Basement Boundary Research Articles

The correctness of the finite-difference problems of the time- and wave fields continuation for the migration image of the basement boundary

In modern seismic exploration, the migration procedure plays an important role for further interpretation of the observed data. It is the migration that makes it possible to display the deep structure of the geological section based on the dynamic characteristics of the registered wave fields.
 When processing WARRP (wide angle reflection/refraction profiling) seismic data, standard migration methods are ineffective, which is due to the peculiarities of observation systems. All existing migration methods are mainly based on reflected waves, which have a limited registration interval. The wave field on WARRP is observed at distances from sources that reach several hundred kilometers, and the uneven step between receivers is on average 1—3 km. Under such conditions, it is difficult and sometimes impossible to separate the reflected waves.
 The finite-difference reflection/refraction migration developed at the Subbotin Institute of Geophysics of the National Academy of Sciences of Ukraine has proven its effectiveness in constructing migration images of the depth structure of the section based on WARRP data observed in various regions of the world. The main difference of this migration method is the use of refracted waves as reference waves registered in the large offsets. At the same time, there is a question of the correctness of the reproduction of the depth structure of the section on the migration image, which depends on the correctness of the calculation methods. The algorithm of finite-difference reflection/refraction migration contains a continuation of the time and wave fields.
 The article presents a proof of the mathematical correctness of the solution of the eikonal differential equations and the scalar wave equation by the finite-difference method, on which are based the continuation of the time and wave fields, respectively.
 The given comparison of the formed migration images and the velocity models, calculated by the ray-tracing method, of the upper crust along the PANCAKE and TTZ-South profiles allows us to assert that the combination of the results of kinematic and dynamic processing of WARRP data allows to increase the informativeness of their further interpretation

Nankai Forearc Structural and Seismogenic Segmentation Caused by a Magmatic Intrusion off the Kii Peninsula

AbstractThe causes for forearc basin and megathrust rupture zone segmentation are controversial. The Nankai forearc, Japan, is separated into five domains based on topography: Enshu, Kumano, Muroto, Tosa, and Hyuga. The boundaries of these domains correspond to the rupture limits of large earthquakes. We examined the geologic structure of the boundary region between the Kumano and Muroto domains off the Kii Peninsula using multichannel seismic reflection data to evaluate the role of upper plate composition in controlling segmentation. The results suggest that thick cover sediments and underlying accretionary prism are obliquely thrust landward over the igneous basement complex rock in the region of offshore of Cape Shionomisaki and separate the forearc basin. The igneous basement complex rocks directly overlying the plate interface in the hypocentral regions of 1944 Tonankai and 1946 Nankai earthquakes. The 1944 earthquake originated at the base of the complex, and the rupture extent slipped past its basement boundary, whereas the 1946 event nucleated at the updip boundary of the basement complex. The dense igneous rocks might have worked as a heavily loaded barrier on the seismogenic megathrust and separated the rupture area of both the earthquakes. Upper plate geology may be an important factor in controlling seismogenesis in the Nankai Trough and may serve as an example for understanding the controls on megathrust slip in other subduction zones.

Results and prospects of geological mapping of the Arctic shelf of Russia

The results of compiling the sets of the State Geological Map at a scale of 1:1,000,000 for the Arctic continental shelf of Russia are analyzed. Results are summed up, and the main problems of geological mapping are outlined. The results of geological and geophysical studies of the Arctic Ocean are of great importance for deciphering the geological evolution. The Arctic shelf is the widest shelf in the world, while the spreading ocean basin is one of the narrowest and is characterized by anomalous structural features. The main problems of geological mapping include identification the sedimentary cover/folded basement boundary, interpretation the geodynamic evolution of the shelf and adjacent ocean, determining the rates of sedimentation and stratigraphic subdivision of the sedimentary cover due to a small number of key boreholes. It is promising to further study problem areas with unclear features of geological structure as well as small-scale mapping in areas of industrial development on the Arctic continental shelf.

