Rapid Subsidence Research Articles

Fast rates of subduction erosion along the Costa Rica Pacific margin: Implications for nonsteady rates of crustal recycling at subduction zones

At least since the middle Miocene (∼16 Ma), subduction erosion has been the dominant process controlling the tectonic evolution of the Pacific margin of Costa Rica. Ocean Drilling Program Site 1042 recovered 16.5 Ma nearshore sediment at ∼3.9 km depth, ∼7 km landward of the trench axis. The overlying Miocene to Quaternary sediment contains benthic foraminifera documenting margin subsidence from upper bathyal (∼200 m) to abyssal (∼2000 m) depth. The rate of subsidence was low during the early to middle Miocene but increased sharply in the late Miocene‐early Pliocene (5–6.5 Ma) and at the Pliocene‐Pleistocene boundary (2.4 Ma). Foraminifera data, bedding dip, and the geometry of slope sediment indicate that tilting of the forearc occurred coincident with the onset of rapid late Miocene subsidence. Seismic images show that normal faulting is widespread across the continental slope; however, extension by faulting only accounts for a minor amount of the post‐6.5 Ma subsidence. Basal tectonic erosion is invoked to explain the subsidence. The short‐term rate of removal of rock from the forearc is about 107–123 km3 Myr−1 km−1. Mass removal is a nonsteady state process affecting the chemical balance of the arc: the ocean sediment input, with the short‐term erosion rate, is a factor of 10 smaller than the eroded mass input. The low 10Be concentration in the volcanic arc of Costa Rica could be explained by dilution with eroded material. The late Miocene onset of rapid subsidence is coeval with the arrival of the Cocos Ridge at the subduction zone. The underthrusting of thick and thermally younger ocean crust decreased the subduction angle of the slab along a large segment of the margin and changed the dynamic equilibrium of the margin taper. This process may have induced the increase in the rate of subduction erosion and thus the recycling of crustal material to the mantle.

Quaternary Sedimentation and Subsidence History of Lake Baikal, Siberia, Based on Seismic Stratigraphy and Coring

The long, continuous, high-latitude, stratigraphic record of Lake Baikal was deposited in three broad sedimentary environ- ments, defined by high-resolution seismic-reflection and coring meth- ods: (1) turbidite depositional systems, by far the most widespread, characterizing most of the margins and floors of the main basins of the lake, (2) large deltas of major drainages, and (3) tectonically or topographically isolated ridges and banks. Holocene sedimentation rates based on radiocarbon ages vary by more than an order of mag- nitude among these environments, from less than about 0.03 mm/yr on ridges and banks to more than about 0.3 mm/yr on basin floors. Ex- trapolating these rates, with a correction for compaction, yields ten- tative estimates of about 25 and 11 Ma for the inception of rifting in the Central and North basins, respectively, and less than 6 Ma for the 200-m sediment depth on Academician Ridge. The Selenga Delta has the distinctive form of a classic prograding Gilbert-type delta, but its history appears to represent a complex com- bination of tectonism and sedimentation. The central part of the delta is underlain by prograding, shallow-water sequences, now several hun- dred meters below the lake surface. These deposits and much of the delta slope are mantled by fine-grained, deep-water, hemipelagic de- posits whose base is estimated to be about 650,000 years old. Modern coarse-grained sediment bypasses the delta slope through fault-con- trolled canyons that feed large, subaqueous fans at the ends of the South and Central basins. These relations, along with abundant other evidence of recent faulting and the great depths of the Central and South basins, suggest that these two rift basins have experienced a period of unusually rapid subsidence over the last 650,000 years, dur- ing at least part of which sedimentation has failed to keep pace.

Sequence model and its application to a Miocene shelf–slope system in the tectonically active Ulleung Basin margin, East Sea (Sea of Japan)

