Northeast Trending Research Articles

The Anita Shear Zone: A major, middle Cretaceous tectonic boundary in northwestern Fiordland

Abstract A 3‐4 km wide shear zone, here named the Anita Shear Zone, runs parallel to the west coast of northern Fiordland and is marked by amphibolite facies mylonites and intensely deformed gneisses. This shear zone marks a tectonic boundary between the metasediments and meta‐granites of the Tuhua Sequence and the gneisses of the Arthur River Complex and Western Fiordland Orthogneiss. Detailed structural analysis indicates that the Anita Shear Zone has been folded and reoriented into its current steep northeast trend by late folds associated with steeply dipping retrograde shear zones. Removing the effects of the overprinting folds reveals that the Anita Shear Zone was originally subhorizontal or shallowly dipping. The Anita Shear Zone is very similar in many aspects to a middle Cretaceous shear zone in Doubtful Sound which has formed along the same lithological boundary. Both shear zones separate the Tuhua Sequence from the Western Fiordland Orthogneiss, indicating that there is no intrusive contact bet...

COMPRESSIONAL CONTROL ON SEDIMENT AND FACIES DISTRIBUTION SW NAPPAMERRI SYNCLINE AND ADJACENT MURTEREE HIGH, COOPER BASIN

The northeast-trending Nappamerri Syncline and its flanking high to the southeast, the Murteree-Nappacoongee (NM) Trend, show structural development throughout the deposition of the Permo-Triassic Cooper Basin sequences. Pre-existing topography, such as around Moomba, influenced early depositional patterns within the area. Rates of sedimentation were influenced by the rate of uplift of source areas around the basin, largely as a consequence of buckling. Periods of active tectonism resulted in non-deposition and stripping of sediments from uplifted blocks while deposition continued in deeper areas. Hangingwall blocks, pushed up along reverse faults, strongly influenced the areas of sedimentation and the facies being deposited. This is particularly evident along the NM Trend. Phases of uplift were immediately followed by increased sedimentation where fluvial deposition dominated. The effect of displacement along the northwest-trending basement lineaments was to subdivide the northeast trends into compartments which contain similar facies within each time slice. Adjacent compartments may display different facies and different tectonic histories. An inversion episode, particularly evident in the Big Lake/Moomba area, resulted in a reversal of the depocentre and was related to phases of Sakmarian compression.Facies distribution and sandstone percentage maps of the chronostratigraphic units of the Patchawarra Formation suggest that a northeast-trending major channel system entered this part of basin along the NW edge of the Murteree area.

THE SAWPIT STRUCTURE—EVALUATION OF A FRACTURED BASEMENT RESERVOIR PLAY IN THE OTWAY BASIN

The 1992 exploration well Sawpit-1, drilled in the Penola Trough of the western Otway Basin, recovered a minor amount of oil (1.5 BBL) from fractured basement. This oil recovery, the first in the basin from rocks older than the Eumeralla Formation, has opened up a new oil play. Fractured basement reservoirs, which can be prolific producers, require prediction of fracture orientation and intensity to be appraised and developed successfully. Such information was sought from a detailed structural analysis of fault attributes (including dip, vertical displacement and heave) using 3D seismic data acquired over the structure in 1993.This 3D seismic data set, acquired at twice the group interval and less than one-quarter the fold of 2D data in the area, has provided the best imaging to date of the complex rifting events of the Sawpit region. Structural analysis of fault attributes indicates an initial south-southwest direction of extension in the Late Jurassic/Early Cretaceous for the Penola Trough, rather than oblique extension as suggested by some authors. This south-southwest extension orientation favours the formation of moderate to high angle fractures trending west-northwest to northwest. Present day minimum horizontal stress is favourably oriented approximately northeast-southwest for these fractures to be open. Interpretation of the 3D seismic data indicates Sawpit-1 probably intersected northeast trending fractures that are oriented perpendicular to the maximum horizontal compressive stress direction and therefore likely to be closed.A deviated well drilled from the northeast would be optimally oriented to intersect northwest trending basement fractures and test a play that has significant oil potential.

