Results from regional vibroseis profiling: Appalachian ultra-deep core hole site study

Summary. In 1985, 180 km of regional vibroseis profiles were acquired in the Carolinas and Georgia, southeastern United States, as part of the Appalachian Ultra-Deep Core Hole (ADCOH) Site Study. The data quality is excellent, with large-amplitude reflections from faults and crystalline rocks, lower Palaeozoic shelf strata and from within autochthonous Grenville basement. The profiles image the subsurface more clearly than other available data and allow the possibility of alternative interpretations of important elements of the tectonic framework of the southern Appalachians.
The major points in the interpretation are: 1) The Blue Ridge master decollement is at a depth of 2-3 km beneath the Blue Ridge. This thrust increases in dip just NW of the Brevard fault zone. 2) The Brevard fault zone appears to splay from the master decollement at 6 km (2.2 s) near Westminster, S.C., and defines the base of the crystalline Inner Piedmont allochthon. 3) Below the Blue Ridge thrust sheet are images of duplex and imbricate structures ("duplex tuning wedges") connected by other thrust faults that duplicate shelf strata to a thickness of 4–5 km. 4) Subhorizontal reflections from depths of 6 to 9 km may be from relatively undisturbed lower Palaeozoic strata as suggested by others. 5) Eocambrian-Cambrian(?) rift basins in the Grenville basement are also imaged.
The ADCOH data were originally recorded with 14–56 Hz bandwidth and 8 s length, but an extended Vibroseis correlation was used to produce 17 s data length revealing reflections from within the upper crust. Below 8 s, reflections from within the Grenville basement become weak, but are observable as late as 13 s; however, these Moho (?) reflections are generally short segments.     

Contrasting soils and landscapes of the Piedmont and Coastal Plain, eastern United States

The Piedmont and Coastal Plain physiographic provinces comprise 80 percent of the Atlantic Coastal states from New Jersey to Georgia. The provinces are climatically similar. The soil moisture regime is udic. The soil temperature regime is typically thermic from Virginia through Georgia, although it is mesic at altitudes above 400 m in Georgia and above 320 m in Virginia. The soil temperature regime is mesic for the Piedmont and Coastal Plain from Maryland through New Jersey. The tightly folded, structurally complex crystalline rocks of the Piedmont and the gently dipping “layer-cake” clastic sedimentary rocks and sediments of the Coastal Plain respond differently to weathering, pedogenesis, and erosion. The different responses result in two physiographically contrasting terrains; each has distinctive near-surface hydrology, regolith, drainage morphology, and morphometry.The Piedmont is predominantly an erosional terrain. Interfluves are as narrow as 0.5 to 2 km, and are convex upward. Valleys are as narrow as 0.1 to 0.5 km and generally V-shaped in cross section. Alluvial terraces are rare and discontinuous. Soils in the Piedmont are typically less than 1 m thick, have less sand and more clay than Coastal Plain soils, and generally have not developed sandy epipedons. Infiltration rates for Piedmont soils are low at 6–15 cm/h. The soil/saprolite, soil/rock, and saprolite/rock boundaries are distinct (can be placed within 10 cm) and are characterized by ponding and/or lateral movement of water. Water movement through soil into saprolite, and from saprolite into rock, is along joints, foliation, bedding planes and faults. Soils and isotopic data indicate residence times consistent with a Pleistocene age for most Piedmont soils.The Coastal Plain is both an erosional and a constructional terrain. Interfluves commonly are broader than 2 km and are flat. Valleys are commonly as wide as 1 km to greater than 10 km, and contain numerous alluvial and estuarine terrace sequences that can be correlated along valleys for tens of kilometers. Coastal Plain soils are typically as thick as 2 to 8 m, have high sand content throughout, and have sandy epipedons. These epipedons consist of both A and E horizons and are 1 to 4 m thick. In Coastal Plain soils, the boundaries are transitional between the solum and the underlying parent material and between weathered and unweathered parent material. Infiltration rates for Coastal Plain soils are typically higher at 13–28 cm/h, than are those for Piedmont soils. Indeed, for unconsolidated quartz sand, rates may exceed 50 cm/h. Water moves directly from the soil into the parent material through intergranularpores with only minor channelization along macropores, joints, and fractures. The comparatively high infiltration capacity results in relatively low surface runoff, and correspondingly less erosion than on the Piedmont uplands.Due to differences in Piedmont and Coastal Plain erosion rates, topographic inversion is common along the Fall Zone; surfaces on Cenozoic sedimentary deposits of the Coastal Plain are higher than erosional surfaces on regolith weathered from late Precambrian to early Paleozoic crystalline rocks of the Piedmont. Isotopic, paleontologic, and soil data indicate that Coastal Plain surficial deposits are post-middle Miocene to Holocene in age, but most are from 5 to 2 Ma. Thus, the relatively uneroded surfaces comprise a Pliocene landscape. In the eastern third of the Coastal Plain, deposits that are less than 3.5 Ma include alluvial terraces, marine terraces and barrier/back-barrier complexes as morphostratigraphic units that cover thousands of square kilometers. Isotopic and soil data indicate that eastern Piedmont soils range from late Pliocene to Pleistocene in age, but are predominantly less than 2 Ma old. Thus, the eroded uplands of the Piedmont “peneplain” comprise a Pleistocene landscape.     

