thus implying movements on a broader scale.

Brine expulsion, fluid transport and metallization spanning 2.0 Gyr in basins of southern and central Africa

AbstractMineral provinces in southern and central Africa are strongly controlled by major structural trends, the alignment of sedimentary basins and metamorphically induced thermal regimes deep in the crust. Ore deposits are preferentially located on cooler margins of orogenic belts and are ultimately end‐products of fluid expulsion out of the deeper parts of orogenic axes. Metamorphic and structural vectors within orogenic belts adjacent to major cratons show a trend of high‐grade thermal overprinting in the cores of axes and lower metamorphic regimes acting on the distal margins of orogens (e.g. foreland basins). This apparent pattern is considered to have importance in the expulsion of at least three generations of mineralizing fluids beginning with exhalative migration during diagenesis and culminating much later in thrust‐controlled expulsion onto adjacent craton margins. Fluids within the hydrosphere that accumulate initially through topographic gradients in the sediments mixed with components of the mantle (CO2). After storage within the crust, migration, enforced by metamorphic processes, transferred fluids out of, and away from, high thermal regimes in the axes of belts, leading to their present preservation around the margins of the major cratonic nuclei.

The Petrology and Geochemistry of Vesteris Seamount, Greenland Basin--an Intraplate Alkaline Volcano of Non-Plume Origin

Vesteris Seamount is a solitary alkaline volcano in the Greenland Basin some 280 km NW of Jan Mayen. Topographic and geophysical studies have shown no sign of an associated plume trace. Evidence from ash layers in sediment cores around the volcano and dating of dredged samples show that it has been active in Quaternary times. The lavas from Vesteris studied here consist of basanites, tephrites, mugearite, and alkali basalts. Crystal fractionation models are consistent with the generation of the tephrites and mugearite from a basanitic parent. Extensive kaersutite fractionation is required late in the fractionation sequence to produce the extreme mugearite composition. Na-Al-Fe-rich green cores to many clinopyroxene phenocrysts at Vesteris suggest a fractionation history beginning at high pressure in the mantle. Differences between Vesteris and Jan Mayen in the ratios of highly incompatible trace elements such as Ce/Pb and Rb/Cs, which will not normally be fractionated from one another during mantle melting, suggest that the two are not derived from the same source. Relatively unradiogenic Sr isotope ratios (compared with Bulk Earth), and highly incompatible trace element patterns similar to those for St. Helena, suggest that Vesteris magmas are derived from a depleted, asthenospheric source. We propose that the Vesteris basanites are very low degree partial melts (˜1%) of this source, most probably those which give rise to the seismic low-velocity zone (LVZ). Such small-degree melts may preferentially tap small-scale heterogeneities in the asthenosphere. Vesteris lies at the intersection of two major structural trends in the Greenland Basin—(1) a zone of major reorientation of spreading direction on the Mohns Ridge north of Jan Mayen and (2) the extension of the Kolbeinsey Ridge axis. We propose that a combination of the extensional stress fields related to these two lineaments produces sufficient dilation of the lithosphere at Vesteris to allow magmas from the LVZ to reach the surface.

Seismicity, active faults and stress field of the North Muya Region, Baikal Rift: New insights on the rheology of extended continental lithosphere