Magnetic Structure and its Tectonic Implication Around Longmenshan Fault Zone Revealed by EMAG2v3

The formation of magnetic minerals is bound up with the tectonic evolution history, whereupon the distribution of magnetic anomalies has great meanings for regional tectonics. In this study, we use the latest global lithospheric magnetic field model EMAG2-v3, processed by various techniques including reduction to the pole (RTP), upward continuation, derivations, Euler deconvolution, estimation of total magnetization direction, and Curie point depth (CPD), to unveil the tectonics around Longmenshan fault zone (LFZ). LFZ is clearly displayed as a positive and negative anomaly transition zone in RTP anomalies and acts as a magnetic basement boundary. The Sichuan Basin (SB), located to the east of LFZ, is marked by strong magnetic basement and NE-strike banded induced positive anomalies which are associated with the Neoproterozoic magmatic activity. The banded shape, absence of radial pattern of anomalies, and existence of fossil subduction zone supports that the magnetic basement was formed in arc environment. The CPD in SB estimated by radial average power spectral is 30–51 km, which allows magnetic minerals in deep crust or even in lithospheric mantle to exhibit high magnetizations. The Songpan-Ganzi fold belt (SGFB), in contrast, is located to the west of LFZ and covered by thick and low-susceptibility Triassic deposits of flysch, manifests as weak negative anomalies caused by relatively shallow CPD and widespread remanent magnetization. Significant positive anomalies, appearing around the Manai and Rilonguan granitic massifs, indicate a strong magnetic basement beneath SGFB, which is conjectured as westward extension of the Yangtze Block at depth.

Deep displacement monitoring and foundation base boundary reconstruction analysis of diaphragm wall based on ultra-weak FBG

The instability or destruction of diaphragm walls in underground engineering will cause significant economic losses and casualties. The distributed and high-precision monitoring of diaphragm wall deformation can provide reliable and detail data for disaster early-warning as well as economical design. However, recording continuous deformation of the diaphragm wall using traditional monitoring technologies remain challenging. Moreover, the basement movement of diaphragm wall is rarely considered in previous studies. In order to overcome the shortcomings of the current displacement monitoring methods and deal with the basement boundary issues, a new base boundary reconstruction (BR) was proposed and the ultra-weak Fiber Bragg grating (UWFBG) technology was adopted to monitor the displacement of the diaphragm wall. For validation, a laboratory test based on ultra-weak FBG was carried out and the feasibility of the BR method was demonstrated. Subsequently, the ultra-weak FBG and the BR method were applied to Shanghai metro line 18. The results showed that the monitoring technology and analysis method proposed in this paper can obtain detailed deformation information of the diaphragm wall in real-time, which not only provides an advanced monitoring method for the deep deformation of diaphragm wall, but also solves the deformation calculation issue with the base boundary condition changes involved.

3D hybrid imaging based on gravity migration and regularized focusing inversion to predict the Poyang Basin interface

Describing and understanding the basement relief of sedimentary basins is vital for oil and gas exploration. The traditional method to map an interface in each spatial direction is based on 3D modeling of gravity Bouguer anomalies with variable lateral and vertical density contrasts using a priori information derived from other types of geoscience data sets as constraints (e.g., well and/or seismic data). However, in the preexploration stage, vertical gravity [Formula: see text], which is sometimes the only available geophysical data, is typically used to recover smooth density contrast distributions under a generic set of constraints. Apparently, the use of the [Formula: see text] component is not sufficient to produce geologically reasonable interpretations with high resolution. To address this, we have developed a novel process of hybrid inversion, combining gravity migration and inversion using the same [Formula: see text] data set, to distinguish the complicated interface between basement and sedimentary basin rocks from a full-space inverted density distribution volume. First, a 3D-migrated model delineating the basic sedimentary basin structure is derived using a focusing gravity iterative migration method, where a priori information is not necessary. Subsequently, under the framework of the regularized focusing conjugate inversion algorithm, a high-resolution density contrast model is inverted for delineation of the basement boundary by integrating the 3D-migrated density model as a priori information. We examine the method using one synthetic example and a field data case, of which a transformed resolution density matrix is developed from logarithmic space to qualitatively evaluate the practical resolutions. The high resolution of the density distribution of the Cretaceous basement with a clear interface is achieved and verified by limited seismic data and strata markers in limited wells.