Rumpftreppen in Southernmost Norway

In several publications Evers (see especially 1941) described ‘Piedmont’ or ‘Rumpftreppen’ in Norway. He claimed to have found the Treppen in many parts of the country at surprisingly regular height intervals: at 200 m, 400 m, 600 m, 800 m, 1000 m, and 1200 m above sea level, at sea level and 200 m below sea level. Evers recognized ‘Rumpftreppen’ north of Kristiansand in the southernmost part of Norway at 200 m, 600 m, 800 m, 1000 m, and 1200 m elevation (Evers 1941, p. 38). This part of Norway is underlain by crystalline Precambrian rocks. Rolling hills with a low relief between more deeply incised north-south-trending main valleys characterize the landscape. The hills are flat-topped or slightly rounded. A smooth surface, which passes through the highest hill-tops, rises gradually from the coast to the high mountain plateau of central southern Norway (Fig. 1). The surface is also shown by profiles at right angles to the coast line (Fig. 2). Towards the northwest this ‘summit surface’ passes into the mountain plateau which follows the sub-Cambrian ‘peneplain’ in Ryfylke (Andersen 1954). Andersen (1960) therefore suggested that the ‘summit surface’ in southernmost Norway was probably the tilted sub-Cambrian ‘peneplain’ slightly lowered by erosion. The only extensive distinct younger erosion surface (shelf) that could be clearly recognized lies on the coast approximately at sea level (Fig. 3).     

Results of recent COCORP profiling in the southeastern United States

Summary. Recently acquired COCORP profiles in the southeastern United States show that: 1) Reflections associated with the Appalachian detachment are prominent beneath the Inner Piedmont of western Georgia, but do not extend further southeast beneath the Pine Mountain belt. 2) The Brunswick magnetic low is associated with a broad zone of crustal-penetrating dipping reflections that probably marks the Alleghanian suture in the southeastern U.S. 3) The South Georgia basin is a composite feature consisting of several half-graben, locally containing >5 km of Triassic - E. Jurassic basin fill. These basins occur within the interior of the Alleghanian orogen, but are not specifically associated with Alleghanian suture. 4) Across-strike crustal thickness variation, and distribution and character of lower-crust and Moho reflections in the Southern Appalachians is grossly similar to that observed in other parts of the Appalachian/Hercynian orogenic belt. Global comparisons suggest that these regional variations are a consequence of post-collisional extensional tectonics, rather than a primary (Palaeozoic or older) feature of the orogenic belt.     

