Ophiolitic Basement Research Articles

Review: Geological Structure impacts to hydrocarbon potential and active faults in the East Java Basin, Indonesia

Review of the geological structure patterns in the East Java Basin (EJB) to understand the geodynamics has implicated to the hydrocarbon potential and active faults. However, the impact of those structures on hydrocarbon potential and active faults is unclear. This study reviewed structure patterns using surface and subsurface data, GPS, seismicity and tremors in the East Java Basin, Indonesia and Nankai Trough, Japan. In EJB, Indonesia, the tectonic setting is constrained by the Rembang Fault. The north of the Rembang Fault, the pattern exhibits NE-SW structures, while to the south, it shows W-E structures. The results indicate that the upper crust (including ophiolitic basement) has greater density to the north than to the south. Thus, vertical motion of the crust is more dominant than lateral motion to the north of the Rembang Fault. This vertical motion may trigger the reactivation of the Meratus Fault (weak zone or as active fault) located on the northern platform (e.g., the Bawean earthquake on March 22, 2023). Conversely, to the south of the Rembang Fault, there is a significant hydrocarbon potential associated with W-E structures. Those structures could form by subduction and collision tectonic. Similary, tectonic backstop may account for presence of structures in Nankai Trough, Japan.

Geoelectrical subsurface characterization: A case study of saltwater intrusion in Kudat, Sabah

Saltwater intrusion is one of the primary sources of groundwater contamination, especially near the coastal line, and it is crucial to monitor saltwater intrusion to protect fresh groundwater. Due to the proximity of the coastal line, the area within Kudat Peninsula that was formed by the ophiolite basement, Kudat Formation Chert-Spilite Formation, and alluvium deposit is susceptible to saltwater intrusion. In this study, two survey lines of geoelectrical resistivity and induced polarization imaging survey with a total length of 200 m were carried out. The Wenner configuration was used to characterize the subsurface of the Kudat Peninsula. The interpolation between geoelectrical resistivity and chargeability profiles provided better interpretations of the subsurface environment. The saltwater intrusion zone displays a resistivity value between 0 – 5.0 Ωm and a chargeability value between 0 – 3.0 ms. The results showed that saltwater intrusion was detected at depths as shallow as 1.25 m to 36.90 m and intruded as far as 218.00 m into the mainland, from the coastline.

Dynamic processes of upper plate deformation at the New Guinea Trench

The interactions between arc-continent collision and subduction zones are still poorly understood. Here, we use 2D seismic data to document upper plate deformational features and identify different structural styles along the New Guinea Trench, western Pacific, and discuss their broader significance. In the western section, normal faults and negative flower structures occur within the strata of the upper plate. An unconformity represents the signal of tectonically driven subsidence. In the eastern section, both thick-skinned and thin-skinned thrusting are found within the frontal upper plate. In the central section, rare extensional or shortening deformation is identified within the forearc basement and overlying strata. Horizontal, thick sedimentary sequences within the Caroline subbasin have been subducted beneath the upper plate. This suggests that Miocene subduction in the eastern section led to the formation of a thrust-deformed ophiolitic basement in the upper plate. Since the Miocene, strike-slip tectonic processes dominated in the upper plate, causing compressional deformation in the strata in the eastern section and extensional to transtensional deformation in the strata in the western section. The strike-slip tectonic processes might have been weakened in the upper plate in central section due to the incoming Eauripik Rise. Over geologic time, arc-continent collision processes and related changing of regional tectonic forces can have a long-lived influence on shaping and deforming subduction systems.

