Ophiolitic Basement Research Articles

Origin of the Upper Cretaceous–Tertiary sedimentary basins within the Tauride–Anatolide platform in Turkey

A number of sedimentary basins formed within the Tauride–Anatolide Platform of Anatolia during the Late Cretaceous–Tertiary period. Previous studies have proposed different tectonic and evolutionary models for each basin. Geological characteristics of the basins, however, suggest that all these basins are of the same origin and that they followed a similar evolutionary model to one another. Basin development within the Tauride–Anatolide Platform took place in a post-collisional environment following the northward subduction of the northern Neotethys ocean beneath the Pontides. The closure of the northern Neotethys ocean ended with collision of the Tauride–Anatolide Platform with the Pontide volcanic arc and resulted in large bodies of oceanic remnants thrust over the Tauride–Anatolide Platform as ophiolite nappes. Formation of the sedimentary basins followed the emplacement of the ophiolite nappes as they formed as piggy-back basins on top of the underlying thrust ophiolite basement.

3‐D crustal velocity structure at the rift tip in the Western Woodlark Basin

We present a three‐dimensional (3‐D) crustal velocity model for part of the western Woodlark Basin in the SW Pacific, where rifting of a continental arc transitions to seafloor spreading. Velocities to ∼14 km depth are constrained by regularized inversion of first arrival traveltimes recorded at 14 ocean bottom instruments from a network of airgun profiles within an 80 km × 80 km area. The 3‐D velocity model images the axis and flanks of the rift basin adjacent to the westernmost spreading segment. Velocity contours that dip north at ∼10° parallel seismic reflectors from within a thick ophiolitic basement. High velocities (7.5 km/s) at the base of our model and preliminary analysis of earthquake P‐wave arrival times are consistent with an average crustal thickness of 15–20 km. Anomalously shallow upper mantle (8 km/s) velocities occur 6 km south of the spreading center at a depth of 3 km below the seafloor, and in the western part of the model, ∼30 km east of the D'Entrecasteaux Islands, at a depth of 6 km below the seafloor.

Discussion of a Miocene collisional belt in north Borneo: uplift mechanism and isostatic adjustment quantified by thermochronology

Scientific editing by Alex Maltman. John Milsom & Robert Holt write: Knowledge of the gravity field is vital to any study of isostasy. This was acknowledged by ⇓Hutchison et al. (2000) when they noted that the work of ⇓Holt (1998) placed constraints on their geological model for Sabah. However, they went on to ignore these constraints, arguing that the depth of the crustal root beneath the Western Cordillera might have been overestimated due to ‘the model assumption of an excessively thick ophiolitic basement’ and that the presence of thick crust beneath Sabah was ‘in conflict with the outcropping geology’. The Hutchison et al. interpretations of the deep structure of Sabah were illustrated in a series of cross-sections in which some features (notably the topography) were amplified for the sake of clarity. There was, however, no suggestion that any features would be underemphasized, and the depth scales, although described as only ‘rough guides’, indicated normal 5–10 km thicknesses for oceanic crust. Therefore, and in the light of the comments quoted above, the thin crust shown throughout Sabah and the generally somewhat greater thickness of the extended continental crust of the Dangerous Grounds must be regarded as essential elements of the model. The Late Miocene cross-section (⇓Fig. 1), showing the end of orogeny in Sabah, is a guide to the Hutchison et al. view of the present-day situation. Crust is less than 30 km thick beneath the Western Cordillera and less than 20 km thick elsewhere. We contend that these values are incompatible with the gravity field, and that it is no more acceptable to present a geological interpretation that ignores geophysical data than a geophysical interpretation that ignores geology. Some way must be found of reconciling the two. Fig. 1. Sabah in the Late Miocene, from ⇓Hutchison et al. (2000, fig. 6b). …

Tectonic evolution of the Jurassic–Cretaceous Great Valley forearc, California: Implications for the Franciscan thrust-wedge hypothesis

