Arc Basement Research Articles

Structure and tectonics of the upper Cenozoic Puerto Rico‐Virgin Islands carbonate platform as determined from seismic reflection studies

The Puerto Rico‐Virgin Islands carbonate platform was deposited over an area of 18,000 km2 from early Oligocene to Holocene on top of an inactive and subsiding Cretaceous‐earliest Oligocene island arc. Regional single‐channel and multichannel seismic reflection lines presented in this study provide the first information on the regional stratigraphy and structure of this platform that has previously been known mainly from onshore stratigraphic sections of a relatively small (2250 km2) portion of the platform exposed by late Neogene tectonic uplift along the north coast of Puerto Rico. Seismic reflection lines are used to map the thickness of the carbonate platform strata and to correlate this thickness with onshore outcrop and well data from northern and southern Puerto Rico, St. Croix (U.S. Virgin Islands), and the Saba Bank. Limestone thickness variations from a little over 2 km to almost zero are used to subdivide the Puerto Rico‐Virgin Islands platform into five distinct carbonate provinces: (1) north Puerto Rico area including the onshore exposures; (2) Virgin Islands area; (3) St. Croix and Saba Bank area; (4) south Puerto Rico area; and (5) Mona Passage area. Carbonate thickness and structural information from each area are used to test five previously proposed models for the deformation and vertical movements of the platform. The most prominent feature of the platform in the Puerto Rico‐Virgin Islands area is a large, east‐west trending arch. The northern limb of this arch exhibits a smoother, more uniform dip than the steeper, more abruptly faulted, southern limb. The core of the arch is responsible for the exposure of arc basement rocks on Puerto Rico. The origin of this arch, which occurs over a 300 km wide area, is best explained by north‐south shortening and arching, caused by interaction at depth of subducted slabs of the North America and Caribbean plates. Other important evidence for this model can be found in the Benioff zones observed in the earthquake profiles. Loading of the Caribbean plate results in downward flexing of the North America plate and causes the 4 km subsidence of the carbonate platform north of Puerto Rico.

Supra‐Subduction Zone Ophiolite Formed in an Extensional Forearc: Trinity Terrane, Klamath Mountains, California

The Paleozoic Trinity terrane is a massif of ultramafic rock and mafic intrusive complexes that forms the basement of eugeoclinal rocks in the eastern Klamath Mountains. The origin of the Trinity terrane has been particularly enigmatic and several mutually exclusive origin models have been proposed: (1) mantle diapir; (2) slice of obducted monogenetic oceanic lithosphere; (3) monogenetic backarc‐basin lithosphere; (4) forearc lithosphere; and (5) volcanic arc basement. We have compiled new and existing field, petrographic, geochronologic, isotopic, and whole‐rock chemical data for the Trinity terrane to evaluate these models and conclude that the Trinity terrane is an supra‐subduction zone ophiolite whose mafic intrusive complexes record subduction‐related magmatism and forearc extension that occurred during the inception of intraoceanic subduction. According to this extensional forearc model, all four terranes in the eastern Klamath Mountains evolved principally during the inception and evolution of a si...

Introduction to the special volume on the Izu-Bonin - Mariana arc system

Introduction to the special volume on the Izu-Bonin - Mariana arc system

Structural insight into the south Ryukyu margin: effects of the subducting Gagua Ridge

This study presents three multi-channel deep seismic reflection profiles located in the south Ryukyu margin between 122°30′E and 123°30′E, where a N-S-trending oceanic ridge, the Gagua Ridge, is entering the subduction zone, for the purpose of examining the effects of ridge subduction on structures of the forearc region. Structural features which correspond to different stages of the oblique ridge subduction are observed. East of 123°E, a short-lived sequence of indentation, tunneling, then resumption of frontal accretion occurred in the accretionary wedge (the Yaeyama Ridge) as the subducted portion of the Gagua Ridge swept the overriding Ryukyu margin from below along the northwesterly convergent direction. Under the forearc basin, the subducted portion of the Gagua Ridge is uplifting the arc basement to form the Nanao Basement Rise which separates the sedimentary strata of the Nanao and East Nanao forearc basins. Results from this study suggest that the oblique subduction of the Gagua Ridge has not only affected accretionary wedge structures but also the arc basement of the south Ryukyu margin.

