Dipping Reflectors Research Articles

Early Cenozoic crust at the Norwegian continental margin and the conjugate Jan Mayen Ridge

Multichannel seismic profiles at the Vøring Plateau Margin off Norway and the northern Jan Mayen Ridge have provided a framework for the early Tertiary evolutionary history of these areas which were adjacent prior to the opening of the Norwegian Sea. We propose that their Paleogene evolution is quite similar and that the two acoustic basement reflectors, EE and JO, were formed in the early Eocene by extrusion of flow basalts. From the time of opening to about anomaly 23 time, oceanic seafloor was generated by Icelandic‐type spreading. During that time, the Central Jan Mayen Fracture was the main transform. The seaward dipping reflector sequences are flow basalts generated by the Icelandic spreading. The existence of less well developed dipping sequences seaward of the primary ones probably reflects that these regions submerged later because of higher initial elevation. During the early Eocene subaerial spreading episode, which is related to the North Atlantic Volcanic Province, basalt flows also covered the adjacent thinned and heavily intruded continental crust. It is proposed that the continent‐ocean boundary is located just landward of anomaly 24B, a short distance seaward of the Vøring Plateau Escarpment. Reflector K is only recognized landward of this boundary in a region of “transitional crust.” The escarpment experienced syndepositional faulting, and the most elevated parts of the Vøring Marginal High did not subside below shallow water depths before Oligocene/Miocene time. We suggest that the Vøring Margin and the Jan Mayen Ridge represent “volcanic” passive margins. The evolution of this margin type may relate to initial uplift due to rifting in previously thinned crust.

Seismic structure of a seaward-dipping reflector sequence southwest of Rockall Plateau

A seismic refraction line was obtained with a sonobuoy and airgun source over a seaward dipping reflector sequence on the southwest flank of the Rockall Plateau. The record section has been modelled using synthetic seismograms to obtain a well-constrained velocity/depth structure down to about 4 km below the top of the dipping reflector sequence. The smooth increase of velocity in the upper 1.5 km probably is due to the downward closure of cracks and voids. The base of the dipping reflector sequence beneath the profile is associated with the bottom of a relatively rapid increase in velocity from about 5.5–6.5 km s−1. A similar association of the base with a velocity increase from about 5.5–6.5 km s−1 off Queen Maud Land and under the Vøring Plateau suggests it may be a general feature of seaward dipping reflector sequences.

Crustal refraction surveys across the Trans‐Hudson Orogen/Williston Basin of south central Canada

Eight reversed seismic refraction profiles have been recorded across the Phanerozoic Williston sedimentary basin that overlies the margins of the Superior and Wyoming Archean cratons and the Proterozoic Trans‐Hudson orogen. These data have been modeled using two‐dimensional ray‐tracing techniques to match the times and amplitudes of primary and coherent secondary arrivals. Within the southern extension of the Superior craton margin (Thompson belt) a high‐velocity (∼ 6.4 km/s) lid overlies a narrow region of high electrical conductivity, and a zone of westerly dipping reflections is nearly coincident with a weak velocity discontinuity that seems to delineate the western edge (Thompson fault) of the craton margin. Complex zones of low‐velocity material and the North American Central Plains electrical conductivity anomaly are concluded to lie within the southern extension of the Reindeer‐South Indian Lakes terrain, a component of the Trans‐Hudson orogen. A high‐velocity (≳ 7.0 km/s) lower crustal layer, which represents a zone of complexity separating “normal” crust from the upper mantle, extends throughout the region. The crust is relatively thick, with depths to Moho varying smoothly between 41 and 48 km. There is no evidence in the relatively low‐resolution seismic refraction data for abrupt changes in the level of the Moho, but there may be a minor (3–4 km) thinning of the crust near the center of the Williston Basin.

Seismic reflection measurements behind the Hikurangi convergent margin, southern North Island, New Zealand.

The active Australian-Pacific plate boundary passes through New Zealand. In the north, the Pacific plate subducts beneath the Australian plate with an accretionary wedge forming the eastern continental (Hikurangi) margin of the North Island. The structure of the region behind the Hikurangi margin changes from the extensional back-arc basin under central North Island to a postulated crustal downwarp under the southern North Island. A 100 km long multichannel seismic reflection profile was recorded across the region of crustal downwarp. The data show discontinuous coherent reflectors dipping westwards at the east end of the profile, and east dipping reflectors at the west end, from depths of 9 to 15 s two way time. Simple hand migration of these events indicate that the east dipping reflectors, interpreted as the base of the Australian plate crust, abut against the west dipping reflectors which are interpreted as marking the top of the subducted Pacific plate. Detailed earthquake hypocentre locations in the area show a dipping zone of high seismicity, the top of which coincides closely with the west dipping events, thus supporting this interpretation.

