Research Article| May 01, 1991 Southwest U.S.-East Antarctic (SWEAT) connection: A hypothesis E. M. Moores E. M. Moores 1Department of Geology, University of California, Davis, California 95616 Search for other works by this author on: GSW Google Scholar Author and Article Information E. M. Moores 1Department of Geology, University of California, Davis, California 95616 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1991) 19 (5): 425–428. https://doi.org/10.1130/0091-7613(1991)019<0425:SUSEAS>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation E. M. Moores; Southwest U.S.-East Antarctic (SWEAT) connection: A hypothesis. Geology 1991;; 19 (5): 425–428. doi: https://doi.org/10.1130/0091-7613(1991)019<0425:SUSEAS>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract A hypothesis for a late Precambrian fit of western North America with the Australia-Antarctic shield region permits the extension of many features through Antarctica and into other parts of Gondwana. Specifically, the Grenville orogen may extend around the coast of East Antarctica into India and Australia. The Wopmay orogen of northwest Canada may extend through eastern Australia into Antarctica and thence beneath the ice to connect with the Yavapai-Mazatzal orogens of the southwestern United States. The ophiolitic belt of the latter may extend into East Antarctica. Counterparts of the Precambrian-Paleozoic sedimentary rocks along the U.S. Cordilleran miogeocline may be present in the Transantarctic Mountains. Orogenic belt boundaries provide useful piercing points for Precambrian continental reconstructions. The model implies that Gondwana and Laurentia rifted away from each other on one margin and collided some 300 m.y. later on their opposite margins to form the Appalachians. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
ABSTRACT Ophiolite complexes represent fragments of ocean crust and mantle formed at spreading centers and emplaced on land. The setting of their origin, whether at mid-ocean ridges, back-arc basins, or forearc basins has been debated. Geochemical classification of many ophiolite extrusive rocks reflect an approach interpreting their tectonic environment as the same as rocks with similar compositions formed in various modern oceanic settings. This approach has pointed to the formation of many ophiolitic extrusive rocks in a supra-subduction zone (SSZ) environment. Paradoxically, structural and stratigraphic evidence suggests that many apparent SSZ-produced ophiolite complexes are more consistent with mid-ocean ridge settings. Compositions of lavas in the southeastern Indian Ocean resemble those of modern SSZ environments and SSZ ophiolites, although Indian Ocean lavas clearly formed in a mid-ocean ridge setting. These facts suggest that an interpretation of the tectonic environment of ophiolite formation based solely on their geochemistry may be unwarranted. New seismic images revealing extensive Mesozoic subduction zones beneath the southern Indian Ocean provide one mechanism to explain this apparent paradox. Cenozoic mid-ocean-ridge–derived ocean floor throughout the southern Indian Ocean apparently formed above former sites of subduction. Compositional remnants of previously subducted mantle in the upper mantle were involved in generation of mid-ocean ridge lavas. The concept of historical contingency may help resolve the ambiguity on understanding the environment of origin of ophiolites. Many ophiolites with “SSZ” compositions may have formed in a mid-ocean ridge setting such as the southeastern Indian Ocean.
The Jurassic Humboldt igneous complex in west-central Nevada consists of a comagmatic suite of intrusive and extrusive rocks and is tectonically intercalated with Triassic to Lower Jurassic shelf sequence and basinal successions. Its plutonic rocks include, from bottom to top, olivine-gabbro, melatroctolite, hornblende gabbro, microgabbro, and diorite transitional upward into quartz diorite, tonalite-granodiorite, and monzonite. Contacts between these plutonic subunits are commonly gradational, but mutual intrusive relations, characterized by the existence of brecciated and altered zones and xenoliths, are also common. Mafic to felsic plutonic rocks are cut by generally N–S to NW–SE striking dikes that form local dike swarms with one- and two-sided chilled margins. Dikes are composed of fine- to medium-grained rocks ranging in composition from basalts to andesites and feed into and/or are overlain by extrusive rocks consisting of lava flows intercalated with volcanic tuff and breccia. Lava flows at stratigraphically lower levels are more mafic and locally display pillow shapes reminiscent of submarine lava flows, whereas lava flows at higher levels are more felsic and are commonly interleaved with a fine-grained tuffaceous material. Volcanic rocks range in composition from basalts, basaltic andesites, andesites, to latites and dacites and mineralogically and texturally are similar to the dikes. The major element compositions of the analyzed rocks suggest relatively evolved basaltic magmas, whereas strongly incompatible trace element ratios (e.g., Ce/Ta) have high values typical of subduction related magmas. Lavas, dikes, and gabbros commonly display similar rare earth element (REE) patterns, although more felsic rocks are light rare earth (LREE) enriched, suggesting a cogenetic suite of rocks. These REE patterns are characteristic of basaltic andesites from volcanic arcs and suggest, coupled with field relations, that the rocks of the Humboldt complex might have evolved from subduction originated magmas in a volcanoplutonic arc setting. The tectonic nature of the contact between the Humboldt complex and the underlying Triassic-Jurassic sedimentary strata indicates that it was displaced eastward from its original arc environment following its igneous evolution. Both the Humboldt complex and the sedimentary strata are intruded at all structural levels by numerous northeast-striking dikes and dike swarms, which strongly altered and metasomatized their country rocks. These dike rocks are Miocene in age and have geochemical characteristics distinctly different from those of the Humboldt rocks.