In a broader application of sequence stratigraphic concept to a tectonically active margin setting, this study presents a sequence model that considers all three controls on sequence development (i.e. eustasy, tectonic movement and sediment supply) as independent variables. The model introduces six sequence types (A to F) including type 1 and type 2 sequences defined in the original Exxon scheme. Each sequence shows a variety in number and stacking pattern of its constituent parasequence sets reflecting combined effects of accommodation change and sediment supply. This model is applied to a seismic sequence analysis of the shelf–slope system (middle to upper Miocene) in the southwestern margin of Ulleung Basin which has experienced significant crustal deformation during the Tertiary back-arc opening and subsequent closing of the East Sea (Sea of Japan). The model application delineates four sequence types whose development is closely associated with the tectonic evolution of the Ulleung Basin margin. During the back-arc opening (early to middle Miocene), type A and B sequences were emplaced as a result of steady creation of accommodation space due to a rapid subsidence combined with a tectonic-controlled high to moderate rate of sediment supply. The sequences associated with the extensional tectonism are characterized by active progradation and aggradation without forced regressive phases. In the initiation stage of back-arc closure (middle to late Miocene), subsidence rates were significantly reduced because of a widespread contractional deformation, while subaerial erosion of the uplifted thrust belt resulted in an increase in sedimentation rate. As a result, steady prograding type-E sequences were formed by alternating normal and forced regressions. During the quiescent phase of back-arc closure in the late Miocene, rise-dominant fluctuating relative sea-level change and moderate to low sediment supply gave rise to type-F sequences (similar to type-1 sequences of the Exxon group) reflecting a major control of eustatic sea-level change.

Cenozoic stratigraphy and subsidence history of the South China Sea margin in the Taiwan region

AbstractSeismic reflection profiles and well data are used to determine the Cenozoic stratigraphic and tectonic development of the northern margin of the South China Sea. In the Taiwan region, this margin evolved from a Palaeogene rift to a latest Miocene–Recent foreland basin. This evolution is related to the opening of the South China Sea and its subsequent partial closure by the Taiwan orogeny.Seismic data, together with the subsidence analysis of deep wells, show that during rifting (∼58–37 Ma), lithospheric extension occurred simultaneously in discrete rift belts. These belts form a >200 km wide rift zone and are associated with a stretching factor, β, in the range ∼1.4–1.6. By ∼37 Ma, the focus of rifting shifted to the present‐day continent–ocean boundary off southern Taiwan, which led to continental rupture and initial seafloor spreading of the South China Sea at ∼30 Ma. Intense rifting during the rift–drift transition (∼37–30 Ma) may have induced a transient, small‐scale mantle convection beneath the rift. The coeval crustal uplift (Oligocene uplift) of the previously rifted margin, which led to erosion and development of the breakup unconformity, was most likely caused by the induced convection.Oligocene uplift was followed by rapid, early post‐breakup subsidence (∼30–18 Ma) possibly as the inferred induced convection abated following initial seafloor spreading. Rapid subsidence of the inner margin is interpreted as thermally controlled subsidence, whereas rapid subsidence in the outer shelf of the outer margin was accompanied by fault activity during the interval ∼30–21 Ma. This extension in the outer margin (β∼1.5) is manifested in the Tainan Basin, which formed on top of the deeply eroded Mesozoic basement. During the interval ∼21–12.5 Ma, the entire margin experienced broad thermal subsidence. It was not until ∼12.5 Ma that rifting resumed, being especially active in the Tainan Basin (β∼1.1). Rifting ceased at ∼6.5 Ma due to the orogeny caused by the overthrusting of the Luzon volcanic arc.The Taiwan orogeny created a foreland basin by loading and flexing the underlying rifted margin. The foreland flexure inherited the mechanical and thermal properties of the underlying rifted margin, thereby dividing the basin into north and south segments. The north segment developed on a lithosphere where the major rift/thermal event occurred ∼58–30 Ma, and this segment shows minor normal faulting related to lithospheric flexure. In contrast, the south segment developed on a lithosphere, which experienced two more recent rift/thermal events during ∼30–21 and ∼12.5–6.5 Ma. The basal foreland surface of the south segment is highly faulted, especially along the previous northern rifted flank, thereby creating a deeper foreland flexure that trends obliquely to the strike of the orogen.

Tectonic controls on spatio‐temporal development of depositional systems and generation of fining‐upward basin fills in a strike‐slip setting: Kyokpori Formation (Cretaceous), south‐west Korea