Lithoprobe seismic reflection profiling across Vancouver Island: results from reprocessing

Summary. Multichannel seismic reflection sections recorded across Vancouver Island have revealed two extensive zones of deep seismic reflections that dip gently to the northeast, and a number of moderate northeasterly dipping reflections that can be traced to the surface where major faults are exposed. Based on an integrated interpretation of these data with information from gravity, heat flow, seismicity, seismic refraction, magnetotelluric and geological studies it is concluded that the lower zone of gently dipping reflections is due to underplated oceanic sediments and igneous rocks associated with the current subduction of the Juan de Fuca plate, and that the upper zone represents a similar sequence of accreted rocks associated with an earlier episode of subduction. The high densitylhigh velocity material between the two reflection zones is either an underplated slab of oceanic lithosphere or an imbricated package of mafic rocks. Reprocessing of data from two of the seismic lines has produced a remarkable image of the terrane bounding Leech River fault, with its dip undulating from > 60 near the surface to 20 at 3 km depth and -38 at 6 km depth.

Crustal structure beneath exposed accreted terranes of Southern Alaska

Summary. The crustal structure beneath the exposed terranes of southern Alaska has been explored using coincident seismic refraction and reflection profiling. A wide-angle reflector at 8-9 km depth, at the base of an inferred low-velocity zone, underlies the Peninsular and Chugach terranes, appears to truncate their boundary, and may represent a horizontal decollement beneath the terranes. The crust beneath the Chugach terrane is characterized by a series of north-dipping paired layers having low and high velocities that may represent subducted slices of oceanic crust and mantle. This layered series may continue northward under the Peninsular terrane. Earthquake locations in the Wrangell Benioff zone indicate that at least the upper two low-high velocity layer pairs are tectonically inactive and that they appear to have been accreted to the base of the continental crust. The refraction data suggest that the Contact fault between two similar terranes, the Chugach and Prince William terranes, is a deeply penetrating feature that separates lower crust (deeper than 10 km) with paired dipping reflectors, from crust without such reflectors.

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 AppalachiadHercynian 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.

Cocorp deep seismic reflection traverse of the interior of the North American Cordillera, Washington and Idaho: Implications for orogenic evolution

Deep seismic reflection data collected by the Consortium for Continental Reflection Profiling along a transect of the interior of the northwestern U. S. Cordillera indicate the presence of crustpenetrating faults; these faults developed during an orogenic cycle consisting of Jurassic to Paleocene accretion‐related crustal thickening followed by Eocene crustal thinning. The reflection data also suggest that the Moho is a young, dynamic feature which developed a relatively smooth, flat geometry beneath a complexly deformed crust, probably during an Eocene episode of crustal extension. The profiles reveal prominent west dipping reflection sequences (and associated diffractions) interpreted as a Mesozoic thrust system of crustal dimensions that linked accretion‐related shortening in the western Cordillera with the thrust belt in the eastern Cordillera, east dipping reflections that crosscut the west dipping reflections in the middle and lower crust and are interpreted as ductile deformation zones that accommodated Eocene crustal extension, and essentially flat Moho reflections at 33–35 km beneath complexly deformed crust. These features are consistent with a sequence of (1) late Mesozoic crustal imbrication of transitional crust beneath the easternmost accreted terrane which overrode the North American passive continental margin along a Jurassic thrust (2) propagation of crust‐penetrating thrusts to shallower levels in the east, where they probably formed the sole of the Rocky Mountain Thrust Belt (3) Eocene extension that disrupted these thrusts and led to uplift of core complexes along detachments which fed into broad deformation zones in the lower crust and (4) development of a flat Moho geometry during Cenozoic extension and magmatism. These data provide new information about the structure of the entire crust in the Cordilleran interior and offer a working model, both for evolution of Cordilleran core complexes and possibly for other collisional orogens.