New U-Pb zircon ages of plagiogranites from the Coastal Complex ophiolite and Twillingate batholith, Newfoundland: Evidence for the oldest and overlapping silicic magmatism in the nascent Cambrian peri-Laurentia forearc and arc terranes

The Coastal Complex (CC) and Bay of Islands Complex (BOIC) represent two supra-subduction zone (SSZ) forearc ophiolitic assemblages of different ages within the Taconic Humber Arm Allochthon of the Western Newfoundland Appalachians. To further constrain age relationships between the CC and BOIC and earliest initiation of Iapetus subduction, we acquired LA-ICP-MS U-Pb zircon ages for eleven plagiogranites sampled along strike of the CC belt. These ages combined with three prior legacy ages establish an age range of 514.3–502.7 Ma for the six CC plagiogranite plutons. The oldest precise ca. 514.3 Ma CC pluton age represents the oldest Taconic arc-related plutonic activity yet documented in Western Newfoundland and provides a minimum age for the intruded CC ophiolite basement and initiation of westerly vergent subduction within the peri-Laurentian Iapetus Ocean. This confirms the Early Cambrian CC mafic–ultramafic basement is significantly older than the juxtaposed SSZ BOIC, which formed by later rifting of the CC forearc along younger forearc spreading center in the Late Cambrian by ca. 488.3 Ma. After subduction initiation a prolonged early period CC forearc silicic magmatism lasted at least ∼ 11.6 Ma until ca. 502.7 Ma, a range consistent with age spans of magmatism in modern juvenile forearcs. We also reinvestigated the Central Newfoundland Twillingate Granite batholith thought to be one of the earliest major felsic batholiths formed within the adjacent juvenile arc massif of the Notre Dame Subzone, part of the same peri-Laurentian arc. We determined a precise 3-sample composite age of 504.3 ± 1.8 Ma that signals the formation of earliest Notre Dame arc edifice developed by Middle Cambrian. It overlaps the end-stage CC forearc plagiogranite magmatism and places a minimum mid-Cambrian age constraint on intruded ophiolitic arc tholeiitic and boninitic pillow lavas of the Sleepy Cove and correlated Lushs Bight volcanics, suggesting basement continuity with the CC.

Imaging crustal features and Moho depths through enhancements and inversion of gravity data from the Philippine island arc system

The Philippine archipelago is a complex island arc system, where many regions still lack geopotential field studies. The high-resolution isostatic anomaly and free-air anomaly digital grids from the World Gravity Map (WGM) were processed and analyzed to present a general discussion of the Philippines’ gravity signatures and contribute to understanding its regional geology and tectonics. The isostatic anomaly map was continued upward to investigate the high-density ophiolitic basement rocks and low-gravity sedimentary basins at depth. The first vertical derivative (1VD) filter was applied to the free-air anomaly grid map to locate regional structures represented by density contrast boundaries. The depth to the top of the Moho and basement rock over the Sulu Sea was computed using the two-dimensional (2-D) radially averaged power spectrum analysis. Three-dimensional (3-D) gravity inversion was applied to some major sedimentary basins in the Philippines to present 3-D subsurface density contrast models. The interpreted gravity maps highlighted prominent geologic features (e.g., trench manifestation, ophiolite distribution, basin thickness). The negative isostatic anomalies (< 0 mGal) represent the thick sedimentary basins, while the moderate signatures (0 to 80 mGal) correspond to the metamorphic belts. The distinct very high-gravity anomalies (> 80 mGal) typify the ophiolitic basement rocks. The gravity data’s upward continuation revealed contrasting deep gravity signatures; the central Philippines with continental affinity (with 20–35 mGal) was distinguished from the regions with oceanic affinity (with 45–200 mGal). The 1VD map over the Sulu Sea showed anomalies associated with shallow features dominantly related to the Cagayan Ridge. The 2-D radially averaged power spectrum analysis exposed gravity anomalies with tectonic significance (e.g., basement characterization, Moho depth estimation). The estimated average Moho depth in the Sulu Sea is from 12 to 22 km, while the average basement depth is within the range of 5 to 11 km. Lastly, the 3-D subsurface density contrast models characterized the very low-density zones representing the deep (> 7 km) sedimentary basins in the northern Cagayan Valley and southern Central Luzon basins. Furthermore, thin (~ 3.5 km) sedimentary formations are inferred for the low-density areas in northern Agusan-Davao and eastern Cotabato basins.

