Oceanic Forearc Research Articles

Neotethyan Ankara Melange, central Turkey: Formation by accretion of seamounts and supra-subduction zone ophiolites in an oceanic fore-arc setting

The Neotethyan Ankara Melange is a global reference for oceanic accretionary processes preserved in a collisional orogen. Here we show, based on eight representative outcrops around Ankara, that the melange encompasses stratigraphically coherent volcanic-sedimentary successions of mainly Late Jurassic-Early Cretaceous age. Our results indicate that the melange has a relatively organised tectonostratigraphy, in contrast to previous interpretations that emphasised its chaotic nature. Coherent volcanic-sedimentary successions of Late Jurassic-Early Cretaceous age are interpreted as flank facies of a large oceanic seamount (probably plume related) and its capping carbonate platform (atoll), including reef-and back-reef facies. Remnants of other seamounts are dated as Late Triassic-Early Jurassic and Late Cretaceous. Variably dismembered supra-subduction zone ophiolites of latest Early Jurassic-Middle Jurassic age, including boninites, formed in an oceanic fore-arc setting, similar to that of the Cenozoic Izu-Bonin fore arc, NW Pacific. The melange in the Ankara region partly accreted during the Early Cretaceous (pre-Albian) in response to collision of the inferred large seamount with the oceanic (ophiolitic) fore arc. During the collision which involved subduction-erosion slices of proximal–distal seamount flank lithologies accreted sequentially, whereas the seamount core subducted. Also during the collision, the distal outer edge of the ophiolitic fore arc, mainly mantle harzburgite, detached from the over-riding lithosphere and was incorporated into the melange. Structural evidence emphasises the importance of tectonic processes (rather than olistostromes) in the melange accretion. The accretionary prism in the outcrops studied was covered by arc-related sediments and volcanics during the Late Cretaceous and deformed by collision-related processes during latest Cretaceous-Paleogene.

Melt Migration and Interaction in a Dunite Channel System within Oceanic Forearc Mantle: the Yushigou Harzburgite–Dunite Associations, North Qilian Ophiolite (NW China)

AbstractHarzburgite–dunite associations in the Yushigou ophiolitic complex (North Qilian suture, NW China) offer insights into melt migration and melt–rock interaction above a subduction zone. Based on detailed petrographic and in situ analyses, we propose a three-stage model to describe the history of melt channel formation in oceanic arc mantle. In the first stage, high-degree melt extraction was followed by widespread infiltration of small-volume melts, which formed the harzburgite that makes up most of the Yushigou complex. The Yushigou harzburgites thus have highly refractory major-element compositions (olivine Fo = 91·1–93·2, spinel Cr# [atomic 100 × Cr/(Cr + Al)] = 55·2–62·9, whole-rock Mg# = 90·5–92·0, Al2O3 = 0·29–0·74 wt%, CaO = 0·35–0·54 wt%, and low Na2O = 0·02–0·05 wt%) but slight enrichment in the light rare earth elements (LREE). In the second stage, high-volume focused melts infiltrated into the mantle wedge and produced two types of dunite, with high-Cr# (mainly 63·1–73·6) versus low-Cr# (23·6–33·7) spinel. In the low-Cr# dunites, spinels have significantly fewer inclusions and lower Fo values (88·7–90·1) in olivine than in the high-Cr# dunite (Fo = 89·9–90·5), together with higher modal abundances of interstitial sulfides. The two groups of dunite show similar extremely low TiO2 (<0·01 wt%) but different igneous clinopyroxene trace-element patterns (ΣLREE/ΣHREE ≈ 1 in low-Cr# dunite versus ΣLREE/ΣHREE >10 in high-Cr# dunite), indicating distinct differences in the infiltrating melts. The low-Cr# dunite is a cumulate from an anhydrous Al- and S-enriched basaltic melt, whereas the high-Cr# dunite was produced by reaction of harzburgite with a carbon-bearing, alkaline hydrous silicate magma. The confluence of these melt migrations finally formed a channelized dunite network, and later magmatic activity was restricted to these channels. Pulsated melt supply after chromite crystallization formed pervasive sieve-textured rims around spinel in both kinds of dunite, but these are rare in the harzburgite. The third stage was marked by fluid-dominated metasomatism recorded by millimeter-scale veined conduits, which contain carbonate (dolomite and magnesite), amphibole, phlogopite and compound crystal assemblages in both kinds of dunite. CH4–N2–graphite-dominated fluid inclusions are widespread in all lithologies, recording the volatiles transported during the last metasomatic event. The Yushigou complex thus provides a detailed example of interaction between multiple batches of melt or fluid and a lithospheric mantle wedge dunitic channel system. The results of this study further suggest that the formation of podiform chromite requires melt participation, and the nature and origins of the melts can be diverse.

