Forearc Basin Development Research Articles

Tectono‐Stratigraphic Insights on the Dynamics of a Complex Subduction Zone, Northern Peruvian Forearc

AbstractTwo main types of subduction are recognized around the world: accretionary and erosive. The northern Peruvian margin is a well‐known example of a margin subjected to subduction erosion, but to date the along‐margin variability and temporal changes in subduction process and forearc basin evolution have not been characterized in detail. Interpretation of regional seismic lines and integration of oil‐industry wells and seafloor data captures the erosive nature of subduction underneath the forearc with only a minor accretionary component to the north. Episodes of uplift driven by plate coupling were followed by normal faulting/extensional collapse due to plate decoupling. This mechanism explains the dominance of normal faulting across the forearc until the Oligocene with a slight reactivation within the Miocene. The subduction history is complex and includes a reduction in plate convergence rate related to forearc crustal shortening, represented by large‐scale structures including the Peru fault (reactivated) and the Illescas fault‐propagation anticlines of the Northwest Peru transpressional system. This crustal deformation started in the Miocene. Integration with magnetic anomaly data indicates that activity of the present‐day transpressional system driven by tectonic escape of the Nazca Sliver toward the northeast, may explain the seismicity gap in southern Ecuador and northern Peru. An evolutionary model of the northern Peruvian margin shows how subduction zone geodynamics left its erosive fingerprint in the forearc basin configuration.

Turonian highly differentiated pyroclastic xenoliths and Cretaceous sedimentary record challenge mantle-plume view of Costa Rican forearc basement successions

The Mesozoic basaltic basement of southern Central America is believed to consist of proxy sections of mantle-plume events leading to the Caribbean igneous province. A significant xenolith assemblage of radiolarite, tuffite, and Turonian (∼93 Ma) highly differentiated pyroclastic rock discovered in basalt of the Costa Rican forearc basement fundamentally challenge this long-held view, revealing bimodal Cretaceous island-arc activity and forearc basin development instead. The pyroclastic materials originally accumulated with monotonous radiolarite deposits in a deep-marine forearc basin slope environment to build part of the Mesozoic sedimentary cover of the evolving arc. Xenolith formation was related to widespread Turonian to Campanian intrusive basaltic activity on the Nicoya Peninsula that affected Albian-Turonian and Berriasian-Barremian radiolarite and pyroclastic deposits as well as Jurassic siliceous strata. Strong correlation with Cretaceous biotite- and hornblende-bearing pyroclastic beds reveals explosive calc-alkaline volcanic arc episodes alongside basaltic activity linked to marine forearc basin development. These materials provide decisive evidence of explosive calc-alkaline volcanic activity of the Cretaceous Costa Rican arc as well as the probable intraoceanic-arc nature of its basaltic forearc basement and a ∼90 Ma aged komatiite suite found at Tortugal. The resulting tectonic scenario shows the Caribbean Plate was separated in southern Central America from any potential Pacific or Galápagos hotspot sources by a subduction zone. This favours in-place origin of the Caribbean Plate and its igneous province between the Americas. The findings also reveal the unsuitability of basaltic forearc basement successions to deduce with certainty mantle-plume events and hotspot evolutions in adjoining oceanic domains.

Structural-morphological and sedimentary features of forearc slope off Miyagi, NE Japan: implications for development of forearc basins and plumbing systems

Multibeam (MB) and subbottom profile (SBP) data along the forearc slope off Miyagi, Japan, are combined to investigate structural-morphological and sedimentary structures along the forearc slope of the Japan Trench subduction zone. In addition to dip-oriented slope gullies, the MB data image a nearly dip-perpendicular slope trough bounded by a fault scarp landward of the trench-slope break. Seaward of the trench-slope break, the subbottom mostly contains normal faults that dip in opposite directions. The SBP data show not only unconformity and sliding surfaces, but also underfilled (forearc trough) and filled structures that may reflect the most recent forearc subsidence and basin filling. We propose these forearc trough and associated filling structures may indicate the earliest developments of modern isolated basins. Thus, a model of forearc basin development from slope gully to slope trough, isolated basins and forearc basin, transferred by active structures, could be proposed. Observed seafloor liquefaction in the SBP data may represent a near-surface seep structure of a plumbing system that enables hydrosphere-mantle fluid migration plausibly activated by earthquake cycles.