Seismic stratigraphy of a buried volcanic arc, Northland, New Zealand and implications for Neogene subduction

For over 6 million years in the Early Miocene, the Northland Peninsula of New Zealand's North Island was the site of a volcanic arc, comprising north-westerly trending twin belts of marine and subaerial andesitic volcanoes. Approximately 50 volcanic edifices, from major massifs to small cones, have been newly identified on marine seismic profiles of the western belt, which is largely buried. The volcanoes were important point sources of sediment, and the volcaniclastic deposits of each of the major and intermediate sized volcanoes form distinct seismic sequences. The three main elements of the volcanic sequence are (i) a main body, (ii) a sloping apron and (iii) a wide, almost flat ring plain. The downlap of the oldest discernible internal clinoform reflector, relative to a known regional marker reflector underlying it, indicates the approximate time of onset of activity, whereas downlap of the top bounding reflector relative to those of other volcanic sequences provides a relative order of extinction. Sequence stratigraphic principles and super-position determine relative stratigraphic hierarchy. The volcanoes studied here were active only in the Early Miocene. With few exceptions, most were active by 22 Ma and extinct by 16 Ma, with many extinct by 19 Ma. There was no pattern in their order of inception or extinction, although a few became dominant, burying or surrounding their neighbours. At the south-eastern end of the chain there is a rapid change over a short distance in both age and orientation, from the northwest trending Early Miocene volcanism in the Northland Basin to north-east trending Middle and Late Miocene volcanism in the contiguous Taranaki Basin. The Northland volcanoes are interpreted to have been an arc facing the south-westwards subducting South Fiji Basin, whereas the Taranaki volcanoes are interpreted to have been part of a separate, Pacific-facing volcanic arc—the Lau-Colville Ridge.

Major northwest-directed Caledonian thrusting and folding in Precambrian rocks, northwest Mayo, Ireland

AbstractStructural mapping of the North West Mayo Inlier has investigated the kinematics of ductile fault and shear zones within the Dalradian and pre-Dalradian rocks. The area is characterized by an intense east to southeast trending elongation direction. Pre-Caledonian deformation is preserved as west-northwest trending sinistral mylonite zones. These are cross-cut by Grenville pegmatites and by undated mafic dykes. Subsequent deformation has been influenced by the majorbasement structures. During the Caledonian deformation, the pre-existing sinistral structures were reactivated coaxially as steep sinistral shear zones, deforming the mafic dykes. Major northwesterly verging thrusts are widespread in flat-lying zones which occur over much of the inlier. These are interpreted as D2 in age. The S2 fabric is the dominant fabric and contains an east to northeast trending mineral lineation. The southeastern part of the inlier is characterized by very large scale (wavelengths of kilometres) reclined D2 folding. This is interpreted as a crustal scale ramp zone. In the vicinity of this ramp there is evidence of earlier (F1) folds which are possibly the product of ramprelated deformation. All of these structures are interpreted as the product of northwest verging thrusting with crustal scale frontal and lateral ramps.

Topography and tectonics of the central New Madrid seismic zone: Results of numerical experiments using a three‐dimensional boundary element program

We present results of a series of numerical experiments designed to test hypothetical mechanisms that drive deformation in the New Madrid seismic zone. Experiments are constrained by subtle topography and the distribution of seismicity in the region. We use a new boundary element algorithm that permits calculation of the three‐dimensional deformation field. Surface displacement fields are calculated for the New Madrid zone under both far‐field (plate tectonic scale) and locally derived driving strains. Results demonstrate that surface displacement fields cannot distinguish between either a far‐field simple or pure shear strain field or one that involves a deep shear zone beneath the upper crustal faults. Thus, neither geomorphic nor geodetic studies alone are expected to reveal the ultimate driving mechanism behind the present‐day deformation. We have also tested hypotheses about strain accommodation within the New Madrid contractional step‐over by including linking faults, two southwest dipping and one vertical, recently inferred from microearthquake data. Only those models with step‐over faults are able to predict the observed topography. Surface displacement fields for long‐term, relaxed deformation predict the distribution of uplift and subsidence in the contractional step‐over remarkably well. Generation of these displacement fields appear to require slip on both the two northeast trending vertical faults and the two dipping faults in the step‐over region, with very minor displacements occurring during the interseismic period when the northeast trending vertical faults are locked. These models suggest that the gently dipping central step‐over fault is a reverse fault and that the steeper fault, extending to the southeast of the step‐over, acts as a normal fault over the long term.