Interpretation of seismic reflection data and restoration of depositional geometries of Cretaceous forearc basin strata in the northwest Great Valley of California provide important controls on structural reconstructions of the western margin of the Sacramento Valley and northern Coast Ranges. Monoclinal eastward dips of Great Valley Group strata and fault systems striking northwest-southeast, which are features proposed as evidence for a west-dipping blind Great Valley–Franciscan sole thrust and related backthrusts, instead are expressions of bedding geometry that resulted from folding of the Paskenta and related synsedimentary normal faults, depositional onlap, and a major structural-stratigraphic discontinuity. The discontinuity separates east-dipping Aptian and younger Great Valley Group strata from beds of lower Great Valley Group and Coast Range ophiolite that were deformed and erosionally or structurally truncated by mid-Cretaceous time. Dip divergence imaged between the supracrop and subcrop of the discontinuity is not unique to the ancient Great Valley forearc, but is also observed in modern forearc basins. Advocates of the Franciscan thrust-wedge model have also proposed that west-dipping, shingled patterns of seismic events imaged beneath the Sacramento Valley are imbricate thrust slices of the Great Valley Group. This hypothesis, however, is incompatible with borehole, potential-field, and seismic-refraction data that characterize the Sacramento Valley basement as ophiolitic. Seaward-dipping reflections in the ophiolitic basement of the Sacramento Valley are analogous to layering developed in the oceanic crust of volcanic rifted margins or generated along midocean ridges. Thus, late-stage tectonic mechanisms are not required to interpret a forearc that owes much of its present-day bedding architecture to processes coeval with deposition. Thickening of the Great Valley Group stratigraphic section (Valanginian–Turonian) in the hanging walls of the Paskenta, Elder Creek, and Cold Fork fault zones, combined with attenuation or complete omission of preextensional units (including the Coast Range ophiolite) and geometric evidence based on seismic reconstructions, suggest that these faults are Jurassic–Cretaceous normal faults that developed in a submarine setting. Down-structure views of the Great Valley outcrop belt simplify otherwise complex map relations and portray the Paskenta and related faults as half-graben bounding faults that accommodated significant northwestward tectonic transport of hanging-wall rocks. It is significant that these faults sole into the Coast Range fault, an enigmatic forearc structure that juxtaposes rocks of the Franciscan Complex (blueschists) with rocks of the Coast Range ophiolite and Great Valley Group that have sustained only zeolite-grade metamorphism. Discovery of Jurassic–Cretaceous crustal-scale extension in the Great Valley forearc suggests that a significant part of Coast Range fault-related attenuation developed early in the history of the subduction complex.

Age and MORB geochemistry of the Sabah ophiolite basement

A late Jurassic to early Cretaceous (Neocomian) age is most likely for the ophiolite basement of Sabah, consistent with the Barremian-Aptian age of the overlying ribbon cherts. Attempts at dating have been confined to the K:Ar method, unfortunately unsuited to this rock suite because of its low potassium and high atmospheric argon contents. Our data confirm the need for interpreting part of the extensive ophiolite terrane of the Segama Highlands as consisting of continental lithosphere of Jurassic age - only limited exposures of calc-alkaline granite are known in the Litog Klikog Kiri vicinity ofUlu Segama. The ophiolite suite of the Labuk and Segama Highlands is oflow-K tholeiitic affinity and the trace element geochemistry indicates MORB characteristics, typical of magma formation at a mid ocean ridge spreading centre. Before uplift, the Sabah ophiolitic basement may be interpreted as having formed an integral part of either the western Pacific or the eastern Indian Ocean, extant beneath sea level in the Argo Abyssal Plain west of Australia.

Crustal structure across the Pacific margin of Nicaragua:evidence for ophiolitic basement and a shallow mantle sliver