Study on thermal parameters of asphalt concrete for countermeasures against high surface temperature of pavement in tunnel

It is reported that the surface temperature of asphalt pavement in a road tunnel in an urban area in summer days gets hot especially when the tunnel is long and the traffic volume is heavy. Since the asphalt pavement absorbs a large amount of heat generated by vehicles and keeps it for a long time, a high-temperature environment in the tunnel also continues for a long time. This phenomenon brings uncomfortableness for users, especially for motor-bikers and maintenance workers. On this background, this study aims to estimate the thermal parameters of the various types of asphalt concrete and investigate their effects on a high-temperature pavement surface. Six types of asphalt concrete specimens were tested in a wind-tunnel where a hot temperature air inside was generated by a forced circulation boiler system. Thermal parameters: specific heat capacity, heat conductivity, heat diffusivity, and heat transfer coefficients were analysed using a one-dimensional partial differential heat transfer equation and a finite difference approximation method. Additionally, the effects of different basement boundary conditions and water spray on the surface temperature were investigated to find effective countermeasures to decrease the surface temperature of asphalt concrete in the tunnel.

Microtremor response of the Cheongcheon dam in Korea

Microtremors were analysed using the reference-dependent horizontal spectral (H/H) method and the horizontal-to-vertical spectral ratio (H/V) method to estimate site effects of the Cheongcheon earthen dam in Korea. Seismic vibrations were recorded on the dam’s crest, at the toe of the dam, and in a downstream borehole using three-component accelerometers. The H/H peaks for crest versus toe-of-dam accelerations occurred at 3.1, 6.7 and 13.1 Hz with ratios of 6.9, 11.4 and 8.9, respectively. The H/V peaks for the crest of the dam occurred at 3.0, 7.0 and 13.4 Hz with ratios of 6.7, 7.8 and 5.6, respectively. The peak near 3 Hz may correspond to depth to bedrock, whereas the other peaks at higher frequencies may reflect the geometrical effect of the dam or overtone responses to the thickness of dam fill overlying the clay core. For the toe data, from the H/V spectral ratio method, the basement boundary appeared as double peaks near 8 and 10 Hz with corresponding amplification factors of 5.2 and 6.2. These may indicate a gradual change in velocity across the basement boundary at ~10 m depth. The third resonance, which occurred at 15 Hz, may correlate with the refraction boundary at 5–6 m depth in the overburden layer. Both the frequencies and magnitudes of resonance derived from the H/H and H/V methods are reasonably well matched to the theoretical response curves computed by the reflection and transmission matrix and the two-dimensional finite difference methods.

Intraplate volcanism of the western Pacific: New insights from geological and geophysical observations in the Pigafetta Basin

AbstractUnderstanding intraplate volcanism is a key to deciphering the Earth's magmatic history. One of the largest intraplate volcanic events occurred during the mid‐Cretaceous, roughly 75–125 Ma in the western Pacific. To investigate the origin of this volcanism we present the first comprehensive study of volcanism in the Pigafetta Basin using seismic surveys, magnetic and gravity modeling, and Ocean Drilling Program (ODP) drill core and well log data from Site 801. Our results show that intraplate volcanism in the Pigafetta Basin coincides with the rest of the western Pacific seamount provinces, supporting the previously suggested plumelets scenario for the origin of intraplate volcanism during the mid‐Cretaceous volcanic events. Our magnetic modeling suggests that the late‐stage volcanism does not overprint the remanant magnetization acquired by the Jurassic ocean crust in the Pigafetta Basin, and hence, marine magnetic anomalies recorded in the Jurassic basement are preserved. Also, the formerly identified Rough‐Smooth Boundary (RSB) is indistinguishable from any other rough‐smooth topographic boundaries throughout the survey area suggesting that the RSB is unlikely to be a Cretaceous sill‐Jurassic basement boundary. Lastly, the apparent ages and spatial distribution of volcanic features suggests a dynamic history of hydrothermal circulation in the Pigafetta Basin, indicating that hydrothermal circulation was ongoing well past 100 Ma.