Tertiary metamorphic core complexes in Sonora, northwestern Mexico

Several ranges encompassing more than 35,000 km2 of Sonora, Mexico, contain distinctly lineated and foliated granitic and metamorphic rocks that constitute the lower plates of metamorphic core complexes. Penetrative deformation is characterized by gently dipping mylonitic foliation across which northeast trending stretching lineation is everywhere developed. Prominent northwest trending fractures, dikes, and normal faults are orthogonal to the lineation. Most kinematic indicators in lower plate mylonitic rocks record top‐to‐the‐southwest sense of shear. Upper plate stratigraphic sequences include Mesozoic supracrustal rocks, Tertiary volcanic and sedimentary rocks, and allochthonous Precambrian basement. Tilted blocks of upper plate strata generally overlie the mylonites along gently dipping detachment faults. Previously published U‐Pb and K‐Ar ages from lower plate granitic orthogneisses, upper plate volcanic sequences, and crosscutting dikes constrain the time of mylonitic deformation and detachment faulting in several of these areas to late Oligocene‐early Miocene. Partitioning of extensional strain in Sonora was influenced by pre‐Tertiary crustal structure. The belt of core complexes developed across two contrasting blocks of continental crust separated by the N60°W striking Mojave‐Sonora megashear. Portions of the southern Papago block (northeast of the megashear) consisting of Jurassic magmatic arc rocks and Upper Jurassic‐Cretaceous siliciclastic and carbonate strata resting upon a concealed, tectonically fragmented Precambrian basement were especially susceptible to crustal attenuation. Some core complexes of the southern Papago block occur within zones trending northwest that may coincide with Late Jurassic lineaments. In the Caborca block (southwest of the megashear), core complex‐related rocks and structures have not been identified where surface exposures of Middle Proterozoic basement and overlying Upper Proterozoic‐Paleozoic platform strata are common. However, extensional mylonitic fabrics are locally developed along the margins of a Tertiary two‐mica granite batholith. Core complexes on both sides of the megashear appear to be preferentially developed where Tertiary granites have intruded regions of crust with basement disrupted by pre‐Tertiary structures. Sonoran core complexes preserve an extensional tectonic history comparable with that described from core complexes farther north in the United States and Canadian Cordillera. The timing of mid crustal extension in Sonora (25–18 Ma) is contemporaneous with the timing of core complex development in Arizona, Nevada, and Utah. Extension occurred later in these areas than in the Pacific Northwest‐British Columbia region but earlier than in the Mojave Desert‐Death Valley region. Middle Tertiary mylonitic fabrics of similar style and orientation have not been recognized farther south in Mexico. The southern terminus of the mid‐Tertiary Cordilleran core complex belt appears to be in Sonora.

Tectonics of the north-western Gulf of Oman and the Arabian continental margin as indicated by magnetic data

North-south, north-east and north-west sets of magnetic lineations are outlined in the north-western part of the Gulf of Oman. The north-south lineations have previously been modelled to be of Early Cretaceous age. The north-east set forms a swarm parallel to the mapped onshore Dibba transform fault which, in turn, is represented by north-east trending total intensity magnetic anomalies. The Dibba Zone magnetic pattern correlates with a fracture zone in the eastern part of the Emirate of Abu Dhabi. The north-west transverse magnetic set may have a counterpart in the adjacent Hatta Zone, northern Oman Mountains. The sense of displacement about the north-east lineations suggests a dextral motion similar to that along the Zendan Fault in Makran (Iran). The Dibba-Zendan configuration has apparently persisted since the Late Cretaceous and may have shielded the Oman-Makran sector from compressive stresses. This preserved a relict of Tethyan crust in the present day Gulf of Oman. The north-east and north-west magnetic sets (apparently traces of leaky transform faults) are suggested to represent continental fractures (of Precambrian-Early Palaeozoic age) which extended into that part of the Gulf of Oman during accretion of the Neotethyan ophiolites. Both the offshore and onshore north-east trending magnetic axes may support a leaky transform fault model.