The Pre‐Obduction to Post‐Obduction Evolution of the Sivas Ophiolite (Turkey) and Implications for the Precollisional History of Eastern Anatolia

AbstractThe Anatolian block preserves remnants of Tethyan oceanic basins consumed by north dipping subduction zones until the Late Cretaceous prior to Paleogene collision. The Sivas Basin, which belongs to the Maastrichtian to Cenozoic Central Anatolian basins, is located in a key position limited to the north and the south by respectively the Pontides and Tauride ophiolitic bodies and to the west by the Kırşehir block. This study focuses on the southern margin of the Sivas Basin, where an obducted ophiolite is capped by Maastrichtian‐Paleocene sediments. We present new field observations, with U‐Pb zircon dating on magmatic rocks and geochemical analyses to (1) unravel the pre‐obduction nature and origin of the ophiolitic basement and (2) describe the post‐obduction tectonosedimentary evolution. The pre‐obduction evolution shows that (i) the Sivas ophiolite is characterized by serpentinized peridotites, with minor magmatic intrusions, (ii) the top of the serpentinized mantle is characterized by a cataclastic deformation with ophicalcites interpreted as an extensional detachment fault, (iii) the U‐Pb zircon dating of two magmatic intrusions yield age of 91.49 ± 0.8 Ma and 72.7 ± 0.5 Ma, and (iv) petrological and geochemical data show that the magmatic intrusions were affected by hydrothermal metamorphism. These data suggest that the Sivas ophiolite may have recorded forearc hyperextension in frame of a Late Cretaceous suprasubduction zone. The post‐obduction evolution is characterized by the deposition of a Maastrichtian‐Paleocene carbonate platform on the ophiolite, followed by clastic sediments containing reworked ophiolitic and Tauride Mesozoic clasts.

Geology of the Central Sivas Basin (Turkey)

ABSTRACT This paper presents a revised geological map at the 1/50,000 scale of the Central Sivas Basin together with a synthetic stratigraphic chart and cross-sections. The map covers an area of approximately 9840 km² within the Eastern Anatolian orogen. The structure of the studied area is dominated by three major tectonic domains: (i) to the south, a north-verging thrust wedge involving Maastrichtian – Eocene sediments deposited onto an ophiolitic basement, (ii) in the center an Oligo-Miocene domain shaped by salt tectonics detached above the thrust wedge, along a late Eocene salt layer, and, (iii) to the north the Pliocene depocenter onlapping onto the Kırşehir basement. The central halokinetic domain exhibits two generations of minibasins (respectively, early Oligocene and late Oligocene to late Miocene), separated by an evaporite canopy. The map includes new stratigraphic correlations for the pre-salt stratigraphy and improve the comprehension of the southern fold-and-thrust-belt.

Electrical conductivity of a locked fault: investigation of the Ganos segment of the North Anatolian Fault using three-dimensional magnetotellurics

This study attempts to reveal the fault zone characteristics of the locked Ganos Fault based on electrical resistivity studies including audio-frequency (AMT: 10,400–1 Hz) and wide-band (MT: 360–0.000538 Hz) magnetotellurics near the epicenter of the last major event, that is, the 1912 Mürefte Earthquake (Mw 7.4). The AMT data were collected at twelve stations, closely spaced from north to south, to resolve the shallow resistivity structure to 1 km depth. Subsequently, 13 wide-band MT stations were arranged to form a grid enclosing the AMT profile to decipher the deeper structure. Three-dimensional inverse modeling indicates highly conductive anomalies representing fault zone conductors along the Ganos Fault. Subsidiary faults around the Ganos Fault, which are conductive structures with individual mechanically weak features, merge into a greater damage zone, creating a wide fluid-bearing environment. This damage zone is located on the southern side of the fault and defines an asymmetry around the main fault strand, which demonstrates distributed conduit behavior of fluid flow. Ophiolitic basement occurs as low-conductivity block beneath younger formations at a depth of 2 km, where the mechanically weak to strong transition occurs. Resistive structures on both sides of the fault beneath this transition suggest that the lack of seismicity might be related to the absence of fluid pathways in the seismogenic zone.Graphical .

