Result Of Subduction Research Articles

Detailed look at final stage of plate break‐off is target of study in Romania

Geophysical experiments next year in Romania may provide insight into a common but short‐lived seismic process that can be observed and understood at only one spot on Earth at present. About 150 stations will be set up in the Vrancea area in the southeast Carpathian Mountains to, in effect, record the terminal phase of the detachment of a subducting slab of oceanic lithosphere. This is a major regional tomographic study using a large number of broadband seismometers, which will operate for 6 months. Images will be used for hazard assessment as well as for a delineation of detachment history.Active subduction of oceanic lithosphere at convergent plate boundaries involves earthquakes, magmatism, metamorphism, and deformation—some of the most vivid manifestations of any plate tectonic process. The initiation and termination of subduction, however, remains relatively poorly understood. When convergence of lithospheric plates ceases and the suction force of the subducting plate becomes negligible, the subducting slab moves into an almost vertical position. If subduction occurs in an arcuate geometry, the slab is likely to be segmented.

The tectono-metamorphic evolution of gneiss complexes in the Middle Urals, Russia: a reappraisal

The Middle Urals are characterized by a major virgation in the linear trend of the Urals orogen, and represent the most highly contracted part of the late Palaeozoic collisional belt. This part of the orogen is dominated by metamorphic complexes and major fault and shear zones. The Main Uralian Fault zone (MUF), the east-dipping suture of the orogen containing low-grade metamorphic rocks, separates the Sysert Complex in the east from the Ufaley Complex in the west. The Sysert Complex in the hanging wall of the MUF consists of intensely deformed gneisses, granitic intrusions and a metamorphosed mélange zone. Tectonic and isotopic investigations suggest the following stages for the evolution of the Sysert Complex: (a) pre-orogenic rifting and magmatism during Ordovician and Silurian times; (b) oceanic closure, island arc formation related to convergence and subduction during Devonian times; (c) major ductile deformation under amphibolite facies conditions related to NW-directed thrusting associated with crustal stacking during collision in Carboniferous times; (d) exhumation and contractional intracontinental tectonics during Permian times; and (e) closing of isotope systems related to cooling and the end of orogenic shortening through Triassic times. The Ufaley Complex, in the footwall of the MUF, is interpreted as an east-dipping crustal stack that records an amphibolite facies Uralian metamorphism. Lithologically the complex can be divided into pre-orogenic European basement (West Ufaley) and intensely deformed Palaeozoic metasediments and amphibolites (East Ufaley). High-pressure relics in the East Ufaley Complex are interpreted to be the result of subduction, whereas intense ductile deformation is related to overthrusting onto West Ufaley. The West Ufaley Complex is composed of gneisses, amphibolites, migmatites and granitic intrusions and has been thrust onto Devonian limestones along a major shear zone. In both Sysert and Ufaley Complexes, NW-trending stretching lineations and top-to-the-NW kinematic indicators suggest an oblique plate convergence with a significant sinistral component. The MUF is interpreted as a major normal fault that developed congruent with continental subduction and that compensated lithospheric thickening and the rapid exhumation of subducted crust in the footwall.

Large‐magnitude Oligo‐Miocene extension in southern Sonora: Implications for the tectonic evolution of northwest Mexico

New structural and 40Ar/39Ar data from the Santa Rosa area, southeastern Sonora shed light on the magnitude and timing of extensional tectonism. Episodic magmatic activity from the Late Cretaceous to the Miocene is recorded by a 3–4 km thick structural/stratigraphic section that includes a composite Laramide granodiorite batholith and its andesitic wall rocks, early to middle Tertiary rhyolite ignimbrites and silicic domes, and late Oligocene to middle Miocene mafic lavas interbedded with tuffaceous sediment and fanglomerate. The older units are complexly faulted and steeply tilted by several generations of normal faults. Palinspastic reconstructions indicate cumulative extension of ∼90%. Early gently dipping, NW striking normal faults document major extension oriented −N50°E. Angular unconformities and growth fault relations within Oligo‐Miocene sequences bracket this extension between 26 and ∼20 Ma. Younger, widely spaced NS to NNW trending, high‐angle normal faults cut the previously faulted and tilted sections and produced modest (10–15%) E‐W extension that is largely bracketed between 20 and 17 Ma. The magnitude of Neogene extension in this region is greater and the timing is older than previously recognized. Reconnaissance of other areas between the Sierra Madre Occidental and the coast at this latitude suggests that most extension in Sonora occurred between ∼27 and 12 Ma, while remnants of the Farallon plate were still being subducted. Plate‐tectonic models that predict substantial extension in Sonora during proto‐Gulf (∼10 – 5 Ma) transtensional deformation may need to be reevaluated. An alternative model, presented here, is that Baja California began moving with Pacific plate motion shortly after the ∼11 Ma termination of subduction and that northwest motion of Baja relative to mainland Mexico might total 500 km, distributed across the previously extended Oligo‐Miocene magmatic arc.