Seismic wide-angle measurements across the Pacific margin of Nicaragua were carried out using ocean bottom hydrophones and land stations recording marine airgun shots. The structure and the P-wave velocity of the subducting Cocos and overriding Caribbean Plates were determined by modelling wide-angle data and further constrained by coincident seismic reflection, borehole and gravity data. The oceanic crust of the Cocos Plate is 5.5 km thick, with a thin pelagic sediment cover. The plate boundary can be traced to 40 km depth and is generally similar to configurations derived earlier from the local seismic network. A major feature of the upper plate is an 80-km-wide high-velocity, high-density rock unit reaching from the front of the margin to about the middle of the shelf. This wedge-shaped unit is 15 km thick beneath the shelf edge and is composed of a 5-km-thick upper part with velocities increasing from 3.5 km s−1 near the trench to 5.2 km s−1 at the shelf break overlying a 10-km-thick lower part with velocities of 6.0 km s−1. Analysis of the gravity field requires densities of 2.6–2.7 g cm−3 for the upper part and 2.9 g cm−3 for the lower part of this unit. These velocities and densities suggest that the margin wedge is composed of ophiolitic rock similar to those sampled offshore Guatemala and exposed on the Nicoya Peninsula, Costa Rica. The velocity structure of this ophiolitic unit ends about 50 km offshore. Landward, the basement underneath the Sandino Basin is formed by older igneous rock drilled beneath upper Cretaceous sedimentary rock onshore Nicaragua. Beneath the ophiolitic basement we find a sliver with velocities typical of mantle material that begins at depths of 12–18 km and coincides with the down-dip limit of the seismogenic zone. Mantle densities are required for the sliver to match the gravity data. In a tectonic reconstruction, the suture of an oceanic plateau on the Farallon Plate against the Chortis Block in upper Cretaceous time is suggested. Suturing left the former trench and margin in deep water, consistent with the late Cretaceous to Palaeocene deep-water sediment of the Rivas Formation at the base of the Sandino Basin. Suture of the Farallon Plate and Chortis Block might have initiated the strike-slip movement along the Motagua–Polochic Fault System. The development of a new subduction zone in Eocene–Oligocene times left the ophiolitic basement and a sliver of oceanic mantle attached to the Chortis Block and shifted the volcanic arc about 70 km southwestwards, close to its present position.

Slow spreading accretion and mantle denudation in the Mirdita ophiolite (Albania)

The Mirdita ophiolite in Albania occupies a N‐S corridor which escaped most Alpine and Cenozoic deformation, possibly due to a thick ophiolitic basement. The sheeted dike complex strikes NS and dips steeply, indicating that the ridge was oriented parallel to the NS corridor and that the ophiolite has not been significantly tilted, although differential motion between individual massifs cannot be excluded. Classically, the western massifs have been considered “lherzolitic” and the eastern massifs “harzburgitic”. Detailed structural mapping reveals that the deep mantle section was harzburgitic and that the major differences between the two are restricted to the uppermost mantle and lower crustal section. These are typically “ophiolitic” in the eastern massifs, being composed of a thick dunitic transition zone rich in basaltic impregnations and chromite deposits overlain by a lower crust of layered gabbros. In contrast, in the western massifs the uppermost mantle is composed of highly strained to mylonitic lherzolites which originate from more depleted harzburgites by impregnation and tectonic dispersion of melt during deformation occurring at 1000°–800°C. Layered gabbros are locally absent, and the crust can be reduced to diabase dikes or sills and extrusives. The diabase intrusions are locally sheared together with peridotites and metamorphosed to amphibolites. The contrast between the eastern and western massifs is ascribed to successive episodes of magmatic and amagmatic spreading in a slow spreading environment. The low‐T, high strain deformation of the western massifs is localized in the dome‐shaped envelopes of these massifs. This structure, and even the present‐day topography of the western massifs, evoke the “turtleback” domes described along the Mid Atlantic Ridge (MAR) and explained by mantle denudation [Tucholke et al., 1998].

Regional implications of U/Pb SHRIMP age constraints on the tectonic evolution of New Caledonia

U/Pb SHRIMP ages for zircons from plagiogranites indicate a Late Carboniferous age of formation for ophiolitic basement of the Koh terrane in the Central Chain Mountains of New Caledonia. Samples from ophiolites at Koh and Koua yielded ages of 302±7 Ma and 290±5 Ma, respectively. The similarity of these ages to those of plagiogranites in the Dun Mountain Ophiolite Belt/Maitai terrane of New Zealand, and the comparable structural positions of the two terranes, potentially constrain reconstructions of the Cretaceous SW Pacific margin. Detrital zircons from late Mesozoic sediments that overlap the Koh terrane also provide potential constraints on the location of New Caledonia prior to its break-up and dispersal from eastern Australia.

