Showing papers on "Terrane published in 1985"

Pan-African microplate accretion of the Arabian Shield

Abstract: The late Proterozoic Arabian Shield is composed of at least five geologically distinct terranes (microplates) separated by four ophiolite-bearing suture zones. Three ensimatic island-arc terranes occur in the western shield, whereas the two terranes in the eastern shield have continental affinities. The western two sutures are island-arc-island-arc joins, whereas the eastern two sutures collectively form a major coilisional orogenic belt. Accretion of the five terranes to form an Arabian neocraton occurred from 715 to 630 Ma. After accretion, intracratonic deformation and magmatism related to collision continued and resulted in the formation of molasse, intermediate to silicic volcanic rocks, peralka-line to peraluminous granites (640-570 Ma), and a major left-lateral wrench fault system (∼630-550 Ma) which displaced the northern part of the Arabian neocraton ∼250 km to the northwest. These tectonic events represent the accretion of the Arabian portion of Gondwanaland during the Pan-African event.

Pericollisional Strike Slip Faults and Synorogenic Basins, Canadian Cordillera

Abstract: During large scale convergence of oceanic and arc type lithospheric fragments towards a cratonic promontory along western North America from Middle Jurassic through Paleogene time non subductable crust of the approaching Pacific realm was deflected dextrally northward or sinistrally southward from this reverse indenter in the California Nevada region Paleontologic and paleomag netic data suggest oblique dextral displacements on the order of 1 500 to 2 000 km for the accreted terranes in the western Cordillera of Canada These dextral displacements were first concentrated along closing sutures from Middle Jurassic to Early Cretaceous time later they were also taken up by peri collisional fault zones which propagated into the western parts of the Cordilleran thrust belt and involved the Coast Plutonic Complex mid Cretaceous to Paleocene A subduction in the thrust belt and inferred B subduction west of the Coast Plutonic Complex were thus accompanied by dextral displacements within the Omineca and Coast fault arrays respectively imparting northwest directed stretching fabrics onto ductile metamorphic or igneous rocks and discrete fault strands on high level crustal rocks The convergent strike slip fault motions in the Canadian Cordillera created mainly sedimentary source areas rather than subsiding basins Pericollisional basins that did receive clastic materials from zones of oblique convergence were I marginal basins in the process of closing 2 relict or tectonically overloaded depressions on accreting terranes 3 foreland basins created by thrust prop agation in the miogeoclinal succession and 4 small pull apart or restraining bend depressions near high angle strike slip faults Basins in the accreted terrane complexes west of the Cordilleran thrust belt received most of their detrital material from exposed volcanic plutonic and oceanic sedimentary rocks the predominantly turbiditic basin fill suffered repeated deformation high sustained heat flow and intrusive activity The foreland basin to the east of the thrust belt on the other hand received most of its detrital input from carbonate and quartz rich clastic rocks of the miogeocline and metamorphosed equivalents the predominantly shallow water clastic deposits of the foreland basin experienced considerably less deformation and thennal alteration than the varied sedimentary assemblages of the accreted belt

Rapid production of continental crust 1.7 to 1.9 b.y. ago: Nd isotopic evidence from the basement of the North American mid-continent

Abstract: Nd isotopic analyses of Precambrian granitic rocks from the central United States were made to determine crust-formation ages and thereby to investigate the structure and evolution of central North America. Samples of anorogenic granites that have 1.4-b.y. crystallization ages yield model TDM ages of 1.77 to 1.98 b.y. that indicate a previously undocumented 0.4- to 0.6-b.y.-old precrystallization crustal history. These and previously published data approximately define the boundaries of a large segment of continental crust that was formed from the mantle during a relatively short interval ∼1.9 b.y. ago. Both 1.4- and 1.8-b.y.-old samples from the Penokean terrene of central Wisconsin yield distinctly older TDM model ages of 2.1 to 2.3 b.y. Approximately 1.1-b.y.-old samples from the Llano uplift in Texas yield crust-formation ages of 1.3 to 1.4 b.y. The data appear to delineate provinces with well-defined, nonoverlapping crust-formation ages that young to the south, indicating episodic formation of the North American continental crust during Proterozoic time. The initial 143Nd/144Nd ratios of the 1.4-b.y.-old granites are varied ( ϵ Nd = +4.8 to −2.0). Most of the samples analyzed probably represent crustal melts, but the existence of some high initial ϵ Nd values indicates an admixture of mantle-derived material. The isotopic patterns suggest that the 1.4-b.y.-old “anorogenic” plutonism may be an inland manifestation of subduction activity related to formation of Llano crust. The data also indicate that the Penokean terrane is not entirely 1.8-b.y.-old, mantle-derived material, nor simply reworked Archean crust. It may represent a mixture of Archean and juvenile mantle material added during the 1.84 Ga Penokean orogeny. Patterns of continental growth deduced from this study emphasize the episodicity of crust formation. Construction of an apparent crustal growth curve for North America indicates that the average age of the continent is ∼2.2 b.y., which is significantly older than the average age of ∼ 1.5 b.y. derived from Rb-Sr studies.