Subduction model for the origin of some diamonds in the Phanerozoic of eastern New South Wales

Eastern New South Wales has produced over 500 000 carats of diamonds, mostly from the Copeton‐Bingara area in the north. A local derivation is indicated by their distinct tribal character and lack of surface damage, while carbon isotopic values and composition of mineral inclusions are unlike those in diamonds from conventional diamond‐producing areas, for example Yakutia (Siberia), Kapvaal (South Africa), or Argyle (Western Australia). The eastern New South Wales tectonic setting is also unlike that for ‘conventional’ diamonds, involving a Phanerozoic sequence of accreted subduction terranes, with relatively thin hot crust. A subduction diamond model is developed to explain the origin and geology of eastern New South Wales diamonds. This model involves prolonged subduction of mature oceanic crust, resulting in the development of a low‐temperature metamorphic window into the diamond stability field within the downgoing slab at half the depth required by conventional models. The diamonds are preserved at depth by termination of subduction, and brought to the surface by obduction or by entrainment in suitable magmas. The type of diamond formed depends on the original source rock (diamond affinity is shown in brackets): carbonaceous‐rich and carbonaceous‐poor trench sediments (CaFeMgMn‐silicate, eclogitic), graphite‐poor basalt and gabbro (eclogitic) and lherzolite (peridotitic). The model predicts that the New South Wales diamonds are young (Phanerozoic) and that their features, age, associated minerals and geographic distribution relate to New South Wales tectonic provinces. The subduction diamond model extends the range of indicator minerals to include macro‐diamond, corundum, Na‐garnet, with a new series of carrier magmas (basanite, nephelinite, leucitite). Unsuccessful previous exploration in New South Wales, based on conventional models, has produced some positive results when reinterpreted in terms of the subduction diamond model. A re‐evaluation of diamond potential of eastern New South Wales is warranted.

Post-collision magmatism and tectonics in northwest Anatolia

A suite of biotite-hornblende granodiorite intrusions has been emplaced into blueschist-facies metasediments in northwest Anatolia, following collision between two continental margins, now represented by the Tavsanli and Sakarya zones. The 40Ar/39Ar ages of phengites and glaucophanes from the blueschists, metamorphosed under unusually high P-low T conditions (P=20±2 kbar, T=430±30° C), suggest that metamorphism apparently occurred over a period spanning at least 20 Ma from 108 to 88 Ma. Post-tectonic granodiorites were emplaced during the Eocene (53 to 48 Ma) resulting in a cordierite and andalusite-bearing thermal aureole, indicative of pressures of ∼3 kbar. Trace-element systematics of the granodiorites are consistent with a derivation either from mantle-derived magmas by fractional crystallisation in shallow magma chambers, or from anatexis of crustal lithologies of internediate composition at pressures <10 kbar. The preservation of high P-low T assemblages in the blueschists together with the range of ages determined for blueschist-facies metamorphism are indicative of rapid exhumation of delaminated fragments from a subducted continental margin. However decompression melting of the crust is unlikely to have been a significant cause of magmatism, both because exhumation of the blueschists from deep crustal levels predated magmatism by at least 25 Ma, and because of the small melt fraction (<0.1) that may be generated in crustal lithologies by this process. Melting in the mantle wedge is required either to generate a primary melt for the derivation of magmas of intermediate composition or to provide an advective heat source for crustal melting. The cause of melt formation in the upper mantle may be related to the termination of subduction following collision during the Mid-Eocene.

The Proterozoic Sinclair Sequence in southern Namibia: intracratonic rift or active continental margin setting?