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

Initiation and evolution of forearc basins in the Central Myanmar Depression

AbstractThe forearc basin in Myanmar is significant in understanding the development of continental forearc basins. We present stratigraphic, sandstone petrographic, and U-Pb detrital data from Upper Cretaceous–Eocene strata of Chindwin and Minbu sub-basins in the Central Myanmar Depression. The Upper Cretaceous lower Kabaw Formation consists of turbiditic conglomerate, sandstone, and mudstone in the Minbu sub-basin. The composition of conglomerates are mainly schist and subordinate quartz. Prominent detrital zircon age probability peaks are between 260 and 223 Ma, similar with that of Upper Triassic Pane Chaung turbidites and Kanpetlet schist on the West Burma plate. In the upper Kabaw Formation, turbiditic volcanic-rich sandstones have major age populations ranging from 103 to 70 Ma in both Minbu and Chindwin sub-basins. The Paleocene slope environment Paunggyi Formation, which overlies the Kabaw Formation, mainly consists of conglomerate, sandstone, mudstone, and tuff beds in the Minbu sub-basin. In contrast, the Paunggyi Formation in the Chindwin sub-basin is composed of sandstone and mudstone; major detrital zircon age populations from the Paunggyi Formation are between 100 and 60 Ma. Eocene strata in both basins are composed mainly of shallow marine to delta sandstone and mudstone. Major detrital zircon age populations are 100–36 Ma and 600–500 Ma. The Late Cretaceous–Eocene ages from Upper Cretaceous–Eocene strata overlap with igneous crystallization ages from the Western Myanmar Arc. We propose that the Chindwin and Minbu sub-basins developed as parts of a forearc basin along the west flank of Western Myanmar Arc (present coordinate). The forearc basin initiated in Albian time atop the continental West Burma plate due to the formation of a structural high along the western margin of West Burma plate.

Kinematic and thermal evolution of the Haymana Basin, a fore-arc to foreland basin in Central Anatolia (Turkey)

Gondwana (Tauride/Kırşehir blocks) and Eurasia (Pontides) derived continental blocks delimit the Haymana basin, central Turkey, to the south and the north, respectively. The boundaries of these blocks define the Izmir-Ankara-Erzincan and Intra-Tauride Suture zones which are straddled by a number of Late Cretaceous to Oligocene marine to continental basins. The Haymana Basin is located at the junction of the IAESZ and ITSZ and comprises Upper Cretaceous to Middle Eocene basin infill deposited in response to the interaction of these blocks. The basin provides a unique opportunity to unravel spatio-temporal relationships related to the timing of late stage subduction history of Neo-Tethys Ocean and subsequent collision of the intervening continental blocks. We have conducted a multidisciplinary study in the region that includes mapping of major structures combined with fault kinematic analyses. E-W striking folds dominate the basin, cross-section balancing of these structures indicates around 25% roughly N-S shortening in the region. Paleostress studies indicate that the basin was initially subjected to N-S to NNE-SSW extension until the middle Paleocene (phase 1) and then N-S directed syn-depositional compression and coeval E-W directed extension until the middle Miocene (phase 2) implying strike-slip deformation and pure shear shortening in the basin. These different deformation phases are attributed to first fore-arc (subduction) basin development then foreland (collision) stages of the basin. Apatite (U-Th)/He dating of 5 samples indicate that exhumation of the SE segment of the basin started in early Oligocene, whereas the NW segment of the basin exhumed in the early Miocene. The differential uplift is possibly related to progressive north-westwards movement of Dereköy basin bounding fault at the north. We propose that the Haymana basin evolved from extensional forearc basin during the late Cretaceous to early Paleocene and foreland basin after the terminal subduction and subsequent collision of Tauride and Pontide blocks.