The eastern Baikal rift is characterized by a succession of young en echelon half‐grabens distributed in a 300‐km‐wide zone of high seismic activity. In this study we present the overall seismotectonic setting of the north Muya region, which is an along‐strike transfer zone between two en echelon dip‐slip fault systems. Using both regional and local seismological networks, we refine the hypocenters and the single‐event focal mechanisms of 704 earthquakes recorded during 3 years, including the main period of activity of a very dense cluster, the Angarakan swarm. Our final hypocenter locations, obtained after determining a region‐specific velocity model and applying a master event technique, allow us to observe a widespread and unusual seismic deformation throughout the upper 30 km of the continental crust. Deep earthquakes (15–30 km) define a planar surface related to a large north‐dipping basement fault (Upper Muya), and also occur in a small cluster at 23 km depth at the intersection of two major structural trends. Along more than 150 km of strike length from west to east, maximum focal depths continuously increase from 20 to 30 km, whereas fault dip concomitantly increases from 30° to 55°. On the other hand, most activity of the Angarakan swarm is confined in the “classical” depth range of 0–15 km and depicts a narrow and steeply south‐dipping seismic band related to a very recent fault scarp at the surface (eastern Kovokta). This fault geometry, which is observed at depth and in the field, and the opposite tilting of regularly 30‐km‐spaced horsts and grabens, reveal an asymmetrical rift system. The Agreement of fault lengths estimated from field studies with rupture lengths estimated from large historical earthquakes strengthens the inference that large earthquakes may occur near the base of the seismogenic layer of the crust, i.e., 30 km. Most of the 39 focal mechanisms determined in the study region show a dominant normal dip‐slip displacement on nodal planes striking in the main inherited structural direction WSW‐ENE, whereas some limited pure strike‐slip faulting mainly occurred in the highly fractured Angarakan zone. From inversion of 37 focal mechanisms, we deduce a tensional stress tensor with a σ3 axis striking N160° and a σ2 axis slightly compressional (shape factor R of 0.4). Comparing these results with stress tensors computed along the eastern Baikal rift and with the focal mechanisms of three strong earthquakes that occurred in the Muya region during this century shows that small‐magnitude earthquake data give consistent and accurate constraints on the regional stress regime. Whether this present stress tensor remained stable since the beginning of opening of the Baikal rift and during Quaternary is unknown. Our results confirm that continental lithosphere submitted to rapid extension in an early stage of rifting may retain significant rigidity.

Windows-based high-precision EMG analysis tool

The South China Sea is a mid-late Tertiary marginal basin. The magnetic anomaly lineations in the eastern part of the basin trend approximately east-west [1,2], suggesting a north-south direction of spreading. In the spring of 1985, two cruises on the French research vessel “Jean Charcot” provided Sea Beam coverage, seismic reflection, magnetic and gravity profiles. The Sea Beam data exhibit two major structural trends: scarps striking N50°E± 15°, interpreted as normal faults, and scarps striking N140°E± 15°, interpreted as fracture zones. This fabric implies a northwest-southeast direction of spreading, up to about 100 km north and south of the inferred spreading axis [3]. Dense Sea Beam coverage of a roughly 1° square area northwest of the Scarborough Seamounts chain shows that the emplacement of these seamounts was, at least in its initial stage, controlled by faulting in two orthogonal directions, N50°E and N140°E. Magnetic and gravimetric maps of the same area also reveal anomalies trending roughly N50°E, which are disrupted by transform zones striking N140°E. This detailed study indicates that the fracture zones may be closely spaced (less than 20–30 km) east of Macclesfield Bank. Furthermore, magnetic anomalies identified as 6 and 6a (20 Myr) along two north-south profiles located at both edges to the north of this detailed study area may be correlated with the N80°E trend characteristic of such anomalies in the eastern part of the South China Sea. The east-west trend of magnetic anomalies 6 and 6a, south of Scarborough seamount chain, recognised by Taylor and Hayes (1983) [1] is incompatible with the trend of the fault scarps observed on Sea Beam data. We infer that progressive, right-lateral offsets of the ridge across closely spaced discontinuities may account for the nearly east-west average trend of some of the magnetic anomalies, and of the Scarborough seamount chain, which represents the location of the relict spreading axis, in spite of a N50°E spreading direction. The whole central part of the basin east of 115°E may have formed in this way, since N130–140°E striking fracture zones are observed on the Sea Beam swaths. This model may correspond to the second of two successive spreading phases, with N-S and NW-SE directions of extension respectively, as was presented by the authors in a previous paper (Pautot et al., 1986 [3]). Alternatively, we may assume that the direction of extension did not change radically during the opening history [3], implying that oblique spreading occurred along N80°E trending ridge segments, creating the N80°E trending scarps and magnetic anomalies observed to the north and to the south of the study area. Such a direction of extension is compatible with mid-late Tertiary left-lateral movements along large N130–150°E strike-slip faults, such as the Red River and Wang Chao faults in South China and Sundaland.