The Sinclair Sequence in southern Namibia comprises largely undeformed and unmetamorphosed volcano-sedimentary successions which have been extensively invaded by high-level intrusions, including dyke swarms, of mafic to felsic character. The westernmost outcrops of the Sinclair Sequence form part of the Awasib Mountain terrain (AMT) and have undergone significant deformation and greenschist facies metamorphism. More intensely deformed and metamorphosed lithologies which underlie the Sinclair Sequence in the AMT are correlated with the Namaqualand Metamorphic Complex (NMC). This “basement” constitutes the initial stage of a continual crustal evolution from the Meso- to Neoproterozoic culminating in the development of the regionally extensive Sinclair Sequence. The development of the Sinclair Sequence took place in three major cycles, the AMT being accommodated lithostratigraphically within the second cycle but chronostratigraphically within the third cycle. Magmatism in the Sinclair Sequence was initiated at least as early as 1.2 Ga and is related to the initiation of subduction in response to continental collision between ∼ 1.3 and 1.2 Ga in an event which is widely represented by deformation and granitoid emplacement in the NMC. The oblique component and low angle of subduction resulted in both compressional (“ice-floe”) and tensional (pull-apart) tectonics, enabling preservation of high-level volcano-sedimentary successions in fault-bounded basins. Alternation of compression and tension enabled calc-alkaline mafic and felsic magmatism in the arc, while bimodal tholeiitic volcanism took place in a developing back-arc basin. Transcurrent movements resulted in slicing up and removal of part of the volcanic arc and accretionary complex. Correlation between the Sinclair Sequence and Koras Group (South Africa) suggests that this active margin may, like its predecessor at 1.3 Ga, extend over a considerable distance (nearly 800 km) in a northwest-trending direction. The broadly synchronous tectonomagmatic activity in the Sinclair Sequence and the adjacent Central Zone of the NMC may, in part, be related to activity at different crustal levels. Termination of subduction and crustal stabilisation appear to have taken place by ∼ 0.9 Ga, prior to earliest Pan-African rifting.

Geochemistry of Palaeozoic–Mesozoic Pacific rim orogenic magmatism, Thurston Island area, West Antarctica

Thurston Island, and the adjacent Eights Coast and Jones Mountains, record Pacific margin magmatism from Carboniferous to Late Cretaceous times. The igneous rocks form a uniformly calc-alkaline, high-alumina, dominantly metaluminous suite; some relatively fractionated granitoids are mildly peraluminous. The magmas were hydrous, a result of subduction. Gabbros have compositions outside the range of mafic volcanic and hypabyssal rocks, as a result of cumulate processes. Trace element compositions of the mafic magmas range from a low La/Yb, Th/Ta end-member close to E-MORB in composition, perhaps contaminated by crust, to a high La/Yb, Th/Ta end-member, close to shoshonite, with strong magmatic arc trace element character. This variation may be a result of mixing of tholeiitic and shoshonitic end-members. Most silicic rocks could have been generated batch-wise from mafic magmas by fractional crystallization of a phenocryst assemblage dominated by plagioclase, pyroxene ± amphibole, as seen in the cumulates. Cessation of magmatism at about 90 Ma approximately coincided with collison of a spreading centre between the Phoenix and Pacific oceanic plates with the continent margin subduction zone. The rifting of New Zealand from West Antarctica and associated extension probably was responsible for emplacement of a coast-parallel Cretaceous dyke swarm.

Active margin processes along the Antarctic Peninsula

The Antarctic Peninsula has a remarkable record of active margin processes, which include subduction with progressive ridge-trench collisions, margin segmentation by major fracture zones, rifting in a hybrid back-arc and sheared plate margin context, fore-arc basin development and glacial-marine controlled trench fill processes. Several facets of these active margin processes both of internal (crustal dynamic) and external origin (climate-controlled) have been documented by a geophysical survey during the Antarktis VI/2 cruise of R.V. Polarstem(October-December 1987). Reflection seismic profiles have been shot over the rift basin of Bransfield Strait, over an elongated sediment-filled trough interpreted as a fore-arc basin, over accretional and progradational slopes, over recent and ancient trench environments and over the facing oceanic domain. In this oceanic domain, different fracture zones have highly contrasting morphological and geophysical expressions. The subduction of a fracture zone like Hero F.Z., characterized by a significant relief possibly related to the presence of buoyant (serpentinite) ridges, may have been a factor of subduction termination for the last segment of the Aluk (Drake) plate; it may also have played a role in the separation of a blueschist-bearing fragment (Smith Island) from the base of the accretionary plate margin and in its lift to the surface. The magnetic anomaly pattern of the oceanic slabs facing the northwestern Peninsula margin shows evidence of an intriguing spreading acceleration, which apparently preceded ridge-trench collision. The same anomaly pattern provides a clue to the stratigraphie interpretation of the oceanic sediment cover and of the frontal part of the prograding, now passive margin south of the South Shetland Island Arc. An apparently broken and tilted oceanic plate fragment, squeezed between the South Shetland Trench and Shackleton Fracture Zone, may argue for the role of transpression associated with the oblique convergence of the Antarctic and Scotia plates.