The birth of a forearc: The basal Great Valley Group, California, USA

AbstractThe Great Valley basin of California (USA) is an archetypal forearc basin, yet the timing, structural style, and location of basin development remain controversial. Eighteen of 20 detrital zircon samples (3711 new U-Pb ages) from basal strata of the Great Valley forearc basin contain Cretaceous grains, with nine samples yielding statistically robust Cretaceous maximum depositional ages (MDAs), two with MDAs that overlap the Jurassic-Cretaceous boundary, suggesting earliest Cretaceous deposition, and nine with Jurassic MDAs consistent with latest Jurassic deposition. In addition, the pre-Mesozoic age populations of our samples are consistent with central North America sources and do not require a southern provenance. We interpret that diachronous initiation of sedimentation reflects the growth of isolated depocenters, consistent with an extensional model for the early stages of forearc basin development.

About this title - Paleozoic–Mesozoic Geology of South Island, New Zealand: Subduction-related Processes Adjacent to SE Gondwana

This volume presents a set of research papers that provide new data and interpretations of the Permian–Triassic terranes of SE Gondwana, now exposed in South Island, New Zealand. Following an introduction for general readers, a historical summary and a review of biostratigraphy, the individual papers primarily focus on the Permian magmatic arc of the Brook Street Terrane, the classic Permian Dun Mountain ophiolite and the Permian–Triassic Maitai Group sedimentary succession. The new results emphasize the role of subduction and terrane displacement adjacent to the Permo-Triassic Gondwana margin, and present fundamental insights into three crustal processes: subduction initiation, supra-subduction zone oceanic crust genesis and forearc basin evolution. The volume concludes with a wide-ranging summary and synthesis of the regional Cambrian to Early Cretaceous tectonostratigraphy of New Zealand's South Island in relation to the wider areas of Zealandia, East Australia and West Antarctica. The volume will interest geoscientists, including stratigraphers, sedimentologists, palaeontologists, igneous petrologists, geochemists, geochronologists and economic geologists, and is aimed at professional geologists and advanced students of geology.

Controls on forearc basin formation and evolution: Insights from Oligocene to Recent tectono-stratigraphy of the Lower Magdalena Valley basin of northwest Colombia

Abstract Mechanisms of forearc basin evolution remain not very well understood, mostly because of insufficient resolution of their temporal evolution and potential drivers. The convergent margin of northwest Colombia where the San Jacinto and Lower Magdalena Valley forearc basins are located, offers a unique opportunity to study such mechanisms. The formation of the Lower Magdalena amagmatic, forearc basin occurred in a stable setting from the Oligocene to the present, characterized by the slow and nearly orthogonal, low-angle subduction of the Caribbean plateau. We use an exceptional regional database to reconstruct the subsidence, extension, sedimentation and paleo-geographic history of the Lower Magdalena forearc basin. We propose possible mechanisms controlling its evolution, in the absence of major changes in plate kinematics and in a low-angle subduction setting. Following the collapse of a pre-Oligocene magmatic arc, late Oligocene to early Miocene fault-controlled subsidence allowed initial basin fill at relatively low sedimentation rates. After the connection of the Lower and Middle Magdalena valleys, the proto-Magdalena river in the north and the proto-Cauca river in the south both started delivering high amounts of sediments in middle Miocene times. Fault controlled subsidence was gradually replaced by subsidence due to increased sedimentary load. Increase in sediment flux would have also caused the formation of an accretionary prism, weakened the plate interface through lubrication with fluid-rich sediment and initiated underplating, with the development of forearc highs in the San Jacinto area. Inherited basement structures allowed the tectonic segmentation of the basin with the formation its two depocenters (Plato and San Jorge). Our results highlight the fundamental role of sediment flux, of the basement structure and of flat subduction on the evolution of forearc basins such as the Lower Magdalena.