India‐Asia convergence: Insights from burial and exhumation of the Xigaze fore‐arc basin, south Tibet

AbstractThe composite fore‐arc/syncollisional Xigaze basin in south Tibet preserves a key record of India‐Asia collision. New apatite fission track and zircon (U‐Th)/He data from an N‐S transect across the preserved fore‐arc basin sequence near Xigaze show a consistent northward Late Cretaceous to middle Miocene younging trend, while coexisting apatite (U‐Th‐Sm)/He ages are all Miocene. Corresponding detrital zircon U‐Pb data are also reported for constraining the Cretaceous depositional ages of the Xigaze basin sequence in the region. Thermal history modeling indicates that the basin experienced northward propagating episodic exhumation, along with a northward migration of the depocenter and a pre‐existing Cenozoic syncollisional basin sequence which had been removed. In the southern part, fore‐arc exhumation commenced in the Late Cretaceous (~89 ± 2 Ma). Following transition to a syncollisional basin in the Paleocene, sedimentation in the central and northern Xigaze basin continued until the latest Eocene (~34 ± 4 Ma). Ongoing folding and thrusting (e.g., Great Counter Thrusts) caused by progressive plate convergence during late Oligocene‐early Miocene time resulted in regional uplift and considerable basin denudation, which fed two fluvial basins along its northern and southern flanks and exposed the basement ophiolite. Subsequent incision of the Yarlung River resulted in Miocene cooling in the region. Different episodes in the exhumation history of the Xigaze basin, caused by thrusting of an accretionary wedge and ophiolitic basement, can be linked to changes in India‐Asia convergence rates and the changing subduction pattern of the Indian and Neo‐Tethyan slabs.

Mineralogy, Geochemistry, and Genesis of Bentonites in Miocene Volcanic-Sedimentary Units of the Ankara-Çankiri Basin, Central Anatolia, Turkey

AbstractWidespread alteration in the Miocene lacustrine volcanic/sedimentary rocks of the Ankara-Çankiri basin of central Anatolia has resulted in the formation of sizeable (economic) quantities of bentonite deposits. No detailed characterization of the geological, mineralogical, and geochemical properties or the depositional environments of these primary and secondary bentonite deposits has been carried out to date. The present study was undertaken to close this knowledge gap. Two possible hypothetical processes were examined to explain the genesis of the bentonites: 1) The bentonites were formed by the devitrification of volcanic glass in a lacustrine setting; and 2) The bentonites were formed by the chemical weathering of previously deposited volcaniclastic sediments and ophiolitic materials. The characteristics of the bentonites were examined using X-ray diffractometry, scanning and transmission electron microscopy, energy dispersive spectroscopy, and chemical analyses of major and trace elements. The Ankara-Çankırı bentonites are found gradationally interbedded with parent Miocene volcanic and volcaniclastic rocks. These bentonites were deposited in a shallow lacustrine setting based on observed desiccation cracks, locally enclosed coal seams, plant rootlets, gypsum lenses, yellow sulfate-like fracture infillings, and ferric iron oxide stains. Smectite resulted from the chemical weathering of feldspar and possibly also the weathering of biotite and hornblende. This smectite was precipitated in situ on partially dissolved glass and feldspar. The average major-element composition of the smectite-rich clay fractions yielded the following montmorillonitic smectite structural formula: (Na0.33Ca0.31K0.18) (Al2.35Fe0.80Mg0.78)(Si7.79Al0.21)O20(OH)4.The interlayer cation occupancy in the smectite-rich clay fractions was based on the use of Na+/(Na++Ca2+) ratios and showed a composition between a Ca-smectite and a Na-smectite. The relative increases in several groups of elements according to the LREE/(MREE+HREE) ratio, Al2O3, the sum of Ni+Co+Cr, the sum of Fe2O3+MgO+TiO2, the positive correlation between Rb+Ba and Na2O+K2O, Sr and Rb, Rb/Sr and Zr, Zr/Sm and SiO2, the negative Eu anomaly, and the field and petrographic observations further showed that the Si, Al, Fe, and Mg required to form smectite were mainly supplied from the decomposition of feldspars, amphiboles, and volcanic glass from volcanic materials and were partially supplied from the chemical weathering of ophiolitic basement units. The bentonite deposits examined in this study are mainly primary bentonites derived from volcanics and local secondary bentonites from previously deposited volcaniclastic sediments and ophiolitic materials.