Siliceous microfossils from Nias Island: their significance for the Tertiary paleoceanography of the northeast Indian Ocean

Nias Island, located in the eastern Indian Ocean, is part of the emergent, non-volcanic outer-arc ridge which marks the western margin of the Sunda Forearc Basin of northwestern Sumatra. The island is structurally divided into three main sub-basins and a basement high by three prominent SE-striking faults. Geologically it consists of three principal units; basement ophiolite complex, Oligocene to Recent sedimentary rocks and diapiric mélange. The recovery of indigenous radiolarians from a chert sample in the Mujoi Sub-basin constrains the oldest age of emplacement of the ophiolitic basement as Middle Eocene. The finding of Miocene diatomite in the Lahewa Sub-basin is interpreted as the product of upwelling effects and probably coincides with one of the cooling intervals recognized from other parts of the world.

Ophiolitic basement to a forearc basin and implications for continental growth: The Coast Range/Great Valley ophiolite, California

We present a compilation of 18 published models from the length of the Great Valley forearc basin, California, based on seismic reflection, borehole, seismic refraction, gravity, and aeromagnetic data to address long‐standing questions about the nature of the basement to the Great Valley, its origin, its tectonic history, and it's mechanism of incorporation into the North American continental margin. The geophysical models permit a 700‐km‐long, 70‐km‐wide, complete ophiolite sequence beneath the entire Great Valley. In the northern Great Valley, the ophiolite is overlain by ophiolitic breccia, the ophiolite crust is 7–8 km thick, and the mantle section is mostly unserpentinized. Beneath the southern Great Valley, there is no ophiolitic breccia, the crust may be up to 10 – 12 km thick, and the mantle section, if present at all, is serpentinized to such a degree that it cannot be distinguished from Sierran basement or mafic ophiolite members on the basis of velocity or density data. Geochemical, petrological, and paleomagnetic data support suprasubduction zone ophiolite formation at North American paleolatitudes, and geological data and geophysical models are consistent with ophiolite formation by back arc spreading behind an east facing arc. In the north, this was apparently followed by obduction of back arc crust onto older continental basement during the Late Jurassic Nevadan orogeny. In the south, the newly formed intraoceanic arc and back arc apparently collided with the continental margin during the Nevadan orogeny but were not obducted onto it. Instead, the arc and back arc “docked” with the continental margin leaving the arc itself to become the basement to the Great Valley basin. Cretaceous Sierran magmatism then intruded plutons beneath the docked ophiolite and mafic arc. Irrespective of the detailed accretionary history, our cross sections show a rapid pulse of continental growth by ophiolite accretion of more than 500 km³ km−1 in less than 10 Myr.

Age and origin of the Boil Mountain ophiolite and Chain Lakes massif, Maine: implications for the Penobscottian orogeny

The Boil Mountain ophiolite complex of west-central Maine is widely interpreted to mark the Lower Ordovician Penobscottian suture between the Dunnage, Chain Lakes, and Gander terranes. The ophiolite consists of two distinct volcanic groups, including a lower island-arc tholeiite sequence and an upper mid-ocean-ridge basalt sequence. A new Middle Ordovician 477 ± 1 Ma U–Pb age on a tonalite sill that intrudes the lower volcanic–gabbroic sequence is younger than other ca. 500 Ma age constraints for the ophiolite and represents a maximum age for the ophiolite prior to final emplacement over gneissic rocks of the Chain Lakes massif. A comparison of ages and paleogeography of the Boil Mountain ophiolite with ophiolitic sequences in Quebec and Newfoundland indicates that the Taconian and Penobscottian orogenies and ophiolite obduction occurred simultaneously, although on different margins of the Iapetus Ocean. The Taconian ophiolite sequences were obducted onto the Appalachian margin of Laurentia during its collision with the Notre Dame – Bronson Hill belt in the Middle Ordovician, whereas the Boil Mountain ophiolite was obducted onto the Gander margin of Gondwana during its collision with the Exploits subzone – Penobscot arc of the Dunnage terrane in the Lower – Middle Ordovician. We suggest that the lower volcanic–gabbroic sequence of the Boil Mountain ophiolite represents the fore-arc ophiolitic basement to the Penobscot arc. Middle Ordovician rifting of the Penobscottian orogenic collage on the Gander margin formed a new volcanic sequence (Popelogan arc) in front of a growing back-arc basin, and erupted the upper tholeiitic sequence of the Boil Mountain ophiolite in a back-arc-basin setting. The tonalité sill formed during this event by partial melting of the lower volcanic–gabbroic sequence. Spreading in this back-arc basin (Tetagouche basin) brought a fragment of the Gander margin (Chain Lakes massif), along with an allochthonous ophiolitic cover (Boil Mountain complex) across Iapetus, where it collided with the Taconic modified margin of North America in the Late Ordovician and was then intruded by the Ashgillian Attean pluton.