Sedimentary evidence relating to the tectonic evolution of the Lau Basin, SW Pacific, from ODP Sites 834–839 (ODP Leg 135)

Abstract Six sites were drilled during Ocean Drilling Program Leg 135 in the western Lau backarc basin in the southwest Pacific (Sites 834–839). These sites are all located in basins within a horst and graben terrain and form an approximate transect across the rifted arc basement onto crust considered to have been formed at the East Lau Spreading Centre. The sedimentary sequences recovered from these backarc sites range in age from the late Miocene to the Holocene; they consist primarily of a lower succession of volcaniclastic sediment gravity-flow deposits interbedded with hemipelagic clayey nannofossil oozes and nannofossil clays, overlain by a distinctive upper succession of hydrothermally stained hemipelagic, and locally redeposited, clayey nannofossil oozes. The volcaniclastic sediment gravity-flow deposits are predominantly massive, proximal, vitric gravels, sands and silts, that are mainly locally derived from adjacent basement ridges and intrabasin seamount volcanoes. At Site 835, which was drilled in a small extensional sub-basin in the oldest part of the Lau Basin, the upper clayey nannofossil ooze sequence is anomalously thick and rigorous sedimentological analysis shows that much of this sequence is redeposited. Thick clayey nannofossil ooze turbidite muds are identified that closely resemble the enclosing clayey nannofossil ooze hemipelagites. These thick turbidite muds are associated with mudclast conglomerates, interpreted as muddy debris-flow deposits, and a number of coherent rafted blocks of older hemipelagic sediment. These allochthonous deposits testify to several episodes of instability in the sub-basin that may be related to large-scale tectonic activity caused by the southward passage of ocean-ridge propagator tips past the latitude of the drillsite. Episodes of increased sediment deposition, due to increased frequency of turbidite emplacement, are evident at all of the Lau Basin backarc drillsites, and these correlate moderately well with the closest approach of the propagating ridge tip to each sub-basin. The research presented illustrates how turbidites, once identified, can be used to decipher complex tectonic histories, and demonstrates the importance of local tectonic controls on sediment redeposition in backarc basins.

The Border Ranges fault system in Glacier Bay National Park, Alaska: evidence for major early Cenozoic dextral strike-slip motion

The Border Ranges fault system of southern Alaska, the fundamental break between the arc basement and the forearc accretionary complex, is the boundary between the Peninsular–Alexander–Wrangellia terrane and the Chugach terrane. The fault system separates crystalline rocks of the Alexander terrane from metamorphic rocks of the Chugach terrane in Glacier Bay National Park. Mylonitic rocks in the zone record abundant evidence for dextral strike-slip motion along north-northwest-striking subvertical surfaces. Geochronologic data together with regional correlations of Chugach terrane rocks involved in the deformation constrain this movement between latest Cretaceous and Early Eocene (~50 Ma). These findings are in agreement with studies to the northwest and southeast along the Border Ranges fault system which show dextral strike-slip motion occurring between 58 and 50 Ma. Correlations between Glacier Bay plutons and rocks of similar ages elsewhere along the Border Ranges fault system suggest that as much as 700 km of dextral motion may have been accommodated by this structure. These observations are consistent with oblique convergence of the Kula plate during early Cenozoic and forearc slivering above an ancient subduction zone following late Mesozoic accretion of the Peninsular–Alexander–Wrangellia terrane to North America.

Magmatic arcs and associated gold and copper mineralization in Indonesia

Gold mineralization in Indonesia formed in andesitic arcs which were active for intervals of between 3 and 20 My from the Cretaceous to Pliocene. Fifteen major arcs are identified with a total on land extent of over 15,000 kms. Known orebodies and major prospects are confined to six arcs within the mid-Tertiary to Pliocene age range. In Indonesia, these arcs total approximately 7,000 kms in length and contain combined historical production and current resources in excess of 2,500 tonnes of gold and 20 million tonnes of copper. Individual arcs or segments of arcs are characterized by specific types of mineralization reflecting both arc basement related to earlier collisions and reversals in tectonic polarity, and erosion level. Porphyry copper-gold, skarn, and high sulphidation enargite-gold deposits formed in both island arc and continental settings where intrusive stocks ascended to shallow depths near the base of a coeval volcanic sequence. Low sulphidation epithermal deposits, largely confined to the western Sunda-Banda and Central Kalimantan continental arcs, formed preferentially in the lower part of the volcanic pile and sometimes at the basement surface. The low sulphidation deposits may be underlain at depths of several kilometres by sills which provided heat for the hydrothermal systems; the deposits probably formed during late-stage extension on low-angle faults. Sediment-hosted mineralization developed where low sulphidation fluids encountered favourable calcareous marine sedimentary rocks at epithermal depths. The distinctive gold-silver-barite±base metal deposits of eastern Indonesia may have formed by venting of high sulphidation fluids into restricted shallow marine basins. As more information becomes available and understanding of arc systems increases, the proposed relationships of deposit type to upper basement and stage of arc evolution, and deposit abundance to erosion level, are likely to become increasingly useful to regional and district scale assessments of mineral potential, both in Indonesia and in arc environments elsewhere.