Jurassic

Abstract The interplay of regional or global sea-level changes and continuing crustal extension related to rifting in the North Atlantic and Tethyan megarifts dominated the palaeogeographical evolution of the area. The sea-level rise that commenced in the Rhaetian continued into the Jurassic to usher in a new phase of predominantly marine sedimentation across northwest Europe. The climate was warm and humid, the seas generally shallow. Land areas became well vegetated, as attested by the abundance of fossil driftwood in marine as well as non-marine sediments. The region probably lay about 10° south of present latitudes, in an area of overlap of Tethyan and Boreal marine realms. For much of the Jurassic, sea levels continued to rise but two major falls occurred, coinciding with accelerated extensional movements in the area. The first was during the mid-Jurassic ('mid-Cimmerian phase') when there was a major regional upwarp centred on the North Sea, and the second across the Jurassic/Cretaceous boundary ('late Cimmerian phase') when a global fall in sea level coincided with an important phase of rifting and block faulting. Major structural trends generally follow previously established lines. The basic structural framework for the Jurassic maps has been compiled from numerous sources, including Andrews &Brown (1987); Dunning (1985); Evans et al. (1982); Gardiner &Sheridan (1981); Van Hoorn (1987); Thomas et al. (1985); Whittaker (1985); Ziegler (1982, 1987); and papers in Brooks &Glennie (1987). Fault trends are generalized and selected to pick out the main structural features controlling the geography; their occurrence on

Shear-controlled evolution of the Red Sea: pull apart model

Results of seismic and other geophysical investigations suggest that strike-slip processes controlled the break-up of the Arabian plate from Africa and initiated the Red Sea Rift. Early oceanisation was facilitated by nucleation of pull apart basins and massive intrusives. The evolution of the Red Sea has gone through different stages. It was a zone of structural weakness already during the Pan-African orogeny approximately 600 Ma. A major reactivation, however, that gradually led to the present-day configuration was initiated during the late Oligocene with intense magmatic activity and the development of a continental rift. Wrench faulting played a key role in the early evolution of the Red Sea, as it shaped most of its western flank as a sharp plate boundary and resulted in the generation and rapid oceanisation of linearly arranged pull apart basins. Spatial distribution of these basins reflects the geometry of the strike-slip zone, which was controlled by pre-existing fault systems like the Najd Shear System, the Central African Fault Zone or the Onib-Hamisana and Baraka suture zones. Strike-slip motion along the latter zones of weakness influenced mainly the Egyptian and Sudanese coastal areas. Arabia was therefore separated from Africa by oceanisation in those regions, where pull apart basins developed. They were still connected in the in-between segments by stretched continental crust. With Arabia as the “moving” and Africa as the “stable” plate the eastern Red Sea flank was formed by pure shear through stretching, thinning and diffuse extension. As a consequence, the eastern and western flanks of the Red Sea are asymmetrical. The acceleration of the movement of Arabia in early/middle Miocene could no longer be accommodated by the opening in the Gulf of Suez and consequently the Dead Sea strike-slip fault developed approximately 14 Ma ago. Since plate motion was still oblique to the major structural trends, the pull apart evolution on the western flank continued and the oceanisation was accentuated, while in the north the eastern flank developed en-echelon fractures of “Aqaba-orientation”. Seafloor spreading commenced 5 Ma ago at parts of the central and southern Red Sea. This final stage is also responsible for the bathymetry with maximum depth values of 3000 m in the seafloor spread areas and a much shallower basin in the north. The metalliferous “Red Sea deeps” are either associated with this infant mid-ocean ridge or with the en-echelon fractures in the north.

A Pull-Apart Model for the Red Sea Without a Pole of Rotation in the Separation of Arabia and Africa

Wide-angle seismic surveys in the Red Sea and adjacent areas revealed fundamental asymmetries of the conjugate continental margins: the Western flank shows large areas of old oceanic crust, whereas the whole eastern flank is underlain by stretched continental crust. In addition, the African margin appears very abrupt over long sectors of the northern and central Red Sea. Gravity, magnetic, and heat flow density observations reveal the controlling structural trends, spreading history, and locations of recent magmatic intrusions, They confirm the seismic results and provide information on the entire basin. These geophysical results have implications for Red Sea evolution models: strike-slip processes controlled the breakup of the Arabian plate from Africa. The geometry of the early strike-slip zones reflects the configuration of major preexisting fault systems such as the Najd fault system, the Onib-Harnisana and Baraka suture zones, the Central African fault zone, or the Marda line. Further evolution introduced en echelon fracture systems of Aqaba orientation as major structural trends in the northern Red Sea, whereas symmetrical extension by sea floor spreading began in the axial trough of the southern Red Sea. The central Red Sea represents the zone of transition between these two contrasting domains. The significant changes acrossmore » as well as along the rift cannot be explained by using a single pole of rotation to describe the separation of Arabia from Africa. Instead, several segments of both flanks have to be considered independently.« less

