Academic literature on the topic 'Eastern Cordillera boundary'

Abstract The Internal-External Zone boundary (IEZB) in the eastern Betic Cordillera partially coincides with the Cadiz-Alicante Accident, mainly a major transcurrent fault of N060E direction. The study of an area located along the IEZB at a point where it separates from the Cadiz-Alicante accident has provided details concerning the geodynamic evolution of the cordillera at the moment of its structuration. Here the Internal Zone, consists of rocks assigned to the Malaguide Complex, dating its last sedimentation to the Aquitanian, and deposits assigned to the Vinuela Group (early-middle Burdigalian). The nappes of the Internal Zone were emplaced during the latest Aquitanian and the Vinuela Group (here the El Nino Formation) sealed it but was afterwards affected by the collision with the External Zone. On the other side of the boundary, the External Zone comprises two tectonic units: the Penarrubia Unit (late Cretaceous-middle Burdigalian), which is made up mainly of limestones and marls, and the El Frances Chaotic Complex composed by a set of different lithologies, all from the External Zone in a marly matrix that could be interpreted as a collisional melange formed in the early-middle Burdigalian. The contact between the two domains corresponds to a backthrust of the External Zone over the Internal Zone which occurred in the middle Burdigalian. The deposits sealing the IEZB are dated by calcareous nannofossils and planktonic foraminifera as late Burdigalian, and comprise clasts from both domains.

The boundary between the external zones (Rocky Mountains) and the internal zones of the eastern Canadian Cordillera is marked by a Tertiary half graben, the Rocky Mountains Trench (RMT). In the south Cordillera, east of the Shuswap metamorphic complex, the fault limiting the trench is superimposed on an early major thrust, the Late Jurassic Purceli thrust. On approaching this discontinuity, the ductile deformation of the Miette Group, a detrital Precambrian suite, is characterized by a subvertical foliation and a subhorizontal stretching lineation parallel to the fold axes. The deformation intensity, its noncoaxial characters, and its geographic extension are interpreted as resulting from a dextral crustal shear, parallel to the mapped trace of the Purcell thrust and RMT. The dextral slip is deduced from a microtectonic analysis of the observed rotational criteria and is consistent with the small angle occurring between the directions of the linear structure (stretching lineations and fold axes) and those of adjacent discontinuities. The Middle Cretaceous age (100–78 Ma) attributed to this deformation is based on the age of syn- to late-tectonic metamorphic minerals as dated by the 39Ar–40Ar method. A kinematic model involving vectorial decomposition of an oblique convergence is proposed, suggesting the simultaneous occurrence, in the Middle Cretaceous, of two suborthogonal conjugated movement directions, respectively parallel and normal to the general Cordilleran trend. [Journal Translation]

The transition from the crust to the mantle beneath the Canadian portion of the North American Cordillera varies in depth, geometry, and tectonic age across the orogen. These variations are rarely spatially related to the positions of morphologic or tectonic belts based on surface geology, nor to nearly 25 km of structural relief identified in outcrop and on seismic reflection data. The Moho in this region is thus interpreted to be a long-lived feature, perhaps as old as Proterozoic in the eastern part of the Cordillera, that probably has been active as a structural boundary during periods of crustal contraction and subsequent crustal stretching. Recognition of the Moho and lower crust as a zone of localized tectonic activity provides a partial explanation for the problem of where regional detachments that underlie the foreland thrust and fold belt go as they project westward to deep structural levels beneath the interior of the orogen: they likely project to the base of the crust, where they flatten and cause imbrication of crustal rocks.

Abstract. All major mountain ranges are assumed to have been subject to increased uplifting processes during the late Miocene and Pliocene. Previous work has demonstrated that African uplift is an important element to explain Benguela upper-ocean cooling in the late Miocene–Pliocene. According to proxy records, a surface ocean cooling also occurred in other eastern boundary upwelling regions during the late Neogene. Here we investigate a set of sensitivity experiments altering topography in major mountain regions (Andes, North American Cordillera, and southern and East African mountains) separately with regard to the potential impact on the intensity of near-coastal low-level winds, Ekman transport and Ekman pumping, and upper-ocean cooling. The simulations show that mountain uplift is important for upper-ocean temperature evolution in the area of eastern boundary currents. The impact is primarily on the atmospheric circulation which is then acting on upper-ocean temperatures through changes in strengths of upwelling, horizontal heat advection and surface heat fluxes. Different atmosphere–ocean feedbacks additionally alter the sea surface temperature response to uplift. The relative importance of the different feedback mechanisms depends on the region, but it is most likely also influenced by model and model resolution.