Provincias magmaticas acidas y evolucion tectonica de Gondwana: Andes chilenos (28-31°S)

ABSTRACT. The batholiths of the Andean Cordillera in the Atacama and Coquimbo Regions (28-31°S) reflect tectonic processes associated with the evolving margin of the Gondwana supercontinent. The batholiths consist of the Carbonilerous-Lower Permian Elqui Superunit which records the final assembly of Gondwana, and the Permian-Triassic (Lower Jurassic?) Ingaguas Superunit, which coincides with the period of stability of the supercontinent. These events are tied to those occurring along the Gondwana margin from Peru to Australia. The oldest Elqui unit, Guanta, is composed of calc-alkaline tonalites and granodiorites formed along an active continental margin. These granitoids are intruded by leucocratic peraluminous granitoids (Cochiguas and El Volcan units) derived from melting of variable crustal sources. Some of these granitoids have trace element signatures consistent with a high pressure residual mineralogy suggesting that they formed by melting of a thickened crust. All of the Elqui granitoids are mesozonal and show evidence of contemporaneous and post-emplacement deformation. Their uplift is contemporaneous with a compressional deformation (San Rafael Phase) in the Argentine foreland. The Ingaguas Superunit is an association of epizonal, post-collisional, intrusives which include granitoids derived from deep levels in a garnet-bearing thickened crust (Los Carricitos Unit) and hypersilicic, calc-alkaline to transitional A-type granites, indicating extensive crustal melting of a garnet-poor crust. These granites and the synchronous Pastos Blancos Rhyolites are part of the Choiyoi Magmatic Province which extends for more than 2,500 km along the Central and Southern Andes. Regional considerations suggest that the uplift of the Elqui Superunit and the San Rafael Phase could have resulted from the oblique collision of an allochthonous block to the west in the mid-Permian. This collision could explain crustal thickening and the termination of subduction and block rotations in the Argentine Frontal Cordillera. The removal of the inactive subducted plate would favor decompressional melting, generating large volumes of basalt that could accumulate at the base of the crust and produce the crustal melting that formed the Choiyoi Province. A similar Permian collision has also been suggested in the New England Fold 8elt in Australia. These collisions coincide with the end of the rapid movement of Gondwana relative to the South Pole. In the central part of the Gondwana margin from Patagonia to Antarctica, subduction continued until the Jurassic. 'Thermal blanketing' of the mantle by the stationary supercontinent resulted in accumulation of basaltic magmas at the crust-mantle boundary. The basaltic magmas accumulated below the collage of arc magmatic rocks and exotic blocks accreted in the Paleozoie produced extensive melting forming the Gondwana granite-rhyolite provinces. These events preceded the Jurassic dispersal of the Gondwana supercontinent in this region.

Subnormal Cenozoic geothermal gradients in the extinct Sierra Nevada magmatic arc: Consequences of Laramide and Post‐Laramide shallow‐angle subduction

From 120 to 80 Ma, the Sierra Nevada of California was the site of a vigorous, subduction‐generated magmatic arc. At about 80 Ma, arc magmatism left the Sierra and began to migrate far to the east into the interior of the North American plate, where sporadic magmatism occurred during the latest Cretaceous to early Tertiary Laramide orogeny. This migration of magmatism inland has generally been attributed to a shallowing in the angle of descent of the subducting plate. Apatite and sphene fission track thermochronology data indicate that geothermal gradients in the Sierra remained relatively high for 5–10 m.y. after the cessation of Sierran arc magmatism, then decreased to about one‐fifth or less of previous values over about the next 10 m.y. The timing, magnitude, and time scale of this cooling are consistent with conductive cooling of the Sierran lithosphere in response to initiation of unusually shallow underflow of a cold subducting plate below. Dimensional analysis of the equation for conductive heat transfer shows that the thickness of the Sierran block above the shallowly subducting plate would be the dominant factor controlling the expected timing and time scale of subduction‐induced cooling. Modeling of the cooling, supplemented by modern crustal thickness data, yields a best estimate of the Laramide depth to the top of the subducting plate of about 35–50 km, with depths of about 35–60 km permitted. These depths are much less than the likely ≈ 120 km depth of the subducting plate beneath the Sierra during Sierran arc magmatism and thus support the shallow subduction model. Apatite fission track length data allow estimation of paleotemperatures experienced by the Sierran samples in the Tertiary. The estimated paleotemperatures and their variation with sample elevation suggest that even the highest‐elevation samples were buried significant amounts (perhaps 2–3 km) at ≈30–15 Ma and that geothermal gradients at that time were ≈ 5°–15°C/km. Early Tertiary values for gradients were probably similar or lower. The low gradients are an expected consequence of moderately shallow subduction continuing beneath the Sierra through the Cenozoic until the shift to San Andreas transform motion in the late Cenozoic. The evidence for low Cenozoic gradients in the Sierra, in conjunction with evidence for low Late Jurassic to Late Cretaceous gradients in the adjacent Franciscan accretionary prism and Great Valley forearc basin, suggests that gradients in the forearc of the Franciscan subduction zone were continuously subnormal from the initiation of Franciscan subduction in the Late Jurassic until the termination of subduction in the late Cenozoic. This suggests that subduction at the Franciscan trench was sufficiently fast and continuous to maintain continuously low gradients. If gradients remained continuously subnormal, high‐P/T metamorphism in the Franciscan accretionary prism may have occurred throughout the period of subduction and high‐P/T rocks may have resided at depth for extended periods without being overprinted by normal‐P/T conditions.