A seismic study of deep geological structure in the Bristol Channel area, SW Britain

Summary. Results from eight seismic refraction lines, 35–90 km long, in the Bristol Channel area are presented. The data, mostly land recordings of marine shots, have been interpreted mainly by ray-tracing and time-term modelling. Upper layer velocities through Palaeozoic rocks usually fall within the range 4.8–5.2 km s⁻¹. Below the Carboniferous Limestone with a normal velocity of 5.1–5.2 kms⁻¹, the Old Red Sandstone with a velocity of 4.7–4.8 kms⁻¹ acts as a low velocity layer, as do parts of the underlying Lower Palaeozoic succession. In the central South Wales/Bristol Channel area and the Mendips, a 5.4–5.5 km s⁻¹ refractor is correlated with a horizon at or near the top of the Lower Palaeozoic succession. Under the whole area, except for north Devon, a 6.0–6.2 km s⁻¹ basal refractor has been located and is correlated with Precambrian crystalline basement rocks. In general, this refractor deepens southwards from a series of basement highs, which existed before the major movements of the Variscan orogeny in South Wales, resulting in a southerly thickening of the pre Upper Carboniferous supra-basement sequence. In north Devon, a 6.2 km s⁻¹ refractor at shallow depth, interpreted as a horizon in the Devonian or Lower Palaeozoic succession, overlies a deep reflector that may represent the Precambrian crystalline basement.     

西南极乔治王岛北海岸第三纪火山岩的岩石化学和地球化学研究

乔治王岛北海岸的第三纪火山岩包括菲尔德斯半岛组（ＦＰＧ）熔岩、Ａｄｍｉｒａｌｔｙ湾组（ＡＢＧ）岩颈和Ｗｅｇｇｅｒ峰组（ＷＰＧ）侵入岩。大多数中基性熔岩为斑状结构，斜长石和普通辉石是岩石中最主要的斑晶相。安山岩中尚含有少量斜方辉石，英安岩质熔岩则为细晶质结构。岩颈相岩石的物质组成与熔岩相似，但斑晶含量往往较少。岩石化学成分表明，所有岩石均富铝，为亚碱性火山岩，属于钙碱性岩石系列。岩石富集大离子亲石元素，轻稀土元素略为富集，稀土元素总含量落在钙碱性岩石的含量范围之内并基本随硅含量的增加而增加。岩石具有很低的８７Ｓｒ／８６Ｓｒ比值（０．７０３４左右）和较高的１４３Ｎｄ／１４４Ｎｄ比值（０．５１２７８－０．５１２８８），εＮｄ平均值在＋６左右。８７Ｓｒ／８６Ｓｒ比值与Ｓｉ、Ｋ、Ｒｂ、１／Ｓｒ等元素呈水平线性关系。在Ｃｅ／Ｙｂ－Ｃｅ和Ｔｈ／Ｔａ－Ｔｈ图解上所有岩石的投影点分布趋势相同，趋势线的斜率＞０。上述特征表明，乔治王岛北海岸第三纪火山岩均由上地幔物质直接生成而未受壳源物质的混染。推测在古太平洋板块向南极板块下俯冲的过程中，上地幔发生的不同程度部分熔融所生成的玄武岩岩浆从深部岩浆囊沿构造薄弱带穿过地壳直接喷出形成     

Extent and Chronology of the Ross Sea Ice Sheet and the Wilson Piedmont Glacier along the Scott Coast at and Since the Last Glacial Maximum

During the last glacial maximum, a coalescent ice mass consisting of the grounded Ross Sea ice sheet and an expanded Wilson Piedmont Glacier covered the southern Scott Coast. This coalescent ice mass was part of a larger grounded ice sheet that occupied the Ross Sea Embayment during the last glacial maximum. Deglaciation of the western Ross Sea Embayment adjacent to the southern Scott Coast was delayed until shortly before 6500 ¹⁴C yr bp , aconclusion based on ages of marine shells from McMurdo Sound, a relative sea-level curve, and algae that lived in ice-dammed lakes. Therefore, most recession of grounded ice in the Ross Sea Embayment occurred in mid to late Holocene time, after deglacial sea-level rise due to melting of Northern Hemisphere ice sheets essentially was accomplished. Rising sea level alone could not have driven grounding-line retreat back to the present-day Siple Coast.     