A Regional Subsurface Study of the Expanded Yegua Formation in the Houston Embayment of the Upper Texas Gulf Coast: ABSTRACT

The Eocene Yegua Formation, one of a series of Tertiary clastic wedges, thickened significantly basinward of the platform margin as it prograded across the Houston diapir province of southeastern Texas. Sandstones of the expanded Yegua represent neritic, distal mouth-bar and shelf environments downdip from the strandline. Structural movements (growth faulting and salt diapirism) were contemporaneous with deposition and exerted a strong influence on patterns of sand dispersal, while sedimentation in turn mobilized the underlying salts and triggered faulting over the unstable shelf edge. Major depocenters became established along strike-parallel growth faults, sourced by dip-oriented feeder systems from the northwestern shelf. The faults, which trend northeast through Liberty and Hardin counties, served to localize thicker sand sections on their downthrown side. Salt diapirism created a major depocenter over a large withdrawal syncline in south-central Liberty County, assumed to be the source for most piercement features in the study area. Thick sand sections also accumulated in other low areas due to salt withdrawal or sediment loading and to damming behind growing domes. In contrast, salt domes and ridges were generally positive topographic highs, which channeled sand into interdomal conduits. Diapirism has therefore compartmentalized the depocenters. Multiple Yegua sandstones now occur in severalmore » favorable structural positions (especially along the shelf-edge growth faults and over the central salt-withdrawal syncline), and these areas of thick sand accumulation constitute primary targets for petroleum exploration.« less

Late Silurian – Early Devonian rifting during dextral transpression in the southern Gaspé Peninsula (Quebec): petrogenesis of volcanic rocks

The Upper Silurian – Lower Devonian volcanic rocks in the southern Gaspé Peninsula of the Quebec Appalachians crop out at the northeast end of the Connecticut Valley – Gaspé Synclinorium. These shallow marine and subaerial sequences reach a thickness of up to at least 2000 m and comprise two groups: (1) the Late Silurian volcanic rocks, which are mainly transitional alkalic–tholeiitic basalts with steeply sloping REE patterns; (2) the Early Devonian volcanic rocks, which include a significant proportion of intermediate rocks in addition to tholeiitic basalts. Compared with the Silurian rocks, the Devonian basalts have lower abundances of strongly incompatible trace elements such as Ba, Th, Ta, Nb, and light REE and relatively flat heavy REE patterns. Basalts of both groups display negative Nb and Ta anomalies (relative to Th and La).Although the basalts of both sequences were derived from lithospheric mantle, the Silurian basalts were generated from garnet peridotite at ~ 80 km depth while the Devonian basalts appear to have resulted from a larger degree of melting of spinel peridotite at a shallower depth (~ 60 km). Devonian intermediate rocks are probably the result of mixing of the basaltic magma with upper crustal material through assimilation – fractional crystallization processes. The basalts are interpreted to have formed in a northwest-trending rift zone located in the Quebec Reentrant during dextral transpression along the Appalachian Orogen. Rotation during and after the volcanism reoriented the rift zone to a northeast trend. The high density layer at the base of the crust under the Magdalen Basin may be the former magma chamber for the Silurian–Devonian volcanism. The change from transitional to tholeiitic volcanism at the Silurian–Devonian boundary suggests that the stretching value (ratio of final to initial surface area) increased from < 2 to > 2 at that time. This boundary is also coincident with the Salinic disturbance that is inferred to have been produced by erosion of the thermally uplifted block associated with rifting.

The Dog Bay Line: a major Silurian tectonic boundary in northeast Newfoundland

The Dog Bay Line separates different Silurian rock groups in northeast Newfoundland. West of the line, terrestrial volcanic rocks and sandstones (Botwood Group) overlie marine greywackes and conglomerates (Badger Group). East of the line, red sandstones overlie shallow marine shales and limestones (Indian Islands Group). Throughout Dog Bay, the line is marked by a disrupted zone of dark grey to black shales, volcanic rocks, and gabbros. Pervasive dextral, transpressive ductile deformation followed by successively more brittle extension with renewed dextral movements mark the northwest side of the line on the coast.The Dog Bay Line is traceable for 100 km and it is open-ended. Dextral offset is deduced to be many tens of kilometres. The line trends northeast, parallel to outcrop belts, and both the line and outcrop belts are curved eastward at the coast. The Mount Peyton Batholith, dated at 420 ± 8 Ma, apparently cuts the line.The Dog Bay Line occurs within the Dunnage Zone whose Cambrian–Ordovician rocks represent vestiges of the Iapetus Ocean. Northwest of the line, the Silurian rocks were deposited on Ordovician rocks already accreted to Laurentia. Southeast of the line, the Silurian rocks were deposited on Ordovician rocks already amalgamated with the continental Gander Zone. Timing of major movement and a Silurian marine to terrestrial depositional change recorded on both sides of the line agree within error with isotopic ages for the onset of plutonism, regional deformation, and metamorphism in central Newfoundland. The Dog Bay Line may mark the terminal Iapetus Ocean.