Late Precambrian-Early Palaeozoic evolution of part of the Avalon terrane in southern New Brunswick, Canada

Abstract An important Proterozoic to early Ordovician record of the Avalon terrane is exposed between the Wheaton Brook Fault and the Bay of Fundy in southern New Brunswick, Canada. Ductilely mobilized tonalitic ?basement of possibly Helikian age (Brookville gneiss) a platform carbonate-quartzite cover (Green Head Group) containing ?mid-Riphaean stromatolites. ?Unconformably overlying carbonate slump breccias and turbidites (Martinon Formation) are interpreted to record the collapse of this platform and may be broadly contemporary with minor mafic magmatism and a mid-Hadrynian ( c. 800 Ma) tectonothermal event apparently recorded in the Brookville gneiss. However, the association may equally reflect the onset of late Hadrynian ( c. 625–550 Ma) subduction recorded in calc-alkaline granitoid plutonism (Golden Grove suite) and associated arc-related volcanism (Coldbrook Group). The broadly coeval? emplacement of bimodal sheeted dykes (Kingston complex) appears to have occurred under conditions of sinistral transtension suggesting subduction was oblique. Mafic dykes are mainly continental tholeiites and the complex is interpreted to represent the floor of a volcanic arc rift. Major ductile shear zones of dextral sense, which border the dyke complex, may record late Precambrian rift closure but were active in the mid-Palaeozoic. Post-tectonic ‘Eocambrian’ volcanogenic redbeds and continental rift volcanics pass disconformably into Cambro-Ordovician sandstones and shales of the Saint John Group which contain shallow-marine Acado-Baltic fauna of the Iapetus cycle and represent an overstep sequence to Avalon’s Precambrian evolution. Termination of subduction and the tectonic transition to Cambro-Ordovician platformal conditions lacks evidence of a major collisional event. Instead, they coincide with the break-up of a late Precambrian supercontinent that was responsible for the inception of the Iapetus cycle and are likely to reflect the major plate reorganizations that would accompany such an event.

Constraints on uplift in the Franciscan Subduction Complex from apatite fission track analysis

The Franciscan accretionary prism of California contains large belts of sediments that were subducted to depths of 10 to 30 km, metamorphosed there under high‐pressure, low‐temperature conditions, and later uplifted back to the surface. Reconnaissance apatite fission track data (40 samples) place constraints on the uplift histories of these and related lower‐pressure rocks. Franciscan metagreywackes from a few areas (parts of the Diablo Range, parts of the northern Coast Ranges' Eastern and Central belts) retain imprecise fission track evidence of cooling in the middle or Late Cretaceous(?). Using annealing temperatures of fission tracks in apatite and assuming 9°C/km subduction‐period geothermal gradients (estimated from blueschist mineralogy, paleogeothermal gradients in the Great Valley forearc basin, plate motion reconstructions, and other data), these rocks were uplifted through about the 12 km depth level at that time. Since some of these rocks were probably metamorphosed at depths of 15–25 km, substantial uplift had occurred by the middle or Late Cretaceous(?). Rocks from several areas retain fission track evidence of cooling in the early to middle Tertiary, including the Diablo Range (roughly 32 Ma), San Francisco (about 43 Ma), Chiles Creek in Napa County (about 45 Ma), and parts of the northern Coast Ranges' Eastern and Central belts (early to middle Tertiary). Great Valley forearc basin sandstones in the Clear Lake outlier retain evidence of cooling at roughly 35 Ma. Assuming 8.5°–10.5°C/km gradients, rocks from these areas were buried at least 8–10 km in the early or middle Tertiary and experienced at least a few kilometers of uplift at that time. In the northern Coast Ranges, two areas retain evidence of cooling in the late Tertiary (Coastal belt at roughly 20–10 Ma; Yolla Bolly Mountains in Eastern belt at roughly 20 Ma). Assuming 12°C/km gradients, they were buried at least 7 km in the late Tertiary and were uplifted at least a few kilometers at that time. The data thus suggest in general that uplift was occurring in various areas of the Franciscan forearc more or less continuously thoughout much of the period that subduction was occurring. This progressive uplift is most compatible with models where uplift is driven by synsubduction underplating of subducted sediments at the base of the accretionary prism, because those models predict continuing, long‐term uplift throughout the period of subduction but also allow episodicity in uplift as underplating rates and locations vary.Modern geothermal gradients in most Franciscan outcrop areas are moderately high (about 28°–35°C/km), a consequence of latest Cenozoic termination of subduction and development of the San Andreas transform margin. Franciscan and Great Valley samples show no fission track evidence of latest Cenozoic exposure to high temperatures (&gt;100°C). Therefore they were only shallowly buried during the postsubduction period, and postsubduction uplift has been relatively minor (maximum of about 3.2 km).