Origin and tectonic evolution of the Maclaren and Wrangellia terranes, eastern Alaska Range, Alaska

Abstract: Major portions of the eastern Alaska Range, south of the Denali fault, in the McCarthy, Nabesna, Mount Hayes, and eastern Healy quadrangles, consist predominantly of the Maclaren and Wrangellia tectono-stratigraphic terranes. The Maclaren terrane consists of the Maclaren Glacier metamorphic belt and the regionally deformed and metamorphosed East Susitna batholith. The Maclaren Glacier metamorphic belt is composed of argillite, metagraywacke, and sparse andesite flows that are progressively regionally metamorphosed from lower greenschist facies to middle amphibolite facies near the East Susitna batholith. The East Susitna batholith is composed of gabbro, quartz diorite, granodiorite, and sparse quartz monzonite. Isotopic ages are as old as a K-Ar hornblende age of 87.5 m.y., possibly reset, and a U-Pb zircon age of 70 m.y. The batholith is intensely deformed and regionally metamorphosed under conditions of the middle amphibolite facies. The Wrangellia terrane is divided into two subterranes: (1) the Slana River subterrane, composed of late Paleozoic andesite to dacite flows, tuff, limestone, and argillite, unconformably overlying massive basalt flows of the Triassic Nikolai Greenstone, Late Triassic limestone, and younger Mesozoic flysch; and (2) the Tangle subterrane, a deeper-water equivalent of the Slana River subterrane, composed of late Paleozoic and Early Triassic aquagene tuff, chert, minor andesite tuff and flows, limestone, unconformably overlying pillow basalt and massive basalt flows of the Triassic Nikolai Greenstone, and Late Triassic limestone. Both subterranes are intruded by locally extensive gabbro and diabase dikes and by cumulate mafic and ultramafic sills. Less extensive terranes (two) are the Clearwater terrane, a sequence of intensely deformed chlorite schist, muscovite schist, marble, and greenstone of Late Triassic age; and an unnamed terrane of ultramafic and associated rocks of presumable Paleozoic or Mesozoic age. Each terrane or subterrane generally has (1) a distinctive time-stratigraphic sequence reflecting a unique geologic history; (2) a missing provenance for bedded sedimentary or volcanic rocks; and (3) bounding thrust or strike-slip faults, interpreted as accretionary sutures. The Maclaren and Wrangellia terranes are juxtaposed along the Broxson Gulch thrust, which consists of an imbricate series of north-dipping thrust faults. Paralleling the Broxson Gulch thrust, a few kilometres to the south, is the north-dipping Eureka Creek thrust, along which are juxtaposed the Slana River and Tangle subterranes. The Maclaren terrane is correlated with the Kluane Schist and the Ruby Range batholith in the southern Yukon Territory, which represent the northward extension of the Taku and Tracy Arm terranes. If correct, this correlation defines a minimum displacement of the Maclaren terrane along the Denali fault of ∼400 km. The Maclaren terrane is interpreted to have formed in a synorogenic Andean-type arc setting on the west margin of Mesozoic North America in the middle to late Mesozoic and early Cenozoic. The Wrangellia terrane is interpreted to have initially formed in an island-arc setting during the late Paleozoic. Subsequently in the Late Triassic, the Wrangellia terrane underwent rifting near the paleoequator, with formation of the Nikolai Greenstone and associated mafic and ultra-mafic igneous rocks. In the middle and late Mesozoic, Wrangellia migrated toward, and was accreted during, the middle Cretaceous to the Maclaren terrane along the Broxson Gulch thrust. Subsequent dispersion of both the Maclaren and Wrangellia terranes along the Denali fault and the Broxson Gulch thrust commenced during the early Tertiary and continues through the present.