Abstract The Kyokpori Formation (Cretaceous), south‐west Korea, represents a small‐scale lacustrine strike‐slip basin and consists of an ≈ 290 m thick siliciclastic succession with abundant volcaniclasts. The succession can be organized into eight facies associations representing distinctive depositional environments: (I) subaqueous talus; (II) delta plain; (III) steep‐gradient large‐scale delta slope; (IV) base of delta slope to prodelta; (V) small‐scale nested Gilbert‐type delta; (VI) small‐scale delta‐lobe system; (VII) subaqueous fan; and (VIII) basin plain. Facies associations I, III and IV together constitute a large‐scale steep‐sloped delta system. Correlation of the sedimentary succession indicates that the formation comprises two depositional sequences: the lower coarsening‐ to fining‐upward succession (up to 215 m thick) and the upper fining‐upward succession (up to 75 m thick). Based on facies distribution, architecture and correlation of depositional sequences, three stages of basin evolution are reconstructed. Stage 1 is represented by thick coarse‐grained deposits in the lower succession that form subaqueous breccia talus and steep‐sloped gravelly delta systems along the northern and southern basin margins, respectively, and a sandy subaqueous fan system inside the basin, abutting against a basement high. This asymmetric facies distribution suggests a half‐graben structure for the basin, and the thick accumulation of coarse‐grained deposits most likely reflects rapid subsidence of the basin floor during the transtensional opening of the basin. Stage 2 is marked by sandy black shale deposits in the upper part of the lower succession. The black shale is readily correlated across the basin margins, indicating a basinwide transgression probably resulting from large‐scale dip slip suppressing the lateral slip component on basin‐bounding faults. Stage 3 is characterized by gravelly delta‐lobe deposits in the upper succession that are smaller in dimension and located more basinward than the deposits of marginal systems of the lower succession. This lakeward shift of depocentre suggests a loss of accommodation in the basin margins and quiescence of fault movements. This basin evolution model suggests that the rate of dip‐slip displacement on basin‐margin faults can be regarded as the prime control for determining stacking patterns of such basin fills. The resultant basinwide fining‐upward sequences deviate from the coarsening‐upward cycles of other transtensional basins and reveal the variety of stratigraphic architecture in strike‐slip basins controlled by the changes in relative sense and magnitude of fault movements at the basin margins.

Long term results with the BiCONTACT system--aspects to investigate and to learn from

The BiCONTACT femoral stem for cementless fixation is being used without any technical modification after 15 years. The long-term results should be evaluated in this study. A consecutive series was continuously monitored in a prospective follow-up study. A survival analysis was performed, clinical results were rated according to the Harris score. There were 236 patients with 250 total hip replacements (THR); mean age at time of implantation was 58.2 years. Indications for THR included osteoarthritis (62.4%), dysplasia (16.8%), trauma (8.4%) and femoral-head necrosis (16.8%). Average time of follow-up evaluation was 8.9 years (range 7.4-10.7 years). At follow-up, 27 patients had died and two could not be located. Seven patients were revised--two for infection, one for recurrent dislocation, two for component undersizing with rapid subsidence, and one for aseptic loosening of a varus-malaligned stem; one radiologically well-fixed stem had been revised during acetabular revision. Survival estimate showed an overall survival rate of 97.1% after 11 years (confidence limits: 98.7% upper and 93.6% lower). Radiologically, tiny reactive lines (< 2 mm) were present in the distal zones of the femoral shaft, but no radiolucencies could be found in the proximal anchoring zone. Migration analysis with Ein-Bild-Röntgen-analyse/femoral component analysis (EBRA/FCA) demonstrated a very small amount of migration: in 31.0%, the overall migration was between 0.5 and 1 mm after 120 months; 8.5% had an absolute amount of subsidence exceeding 2 mm after 120 months (one case more than 3 mm). Mean subsidence was 0.2 mm after 3 months and 6 months, 0.3 mm after 12 months, and reached 0.5 mm after 10 years. An initial small amount of subsidence could be detected in 45.1%, and 15.5% had a late onset of subsidence. Continuous sinking could be found in 12.7%, while 26.8% had irregular patterns of migration. Clinical results were somewhat compromised by a higher-than-average rate of cup loosening (uncoated threaded cup). The average Harris hip score at follow-up was 84.3 points. Interestingly, no femoral osteolysis could be detected, even in cases with severe acetabular osteolyses, indicating sealing of the stem interface by tight osseointegration of the proximally-coated stem.

Geochemistry and origins of natural gases in the Yinggehai and Qiongdongnan basins, offshore South China Sea

