Ordovician collision of the Argentine Precordillera with Gondwana, independent of Laurentian Taconic orogeny

The Argentine Precordillera, a rifted fragment of Laurentian crust and sedimentary cover, collided with Gondwana in Middle Ordovician time; the time of collision (Ocloyic orogeny) is similar to that of the Taconic orogeny of eastern Laurentia. Three hypotheses have been proposed to explain Ordovician docking of the Precordillera with western Gondwana: (A) the Precordillera microcontinent was rifted from Laurentia in Cambrian time and, following solitary drift, collided with Gondwana, independent of the Laurentian Taconic orogeny; (B) a continent–continent collision of Laurentia with Gondwana, producing a continuous Taconic–Ocloyic orogenic belt, was followed by rifting that left the Precordillera attached to Gondwana; and (C) the Precordillera at the tip of a distal plateau on greatly stretched Laurentian crust collided with Gondwana and subsequently separated from Laurentia.Contrasts in several aspects of Taconic and Ocloyic orogenic history provide for discrimination between the microcontinent and continent–continent-collision hypotheses. Stratigraphic gradients and lithologic assemblages within the synorogenic clastic wedges are incompatible with a single continuous orogenic belt, which, in palinspastic location, places the thin, fine-grained southern fringe of the Taconic clastic wedge adjacent to the thickest and coarsest part of the Ocloyic clastic wedge. Separate temporal and spatial distribution patterns of volcanic ash (bentonite) beds in Laurentia and the Precordillera indicate originally separate dispersal systems. Late Ordovician Hirnantian Gondwanan glacial deposits in the Precordillera indicate substantial latitudinal separation from Laurentia. Post-collision faults with large vertical separation in the Precordillera have no coeval counterparts on the Laurentian foreland. These contrasts indicate originally separate (not initially continuous, and subsequently dismembered) orogenic belts, favoring the microcontinent hypothesis and eliminating the continent–continent-collision hypothesis.Initial Taconic tectonic loading near the southern corner of the Alabama promontory of Laurentia and the lack of post-Taconic extension there are inconsistent with the tectonic history required by the plateau hypothesis, but are consistent with the tectonic history required by the microcontinent hypothesis. Paleobiogeography, distribution of bentonite beds, and the Hirnantian glacial deposits, all indicate wide separation (Iapetus Ocean) between the Precordillera and southern Laurentia at the time of the Ocloyic and Taconic orogenies, further favoring the microcontinent hypothesis.     

Regional Seas Towards Sustainable Development

Reports

Regional Seas Towards Sustainable Development     

西秦岭勉略带陆内构造变形研究

秦岭造山带勉略缝合带是古特提斯洋盆向北俯冲形成的华北与华南最后拼接带。这个主缝合带俯冲-碰撞过程中以由北向南的一系列韧性逆冲推覆构造为特征,形成由前泥盆系、泥盆-石炭系和蛇绿混杂岩等不同构造岩片叠置的复杂构造带,碰撞时代从245Ma一直延续到230Ma左右。最近,作者对勉略缝合带内发育的韧性和脆性左行走滑剪切变形进行了研究,结果表明这些顺造山带的左行韧性走滑剪切变形带的变形时代为223±2Ma,与碰撞后花岗岩所确定的碰撞后构造环境的起始时间(225Ma)一致,显示这些韧性走滑剪切变形带是勉略带陆内变形初期变形产物。亦即华北、扬子大陆碰撞之后很快就转入陆内变形阶段,并且是以顺造山带的侧向走滑位移为主要变形方式。勉略带内顺造山带的脆性左行走滑断层的发育,表明这种顺造山带的侧向位移过程从深部到地壳浅层是一致的。因此,大陆碰撞在直接碰撞之后很快转变为顺造山带的侧向走滑位移为主的陆内变形,这种位移可能表现为两个大陆碰撞后的相对走滑,或是碰撞带中强烈变形部分顺造山带的侧向挤出,从而消减了正向碰撞所造成的地壳缩短和增厚。     