Detrital record of the Gravina arc, southeastern Alaska: Petrology and provenance of Seymour Canal Formation sandstones

Sandstones of the Upper Jurassic to mid-Cretaceous Seymour Canal Formation (northern Gravina belt, southeastern Alaska) are volcaniclastic wackes. They exhibit a variably developed solution cleavage and zeolite- to low-greenschist-grade metamorphic assemblages. Point counts of 37 lithofeldspathic sandstones, spanning the unit9s stratigraphic range, yield framework modes typical of detritus shed from undissected arc sources (Q 4 F 40 L 56 , Qm 3 F 40 Lt 57 , Qm 8 P 91 K 1 , Qp 3 Lvm 91 Lsm 6 ) with only minor detritus characteristic of older terranes or continental-margin sources. Sandstones of the dominant thinly bedded turbidite lithology were derived almost exclusively from the contemporaneous Gravina arc. In contrast, sandstones of channelized conglomeratic facies are richer in quartz, sedimentary, metasedimentary, and plutonic detritus derived from basement terrane(s). The compositional heterogeneity and grain size of conglomeratic facies are consistent with derivation from local source areas, and we interpret them to reflect minor unroofing of the Alexander terrane and the superposed Gravina arc, mostly during Early Cretaceous time. A distinctive, although volumetrically minor, clinopyroxene-rich sandstone petrofacies signals a change in modal expression of source-area volcanism during the Cretaceous, and it probably records initial eruption of pyroxene-rich basalts of the mid-Cretaceous Douglas Island Volcanics. Supracrustal records preserved in the Nutzotin Mountains sequence, Dezadeash flysch, and Gravina sequence are also dominated by shallow-level arc detritus, although coeval supracrustal units on the Insular terrane of British Columbia reflect deeper incision of arc basement, possibly due to along-strike variation in arc geometries or tectonics of the Jura- Cretaceous Insular terrane.

Pre-spread rifting and heterogeneous volcanism in the southern Havre Trough back-arc basin

The Havre Trough is an active back-arc basin associated with Pacific—Australian plate convergence. Two segments within the southern Havre Trough (SHT), surveyed with gloria, Sea beam and SYS09 swath data, now provide the first detailed data to reliably document the structure and morphology of this complex back-arc region. At the southern segment (35°40′–37°S), along a strike length of some 120 km, the SHT structure comprises a series of segmented, en echelon, generally flat floored, sediment filled, axial rift grabens, flanked by ridge and basin rift block topography. Prominent fault escarpments, with relief of 500–1000 m and a marked zig-zag arrangement, mark the outer margins of the rift system. Within the rift grabens extensional tectonism is pervasive. Extensive shallow seismicity within the SHT can only partly be associated with the rift graben system at present. Youthful, constructional volcanic terrains within the rift grabens are not common, and invariably have a restricted areal extent and a relief < 400 m. Neovolcanism is fault controlled, being apparently restricted to the rift escarpments. Lavas within the rifts display a marked source heterogeneity as evinced by the eruption of back-arc basin and island arc basalts on the western and eastern escarpments, respectively, even though separated by as little as 14 km. An identical rift morphology has been identified at the northern (33°–34°S) site. Data from both sites show individual rifts are 15–35 km in length, 6–10 km wide, and have axial depths of 3100–4000 m. Apart from at least one 15–20 km wide, cross trough ridge, available data indicate the rift graben system is contiguous between 33°S and 37°S. This rift graben morphology, by analogy with other known rifting back-arc basins, is considered here not to be consistent with true oceanic back-arc spreading. Rather it is argued that the SHT, possibly indeed all of the Havre Trough, is undergoing back-arc rifting prior to true spreading. Further, the rifting tectonic fabric within the SHT, and in the onshore Taupo to the south, extension is consistently dextrally oblique relative to the bounding Colville and Kermadec Ridges. The degree of obliquity increases from south to north. If accepted that the SHT is rifting rather than spreading, magnetic anomaly data within the trough do not record the accretion of new oceanic crust, and consequently can no longer be used to establish the “spreading” history of the SHT. The SHT magnetic anomalies are here interpreted as “pseudo-linear” magnetic anomalies resulting from the generally irregular spatial and temporal emplacement of magnetic sheeted lava and dike intrusives between older, low magnetisation arc basement rift blocks flanking the axial rift. Proxy data from onshore North Island, New Zealand, and offshore SHT suggest that the rate of extension and age of initiation of back-arc rifting are, between 35° and 37°S, respectively, 15–20 mm yr −1 and 5 Ma.