Sedimentological and structural processes operative along a metalliferous catena from sandstone-hosted to unconformity-related Pb-Cu-Zn deposits in an epicontinental basin, SE Germany

The sandstone-hosted Pb-Zn deposits worldwide, including those in the Middle Triassic series of the epicontinental basin in SE Germany, were targeted upon in a literature review by Bjørlykke and Sangster in 1981, treating each mineralization as a separate entity. A different approach based on structural geology, sediment mineralogy and geochemistry (major and trace elements, C-, O-, S- and Pb isotopes) has been taken to study a “metalliferous catena” from sandstone-hosted to unconformity-related Pb-Cu-Zn deposits. The resultant model encompasses five reference types of base metal mineralization lined up along a NW-SE transect perpendicular to the various facies zones of the basin and covering the entire sequence of mineralizing processes from marine-diagenetic processes in the basin center through the epigenetic mineralization finding an outlet in the unconformity-related vein-type deposits close to the basement boundary fault at the edge of the epicontinental basin.The stratiform “Bleiglanzbank-type” (stage I) is confined to the basin center of the Middle Triassic Keuper Basin. Galena derived from bacterial reduction of sulfate and from Pb originating from a marine source of strongly homogenized average basement Pb. The Pb ore evolved in an extensional geodynamic regime. The marine-diagenetic “Bleiglanzbank-type” mineralization can be characterized as a marker stage for a Pb-enriched basin fill.The sulfidic and non-sulfidic stratabound Zn-Cu-Pb mineralization named “Weiden-type” is still dominated by Pb sulfide with only subordinate amounts of cerussite. The ore developed more landward in the epicontinental basin, in a sabkha to alluvial-fluvial depositional environment. The Pb model ages are significantly higher than those of the “Bleiglanzbank-type “ore. The reason for the chemical and facial differences in the epicontinental basin are accounted for by a barrier resultant from synsedimentary faulting around 230Ma. It is a true dia- (to epigenetic) mineralization whose fluid migration is explained by a flood recharge model. Basin dynamic and structural geology changed from extensional to moderately compressional.Two epigenetic, Pb-dominated sandstone-hosted ore types may be defined in the region, the flexurebound sulfidic and non-sulfidic Pb-(Cu-Zn) mineralization of “Eschenfelden-type” (stage III) is structure-related and superimposed on the afore-mentioned “Weiden-type” and the “Freihung-type” (stage IV). In the latter the flexure no longer existed and has turned into a reverse fault while the Pb mineralization became ore grade. The compressional regime dominated the block faulting in this ore type being located more landward. The maximum horizontal stress exerted from the basement onto the platform sediments was reached during the Lower Cretaceous resultant in a stacked pattern of Mesozoic beds and the development of an unconformity truncating the Jurassic beds. Intraformational recycling of Pb was at a maximum around the reverse faults later than 85Ma when reverse faults conduced to an overstepping of Triassic sediments, containing “Weiden-type” Pb protore (Roll front-like intraformational recycling of Pb). The 206Pb/207Pb model ages of “Weiden-“, “Eschenfeld- “and “Freihung-type” ore mineralization are almost identical. Investigations focused on the temperature of formation yielded 110° to 150°C for the sulfides and 141° to 169°C for the carbonate minerals. In this compressional regime, heat has been generated by friction along the fault planes as a function of dip angle and depth, reducing the grain size and delivering elements to the intrastratal solutions. The mineralized structures tend to strike NW-SE and in plan view are shaped concave towards the basement from which the maximum horizontal pressure was exerted onto the platform sediments.The unconformity-related sulfidic Pb-( Zn) mineralization of “Erbendorf-type” (stage V) formed in an extensional regime creating a strike-slip fault system at temperature between 132°C and 200°C. The Pb isotope signature is different from the ore types dealt with previously as far as the U series is concerned and closely resembles the sulfidic members from the sandstone-hosted Pb deposits which both have a common source for Th in monazite. Intraformational recycling in sandstone-hosted Pb deposits and concentration of Pb in unconformity –related vein-type deposits tapped the same element source and belong to the “metalliferous catena” of Pb ore crossing the basin-basement boundary or unconformity, respectively. Sandstone-hosted and unconformity-related mineral deposits are not separate entities but two sides of the same coin, the geodynamic evolution of an epicontinental basin. They are a function of the position within the “metalliferous catena” from the basin center to its edge.