Seismic refraction data recorded along a 330 km cross-strike profile through the eastern Insular and southernmost Coast belts of the Canadian Cordillera are interpreted using an iterative combination of traveltime inversion and amplitude forward modelling. The resultant model is characterized by large lateral variations in velocity. The most significant of these variations is a decrease in upper and middle crustal velocities to the east of the surface trace of the Harrison fault, which likely represents the transition from crust of the Insular superterrane to that of the Intermontane superterrane. This interpretation is consistent with some present geological models that place the possible (probable) location of the suture between the two superterranes less than 20 km east of the Harrison fault. Velocities at the base of the upper crust average 6.4 and 6.2 km/s west and east of the fault, respectively. Mid-crustal velocities average 6.6–6.9 km/s to the west and 6.35–6.45 km/s to the east of the fault. Lower crustal velocities also decrease slightly to the east. Other features of the velocity model include (i) a thin near-surface layer with velocities between 2.5 and 6.1 km/s; (ii) upper crustal thickness of 12.5 km, thinning to 8 km at the eastern boundary of the Western Coast Belt (WCB); (iii) high velocity (6.6–6.9 km/s) mid-crustal layer west of the Harrison fault extending to 21 km depth; (iv) high-velocity (6.75–7.1 km/s) lower crustal layer; (v) low-velocity gradient upper mantle with depth to Moho at 34–37 km beneath most of the Coast Belt, decreasing to 30 km beneath the eastern Insular Belt, a depth much less than previous estimates. The inferred crustal velocity structure beneath the WCB is consistent with the three-layer electrical conductivity structure for this area derived from magnetotelluric surveys. The association of high resistivities with the upper crust suggests that the upper 8–12 km represents the massive cover of plutonic rocks which characterizes the WCB. Middle and lower crustal velocities beneath the WCB are consistent with Wrangellian velocities found beneath Vancouver Island, suggesting Wrangellia may extend at depth eastward as far as the Harrison fault.

The fault-bounded Cianzo basin represents a Cenozoic intermontane depocenter between the Puna plateau and Eastern Cordillera of the central Andean fold-thrust belt in northern Argentina. New characterizations of fold-thrust structure, nonmarine sedimentation, and sediment provenance for the shortening-induced Cianzo basin at 23°S help constrain the origin, interconnectedness, and subsequent uplift and exhumation of the basin, which may serve as an analogue for other intermontane hinterland basins in the Andes. Structural mapping of the Cianzo basin reveals SW and NE-plunging synclines within the >6000 m-thick, upsection coarsening Cenozoic clastic succession in the shared footwall of the N-striking, E-directed Cianzo thrust fault and transverse, NE-striking Hornocal fault. Growth stratal relationships within upper Miocene levels of the succession indicate syncontractional sedimentation directly adjacent to the Hornocal fault. Measured stratigraphic sections and clastic sedimentary lithofacies of Cenozoic basin-fill deposits show upsection changes from (1) a distal fluvial system recorded by vi fine-grained, paleosol-rich, heavily bioturbated sandstones and mudstones (Paleocene‒Eocene Santa Bárbara Subgroup, ~400 m), to (2) a braided fluvial system represented by cross-stratified sandstones and interbedded mudstones with 0.3 to 8 m upsection-fining sequences (Upper Eocene–Oligocene Casa Grande Formation, ~1400 m), to (3) a distributary fluvial system in the distal sectors of a distributary fluvial megafan represented by structureless sheetflood sandstones, stratified pebble conglomerates and sandstones, and interbedded overbank mudstones (Miocene Río Grande Formation, ~3300 m), to (4) a proximal alluvial fan system with thick conglomerates interbedded with thin discontinuous sandstone lenses (upper Miocene Pisungo Formation, ~1600 m). New 40Ar/39Ar geochronological results for five interbedded volcanic tuffs indicate distributary fluvial deposition of the uppermost Río Grande Formation from 16.31 ± 0.6 Ma to 9.69 ± 0.05 Ma. Sandstone petrographic results show distinct upsection trends in lithic and feldspar content in the Casa Grande, Río Grande, and Pisungo formations, potentially distinguishing western magmatic arc (Western Cordillera) sediment sources from evolving eastern thrust-belt sources (Puna‒Eastern Cordillera). In addition to growth stratal relationships and 40Ar/39Ar constraints, conglomerate clast compositions reflect distinct lithologic differences, constraining the activation of the Cianzo thrust and coeval movement on the reactivated Hornocal fault. Finally, U-Pb geochronological analyses of sandstone detrital zircon populations in conjunction with paleocurrent data and depositional facies patterns help distinguish localized sources from more distal sources west of the basin, revealing a systematic eastward advance of Eocene to Miocene fold-thrust deformation in the central Andes of northern Argentina.<br>text