Dipping reflectors in the Norwegian Sea—ODP Leg 104 drilling results

The following is an extended abstract of a paper presented at the Marine Studies Group meeting ‘Structure of Continent-Ocean Boundaries’ , 11 December 1985. Ocean Drilling Program (ODP) Leg 104 successfully completed a number of deep drill holes on the Outer V0ring Plateau and the Vering Basin during July and August 1985 (Fig. 1; Eldholm, Thiede, Taylor et al. 1986). One of the principal objectives of the leg was to drill and sample a thick oceanward-dipping wedge of seismic reflectors, known to characterize much of the multichannel seismic (MCS) profile data recorded across ocean-continent transitions in the NE Atlantic north of 55°N. Elsewhere, these reflectors have previously been the target of deep drilling during Legs 48 and 81 of the International Phase of Ocean Drilling IPOD (Montadert, Roberts et al. 1979; Roberts, Schnitker et al. 1984). These efforts met with some success, sampling only the uppermost part of the sequence and identifying a series of tholeiitic lava flows. Over the Outer V~ring Plateau, a number of MCS profiles reveal a distinct change in seismic character at the base of the well-stratified dipping reflector sequence, where an irregular surface characterized by a band of low-frequency, high-amplitude reflectors occurs. This surface is referred to as K. Prior to Leg 104, therefore, relatively little was known regarding the variation in petrographic character of the flows at depth, their evolution and origin, and particularly the character of the material below the reflector sequence. A single deep drillhole (642E) successfully recovered a section through the

A new formulation of dip and depth to a dipping reflector beneath V=V0(1+KZ)

The case where the velocity increases linearly with depth over a dipping reflector is considered. The underlying medium is assumed to have a constant velocity. On the time‐distance graph related to this model there is an end point which corresponds both to the direct seismic ray tangent to the reflector and to the maximum reflection distance in the first medium. Starting from this point, a method of finding the dip and depth is developed and a synthetic example is given.

Discussion on the development of rift basins illustrated by the structural evolution of the Oseberg feature, block 30/6, offshore Norway

D. S outter and J. S. R itchie write: We have the following reservations regarding the model presented by Badley et al. 1. Whilst agreeing with Badley et al. on the existence of an at least early Triassic, probably Permian, ‘pre-rift’ unconformity, we question their interpretation of tilted fault blocks below this unconformity, on the evidence presented in their fig. 2, on two grounds: (a) Badley et al. state that supposed sub-unconformity reflectors dip at a steeper angle than their picked unconformity. We question their picking of the unconformity downdip of the crestal structure west of fault B on fig. 2 and believe that they have deviated on to an onlapping reflector, thus making the dip of the unconformity appear less steep. (b) We believe that the picks below the unconformity reflector, in particular those between shot-points 1400 and 1500, are in fact sea-bed multiples. On the evidence of other seismic lines to which we have access, we think that the ‘pre-rift’ unconformity may well be faulted crystalline basement in this region (i.e. faulting of Permian age). 2. According to Badley et al.’s model, it is regarded as significant that Hauterivian sediments are found in what they term the Oseberg ‘back basin’, and yet coeval sediments are found west of the 30/6 structure towards the Viking Graben axis at a present-day structural level more than 1400 m lower. Further, it is suggested that migration of fault movement progressed eastward away from the graben axis, and that faults F and G of fig.

East-West Antarctic boundary in McMurdo sound

Abstract Common-depth-point seismic reflection profiling indicates that the crust beneath western McMurdo Sound is capped by a thin veneer of layered reflectors. The absence of deep, layered reflectors suggests the crust beneath the coast is made up primarily of intrusions which we associate with plutons generated during the Ross Orogeny. The layered reflectors dip and thicken to the east, away from the coast, where they are found at two-way reflection times of 7 s, corresponding to depths of 14–16 km. Reflection data support earlier refraction, gravity, and magnetic interpretations that indicate fundamental differences in the crust beneath McMurdo Sound and the Transantarctic Mountains. Differences may be due to early Paleozoic subduction of the Ross Embayment crust beneath the Transantarctic Mountains during the Ross Orogeny. Orogeny has produced an over-thickened crust beneath the Ross orogenic belt which was followed by several periods of reactivation including the Jurassic thermal event and the uplift of the present-day Transantarctic Mountains in early Tertiary time. The presence of the McMurdo Volcanics and preliminary interpretations of reflection data suggest that the Sound is now being thinned by processes of extension.