Fault plane determinations for three small earthquakes along the San Jacinto Fault, California: Search for cross faults

Using rupture directivity and aftershock locations, three earthquakes (ML 4.2 – 4.3) were studied to determine which of the two nodal planes of the focal mechanism was the rupture plane. All three events occurred near the San Jacinto fault zone during December 1989 and February 1990 and had a large component of strike‐slip motion with nodal planes oriented in the northeast and northwest directions. Smaller earthquakes were used as empirical Green functions to deconvolve the path and site complications from the short‐period P waveforms. The resultant far‐field source time functions were inverted for slip on a finite fault and tested to see which nodal plane provided a better fit to the data. The results of the waveform inversion showed that the earthquake near the Anza Gap (December 2, 1989) and the earthquake near San Bernardino (December 28, 1989) ruptured northwest trending planes. The earthquake near Buck Ridge (February 18, 1990) appeared to rupture a northeast trending plane, but the waveform results for this event were not considered as reliable. Available aftershock data for the three events were also examined using master event relocations. The aftershock sequence following the Buck Ridge event showed a clear lineation trending toward the northeast. The results of this study indicate that the Anza and San Bernardino earthquakes ruptured areas on or parallel to the San Jacinto fault, while the Buck Ridge event ruptured a northeast trending cross fault. In addition to determining the fault plane, the waveform analysis resolved details of the slip distribution for these relatively small earthquakes.

Saxothuringian zone in Germany and Poland: differences and common features

The classical correlation by kossmat (1927) of the Saxothuringian belt of Germany with the West Sudetes of Poland is re-evaluated with special reference to the crystalline massifs of Munchberg and the Gory Sowie and the surrounding areas. Important common features are: Cambro-Ordovician magmatism and sedimentation; Silurian to Devonian radiolarian cherts in the low grade to unmetamorphosed units; Early Devonian (and older?) pressure-dominated metamorphism followed by rapid uplift; the incorporation of high pressure and mantle rocks; and Early Carboniferous mass flow deposits with ‘Saxothuringian’ clasts. Differing features in the West Sudetes are: tectonic contacts in and around the Gory Sowie are mostly subvertical and mainly (semi-)brittle (in contrast with subhorizontal and ductile contracts in the Munchberg); the Gory Sowie has undergone late re-equilibration of the earlier tectonic fabrics; an ophiolite juxtaposed with the Gory Sowie crystallized in (?) Devonian time and became only statically metamorphosed, as opposed to the regionally metamorphosed Cambro-Ordovician mafic rocks of the Saxothuringian belt; localized depocenters with Upper Devonian to Lower Permian elastic rocks relating to north-west trending strike-slip faults instead of the north-east trending cylindroidal foreland basin in the Saxothuringian. It is possible that the West Sudetes originated as an easterly continuation of the Saxothuringian belt, but then became involved, from the Late Devonian onwards, in a strike-slip system parallel with the margin of the East European Platform.

Glacial dispersal of rock debris in central Gaspésie, Quebec, Canada

The lithological content of tills in central Gaspésie is evaluated by pebble counting of 231 samples collected in excavation pits and containing 200 pebbles each. The results are used here to establish the pattern of debris dispersal and to infer the glacial history of the area. The dispersal pattern is characterized by well-defined southeasterly (160–170°) and northeasterly (40–60°) trending trains. Half-distance values of glacial transport along the trains range from 5 to 9 km for both directions, suggesting ice flow events of considerable magnitude. The volume of material in the trains represents 1–6 m of glacial erosion of the bedrock. Glacial cirques and short U-shaped valleys, about 100–200 m deep, are incised into the McGerrigle Mountains granite pluton as well as the adjacent metabasalt. The corresponding trains are aligned with these erosional features, indicating that their clast content was derived from those features during an early Alpine Glacier Phase. The southeasterly trending dispersal trains are associated with an invasion of central Gaspésie by the Laurentide Ice Sheet during the Early Wisconsinan, whereas the northeasterly trending trains are associated with a local centre of outflow over Gaspésie during the Late Wisconsinan.