Four gas fields and a number of gas-bearing structures have been discovered in the Yinggehai and Qiongdongnan basins offshore South China Sea. Chemical and isotopic data indicate the presence of at least three genetic groups of gases in these basins: biogenic gas, thermogenic gas, and gases with mixed origin. Thermogenic gases produced from the Yacheng field are characterized by relatively high contents of benzene and toluene, relatively low δ 13C values of toluene, and high abundance of bicadinanes and oleanane in the associated condensates, showing good correlation with the coal-bearing sequence of the Yacheng Formation in the Qiongdongnan Basin. In contrast, the gases from the DF1-1 and LD gas fields contain high amounts of N 2 and CO 2, low to moderate amounts of benzene and toluene, with relatively high δ 13C values of toluene. These characteristics correlate well with the Miocene neritic shales in the Yinggehai Basin. Analyses on potential source rock samples indicate excellent gas source potential for both the coal-bearing sequence in the Yacheng Formation and the Miocene neritic shales containing type III∼II2 kerogens. As the result of recent rapid subsidence and sedimentation, high temperature and overpressure systems are well developed in the Yinggehai and Qiongdongnan basins. The rapid heating resulted in advanced maturation of organic matter deposited in normally pressured and overpressured strata, whereas the strongly overpressured systems led to retarded organic matter maturation, postponing the time when the source rocks reached peak gas generation. Results of 1-D modeling indicate that the coal-bearing Yacheng Formation in the Yacheng Sag reached peak gas generation during mid Miocene-Pliocene time, after the deposition of upper Oligocene to lower Miocene reservoir rocks. The lower-mid Miocene marine shales reached their peak gas generation stage during the Pliocene-Quaternary in the central Yinggehai basin, supplying abundant gases to charge the Pliocene reservoirs. More importantly, focused-episodic gas migration significantly increased the hydrocarbon expulsion efficiencies of source rocks, and thus provided favorable conditions for the accumulation of large amounts of gases in the diapiric structures in a very short geological time. The gas–source relationship for the Yacheng field suggests dominantly short–distance gas migration and thus strong source facies controls on the geographic distribution of gas fields within the Qiongdongnan Basin.

Geochemistry of petroleum systems in the Niuzhuang South Slope of Bohai Bay Basin. Part 2: evidence for significant contribution of mature source rocks to “immature oils” in the Bamianhe field

The chemical compositions of 28 representative crude oils and oil sand samples collected from the Bamianhe region of the Bohai Bay Basin, eastern China were studied in detail in order to understand their genetic relationships, biodegradation ranking and thermal maturity level. Molecular geochemical parameters indicate the presence of one single oil family in the Cenozoic reservoirs. A number of biomarker features support deposition of the source rocks for these oils under conditions of enhanced salinity/stratified water columns. The extremely low maturity levels of these “immature oils”, as indicated by sterane isomerization ratios, were not supported by other maturity parameters. The genetic relationship between the oils and the Es 4 and Es 3 members of the Eocene-Oligocene Shahejie Formation present in the deeper part of the Niuzhuang Sag, but not those along the Niuzhuang South Slope was suggested using a wide range of molecular parameters. Variation in the sterane concentrations observed from potential source rocks with different maturity levels in the study area suggests a high probability of migration contamination of mature oils by immature bitumens along migration pathways or from intra-reservoir shales. Variation in the level of oil biodegradation could also create anomalies in the use of gross oil compositions as an indicator of oil maturity. Although the recent rapid subsidence of the Bohai Bay Basin and the saline depositional environment of one of the potential source rocks favor oil generation at relatively low maturity levels, most of the economic oils discovered in the Bamianhe region appear to have derived from conventionally mature source rocks but contain lower maturity fluids admixed with them.

Rates of deformation of an extensional growth fault/raft system (offshore Congo, West African margin) from combined accommodation measurements and 3‐D restoration

AbstractsThe aim of this paper is to quantify the evolution in time and space of the accommodation (space available for sedimentation) in the case of a growth fault structure resulting from gravity‐induced extension comprising a listric fault/raft system located along the West African margin. To achieve this, use was made of an original approach combining two complementary techniques (accommodation variation measurements and 3‐D restoration) in order to quantify vertical and horizontal displacement related to deformation, using a data set made up of a 3‐D seismic survey and well logs. We applied sequence stratigraphic principles to (i) define a detailed stratigraphic framework for the Albo‐Cenomanian and (ii) measure subsidence rates from accommodation variations. 3‐D restoration was used to (iii) reconstruct the evolution of the 3‐D geometry of the fault system. The rates of horizontal displacement of structural units were measured and linked to successive stages in the growth of the fault system.Subsidence of the structural units exhibits three scales of variation: (1) long‐term variation (10 Ma) of c. 80 m Ma−1 for a total subsidence of about 1400 m, compatible with the general subsidence of a passive margin, and (2) short‐term variations (1–5 Ma) corresponding to two periods of rapid subsidence (about 150–250 m Ma−1) alternating with periods of moderate subsidence rate (around 30 m Ma−1). These variations are linked to the development of the fault system during the Albian (with downbuilding of the raft and development of the initial basin located in between). During the Cenomanian, the development of the graben located between the lower raft and the initial basin did not seem to affect the vertical displacements. (3) High‐frequency variations (at the scale of genetic unit sets) range between −50 and 250 m for periods of 0.2–2 Ma. Accommodation variations governed these cycles of progradation/retrogradation rather than sediment flux variations. In addition, the nine wells display a highly consistent pattern of variation in accommodation. This suggests that the genetic unit sets were controlled at a larger scale than the studied system (larger than 20 km in wavelength), for example, by eustatic variations. Translation rates are between 3 and 30 times higher than subsidence rates. Therefore, in terms of amplitude, the main parameter controlling the space available for sedimentation is the structural development of the fault system, that is to say, the seaward translation of the raft units, itself resulting from a regional gravity‐driven extension.