MELTWATER FEATURES THAT SUGGEST MIOCENE ICE-SHEET OVERRIDING OF THE TRANSANTARCTIC MOUNTAINS IN VICTORIA LAND, ANTARCTICA

ABSTRACT. We illustrate here spectacular meltwater features associated with outburst floods beneath an ice sheet that overrode the Transantarctic Mountains in southern Victoria Land. Because of long-term hyperarid polar climate, these features are part of an ancient landscape preserved for about 14 million years. Some channels are associated with areal scouring of basement rocks extending from sea level to as much as 1200–2100 m elevation in coastal regions. Scablands with scallops, potholes and plunge pools are cut in Beacon Super group sandstones and Ferrar Dolerite and cover wide areas of high western plateaus near the mountain crest. Subglacial channel systems commonly originate near divides and converge downhill toward the northeast. We argue that the landforms were created beneath a major Antarctic Ice Sheet that submerged the whole area, with the possible exception of the high peaks of the Royal Society Range, as it flowed northeastward toward the outer Antarctic continental shelf. Areal scouring, associated with warm-based regimes, is restricted to the lower slopes close to the coast. In the higher terra in, meltwater channels and scabland alongside preserved patches of regolith are best explained by the breaching of cold-based ice on the mountain rim by subglacial melt water outbursts. Melt from warm-based ice, along with subglacial lakes trapped upstream of the mountain rim, are possible sources of the meltwater necessary to form the channel systems and scablands.     

The geological evolution of southern McMurdo Sound — new evidence from a high-resolution aeromagnetic survey

Magnetic anomaly data are presented from a new helicopter-borne high-resolution aeromagnetic survey in southern McMurdo Sound. Anomaly data have been acquired at a common 305 m elevation above the McMurdo and southern McMurdo ice shelves and draped over the volcanic islands that pin them. The resulting anomaly patterns provide a significant advance in the understanding of the rift related geology beneath the floating ice shelves. More extensive Erebus Volcanic Province (McMurdo Volcanic Group) rocks are indicated along with a significant blanket of glaci-volcaniclastic sediment on the seafloor between the volcanic islands in southern McMurdo Sound. These glaci-volcaniclastic sediments are inferred to originate from former grounding of the southern McMurdo Ice Shelf as a marine ice sheet. A strong N–S fabric is also observed in the anomaly data suggesting that the rift structure observed in the Victoria Land basin persists to the south beneath the McMurdo and southern McMurdo ice shelves. W–N–W transfer faults identified within the Transantarctic Mountain rift flank to the west are not obvious in the aeromagnetic data set, implying that the 'Discovery Accommodation Zone' may be restricted to the region between a southward extension of the range bounding fault that marks the limit of the Victoria Land Basin and the right lateral offset in the Transantarctic Mountain front in southern Victoria Land.     

Geochemistry of the late Proterozoic Kapp Hansteen igneous rocks of Nordaustlandet, Svalbard

Proterozoic igneous rocks occur in three areas in Nordaustlandet, Svalbard, and are found in the upper part of the Lower Hecla Hoek succession, the Botniahalvøya Supergroup. The rocks have been called porphyrites in Botniahalvøya, metadiabases in Prins Oscars Land and quartz porphyries in both areas as well as in the Sabinebukta area. All rocks have been metamorphosed under the greenschist facies conditions. The porphyrites are calc-alkaline acid andesites and dacites of medium to high K2O type, possibly showing a transition to tholeiitic series. The quartz porphyries are calc-alkaline rhyolites of high K20 type. The metadiabases are subdivided into two: the basic dykes of low K20 type and relatively high Fe tholeiite series, while the main bodies are acid andesites of medium to high K20 and low Fe tholeiite series. The basic dykes fall in the oceanic rock field of the Tiø2-K20-P20s diagram, and are most likely belonging to the island arc type volcanism. The metadiabases of main bodies and the porphyrites, and possibly the quartz porphyries, are chemically continuous. The medium to high K20 contents, and their Tiø2-K20-P2O5 ratios suggest that these three rock groups are non-oceanic and resemble the rock associations of the areas having thick continental crust. This conclusion agrees with the reported high initial Sr^87/86 ratios and the existence of a distinct unconformity at the base of this volcanogenic succession.     