The role of early formed structures on the development of the world class St Ives Goldfield, Yilgarn, WA

A revised structural interpretation for the Victory to Kambalda area of the world class St Ives Goldfield in the Archean Yilgarn Craton has mapped out the distribution of WNW-trending faults within this area of the field. These previously cryptic WNW-trending structures had been identified in gravity data, and also by isopach thickness variations. The WNW-trending faults acted as transfers syn-gold mineralization, although only discrete segments of these faults were active during the main stage of gold mineralization. Where mineralized, the faults transferred strain from a complex combination of block-on-block movement associated with thrusting and strike-slip movement on NW- and N-trending faults. Along some segments N-trending mineralized faults terminate against the WNW-trending faults. Many of the WNW-trending faults correlate with major strike changes on regional and camp-scale faults and they are domain boundaries for the critical N-trending fault segments that host high-grade gold within contractional jogs. The WNW-trending faults also show evidence for an older deformation history prior to main-stage gold, which may extend back to early basin development associated with ultramafic and mafic volcanism. They are inferred to have been a series of early WNW-trending normal faults and breached relay ramps associated with oblique rifting along an older NNW-trending basement boundary.

Geochemical variation along and across the Central American Miocene paleoarc in Honduras and Nicaragua

An analysis of 70 samples, collected from Miocene-age ignimbrites in 68 locations in Nicaragua and Honduras, indicates that geochemical processes and a subduction zone geometry similar to those in the modern arc were also present in the paleoarc. Samples were plotted along two transects, parallel and perpendicular to the inferred paleoarc. Most of the oxide plots exhibit flat profiles along the arc from NW to SE, except for SiO 2, which decreases. FeO, CaO, TiO 2, and MgO increase from NW to SE. There are no clear trends in the oxides from SW to NE, except K 2O. K 2O increases to the SW. Most trace elements show little to no variation along the province from NW to SE, with the exception of La, Ce, Rb, Ba, Th, Nb, Ta, Pb, and Sr, which all appear to decrease in abundance from NW to SE. While Ba increases to the SW, toward the paleotrench, it also shows remarkable variation parallel to the arc, with highs in Nicaragua, and decreasing concentrations to the NW and SE. This trend is especially apparent in Ba/La with lows near 1 in the NW and highs approaching 80 in Nicaragua. This trend is similar to that of the modern arc, which suggests that the geometry of the arc in the Miocene was very similar to that of the modern arc. In addition, the data also becomes more scattered to the southeast as a basement boundary is crossed (determined by gravity-anomaly mapping), probably due to a higher degree of contamination by continental crust.

Tectonics of the San Gabriel Basin and surroundings, southern California

The SanGabriel Basin is a Pliocene–Pleistocene Transverse Ranges structure bounded by the San Gabriel Mountains on the north, the San Jose and Puente Hills on the east, and the Repetto and Montebello Hills on the west. A basement boundary between gneissoid rocks of San Gabriel Mountains aspect and Peninsular Ranges rocks strikes east across the basin and connects with the San Jose fault. This boundary may represent the eastward continuation of the Canton fault, a middle Miocene precursor of the San Gabriel fault system with up to 30 km of right slip. Canton fault displacement is largely canceled by ∼22.5 km of left slip on a precursor to the Raymond fault such that an Oligocene dacite in the San Jose Hills and clasts derived from it show little evidence of large horizontal displacement between the San Gabriel Mountains and Puente Hills. Clockwise rotation in the middle Miocene probably influenced the distribution of lateral slip. The fill in the northeast-trending Pliocene–Pleistocene San Gabriel Basin is a basal shallow-marine sequence overlain by the nonmarine Duarte Conglomerate, in contrast to the deep-water Fernando Formation of the Los Angeles Basin. The paleoslope trends south in the direction of the Repetto and Puente Hills, evidence that uplift of these hills postdated deposition. The triangular San Gabriel Basin is currently a stable block bounded by the right-slip, northwest-striking East Montebello fault on the west and the left-slip(?), northeast-striking Walnut Creek fault on the east. Farther east, the San Jose and Puente Hills are underlain by an east-trending active fold belt, including the San Jose reverse fault and anticline, the Walnut anticline, and the Puente Hills anticline adjacent to the right-lateral Whittier fault. The East Montebello fault is bounded on the west by the Elysian Park anticlinorium, a fold belt in downtown Los Angeles. The left-slip Holocene Raymond fault joins the Sierra Madre reverse fault at its segment boundary marked by the Clamshell-Sawpit fault. The triangular stable block between east-trending fold belts is bounded on the south by the Puente Hills blind thrust and on the north by the Sierra Madre fault, documenting north-south Quaternary convergence. East-west extension is also indicated, although not confirmed by geodetic measurements.