Structure of the SE Greenland margin from seismic reflection and refraction data: Implications for nascent spreading center subsidence and asymmetric crustal accretion during North Atlantic opening

Seismic reflection and refraction data from the SE Greenland margin provide a detailed view of a volcanic rifted margin from Archean continental crust to near‐to‐average oceanic crust over a spatial scale of 400 km. The SIGMA III transect, located ∼600 km south of the Greenland‐Iceland Ridge and the presumed track of the Iceland hot spot, shows that the continent‐ocean transition is abrupt and only a small amount of crustal thinning occurred prior to final breakup. Initially, 18.3 km thick crust accreted to the margin and the productivity decreased through time until a steady state ridge system was established that produced 8–10 km thick crust. Changes in the morphology of the basaltic extrusives provide evidence for vertical motions of the ridge system, which was close to sea level for at least 1 m.y. of subaerial spreading despite a reduction in productivity from 17 to 13.5 km thick crust over this time interval. This could be explained if a small component of active upwelling associated with thermal buoyancy from a modest thermal anomaly provided dynamic support to the rift system. The thermal anomaly must be exhaustible, consistent with recent suggestions that plume material was emplaced into a preexisting lithospheric thin spot as a thin sheet. Exhaustion of the thin sheet led to rapid subsidence of the spreading system and a change from subaerial, to shallow marine, and finally to deep marine extrusion in ∼2 m.y. is shown by the morphological changes. In addition, comparison to the conjugate Hatton Bank shows a clear asymmetry in the early accretion history of North Atlantic oceanic crust. Nearly double the volume of material was emplaced on the Greenland margin compared to Hatton Bank and may indicate east directed ridge migration during initial opening.

Middle and Late Eocene Great Australian Bight lithobiostratigraphy and stepwise evolution of the southern Australian continental margin

Middle to Upper Eocene carbonates recovered at ODP Leg 182 sites from the Great Australian Bight comprise three packages: (i) a 43–40 Ma quartzose limestone and packstone package; (ii) a 39–37 Ma wackestone package with ooze; and (iii) an Upper Eocene fine‐grained wackestone package mainly 36.5–35 Ma in age. Bounding these sediment packages are unconformities inferred to have occurred at ca 43, ca 39, ca 37 and ca 34 Ma, coinciding with four major third‐order sequence boundaries. Rapid changes in sea‐floor spreading probably caused these unconformities, while a stable high sea‐level between spreading pulses led to sediment packaging from offshore to coastal basins. The regional sea‐level changes in these Middle to Late Eocene times, when glacioeustatic influence was minimal, were mainly driven by plate tectonics. Biofacies indicate a rapid subsidence, faster offshore than nearshore, during the first phase of accelerated sea‐floor spreading between Australia and Antarctica around 43 Ma. Most significant environmental changes affecting sedimentation along the southern Australian margin at that time were the influx of warm waters from the Indian Ocean and the initial development of a stratified water mass in the deepening and widening Australo‐Antarctic Gulf. The warm (surface) water supported plankton from the subtropics and migratory and endemic larger benthos including foraminifers,while the fertile deeper waters were sites of chert accumulation. The bloom of carbonate‐generating bryozoans contributed to widespread carbonate sedimentation that progressed towards the southeastern margin ∼4 million years later. Subsequent carbonate packaging and unconformities, reflecting a stepwise accelerated sea‐floor spreading pattern, persisted into the terminal Eocene before the onset of global glaciation.