The record of Himalayan erosion preserved in the sedimentary rocks of the Hatia Trough of the Bengal Basin and the Chittagong Hill Tracts,Bangladesh

The Cenozoic sedimentary succession of Bangladesh provides an archive of Himalayan erosion. However, its potential as an archive is currently hampered by a poor lithostratigaphic framework with limited age control. We focus on the Hatia Trough of the Bengal Basin and the adjacent fold belt of the Chittagong Hill Tracts which forms the outermost part of the west‐propagating Indo‐Burmese wedge. We present a basin‐wide seismic stratigraphic framework for the Neogene rocks, calibrated by biostratigraphy, which divides the succession into three seismically distinct and regionally correlatable Megasequences (MS). MS1 extends to NN15‐NN16 (ca. 2.5–3.9 Ma), MS2 to NN19‐NN20 (ca. 0.4–1.9 Ma) and MS3 to present day. Our seismic mapping, thermochronological analyses of detrital mineral grains, isotopic analyses of bulk rock, heavy mineral and petrographic data, show that the Neogene rocks of the Hatia Trough and Chittagong Hill Tracts are predominantly Himalayan‐derived, with a subordinate arc‐derived input possibly from the Paleogene IndoBurman Ranges as well as the Trans‐Himalaya. Our seismic data allow us to concur with previous work that suggests folding of the outer part of the west‐propagating wedge only commenced recently, within the last few million years. We suggest that it could have been the westward encroachment and final abutment of the Chittagong Hill Tracts fold belt onto the already‐uplifted Shillong Plateau that caused diversion of the palaeo‐Brahmaputra to the west of the plateau as the north‐east drainage route closed.     

The dynamical origin of subduction zone topography

Summary. Subduction zones are expressed topographically by long linear oceanic trenches flanked by a low outer rise on the seaward side and an island arc on the landward side. This topographic structure is reflected in free air gravity anomalies, suggesting that much of the topography originates from dynamical forces applied at the base of the crust. We have successfully reproduced the general topographic features of subduction zones by supposing that the stresses generated by the bending of the viscous lower lithosphere as it subducts are transmitted through the thin elastic upper portion of the lithosphere. The trench is due to a zone of extensional flow (associated with low pressure) in the upper part of the viscous lithosphere.
The stresses in the subducting slab are computed using a finite element technique, assuming a Maxwell viscoelastic constitutive relation. Various dips (10 to 90°) were investigated, as well as depth dependent and non-Newtonian (power law, n = 3) viscosities. Observed subduction zone dimensions are well reproduced by these models. The effective viscosity required at mid-depth in the lithosphere is about 6 × 10²² P. This low value is probably due to the stress dependence of the effective viscosity. However, these models also show that the topography of the subduction zone depends primarily upon the geometry of the subducting slab (dip, radius of curvature of the bend) rather than upon its rheology. Shear stresses beneath the trench reach maxima of approximately 50 MPa. An interesting feature of some solutions is a dynamically supported bench or platform between the trench and island arc.     

Two major unconformities beneath the Neoproterozoic Murchisonfjorden Supergroup in the Caledonides of central Nordaustlandet, Svalbard