Quaternary transfer faulting and belt front deformation at Pakuashan (western Taiwan)

The arcuate Pakuashan anticline is located in the outermost front units of the Western Foothills of Taiwan. This oblique feature of the deformation front is investigated in terms of combined morphostructural analysis, based on imagery and digital elevation model as well as microtectonic analysis of fault slip data. A subsurface structural study based on available seismic and well data was also carried out, resulting in improved mapping of the Neogene series and associated structures. This mapping allowed construction of several along‐strike cross sections. Such combined analyses revealed that the transverse Pakuashan fold is located above a major transfer fault zone. This active fault zone accommodates differential westward propagation of thrust units; its kinematic evolution is principally controlled by the geometry of the foreland Peikang High, behaving as a buttress for the west verging thrust sheets. A preliminary analytical model of the oblique thrusting at Pakuashan is based on similar cases studied by Apotria et al. [1992]. It involves quaternary transfer faulting accommodating the motion of connected thrust sheets, moving over oblique ramps linked to a preexisting major basement boundary (the hinge fault of the Peikang High). This analytical modeling accounts for the occurrence of local extension at the intersection of oblique ramps in the southern Pakuashan. Numerous complementary structural and tectonic evidences led us to establish a complete deformation model, involving local rotation in southern Pakuashan which caused differential slips in northern Pakuashan, resulting in tear faulting. These evidences include large extension at the intersection of oblique ramps, distributed extension in the transverse zone, regional wrench deformation, absence of major reorientation of local stress inside the transverse zone, along‐strike variation of structural styles coupled with low to high uplift rate from the Northern to the Southern part of the Pakuashan fold. Thus a synthetic reconstruction of the Pakua Transfer Fault Zone evolution is proposed, as a typical example of active transfer faulting, evolving gradually from a primary tear fault with a slight curvature to the left‐lateral tear fault or transfer fault that offsets two distinct frontal thrust‐and‐fold sheets.

Crustal magnetic structure of the Irish Sea region: evidence for a major basement boundary beneath the Isle of Man

Abstract Imaging and modelling of regional aeromagnetic data indicate a significant change in crustal magnetization across a boundary which extends northeast-southwest across the Irish Sea. A concealed magnetic basement is interpreted to underlie the area to the southeast of the boundary at mid-crustal levels, whereas relatively non-magnetic rocks lie at these levels to the northwest. The boundary does not coincide with the Iapetus Suture as defined by seismic evidence in the Irish Sea and palaeontological evidence on the east coast of Ireland. Instead, it appears to extend towards the northern margin of the Precambrian rocks of southeast Ireland. Its orientation swings sharply in the vicinity of the Isle of Man from north-northeast-south-southwest in the south to east-northeast-west-southwest in the northeast. There is an apparent correlation between the magnetic basement structure and an overlying, shallow, anomalous zone along the axis of the island which is revealed by gravity and high-resolution aeromagnetic data. Modelling indicates that shallow and deep structures may be related if both are assumed to dip to the northwest, a dip direction supported by seismic reflection data acquired immediately to the south of the island. The preferred interpretation is that the mid-crustal magnetic block is of Precambrian age and represents the northern part of an Avalonian basement which was assembled in late Precambrian-early Cambrian times. The geometry of the boundary beneath the Isle of Man may have been inherited from the pre-existing Avalonian basement architecture. Planes of weakness could have been reactivated as major, early Palaeozoic extensional structures and subsequently been activeunder a compressional regime during the closure of the Iapetus Ocean and the Acadian Orogeny.