Tectonics and Terranes of the Southeastern Caribbean

The southeastern Caribbean plate (Ca) is comprised of the following terranes: Tobago, Grenada Basin, St. Vincent, Araya-Margarita, and Paria-Trinidad-Barbardos (PTB). All are alient relative to South America (SA) east of Caracas except for PTB, which is of continental provenance and parautochthonous and lies within the principal movement zone of the Ca-Sa plate boundary. The Tobago terrane extends between the eastern Venezuelan coastline and the Grenada Basin. On its south, the Tobago terrane overrode PTB and the South American passive margin during Neogene oblique collision. The Mesozoic tectonostratigraphy of the Tobago terrane is not unlike that of the Colombian Basin, suggesting the Tobago may belong to the Pacific-derived Caribbean plate. The Grenada Basin terrane consists of Eocene and older oceanic crust that now occupies the southern Lesser Antilles arc platform and the southern Grenada Basin. Such crust abducted southward below the Tobago terrane in mid-Cenozoic time, probably taking up boundary-normal shortening during oblique collision of the Ca and Sa plates. The oceanic crust of the GB terrane arose by backarc spreading of unknown orientation. The St. Vincent terrane extends north in the Antilles from St. Vincent; it is defined by thick crust, perhaps an old arc basement. The Araya-Margarita terrane ismore » a probable subduction complex of Mesozoic age of metamorphism that has been transported far eastward from an unknown site with the Ca plate.« less

Late Cretaceous-Cenozoic Basin Framework and Petroleum Potential of Panama and Costa Rica

Despite its location between major petroleum provinces in northwestern South America and northern Central America, there is a widespread negative perception of the petroleum potential of Panama and Costa Rica in southern Central America. Several factors may contribute to this perception: (1) the on and offshore geology of many areas has only be studied in a reconnaissance fashion; (2) sandstone reservoirs and source rocks are likely to be of poor quality because Upper Cretaceous-Cenozoic sandstones are eroded from island arc or oceanic basement rocks and because oil-prone source rocks are likely to be scarce in near-arc basins; and (3) structural traps are likely to be small and fragmented because of complex late Cenozoic thrust and strike-slip tectonics. On the other hand, onshore oil and gas seeps, shows and small production in wildcat wells, and source rocks with TOC values up to 26% suggest the possibility of future discoveries. In this talk, we present the results of a regional study using 3100 km of offshore seismic lines kindly provided by industry. Age and stratigraphic control of offshore lines is constrained by limited well data and detailed field studies of basin outcrops in coastal areas. We describe the major structures, stratigraphy, andmore » tectonic history of the following areas: Gulf of Panama and Gulf of Chiriqui of Panama and the Pacific and Caribbean margins of Costa Rica.« less

Evolution of the lower arc crust in Kohistan, N. Pakistan: temporal arc magmatism through early, mature and intra-arc rift stages