Strontium and oxygen isotopic variations in Mesozoic and Tertiary plutons of central Idaho

Abstract: Regional variations in initial 87Sr/86Sr ratios (r i) of Mesozoic plutons in central Idaho locate the edge of Precambrian continental crust at the boundary between the late Paleozoic-Mesozoic accreted terranes and Precambrian sialic crust in western Idaho. The r i values increase abruptly but continuously from less than 0.704 in the accreted terranes to greater than 0.708 across a narrow, 5 to 15 km zone, characterized by elongate, lens-shaped, highly deformed plutons and schistose metasedimentary and metavolcanic units. The chemical and petrologic character of the plutons changes concomitantly from ocean-arc-type, diorite-tonalite-trondhjemite units to a weakly peraluminous, calcic to calcalkalic tonalite-granodiorite-granite suite (the Idaho batholith). Plutons in both suites yield Late Cretaceous ages, but Permian through Early Cretaceous bodies are confined to the accreted terranes and early Tertiary intrusions are restricted to areas underlain by Precambrian crust. The two major terranes were juxtaposed between 75 and 130 m.y. ago, probably between 80 and 95 m.y. Oxygen and strontium isotopic ratios and Rb and Sr concentrations of the plutonic rocks document a significant upper-crustal contribution to the magmas that intrude Precambrian crust. Magmas intruding the arc terranes were derived from the upper mantle/subducted oceanic lithosphere and may have been modified by anatexis of earlier island-arc volcanic and sedimentary units. Plutons near the edge of Precambrian sialic crust represent simple mixtures of the Precambrian wall-rocks with melts derived from the upper mantle or subducted oceanic lithosphere with r i of 0.7035. Rb/Sr varies linearly with r i, producing “pseudoisochrons” with apparent “ages” close to the age of the wall rocks. Measured δ 18O values of the wall rocks are less than those required for the assimilated end-member by Sr-O covariation in the plutons, however, indicating that wall-rock δ 18O was reduced significantly by exchange with circulating fluids. Metasedimentary rocks of the Belt Supergroup are similarly affected near the batholith, documenting a systematic depletion in 18O as much as 50 km from the margin of the batholith. Plutons of the Bitterroot lobe of the Idaho batholith are remote from the accreted terranes and represent mixtures of Precambrian wall-rocks with melts dominated by continental lower crust (r i>0.708) rather than mantle. “Pseudoisochrons” resulting from these data are actually mixing lines that yield apparent “ages” less than the true age of the wall rocks and meaningless “ri”. Assimilation/ fractional-crystallization models permit only insignificant amounts of crystal fractionation during anatexis and mixing for the majority of plutons of the region.

Sedimentation and tectonics in the Scottish Dalradian

Abstract: Synopsis Sedimentological research on the Scottish Dalradian has progressed from the recognition of sedimentary structures in the 1930s, via the identification of sedimentary facies from the 1950s onwards, to the integration, in the 1970s, of sedimentological data with that from studies of stratigraphy, tectonics and volcanism. This has now led to an understanding of the pre-orogenic evolution of the Dalradian terrane in terms of progressive lithospheric stretching associated with the break-up of the Proterozoic Supercontinent. The Appin and Argyll Groups were deposited on the NW side of a late Precambrian marine gulf which developed over a complex zone of crustal thinning between the Laurentian and Baltic parts of the Super-continent. As extension accelerated, subsidence rates increased and the Dalradian area of the gulf evolved from a relatively shallow shelf into a series of turbidite basins. Thinning of the lithosphere gave rise, in Argyll Group times, to locally intense igneous activity. Subsequently, complete continental rupture along the gulf axis led to the birth of the Iapetus Ocean. By Southern Highland Group times the Dalradian terrane had become part of the new, thermally-subsiding, Laurentian continental margin. One can envisage the geometry and facies variations of many horizons within the Dalradian in terms of a pattern of numerous fault blocks defined by listric normal faults, dipping SE towards the site of continental rupture, and NW–SE trending transfer faults, which divided the gulf and subsequent margin into a series of compartments. It was movements on these faults that largely controlled Dalradian stratigraphic evolution. For example, pulses of rapid stretching, and consequent fault activity, produced basin-deepening sequences which mark the base of the Easdale and Crinan Subgroups.