Geochemistry of semipelitic schist of Isanlu area, south west Nigeria: implication for the geodynamic evolution of the Egbe-Isanlu schist belt

Field and geochemical studies carried out on the semipelitic quartz-mica schist of Isanlu area, SW Nigeria have been used to speculate on the geodynamic activity prevalent in the Egbe-Isanlu palaeobasin at the time its sediment was deposited. Basement rock exposures in the Egbe-Isanlu schist belt are dominated by metasedimentary rocks, chiefly, quartz-mica schist with small occurrences of quartzite, marble and silicate facies iron-formation. These metasedimentary rocks are interbanded with metaigneous rocks such as talc schist and amphibolite, all of which are intruded in places by granitoids. Chemical data on major and trace elements of the sheared and unsheared varieties of quartz – mica schist from the Isanlu area, reveal a composition comparable to that of semipelitic metasediment. However, the sheared variety shows elevated values of Na2O, MnO and Nb, and depleted mean values of FeO and MgO relative to the composition of normal metasediments. This enrichment – depletion signature is probably a consequence of the movement of metamorphic remobilised fluids along these shear zones during the Pan-African or earlier events. A further geochemical characterisation of the rock indicates that its protolith was an immature sub-greywacke to greywacke-type sediment derived from a predominantly granitic provenance. This immature nature of the protolith sediment suggests rapid subsidence of the basin during its genesis, and/or tectonic instability in the surrounding environment from which the sediment was derived. Nonetheless, field observations indicate the absence of typical deep-water sediments and a lack of regional proximal-distal facies variation suggesting a limited depth and width for the basin. The presence of shallow, stable shelf-type sediments such as carbonates and iron-formation (some of which contain carbonaceous material) suggests a change in tectonic regime at some point in the evolution of the basin from a rapidly sinking, to a stable shallow basin. KEY WORDS: Geochemical studies, Semipelitic schist, Geodynamic activities, Rapidly sinking basin [Global Jnl Geol. Sci. Vol.1(2) 2003: 113-128]

The Role of Soil Organic Carbon in Maintaining Surface Elevation in Rapidly Subsiding U.S. Gulf of Mexico Coastal Marshes

Subsidence is a primary factor governing marsh deterioration in Mississippi River deltaic plain coastal marshes. Marsh surface-water level relationships are maintained primarily through soil organic matter accumulation and inorganic sediment input. In this study we examined the role of soil organic matter accumulation in maintaining marsh elevation in a brackish Spartina patens marsh. Measured rates of soil organic accumulation were compared to plant biomass production and soil respiration (carbon dioxide and methane emission) at the study sites. The study demonstrated the importance of plant biomass production to soil organic carbon accumulation in maintaining viable Spartina patens marshes in sediment-deficient coastal environments. The role of Mississippi River freshwater reintroduction in maintaining conditions for organic accretion is discussed.

Causes of hot-spot wetland loss in the Mississippi delta plain

Field surveys and sediment cores were used to estimate marsh erosion and land subsidence at Madison Bay, a well-known wetland loss hot spot in coastal Louisiana. Former marshes of Madison Bay are under about 1 m of water. Nearly two-thirds of the permanent flooding was caused by rapid subsidence in the late 1960s, whereas the other third was caused by subsequent erosion. Subsidence rates near Madison Bay since the 1960s (20 mm/yr) are an order of magnitude greater than deltaic subsidence rates averaged for the past 4004000 yr (2 mm/yr).The rapid acceleration and unexpected decline in wetland losses in the Mississippi delta plain are difficult to explain on the basis of most physical and biogeochemical processes. There are, however, close temporal and spatial correlations among regional wetland loss, high subsidence rates, and large-volume fluid production from nearby hydrocarbon fields. The decreased rates of wetland loss since the 1970s may be related to decreased rates of subsidence caused by significantly decreased rates of subsurface fluid withdrawal.Annual fluid production from the Lapeyrouse, Lirette, and Bay Baptiste fields that encompass Madison Bay accelerated in the 1960s, peaked about 1970, and then declined abruptly. Large decreases in pore pressure in the Lapeyrouse field have likely altered subsurface stresses and reactivated a major fault that coincides with the wetland loss hot spot. Therefore, wetland losses at Madison Bay can be closely linked to rapid subsidence and possible fault reactivation induced by long-term, large-volume hydrocarbon production.