Two unconformities have been found in central Nordaustlandet. New mapping has located a major unconformity at the base of the Neoproterozoic Murchisonfjorden Supergroup, with quartzites and basal conglomerates of the Djevleflota Formation unconformably overlying dark phyllites of the Helvetesflya Formation and metavolcanic rocks of the Svartrabbane Formation. A second unconformity separates the Helvetesflya from the Svartrabbane formations. These rocks were isoclinally folded, metamorphosed in lower greenschist facies, and, apparently, syntectonically intruded by Grenville-age granites, prior to uplift, erosion and Neoproterozoic deposition. Caledonian tectonothermal activity, as recorded in the Neoproterozoic strata , appears to vary very little across Svalbard's Eastern Terrane from Ny Friesland, in the west, to Murchisonfjorden in western Nordaustlandet and, via Wahlenbergfjorden, to the central Nordaustlandet area, described here. Upright folds with associated high angle, usually E-dipping cleavages, characterise the Caledonian deformation over an east-west distance of about 100 km. This evidence allows the possibility that the pre-Devonian basement, to the east of Nordaustlandet, beneath the northern Barents Sea (Barentsia), may be composed of Grenville-age complexes little influenced by Caledonian tectonothermal activity. Alternatively, Barentsia is dominated by Caledonian hinterland tectonics, with extensive middle Paleozoic tectonothermal reworking of a Precambrian basement.     

An Upper Proterozoic unconformity in northern Wedel Jarlsberg Land, southwest Spitsbergen: Lithostratigraphy and tectonic implications

Recent field studies of Upper Proterozoic rocks in northern Wedel Jarlsberg Land, southwest Spitsbergen, have shed new light on the pre-Caledonian evolution of the region. A regional angular unconformity divides the greenschist-facies metasedimentary rocks into two distinct tectono-siratigraphic sequenees. The sub-unconformity (Nordbukta) sequence, exposed in the southwestern part of the study area, consists mainly of quartzites, phyllites and dolomites, and may be correlative with Proterozoic rocks exposed east of Recherchebreen (Magnethøgda sequence) and south of Torellbreen (Dcilegga sequence). The Nordbukta sequence was affected by large-scale recumbent folding during late (?) Proterozoic tectonism. Strata above the unconformity (Dunderbukta-Recherchefjorden sequence) include conglomerates, dolomites, green and black phyllites, meta-basalts and Vendian (?) diamictites, with laterally complex depositional relationships. The continuation of this sequence south of Torellbreen is the Sofiebogen Group in the Hornsund area. The apparent continuity of both sub- and supra-unconformity Proterozoic rocks across Recherchebreen and Torellbreen is not compatible with the hypothesis that a major late Devonian strike-slip terranc boundary lies beneath these glaciers.     

Photointerpretation of Tertiary structures in platform cover strata of interior Oscar II Land, Spitsbergen

A structural map of Oscar II Land compiled using black and white, stereo-pair air photos and other available information sources suggests the existence of three structural zones within the deformed platform cover strata. The western zone consists of basement involved overthrusts of complex geometry in the north large-scale folds of platform cover strata with a stepped, down to the east profile in the south. The central zone consists primarily of folds within Kapp Starostin Fm. and overlying Triassic strata that likely formed above a subhorizontal décollement within Gipshuken Fm. strata. Farther to the east thrusts emplace Permian Kapp Starostin Fm. strata over Triassic strata. This zone probably represents stratigraphic climb of the basal decollement. Conservative estimates of shortening for the central and eastern zones are in the 15-25% range. The contribution of the western zone structures to overall Tertiary shortening depends on the geometry of faults underlying the stepped folds and is uncertain at present.
Changes along strike are in part due to differential levels of erosion and changes in transport direction, but also probably important was variable thickness of the platform cover strata involved (less to the north) and the availability of incompetent horizons along which thrust-fiats could form. Two areas with anomalous structural trends may represent oblique ramps.     