Proterozoic Anorthosite Massifs

Abstract: Major anorthositic massifs intruded stable cratonic crust in the North Atlantic region about 1.4 to 1.7 Ga ago. Similar (meta-) anorthosite massifs (0.9–1.6 Ga) in Grenville and Sveconorwegian terranes exhibit varying degrees of deformational and metamorphic overprint and consequently have ambiguous tectonic settings. Close correspondence of rock and mineral associations in both types points to common origins. Mineral assemblages, chemical, and isotopic data for pristine and meta-anorthosite massifs strongly imply varying degrees of interaction between mantle-generated magmas and deep continental crust occurred prior to final emplacement. Olivine-bearing massifs consistently show less evidence of crustal assimilation than olivine-free massifs. Parent magmas of anorthosite massifs were LREE-enriched with moderately high Fe/Mg, consistent with fractionated basic magmas or their derivatives. Recently reported Nd and Sr isotopic relationships imply that the parent magmas were not simply partial melts of undepleted subcontinental mantle. Appropriate dry, plagioclase-saturated or -supersaturated magmas can be generated on a large scale only within a depth interval ~35–50 km, embracing the lowermost continental crust and upper mantle. Fractionation of pyroxenes ±olivine was an important factor in determining the chemistry of parent magmas and in decreasing their densities prior to intrusion into the continental crust. The high level intrusions of Fennoscandia and the East European platform are not obviously related to crustal rifting. Widespread non-orogenic granitic suites, including rapakivi types, span a similar time interval and provide supporting evidence for local heat sources perhaps related to subcratonic mafic magma ponding. This unique period of geological history may mark a transition to Phanerozoic tectonic regimes in which large scale magmatism became increasingly confined to plate margins and intracontinental rift zones.

Geochemical evidence for the tectonic setting of the Coast Range ophiolite: A composite island arc–oceanic crust terrane in western California

Abstract: The Middle to Late Jurassic age Coast Range ophiolite (CRO) of California contains two geochemically distinct volcanic rock associations that formed in different tectonic settings. Volcanic rocks from the southern CRO (Point Sal, Cuesta Ridge, Stanley Mountain, Llanada, Quinto Creek, and Del Puerto) and parts of the northern CRO (Healdsburg, Elder Creek) are similar to low-K tholeiites and calc-alkaline rocks of the island-arc suite. The thin volcanic sections of these ophiolite remnants suggest formation by intra-arc rifting. In contrast, volcanic rocks from Stonyford seamount and Paskenta in the northern CRO are transitional subalkaline metabasalts with geochemical characteristics similar to enriched mid-ocean ridge basalts or ocean-island tholeiites. These rocks are associated with Tithonian radiolarian cherts and may be part of the Franciscan Complex. Alternatively, they may represent a change in tectonic setting within the CRO during the Late Jurassic. Regardless, the CRO as currently conceived cannot be considered a single terrane with one mode of origin.

Evolution of the Archean Continental Crust

Abstract: Some 20 years of plate tectonic theory, combined with new insights into the fine structure of the lithosphere, the application of multielement geo­ chemical and isotopic studies, paleomagnetism, and geophysical modeling of mantle processes, have profoundly influenced present thinking on the origin and evolution of the Earth's early continental crust; previously, our knowledge of the continental crust was based almost exclusively on field geological observations. Although there is now general agreement on how the Earth worked for the last 200 m.y. because of observable evidence in the oceans and continents (e.g. Bird 1980, Condie 1982), it has proved difficult to extend this history into more ancient times in view of the lost oceanic record and the ambiguity and complexity of the pre-Mesozoic rock relationships in the continents (Dewey 1982). However, preserved characteristic rock as­ semblages uniquely identifying modern-type Wilson-cycle processes (i.e. opening and closure of oceans underlain by oceanic crust) have now been recognized in continental terranes as old as �900 m.y., and these assemblages provide strong evidence for the conclusion that the present global tectonic regime has governed the evolution of the lithosphere at least since the late Precambrian (Kroner 1977, 1981a,b; Goodwin 1981). Profound disagreement on the older crustal history, however, prevails to the present day, since typical features characterizing Phanerozoic accretionary terranes (such as obducted ophiolites, blueschists, and Franciscan-type melanges) have not been found in more ancient regions. Thus, two types of evolutionary models have been developed. One type postulates uniformitarian development back to the earliest Archean (Burke

Cenozoic collision of the Lesser Antilles Arc and continental South America and the origin of the El Pilar Fault