Subduction Polarity in Ancient Arcs: A Call to Integrate Geology and Geophysics to Decipher the Mesozoic Tectonic History of the Northern Cordillera of North America: REPLY

Subduction Polarity in Ancient Arcs: A Call to Integrate Geology and Geophysics to Decipher the Mesozoic Tectonic History of the Northern Cordillera of North America: REPLY

Chapter 7 Patuki and Croisilles melanges in South Island, New Zealand: genesis related to Permian subduction–accretion processes

Abstract The late Early Permian ( c. 278–270 Ma) supra-subduction zone (SSZ) Dun Mountain ophiolite is bordered to the east by the Pataki Melange and to the NE by the Croisilles Melange. In the south, the ophiolite passes into a dismembered incipient oceanic arc (Otama Complex). The above units represent an oceanic forearc generated above a west-dipping subduction zone. Terrigenous sediment reached the subduction trench after the Mid-Permian(?) docking of the oceanic forearc with the long-lived SE Gondwana active continental margin. Mixed terrigenous–volcaniclastic turbidites accumulated in the trench prior to and during melange accretion. Fragments of the overriding oceanic forearc (and incipient arc, locally) detached and mixed to form melange and broken formation. Despite some individual features (e.g. of the basalt chemistry), the Patuki and Croisilles melanges are interpreted as originally representing a single Permian trench–accretionary complex. The more distal (easterly) part was sliced into the adjacent accretionary complex of the Caples Terrane to form the Croisilles Melange (and equivalent Greenstone Melange) probably after the Triassic. The South Island melanges exemplify accretionary processes in which igneous and sedimentary rocks were detached from the overriding plate by subduction–erosion, together with accretion, including seamount material from the subducting oceanic plate, with implications for melanges elsewhere.

The ‘subduction initiation rule’: a key for linking ophiolites, intra-oceanic forearcs, and subduction initiation

We establish the ‘subduction initiation rule’ (SIR) which predicts that most ophiolites form during subduction initiation (SI) and that the diagnostic magmatic chemostratigraphic progression for SIR ophiolites is from less to more HFSE-depleted and LILE-enriched compositions. This chemostratigraphic evolution reflects formation of what ultimately becomes forearc lithosphere as a result of mantle melting that is progressively influenced by subduction zone enrichment during SI. The magmatic chemostratigraphic progression for the Izu–Bonin–Mariana (IBM) forearc and most Tethyan ophiolites is specifically from MORB-like to arc-like (volcanic arc basalts or VAB ± boninites or BON) because SI progressed until establishment of a mature subduction zone. MORB-like lavas result from decompression melting of upwelling asthenosphere and are the first magmatic expression of SI. The contribution of fluids from dehydrating oceanic crust and sediments on the sinking slab is negligible in early SI, but continued melting results in a depleted, harzburgitic residue that is progressively metasomatized by fluids from the sinking slab; subsequent partial melting of this residue yields ‘typical’ SSZ-like lavas in the latter stages of SI. If SI is arrested early, e.g., as a result of collision, ‘MORB-only’ ophiolites might be expected. Consequently, MORB- and SSZ-only ophiolites may represent end-members of the SI ophiolite spectrum. The chemostratigraphic similarity of the Mariana forearc with that of ophiolites that follow the SIR intimates that a model linking such ophiolites, oceanic forearcs, and SI is globally applicable.

Ultra-refractory mantle xenoliths from ocean islands: How do they compare to peridotites retrieved from oceanic sub-arc mantle?