The birth of the Xigaze forearc basin in southern Tibet

The stratigraphic succession of a forearc basin provides crucial information on the history of a convergent plate margin. In particular, it helps to establish the origin of the underlying ophiolites and to unravel the earliest evolutionary stage of arc-trench systems, which remain poorly understood. The Xigaze forearc basin in southern Tibet is one of the best examples of a fossil forearc basin. This study illustrates detailed stratigraphic and high-precision SIMS U–Pb zircon geochronological and Hf isotopic data from the Chongdui Formation, representing the very base of the Xigaze forearc-basin succession, and reconstructs when and how the basin was formed. The Chongdui Formation includes tuffaceous chert and siliceous mudrocks deposited directly on top of pillow basalts of the Xigaze ophiolite and conformably overlain by volcaniclastic turbidites. Tuff layers are interbedded throughout the unit, and their U–Pb zircon ages range from 119 to 113 Ma in the lower member and from 113 to 110 Ma in the upper member, broadly consistent with the established radiolarian biostratigraphy. U–Pb ages and Hf isotope signatures of zircons contained in both tuff layers and turbiditic sandstones indicate clear affinity with magmatic rocks of the Lhasa terrane. Direct depositional and chronostratigraphic relationship with the underlying oceanic crust, dated between 131 and 124 Ma, proves that the Xigaze ophiolite is the basement of the Xigaze forearc basin. After an initial prolonged stage of starved siliceous sedimentation, influx of terrigenous detritus began at 113–110 Ma, reflecting the onset of topographic growth and erosion of the Lhasa terrane in response to intense magmatic activity. Formation of the ophiolitic basement during the early stage of subduction and the subsequent topographic growth of the arc source induced by subduction-related magmatism are thus two critical factors for the birth of the Xigaze forearc basin. Similar stratigraphies were identified in the Great Valley and Luzon Central Valley forearc basins, suggesting that the initial geodynamic evolution of the Xigaze forearc basin may be common to many other forearc basins worldwide.

Zircon U-Pb ages and petrologic evolution of the English Peak granitic pluton: Jurassic crustal growth in northwestern California