The southern termination of the Philippine Trench

Recent studies of the southern end of the Philippine Trench using GLORIA sidescan sonar and single channel seismic data and geological studies on the island of Halmahera suggest that the trench is in the process of propagating south and that some of the ESE-WNW convergence is transferred via a broad NE-SW zone of dextral strike-slip across northern Halmahera into the Molucca Sea Collision Zone. NE-SW ridges and lineaments on the seafloor can be traced into major faults and structural trends on land. The bathymetric expression of the Philippine Trench is lost where it meets an oceanic plateau on the Philippine Sea Plate. This elevated and probably thickened plateau appears to have inhibited any further propagation of the trench southwards. To the north of the plateau there is a well-developed accretionary prism, but to the south deformation of sediments on the seafloor is less intense. A prominent ridge, with sediments passively banked up against it marks the southern limit of deformation associated with the Philippine Trench. To the south lies the eastern Halmahera-Waigeo Ophiolite Terrane, an area of shallow water and islands underlain by ophiolitic basement between Halmahera and the Sorong Fault Zone. There are no bathymetric or structural features to indicate any form of active link between the Philippine Trench and the Sorong Fault through or along the northern side of this ophiolite terrane. There is no earthquake evidence for any form of fault linking the Philippine Trench to either the Palau Trench or the Sorong Fault, but there are numerous earthquake epicentres reported between the southern end of the trench and the Molucca Sea. The island of Halmahera is considered to lie in a diffuse boundary zone at the margin of the Philippine Sea Plate. Continued collision of the opposing arcs in the Molucca Sea will ultimately prevent further E-W convergence between Halmahera and Sulawesi. It is predicted that either the Philippine Trench will propagate south along its present line to meet the Sorong Fault or that another trench will develop further to the east. In either case the arc and ophiolite terrane of Halmahera might then be accreted onto the Eurasian margin.

A stochastic theory of community food webs. VI. Heterogeneous alternatives to the cascade model

This paper considers 14 models as possible explanations of three major structural trends observed in 113 community food webs: (1) The average proportion of species that are top species is roughly constant in webs with different total numbers of species. The same holds for the average proportions of intermediate and basal species. (2)The average proportion of links that are intermediate-top links is roughly constant in webs with different total numbers of species. The same constancy of proportions in webs with different numbers of species holds for the three other kinds of links. (3) The frequency distributions of chain lengths are unimodal with modes usually between two and five links. One model, the cascade model, explains these trends and other structural trends. The other 13 models relax in various ways the cascade model's assumption that the probability of predation is the same between every pair of potential predator and potential prey and its assumption that the average probability of predation is inversely proportional to the total number of species in the web. Numerical comparisons of the observed structural trends with the model predictions show that the only viable alternative to the cascade model is the predator-dominant, constant-column-sum, linear-link-scaling model. This model differs from the cascade model in assuming that the expected number of prey species of any potential predator is the same as that for any other potential predator. The biological lesson suggested by these findings is that potential predators appear to have a mean number of prey species that is closer to constant than to linearly increasing with rank in the cascade model. Models that assume that predation probabilities are determined primarily by the prey species or by the distance between species in a cascade make predictions that are inferior descriptions of the major structural trends. Models that assume the mean number of trophic links increases as the 1.35 power of the total number of species make predictions that are inferior to those of models that assume the mean number of trophic links increases linearly with the total number of species.