Northeast-trending Early Proterozoic dykes of southern Superior Province: multiple episodes of emplacement recognized from integrated paleomagnetism and U–Pb geochronology

Integrated paleomagnetic and U–Pb geochronologic studies have been conducted to establish the paleomagnetic directions and ages of Early Proterozoic tholeiitic dykes of northeast trend in the southern Superior Province, previously referred to collectively as Preissac dykes. It is demonstrated that they are readily separated on the basis of paleomagnetism into subsets, referred to as the Biscotasing and Senneterre swarms. In addition a pair of unnamed dykes may be associated with the north-and northwest-trending Matachewan swarm farther west.Biscotasing dykes have a down-west magnetization of single polarity with a corresponding paleopole at 27.8°N, 136.7°W (dm = 12.3° and dp = 9.4°). Senneterre dykes carry an up-north (or occasionally down-south) direction with corresponding paleopole at 15.3°S, 75.7°W (dm = 7.0°, dp = 4.4°). The Senneterre direction is indistinguishable from the primary N1 remanence direction that dominates the magnetization of Nipissing sills of the Southern Province. Paleomagnetic field tests described herein or in earlier studies indicate that Biscotasing and Senneterre directions are primary and, hence, that two ages of intrusion are involved, with the age of Senneterre dykes coinciding with the intrusion of most Nipissing sills. U–Pb dating of baddeleyite conducted at a paleomagnetic sampling site yields an age of 2214.3 ± 12.4 Ma for the Senneterre swarm, indistinguishable from the age of 2217.2 ± 4 Ma reported from an N1 Nipissing sill site in another study. A U–Pb age on baddeleyite and zircon of 2166.7 ± 1.4 Ma was obtained from a paleomagnetic site in the Biscotasing swarm. The primary paleopoles for the Senneterre, Nipissing, and Biscotasing rocks define a direction of polar wander opposite to that of the most widely used polar wander paths for North America for this period, suggesting that these paths should no longer be used.

Tomographic study of upper mantle atttenuation in the Pacific Ocean

The quality factor Q has been measured in the Pacific ocean along many paths using long period seismograms of the Geoscope and IDA networks. The Love and Rayleigh phase velocity and QR−1 have been regionalised in the period range 60s to 200s, and then simultaneously inverted at depth to obtain 3D images of S‐wave velocity and attenuation Qβ−1 in the depth range 60–300km. Up to 100s, three very attenuating zones appear, one under the East Pacific Rise (E.P.R), another one around Hawaii, a third one east of Kermadec. By inversion, those three anomalies are found stable down to 160km. Below 200km, lateral variations are less intense and differently distributed; the maximum attenuation seems to concentrate along a northeast trend in the North Central Pacific.

Tectonic deformation in the New Madrid Seismic Zone: Inferences from map view and cross‐sectional boundary element models

The lack of instrumental recordings and obvious fault scarps associated with the 1811–1812 New Madrid earthquakes necessitates examination of more subtle indicators of the geometry and type of faulting associated with these events. Morphologic and geologic features and the distribution of modern seismicity are used to infer the slip distribution and type of faulting (strike‐ or dip‐slip), number, strike, length, and width of the major faults in the New Madrid seismic zone (NMSZ). This is accomplished through two‐dimensional boundary element modeling of the strain field arising from slip on hypothetical faults that is driven by either coseismic or uniform regional strains. Tectonic deformation is reflected in (1) the seismicity, (2) the Lake County uplift, (3) Reelfoot Lake, (4) the fractured rocks of the Blytheville arch, and (5) the St. Francis Sunk Lands. Many of these features can be qualitatively explained as resulting from tectonic deformation due to slip on two left‐stepping right‐lateral strike‐slip faults that are coincident with the northeast trending zones of seismicity and the Blytheville arch. The morphology appears to be, at least in part, a consequence of major earthquakes that rupture these faults. The locations of the 1811–1812 and largest post‐1812 earthquakes and the models are consistent with a process in which the 1811–1812 earthquakes relieved accumulated regional shear strain causing the greatest post‐1812 shear strains to exist at the ends of the fault zone. Modeling results also suggest that the numerous small earthquakes in the NMSZ are not aftershocks of the 1811–1812 earthquakes but instead represent continuous localized adjustments to a uniform regional strain field. The Bootheel lineament does not appear to be significant in shaping the morphology, geologic structure, and pattern of seismicity in the NMSZ. The absence of fault scarps and coherence of shallow reflectors overlying the Blytheville arch suggests that the major NMSZ faults may not have ruptured to the surface during the 1811–1812 earthquakes. The inferred lengths of the 1811–1812 earthquake ruptures also suggest that their sizes may have been overestimated. Model‐predicted subsidence within the St. Francis Sunk Lands indicates that tectonic deformation also may have influenced alluvial processes in the NMSZ.