Seismic parameters for transversely isotropic media

One of the most important problems in exploration seismology is to relate the surface seismic measurements with the subsurface geologic parameters. The concept of wavefront curvature has been in extensive use for this purpose. Byun (1982) developed relationships between several measurable seismic parameters (e.g. geometrical spreading and normal moveout velocity) and parameters of the media with elliptical velocity dependencies. This paper extends the wavefront curvature concept to more general, transversely isotropic media. After a brief discussion on ray tracing, a procedure is developed to describe the local properties of the ray based on an elliptical surface fit to the actual wave surface. The apparent velocities of the elliptical fit are then used to generalize the seismic parameters developed in Byun (1982). Simple numerical experiments are given to demonstrate the explorational significance of the theory. It is shown that the measurements of the normal moveout velocity are not sufficient to estimate the velocity structure of the transversely isotropic medium. The ‘side-slip’ effect can lead to significant errors in depth-mapping dipping reflectors.

Intracrustal complexity in the United States midcontinent: Preliminary results from COCORP surveys in northeastern Kansas

Unusually clear indications of complex structure in the mid-to-lower crust is revealed by seismic reflection surveys in northeastern Kansas. This complexity contrasts markedly with the layer-cake simplicity of both the overlying sedimentary cover and most previous crustal models for the central United States. Seismic sections collected by COCORP (Consortium for Continental Reflection Profiling) as part of a major east-west traverse across the Neniaha Ridge and Midcontinent Geophysical Anomaly indicate that below a thin, relatively flat layered Paleozoic sedimentary section, the deep crust is characterized by numerous dipping and arcuate reflections and diffractions. In many places layered and crosscutting, these reflections suggest convoluted three-dimensional folded, faulted, and intruded structures. Specific identification of these deep features may be possible if future surveys can trace them to accessible depths. The basement above these reflection complexes contains significantly fewer reflections—consistent with, but not necessarily diagnostic of, the granitic terrane that dominates basement drill-hole samples in the region. Among the events at these shallower basement depths are several east-dipping reflections, some of which may be major faults. Travel times corresponding to expected Moho depths (about 36 km) are characterized less by specific reflections than by an apparent decrease in the density and number of reflections. While evidence of crustal heterogeneity is common among deep reflection studies, the Kansas seismic results outlined in this brief report stand out as being unusually clear representations of such.

Structure of the Sunda Trench lower slope off sumatra from multichannel seismic reflection data

Multichannel seismic reflection profiles across the Sunda Trench slope off central Sumatra reveal details of subduction zone structure. Normal faults formed on the outer ridge of the trench offset deep strate and the oceanic crust, but die out upsection under the trench sediments. At the base of the inner trench slope, shallow reflectors are tilted seaward, while deeper reflectors dip landward parallel to the underlying oceanic crustal reflector. Intermediate depth reflectors can be traced landward through a seaward-dipping monocline. We interpret this fold as the shallow expression of a landward-dipping thrust fault at depth. Landward of this flexure, relatively undeformed strata have been stripped off the oceanic plate, uplifted 700 meters, and accreted to the base of the slope. The oceanic crust is not involved in the deformation at the toe of the slope, and it can be observed dipping landward about 25 km under the toe of the accretionary prism.

Discussion on “Diffraction response for nonzero separation of source and receiver,” by John R. Berryhill (GEOPHYSICS, October 1977, p. 1158–76, and Discussion on “Diffractions for arbitrary source‐receiver locations,” by A. W. Trorey (GEOPHYSICS, October 1977, p. 1177–82)

Study of simple synthetic seismograms convinced both authors that the diffracted pulse shape shows virtually no variation with source‐receiver offset if the midpoint is held constant. First, I wish to corroborate their findings by means of a simple example and at the same time suggest that conclusions of both authors apply strictly only to planar (and planar dipping) reflectors. Secondly, I wish to show that because of the invariant behavior of the pure diffraction pulse and because of the equally invariant behavior of the pure reflection pulse, the shape of the reflected pulse for depth points close to the edge of the reflector can not be offset invariant.