Tertiary and quaternary magmatism in Mindanao andLeyte (Philippines): geochronology, geochemistry and tectonic setting

A set of 230 igneous rock samples from Leyte and Mindanao (Philippines) has beenstudied for their major and trace element chemistry, mineralogy and geochronology ( 40K- 40Ar dating). Several volcanic sectors have been distinguished according to their geographic position, tectonic framework and geologic history. (a) In Leyte, the ophiolitic basement at 50 Ma is MORB-type but characterized by negative Nb 0 anomalies typical of back-arc basalts. Miocene (20 and 11 Ma) and Late Pliocene to Quaternary magmas (2.5−0.4) show typical island are talc-alkaline characteristics. In Eastern Mindanao, Eocene (47−46 Ma) magmatic rocks are arc-tholeiitic, while those of Miocene (18−6 Ma) and Early Pliocene (4.5 Ma) age are generally talc-alkaline. Dacites of 12 Ma age have adakitic compositions. The young ( > 3 Ma) volcanics in Surigao and North Davao are mostly adakitic, although talc-alkaline basalts and basaltic andesites dated at 0.9 Ma overlie adakites in Surigao. (b) In Daguma, Oligocene (30 Ma) arc tholeiitic diorites intrude older arc sequences. Miocene0 (17.0−7.7 Ma) dikes and flows in both Daguma and Sarangani Peninsula are mostly talc-alkaline andesites, although minor adakites dated back to 18 Ma have been found overlying undated breccias in Daguma. In southeast Daguma, the tale-alkaline volcanic substratum of Pliocene-Recent volcanoes is 8−6 Ma old. Quaternary (1.83 Ma to present) andesitec and dacitic volcanics in Mt Parker and Mt Matutum (southwest Daguma) are either talc-alkaline or adakitic, while Mt Blit (northwest Daguma) is built-up by 2.5-Ma-old HFSE-enriched basalts, 1-Ma-old normal talc-alkaline andesites and 0.8-Ma-old adakites. (c) In Central Mindanao, Miocene andesites yield ages of 20 and 16 Ma. Pliocene (2.5 Ma) 0 volcanism is dominated by talc-alkaline and HFSE-enriched basalts and basaltic andesites. Quaternary lavas (I Ma to present) range in character from tale-alkaline to shoshonitic and exhibit wide variations in their trace element behavior. (d) Miocene lavas of 19−11 Ma age in Zamboanga are mostly talc-alkaline, though a lone 0 18-Ma-old adakite was also encountered. Pliocene-Quaternary basalts and basaltic andesites dated back to 2.2−1 Ma are HFSE-enriched, such that some display positive Nb anomalies in spidergrams. Adakites of 3.8−0.7 Ma age are located nearer the Sulu trench. The compositional variety of Tertiary-Recent island arc magmatism in Leyte and Mindanao couldbe linked to the tectonic settings prevailing during each magmatic phase and is here utilized to refine paleokinematic reconstruction of the tectonic history of the Philippine archipelago. The identification of adakites at certain time periods is used to date timings of subduction initiations and arc polarity reversals. Major and abrupt changes in geochemical compositions are well exhibited by the Pliocene-Pleistocene volcanism, involving the collision-related shift from talc-alkaline to shoshonitic volcanism in Central Mindanao and adakite production in all the volcanic sectors defined. The origin of the HFSE-enrichments among lavas younger than 3 Ma could be attributed to the presence in their mantle source of an OIB component, or, alternatively, to the enrichment of this mantle source through percolation of slab-derived melts.