In the central Klamath Mountains, the English Peak plutonic complex (EPC) invaded the faulted contact between the outboard Eastern Hayfork and inboard North Fork terranes of the Western Paleozoic and Triassic Belt (WTrPz). This calc-alkaline igneous complex is composed of two small, ~1–2-km-diame­ter, relatively mafic satellitic plutons peripheral to the younger, much larger, ~10–15-km-diameter English Peak zoned granitic pluton. The EPC magmas were mantle derived and reflect temporary residence and mixing at various depths in the overlying crust, with initial storage and modification near the Moho, and uppermost crustal emplacement at 5–10 km depths. Phase assemblages suggest pre-emplacement magma storage at a depth of ~20–25 km for the early satellitic plutons, versus ~15–20 km for samples from the larger zoned granitic pluton. We obtained zircon U-Pb geochronologic results (reported as internal and external weighted-mean 207 Pb-corrected 206 Pb/ 238 U ages, 95% confidence level) from seven samples in the complex via laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS). The 172.3 ± 2.0 [3.7] Ma Uncles Creek and 166.9 ± 1.6 [3.4] Ma Heiney Bar satellitic plutons range from gabbro–quartz diorite to granodiorite in bulk-rock composition. The main English Peak pluton consists of an early stage of gabbro-tonalite (three samples: 160.4 ± 1.1 [3.1] Ma, 158.1 ± 1.1 [3.1] Ma, and 158.0 ± 1.2 [3.1] Ma) and a late stage (two samples: 156.3 ± 1.3 [3.1] Ma and 155.3 ± 1.2 [3.0] Ma) passing inward from tonalite through granodiorite to a central zone of granite. The 172 Ma age of the Uncles Creek pluton makes it coeval with Middle Jurassic Western Hayfork arc magmatism. In contrast, Heiney Bar and the main English Peak igneous ages overlap some of the oldest and youngest components, respectively, of the Middle to Late Jurassic Wooley Creek plutonic suite. Study of this multiple-intrusion complex provides an illuminating example of the gradual intermediate-to-felsic modification of the upper crust in the central Klamath Mountains. Inherited zircon ages of ca. 172 Ma in two other EPC samples indicate potential Middle Jurassic crustal sources or contaminants. Geochronologic correlation of the EPC with geologic histories of other Klamath terranes provides fresh insights for understanding spatial and temporal elements of Middle to Late Jurassic arc magmatism in the Klamath Mountains sector of the Cordilleran margin. This igneous activity illuminates some petrotectonic processes whereby accreted ophiolitic basement terranes were modified and incorporated into the evolving Jurassic continental crust. It took place prior to the earliest Cretaceous onset of westward transport of the stack of Klamath allochthons relative to the active Jura-Cretaceous Sierran calc-alkaline arc.

Petrography and geochemistry of Cenozoic sedimentary sequences of the southern Samar Island, Philippines: Clues to the unroofing history of an ancient subduction zone

The Cenozoic sedimentary sequences of southern Samar Island in eastern Philippines were examined to understand the unroofing history of an ancient arc terrane. Petrographic and geochemical data revealed varying degrees of inputs from the ophiolite basement and differences in modal compositions. The sedimentary units are mostly made up of lithic fragments. The Late Oligocene to Early Miocene Daram Formation contains more chert and volcanic fragments whereas the late Middle Miocene to Early Pliocene Catbalogan Formation is dominantly composed of ultramafic components. These variances are correspondingly reflected in the geochemical signatures of these two sedimentary formations. The Catbalogan Formation clastic rocks have higher volatile-free MgO and Fe2O3 values (average: 8.4% for both oxides) compared to the Daram Formation samples (average: 5.1 and 6.3%, respectively). Geochemical variations are also reflected in the Co, Cr and Ni values: the Catbalogan Formation samples reflect higher concentrations (Co: 15–57ppm; Cr: 231–1094ppm; Ni: 84–484ppm) compared to the Daram Formation samples (Co: 24–32ppm; Cr: 234–418ppm; Ni: 212–323ppm). These observations suggest that the Daram Formation eroded and transported more of the crustal portions of the ophiolite, while the younger Catbalogan Formation represents a later exhumation and subsequent erosion of the ultramafic section. An oceanic island arc (OIA) setting is proposed for the two formations based on several tectonic discrimination diagrams (e.g., Th-La-Sc, La vs. Th). The OIA signature is further supported by their smooth chondrite-normalized rare earth element (REE) patterns with no obvious Eu anomaly as well as LREE enrichment which are typical of sediments deposited in OIA setting. Based on the dominantly ophiolitic provenance of the Daram and Catbalogan formations, the post-emplacement history of the nearby Samar Ophiolite is constrained during the Late Oligocene to Early Pliocene period.