Abstract: It is proposed that the Cenozoic tectonic record of the southern Lesser Antilles arc and northeastern continental South America can be explained by ongoing right-oblique collision between the arc and continent. The collision has proceeded by the transport of the leading edge of the arc across the slope and outer shelf of a former north facing passive margin of the South American continent. The overriding began in the study region near the Gulf of Cariaco in eastern Venezuela in late Eocene or Oligocene time and has migrated with a generally SE vector. Suturing has occurred between the arc and continent after the attainment of a critical distance of overlap; today's point of suturing lies in the Paria Peninsula. East of there, overriding continues. Major tectonic elements engaged in or created by the collision are the southern Lesser Antilles magmatic arc, forearc basin, the Araya-Tobago terrane, a South American foreland thrust and fold belt, and a foreland basin. The Araya-Tobago terrane is thought to consist of sediments of South American provenance that were accreted to the Lesser Antilles forearc during its transit of an ocean basin and the continental slope and outer shelf. The emplacement of the magmatic arc and the Araya-Tobago terrane caused tectonic imbrication of shelf strata to propagate ahead of the arc front as a foreland thrust and fold belt. Tectonic loading of the shelf also caused subsidence of a major foreland basin on the continentward side of the thrust belt. It is proposed the El Pilar fault exists between the Gulf of Cariaco and the Paria Peninsula as an active right slip fault but not east of Paria. It is not a throughgoing transform fault between the South American and Caribbean plates. The El Pilar fault exists where the overlapping arc and the continent are sutured and takes up a suture-parallel component of convergence between arc and continent. The eastern tip of the fault propagates east with the point of suturing. Reconstructions of the Cenozoic collision of the Lesser Antilles arc and the South American continent suggest that the arc lay somewhat north and west of its present position in the Eocene. This conclusion differs from that of plate reconstructions that assume that the arc was the leading edge of a Caribbean plate that has moved east from Pacific longitudes since the Eocene.

Mesozoic paleomagnetism and northward translation of the Baja California Peninsula

Abstract: Paleomagnetic data from Cretaceous igneous and sedimentary rocks of the Peninsular Ranges and Continental Borderland of the Baja California Peninsula yield an average pole position of 83°N, 344°E (A95 = 3.4°), suggesting that the peninsula as a whole has moved northward ∼11° and rotated 32° clockwise, with respect to the North American craton, since Cretaceous time. Only a small portion of this movement is due to the Neogene opening of the Gulf of California. Results from stratified rocks giving both positive and negative fold tests are included in the average Cretaceous pole. The rocks that fail fold tests have been deformed and remagnetized by nearby intrusives that are only slightly younger; unconnected directions from these sites closely match corrected directions from sites where the fold tests are positive. Oceanic rocks were also sampled in the Continental Borderland from Upper Triassic and Upper Jurassic tectonostratigraphic terranes which are apparently allochthonous with respect to the western Baja Peninsula and each other. These oceanic rocks show magnetizations indicating low paleolatitudes, although the hemisphere of origin is presently unknown. The Cretaceous rocks of the Baja California Peninsula may have originally formed adjacent to the now-truncated continental margin along the northern Middle America Trench. Northward transport of the peninsula probably occurred by transcurrent faulting associated with oblique subduction of the Farallon plate beneath western North America between Late Cretaceous and late Miocene time.

Middle Oligocene oceanic crust of South China Sea jammed into Mindoro collision zone (Philippines)

Abstract: Mindoro Island, south of Luzon (Philippines), is a complex junction between the Manila Trench and the collision zone of the North Palawan block with the western Philippines mobile belt. Middle Oligocene ophiolites recognized in this suture are coeval with the oldest magnetic anomalies of the South China Sea basin. These ophiolites are part of a complex pile of terranes thrust above the North Palawan block at the lower–middle Miocene boundary. These ophiolites are interpreted as fragments of South China Sea oceanic crust jammed between two distinct continental blocks during the counterclockwise rotation of Luzon.

The Scottish paratectonic Caledonides

Abstract: Synopsis The southern margin of the Laurasian continent has been destructive or strike-slip from at least 576 Ma to c. 360 Ma. There may have been a pre- or early Arenig obduction of ophiolite onto it, and this ophiolite now provides the base to the Ordovician part of the Highland Border Complex and could have been responsible for regional metamorphism in Dalradian or related rocks. At c. 480 Ma there was certainly final obduction of the Ballantrae Complex, which formed in an oceanic setting some distance south of its present location. This ophiolite was initially obducted over olistostromes and formed the basement to a later fore-arc basin, the proximal part of which is now exposed at Girvan. The Midland Valley, a Palaeozoic plutonic—volcanic arc, founded on a metamorphic basement, supplied sediments to the fore-arc—accretionary prism on its southern margin. This metamorphic basement is unlikely to have been of local Dalradian type. Growth of the paratectonic zone involved thrusting and probably strike-slip accretion of possibly 6 terranes. Ophiolites and the accretionary prism of the Southern Uplands were thrust NW onto Midland Valley basement. The whole of the paratectonic zone was initially juxtaposed against the Moine—Dalradian terrane by either strike-slip or thrust movements, but the latest interaction between the Midland Valley and Dalradian terranes involved thrusting. Future investigations into the timing and extent of thrust and strike-slip activity will greatly increase our understanding of continental growth in this region.