The rise and fall of the Faroe–Shetland Basin: evidence from seismic mapping of the Balder Formation

Seismic mapping of the Faroe–Shetland Basin from three-dimensional surveys provides striking evidence for uplift of the southern Faroe–Shetland Basin in the late Palaeocene and rapid subsidence in the early Eocene. The seismic reflector at the base of the Balder Formation follows a regional unconformity surface, which records erosion of a major branching drainage network into the underlying Palaeocene section, following a major base level fall. The topography of this surface was subsequently in-filled following marine transgression, and deep-water conditions returned to the basin in the early Eocene. We attribute base level changes to the influence of the proto-Iceland mantle plume beneath the lithosphere, providing support until continental break-up west of the Faroe Islands.

Stand composition and structure across a changing hydrologic gradient: Jean Lafitte National Park, Louisiana, USA

We report the results of an intensive study of forest structure and composition across a 1.4-m elevation gradient from the top of a natural levee into the backswamp of Bayou Des Familles, Jean Lafitte National Park, Jefferson Parish, Louisiana, USA. At the southernmost edge of the great bottomland hardwood forest of the Lower Mississippi Alluvial Valley (LMAV), forests of the Bayou Barataria-Des Familles distributary are undergoing rapid subsidence with resulting increased flood frequency, depth, and duration. We used data from a 4.6-ha permanently marked plot to examine patterns of distribution and regeneration in forest trees. Non-metric multidimensional scaling ordination of 23 quadrats (20×100 m) from this plot showed variation in forest composition across this 1.4-m elevation gradient corresponding to bottomland hardwood forest Zones III (semipermanently flooded) through V (temporarily flooded). A comparison of the size-frequency distributions of common species in upper, middle, and lower sectors of the gradient revealed deficient and poor recruitment in Quercus virginiana, Acer negundo, Celtis laevigata, and Salix nigra and episodic regeneration in Liquidambar styraciflua, Taxodium distichum, and Quercus nuttallii. Recruitment of the exotic species, Sapium sebiferum, is occurring at the low end of the gradient, as well as in canopy gaps throughout the gradient. Logistic regressions of sapling ( -10 cm dbh) size classes as a function of elevation showed that saplings of L. styraciflua, Q. nigra, and U. americana occur at higher elevations than do adult trees of the same species, evidence of the rate of hydrologic change in this forest. A fourth species, Acer rubrum, resprouts vigorously under rising water levels and may be an effective competitor with more light-demanding, flood-tolerant species at low elevations.

Late Quaternary glacial development of the mid-Norwegian margin—65 to 68°N

We have studied the younger part of the large Plio-Pleistocene prograding wedge on the northern part of the mid-Norwegian margin, using high-resolution seismic data. The wedge is mainly composed of thick glacigenic debris flow packages interbedded with hemipelagic and contouritic sediments. Age constraints of the seismic stratigraphy obtained through correlation to ODP 644A show that the Fennoscandian Ice Sheet advanced to the shelf break during Marine Isotope Stages (MIS) 2 (ca. 15–22 ka BP), 6 (128–186 ka BP), 10 (339–362 ka BP), 12 (423–478 ka BP) and 14 (524–565 ka BP), and to the inner shelf during MIS 8 (245–303 ka BP). The stratal geometry and sedimentary pattern locally indicate rapid subsidence (up to ca. 1.2 m/ka) of the outer shelf and slope area since 350 ka BP. The subsidence is most likely a response to the rapid deposition of glacigenic sediments during the Plio-Pleistocene. It is inferred that sediment loading was an important factor behind the diapirism in the Vema Dome area.