Paleomagnetism of the Samar Ophiolite: Implications for the Cretaceous sub-equatorial position of the Philippine island arc

Samar island in the eastern part of Central Philippines is underlain by a complete ophiolite suite, the Samar Ophiolite. We present the first geochronological and paleomagnetic data for the Samar Ophiolite. Whole rock K–Ar dating of two basalt samples yielded an age of 100.2±2.7Ma and 97.9±2.8Ma. Thirteen sites in four localities yielded characteristic remanent magnetization with in situ direction of D=340°, I=−24°, k=15, α95=11° and tilt-corrected direction of D=342°, I=−27°, k=15, α95=11°. These values suggest that the ophiolitic basement rocks of Samar formed in the Late Cretaceous at a paleolatitude of 14°S±6°. The paleolatitude is several degrees south of the sub-equatorial positions calculated for the three other Mesozoic ophiolites of the Philippine Mobile Belt (PMB) whose paleomagnetism had been previously studied. The PMB ophiolites in eastern and central Philippines share a common age, geochemistry and paleolatitude with the Halmahera Ophiolite, suggesting that they originated from a Mesozoic supra-subduction zone that spanned a few degrees north of the equator to around 15°S.

Provenance of Late Carboniferous to Jurassic sandstones for southern Taimyr, Arctic Russia: A comparison of heavy mineral analysis by optical and QEMSCAN methods

Sandstone framework-grain petrography, optical and QEMSCAN (Quantitative Evaluation of Minerals by Scanning Electron Microscopy) heavy mineral analysis carried out on 40 samples collected from east and west southern Taimyr are used to constrain the provenance and tectonic history of Late Carboniferous to Late Jurassic siliciclastic sequences. The tectonic settings of provenance evolved gradually from a mix of volcanic arc and recycled orogen to craton interior. Much of the detritus in the Late Paleozoic to Mesozoic siliciclastic succession came from proximal sources with contributions from multi-type source rocks including acid igneous rocks, basalts, sedimentary rocks and low to medium-grade metamorphic rocks. Carboniferous to Permian sandstones contain low-diversity suites of heavy minerals, including apatite, tourmaline, zircon, rutile, Cr-spinel, monazite and titanite. Cr-spinel indicates probable influx from exposed ophiolitic basement. Abundant euhedral zircon and apatite suggest a volcanic arc source related with Uralian collision. The appearance of garnet in the early Triassic signals the unroofing of a metamorphic source. The abrupt increase of clinopyroxene in Middle to Late Triassic sandstones indicates the influx of detritus from basic rocks related with Siberian Trap magmatism. The decrease of Cr-spinel and an abundance of staurolite in Jurassic samples indicate that unroofing of an ophiolitic source ceased and that stripping of a different thrust sheet containing plenty of staurolite-bearing metamorphic rocks commenced.

Crustal thickness beneath Central and East Java (Indonesia) inferred from P receiver functions

We compute receiver functions from teleseismic earthquake data recorded at the dense MERAMEX network in Central and East Java, Indonesia. In order to map the Moho depths H and the bulk crustal vP/vS-ratio κ we apply the H–κ stacking technique of Zhu and Kanamori (2000). Though the interpretation of the results is hampered by the high complexity of the recorded waveforms, in the context of a high noise level and of the heterogeneous assemblage of continental fragments and ophiolitic suture zones forming Southeast Asia, we were able to derive an average crustal thickness of about 34km. Support for the assumption of ophiolitic basement in the center of the island related to the Meratus suture zone comes from shallowing of the Moho to about 30km beneath the Kendeng zone. North and west of the Kendeng zone the Moho is imaged at anomalous depths down to 39km. The observed anomalies line up along the hypothetical boundary between the continental SW Borneo fragment and the Meratus suture and are indicative of crustal thickening caused by overthrusting and compressional deformation in the context of a former collision zone. Resulting vP/vS-ratios cannot be inferred reliably due to the complexity of the waveforms. Our results do not corroborate the hypothesis of a major structural change along the southern coast at the division between Central and East Java. Likewise, we do not find clear evidence for the existence of the postulated Muria-Progo lineament.