Geometry of fold arrays in the Silesian-Cracovian region of southern Poland

Abstract A zone of deformation characterized by en echelon folds extends along the northeast margin of the Upper Silesian Coal basin in southern Poland. The zone is well exposed by mining in numerous collieries where the deformation involving Upper Carboniferous strata can be observed. Folds and associated faults form a structural pattern consistent with an origin in dextral convergent wrenching on both the principal deep-seated fractures and the subsidiary faults affecting crystalline basement beneath this part of the basin. The zone is cross-cut by sublatitudinal fold belts which, according to their geometrical parameters, seem to result from alternate transcurrent shearing along west-east oriented fractures in the basement, sinistral in one case and dextral in the other. Among folds caused by the concentrated operation of a force couple whose vectors lie in the horizontal plane, there are examples of bending folds (forced folds) in each array resulting from vertical components of movement of deep-seated fault blocks. The dimensions of the folds change with depth, and the axial trend and the inclination of axial surfaces differ with distance from the basement fault trace. The length of fold segments strongly depends on axes orientation. Widely distributed evidence of interlayer slip demonstrates a variable spatial distribution of tectonic-transport direction in the sedimentary sequence. The above-mentioned features and other geometrical attributes of the fold arrays enable the dynamics of the forcing structures during Variscan tectogenesis to be determined and illustrate a hierarchical structure of the basement boundary zones. Examples from the Silesian-Cracovian region suggest that bending folds (forced folds) can sometimes originate in conditions provided by a strike-slip regime.

Seafloor spreading history of the western North Atlantic Basin derived from the Keathley sequence and computer graphics

An analysis of sea-surface magnetic profiles and an aeromagnetic contour chart west of Bermuda yields a new tectonic framework for the Keathley (M-series) seafloor spreading magnetic anomaly sequence. Changes in spreading direction derived from the western Keathley sequence occur at times corresponding to M24, M21 and M11, of which M11 is a time of continental breakup. The models facilitate new identifications of the anomalies between M16 and M4, obviating the need for anomalously slow spreading between M11 and M4, previously suggested by other workers and by us, and placing the location of the well-defined rough-smooth basement boundary at M11. The models show that spreading rates contemporaneously vary among profiles collected between flowlines as determined by a recently presented kinematic history. This strongly suggests that individual ridge axis segments act independently of another in detail. Our reconstruction technique records in map form where the ridge crest has made significant departures from perfect symmetry and a predictable spreading rate during its evolutionary history. Among eight profiles collected parallel to seafloor spreading flowlines in the western North Atlantic encompassing only 2700 km of “normal” oceanic crust we can identify more than fourteen ridge-jump events. There is evidence for a small (≈ 30 km) ridge jump to the east of many ridge axis segments at M24. This is supported in the eastern North Atlantic conjugate crust of the Canary basin where M25 and M24 are poorly represented. A series of four ridge jumps to the east along one paleoridge axis segment now located at 36°N, 65°W is related to the passage of the Mesozoic Mid-Atlantic Ridge crest over the Verde hotspot at ≈ 137 m.y. B.P. The Project Magnet magnetic anomaly contour chart for the area between 29° N and 40° N has been used to determine stage poles of motion for processes occurring at the ridge crest related to its segmentation, which have a component oblique to spreading between chrons M21 and M11. These trends may relate to the motion of the ridge crest over the mantle.