Regional variations in South America’s weather and climate reflect the atmospheric circulation over the continent and adjacent oceans, involving mean climatic conditions and regular cycles, as well as their variability on timescales ranging from less than a few months to longer than a year. Rather than surveying mean climatic conditions and variability over different parts of South America, as provided by Schwerdtfeger and Landsberg (1976) and Hobbs et al. (1998), this chapter presents a physical understanding of the atmospheric phenomena and precipitation patterns that explain the continent’s weather and climate. These atmospheric phenomena are strongly affected by the topographic features and vegetation patterns over the continent, as well as by the slowly varying boundary conditions provided by the adjacent oceans. The diverse patterns of weather, climate, and climatic variability over South America, including tropical, subtropical, and midlatitude features, arise from the long meridional span of the continent, from north of the equator south to 55°S. The Andes cordillera, running continuously along the west coast of the continent, reaches elevations in excess of 4 km from the equator to about 40°S and, therefore, represents a formidable obstacle for tropospheric flow. As shown later, the Andes not only acts as a “climatic wall” with dry conditions to the west and moist conditions to the east in the subtropics (the pattern is reversed in midlatitudes), but it also fosters tropical-extratropical interactions, especially along its eastern side. The Brazilian plateau also tends to block the low-level circulation over subtropical South America. Another important feature is the large area of continental landmass at low latitudes (10°N–20°S), conducive to the development of intense convective activity that supports the world’s largest rain forest in the Amazon basin. The El Niño–Southern Oscillation phenomenon, rooted in the ocean-atmosphere system of the tropical Pacific, has a direct strong influence over most of tropical and subtropical South America. Similarly, sea surface temperature anomalies over the Atlantic Ocean have a profound impact on the climate and weather along the eastern coast of the continent. In this section we describe the long-term annual and monthly mean fields of several meteorological variables.

There are large climatic differences among the boreal regions of the world. The extreme continental climates of central Siberia, with a mean annual temperature of –11°C or colder and precipitation of only 150 mm, for example, contrasts strikingly with the semicoastal climate of Newfoundland, with a mean annual temperature of +5°C and precipitation of 1400 mm. Yet both are considered boreal. This wide range in mean annual temperatures translates into large variation in the soil thermal conditions. Although much of the northern region of the boreal forest is underlain by continuous and discontinuous permafrost, southern regions are entirely permafrost-free. Boreal Canada has been classified into four major ecoclimatic provinces (Ecoregions Working Group 1989). The Subarctic Ecoclimatic Province extends from treeline in northern Canada south to the border with continuous stands of closed spruce. It ranges from the highly continental areas of northern Yukon Territory to the wetter and somewhat warmer regions of the Labrador Peninsula. The Boreal Ecoclimatic Province includes the main body of the boreal forests of Canada from the Mackenzie River east to Newfoundland. It is a complicated province that has been divided into High, Mid-, and Low Boreal, with a wide range of climate conditions. The Subarctic Cordilleran Ecoclimatic Province occurs only at higher elevations in western Canada. Forested areas in this region are usually restricted to valley bottoms or low, south-facing slopes. The Cordilleran Ecoclimatic Province includes the mountain ranges along the west coast and the continental divide from Montana to Alaska and from the Yukon River south to the boundary with the coastal forests. The boreal portion of this province has climates similar to that of the eastern section of the Interior Highland Ecoregion of Alaska (Fig. 2.3, Gallant et al. 1995). Alaska does not fit well into these Canadian ecoclimatic provinces because of differences in elevation, the effects of the two east-west-oriented mountain ranges (the Alaska and Brooks Ranges), and the coastal influences of the Bering Sea to the west and Cook Inlet to the south (Fig. 1.1; Hopkins 1959, Hare and Ritchie 1972). Hammond and Yarie (1996) separated Alaska into 35 ecoclimatic regions, of which nine include areas of boreal forest.