Constraints of Sea Beam data on crustal fabrics and seafloor spreading in the South China Sea

The South China Sea is a mid-late Tertiary marginal basin. The magnetic anomaly lineations in the eastern part of the basin trend approximately east-west [1,2], suggesting a north-south direction of spreading. In the spring of 1985, two cruises on the French research vessel “Jean Charcot” provided Sea Beam coverage, seismic reflection, magnetic and gravity profiles. The Sea Beam data exhibit two major structural trends: scarps striking N50°E± 15°, interpreted as normal faults, and scarps striking N140°E± 15°, interpreted as fracture zones. This fabric implies a northwest-southeast direction of spreading, up to about 100 km north and south of the inferred spreading axis [3]. Dense Sea Beam coverage of a roughly 1° square area northwest of the Scarborough Seamounts chain shows that the emplacement of these seamounts was, at least in its initial stage, controlled by faulting in two orthogonal directions, N50°E and N140°E. Magnetic and gravimetric maps of the same area also reveal anomalies trending roughly N50°E, which are disrupted by transform zones striking N140°E. This detailed study indicates that the fracture zones may be closely spaced (less than 20–30 km) east of Macclesfield Bank. Furthermore, magnetic anomalies identified as 6 and 6a (20 Myr) along two north-south profiles located at both edges to the north of this detailed study area may be correlated with the N80°E trend characteristic of such anomalies in the eastern part of the South China Sea. The east-west trend of magnetic anomalies 6 and 6a, south of Scarborough seamount chain, recognised by Taylor and Hayes (1983) [1] is incompatible with the trend of the fault scarps observed on Sea Beam data. We infer that progressive, right-lateral offsets of the ridge across closely spaced discontinuities may account for the nearly east-west average trend of some of the magnetic anomalies, and of the Scarborough seamount chain, which represents the location of the relict spreading axis, in spite of a N50°E spreading direction. The whole central part of the basin east of 115°E may have formed in this way, since N130–140°E striking fracture zones are observed on the Sea Beam swaths. This model may correspond to the second of two successive spreading phases, with N-S and NW-SE directions of extension respectively, as was presented by the authors in a previous paper (Pautot et al., 1986 [3]). Alternatively, we may assume that the direction of extension did not change radically during the opening history [3], implying that oblique spreading occurred along N80°E trending ridge segments, creating the N80°E trending scarps and magnetic anomalies observed to the north and to the south of the study area. Such a direction of extension is compatible with mid-late Tertiary left-lateral movements along large N130–150°E strike-slip faults, such as the Red River and Wang Chao faults in South China and Sundaland.

Extension of the Eyre Peninsula Conductivity Anomaly

Twenty-two digitally recording fluxgate magnetometers measuring short-period natural time variations of the geomagnetic field were deployed during 1988 at 54 sites to define the northward extension of the Eyre Peninsula Conductivity Anomaly (EPCA). The southern portion of this zone of high electrical conductivity was initially mapped during the early 1980s; it trends inland just to the east of north from the southern extremity of the peninsula, at nearly 90� to the continental shelf edge.The 1988 data show that the axis of the anomaly in the north initially swings to the east, and then turns NNW, following the major structural trends in the Precambrian basement of the Gawler Block. Short-period geomagnetic variations observed in stacked magnetograms display a significant amplitude and phase change in the vertical (Z) and total field (F) components across the anomaly axis, while the horizontal variation fields (D and H) are enhanced along the axis. These features are consistent with a concentration of current in the crust.Transfer-functions relating the anomalous vertical to normal horizontal geomagnetic variation fields are expressed in the form of vectors and hypothetical event contours, and these clearly delineate the surface position of the anomaly axis.The EPCA is coincident with a broad band of seismic activity, and a continuous conductive zone such as this must be formed along a significant structure within the Precambrian crust. The zone probably delineates the boundary between the essentially non-conductive Archean crystalline basement to the west and the multiply deformed Proterozoic metasediments of the Hutchison Group to the east. Its presence is important in understanding the tectonic development of this potentially economic zone.Zones such as the EPCA can also introduce errors of several nT into aeromagnetic surveys; data from closely-spaced magnetometer arrays may be used to accurately predict and eliminate such errors.

