Geological evolution of two crustal scale shear zones. Part I. The Rand thrust complex, northwestern Mojave Desert, California. Part II. The Magdalena metamorphic core complex, north central Sonora, Mexico

The geology and structure of two crustal scale shear zones were studied to understand the partitioning of strain within intracontinental orogenic belts. Movement histories and regional tectonic implications are deduced from observational data. The two widely separated study areas bear the imprint of intense Late Mesozoic through Middle Cenozoic tectonic activity. A regional transition from Late Cretaceous-Early Tertiary plutonism, metamorphism, and shortening strain to Middle Tertiary extension and magmatism is preserved in each area, with contrasting environments and mechanisms. Compressional phases of this tectonic history are better displayed in the Rand Mountains, whereas younger extensional structures dominate rock fabrics in the Magdalena area.

In the northwestern Mojave desert, the Rand Thrust Complex reveals a stack of four distinctive tectonic plates offset along the Garlock Fault. The lowermost plate, Rand Schist, is composed of greenschist facies metagraywacke, metachert, and metabasalt. Rand Schist is structurally overlain by Johannesburg Gneiss (= garnet-amphibolite grade orthogneisses, marbles and quartzites), which in turn is overlain by a Late Cretaceous hornblende-biotite granodiorite. Biotite granite forms the fourth and highest plate. Initial assembly of the tectonic stack involved a Late Cretaceous? south or southwest vergent overthrusting event in which Johannesburg Gneiss was imbricated and attenuated between Rand Schist and hornblende-biotite granodiorite. Thrusting postdated metamorphism and deformation of the lower two plates in separate environments. A post-kinematic stock, the Late Cretaceous Randsburg Granodiorite, intrudes deep levels of the complex and contains xenoliths of both Rand Schist and mylonitized Johannesburg? gneiss. Minimum shortening implied by the map patterns is 20 kilometers.

Some low angle faults of the Rand Thrust Complex formed or were reactivated between Late Cretaceous and Early Miocene time. South-southwest directed mylonites derived from Johannesburg Gneiss are commonly overprinted by less penetrative north-northeast vergent structures. Available kinematic information at shallower structural levels indicates that late disturbance(s) culminated in northward transport of the uppermost plate. Persistence of brittle fabrics along certain structural horizons suggests a possible association of late movement(s) with regionally known detachment faults. The four plates were juxtaposed and significant intraplate movements had ceased prior to Early Miocene emplacement of rhyolite porphyry dikes.

In the Magdalena region of north central Sonora, components of a pre-Middle Cretaceous stratigraphy are used as strain markers in tracking the evolution of a long lived orogenic belt. Important elements of the tectonic history include: (1) Compression during the Late Cretaceous and Early Tertiary, accompanied by plutonism, metamorphism, and ductile strain at depth, and thrust driven? syntectonic sedimentation at the surface. (2) Middle Tertiary transition to crustal extension, initially recorded by intrusion of leucogranites, inflation of the previously shortened middle and upper crustal section, and surface volcanism. (3) Gravity induced development of a normal sense ductile shear zone at mid crustal levels, with eventual detachment and southwestward displacement of the upper crustal stratigraphy by Early Miocene time.

Elucidation of the metamorphic core complex evolution just described was facilitated by fortuitous preservation of a unique assemblage of rocks and structures. The "type" stratigraphy utilized for regional correlation and strain analysis includes a Jurassic volcanic arc assemblage overlain by an Upper Jurassic-Lower Cretaceous quartz pebble conglomerate, in turn overlain by marine strata with fossiliferous Aptian-Albian limestones. The Jurassic strata, comprised of (a) rhyolite porphyries interstratified with quartz arenites, (b) rhyolite cobble conglomerate, and (c) intrusive granite porphyries, are known to rest on Precambrian basement north and east of the study area. The quartz pebble conglomerate is correlated with the Glance Conglomerate of southeastern Arizona and northeastern Sonora. The marine sequence represents part of an isolated arm? of the Bisbee Basin.