Model for the evolution of the Avalonian-Cadomian belt

Research Article| August 01, 1989 Model for the evolution of the Avalonian-Cadomian belt J. Brendan Murphy; J. Brendan Murphy 1Department of Geology, St. Francis Xavier University, Antigonish, Nova Scotia B2G 1C0, Canada Search for other works by this author on: GSW Google Scholar R. Damian Nance R. Damian Nance 2Department of Geological Sciences, Ohio University, Athens, Ohio 45701 Search for other works by this author on: GSW Google Scholar Author and Article Information J. Brendan Murphy 1Department of Geology, St. Francis Xavier University, Antigonish, Nova Scotia B2G 1C0, Canada R. Damian Nance 2Department of Geological Sciences, Ohio University, Athens, Ohio 45701 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1989) 17 (8): 735–738. https://doi.org/10.1130/0091-7613(1989)017<0735:MFTEOT>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 J. Brendan Murphy, R. Damian Nance; Model for the evolution of the Avalonian-Cadomian belt. Geology 1989;; 17 (8): 735–738. doi: https://doi.org/10.1130/0091-7613(1989)017<0735:MFTEOT>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 The inception and termination of the ca. 670-550 Ma Avalonian-Cadomian orogenic cycle are interpreted in terms of the amalgamation and breakup of a late Precambrian supercontinent. Synorogenic, arc-related volcano-sedimentary and granitoid rocks are attributed to oblique, southward-directed subduction along the margin of the supercontinent after its amalgamation. Breakup of the supercontinent and inception of the Iapetus cycle led to termination of subduction and its replacement by transform activity. The resulting closure of late Precambrian arc-related basins and the development of Early Cambrian platform successions with Acado-Baltic fauna accompanied the formation of Iapetus, but took place on the active margin of a preexisting ocean and do not record the evolution of its rifted European flank. 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.

The Iapetus suture in the British Isles

AbstractThe position of the Iapetus Ocean suture can be traced between Ordovician fossil localities in the Southern Uplands and the Lake District which contain, respectively, distinct North American and European faunas. The Southern Uplands contains North American Caradoc faunas in the Northern Belt, and is considered to have been accreted onto the Laurentian (North American) margin between the Llandeilo and the end of the Wenlock. Deep seismic reflection profiling shows that a surface, probably parallel to the suture zone, dips down to the northwest from the Solway Line. In eastern Ireland, the surface trace of the suture coincides with the Navan Fault, which separates the Longford-Down massif from Ordovician sediments containing European faunas. The Navan–Silvermines Fault may represent the surface expression of the suture, but it is masked by Wenlock turbidites, which appear to have crossed over the plate boundary. Northward subduction of continental crust below the margin of Laurentia during the late Silurian may explain the influx of turbidite fans from the north or north west onto depressed crust to the south of the suture. Termination of subduction coincides with the widespread Acadian Orogeny in the Emsian (at 395±5 Ma).

Subnormal geothermal gradients in the Great Valley Forearc Basin, California, during Franciscan Subduction: A fission track study