Style of Faults and Associated Fractures in Austin Chalk: Northern Extension of Balcones Fault Zone, Central Texas: ABSTRACT

ABSTRACT Distributions, geometries, and densities of faults and associated fractures in the Cretaceous Austin Chalk were studied in outcrop within the northernmost extension of the Balcones Fault Zone in Ellis and northern Hill Counties, Texas. Description of the fracture systems may be applicable to hydrocarbon exploration and production from this unit and to locating the proposed Dallas-Fort Worth Area Superconducting Super Collider site in Ellis County. Inactive normal faults with throws of less than 30 m compose this northernmost extension of the fault zone. Most of the major faults strike northeastward to east-northeastward, oblique to the regional strike of bedrock, and dip 45° to 80° northwest and southeast. Smaller faults strike northwestward. Calcite mineralization occurs along most of the fault planes, and slickensides indicate normal to slightly oblique slip. Developed within the fault zone are a series of narrow grabens that are 250 to 600 m wide and as much as 5.5 km long. In plan view these small grabens occur either as (1) depressed strips of bedrock bounded by subparallel hinge faults that increase in displacement to the northeast or as (2) irregularly subsided blocks bounded by double hinge faults that increase in displacement toward the central part of the trough and pinch out toward the northeast and southwest. Areas of high fracture density within the Austin Chalk occur adjacent to the major faults and within gently warped beds located where faults terminate along strike. Narrow zones characterized by different fracture styles and densities occur subparallel to the major structures. These zones vary in width and are composed of (1) complex minor faults (2 m wide), (2) minor faults and joints (15 to 90 m wide), or (3) abundant joints (60 to 140 m wide). Most minor faults have throws of less than 3 m and strike subparallel to an associated major fault or flexure, although some minor faults strike subperpendicular to major structural trends. Minor faults dip between 40° and 70° and are usually partly filled with calcite. Most systematic joints are nearly vertical and also strike subparallel to an associated major fault or flexure. Joints are only rarely filled with calcite, and abutting relationships suggest that minor faults predate joints. Average fracture spacing near faults or within flexures for 0.5- to 1-m-thick chalk beds generally ranges from 0.5 to 1.3 m. Locally joint densities may be as high as 10 joints/2 m. Minor faults and many joints cut through adjacent beds of different thicknesses.

Heat flow at the proposed Appalachian Ultradeep Core Hole (ADCOH) Site: Tectonic implications

The heat flow in northwestern South Carolina at the Appalachian Ultradeep Core Hole (ADCOH) site area is approximately 55 mW/m². This data supplements other data to the east in the Piedmont and Atlantic Coastal Plain provinces where heat flows > 55 mW/m² are characteristic of post‐ and late‐synmetamorphic granitoids. Piedmont heat flow and heat generation data for granites, metagranites, and one Slate Belt site, in a zone approximately parallel to major structural Appalachian trends, define a linear relation. Tectonic truncation of heat‐producing crust at a depth of about 8 km (a depth equal to the slope of the heat flow‐heat production line) is proposed to explain the linear relation. Using the value of reduced heat flow estimated from this empirical relation, and assuming thicknesses of heat‐producing crust defined by new ADCOH seismic data, the heat flow and heat production at the ADCOH site are consistent with a depth to the base of the Inner Piedmont crystalline allochthon of about 5.5 km. Seismic data at the ADCOH site confirm that the Inner Piedmont is tectonically truncated at about 5.5 km by the Blue Ridge master decollement. Temperatures at 10 km at the ADCOH site are predicted to be less than 200 °C.

La marge meridionale de la Mer d'Alboran: Caracteres structuro-sedimentaires et evolution recente

High-resolution seismic reflection profiles (2000 km) were collected across the southern continental margin and major structural trends of the Alboran Sea. Seismic units are dated on the basis of well-known marine Neogene onshore sections and offshore dredged samples. A better assessment of margin structuration and Neogene chronology is made. A conformable succession of Pliocene-Quaternary and older series provides evidence for the non-evaporitic nature of the Upper Miocene, similar to the facies from the onshore Neogene Boudinar and Melilla basins. It also suggests that Alboran Ridge is in part sedimentary rather than only volcanic. This Ridge appears as an “en echelon” anticlinal complex. Conformable Upper Miocene and Lower Pliocene-Quaternary strata are folded and faulted. Primarily important manifestation of the movements is dated as Pliocene, but the main vertical relative movements are of Quaternary to recent age as emphasized by well-developed divergent strata lying above. Submarine volcanism, probably Miocene in age, often controls the morphology and structure of the eastern Moroccan continental margin and adjacent south Alboran Basin. The latter one is characterised by a thick sedimentary accumulation; it is tectonically bounded, except on the southwestern part seaward of the Bokkoyas cliffs. Vertical movements occur during much of Plio-Quaternary time, prevailing during the Quaternary. The structure and related morphology of the study area are mainly the result of neotectonic events.