Major variations in vitrinite reflectance and consolidation characteristics within a post-middle Miocene forearc basin, central Japan: A geodynamical implication for basin evolution

Forearc basin sediments near the ocean-ward margin preserve tectonic information related to plate subduction. The post-middle Miocene Boso forearc basin, central Japan, records major differences in structure, paleo-maximum temperature, and consolidation state between below (Miura Group) and above (Kazusa Group) the Kurotaki Unconformity, which formed at ca. 3Ma. Many fault systems below the unconformity are characterized by a disaggregation-band-like inner fabric that apparently formed soon after sedimentation, whereas there are few of this type of fault system above the unconformity. Vitrinite reflectance values (Ro) are 0.38%–0.44% and 0.22%–0.25% below and above the unconformity, respectively. The consolidation yield stress (pc) in the Miura Group (23.7MPa in the Anno Formation; 31.0MPa in the Amatsu Formation) is much greater than that in the Kazusa Group (7.5, 7.6 and 9.6MPa in the Umegase, Ohtadai and Kiwada formations, respectively). These clear differences in vitrinite reflectance and consolidation characteristics above and below the unconformity are attributed to a forearc basin evolution, which resulted in the Miura Group being geothermally matured, tectonically compacted, uplifted, and eroded (500m in maximum) before sedimentation of the Kazusa Group. The forearc basin, especially near the trench-slope break, records structural and physical properties reflecting the plate-tectonic environment and the development of the trench-slope.

Age and composition of the Rebang Co and Julu ophiolites, central Tibet: implications for the evolution of the Bangong Meso-Tethys

Subduction of the Bangong Meso-Tethys and collision between the Lhasa and Qiangtang blocks were important for the growth of Tibetan crust, as well as the development of super-large porphyry copper deposits in central Tibet. However, the initiation and closure timing, and nature and structure of the Bangong Meso-Tethys are still poorly constrained. Petrology, geochronology, and geochemistry of the Rebang Co and Julu ophiolites in the western part of the suture zone are used to constrain the tectonic history and structure of the Bangong Meso-Tethys. These two suites consist of ultramafic rocks, gabbro and diabase dikes, pillow basalts, and radiolarian cherts. Zircon U–Pb dating of zircon grains of gabbros yields well-defined weighted mean ages of 161.5 ± 1.5 Ma and 103.8 ± 3.9 Ma for the Rebang Co and Julu suites, respectively. Both of ophiolitic suites have mid-oceanic ridge basalt and island-arc tholeiite affinities, similar to back-arc basin crust. We suggest these suites formed above an intra-oceanic subduction zone at least during Middle Jurassic through mid-Cretaceous times. Along with coeval subduction beneath the southern Qiangtang margin, as well as fore-arc basin development, the Bangong Meso-Tethys has a complicated structure and history. Clearly, subduction in the Bangong Meso-Tethys was still active during mid-Cretaceous time and the Meso-Tethys did not close until Late Cretaceous time.

A Panoptic View of Western Margin of Sundaland: Causes of Seismic Vulnerability of Sumatra

Abstract The western margin of Sundaland is affected by two major tectonic events, the India-Eurasia collision and subduction along the Burmese-Andaman-Sunda Arc. Geotectonic analysis of the Burmese-Andaman-Sunda Arc areas reveals that it represents an arc setting exposed both in land and sea environment. Within this broad arc setting, there are individual segments that illustrate discernible trends in terms of structural, seismic and geological characteristics. The Equatorial Region around Sumatra has some peculiar geological/geophysical characteristics that make it a tectonic hotspot. Here a relationship between the active volcanism and the development of forearc basins is established. It is also proposed that the downgoing oceanic plate is more strongly coupled to the overlying plate where it is youngest (~ 40 Ma), has the highest temperature and is topographically most elevated with highest seismic activity. In particular, an increase in convergence rate and presence of youngest oceanic crust appear to be the main controlling factor underpinning the tectonics and surge of recent seismic activity in Sumatra.