Outboard parts of forearcs commonly have subnormal geothermal gradients because relatively cold lithosphere subducts beneath them. The strata of the 10‐ to 15‐km‐thick Late Jurassic‐Tertiary Great Valley forearc basin accumulated in the forearc of the Franciscan subduction zone of coastal California. In this study, paleogeothermal gradients in this basin were reconstructed to constrain the tectonic history of the convergent plate margin.Backstripping was used to reconstruct the burial histories of outcropping basin strata, and apatite fission track analysis was used to determine the maximum burial temperatures experienced by 64 rock samples. Plots of maximum burial temperatures versus maximum burial depths indicate earliest Tertiary (time of deepest burial) forearc basin geothermal gradients of about 9°C/km. Backstripping shows that strata from the lower parts of two sections were deeply buried (&gt;6.5 km) for the period 90–65 Ma without experiencing high temperatures (≥105°C), indicating that gradients were at least moderately subnormal throughout that period. The data permit gradients no greater than about 15°C/km at 90 Ma, decreasing to no greater than about 9°C/km at 65 Ma. Lower strata in another section were probably deeply buried without experiencing high temperatures from 130 or 120 Ma until the latest Cretaceous or early Tertiary, indicating that subnormal gradients probably existed throughout that period. K‐Ar ages of Franciscan Complex blueschists indicate subnormal gradients in the California forearc in the Late Jurassic and Early Cretaceous, fission track data from the Sierra Nevada indicate subnormal gradients in the early and middle Cenozoic, and modern heat flow patterns indicate subnormal gradients in the late Cenozoic. Together, these indicators suggest that thermal gradients in the forearc were continuously subnormal from the initiation of Franciscan subduction in the Late Jurassic until the termination of subduction in the late Cenozoic.Gradients in the range that would permit blueschist facies metamorphism in the Franciscan Complex were thus long‐lived. The broad range of K‐Ar ages noted for Franciscan blueschists need not reflect multiple, discrete metamorphic events, but are also consistent with a continuous, extended period of high P/T conditions. Blueschists need not have been uplifted soon after metamorphism to avoid overprinting by normal P/T conditions, but rather may have remained at depth for extended periods.Preliminary modeling indicates that an interruption in subduction of longer than 5–20 m.y. should cause detectable heating of forearc basin strata, while a much shorter interruption should heat accretionary wedge rocks. Since there is no evidence of such heating, it appears that convergence at the Franciscan trench was essentially continuous. If allochthonous terranes traveled northward parallel to the California margin during the Late Jurassic to late Cenozoic period of Franciscan subduction, they apparently did so during oblique subduction, not during extended interludes of pure transform motion.

Stratigraphy of Northeastern Cady Mountains and Its Implications for Cenozoic Volcanic Evolution of Mojave Desert: ABSTRACT

The Cenozoic rocks of the northeastern Cady Mountains can be divided into two separate sequences--an unnamed volcanic succession in the eastern Cady Mountains, and the volcanic and sedimentary strata of the Hector Formation in the northern Cady Mountains. In the eastern Cady Mountains, approximately 3,280 ft (1,100 m) of volcanic flows, flow breccias, domes, and laharic breccias overlie a basement complex of Mesozoic quartz diorite and quartz monzonite. These volcanic rocks have a continuous compositional variation from basaltic andesite to rhyodacite, although dacites and rhyodacites predominate. The age of this sequence is probably Oligocene and/or Miocene. In the northern Cady Mountains, approximately 2,395 ft (730 m) of strata assigned to the Hector Formation (Oligocene? to Miocene) lie unconformably on a basement complex of Mesozoic quartz monzonite and metamorphic rocks. The upper part of the Hector Formation also unconformably overlaps the volcanic rocks of the eastern Cady Mountains. The Hector Formation consists of alluvial sandstone and conglomerate plus lacustrine and paludal sandstone, siltstone, mudstone, and limestone. Laharic breccias, ash-flow and air-fall tuffs, and andesitic to basaltic lava flows are interbedded with the sedimentary rocks. Geochemical data indicate that the volcanic rocks from both areas are part of a calc-alkalic suite. The model proposed by various authors for the Cenozoic tectonic evolution of the western United States suggests that there should have been a transition from andesitic-type volcanism to basaltic-type volcanism, concurrent with the termination of subduction and the initiation of regional faulting. No such volcanic transition, however, can be identified within the northeastern Cady Mountains, nor has one been clearly identified in other late Oligocene to Miocene volcanic sequences within the Mojave Desert region. If this type of transition did take place within the Mojave Desert, it must have occurred after 18.6 ± 0.2 m.y.B.P. This is the date obtained from the youngest dated volcani rock that is unquestionably part of the calc-alkalic suite observed in the northeastern Cady Mountains. End_of_Article - Last_Page 1695------------

D'Entrecasteaux Zone, trench and western chain of the central New Hebrides island arc: Their significance and tectonic relationship

The absence of a trench and the existence of a separate, western chain of islands in the central New Hebrides island arc are a consequence of earlier tectonic evolution, and not the result of subduction of the d'Entrecasteux Zone as was previously suggested. Because of tectonic consolidation in the western islands prior to present subduction, a resistant block was created opposing subduction, and a trench never did form, here. The d'Entrecasteux Zone is responsible only locally for additional deformation of the subducting plate, in a way that can be regarded as an initial stage of obduction.