Heat flow in the Piedmont and Atlantic Coastal Plain of the southeastern United States

Seventy‐four new heat flow values have been determined at sites from New Jersey to Georgia, mostly in the Piedmont and Atlantic Coastal Plain provinces. Relatively high heat flows are characteristic of synmetamorphic and postmetamorphic granites exposed in the Piedmont and occurring in the basement beneath the sediments of the Atlantic Coastal Plain. The Piedmont heat flow and heat generation values, determined in granites, metagranites, and one Slate Belt site, lie in a belt approximately parallel to major structural trends in the Appalachians. These values fall on or near the regression line: q(mW/m2) = 29.8 ± 1.5(mW/m2) + 7.8(km) ± 0.475A(μW/m3) (R = 0.97477). It is proposed that the occurrence of a thrust fault that truncates granites at a depth, D, of 7.8 km is directly responsible for this linear relation in the eastern Piedmont. Basement core from beneath the sediments of the Atlantic Coastal Plain was recovered at ten sites. Results from five of these sites plus one site in the Baltimore Gabbro define a second linear relation with a slope (D = 8.0±0.380 km; R = 0.99550) approximately parallel to the Piedmont relation, but with an intercept of 48.2±0.8 mW/m2. These two relations may represent different heat flow provinces in the southeastern United States or the linear relations may simply be a consequence of differences in the thicknesses of the upper heat‐producing layer at least as far east as postmetamorphic granites sampled under the Coastal Plain.

DISCOVERY AND EXPLOITATION OF NEW OILFIELDS IN THE COOPER-EROMANGA BASINS

Since 1975 the level of petroleum exploration in the Cooper-Eromanga basins has undergone an unprecedented expansion due to the discovery and development of an increasing number of oil reservoirs, largely in the Eromanga Basin sequence. The commercial incentive provided by the Commonwealth Government's Import Parity Pricing and excise arrangements have been instrumental in the lead up to and continuation of this series of discoveries.Three types of oil discovery in the Eromanga Basin sequence are evident; firstly, shallow pools above Cooper Basin gas fields; secondly, separate single-field discoveries in areas of limited exploration; and thirdly, as multifield discoveries along major structural trends. Exploitation of the Eromanga Basin oil discoveries has been made possible by a combination of rapid appraisal and development drilling and early commencement of production.The initial Eromanga Basin oil discoveries overlie major Cooper Basin gas fields and were located during appraisal and development drilling of deeper Cooper Basin gas reservoirs. Wildcat and appraisal drilling on Eromanga Basin prospects, such as Wancoocha and Narcoonowie, has upgraded the prospectivity of the Eromanga Basin sequence in the southern Cooper Basin—an area where earlier exploration for Cooper Basin gas was unsuccessful. Significant oil discoveries in Bodalla South 1 and Tintaburra 1, in the Queensland sector of the Eromanga Basin, have extended the range of exploration success and generated considerable interest in lesser known parts of the Eromanga Basin.Three successive phases of Cooper-Eromanga exploration have led to the present high level of success. Early exploration, before 1969, led to the initial discovery and development of Cooper Basin gas fields and was largely supported by the Petroleum Search Subsidy Acts (19571974). The results of the second phase, between 1970 and 1975, provided little encouragement to operators to extend exploration beyond the limits of the then known gas accumulations. In the decade since 1975, the oil potential of the Eromanga and parts of the Cooper Basin sequences has become a major factor in the exploration and development activity of the region. Since 1975, the favourable commercial conditions prevailing under the Import Parity Pricing scheme and the concessional crude oil excise arrangments for production from 'newly discovered' oilfields provided a significant incentive for development and exploitation of the post-1975 oil discoveries.