Cretaceous sedimentation and tectonics, Tyaughton–Methow Basin, southwestern British Columbia

Abstract: The Mesozoic Tyaughton–Methow Basin straddles the Fraser–Yalakom–Pasayten – Straight Creek (FYPSC) strike-slip fault zone between six tectono-stratigraphic terranes in southwestern British Columbia. Data from Hauterivian–Cenomanian basin fill provide constraints for reconstruction of fault displacement and paleogeography.The Early Cretaceous eastern margin of the basin was a region of uplifted Jurassic plutons and active intermediate volcanism. Detritus shed southwestward from that margin was deposited as the marine Jackass Mountain Group. Albian inner to mid-fan facies of the Jackass Mountain Group can be correlated across the Yalakom Fault, suggesting 150 ± 25 km of post- Albian dextral offset. Deposits of the Jackass Mountain Group overlap the major strike- slip zone (FYPSC). If that zone represents the eastern boundary of the tectono-stratigraphic terrane, Wrangellia, then accretion of Wrangellia to terranes to the east occurred before late Early Cretaceous time.The western margin of the basin first b...

Importance of thrust faulting in the tectonic development of northern Victoria Land, Antarctica

Abstract: The present configuration of Wilson Group, Bowers Supergroup and Robertson Bay Group in northern Victoria Land (Fig. 1) has been attributed to high-angle block-faulting1–3 or more recently4 to major strike-slip faulting along the Lanterman and Leap Year Fault zones. In this latter interpretation4, adjacent units have allochthonous relationships and constitute separate geological terranes. We offer additional support for the concept of allochthonous terranes in northern Victoria Land but suggest here an alternative model to explain terrane juxtapositions. Our model emphasizes the role of thrust tectonics and implies severe structural telescoping of entire terranes with significant loss of intervening crust. We further suggest that this occurred during the Lower Palaeozoic Ross Orogeny in response to the collision of Antarctica with a westwardly directed eastern continental block. This requires that the northern Victoria Land segment of Gondwana was a zone of major plate convergence throughout the Cambrian–early Ordovician period.

Evolution of the Yukon-Tanana terrane: Evidence from southeastern Yukon Territory

Abstract: The Yukon-Tanana terrane (YTT) in southeastern Yukon consists predominantly of a mid-Paleozoic volcanic-plutonic (arc?) assemblage built on continental crust. The YTT had experienced strong deformation and metamorphism by Late Triassic time. By mid-Cretaceous the metamorphic complex and Late Triassic clastic rocks derived from it were imbricated with middle and upper Paleozoic “ophiolitic” sheets. The YTT differs markedly from adjacent parts of the North American continental margin in both its depositional and tectonic histories. The composition and tectonic history of the YTT is comparable to that of the Kootenay and Barkerville terranes of southern and central British Columbia from which the YTT may have been displaced.

New COCORP profiling in the southeastern United States. Part II: Brunswick and east coast magnetic anomalies, opening of the north-central Atlantic Ocean

Abstract: New COCORP profiles on the coastal plain of Georgia and northern Florida support the hypothesis that the Brunswick anomaly marks a late Paleozoic suture. They do not support the alternate view that this anomaly is caused by a Mesozoic rift basin. The trend of the Brunswick anomaly relative to the Appalachian gravity gradient indicates that in westernmost Georgia and adjacent Alabama, African basement (Suwannee terrane) is in proximity to autochthonous North American basement (Grenville). Farther east one or more Paleozoic accreted terranes intervene between the North American and African sides of the orogen. Offshore, the Brunswick anomaly closely parallels the east coast magnetic anomaly. This relationship implies that the east coast magnetic anomaly marks not only the present continental/oceanic crustal transition but also the northward continuation of the late Paleozoic suture between North America and Africa. Transitional crust beneath the Carolina Trough, Baltimore Canyon trough, and correlative parts of the African Atlantic margin is thus likely to have formed within a preexisting suture zone. The dramatic change in character of the U.S. Atlantic margin southward from the Carolina Trough to the Blake Plateau probably reflects the fact that two different types of prerift crust are juxtaposed in this region.