Ultra-refractory harzburgitic mantle is abundantly sampled as xenoliths in ocean island magmas. Its occurrence has potentially important implications for both geochemistry and geodynamics, and we therefore need to understand how these unusually refractory compositions form. In this contribution, we compare ultra-refractory spinel harzburgite xenoliths from ocean islands with peridotites collected along active arcs, based on available major element data on rocks and their minerals. The compositions of arc peridotites vary from ultra-refractory to fertile and estimated equilibration temperatures range from about 800 to > 1300 °C. These variations are not directly related to setting (ocean–ocean or ocean–continent collision; forearc, arc or back-arc). In general, however, fertile compositions seem to be more common in continental arcs and to be accompanied by higher equilibration temperatures. Ultra-refractory peridotites appear to be most common along oceanic forearcs and give generally lower temperatures in the range 950–1050 °C. Ultra-refractory harzburgites sampled in subduction settings are chemically very similar to the ultra-refractory peridotites found as xenoliths in ocean islands. Both rock types compare quite well with the residues from experiments on the formation of primitive arc magmas. The formation of such magmas and their ultra-refractory harzburgite residues requires melting beyond the stability of clinopyroxene. Such high degrees of partial melting are probably achieved in two stages. The first stage is likely to take place as decompression melting along mid-ocean ridges or in back-arc spreading centers, whereas the second stage seems to require fluid-fluxed melting in the mantle wedge. Large degrees of melting may also be achieved in other types of settings if temperatures are sufficiently high. However, the common occurrence of ultra-refractory harzburgites together with melts generated from a strongly depleted precursor in active arcs is an argument in support of a subduction related origin of ultra-refractory harzburgites both in arc settings and in ocean islands. This implies that ultra-refractory arc material is common and may be recirculated by mantle convection. Because of their low density relative to “normal”, more fertile asthenospheric material, these ultra-refractory residues may rise as diapirs and intrude shallower parts of the asthenosphere, where they can accrete to younger oceanic plate.

Ultra-refractory Domains in the Oceanic Mantle Lithosphere Sampled as Mantle Xenoliths at Ocean Islands

Many peridotite xenoliths sampled at ocean islands appear to have strongly refractory major element and modal compositions. To better constrain the chemistry, abundance and origin of these ultra-refractory rocks we compiled a large number of data for xenoliths from nine groups of ocean islands. The xenoliths were filtered petrographically for signs of melt infiltration and modal metasomatism, and the samples affected by these processes were excluded. The xenolith suites from most ocean islands are dominated by ultra-refractory harzburgites. Exceptions are the Hawaii and Tahiti peridotites, which are more fertile and contain primary clinopyroxene, and the Cape Verde suite, which contains both ultra-refractory and more fertile xenoliths. Ultra-refractory harzburgites are characterized by the absence of primary clinopyroxene, low whole-rock Al2O3, CaO, FeO/MgO and heavy rare earth element (HREE) concentrations, low Al2O3 in orthopyroxene (generally < 3 wt %), high Cr-number in spinel (0·3–0·8) and high forsterite contents in olivine (averages > 91·5). They are therefore on average significantly more refractory than peridotites dredged and drilled from mid-ocean ridges and fracture zones. Moreover, their compositions resemble those of oceanic forearc peridotites. The formation of ultra-refractory ocean island harzburgites requires potential temperatures above those normally observed at modern mid-ocean ridges, and/or fluid fluxed conditions. Some ultra-refractory ocean island harzburgites give high Os model ages (up to 3300 Ma), showing that their formation significantly pre-dates the oceanic crust in the area. A genetic relationship with the host plume is considered unlikely based on textural observations, equilibration temperatures and pressures, inferred physical properties, and the long-term depleted Os and Sr isotope compositions of some of the harzburgites. Although we do not exclude the possibility that some ultra-refractory ocean island harzburgites have formed at mid-ocean ridges, we favor a model in which they formed in a process spatially and temporally unrelated to the formation of the oceanic plate and the host plume. As a result of their whole-rock compositions, ultra-refractory harzburgites have a very high solidus temperature at a given pressure, low densities and very high viscosities, and will tend to accumulate at the top of the convecting mantle. They may be preserved as fragments in the convecting mantle over long periods of time and be preferentially incorporated into newly formed lithosphere.