The South Caspian Basin: a review of its evolution from subsidence modelling

The basement surface of the South Caspian depression lies at a depth of 20–25 km, making it one of the deepest basins in the world. It occupies the southern, deep-water, part of the Caspian Sea and two adjacent lowlands: the West Turkmenia in the east and the Lower Kura in the west. The basin can be subdivided into several sub-basins with two main depocentres, one in the northern part of the basin, just on the southern flank of the Apsheron Sill, and one, called the Pre-Alborz trough, located in the south-eastern part of the marine basin. The sedimentary fill of the South Caspian Basin has been significantly deformed. Part of it is allochthonous and folded, overlying a ductile detachment zone within the Maikop shale (Oligocene–Early Miocene). The folded succession is unconformably overlain by Upper Pliocene–Quaternary neo-autochthonous sediments. An intense shortening event, related to the NNE–SSW convergence of the Arabian plate with Eurasia, affected the region during the Pliocene–Pleistocene. The thickness of Pliocene–Quaternary sediments alone reaches 10 km. They were deposited in a rapidly subsiding basin and were sourced from the surrounding Caucasus, Alborz, and Kopet-Dagh orogens as well as from the nearby Russian Platform. The thickness of the crust beneath the western central part of the basin is as little as 8 km in the western central part of the basin but exceeds 15 km in the eastern part. Geophysical data and gravimetric modelling provide evidence that the basement of the marine part of the basin comprises a high-velocity, thin complex crust. Subsidence of the basin is in part due to profound thinning of continental crustal or, more likely, to the formation of oceanic crust. This took place in Middle–Late Jurassic times, in the context of back-arc basin development, with possible reactivation during the Cretaceous. However, there remains controversy regarding the timing of oceanic accretion and deep-water deposition in the South Caspian Basin. The present results are based on subsidence analysis complemented by geological data from tectonic units surrounding the South Caspian Basin and on its margins. An additional mechanism, nevertheless, must be invoked to explain the younger, much more rapid Pliocene–Quaternary phase of subsidence that occurred simultaneously with the subsidence of Caucasus-related molasse basins and the uplift and erosion of the Caucasus Orogen. This rapid subsidence phase is probably of compressional origin and a simple elastic model in compression provides comparable amplitudes of subsidence. In addition, the South Caspian Basin is surrounded by orogenically loaded crust that adds to basin downwarping. To the north, the basin is bounded by a subduction zone.

From rifting to Alpine inversion: Mesozoic and Cenozoic subsidence history of some Moroccan basins

Present-day Morocco constitutes the junction between the Atlantic and the Mediterranean domains. Its past evolution was driven by the development and the ultimate closure of the Tethys ocean. In order to constrain the evolution of the subsidence of this western part of the Tethyan margin, five regions of Morocco have been modelled, the Essaouira and Missour basins, the Middle and High Atlas and the High Plateaux. The subsidence history of these regions during the Triassic–Jurassic is linked to the evolution of the Atlantic and Tethyan oceans, from rifting to oceanic accretion. The subsidence history of the Essaouira Basin is the most directly linked to the evolution of the Atlantic Ocean. This basin records strong subsidence from Trias to Liassic time as well as a lesser pulse of subsidence during the Malm. After this time, the subsidence rate in the Essaouira Basin dramatically decreased and its present-day architecture was already acquired in the Upper Cretaceous, except for local uplifts. The Atlas intramarginal-rifted basins show strong subsidence at the beginning of the Lias, and a further episode of rapid subsidence during the Dogger. As soon as the alpine cycle started, topography resulting from tectonic inversion of the Atlas basins modified the pattern of sedimentary distribution, and the products of erosion were deposited in the border basins such as Missour, High Plateaux, Souss and Ouarzazate. The data available to us did not allow us to pinpoint the timing of the Atlas uplift between the Cretaceous and the Present, and we found no arguments to support the most likely Late Cretaceous/Tertiary age for the beginning of this uplift. Analysis of subsidence in the Missour Basin and in the High Plateaux confirms their permanent structural high position during Mesozoic time with respect to the High and Middle Atlas domains, and shows that these basins were individualised only after inversion of the Atlas region.

The tectonic framework of a complex pull-apart basin: seismic reflection observations in the Sea of Galilee, Dead Sea transform

A multi-channel seismic reflection survey consisting of 20 lines with a total length of 180 km was conducted in the Sea of Galilee. The data provide new insights into the Pliocene–Quaternary evolution of the Kinarot–Beit–Shean pull-apart basin (KBSB) along the Dead Sea transform. Two distinct zones are defined beneath the lake: (1) a graben that underlies most of the lake, bounded by steep north–south longitudinal strike-slip faults and (2) shallow pre-rift units underlying the northwestern wider part of the lake. We suggest that before approximately 4 Ma, the KBSB grew due to northward movement of the Korazim Plateau and by crustal stretching along the rift axis. Since the Pliocene (∼4 Ma), lateral slip has been transferred from the southern segment of the basin's western marginal fault to normal faults in the Galilee, and to the eastern margin of the Korazim Plateau by the newly formed, Almagor fault, which makes a restraining bend along the transform. N–S lithospheric stretching below the KBSB has diminished and the Korazim Plateau has changed from being a detached block to a compressional saddle. A phase of rapid subsidence, and formation of a half-graben structure in the northern part of the basin approximately 1 Ma ago was coeval with major deformation in areas adjacent to the KBSB, indicating major reorganization of the plate boundary in the region. Currently, most transform motions are probably taken up along a single fault on the eastern side of the KBSB, implying that the main trough under the Sea of Galilee is in a late stage of growth as a pull-apart.