Pre‐Miocene and post‐Miocene deformation in the Bannockbum basin, Central Otago, New Zealand

Abstract The Bannockburn basin is the southern extension of the Upper Clutha basin, one of a series of prominent northeast‐trending structural and topographic basins in Central Otago. Post‐Miocene structure within the Bannockburn basin is shown up by deformation of an unconformity cut into schist basement beneath Miocene terrestrial sediments. The post‐Miocene deformation has been accommodated largely by flexure, and the structure consists of broad north‐trending synforms and antiforms in schistosity, mimicked by folds in the overlying sediments. The northwestern margin of the basin trends northeast, and the unconformity is steeply dipping or locally overturned, but there is no major fault separating the basin from the range immediately to the west. Minor faults in this area have normal, reverse, or strike‐slip senses of movement. The southwestern margin of the basin trends northwest and is marked by distinct faults which offset the unconformity. The present‐day Bannockburn basin coincides with a complex ...

Hydrocarbon Prospectivity of the Argentine Continental Slope

A rift basin containing stratigraphic and structural closures is developed along the Argentine slope over a distance of some 1000 km and area of 50,000 km[sup 2] in potentially exploitable water depths of 200 to 1500 m. No wells exist on this part of the continental margin. Rifting began during the Late Triassic/Jurassic and ended in Early Cretaceous. The first marine seaways flooded the Rift alluvial plains and lake. In Hauterian-Barrenian time, low circulation marine conditions with oxygen deficiency prevailed up to Rio Grande/Walvis Ridge flooding in part the coastal pull-apart basins. A major marine transgression in the Maastrichtian formed a widespread seal over the entire shelf and slope area. Four play types were identified: (1) a major north-east trending elongate delta system sourced inland from the San Julien (N. Malvinas) Basin and deposited on the southern Patagonian shelf and rise, (2) reworked deltaic barrier sands accumulated along the crest of the Outer Basement Ridge, (3) The Outer Basement Ridge, a major structural closure 400 km long, and (4) a series of margin-parallel rift systems in the offshore region between the 200 m and 1500 m isobaths. Regional seismic data and geological correlations suggest that oil prone source rocks aremore » likely in the Middle and Upper Jurassic and Cretaceous formations. Source rocks are probably mature east and west of the Outer Basement Ridge and in the easterndepocenter of the Colorado Basin. Eastward migration from Jurassic age lacustrine source rocks in the Colorado Basin may have charged traps along the crest of the Outer Basement Ridge. Westward directed migration from deeply buried Aptian age marine source rocks in the Atlantic basins has probably charged prospective stratigraphic and structural traps in a suite of coast-parallel grabens developed on the Argentine continental shelf and slope. The Argentine offshore region, therefore, offers significant encouragement as a productive hydrocarbon province.« less

Structural and Hydrological Parameters Modeling of Wilcox Group, Central Louisiana and Mississippi

ABSTRACT In a 900 square miles area between Louisiana and Mississippi 300 well logs were studies. Three major sand formations in middle Wilcox and five target log picks within upper and middle Wilcox were correlated. The lifthofacies variation appear to be related to the sequence and eustatic boundaries. The temperature and salinity values modeled were also obtained from E-log and BHT (Bottom Hole Temperature) data. The relationships among the temperature and salinity distributions, topography, stratigraphy and subsurface structure are provided. Several real images of 3-D topography, formation structure and stratigraphic boundary were reconstructed by using the well log data. This detailed study suggests that the productive oil fields along the east and west flank of Mississippi Embayment are related to the salinity anomalies, and are controlled by structural and stratigraphic features. From the 3-D maps confident interpretation about the structure and stratigraphy changes with time and basin growth history can be made. For instance, it appears that the Holocene Mississippi River course in the study area is nearly coincident with the main depositional axis of Middle and Late Wilcox time. The current Mississippi River channel, as well as the basin's depositional axis of 20-50 million years ago, appear to have been affected by some regional structural events. Also, a north-south salinity anomaly in the east part of study area is a possible indicator of a northeast trending wrench fault along the Mississippi river. End_of_Record - Last_Page 536-------