H2O subduction beyond arcs

The amount of H2O subducted to postarc depths dictates such disparate factors as the generation of arc and back‐arc magmas, the rheology of the mantle wedge and slab, and the global circulation of H2O. Perple_X was used to calculate phase diagrams and rock physical properties for pressures of 0.5–4.0 GPa and temperatures of 300–900°C for a range of bulk compositions appropriate to subduction zones. These data were merged with global subduction zone rock fluxes to generate a model for global H2O flux to postarc depths. For metasomatized igneous rocks, subducted H2O scales with bulk rock K2O in hot slabs. Metasomatized ultramafic rocks behave similarly in cold slabs, but in hot slabs carry no H2O to magma generation depths because they lack K2O. Chert and carbonate are responsible for minimal H2O subduction, whereas clay‐rich and terrigenous sediments stabilize several hydrous phases at low temperature, resulting in significant postarc slab H2O flux in cold and hot slabs. Continental crust also subducts much H2O in cold slabs because of the stability of lawsonite and phengite; in hot slabs it is phengite that carries the bulk of this H2O to postarc depth. All told, the postarc flux of H2O in cold slabs is dominated by terrigenous sediment and the igneous lower crust and mantle and is proportional to bulk rock H2O. In contrast, in hot slabs the major contributors of postarc slab H2O are metasomatized volcanic rocks and subducted continental crust, with the amount of postarc slab H2O scaling with K2O. The Andes and Java‐Sumatra‐Andaman slabs are the principal suppliers of pelagic and terrigenous sediment hosted H2O to postarc depths, respectively. The Chile and Solomon arcs contribute the greatest H2O flux from subducted continental and oceanic forearc, respectively. The Andean arc has the greatest H2O flux provided through subduction of hydrated ocean crust and mantle. No correlation was observed between postarc slab H2O flux and slab seismicity.

Neoproterozoic (835–720 Ma) Serpentinites in the Eastern Desert, Egypt: Fragments of Forearc Mantle

Most Neoproterozoic ophiolites of the Arabian‐Nubian Shield show compositions consistent with formation in a suprasubduction zone environment, but it has not been clear whether this was in a forearc or back‐arc setting. Ophiolitic serpentinites are common in the Eastern Desert of Egypt, but their composition and significance are not well understood. Here we report new petrographic, mineral, chemical, and whole‐rock compositional data for serpentinites from Wadi Semna, the northernmost ophiolitic serpentinites in the Eastern Desert, and use these to provide insights into the significance of other Eastern Desert serpentinite locales. The Wadi Semna serpentinites are composed essentially of antigorite, chrysotile, and lizardite, with minor carbonate, chromite, magnetite, magnesite, and chlorite, and they were tectonically emplaced. The alteration of chrome spinel to ferritchromite was accompanied by the formation of chloritic aureoles due to the release of Al from spinel. Major‐element compositions indicate that, except for the addition of water, the serpentinites have not experienced extensive element mobility; these were originally CaO‐ and Al2O3‐depleted harzburgites similar to peridotites from modern oceanic forearcs. High Cr# ($$\mathrm{Cr}\,/ ( \mathrm{Cr}\,+\mathrm{Al}\,) $$ ) in the relict spinels ($$\mathrm{average}\,=0.69$$ ) indicates that these are residual after extensive partial melting, similar to spinels in modern forearc peridotites. These characteristics of Wadi Semna serpentinites also typify 22 other Eastern Desert serpentinite localities. We infer that Eastern Desert ophiolitic serpentinites, except perhaps Gebel Gerf, originated by forearc seafloor spreading during subduction initiation associated with the closing of the Neoproterozoic Mozambique Ocean.

Link between SSZ ophiolite formation, emplacement and arc inception, Northland, New Zealand: U–Pb SHRIMP constraints; Cenozoic SW Pacific tectonic implications

New U–Pb age-data from zircons separated from a Northland ophiolite gabbro yield a mean 206Pb/ 238U age of 31.6 ± 0.2 Ma, providing support for a recently determined 28.3 ± 0.2 Ma SHRIMP age of an associated plagiogranite and ∼ 29–26 Ma 40Ar/ 39Ar ages ( n = 9) of basalts of the ophiolite. Elsewhere, Miocene arc-related calc-alkaline andesite dikes which intrude the ophiolitic rocks contain zircons which yield mean 206Pb/ 238U ages of 20.1 ± 0.2 and 19.8 ± 0.2 Ma. The ophiolite gabbro and the andesites both contain rare inherited zircons ranging from 122–104 Ma. The Early Cretaceous zircons in the arc andesites are interpreted as xenocrysts from the Mt. Camel basement terrane through which magmas of the Northland Miocene arc lavas erupted. The inherited zircons in the ophiolite gabbros suggest that a small fraction of this basement was introduced into the suboceanic mantle by subduction and mixed with mantle melts during ophiolite formation. We postulate that the tholeiitic suite of the ophiolite represents the crustal segment of SSZ lithosphere (SSZL) generated in the southern South Fiji Basin (SFB) at a northeast-dipping subduction zone that was initiated at about 35 Ma. The subduction zone nucleated along a pre-existing transform boundary separating circa 45–20 Ma oceanic lithosphere to the north and west of the Northland Peninsula from nascent back arc basin lithosphere of the SFB. Construction of the SSZL propagated southward along the transform boundary as the SFB continued to unzip to the southeast. After subduction of a large portion of oceanic lithosphere by about 26 Ma and collision of the SSZL with New Zealand, compression between the Australian Plate and the Pacific Plate was taken up along a new southwest-dipping subduction zone behind the SSZL. Renewed volcanism began in the oceanic forearc at 25 Ma producing boninitic-like, SSZ and within-plate alkalic and calc-alkaline rocks. Rocks of these types temporally overlap ophiolite emplacement and subsequent Miocene continental arc construction.