Crosscutting structural relationships between the pre-Middle Cretaceous supracrustal section, younger plutons, and deformational fabrics allow the tectonic sequence to be determined. Earliest phases of a Late Cretaceous-Early Tertiary orogeny are marked by emplacement of the 78 ± 3 Ma Guacomea Granodiorite (U/Pb zircon, Anderson et al., 1980) as a sill into deep levels of the layered Jurassic series. Subsequent regional metamorphism and ductile strain is recorded by a penetrative schistosity and lineation, and east-west trending folds. These fabrics are intruded by post-kinematic Early Tertiary? two mica granites. At shallower crustal levels, the orogeny is represented by north directed thrust faulting, formation of a large intermontane basin, and development of a pronounced unconformity. A second important phase of ductile strain followed Middle Tertiary? emplacement of leucogranites as sills and northwest trending dikes into intermediate levels of the deformed section (surficial volcanism was also active during this transitional period to regional extension). Gravitational instabilities resulting from crustal swelling via intrusion and thermal expansion led to development of a ductile shear zone within the stratigraphic horizon occupied by a laterally extensive leucogranite sill. With continued extension, upper crustal brittle normal faults (detachment faults) enhanced the uplift and tectonic denudation of this mylonite zone, ultimately resulting in southwestward displacement of the upper crustal stratigraphy.

Strains associated with the two ductile deformation events have been successfully partitioned through a multifaceted analysis. R_f/Ø measurements on various markers from the "type" stratigraphy allow a gradient representing cumulative strain since Middle Cretaceous time to be determined. From this gradient, noncoaxial strains accrued since emplacement of the leucogranites may be removed. Irrotational components of the postleucogranite strain are measured from quartz grain shapes in deformed granites; rotational components (shear strains) are determined from S-C fabrics and from restoration of rotated dike and vein networks. Structural observations and strain data are compatable with a deformation path of: (1) coaxial strain (pure shear?), followed by (2) injection of leucogranites as dikes (perpendicular to the minimum principle stress) and sills (parallel to the minimum principle stress), then (3) southwest directed simple shear. Modeling the late strain gradient as a simple shear zone permits a minimum displacement of 10 kilometers on the Magdalena mylonite zone/detachment fault system. Removal of the Middle Tertiary noncoaxial strains yields a residual (or pre-existing) strain gradient representative of the Late Cretaceous-Early Tertiary deformation. Several partially destrained cross sections, restored to the time of leucogranite emplacement, illustrate the idea that the upper plate of the core complex bas been detached from a region of significant topographic relief. 50% to 100% bulk extension across a 50 kilometer wide corridor is demonstrated.

Late Cenozoic tectonics of the Magdalena region are dominated by Basin and Range style faulting. Northeast and north-northwest trending high angle normal faults have interacted to extend the crust in an east-west direction. Net extension for this period is minor (10% to 15%) in comparison to the Middle Tertiary detachment related extensional episode.

Littoral vegetation of Lake Tohopekaliga: community descriptions prior to a large-scale fisheries habitat-enhancement project

An extreme dry-down and muck-removal project was conducted at Lake Tohopekaliga, Florida, in 2003-2004, to remove dense vegetation from inshore areas and improve habitat degraded by stabilized water levels. Vegetation was monitored from June 2002 to December 2003, to describe the pre-existing communities in terms of composition and distribution along the environmental gradients. Three study areas (Treatment-Selection Sites) were designed to test the efficacy of different treatments in enhancing inshore habitat, and five other study areas (Whole-Lake Monitoring Sites) were designed to monitor the responses of the emergent littoral vegetation as a whole. Five general community types were identified within the study areas by recording aboveground biomasses and stem densities of each species. These communities were distributed along water and soils gradients, with water depth and bulk density explaining most of the variation. The shallowest depths were dominated by a combination of Eleocharis spp., Luziola fluitans, and Panicum repens; while the deeper areas had communities of Nymphaea odorata and Nuphar luteum; Typha spp.; or Paspalidium geminatum and Hydrilla verticillata. Mineralized soils were common in both the shallow and deep-water communities, while the intermediate depths had high percentages of organic material in the soil. These intermediate depths (occurring just above and just below low pool stage) were dominated by Pontederia cordata, the main species targeted by the habitat enhancement project. This emergent community occurred in nearly monocultural bands around the lake (from roughly 60–120 cm in depth at high pool stage) often having more diverse floating mats along the deep-water edge. The organic barrier these mats create is believed to impede access of sport fish to shallow-water spawning areas, while the overall low diversity of the community is evidence of its competitive nature in stabilized waters. With continued monitoring of these study areas long-term effects of the restoration project can be assessed and predictive models may be created to determine the efficacy and legitimacy of such projects in the future.