Example of a supradetachment basin within a pull-apart tectonic setting: Mormon Point, Death Valley, California

The geological features now exposed at Mormon Point, Death Valley, reveal processes of extension that continue to be active, but are concealed beneath the east side of Death Valley. Late Cenozoic sedimentary rocks at Mormon Point crop out in the hangingwall of the Mormon Point low-angle normal fault zone, a fault zone that formed within a releasing bend of the oblique-slip (right-normal slip) fault zone along the east side of Death Valley. The late Cenozoic sedimentary rocks were part of the valley when the low-angle fault zone was active, but during late Quaternary time they became part of the Black Mountains block and were uplifted. Rocks and structures exposed at Mormon Point are an example of the types of features developed in a releasing bend along the margins of a major pull-apart structure, and in this example they are very similar to features associated with regional detachment faults. The oldest sedimentary rocks in the hangingwall of the Mormon Point low-angle fault zone dip steeply to moderately east or north-east and were faulted and rotated in an extensional kinematic environment different from that recorded by rocks and structures associated with younger rocks in the hangingwall. Some of the younger parts of the late Cenozoic sedimentary rocks were deposited, faulted and rotated during movement on the Mormon Point low-angle normal fault. Progressively, strata are less faulted and less rotated. The Mormon Point low-angle normal fault has an irregular fault surface whose segments define intersections that plunge 18°-30°, N10°-40°W, with a maximum of 22°, N22°W that we interpret to be the general direction of slip. Thus, even though Death Valley trends north, movement on the faults responsible for its formation was at least locally north-northwest. Gouge and disrupted conglomerates along the faults are interpreted to have formed either as adjustments to accommodate space problems at the corners of blocks or along faults that bounded blocks during their displacement and rotation. The younger units of the late Cenozoic sedimentary rock sequence and the geomorphic surfaces developed on them are rarely faulted, not rotated, and overlap the Mormon Point low-angle faults. Active faults cut Holocene alluvium north of the late Cenozoic rocks and form the present boundary between Mormon Point and the Black Mountains. The distribution of active faults defines a releasing bend that mimics the older releasing bend formed by the Mormon Point low-angle fault zone. Rocks and structures similar to those exposed above the Mormon Point low-angle fault zone are probably forming today beneath the east side of Death Valley north-west of Mormon Point.     

Early fill of the Western Irish Namurian Basin: a complex relationship between turbidites and deltas

The Western Irish Namurian Basin developed in Early Carboniferous times as a result of extension across the Shannon Lineament which probably coincides with the lapetus Suture. During the late Dinantian, axial areas of the NE-SW elongate trough became deep, whilst shallow-water limestones were deposited on the flanks. This bathymetry persisted into the Namurian when carbonate deposition ceased. In axial areas, a relatively thick mudstone succession spans earliest Namurian to Chokierian whilst on the northwestern marginal shelf, a thin, condensed Namurian mudstone sequence, in which pre-Chokierian sediments are apparently absent, rests unconformably on the Dinantian. From late Chokierian to early Kinderscoutian, the basin was filled by sand-dominated clastic sediments. Sand deposition began in the axial area with deposition of a thick turbidite sequence, the Ross Formation, which is largely equivalent to the condensed mudstone succession on the flanks. Turbidity currents flowed mainly axially towards the north-east and deposited a sequence lacking well-defined patterns of vertical bed-thickness change. Channels and slide sheets occur towards the top of the formation. The turbidite system seems to have lacked well-defined lobes and stable distributary channels. Overlying the Ross Formation, the Gull Island Formation shows a decreasing incidence of turbidite sandstones at the expense of increasing siltstones. This formation is characterized by major slides and slumps interbedded with undisturbed strata. In the flanking areas of the basin, the formation is thinner, has only a few turbidites in the sequence above the condensed mudstones and contains only one slide sheet. Overall the formation is interpreted as the deposit of a major prograding slope, the lower part representing a ramp upon which turbidites were deposited, the upper part a highly unstable muddy slope lacking any conspicuous feeder channels through which sand might have been transferred to deeper water. Progradation of the slope appears to have been increasingly from the northwestern flank of the trough which is similar to the direction deduced for the overlying deltaic Tullig cyclothem which completes the initial basin fill. Whilst several features of the succession can be explained by envisaging the whole sequence as the product of one linked depositional system, the shifting directions of palaeocurrents and palaeoslope raise problems. The switch from axial to lateral supply casts doubt on the strict application of Walther's Law to the total sequence and seems to demand large avulsive shifts of the delta system on the shelf area to the west.     