Structural evolution of an inner accretionary wedge and forearc basin initiation, Nankai margin, Japan

Core recovered during Integrated Ocean Drilling Program (IODP) Expedition 319 from below the Kumano forearc basin of Japan's Nankai margin provides some of the only in situ samples from an inner accretionary wedge, and sheds light on the tectonic history of a seismically hazardous region. The 84m of core comprises Miocene-age well-bedded muds, silts, and volcaniclastic sediments. Beds increase in dip with depth, and are cut by (i) soft-sediment deformation bands (“vein structures”), (ii) ∼1-cm thick shear zones within ∼10-cm thick regions of high shear strain, and (iii) <1-mm thick slickensided faults which are the youngest structures in the core and highly localized. Microstructural analyses of the shear zones suggest that they formed via multiple increments of shear localization and a mixed granular and cataclastic flow. Kinematic analysis of slip indicators in shear zones further reveals that they formed via north–south shortening. In contrast, the faults cut the shear zones with mixed slip kinematics, and accommodated northwest–southeast shortening, roughly parallel to the modern shortening direction. The entire section was also rotated ∼15° counterclockwise about a roughly vertical axis. Therefore the principle strain axes and stratigraphic section rotated during or postdating development of the major sub-basin (∼5.6–3.8Ma) unconformity, a time that generally coincides with a change in the Philippine Sea plate convergence direction. Forearc basin development therefore postdates a protracted geologic evolution of shear-zone development, tectonic rotations, and inner-wedge development, the last of which coincides with a rheological evolution toward localized frictional faulting.

IODP Expedition 319, NanTroSEIZE Stage 2: First IODP Riser Drilling Operations and Observatory Installation Towards Understanding Subduction Zone Seismogenesis

The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is a major drilling project designed to investigate fault mechanics and the seismogenic behavior of subduction zone plate boundaries. Expedition 319 is the first riser drilling operation within scientific ocean drilling. Operations included riser drilling at Site C0009 in the forearc basin above the plate boundary fault, non-riser drilling at Site C0010 across the shallow part of the megasplay fault system – which may slip during plate boundary earthquakes – and initial drilling at Site C0011 (incoming oceanic plate) for Expedition 322. At Site C0009, new methods were tested, including analysis of drill mud cuttings and gas, and in situ measurements of stress, pore pressure, and permeability. These results, in conjunction with earlier drilling, will provide (a) the history of forearc basin development (including links to growth of the megasplay fault system and modern prism), (b) the first in situ hydrological measurements of the plate boundary hanging wall, and (c) integration of in situ stress measurements (orientation and magnitude) across the forearc and with depth. A vertical seismic profile (VSP) experiment provides improved constraints on the deeper structure of the subduction zone. At Site C0010, logging-while-drilling measurements indicate significant changes in fault zone and hanging wall properties over short (< 5 km) along-strike distances, suggesting different burial and/or uplift history. The first borehole observatory instruments were installed at Site C0010 to monitor pressure and temperature within the megasplay fault zone, and methods of deployment of more complex observatory instruments were tested for future operations. doi:10.2204/iodp.sd.10.01.2010

IODP Expedition 319, NanTroSEIZE Stage 2: First IODP Riser Drilling Operations and Observatory Installation Towards Understanding Subduction Zone Seismogenesis

The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is a major drilling project designed to investigate fault mechanics and the seismogenic behavior of subduction zone plate boundaries. Expedition 319 is the first riser drilling operation within scientific ocean drilling. Operations included riser drilling at Site C0009 in the forearc basin above the plate boundary fault, non-riser drilling at Site C0010 across the shallow part of the megasplay fault system &amp;ndash; which may slip during plate boundary earthquakes &amp;ndash; and initial drilling at Site C0011 (incoming oceanic plate) for Expedition 322. At Site C0009, new methods were tested, including analysis of drill mud cuttings and gas, and &lt;i&gt;in situ&lt;/i&gt; measurements of stress, pore pressure, and permeability. These results, in conjunction with earlier drilling, will provide (a) the history of forearc basin development (including links to growth of the megasplay fault system and modern prism), (b) the first &lt;i&gt;in situ&lt;/i&gt; hydrological measurements of the plate boundary hanging wall, and (c) integration of &lt;i&gt;in situ&lt;/i&gt; stress measurements (orientation and magnitude) across the forearc and with depth. A vertical seismic profile (VSP) experiment provides improved constraints on the deeper structure of the subduction zone. At Site C0010, logging-while-drilling measurements indicate significant changes in fault zone and hanging wall properties over short (&lt; 5 km) along-strike distances, suggesting different burial and/or uplift history. The first borehole observatory instruments were installed at Site C0010 to monitor pressure and temperature within the megasplay fault zone, and methods of deployment of more complex observatory instruments were tested for future operations. &lt;br&gt;&lt;br&gt; doi:&lt;a href="http://dx.doi.org/10.2204/iodp.sd.10.01.2010" target="_blank"&gt;10.2204/iodp.sd.10.01.2010&lt;/a&gt;