Accretion tectonism in the Proterozoic Svecokarelides of the Baltic Shield

Abstract: The duality of early middle Proterozoic terranes in the Baltic Shield reflects fundamental differences in history between coeval mobile belts and the manner of their interaction. The Karelides formed on a continent-ocean transcurrent margin in which offsets developed as arcs and back-arc basins on stretched continental crust. At least one back-arc basin was obducted onto the continental margin, preserved as an allochthonous terrane in the Karelian nappe pile. Remnant arc fragments form accreted exotic terrane in Pohjanmaa. The Svecofennides developed as a succession of island arcs, while the site of subduction moved south, and successive extinct arcs were accreted onto the Archean craton. Arc terrane impingement affected deformation style in the Karelides, which changed from one of major horizontal translation to one dominated by major dextral wrench faults. Analogies can be drawn with the Phanerozoic evolution of the northeast Pacific margin.

Geology of the South Mountains, Central Arizona

Abstract: The South Mountains of central Arizona are a typical, but geologically simple, metamorphic core complex. The western half of the range is underlain by Precambrian metamorphic and granitic rocks, whereas the eastern half is primarily a composite middle Tertiary pluton. The Precambrian terrane is composed of two rock units, Estrella Gneiss and Komatke Granite, both of which were involved in a major episode of middle Proterozoic metamorphism and deformation that produced a steep crystalloblastic foliation. The composite Tertiary pluton has three semicontemporaneous phases: (1) South Mountains Granodiorite, the oldest and most widely distributed phase; (2) Telegraph pass Granite; and (3) Dobbins Alaskite. Both the precambrian terrane and the composite Tertiary pluton have been intruded by numerous, north-northwest-trending, middle Tertiary dikes.

Petrology and tectonic significance of augen gneiss from a belt of Mississippian granitoids in the Yukon-Tanana terrane, east-central Alaska

Abstract: An approximately east-west-trending belt of porphyritic peraluminous granitoids, metamorphosed and deformed to augen gneiss, is exposed for 400 km across the Yukon-Tanana terrane. This belt consists of compositionally, texturally, and isotopically similar, concordant intrusions of augen gneiss in the Big Delta, Eagle, and Tanacross quadrangles of east-central Alaska, in the Fiftymile batholith of west-central Yukon Territory, and in two batholiths in the offset part of the Yukon-Tanana terrane northeast of the Tintina fault in southeastern Yukon Territory. Chemical analyses of augen gneiss from widely separated localities within the Big Delta intrusion suggest that the various samples are related by crystal fractionation. Comparison of major- and trace-element data from concordant layers of augen-poor gneiss and aplitic gneiss with similar data from adjacent augen gneiss suggests that the layers represent co-magmatic sills of more highly differentiated filter-pressed melt. Similarities between elemental abundances, particularly those of rare-earth elements, in augen gneiss from the large bodies in the Big Delta, Eagle, and Tanacross quadrangles, Alaska, and southeastern Yukon Territory suggest a common origin for these bodies. The peraluminous composition of the gneiss, its high initial 87 Sr/ 86 Sr ratio and high Th concentration indicate that the augen gneiss protolith contains a large component of crustal material. Geochronologic studies of augen gneiss in east-central Alaska and southeastern Yukon Territory indicate a Mississippian intrusive age and an early Proterozoic age for the crustal component. The augen gneiss bodies were intruded late in a middle Paleozoic volcanic-plutonic episode, and they may represent a deeply eroded continental magmatic arc. Regional amphibolite-facies metamorphism and mylonitization may have occurred late during the tectonic episode that resulted in intrusion of the porphyritic granitoid protoliths.

Review of radiometric data from the Yukon Crystalline Terrane, Alaska and Yukon Territory

Abstract: The results of more than 20 years of geochronological studies in the Yukon Crystalline Terrane in east-central Alaska and the western Yukon Territory suggest at least six igneous and thermal (metamorphic?) events. Plutonism during Mississippian, Early Jurassic, mid-Cretaceous, Late Cretaceous, and early Tertiary times is indicated. Evidence also indicates that Mississippian, Early Jurassic, late Early Cretaceous, and late Cretaceous thermal (metamorphic?) events have affected parts of the terrane. The western part of the terrane was affected by a significant regional metamorphic event in late Early Cretaceous time, followed by a terrane-wide mid-Cretaceous plutonic event. The pattern of K–Ar ages allows division of the terrane into domains, bounded by northeast-trending lineaments.