Lithospheric thickness beneath the Dabie Shan, central eastern China from S receiver functions

P and S receiver functions obtained from a portable array of 34 broad-band stations in east central China provide a detailed image of the crust–mantle and lithosphere–asthenosphere boundaries (LAB) in the Dabie Shan and its adjacent areas. Clear S -to- P converted waves produced at the LAB show a thin lithosphere beneath the whole study area. Based on our results, the thickest lithosphere of 72 km is observed beneath the southern part of the area within the Yangtze craton, whereas beneath the North-China platform, the lithosphere is only 60 km thick. S receiver functions also reveal, in good agreement with P receiver functions, a maximum depth of the Moho beneath the Dabie Shan orogen at approximately 40 km. Furthermore, we interpret the structural difference at 32° latitude as the probable location of the mantle suture formed between the Yangtze and the Sino-Korean cratons.     

Wave Propagation In A Vertical Transversely Isotropic Medium: Field Experiment and Model Study

Summary. The continent-ocean transition adjacent to Hatton Bank was studied using a dense grid of single-ship and two-ship multichannel seismic profiles. The interpretation of the explosive expanding spread profiles (ESPs) which were shot as part of this survey are discussed here in detail. Extensive seaward dipping reflectors are developed in the upper crust across the entire margin. These seaward dipping reflectors continue northwards on the Faeroes and Vøring margins, where they have been shown to be caused by basaltic lavas, as well as on the conjugate margin of East Greenland. The dipping reflectors are an important feature of the rifting history of the margin and show that extensive volcanism was associated with the extension. The ESPs show clear seismic arrivals out to ranges of 100 km. Wide-angle Moho reflections can be seen on all the lines as well as good mid and lower crustal arrivals. The determination of seismic velocity structure was constrained by ray tracing and by amplitude modelling using reflectivity synthetic seismograms. The results from the ESPs show that there is a thick region of lower crustal material beneath the margin with an unusually high crustal velocity of 7.3–7.4 km s⁻¹. This lower crustal material reaches a maximum thickness of 14 km beneath the central part of the margin and is terminated at depth by the Moho. The lower crustal lens of high-velocity material is interpreted as underplated or intruded igneous rocks associated with the large volumes of extrusive basaltic lavas, now seen as dipping reflectors on the margin.     

Geochemistry of Precambrian basic igneous rocks between St. Jonsfjorden and Isfjorden, central western Spitsbergen, Svalbard

Two types of basic igneous rocks have been mapped in the middle Hecla Hoek succession in the area between St. Jonsfjorden and Isfjorden, central western Spitsbergen: the metadiabase-gabbros from the calc-argillo-volcanic formation and the Trollheimen volcanics from the quartzite-shale formation; both formations are older than the Eocambrian tilloid formation. The former has diabasic and gabbroic textures and occurs as thin discontinuous lensoid masses, while the latter exhibits definite extrusive structures with large amounts of pyroclastics and tuffs and relatively small amounts of solid lavas with abundant amygdules. Both basic rocks have been metamorphosed under the greenschist facies conditions, characterized by actinolite-epidote-biotite-albite assemblage.
The metadiabase-gabbros are moderate-high Fe tholeiites and the Trollheimen volcanics are mainly Na-alkaline, accompanied by a small amount of calk alkaline rocks. The immobile minor and trace element contents indicate that the metadiabase-gabbros are oceanic, similar to the MORB, while those of the Trollheimen volcanics show non-oceanic chemical characteristics and occur in shallow marine, shelf-shelfedge sediments.
The calc-argillo-volcanic formation reveals a shallow marine sedimentary environment and this does not conform with the oceanic nature of the metadiabase-gabbros, occurring in the formation. To overcome this disagreement, an idea of changing tectonic position is postulated to bring a shallow marine sedimentary regime onto a mid-oceanic tectonic zone. This hypothesis supports the concept of active consuming margin tectonics of the Svalbard Caledonides and explains, for example, the occurrence of the high-P metamorphic rocks in the western part of the present area.