PETROGENESIS OF MANTLE PERIDOTITES FROM THE IZU-BONIN-MARIANA (IBM) FOREARC

Serpentinised spinel harzburgites to orthopyroxene-rich spinel dunites recovered during the Ocean Drilling Program (ODP) Leg 195 on top of the South Chamorro Seamount (southern sector of the Mariana forearc, West Pacific Ocean), along with additional spinel harzburgites from Conical and Torishima Seamounts (northern Mariana and Izu-Bonin forearc, respectively), previously collected during the ODP Leg 125, have been investigated to shed light on the nature and evolution of forearc mantle in the intra-oceanic supra-subduction environment. All the samples show a marked heterogeneity in terms of petrographic, mineralogical and geochemical features that suggests a complex, multistage evolution involving, at variable extent, partial melting, reactive porous flow melt migration and subsolidus metamorphic re-equilibration under decreasing T and open system conditions. Geochemical evidence of the interaction between peridotites and various melts/fluids is the ubiquitous enrichment in highly incompatible elements, such as Large Ion Lithophile Elements (LILE). As for the high-T evolution of these peridotites, a three-stages-model is proposed, involving: 1) a former depletion event, during which the IBM forearc peridotites experienced 20-25% polybaric fractional melting during adiabatic upwelling; 2) a second depletion event characterised by a marked impoverishment in modal orthopyroxene, related to the upraise migration of ultra-depleted melts; 3) a late interaction between a relatively small volume of residual melts and the refractory mantle sequence. Oxidation state of the mantle minerals meanly decreases from north (Torishima Seamount) to south (South Chamorro), according to significant different contributions coming from the subducted Pacific Plate. In particular, the absence of a marked oxidation in South Chamorro peridotites suggests that the percolating melts during Stage 2 had not significant slab-derived component. This observation lead us to conclude that a thermal anomaly in the western Pacific mantle rather than the injection of hydrous components must be the “engine” determining the extreme depletion of the oceanic forearc peridotites and the arc formation. In this frame, it is proposed that IBM peridotites during Stage 1 underwent decompression partial melting and contributed to arc volcanism as actual mantle source. Successively, they were emplaced at relatively shallow levels (Stage 2), constituting the top of a strongly refractory mantle column and being percolated by melts produced by plumbing sources of the arc volcanism.

The petroleum potential of inversion anticlines in the Banda Arc

The Banda Arc in eastern Indonesia and newly independent Timor-Leste (East Timor) is the zone of collision between the northern margin of the Australian continent and an oceanic island arc system bordering the Eurasian plate. Structurally, the Banda forearc is a fold and thrust belt, consisting of the imbricated outer edge of the Australian continent, overlain locally by fragments of the precollisional oceanic forearc. The Banda forearc is an established petroleum province in the island of Seram, but the remainder of the forearc has so far yielded no further exploration success despite close geological similarity to Seram. A major disincentive to exploration in the Banda forearc has been a common perception of structural complexity. This complexity may, however, have been overstated, and this paper presents a simple structural model for the evolution of the Banda forearc.Key to this model is the recognition of basement-involved inversion structures in the deeper parts of the collision complex. The inverted graben basins are filled with PermianJurassic continental margin sequences, including high-quality source rocks of Late TriassicEarly Jurassic age, and contemporaneous potential reservoir sequences, sealed by MiddleLate Jurassic shales. These Jurassic shales also act as an important decollement horizon, separating shallow-level structural complexity from a simpler structural style beneath, characterized by large inversion anticlines. The Oseil field in Seram may be located in such an inversion anticline, and comparable prospective structures are identified elsewhere in the Banda forearc, particularly in Timor and the Tanimbar islands.