内蒙古鸡冠山斑岩钼矿床成矿时代和成矿流体研究

内蒙古鸡冠山钼矿床是西拉沐伦钼成矿带上的典型斑岩矿床。矿床产于火山侵入杂岩中,矿化类型以细脉浸染状矿化为主。对矿床5件辉钼矿样品进行了铼-锇同位素分析,获得了151.1±1.3Ma的等时线年龄,表明成矿作用发生在晚侏罗世。成矿作用可划分为三个阶段:早阶段为石英-黄铁矿阶段,发育乳白色石英和粗粒浸染状黄铁矿;中阶段包括早期石英-多金属硫化物亚阶段和晚期石英-萤石-金属硫化物亚阶段;晚阶段为石英-碳酸盐细脉,穿切早、中阶段脉体和矿物组合。鸡冠山钼矿床流体包裹体岩相学研究表明,与成矿有关的包裹体主要有六种类型:富液相、富气相、含子晶多相、含CO2三相、纯CO2及纯液相包裹体。其中,早阶段以富气和富CO2包裹体为主,中阶段多种包裹体共存,晚阶段则主要为富液包裹体。冷热台显微测温和激光拉曼显微探针(LRM)成分分析结果表明,早阶段石英中原生包裹体的均一温度480℃,盐度最高66.75%NaCleqv,包裹体气相成分富含水和CO2,液相成分则以水为主,子晶矿物有石盐、黄铜矿以及指示氧化条件的赤铁矿等,同时也说明成矿流体是富含成矿金属元素的。中阶段早期石英中的流体包裹体均一温度为320～480℃,晚期石英和萤石中的流体包裹体的均一温度为180～320℃。中阶段流体盐度介于4.65%～56.76%NaCleqv。中阶段包裹体含石盐、方解石、黄铜矿、赤铁矿等子矿物,富气相、富液相与含子晶多相包裹体共存,且具有相近的均一温度,而盐度相差悬殊,指示流体发生了沸腾。晚阶段流体的温度降低至100～180℃,盐度则低于10.86%NaCleqv,流体包裹体成分主要为水。鸡冠山钼矿成矿流体演化从早至晚为:从早阶段高温、高盐度、高氧逸度、富CO2、富成矿物质以岩浆热液为主成矿流体,演化至晚期低温、低盐度、无子晶、贫CO2、以大气降水为主的流体。沸腾作用是鸡冠山钼矿形成的重要机制。     

Fault-proximal stratigraphic record of episodic extension and oblique inversion, Bisbee basin, southwestern New Mexico, USA

Tectonic inversion models predict that stratigraphic thickening and local facies patterns adjacent to reactivated fault systems should record at least two phases of basin development: (1) initial extension‐related subsidence and (2) subsequent shortening‐induced uplift. In the central Peloncillo Mountains of southwestern New Mexico, thickness trends, distribution, and provenance of two major stratigraphic intervals on opposite sides of a northwest‐striking reverse fault preserve a record of Early Cretaceous normal displacement and latest Cretaceous–Paleogene reverse displacement along the fault. The Aptian–Albian Bisbee Group thickens by a factor of three from the footwall to the hanging‐wall block, and the Late Cretaceous?–Eocene Bobcat Hill Formation is preserved only in the footwall block. An initial episode of normal faulting resulted in thickening of upper Aptian–middle Albian, mixed siliciclastic and carbonate deposits and an up section change from coarse‐grained deltas to shallow‐marine depositional conditions. A second episode of normal faulting caused abrupt thickening of upper Albian, quartzose coastal‐plain deposits across the fault. These faulting episodes record two events of extension that affected the northern rift shoulder of the Bisbee basin. The third faulting episode was oblique‐slip, reverse reactivation of the fault and other related, former normal faults. Alluvial and pyroclastic deposits of the Bobcat Hill Formation record inversion of the Bisbee basin and development of an intermontane basin directly adjacent to the former rift basin. Inversion was coeval with latest Cretaceous–Paleogene shortening and magmatism. This offset history offers significant insight into extensional basin tectonics in the Early Cretaceous and permits rejection of models of long‐term Mesozoic shortening and orogen migration during the Cretaceous. This paper also illustrates how episodes of fault reactivation modify, in very short distances (<10 km), regional patterns of subsidence, the distribution of sediment‐source areas, and sedimentary depositional systems.     

Distinguishing fault reactivation from flexural deformation in the distal stratigraphy of the Peripheral Blountian Foreland Basin, southern Appalachians, USA

Reactivation of intraplate structures and weak zones within the foreland lithosphere disrupt the modelled geometry and pattern of migration of the flexural wave in foreland basins. In the southern Appalachians (USA), the Middle Ordovician unconformity, irregular Middle Ordovician distal foreland deposition and backstepping of Middle–lower Upper Ordovician carbonate strata have been related to migration of the flexural wave. However, integration of stratigraphy, tectonic subsidence history and composition of palinspastically restored distal foreland strata, using a map of subsurface basement structures as reference, allows us to distinguish an early event of inversion from two events of flexural migration. Sections restoring at very short distances outside the boundaries of a former basement graben have the youngest passive‐margin strata preserved beneath Middle Ordovician (～466 Ma) peritidal to deep lagoonal carbonates with gravel‐size chert clasts. In contrast, sections restoring inside the graben record >470 m of truncation of pre‐Middle Ordovician passive‐margin strata, late onset of deposition (～456 Ma), and subaerial features in carbonate and siliciclastic strata. The lacuna geometry and early patterns of distal foreland uplift and carbonate deposition indicate that inversion of a basement graben in response to Middle Ordovician convergence, rather than a migrating or semi‐fixed forebulge, was the primary control on the early evolution of the distal foreland. Drowning of the carbonate platform in more proximal settings, northeastward onset of deposition on upthrown blocks, and thick accumulation of carbonates in downthrown blocks record northwestward and northeastward flexural wave migration at the Middle–Late Ordovician boundary. In early Late Ordovician, the overall shoaling of carbonate and siliciclastic depocentres and the rise of tectonic subsidence curves indicate hinterlandward migration of flexural uplift. Both events of flexural migration were accompanied by influx of volcanic ash and synorogenic sediments.     