Abstract The middle to lower island-arc crust in southern Kohistan represents a 40 Ma life-span of subduction-related magmatism in an intraoceanic setting. The Kamila amphibolite belt comprises two varieties of metavolcanic amphibolites. One enriched in high-field strength (HFS) and heavy rare-earth (HRE) elements is transitional in character between N- and E-MORB with a minor subduction-related component, and represents the earliest arc basement in Kohistan. The other variety of the metavolcanic amphibolites, together with deformed and amphibolitised intrusive basic plutons, has a transitional tholeiitic to calc-alkaline nature marked by depleted HFS and HRE elements and a distinct negative anomaly for Nb, suggesting emplacement in the early to mature stages of arc growth. The subsequent magmatism in southern Kohistan took place in two stages. The extensive calc-alkaline gabbronorites of the Chilas complex were derived from partial melting of a mantle diapir emplaced into a mature arc or, more probably, during the initial stages of sub-arc splitting. This intra-arc rifting, at its advanced stages, generated tholeiitic picrite to high-Mg basalts which crystallized the ultramafic-mafic-anorthosite (UMA) association and basic dykes of the Chilas complex. The Late Cretaceous accretion of the Kohistan island arc with the Karakoram plate in the north ceased the intraoceanic history of magmatism in southern Kohistan.

Tectonic evolution of the Japanese islands as reflected in modal compositions of Cenozoic forearc and backarc sand and sandstone

Cenozoic tectonic evolution of the Japanese rifted continental arc‐trench system is reflected in the detrital modes of sand and sandstone deposited in forearc and backarc basins sampled by the Deep Sea Drilling Project. At present, the Japan arc is divided into two segments along a complex plate boundary where the Izu‐Bonin Ridge intersects the Japan arc, creating two triple junctions, in front of and behind the ridge. Southwest of the Izu‐Bonin Ridge, quartzofeldspathic Cretaceous forearc sediments were uplifted and recycled into Tertiary forearc deposits in response to strike‐slip movement associated with backarc spreading in the Shikoku basin. Quaternary forearc sections record the first major influx of volcanic detritus to southwest Japan sites. Triple‐junction‐related deformation in central Honshu has produced sand of mixed volcanic/sedimentary provenance, which is funneled by the Suruga Canyon into the Nankai Trough along the southwest Japan margin. North of the triple junction, Oligo‐Miocene forearc sand compositions indicate Oligocene forearc uplift and volcanism, possibly related to initial backarc rifting and formation of the Japan Sea, and subsequent Miocene exposure of arc basement. In contrast, Pliocene and Quaternary forearc sand from northeast Japan is primarily volcaniclastic and documents uninterrupted arc volcanism. Within the backarc region, sand compositions vary from east to west across the Japan Sea. The Asian rifted continental margin and submerged remnants shed quartzofeldspathic sand into the western side of the basin, whereas the Japan arc sheds volcaniclastic sand into the eastern side of the basin.

Pre‐Mid‐Mesozoic tectonic evolution of the Yukon‐Tanana Terrane, Yukon and Alaska

Yukon‐Tanana Terrane (YTT) underlies much of central and western Yukon and east central Alaska. Its history and tectonic evolution, particularly prior to mid‐Mesozoic time, has been largely obscured by younger magmatism and tectonism. The application of geochronological and isotopic techniques over the past decade, together with detailed field studies in certain critical areas of the terrane, has shed new light on the early history of YTT. Much of YTT is a product of episodic continental arc magmatism, with three main pulses in Late Devonian‐Early Mississippian, mid‐Permian, and Late Triassic‐Early Jurassic time. From Late Devonian to mid‐Mississippian time, subduction was north or northeast dipping, but arc polarity was apparently reversed by mid‐Permian time. The main, subhorizontal structural fabric characterizing much of YTT was produced between mid‐Permian time and the onset of renewed magmatism in Late Triassic time and probably reflects a major continent‐continent collision. Although the Triassic‐Jurassic magmatism is also considered to be arc related, it occurred over a very broad area of not only YTT, but also Quesnellia, and the Stikine, Nisling, Cache Creek, and Slide Mountain terranes. This magmatism appears to have coincided with final amalgamation of the Intermontane Superterrane, and the arc polarity and the position and orientation of the associated subduction zone is still controversial. Available evidence suggests that Nisling Terrane is closely related to YTT and mainly consists of older strata that underlie the Devonian and younger units generally considered to be more typical of YTT. There are close similarities between YTT and a number of other “pericratonic” terranes in the central and eastern parts of the Cordillera, and it is likely that these terranes originally formed a single arc and arc basement assemblage which has now been fragmented and dispersed by transcurrent faulting.