Rough-smooth basement boundary in the western North Atlantic basin: Evidence for a seafloor-spreading origin

A distinctive change in the character of acoustic basement has been mapped within the Mesozoic (Keathley) magnetic anomaly sequence in the western North Atlantic basin. This boundary, known as the rough-smooth basement boundary, separates relatively smooth acoustic basement on the older (western) side of the boundary from much rougher basement to the east. South of Bermuda, the boundary is well defined and nearly isochronous, corresponding to anomaly M-12 or M-13 (∼128 m.y. B.P.). North of Bermuda the boundary is diachronous and more transitional, with the change in basement relief occurring in a zone between anomaly M-21 and M-16–age crust (132 to 142 m.y. B.P.). Both the well-defined and transitional changes in basement relief found in the western basin are also identifiable in seismic-reflection records from the eastern (Canary) basin, and an analysis of magnetic-anomaly patterns in the central North Atlantic basin indicates that these changes occurred simultaneously on both sides of the ridge. These results strongly suggest a primary seafloor-spreading origin for the rough-smooth basement boundary, with the greater basement relief associated with slower spreading rates, changing directions of relative plate motion, and ridge jumps of isolated accretionary segments beginning in Late Jurassic–Early Cretaceous time.

Low temperature fluid alteration of oceanic layer 2 basalts, DSDP Leg 37

Penetration of cold seawater into layer 2 of the oceanic crust occurs to at least 600 m, the maximum depth drilled below the sediment-acoustic basement boundary during Leg 37 of the DSDP. The main alteration phases are Mg-saponite, Fe-K rich celadonite, phillipsite, calcite, and hematite. The only exception to this occurs in the form of intensely hydrothermally altered gabbro breccias from an intrusive complex at site 334. There is no mineralogical evidence to suggest an increase in metamorphic grade with depth; however, a variation in mineralogy and alteration intensity, occurs as a function of structure, permeability, and the chemical nature of rock-fluid interaction. On this basis three types of alteration are defined as follows: (A) fracture focussed, oxidative; (B) palagonitic; and (C) non-oxidative, pervasive.Electron microprobe analyses reveal that alteration of fresh glass to palagonite involves the addition of H2O, an increase in total Fe, K2O, TiO2, and possibly SiO2, and the loss of CaO, MgO, Na2O, and MnO.Detailed examination of the mineralogy and chemistry of oxidative alteration suggests that during low temperature sea water–basalt interaction, basalt experiences a net gain in CaO, total Fe and K2O, while SiO2 and MgO appear to have been locally remobilized. These trends are generally consistent with the distribution of secondary phases. The distributions of Mn, Cu, Ni, Zn, Co, and Sr do not appear to have been significantly affected during this process.It is apparent that seawater must have experienced significant Eh lowering during this interaction; however, the occurrence of hematite–magnetite and absence of sulphide in this particular environment requires that seawater was either highly depleted in total sulfur or experienced a significant rise in pH.

Evidence of Post-Pleistocene Faults on New Jersey Atlantic Outer Continental Shelf: GEOLOGIC NOTES

Recently obtained high-resolution seismic profiles (400-4,000-Hz band) show evidence of faults in shallow sedimentary strata near the edge of the Atlantic continental shelf off New Jersey. Apparent normal faults having a throw of about 1.5 m displace sediments to within 7 m of the sea floor. The faults appear to be overlain by undeformed horizontal beds of relatively recent age. Several faults 1 to 2 km apart strike approximately N70°E and dip northwest. The data suggest that the faults are upthrown on the southeast. Projection of the faults on the high-resolution profiles to a nearby multichannel seismic-reflection profile indicates that these shallow faults might be the near-surface End_Page 1112------------------------------ expression of a more fundamental deep-seated fault. Several prominent reflectors in the multichannel records are offset by a high-angle normal fault reaching depths of 4.0 to 5.0 sec (6.0 to 6.5 km). The deep fault on the multichannel line also is upthrown on the southeast. Throws of as much as 90 m are apparent at depth, but offsets of as much as 10 m could be present in the shallower parts of the section that may not be resolved in the multichannel data. The position and strike of these faults coincide with and parallel the East Coast magnetic anomaly interpreted as the fundamental seaward basement boundary of the Baltimore Canyon trough. Recurring movements along such boundary faults are expected theoretically if the marginal basins are subsiding in response to the plate rotation of North America and seafloor spreading in the Atlantic.