Fingerprinting petroleum hydrocarbons in plankton and surface sediments during the spring and early summer blooms in the Galician coast (NW Spain) after the Prestige oil spill

Plankton samples (20-350 microm and >350 microm) collected at three transects along the Galician coast (NW Spain) were analysed for individual aliphatic and aromatic hydrocarbons by GC-MS. Sample collection was performed in April-July 2003, after the Prestige oil spill (November 2002), to determine whether the hydrocarbons released into the water column as a consequence of the spill were accumulated by the planktonic communities during the subsequent spring and early summer blooms. Surface sediments were also collected to assess the presence of the spilled oil, removed from the water column by downward particle transport. Plankton concentrations of PAHs (Sigma14 parent components) were in the range of 25-898 ng g(-1)dw, the highest values being close to coastal urban areas. However, the individual distributions were highly dominated by alkyl naphthalenes and phenanthrenes, paralleling those in the water dissolved fraction. The detailed study of petrogenic molecular markers (e.g. steranes and triterpanes, and methyl phenanthrenes and dibenzothiophenes) showed the occurrence of background petrogenic pollution but not related with the Prestige oil, with the possible exception of the station off Costa da Morte in May 2003, heavily oiled after the accident. The dominant northerly wind conditions during the spring and early summer 2003, which prevented the arrival of fresh oil spilled from the wreck, together with the heavy nature of the fuel oil, which was barely dispersed in seawater, and the large variability of planktonic cycles, could be the factors hiding the acute accumulation of the spilled hydrocarbons. Then, with the above exception, the concentrations of PAHs found in the collected samples, mostly deriving from chronic pollution, can be considered as the reference values for the region.     

Evaluation of anthropogenic and biogenic inputs into the western Mediterranean using molecular markers

Aliphatic hydrocarbons (AHs), sterols (ST), and lipid classes were determined in suspended particles collected in the Catalan Sea, northwestern Mediterranean. Principal Component Analysis of the data set revealed a clustering of samples depending on the sources of organic matter, i.e., coastal influenced, open sea and frontal zone. Terrestrial inputs were recognized in particles collected in the surface waters, at the vicinities of river outflow (i.e., Rhône and Ebro), by a predominance of C₂₉ and C₃₁ n-alkanes, 24-ethylcholest-5-en-3β-ol (S12), and the anthropogenic 5β(H)-cholestan-3β-ol, coprostanol (S1). Phytoplanktonic molecular markers (n-C₁₇, 27-nor-24-methylcholesta-5,22E-dien-3β-ol, cholesta-5,22-dien-3β-ol and 24-methylcholest-5-en-3β-ol) were widespread but relatively more apparent in the open sea and frontal zones. A similar distribution was observed for lipid classes, with higher concentrations of phospholipids, and an enrichment in free fatty acids in the areas influenced by river discharges. Total sterol, the unresolved complex mixture of hydrocarbons and the pristane–phytane ratio were highest at a persistent frontal zone, possibly reflecting the combination of a higher primary productivity and fossil hydrocarbon absorption on to suspended particles. Moreover, vertical profiles exhibited a subsurface concentration maximum at 20–30 m water depth, concurrently with the chlorophyll.     

Comparison of sampling devices for the determination of polychlorinated biphenyls in the sea surface microlayer

Over 30 sea surface microlayer (SML) samples from two contrasting sites in the North Western Mediterranean -- Barcelona (Spain) and Banyuls-sur-Mer (France) -- were collected using three different sampling devices, namely, glass plate, metal screen (MS) and a surface slick sampler (SS), and compared with the corresponding underlying water (16 samples). The distributions of 41 polychlorinated biphenyl congeners (PCBs) were determined in the different phases: particulate (1.17-10.8 SigmaPCB ng L(-1)), truly dissolved (0.080-16.7 SigmaPCB ng L(-1)) and colloidal matter (1.17-43.0 SigmaPCB ng L(-1)), being the last two estimated from the analysis of the apparently dissolved phase. Concentrations of PCBs in the SML were higher than those in the underlying water (ULW), giving rise to enrichment factors (EF=[C](SML)/[C](ULW)) up to first-order of magnitude. The ANOVA statistical approach was used to assess differences of bulk data (e.g. dissolved organic carbon, DOC; particulate organic carbon, POC; suspended particulate matter, SPM) among sampling devices, whilst p-tailed t paired tests were used in order to compare the enrichments obtained for each sampling date. In this respect, no significantly different enrichment factors were found among sampling devices (p < 0.05), although the surface SS showed lower enrichments, probably due to the dilution of the SML with the ULW during sampling. The MS seemed to be the most suitable device for the determination of PCBs in the SML in terms of sampling efficiency under a variety of meteorological conditions.