Abstract The Eocene-Miocene Mesohellenic Trough is an elongate sediment-filled tectonic basin trending NW across central Greece and into Albania. Neotethyan oceanic rocks, including Triassic-Jurassic rift-related volcanic rocks and deep-sea sediments, accretionary mélange and ophiolitic complexes, crop out along its margins. These units were tectonically emplaced onto the Pelagonian microcontinent to the east and the Apulian-African continental margin to the west. In northern Greece, the mid-Jurassic Vourinos ophiolite on the eastern margin of the trough is geographically separated from the synchronous Pindos ophiolite along the western margin by a minimum c. 20 km distance. The sedimentary fill of the trough obscures their presumed subsurface continuation, although magnetic surveys identify thick ‘ophiolitic’ rocks beneath the basin. We interpret these ophiolites as parts of the same oceanic slab, two parts of a single larger oceanic complex we now term the Mesohellenic ophiolite. Comparable ophiolitic complexes to the south (the Koziakas and Othris) and the ophiolites of the Mirdita complex to the north in Albania are considered as members of this same complex. Geological and petrological data from the Vourinos and Pindos ophiolites define intra-slab heterogeneity. Vourinos essentially is a ‘Penrose-style’ ophiolite with ‘supra-subduction’ compositions; the less continuous Pindos ophiolite shows coexisting mid-ocean ridge basalt and island arc characteristics. Ophiolitic rocks that seem to represent geographical overlap between these characteristic associations crop out along their northern (Dotsikos strip) and southern (Mesovouni) margins. Variations in mantle strain conditions across the ophiolitic slab have been mapped, and demonstrate a single orientation of deformation; this is explained by variable strain kinematics that persisted across the ductile-brittle boundary. A continuity from ductile to brittle emplacement structures spans the Mesohellenic Trough, independent of petrological association, and indicates the original relative positions of these ophiolites within the oceanic slab. These structures illustrate tectonic ‘steps’ of obduction from the ridge crest onto the Pelagonian margin to the east, and can be relatively timed by the overlap of magmatism with ductile deformation in different parts of the slab. Hence, rotations of original horizontality are dated to the period preceding cessation of ductile field deformation, while still in the oceanic environment. The morphology defined by these structures and the horizontal rotation of stratigraphy are analogous to a spoon- or scoop-shaped nappe originating in the ductile field at its base, and crossing into the brittle field rapidly at its leading edge (Vourinos), whereas the mylonitic deformation characterizes the ‘trailing’ end (Pindos). Age relations require that geochemical variation between the two complexes must be explained within a model of synchronous generation, possibly with apparent ‘supra-subduction zone’ rifting of originally heterogeneous mantle, or an overlapping series of diverse processes of magma generation in an initially homogeneous mantle. Indications of the original ridge crest directions suggest the operation of several simultaneous spreading centres, separated by transform faults or ‘pseudofaults’. A palinspastic reconstruction of the slab constrains applicable oceanic models and provides the basis of future research.
Abstract The California Coast Ranges and Central Valley form a single transpressional “orogenic float,” with strike-slip and thrust wedging mainly decoupled from each other, but linked through a deep master decollement. In the Coast Ranges, imbricate thrusting has shuffled Mesozoic and Cenozoic rocks into a complex structural stack, which is also transected by strike-slip faults by the San Andreas system. In the northern Coast Ranges, thrusts and related ramp anticlines strike north-south to north-northwest-south-southeast, plunge south-southeast, and are cut locally by west-vergent thrusts. The amplitude of these structures diminishes eastward. Surface and subsurface data lead us to interpret the Sacramento Valley and Coast Range structures as a unified system in which strike-slip and thrust motions are largely decoupled. West-vergent thrusts are backthrusts from a master, east-vergent blind detachment that underlies the entire Coast Ranges and the western Sacramento Valley. The easternmost backthrusts in the Sacramento Valley rise from near the tip of the detachment, and together with it form a thrust wedge. Geophysical, radiometric, structural, and stratigraphic data suggest that this and similar wedges may have been active since at least the Paleocene. At present, the wedge is propagating eastward into the Sacramento Valley, forming rising anticlines above its ramps and backthrusts. Strike-slip faults of the San Andreas system also are detached at the level of themaster decollement, which therefore is an oblique-slip fault with strike-slip displacement increasing westward. Strike-slip movement is mainly decoupled from thrust movement, as shown by the general absence of high topography along the most active strike-slip strands.
The model is derived by equating the Troodos igneous massif of southern Cyprus with oceanic crust formed by sea-floor spreading. Comparison of the thicknesses and physical properties of the units of the Troodos massif with those deduced for the oceanic crust by seismic refraction experiments suggests the following correlation: layer 1—sediments; layer 2—pillow lavas and dikes, the lower part being predominantly dikes; layer 3—a layered plutonic complex of gabbros and minor diorites overlain by dikes; layer 4 (upper mantle)—pyroxenites, interpreted as accumulate phases of the gabbroic complex, grading downward into dunites and harzburgites thought to represent depleted mantle.