Avoided temperatures by young fish [Translation from: Informatsionnyi Byulleten Biologiya Vnutrennikh Vod No.50, 45-47, 1981]

Directed local changes of water temperature for the purpose of controlling the behaviour of fish are based on the knowledge of the characteristics of seasonal-age dynamics of their thermoadaptation possibilities. These possibilities are still inadequately studied especially in relation to avoided temperatures. By the authors the attempt was made to determine zones of avoided temperatures for the young of five species of fish (bream, roach, blue bream, perch, peled) in the summer period of the year, and also to assess the influence on them of additional factors, in particular mechanical driving. In parallel in two-fold repetition were conducted experiments on the determination of selected, shock and lethal temperatures of these fish. Experiments were conducted with fish, caught in the littoral of the Rybinsk reservoir.

Zooplankton - fish interaction in the littoral zone of Nyanza Gulf, Lake Victoria

The zooplankton community of the littoral zone of Nyanza Gulf, Lake Victoria, was studied between June 1998 and June 1999 to identify and quantify various zooplankton groups, and investigate the interactions that occur between them and the littoral fish through the food chain. Zooplankton samples were collected from five stations using a 83 micro-m mesh size plankton net hauled vertically through the water column. Fish samples were obtained by beach seine, except at Gingra (May 1999), where trawl samples were used. Gut/stomach analysis was carried out on the three major commercial species, Lates niloticus (L.), Oreochromis niloticus (L.) and Rastrineobola argentea (Pellegrin).

Extent and biomass of the aquatic chariphytes and mosses in Lake Sevan [Translation from: Rasprostranenie i biomassa kharovykh vodoroslei i mkha v os. Sevan, 1951]

This study focusses on the plants in the open parts of the lake - mostly aquatic charophytes and mosses, in what are called in Lake Sevan (Armenia), the ”zones of chara and moss”. Distribution and other ecological conditions are reviewed. Quantity of chara in the littoral zone of lake Sevan is provided

Bibliographie des lagunes et du littoral de Côte d'Ivoire (Afrique de l'Ouest)

The present bibliography collects references on lagoon and coastal environments in Côte d'Ivoire. It is mainly based on: - the draft bibliography prepared by Charles-Dominique and Durand in 1979, edited in the Archives Scientifiques du Centre de Recherches Océanographiques d'Abidjan (vol. 5 no. 2); - the synthesis on the marine environment, published in 1993 (LeLoeuff, Marchal and Amon-Kothias editors); - and the synthesis on the lagoon environment, published in 1994 (Durand, Dufour, Guiral and Zabi editors). In spite of a careful check of the available documents, it is more than possible that references are lacking or erroneous. That's why this bibliography is still a draft, and the author will be glad to receive complements and/or corrections from lectors. After these contributions, a more comprehensive version will be proposed. Remarks can be sent to this e-mail: arfi@ortsom.orstom.fr or to the postal address.

Contribution à l'étude des caractéristiques morphologiques de l'unité littorale de Côte d'Ivoire, Golfe de Guinée: cas du périmètre littoral de Port-Bouet

This paper presents a large scale (1/10000) bathymetric chart along with the beach (s.l.) and shoreface schemes of Port-Bouet littoral. All these charts and maps contributed to identify the three morphological sub-areas which characterize the whole littoral area of Port-Bouet.

Peuplements zooplanctoniques dans les régions de remontées d'eau du littoral atlantique africain

The introduction is followed by a resume of the biogeographic factors and the principal work. The characteristics of zooplankton in different regions are presented based on regular research in Santa Helena Bay and Walvis Bay and the research carried out by William Scoresby. Certain factors of the digestive system of South African plankton are discussed. The next section concerns research in intertropical and equatorial regions in the Gulf of Guinea. It considers the littoral region of Angola, the Pointe Noire region and discusses the density and complexity of stocks. The last section concerns the zooplankton of Nigeria, Ghana and the Ivory Coast and discusses the grouping of species and compares the results.

La pêche industrielle des poissons démersaux en Côte d'Ivoire. Généralités, localisation, homogéneité relative des zones de pêche ivoiriennes

This paper describes the fishery of Côte d'Ivoire and its evolution to the present day. The location of fishing off Côte d'Ivoire is specified for each area and each type of trawler. A comparison of productivity between the different areas shows disparities that allow to make clearer the repartition of effort of all types of trawlers in each area. The Côte d'Ivoire continental shelf is populated with demersal stocks of homogeneous densities.