Structure of the continental margin of South-eastern Greenland

The paper describes the interpretation of geophysical observations across the Southeastern Greenland continental margin between 58° and 65° N. South of 63°, magnetic anomaly 24 is the earliest recognizable oceanic anomaly. North of 63°, anomalies 23 and 24 cut out against the margin and there is a complementary widening of ocean floor of this age on the opposite Rockall Margin north of Hatton Bank; this is attributed to local westward migration of the spreading axis north of 63° shortly after the split. Airgun and sparker profiles indicate the presence of three major sediment groups. Two groups of Tertiary age are separated by an erosional unconformity beneath the rise north of 62°. The overlying group is interpreted as comprising mainly contour current deposits of later Tertiary age, and the underlying group as lithified oozes of about Eocene age. Below the Tertiary sediments, older seaward dipping reflectors occur between anomaly 24 and the slope. These are interpreted as Mesozoic sediments overlying subsided continental crust. The oceanic-continental crustal boundary is recognized from magnetic anomalies and seismic profiles as occurring to the east of this subsided region, and lies up to 80km seaward of the present scarp which is interpreted as an erosional feature cut by contour currents. Gravity profiles indicate that the main change in crustal thickness associated with the margin occurs landward of the scarp north of 63° but corresponds more nearly to the scarp further south.

A succession of Tertiary strata off Nova Scotia, as determined by dredging

Outcrops of strata ranging in age from Oligocene to Miocene occur on the sides of The Gully, a large submarine canyon near Sable Island. Data from dredge-sampling operations suggest that the exposed stratigraphic section may be as much as 1400 m thick. Analysis of dredge samples shows that the rocks collected are dominantly mudstone with minor components of sand. The exposed section is slightly coarser and more varied in texture toward the bottom than toward the top. Sand fractions are quartz-rich and contain only minor feldspar and rock fragments. Slight variations in accessory minerals may reflect the relative influences of different source rocks. The mineral suite is generally representative of plutonic and metamorphic source rocks. Abundant pyrite internal molds in mudstones suggested that the Eh in these sediments is negative. Illite and kaolinite composed most of the clay-size fraction of outcrop samples, with lesser amounts of montmorillonite.Continuous-seismic reflection surveys show that sub-bottom reflectors dip gently seaward. Below approximately 600 m, no sub-bottom reflections were recorded. Shallower, more discontinuous reflections suggest that locally reworked beds occur on the rim of the canyon.Petrological, structural, and faunal data indicate that the section is part of the Atlantic Coastal Plain Province, which, prior to this investigation, was known to occur only as far north as Georges Bank.

STRUCTURE OF THE LOWER CONTINENTAL RISE HILLS OF THE WESTERN NORTH ATLANTIC

Data acquired during a recent seismic reflection survey of portions of the western North Atlantic have provided new information on the movement and deposition of sediments on the ocean floor. The area surveyed included portions of the lower continental rise hills which form the transition between the continental rise and the abyssal floor east of Cape Hatteras. The lower rise hills are ridges having a general east‐west orientation and an average length of 20 miles. Individual hills have 25 to 50 fathoms relief and are two to three miles wide. Each of the hills has a relatively short, steep out‐crop slope facing seaward and a shallow dip slope. Shallow subbottom reflectors dip opposite to the regional slope; deep reflectors are structurally independent of the upper reflecting layers. In profile, the ridges appear to be imbricate blocks with a maximum thickness of 100 fathoms. Ridges become progressively smaller seaward and are eventually onlapped by the horizontal beds of the abyssal floor. No fault structure is visible. The imbricate structure of individual hills, dip reversals between the continental rise and lower rise hills, dip reversals between the lower rise hills and the abyssal floor, and the apparent recent denudation of sediment cover from the continental rise indicate that the lower rise hills are gravitational glide blocks. Published literature on the engineering properties of marine sediments illustrates progressive gravitational gliding on slopes as low as 1:100.

GEOPHYSICAL HISTORY OF THE LA GLORIA FIELD, JIM WELLS AND BROOKS COUNTIES, TEXAS

This paper presents a historical record of the geophysical activity in the area of the La Gloria Field. The successive stages of geophysical exploration were: Torsion balance survey 1934–1935; correlation reflection seismograph survey 1936; dip reflection seismograph survey, 1938; correlation reflection seismograph survey, 1938; gravity meter survey, 1943–1944. The discovery well, Magnolia’s Sam Maun No. 1, was drilled and completed in 1938–1939, producing initially 165 barrels per day of 65° gravity distillate and 5,646,000 cubic feet of gas through a 5/16 inch choke. Oil and gas in the La Gloria Field are being produced from sands in the Frio formation of Oligocene age. There are a number of sands producing gas‐distillate. Several flank wells produce oil. The field has been unitized and a cycling plant is engaged in processing the gas‐distillate.