Abstract: Ophiolitic Basement to the Great Valley Forearc basin, California, from Seismic and Gravity Data: Implications for Crustal Growth at the North American Continental Margin 

Abstract: Ophiolitic Basement to the Great Valley Forearc basin, California, from Seismic and Gravity Data: Implications for Crustal Growth at the North American Continental Margin 

Ophiolitic basement to the Great Valley forearc basin, California, from seismic and gravity data: Implications for crustal growth at the North American continental margin

The nature of the Great Valley basement, whether oceanic or continental, has long been a source of controversy. A velocity model (derived from a 200-km-long east-west reflection-refraction profile collected south of the Mendocino triple junction, northern California, in 1993), further constrained by density and magnetic models, reveals an ophiolite underlying the Great Valley (Great Valley ophiolite), which in turn is underlain by a westward extension of lower-density continental crust (Sierran affinity material). We used an integrated modeling philosophy, first modeling the seismic-refraction data to obtain a final velocity model, and then modeling the long-wavelength features of the gravity data to obtain a final density model that is constrained in the upper crust by our velocity model. The crustal section of Great Valley ophiolite is 7–8 km thick, and the Great Valley ophiolite relict oceanic Moho is at 11–16 km depth. The Great Valley ophiolite does not extend west beneath the Coast Ranges, but only as far as the western margin of the Great Valley, where the 5–7-km-thick Great Valley ophiolite mantle section dips west into the present-day mantle. There are 16–18 km of lower-density Sierran affinity material beneath the Great Valley ophiolite mantle section, such that a second, deeper, “present-day” continental Moho is at about 34 km depth. At mid-crustal depths, the boundary between the eastern extent of the Great Valley ophiolite and the western extent of Sierran affinity material is a near-vertical velocity and density discontinuity about 80 km east of the western margin of the Great Valley. Our model has important implications for crustal growth at the North American continental margin. We suggest that a thick ophiolite sequence was obducted onto continental material, probably during the Jurassic Nevadan orogeny, so that the Great Valley basement is oceanic crust above oceanic mantle vertically stacked above continental crust and continental mantle.

Gravity fields in eastern Halmahera and the Bonin Arc: Implications for ophiolite origin and emplacement

Classic ophiolites, as exemplified by the Troodos Massif in Cyprus and the Papuan Ultramafic Belt in eastern New Guinea, are large overthrust masses which are generally associated with large positive gravity anomalies. However, similar rocks occurring in extensive fragmented terranes which have also been described as ophiolitic do not produce large gravity effects. The eastern part of the island of Halmahera, in northeastern Indonesia, is an ophiolite of this latter type. On the two eastern arms of the island, a Mesozoic ophiolitic basement is overlain by, and imbricated with, Upper Cretaceous and Paleogene arc volcanic and sedimentary rocks. Bouguer gravity values are generally in the range +50 to +150 mGal and are characterised by steep local gradients indicative of shallow sources. The Bouguer gravity average suggests that the crust is at least 20 km thick, and it must be even thicker if a significant part of the anomalous gravity field is due to the presence of a cold and therefore dense, lithospheric slab within the asthenosphere, associated with the present‐day subduction beneath Halmahera. The absence of any exposures of continental basement rocks or of quartzose sediments in eastern Halmahera suggests that these ophiolites have not been overthrust onto continental crust and that the thickening occurred in an intraoceanic island arc. The Paleogene arc was evidently characterised by volcanism occurring over an unusually wide area. In this it resembles the Izu‐Bonin volcanic arc, which, like Halmahera, has been situated at the margin of the Philippine Sea Plate throughout its history. The gravity field of the Halmahera ophiolite is comparable with that of the Bonin volcanic arc, but there is no Halmahera parallel to the very high gravity fields recorded over the Bonin Islands forearc ridge. The equivalents of this part of the Paleogene arc may be represented by the ophiolitic complexes now distributed along the northern margin of the orogenic belt in New Guinea.