Paleomagnetism of the Dunn Point Formation (Nova Scotia): High paleolatitudes for the Avalon Terrane in the Late Ordovician

Abstract: Volcanic flows of the Late Ordovi- clan Dunn Point Formation contain a generally univectorial magnetization which passes the fold test. Its steep direction (D = 335 o , I = -61 o , k = 79, 95 = 4.2o, paleopole at 2oS, 136oE) is un- like any known for North America or for the Ava- lon terrane for post-Ordovician time, so that a primary age of the magnetization appears to be very likely. The paleoiatitude (42oS) for the Avalon terrane derived from this result is much higher than that predicted for the area on the basis of the cratonic North American apparent polar wander path, and a substantial post-Ordo- vician displacement (>3500 km) of Avalon with respect to the craton can be deduced. In all likelihood the Avalon terrane did not collide with North America until Middle Devonian time. This collision produced the Acadian orogeny. High Ordovician paleolatitudes have also been obtained for northern Africa and for several localities in

The ophiolitic Ingalls Complex, north-central Cascade Mountains, Washington

Abstract: The Ingalls Complex is the largest and most complete of several Middle to Late Jurassic ophiolites in northwestern Washington. It structurally overlies the Skagit metamorphic core of the North Cascades and is intruded by the Late Cretaceous Mount Stuart Batholith. The last truncates major structures of the ophiolite and divides it into two outcrop areas, a main area south of the batholith and a smaller area in a roof pendant. In the main area, the internal structure of the ophiolite is dominated by two wide fault zones characterized by steeply dipping serpentinite melange. Displacement probably was a combination of strike slip and dip slip, with the latter apparently related to diapiric rise of serpentinite. Deformation probably occurred in an oceanic transform fault zone. Ultramafic tectonites predominate in the main outcrop area, including from south to north: (1) harzburgites and dunites; (2) mylonitic iherzolites and hornblende peridotites; and (3) Iherzolites. Unit 2 represents a high-temperature shear zone that may record early movement in the deeper levels of the postulated oceanic fracture zone. Mafites generally occur as tectonic blocks; in the southern fault zone, they underwent ocean-floor-type metamorphism, but to the north, metagabbros experienced dynamothermal amphibolite-facies metamorphism prior to incorporation into the serpentinite melange. Gabbros in the southern fault zone locally intrude ultramafites and in places pass upward into a diabase dike complex. Gabbros and diabases intrude partly pillowed basaltic flows and pillow breccias, with interbeds of argillite, chert, and ophiolite-derived sedimentary breccia. Another sequence of basalt, argillite, chert, and minor limestone is penetratively deformed and may be exotic. In the pendant on the Mount Stuart Batholith, the north-directed Windy Pass thrust (WPT) places the Ingalls on the pelitic Chiwaukum Schist of the Skagit metamorphic core. The Ingalls is imbricated with amphibolite-facies metasedimentary and metaplutonic slices that may be a part of the Skagit core not exposed in the lower plate. Imbrication was synchronous with amphibolite-facies metamorphism of the ophiolite in the pendant, but the postmetamorphic sole thrust (WPT) truncates both the imbricate slices and fabric elements in the Chiwaukum Schist. Thrusting was mid-Cretaceous, apparently postdating high-angle faulting within the main portion of the Ingalls. The WPT marks a major Cretaceous boundary between an oceanic and a metamorphic terrane (Skagit core), and its upper plate represents the only rocks preserved above the Skagit core. The Ingalls is similar in age to ophiolites in California, Oregon, and British Columbia, as well as those in northwestern Washington. The California and Oregon ophiolites (Coast Range, Smartville, Josephine) apparently originated in small, short-lived marginal basins, whereas the Ingalls was deformed in a fracture zone in a wider, longer-lived marginal basin or large ocean.

Implications of magmatic epidote-bearing plutons on crystal evolution in the accreted terranes of northwestern North America

Abstract: Magmatic epidote in a plutonic rock signifies that the rock probably crystallized at pressures of at least 8 kbar (25–30 km depth). In western North America, undeformed Late Cretaceous plutons containing magmatic epidote occur within a mobile belt extending from northern California to southeastern Alaska; this belt has been interpreted as consisting of accreted terranes. Because the present crustal thickness within this belt is about 30 km, the plutons imply a crustal thickness at the time of emplacement of 55–60 km, comparable to that in the central Andes. The plutons further imply paleogeothermal gradients in the host country rock at the time of intrusion of not more than 20 °C/km and may be closer to 12 °C/km. Such cool, thick crust suggests that subduction and piling of thrust sheets were probably the principal processes of crustal accretion in western North America. The implications regarding the depth of crystallization of the plutons can be used to estimate the minimum rate of regional uplift; it is on the order of 1 mm/yr. The rapid uplift of the mobile belt by and large did not affect the craton.