Structural evolution and strike-slip tectonics off north-western Sumatra

Based on new multi-channel seismic data, swath bathymetry, and sediment echosounder data we present a model for the interaction between strike-slip faulting and forearc basin evolution off north-western Sumatra between 2°N and 7°N. We examined seismic sequences and sea floor morphology of the Simeulue- and Aceh forearc basins and the adjacent outer arc high. We found that strike-slip faulting has controlled the forearc basin evolution since the Late Miocene. The Mentawai Fault Zone extends up to the north of Simeulue Island and was most probably connected farther northwards to the Sumatran Fault Zone until the end of the Miocene. Since then, this northern branch jumped westwards, initiating the West Andaman Fault in the Aceh area. The connection to the Mentawai Fault Zone is a left-hand step-over. In this transpressional setting the Tuba Ridge developed. We found a right-lateral strike-slip fault running from the conjunction of the West Andaman Fault and the Tuba Ridge in SSW-direction crossing the outer arc high. As a result, extrusion formed a marginal basin north of Simeulue Island which is tilted eastwards by uplift along a thrust fault in the west. The shift of strike-slip movement in the Aceh segment is accompanied by a relocation of the depocenter of the Aceh Basin to the northwest, forming one major Neogene unconformity. The Simeulue Basin bears two major Neogene unconformities, documenting that differences in subsidence evolution along the northern Sumatran margin are linked to both forearc-evolution related to subduction processes and to deformation along major strike-slip faults.

Latest Cretaceous forearc basin development along an accretionary convergent margin: South-central Alaska

Upper Cretaceous sedimentary strata exposed in south-central Alaska provide insight on tectonic processes that shaped the northern Pacific margin following accretion of the Wrangellia composite terrane, the largest addition of crust to North America over the past 100 m.y. Sedimentologic, geochronologic, biostratigraphic, and petrographic data from the Matanuska Formation permit reconstruction of the tectono-sedimentary history of strata in a forearc basin constructed upon accreted oceanic-arc crust. The Matanuska Formation consists of >3 km of sedimentary strata exposed in the northern Chugach Mountains, Matanuska Valley, and southern Talkeetna Mountains of interior south-central Alaska. Measured stratigraphic sections and lithofacies analyses demonstrate that mass slumps and slides, debris flows, and turbidity currents deposited Campanian–Maastrichtian sandstone, conglomerate, and mudstone on a gullied, trenchward-dipping submarine ramp. Benthic foraminifera, inoceramid bivalves, and Nereites ichnogenera indicate deposition mainly at bathyal water depths. Sandstone and conglomerate petrofacies are characterized by monocrystalline quartz, plagioclase feldspar, and volcanic lithic fragments (Q39F40L21, Qm29F40Lt32, Lm25Lv42Ls32, and Qm42P54K4). Jurassic–Cretaceous arc plutons exposed north of the basin were an important sediment source, based on U-Pb zircon ages of granitoid clasts from conglomerate and detrital zircons from sandstone. Coeval arc plutons were unroofed relatively quickly, judging by the presence of 77–71 Ma detrital zircons in sandstone and 79–77 Ma granitic clasts in conglomerate, together with Maastrichtian (71–65 Ma) ammonite and foraminifera fossils. Sparse Paleozoic–Triassic detrital zircons indicate minor sediment contribution from inboard sources, including the Yukon-Tanana composite terrane and recycled Jurassic–Cretaceous sedimentary strata (Kahiltna assemblage). New data from the upper Matanuska Formation, together with recent studies from age-equivalent strata exposed in the Alaska Range and Wrangell Mountains, provide an exceptional example of basin development along a subduction margin shortly following accretion of an oceanic arc. Forearc basin development was dominated by subsidence and sediment gravity flow deposits enriched in plutonic and volcanic clasts eroded from both remnant- and coeval-arc plutons. Within the arc, newly recognized conglomerate in the northern Talkeetna Mountains records erosion of coeval- and remnant-arc source terranes to the south and Precambrian–Paleozoic sources to the north. Farther inboard, syndepositional shortening prompted thrust-top basin development and accumulation of alluvial-lacustrine strata derived from both the former continental margin to the north and accreted oceanic rocks to the south. Regional subsidence and basin development terminated during late Maastrichtian–early Paleocene time, coincident with subduction of progressively younger oceanic lithosphere inboard of an oceanic spreading center.