Forearc ophiolites: a view from the western Pacific

Abstract Many of the world’s largest ophiolite masses are now being interpreted as remnants of oceanic forearcs stranded on continental margins in the course of arc-continent collision. This interpretation implies a former association between the ophiolite and a volcanic arc at a distance of 100–200 km. The New Caledonia region of the SW Pacific is one of the few areas where there is good evidence for the presence of such an arc. On New Caledonia itself, the Grand Massif du Sud ophiolite has been thrust over the Norfolk Ridge continental fragment, while the coral islands of the Loyalty Group, c. 100 km to the NE, cap large sub-sea edifices with volcanic morphology. New Caledonia can be correlated geologically with New Guinea, despite the considerable width of open ocean that now separates the two. In central New Guinea, where ophiolites emplaced along the northern flank of the main mountain spine lie some 100 km south of exposures of arc-volcanic basement in the north coast ranges, the similarities include the relationship between New Caledonia and the Loyalty Islands. There is, however, no obvious comparable relationship in the case of the largest of the New Guinea ophiolites, the Papuan Ultramafic Belt of the eastern peninsula, which is backed to the north and east by the oceanic Solomon Sea. An associated volcanic arc can be recognized in this area only by assuming a complicated history of collision, post-collision arc-forearc separation and sea-floor spreading, followed by renewed contraction and a very recent and continuing second collision. The case for such a sequence of events can be made on a number of grounds. If the processes responsible are general in their nature, they could explain the apparent absence of arc-volcanic belts in association with many other supposedly forearc ophiolites.

Alteration and mineralization of an oceanic forearc and the ophiolite‐ocean crust analogy

Mineralogical, chemical, and isotopic (O, C, S, and Sr) analyses were performed on minerals and bulk rocks from a forearc basement section to understand alteration processes and compare with mid‐ocean ridges (MOR) and ophiolites. Ocean Drilling Program Hole 786B in the Izu‐Bonin forearc penetrates 103 m of sediment and 725 m into volcanic flows, breccias, and basal dikes. The rocks comprise boninites and andesites to rhyolites. Most of the section was affected by low‐temperature (&lt;100°C) seawater alteration, with temperatures increasing downward. The rocks are partly (5–25%) altered to smectite, Fe‐oxyhydroxide, calcite, and phillipsite, and exhibit gains of K, Rb, and P, loss of Ca, variable changes in Si, Na, Mg, Fe, Sr, and Y, and elevated δ18O and 87Sr/86Sr. Higher temperatures (∼150°C) in the basal dikes below 750 m led to more intense alteration and formation of chlorite‐smectite, corrensite, albite, K‐feldspar, and quartz (±chlorite). A 5 m thick hydrothermally altered and pyritized zone at 815 m in the basal dikes reacted with mixtures of seawater and hydrothermal fluids to Mg‐chlorite, albite, and pyrite, and gained Mg and S and lost Si and Ca. Focused flow of hydrothermal fluids produced serialization halos (Na‐K sericite, quartz, pyrophyllite, K‐feldspar, and pyrite) along quartz veins at temperatures of 200°–250°C. High 87Sr/86Sr ratios of chloritized (∼0.7055) and sericitized (∼0.7065) rocks indicate involvement of seawater via mixing with hydrothermal fluids. Low δ34S of sulfide (−2 to −5.5‰) and sulfate (12.5‰) are consistent with input of magmatic SO2 into hydrothermal fluids and disproportionation to sulfide and sulfate. Alteration processes were generally similar to those at MORs, but the arc section is more intensively altered, in part because of the presence of abundant glassy rocks and mafic phases. The increase in alteration grade below 750 m and the mineralization in the basal dikes are analogous to changes that occur near the base of the volcanic section in MOR and the Troodos ophiolite.

Petrologically diverse basalts from a fossil oceanic forearc in California: The Llanada and Black Mountain remnants of the Coast Range ophiolite