Nephrite and metagabbro in the Haast Schist at Muddy Creek, northwest Otago, New Zealand

Abstract At Muddy Creek, northwest Otago, a pod of gabbro, probably in a matrix of serpentinite, was tectonically emplaced into quartzofeldspathic sediments before the main metamorphism of the Haast Schist. Metasomatic diffusion during pumpellyite actinolite to greenschist facies metamorphism produced talc‐, tremolite‐, chlorite‐, and muscovite‐rich reaction zones at the margin of the pod. Subsequent shearing disrupted the metasomatic sequence and produced nephrite from the tremolite zone precursor. Relict magmatic hornblende and clinopyroxene from the metagabbro have compositions appropriate to a volcanic arc tectonic setting. In contrast, relict pyroxenes from four metabasaltic knockers in the Greenstone Melange have compositions indicating generation of magmas in both ocean‐floor and ocean‐island environments. Ultramafic and mafic rocks at Muddy Creek and in the Greenstone Melange are thought to represent fragments of the ophiolitic basement to the Haast Schist, the tectonic settings of which help cons...

A tertiary fold and thrust belt in the Valle del Cauca Basin, Colombian Andes

Surface geology and heophysical data, supplemented by regional structural interpretations, indicate that the Valle del Cauca basin and adjacent areas in west-central Colombia form a west-vergent, basement-involved fold and thrust belt. This belt is part of a Cenozoic orogen developed along the west side of the Romeral fault system. Structural analysis and geometrical constraints show that the Mesozoic ophiolitic basement and its Cenozoic sedimentary cover are involved in a “thick-skinned” west-vergent foreland style deformation. The rocks are transported and shortened by deeply rooted thrust faults and stacked in imbricate fashion. The faults have a NE-SW regional trend, are listric in shape, developed as splay faults which are interpreted as joining a common detachment at over 10 km depth. The faults carry Paleogene sedimentary strata and Cretaceous basement rocks westward over Miocene strata of the Valle del Cauca Basin. Fold axes trend parallel or sub parallel to the thrust faults. The folds are westwardly asymmetrical with parallel to kink geometry, and are interpreted to be fault-propagation folds stacked in an imbricate thrust system. Stratigraphic evidence suggests that the Valle del Cauca basin was deformed between Oligocene and upper Miocene time. The kinematic history outlined above is consistent with an oblique convergence between the Panama and South American plates during the Cenozoic.A negative residual Bouguer anomaly of 20–70 mgls in the central part of the Valle del Cauca basin indicates that a substantial volume of low density sedimentary rocks is concealed beneath the thrust sheets exposed at the land surface. The hydrocarbon potential of the Valle del Cauca should be reevaluated in light of the structural interpretations presented in this paper.

Evidence for ophiolite obduction, terrane accretion and polyorogenic evolution of the north Scandinavian Caledonides

Recent work on the thrust sheets of the Ofoten-Troms region, north Norwegian Caledonides, has resulted in revision of the tectonostratigraphic correlations providing new constraints on models for terrane accretion and poly-orogenic Cambro-Ordovician and Siluro-Devonian development. Previous studies on Late Ordovician-Early Silurian fossils in metasediments unconformably overlying the Lyngen Ophiolite complex within one of the Troms nappes documented a period of ophiolite obduction prior to the major Siluro-Devonian Scandian orogenic event. Correlation of these units and underlying and overlying thrusts sheets southward into the Ofoten area, 150 km to the south, is demonstrated. This implies that an extensive, completely transported, Upper Ordovician-Lower Silurian basin, developed on top of the obducted ophiolite, composes the bedrock of much of northern Norway. Metamorphism and deformation within these basinal rocks resulted from the Scandian phase of the Caledonian orogeny. The relatively weak metamorphic and deformational imprint on the ophiolitic basement provides a rare opportunity to evaluate the pre-Scandian tectonic evolution of northern Scandinavia. Poly-orogenic evolution, a controversial subject in northern Norway, is documented. The character and relative extent of Cambro-Ordovician and Siluro-Devonian orogenic zones in northern Norway are elucidated. The Balsfjord/Ofoten unconformity formed approximately concomitant with other post-ophiolite obduction unconformities in central and southern Norway, implying an orogen-wide Late Cambrian-Early Ordovician event.