The Cretaceous volcanic-plutonic province of the central Queensland (Australia) coast—a rift related ‘calc-alkaline’ province

ABSTRACTSilicic and minor intermediate and mafic pyroclastics, lavas, and dykes occupy a NW-trending zone through the Whitsunday, Cumberland and Northumberland Island groups, and locally areas on the adjacent mainland, over a distance of more than 300 km along the central Queensland coast. K-Ar and Rb-Sr data indicate an age range of 95–132 Ma, with the main activity approximately between 105–120 Ma; there is, however, evidence for easterly increasing ages. Comagmatic granites, some clearly intrusive into the volcanics, occur together with two localised areas of Triassic potassic granites (229 Ma), that form the immediate basement.The volcanics are dominantly rhyolitic to dacitic lithic ignimbrites, with intercalated surge and bedded tuffs, accretionary lapilli tuffs, and lag deposits. Associated rock types include isolated rhyolitic and dacitic domes, and volumetrically minor andesite and rare basalt flows. The sequence is cut by abundant dykes, especially in the northern region and adjacent mainland, ranging from dolerite through andesite, dacite and rhyolite. Dyke orientations show maxima between NW-NNE. Isotope data, similarities in petrography and mineralogy, and alteration patterns all suggest dyke intrusion to be broadly contemporaneous with volcanism. The thickness of the volcanics is unconstrained, although in the Whitsunday area, minimum thicknesses of &gt;1 km are inferred. Eruptive centres are believed to occur throughout the region, and include at least two areas of caldera-style collapse. The sequences are thus considered as predominantly intracaldera.The phenocryst mineralogy is similar to modern “orogenic” volcanics. Phases include plagioclase, augite, hypersthene (uralitised), magnetite, ilmenite, with less common hornblende, and even rarer quartz, sanidine, and biotite. Fe-enriched compositions only develop in some high-silica rhyolites. The granites range from quartz diorite to granite s.s., and some contain spectacular concentrations of partially disaggregated dioritic inclusions.Chemically, the suite ranges continuously from basalt to high-silica rhyolite, with calc-alkali to high-K affinities, and geochemical signatures similar to modern subduction-related magmas. Only the high-silica rhyolites and granites exhibit evidence of extensive fractional crystallisation (e.g. pronounced Eu anomalies). Variation within the suite can only satisfactorily be modelled in terms of two component mixing, with superimposed crystal fractionation. Nd and Sr isotope compositions are relatively coherent, with εNd + 2·2 to +7·3, and ISr (calculated at 110 and 115 Ma) 0·7031-0·7044. These are relatively primitive, and imply mantle and/or newly accreted crustal magma sources.The two end-members proposed are within-plate tholeiitic melt, and ?low-silica rhyolitic melts generated by partial fusion of Permian (to ?Carboniferous) arc and arc basement. The arc-like geochemistry is thus considered to be source inherited. The tectonic setting for Cretaceous volcanism is correlated with updoming and basin rifting during the early stages of continental breakup, culminating in the opening of the Tasman Basin. Cretaceous volcanism is also recognised in the Maryborough Basin (S Queensland), the Lord Howe Rise, and New Caledonia, indicating the regional extent of volcanism associated with the complex breakup of the eastern Australasian continent margin.

Early Proterozoic ophiolite, central Arizona

The 1.73 Ga Payson ophiolite is a pseudostratigraphic sequence of mafic plutons, dike swarms, sheeted dikes, and submarine basalts that intruded and erupted upon a 1.75-1.76 Ga magmatic-arc complex. The composition of the sheeted dikes is tholeiitic basalt (minor andesite) with island-arc affinities. The submarine basalts are overlain by dacitic breccias and a thick section of turbidites with ca. 1.72 Ga ash beds. The entire sequence was deformed, intruded by ca. 1.70 Ga granites, and unconformably overlain by fluvial to shallow-shelf sediments. Although most of the 1.8-1.6 Ga juvenile crust of Arizona consists of magmatic-arc rocks, the Payson ophiolite is unique and is interpreted to have formed the floor of an intra-arc basin. The ophiolite developed in situ on the older arc basement, as opposed to being thrust over it. The basin was accreted to the continent by ca. 1.70 Ga.