Basalts from two geographically separated fragments of the California Coast Range ophiolite are very different petrographically, mineralogically, and geochemically. Pillow lavas from the Llanada Coast Range ophiolite remnant are mostly Cr-rich and TiO 2 -poor basalts in which Ti does not increase with increasing Fe/(Fe + Mg); some Cr-poor siliceous rocks are also present. Most Llanada rocks are depleted in light rare earth elements (REE), although a few show slight light REE enrichment. Ti/V ratios are arc-like. The Llanada basalts appear to be island-arc tholeiites that have limited boninitic affinities—i.e., products of suprasubduction zone magmatism. Basalts from the Black Mountain Coast Range ophiolite remnant near St. Helena are TiO 2 -rich relative to those from most other Coast Range ophiolite fragments, and TiO 2 increases with increasing Fe/(Fe + Mg). Black Mountain basalts are light REE depleted, REE enriched overall relative to Llanada lavas, and have mid-oceanic-ridge basalt (MORB)-like Ti/V ratios. These characteristics are similar to those of MORB, but elevated Th/Ta ratios suggest a suprasubduction-zone origin for the Black Mountain lavas. These results show that the Coast Range ophiolite is highly diverse, containing island-arc tholeiites and possibly suprasubduction zone–related MORB-like components. In the Coast Range ophiolite as a whole, the apparent preponderance of island-arc tholeiite lavas such as those at Llanada over MORB-like lavas such as those at Black Mountain favors a forearc over a backarc setting for the origin of the ophiolite. Other data suggest that the Coast Range ophiolite locally contains ocean-island basalts (OIBs). Moreover, serpentinite-rich ophiolitic detritus overlying Coast Range ophiolite serpentinite suggests that, in some areas, the basaltic section of the oceanic crust was missing completely, and serpentinized harzburgite was exposed at the sea floor. These relationships are similar to those in the modern Mariana forearc, which exposes a complex amalgam of boninitic and arc-tholeiitic suprasubduction zone–related crust, MORB and OIB crust, and serpentinized harzburgite.

Oblique convergence and deformation along the Kuril and Japan Trenches

The hypothesis that present‐day deformation within the southern Kuril forearc is driven by oblique subduction of the Pacific plate is tested using 397 horizontal slip directions derived from shallow‐thrust earthquakes from the Kuril and Japan trenches for the period 1963–1991. A simple two‐plate model fits the 397 slip vectors significantly worse than a model that permits strike‐slip motion of the southern Kuril forearc relative to the overlying plate. Weighted, mean slip directions along the southern Kuril trench are systematically rotated toward the direction orthogonal to the trench, which implies that the net convergence is partitioned into less oblique subduction and trench‐parallel displacement of the southern Kuril forearc. The angular discrepancy between the observed slip direction and the direction predicted by the NUVEL‐1 Pacific‐North America Euler vector implies that the southern Kuril forearc translates 6–11 mm yr−1 to the southwest relative to the overlying North American plate. These results are consistent with geologically, geodetically, and seismologically observed convergence at the leading edge of the forearc sliver in southern Hokkaido and with previously inferred extension at the trailing edge of the sliver, which is located at the Bussol Strait at 46°N. The percentage of the oblique component of the rigid plate convergence vector that is transferred into forearc strike‐slip motion is only 30%, which may be evidence that the Hokkaido/Honshu bend in the trench hinders southwest forearc motion or that the greater strength of oceanic forearcs impedes motion along forearc strike‐slip faults.

The Coast Range Ophiolite: Polygenetic Crust in a Late Mesozoic Oceanic Fore Arc

The Coast Range Ophiolite: Polygenetic Crust in a Late Mesozoic Oceanic Fore Arc

Petrology and geochemistry of the plutonic rocks of the Halmahera ophiolite, eastern Indonesia, an analogue of modern oceanic forearcs

Abstract Ophiolitic rocks form the basement of the eastern part of the island of Halmahera in eastern Indonesia. The ophiolite is tectonically dismembered but all the elements of a complete ophiolite are present with the uncertain exception of a sheeted dyke complex. The ophiolite was formed in a supra-subduction zone setting before the Late Cretaceous and is interpreted to represent the forearc of a Mesozoic arc whose remnants are now found within and close to the margins of the Philippine Sea Plate. The plutonic rocks of the ophiolite include periotites interpreted as a ‘mantle sequence’ and cumulates interpreted as the deeper parts of an ophiolitic crustal sequence. The mantle sequence includes subordinate lherzolites of relatively enriched chemistry which represent upper mantle previously depleted by the extraction of MORB. A high degree of melting of the lherzolites under hydrous conditions left a mantle residue of depleted harzburgites, which are more refractory than the harzburgites of Troodos and Oman. Abundant cumulates which crystallized from the magma include peridotites and gabbroic rocks rich in orthopyroxene. The chemistry and mineralogy of the cumulates indicate open-system crystallization from a high Si, Mg, low Ti, LREE-depleted magma. Highly calcic plagioclase characteristic of island-arc plutons and found in the cumulate sequence of the Troodos ophiolite is not found in the Halmahera cumulates. The Halmahera ophiolite is interpreted as the onland analogue of the present-day Mariana forearc. Rocks dredged from the Mariana Trench, which like Halmahera is also situated at the margin of the Philippine Sea Plate, are remarkably similar to those of the Halmahera ophiolite.