Zircon U‐Pb geochronology of the Zambales and Angat Ophiolites, Luzon, Philippines: Evidence for an Eocene arc‐back arc pair

Two basement terranes, the Zambales ophiolite in the west, and the Angat ophiolite in the east, are exposed on the island of Luzon, separated by a circa 10 km thick and circa 100 km wide sedimentary basin. The structural and age relationships between the two ophiolitic blocks are central to understanding the geologic and tectonic development of the northern Philippines and evaluating models of terrane evolution proposed for this area of the western Pacific. We analyzed zircons from the Zambales and Angat terranes to better constrain their origin. Two zircon fractions from tonalite in the Acoje block of the Zambales ophiolite give concordant U‐Pb ages at 44.2 (±0.9) Ma. Two zircon fractions from plagiogranite and one fraction from diorite in the Coto block of the Zambales ophiolite give concordant U‐Pb ages of 45.1 (±0.6) Ma. These results provide a Middle Eocene age for the Zambales ophiolite, in agreement with the minimum Late Eocene age of the overlying Aksitero Formation. No age difference is discerned between the arc‐like Acoje block and MORB‐like Coto block of the Zambales ophiolite. Four zircon fractions from two sample sites in the Angat ophiolite give concordant ages of 48.1 (±0.5) Ma. This age is considerably younger than the Late Cretaceous age based on radiolarian fauna derived from a sheeted dike‐pillow lava‐sediment sequence south‐southeast of the main ophiolite. The small age difference between the Zambales and Angat ophiolites suggests a common origin and obviates the need for a major structural discontinuity west of the Southern Sierra Madre beneath the Central Valley of Luzon. The Cretaceous biostratigraphic ages of the ophiolitic rocks southeast of the Eocene Angat ophiolite implies that there are two ophiolitic basements exposed in the Southern Sierra Madre. The relationship between the two ophiolites is constrained by the overlying stratigraphic relations which indicate that an Eocene volcanic arc and associated volcaniclastic apron was built on both the Eocene and Cretaceous ophiolitic basement. This suggests that the Zambales‐Angat ophiolite represents a preserved Eocene back‐arc basin that opened behind an Eocene arc that developed within Cretaceous oceanic basement. In this model, the Zambales‐Angat ophiolites are therefore not allochthonous terranes but part of a single plate, generated in situ, forming part of the autochthonous basement of Luzon.

The Southeast Sulu Sea, a Neogene marginal basin with outcropping extensions in Sabah

The geophysical, drill and dredge details of the Sulu Sea marginal basin allow a correlation with the Neogene geology of eastern Sabah. The basin is modelled to have resulted from early Miocene intra-arc rifting related to the SE-facing subduction system of the NW Celebes Sea. The early stages of the opening of the Sulu Sea were characterized by explosive volcanic activity, and the rifting resulted in extensive olistostrome deposits. These events are seen onland as the Ayer and Tungku formations, and the Kuamut and Garinono formations respectively. Uplift ofthe Crocker Formation to the west provided the source for major quartz sands deposited in Sabah within the Tanjong Formation, and a major delta flowing northeasterly near Sandakan fed the continentally-derived turbidites of the deep Sulu Sea. Since the ophiolitic complex of Sa bah pre-dates the late-Lower Miocene opening of the Sulu Sea marginal basin, it represents the ocean floor upon which was built the volcanic arc, rifting of which gave rise to the Sulu Sea. Remnants of this early Cretaceous and older ophiolitic basement therefore underlie and outcrop within the NW margin of the SE Sulu Sea (Labuk Valley through the Cagayan Ridge), as well as along the Sulu Archipelago, extending into Darvel Day and Ulu Segama. Evidence is presented to show that the ophiolite complex was locally uplifted and eroding in the early Cenozoic.