Columbia River Basalt Plateau—An integrated approach to interpretation of basalt‐covered areas

An interpretation is presented of a 219 km regional profile which traverses the eastern Columbia River Plateau in Washington State. Aeromagnetic, magnetotelluric (MT), and gravity data were first interpreted separately. All three data sets then were satisfied by a single geologic model. The objective of this case study is to illustrate the individual contributions derived from these three geophysical data sets to a final integrated interpretation. The aeromagnetic interpretation has produced regional structural information and data from which rock compositions can be inferred. The MT interpretation shows that the basalt/sediment interface can be determined, and thus a relative sediment thickness can be inferred. The gravity interpretation is dependent upon an additional method to determine either the basalt or basement horizon. In order for the gravity interpretation to approximate depth to basement or sedimentary thickness, the base of the basalt must be determined from another scientific method. From comparison of the regional structural results of the three geophysical techniques we conclude that aeromagnetic or MT data can be used to determine major structural trends. Reasonable rock compositions are also determined from the combined data sets. The interpreter must be aware of the different rock properties measured by each tool when performing an integrated interpretation; comparisons between the various techniques must be based upon similar assumptions. We recommend that detailed, integrated models be included for a thorough evaluation of any basalt‐covered area. The analysis of rock composition and regional structural information thus derived provides a sound basis for a regional tectonic interpretation and subsequent prospect evaluation.

Determination of elastic wave velocity and relative hypocenter locations using refracted waves. II. Application to the Haicheng, China, aftershock sequence

We located the aftershocks of the 4 February 1975 Haicheng, China, aftershock sequence using an arrival time difference (ATD) simultaneous inversion method for determining the near-source (in situ) velocity and the location of the aftershocks with respect to a master event. The aftershocks define a diffuse zone, 70 km × 25 km, trending west-northwest, perpendicular to the major structural trend of the region. The main shock and most of the large aftershocks have strike-slip fault plane solutions. The preferred fault plane strikes west-northwest, and the inferred sense of motion is left-lateral. The entire Haicheng earthquake sequence appears to have been the response of an intensely faulted range boundary to a primarily east-west crustal compression and/or north-south extension. The calculated upper mantle P-wave velocity is 7.6 ± 0.09 km/sec, and the inferred crustal thickness is between 31 and 32.5 km. The low upper mantle velocity and thin crust may be indicative of local lithospheric extension.

Structure of Shumagin Continental Margin, Western Gulf of Alaska: ABSTRACT

The Shumagin continental margin lies between Kodiak Island and Unimak Pass. The oldest rocks known to underlie the margin are highly deformed, deep-water turbidites of Late Cretaceous age (Shumagin Formation). These turbidites were intruded by Paleocene and early Eocene granodiorites. Paleogene sedimentary rocks of the Kodiak region may extend southwest to and underlie at least parts of the Shumagin margin but are not known in outcrop on shelf islands. The Cretaceous and Paleogene(?) rocks form acoustic basement on multichannel seismic reflection data, and are overlain by a basin fill of probable late Miocene and younger age. Mesozoic rocks of the Alaska Peninsula extend seaward only to the inferred location of the Border Ranges fault. The Shumagin margin is characterized by five major structural features or trends: (1) Shumagin basin, containing about 2.5 km of late Miocene and younger strata above acoustic basement; (2) Sanak basin, containing as much as 8 km of dominantly late Cenozoic strata in two subbasins separated by a basement high; (3) Cenozoic shelf-edge and upper-slope sedimentary wedges that are 3-4 km thick and possibly as thick as 6 km; (4) a midslope structural trend, Unimak ridge, that is characterized by numerous surface and subsurface structural highs; and (5) a 30-km wide accretionary complex at the base of the slope. A thin (less than 1-2 km) sediment cover of Miocene and younger age covers the continental shelf areas outside of Shumagin and Sanak basins. The tectonic history of the margin includes: (1) Late Cretaceous or early Tertiary removal of the seaward part of the Cretaceous Alaska Peninsula margin along the Border Ranges fault and accretion of the Shumagin Formation against the truncated margin; (2) Miocene uplift and erosion of the shelf; (3) middle or late Miocene uplift of Unimak ridge; and (4) late Miocene and younger subsidence and infilling of Sanak and Shumagin basins, and subduction-accretion along the Aleutian Trench. End_of_Article - Last_Page 657------------