Temporal Changes in Depositional Facies in Great Valley Forearc Basin of California--Influence of Basin Evolution and Tectonics: ABSTRACT

The Great Valley forearc basin began in the Late Jurassic as a residual forearc basin on top of oceanic crust, and evolved into a composite forearc basin on top of both oceanic and continental crust in the Late Cretaceous and Paleogene. The depositional basin widened through time owing to the westward and upward growth of the subduction complex and the eastward migration of the coeval magmatic arc. Depositional facies reflect changes in shape, size, tectonic activity, and inherited characteristics of the basin. Late Jurassic depositional environments primarily consisted of slope with locally incised channels. The basin was relatively narrow and a steep slope allowed sediment movement from the shoreline on the east directly into the trench to the west. By the Early Cretaceous, a bathymetric barrier was formed by the upward- and outward-building subduction complex, thus trapping arc-derived sediment within the forearc basin. Basin-plain environments dominated in this terraced forearc. The subduction complex continued to grow, and the magmatic arc migrated eastward during the Late Cretaceous, resulting in a wider, composite basin. Complex interbedded submarine fan, slope, and basin-plain facies formed in this setting. Submarine fan systems became larger owing to the concentration of sediment gr vity flows within submarine canyons incised into the widening shelf on the east side of the basin. The basin evolved into a broad ridged forearc and, eventually, into a broad shelved forearc during the Paleogene as the subduction complex emerged above sea level and the forearc basin filled. As a result, Upper Cretaceous submarine fan complexes are overlain by slope facies, which are overlain by shelf facies. Nonmarine environments have persisted following filling of the basin and sequential termination of subduction by the northward migration of the Mendocino triple junction during the Neogene. End_of_Article - Last_Page 727------------

Role of Salinian Block in Evolution of San Andreas Fault System, California

Discrepancies in right-lateral strike-slip separation of pre-Tertiary and Tertiary geologic features along the northern San Andreas fault give rise to two contrasting views of the evolution of the fault system. One model entails two stages of right slip on the San Andreas fault, one Late Cretaceous-early Tertiary and one post-Oligocene. The other model requires significant right slip on multiple faults of the Neogene San Andreas fault system. Several lines of evidence now suggest that major Neogene right slip occurred on several faults of the San Andreas transform-fault system in addition to the San Andreas fault proper. Furthermore, these data indicate that the magnitude of proto-San Andreas right slip probably was far less than previously suggested. Integration of the fault studies and regional geologic relations, especially in the central Salinian block, permits reconstruction of the tectonic evolution of the San Andreas fault system and the central California margin. A relatively modest amount of right slip on a proto-San Andreas fault, possibly the consequence of oblique plate convergence, emplaced granitic basement of the Salinian block west of the fault in Paleocene time. Following an initial rise-trench encounter in the Oligocene, migration of a triple junction from south to north past central California resulted in coordinate termination of subduction and propagation of the San Andreas transform-fault system. Initially, the Salinian block experienced extensional tectonism, perhaps related to the passage of the triple junct on. Right slip on the San Andreas fault and on faults cutting the Salinian block probably began in middle Miocene time. Since a late Miocene change of pole of rotation of the Pacific-North American plate pair, motion has been increasingly absorbed on the San Andreas fault proper.

Metamorphic rocks of the Yap arc-trench system

The Yap trench-arc is a link between the Mariana and Philippine arcs; the latter are both loci of acive volcanism and seismicity but the Yap arc is formed of metamorphic rocks and has had few historic earthquakes. It does not appear to be an active subduction zone. The 8–9 km deep Yap trench has a steep west well, it has little or no sediment ponds on the trench slope. There is no modern volcanism and the few volcanic rocks present on Yap may be large fragments in a tectonic breccia. The main rock type on Yap is greenschist with textural evidence for intense penetrative deformation. The chemistry of these rocks, especially the Mg, Cr, Ni and Co abundances, suggest that the protolith was an ultramafic rock. Rocks from the trench wall east of Yap include amphibolite facies metabasite and metasediments, e.g. (1) Ca-Al-rich hornblende + labradorite + epidote schist; (2) calcite + diopside marble; (3) grossularite-andradite + diopside + vesuvianite calc-gneiss; and (4) diopside + calcite + epidote calc-gneiss. These units (1–4) are overlain by Yap ultramafic schists; a major tectonic break characterized by a polymictic breccia including clasts representative of Yap schists, metabasites, metasediment, pyroxenite and feldspathic gabbro separates the trench lithologies from the ultramafic schist. Other sites sampled in the trench wall have rocks typical of seamounts. It is proposed that a Tertiary(?) subduction zone was terminated when “un-subductible” seamounts blocked the trench. This led to overthrusting, from the west, of seafloor crust and upper mantle. The volcanic arc ceased to exist, some of its fragments are seen in the tectonic breccia. Termination of subduction may be caused by the blocking of subduction zones by large seamount-atoll chains or thickened oceanic crust such as the Caroline Ridge. In some cases this may lead to obduction of oceanic lithosphere.