Multiple generations of forearc mafic–ultramafic rocks in the Timor–Tanimbar ophiolite, eastern Indonesia

We investigated mafic–ultramafic rocks distributed in the Timor–Tanimbar region as a possible modern analogue for the Mediterranean-type ophiolites in the Tethyan system. The geological occurrence suggests that the buoyant subduction of Australian continent uplifted the fragments of newly formed mantle–crust section, which extends to the neighboring preemplaced forearc marginal basins. However, we recognized a large variety of igneous features, which is consistent with the lack of complete succession and the presence of abundant crosscutting structures. All peridotite masses in Timor (Mutis, Atapupu and Dili) are mostly fertile (lherzolitic) in compositions. In addition, we found depleted harzburgite, highly refractory dunite and olivine websterite to occur as minor constituents, which display compositional contrast to those of the lherzolites. Structurally overlying Ocussi volcanics resemble island–arc tholeiite in terms of trace element characteristics, apparently inconsistent with genetic relationship with Timor lherzolite masses. In eastern small islands (Moa and Dai), all types of ophiolitic rocks display varying degrees of island–arc affinities. Cumulate origin of wehrlite and gabbroic rocks in Dai is marked by early crystallization of clinopyroxene and common occurrence of high-calcic plagioclase. Dikes cutting the gabbro sequence have weak island–arc signatures relative to those of Ocussi volcanics. Mildly depleted lherzolite–harzburgite in Moa was intruded by high-Mg andesitic magma, which crystallized hornblende gabbro containing high-Mg orthopyroxene. These petrological and geochemical variations can be best explained by the combination of (1) a temporal change of igneous activity possibly associated with development of forearc basin and (2) the emplacement of spatially different forearc regions in each locality. Unusual occurrence of fertile lherzolite in the forearc setting, generation of high-Mg andesite magmatism, inverted metamorphic grade recorded from associated metamorphic rocks, and formation of marginal basins may be linked to the injection of high-temperature asthenospheric materials into the mantle wedge.

Neogene structures and sedimentation history along the Sunda forearc basins off southwest Sumatra and southwest Java

Twenty multi-channel seismic lines in the southwest Sunda arc margin between Manna and west Java have been studied. This study depicts the structures and stratigraphy of the fore-arc basin since Late Paleogene in relation to the regional tectonic events. The paleomorphology of the Cretaceous continental margin persisted until the Oligocene and the paleoshelf margin of the continent extended north-westward off Sumatra. A residual basin filled with turbidite deposits developed just offshore of this margin. Subsequent fore-arc basin evolution was related to the slow down of subduction rate due to the collision of the Indian and Eurasian Plates in the Eocene. The rising Himalayan orogenic zone shed large amounts of sediment to the Indian Ocean and Sunda Trench beginning in the Late Paleogene which led to the growth of the accretionary prisms and the development of the Neogene fore-arc basin. At least two major structural events can be recognized in the fore-arc basin between Late Oligocene and Pliocene. Back thrust-faulting along the southern border of the fore-arc basin and initiation of the Cimandiri Fault Zone occurred in Late Oligocene, whereas the development of the Sumatra and Mentawai Fault Zones was initiated in Pliocene. Four Neogene sedimentary units can be recognized representing three main transgressive–regressive cycles and basin-fill deposits. The cycles resulted from a complex interplay of tectonically induced basin subsidence, eustatic sea level change and sediment supply related to volcanic activity that became abundant since late Middle Miocene. Turbidite deposition was common along and seaward of the basin slope during sea level lows in late Middle Miocene and Late Miocene. Sediment aggradation and progradation occurred during high sea level still stand and fall, respectively. Basin fills occurred mainly during the Pleistocene.