Subduction of Atlantic aseismic ridges and Late Cenozoic evolution of the Lesser Antilles island arc

A great amount of data concerning the Lesser Antilles arc and the near Atlantic has been reviewed: nature, timing and spatial evolution of the Oligocene to Recent volcanic sequences; the location and scale of uplifted areas; facturation of the arc ridge; the structural pattern of the Central Atlantic oceanic crust; and characteristics of the subduction beneath the Caribbean plate. These data allow us to lay stress on the role of the interaction of underthrust oceanic aseismic ridges with the evolution of the overriding arc. Depending on the degree of gravimetrical compensation (buoyancy) of these features, this interaction can induce effects of the first and second orders of magnitude. With the onset of the Late Oligocene, a buoyant ridge of anomalous mid Cretaceous crust reached the front of the northern half of the Lesser Antilles arc basement. It momentarily stopped the subduction along the whole arc, bringing the arc volcanism (Older arc) to a standstill as it was underplated beneath the eastern rim of the Caribbean lithosphere, and triggering the westward tilting of this part of the arc, with locally dramatic uplifting (La Désirade). After a gap of 9–10 m.y., the volcanic activity resumed in the early Burdigalian (Recent arc), with a westward jump of the volcanic axis north of Martinique (Inner arc, corresponding to the northern part of the Recent arc). Later, the subduction of non-buoyant (gravimetrically uncompensated) ridges (Barracuda, Tiburon and St. Lucia ridges) exerted significant effects, but of a lower extent with regard to their spatial incidence and intensity, on the arc north of St. Lucia. These effects induce a sudden transverse shift of the volcanic front, of only some kilometers in length (forward and backward), centrifugal (longitudinal) migration of the eruptive centers on either side of the arc segment above the ridge, gentle uplifts, and enhance seismic and hydrothermal ( sensu lato) manifestations. An inverse approach of reasoning leads us to infer the existence of a ridge in the southern part of the subducted slab, the “Grenadines crypto-ridge”. The latter might be in its final stage of absorption by the subduction. Emphasis is put on the role played by the aseismic ridges in the volcanic and tectonic evolution of convergent margins, and particularly their control of the thickness of the subducted sediments, thought to be the necessary condition for triggering the usual arc volcanism.

Physical volcanology of the submarine Mariana and Volcano Arcs

Narrow-beam maps, selected dredge samplings, and surveys of the Mariana and Volcano Arcs identify 42 submarine volcanos. Observed activity and sample characteristics indicate 22 of these to be active or dormant. Edifices in the Volcano Arc are larger than most of the Mariana Arc edifices, more irregularly shaped with numerous subsidiary cones, and regularly spaced at 50–70 km. Volcanos in the Mariana Arc tend to be simple cones. Sets of individual cones and volcanic ridges are elongate parallel to the trend of the arc or at 110° counterclockwise from that trend, suggesting a strong fault control on the distribution of arc magmas. Volcanos in the Mariana Arc are generally developed west of the frontal arc ridge, on rifted frontal arc crust or new back-arc basin crust. Volcanos in the central Mariana Arc are usually subaerial, large (> 500 km3), and spaced about 50–70 km apart. Those in the northern and southern Marianas are largely submarine, closer together, and generally less than 500 km3 in volume. There is a shoaling of the arc basement around Iwo Jima, accompanied by the appearance of incompatible-element enriched lavas with alkalic affinities. The larger volcanic edifices must reflect either a higher magma supply rate or a greater age for the larger volcanos. If the magma supply (estimated at 10–20 km3/km of arc per million years at 18° N) has been relatively constant along the Mariana Arc, we can infer a possible evolutionary sequence for arc volcanos from small, irregularly spaced edifices to large (over 1000 km3) edifices spaced at 50–70 km. The volcano distribution and basal depths are consistent with the hypothesis of back-arc propagation into the Volcano Arc.