The structural evolution of the Timor collision complex, eastern Indonesia

The structural style of Timor is consistent with foreland fold belt processes, and zones of frontal accretion and underplating can be recognized in the Australian parautochthonous sequence. The parautochthon is overlain by an allochthonous sequence, which corresponds to the pre-collisional oceanic forearc. In northern Timor the parautochthon has been underplated directly beneath the allochthonous basement. Unlike previous interpretations of Timor in terms of foreland fold belt processes, the Timor foldbelt is here interpreted as having evolved in a fairly straightforward way from the pre-collisional forearc by the sequential addition of thrust slices of Australian continental crust to the front and base of the developing collision complex.

Sedimentation in the Chile Trench: Petrofacies and Provenance

ABSTRACT The Chile Trench study area extends over 2,000 km along the Andean continental margin, from 23°S to 42°S latitude. The QFL composition of Chile Trench sands encompasses the average compositions of modern sands from strike-slip, continental arc, oceanic backarc, and oceanic forearc types of active margins (Valloni and Maynard 1981), and spans the complete spectrum from dissected to undissected magmatic arc settings (Dickinson et al. 1983). Trench sands along North Chile, Central Chile, and the glaciated archipelago of South Chile are derived from a more plutonic source region, or dissected magmatic arc; these sands exhibit abnormally low lithic and calcic plagioclase content, but high quartz and alkali feldspar. In Central Chile, the plutonic provenance is correlated with an ab ence of Quaternary volcanism. Along the Chilean archipelago, Pleistocene glaciation has carved into the magmatic roots of the Andes, effecting arc dissection in spite of active volcanism. In arid North Chile, the volcanic debris of the High Cordillera is trapped onshore in longitudinal forearc basins, allowing an exaggerated contribution from crystalline rocks of the Coast Range. The light and heavy, mineral and lithic composition of trench sands is described by four ideal petrologic assemblages, isolated by Q-mode factor analysis. The Basic Magmatic Arc petrofacies (volcanic lithics, calcic plagioclase, olivine, clinopyroxene, and orthopyroxene), the most chemically unstable and least differentiated assemblage, is derived exclusively from basalts and basaltic andesites in the Quaternary volcanic arc of South Chile. The Acidic Magmatic Arc petrofacies (volcanic lithics, alkali feldspar, biotite, hornblende, and magnetite) is derived from andesitic to rhyolitic volcanics, and granodioritic plutons emplaced during various Mesozoic and Cenozoic magmatic episodes in the history of the Andean margin. The Metamorphic Magmatic Arc petrofacies (mica, alkali feldspar, hlorite, actinolite, epidote, and blue-green hornblende) is derived from ferromagnesian rocks altered to the greenschist facies in the basal marine section of the Andean eugeosyncline and in the Paleozoic basement. The Cratonic Block petrofacies (quartz and the accessory minerals andalusite, garnet, and tourmaline) is derived from mature platform sediments, late-stage granitic intrusions, and high-grade regional metamorphics associated with the South American craton; it is most prominent along northern Chile where the continental crust is thickest and presumably truncated by subduction erosion. Actual petrofacies in the Chile Trench are typically mixtures of the four ideal petrofacies. Their compositions can be related to 1) the lithology and erodibility of the source terranes, 2) the distribution and composition of modern arc volcanism, 3) the prevailing climate in the provenance region, and 4) the structure and morphology of the continental margin and trench. In many cases contemporary volcanism inundates and dominates the petrologic assemblage, diluting contributions from all other source-rock types and forming a low-diversity petrofacies. At high latitudes, where glaciation was extensive, a variety of source-rock terranes were actively denuded and petrofacies of high diversity were deposited in the adjacent trench. The linear frequency of sediment-supply points along the base of the continental slope controls the spatial heterogeneity of petrofacies on the depositional surface in the trench basin. The most homogenous petrofacies, derived from major submarine canyon systems and extensive subaerial drainage basins, may extend hundreds of kilometers along-margin due to axial transport processes within the trench.