Continental growth at convergent margins facing large ocean basins: a case study from Mesozoic convergent-margin basins of Baja California, Mexico

Mesozoic rocks of the Baja California Peninsula form one of the most areally extensive, best-exposed, longest-lived (160 my), least-tectonized and least-metamorphosed convergent-margin basin complexes in the world. This convergent margin shows an evolutionary trend that may be typical of arc systems facing large ocean basins: a progression from highly extensional (phase 1) through mildly extensional (phase 2) to compressional (phase 3) strain regimes. This trend is largely due to the progressively decreasing age of lithosphere that is subducted, which causes a gradual decrease in slab dip angle (and concomitant increase in coupling between lower and upper plates), as well as progressive inboard migration of the arc axis. This paper emphasizes the usefulness of sedimentary and volcanic basin analysis for reconstructing the tectonic evolution of a convergent continental margin. Phase 1 consists of Late Triassic to Late Jurassic oceanic intra-arc to backarc basins that were isolated from continental sediment sources. New, progressively widening basins were created by arc rifting and sea floor spreading, and these were largely filled with progradational backarc arc-apron deposits that record the growth of adjacent volcanoes up to and above sea level. Inboard migration of the backarc spreading center ultimately results in renewed arc rifting, producing an influx of silicic pyroclastics to the backarc basin. Rifting succeeds in conversion of the active backarc basin into a remnant backarc basin, which is blanketed by epiclastic sands. Phase 1 oceanic arc–backarc terranes were amalgamated by Late Jurassic sinistral strike slip faults. They form the forearc substrate for phase 2, indicating inboard migration of the arc axis due to decrease in slab dip. Phase 2 consists of Early Cretaceous extensional fringing arc basins adjacent to a continent. Phase 2 forearc basins consist of grabens that stepped downward toward the trench, filled with coarse-grained slope apron deposits. Phase 2 intra-arc basins show a cycle of (1) arc extension, characterized by intermediate to silicic explosive and effusive volcanism, culminating in caldera-forming silicic ignimbrite eruptions, followed by (2) arc rifting, characterized by widespread dike swarms and extensive mafic lavas and hyaloclastites. This extensional-rifting cycle was followed by mid-Cretaceous backarc basin closure and thrusting of the fringing arc beneath the edge of the continent, caused by a decrease in slab dip as well as a possible increase in convergence rate. Phase 2 fringing arc terranes form the substrate for phase 3, which consists of a Late Cretaceous high-standing, compressional continental arc that migrated inboard with time. Strongly coupled subduction resulted in accretion of blueschist metamorphic rocks, with development of a broad residual forearc basin behind the growing accretionary wedge, and development of extensional forearc (trench–slope) basins atop the gravitationally collapsing accretionary wedge. Inboard of this, ongoing phase 3 strongly coupled subduction, together with oblique convergence, resulted in development of forearc strike-slip basins upon arc basement. The modern Earth is strongly biased toward long-lived arc–trench systems, which are compressional; therefore, evolutionary models for convergent margins must be constructed from well-preserved ancient examples like Baja California. This convergent margin is typical of many others, where the early to middle stages of convergence (phases 1 and 2) create nonsubductable arc–ophiolite terranes (and their basin fills) in the upper plate. These become accreted to the continental margin in the late stage of convergence (phase 3), resulting in significant continental growth.