Intra-arc sedimentation and tectonism: Late Paleozoic evolution of the eastern Klamath terrane, California

Rocks of the eastern Klamath terrane (Redding section) in northern California record deposition, magmatism, and tectonism in an island-arc setting. Volcanism and sedimentation persisted for more than 220 m.y., spanning Devonian to Middle Jurassic time, and yielding thick successions of water-laid volcanic debris and lavas, with interstratified carbonate and clastic sequences. This study focuses on the upper Paleozoic rocks within the section. The oldest rocks studied, within the Upper Devonian(?) and Mississippian Brag-don Formation, contain epiclastic debris that was deposited in a submarine-fan setting. Sandstone compositions indicate that quartzose sedimentary and metasedimentary rocks formed the source area for epiclastic debris and that volcanism persisted throughout sedimentation. Transitional shallow-water facies in the upper part of the Bragdon interfinger with the overlying Baird Formation, which contains abundant volcanic debris. Volcanogenic shelf and strandline deposits characterize the lower part of the formation, and volcaniclastic turbidites characterize the upper part. The Baird Formation is locally interfingered with the overlying Lower Permian McCloud Limestone, which records two stages of Early Permian carbonate platform development: early Wolfcampian and late Wolfcampian to early Leonardian. Only the younger of the two platforms is present within the study area, and it contains more distal facies than those preserved to the south. The strata contain abundant fusulinids and corals which provide age constraints for carbonate-platform development and form the basis for regional biogeographic correlation. The time-transgressive Upper Permian Bollibokka Group (Nosoni and Dekkas Formations) succeeds the McCloud Limestone, comprising thick sequences of volcanogenic debris, intercalated lava, and, in the upper part, lenses of shallow-water limestone. The Upper Permian strata reflect volcanism and rapid subsidence within the are. All upper Paleozoic units were intruded by small gabbroic bodies and diabase dikes which may be partly cogenetic with Upper Permian basalts in the upper Dekkas Formation. The upper Paleozoic succession records local pulses of magmatism and variable subsidence rates within the eastern Klamath arc. Mississippian epiclastic rocks reflect uplift of the volcanic-arc basement, which contained rocks similar to those now exposed in the adjacent Yreka terrane or Shoo Fly Complex. Pulses in volcanism were restricted to mid-Carboniferous and Late Permian time. Diminished volcanism during the Early Permian allowed development of carbonate platforms. Rapid fluctuations of sedimentation and sub-sidence rates; pronounced lateral facies and age variations; accumulation of thick, volcanic-debris aprons during episodic volcanism; and sporadic persistence of volcanism during apparent lulls characterized deposition within this succession and are common processes in modern volcanic arcs. The stratigraphic record is composed predominantly of volcanogenic rocks and lesser carbonates, reflecting events and processes within the arc itself. Evidence for nearby basins or landmasses is sparse. The Mississippian epiclastic rocks, however, which were derived from the arc basement, reflect an ultimate sialic crustal source. Comparison of volcanic-arc evolution with the late Paleozoic evolution of continental margin and basinal rocks to the east indicates possible paleogeo-graphic ties between the eastern Klamath volcanic arc and rocks of the Golconda allochthon or the North American continent.

Cenozoic Sedimentary and Deformational History of Hispaniola, 1: Southeastern Cordillera Central: ABSTRACT

The Cordillera Central approximates an elongate (220 km), elevated (>3 km), thrust-bounded anticline cored by Cretaceous-Paleogene arc rocks and uplifted during Miocene to recent time by convergent strike-slip movements between the North American and Caribbean plates. The southeastern termination of the anticline plunges beneath a thick (>6 km), well-exposed marine clastic sedimentary sequence. Because uplift-related faulting is minimal in this hinge region of the Cordillera Central anticline, the authors have carried out detailed mapping of the area to determine (1) relation of Cretaceous-Paleogene arc basement to overlying Cenozoic sedimentary cover, and (2) Cenozoic deformational history of arc and basin sequences. Mapping has clearly distinguished three superimposed Cenozoic basins lying on arc basement. The lowest basin (basin 1) is Paleocene-Eocene in age and consists of alternations of arc-derived turbidies with interbedded pelagic limestone and red mudstone. In apparent conformity above this basin is an approximately 4-km thick marine clastic sequence of medial Eocene through early Miocene age (basin 2). These sediments consist of fining-upward turbiditic sequence derived from the northwest and northeast. Arc basement and overlying basins 1 and 2 were shortened approximately 25% in a short-lived, northwest-southeast-directed compressional event that resulted in the formation of large open synclines and tightlymore » folded and faulted anticlines with fold amplitudes of 1-6 km. Underformed, medial Miocene sediments of a mixed clastic and carbonate shelf facies (basin 3) unconformably overlie the folded latest Oligocene-early Miocene rocks of basin 2 and thus constrain the age of folding as early to middle Miocene.« less