40Ar/39Ar geochronology of subaerial lava flows of Barren Island volcano and the deep crust beneath the Andaman Island Arc, Burma Microplate

Little was known about the nature and origin of the deep crust beneath the Andaman Island Arc in spite of the fact that it formed part of the highly active Indonesian volcanic arc system, one of the important continental crust forming regions in Southeast Asia. This arc, formed as a result of subduction of the Indian Plate beneath the Burma Microplate (a sliver of the Eurasian Plate), contains only one active subaerial magmatic center, Barren Island volcano, whose evolutional timeline had remained uncertain. In this work, we present results of the first successful attempt to date crustal xenoliths and their host lava flows from the island, by incremental heating 40Ar/39Ar method, in an attempt to understand the evolutionary histories of the volcano and its basement. Based on concordant plateau and isochron ages, we establish that the oldest subaerial lava flows of the volcano are 1.58 ± 0.04 (2σ) Ma, and some of the plagioclase xenocrysts have been derived from crustal rocks of 106 ± 3 (2σ) Ma. Mineralogy (anorthite + Cr-rich diopside + minor olivine) and isotopic compositions (87Sr/86Sr 7.0) of xenoliths not only indicate their derivation from a lower (oceanic) crustal olivine gabbro but also suggest a genetic relationship between the arc crust and the ophiolitic basement of the Andaman accretionary prism. We speculate that the basements of the forearc and volcanic arc of the Andaman subduction zone belong to a single continuous unit that was once attached to the western margin of the Eurasian Plate.

From continent to intra-oceanic arc: Zircon xenocrysts record the crustal evolution of the Solomon island arc

Research Article| December 01, 2014 From continent to intra-oceanic arc: Zircon xenocrysts record the crustal evolution of the Solomon island arc Simon Tapster; Simon Tapster 1Department of Geology, University of Leicester, Leicester LE1 7RH, UK2NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK Search for other works by this author on: GSW Google Scholar N.M.W. Roberts; N.M.W. Roberts 2NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK Search for other works by this author on: GSW Google Scholar M.G. Petterson; M.G. Petterson 1Department of Geology, University of Leicester, Leicester LE1 7RH, UK Search for other works by this author on: GSW Google Scholar A.D. Saunders; A.D. Saunders 1Department of Geology, University of Leicester, Leicester LE1 7RH, UK Search for other works by this author on: GSW Google Scholar J. Naden J. Naden 3British Geological Survey, Keyworth, Nottingham NG12 5GG, UK Search for other works by this author on: GSW Google Scholar Geology (2014) 42 (12): 1087–1090. https://doi.org/10.1130/G36033.1 Article history received: 29 Jun 2014 rev-recd: 24 Sep 2014 accepted: 25 Sep 2014 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Simon Tapster, N.M.W. Roberts, M.G. Petterson, A.D. Saunders, J. Naden; From continent to intra-oceanic arc: Zircon xenocrysts record the crustal evolution of the Solomon island arc. Geology 2014;; 42 (12): 1087–1090. doi: https://doi.org/10.1130/G36033.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The first U-Pb ages from a ca. 26–24 Ma pluton on Guadalcanal, in the intra-oceanic Solomon island arc (southwest Pacific Ocean), reveal Eocene- to Archean-aged zircon xenocrysts. Xenocryst populations at ca. 39–33 Ma and ca. 71–63 Ma correlate with previously obtained ages of supra-subduction magmatism within the arc. A ca. 96 Ma zircon population may be derived from Cretaceous ophiolite basement crust or region-wide continental rift-related magmatism. Xenocryst age populations alternate with periods of oceanic basin formation that fragmented the East Gondwana margin. Early Cretaceous to Archean zircon xenocryst ages imply continental origins and a cryptic source within the arc crust; they may have been introduced by Eocene interaction of a continental fragment with the arc, and concealed by ophiolite obduction. The data demonstrate that continentally derived zircons may be transported thousands of kilometers from their source and added to intra-oceanic arc magmas, a process likely facilitated by cyclical subduction zone advance and retreat. The findings highlight the continuum of arcs that occurs between continental and oceanic end members, and the caution with which zircons should be used to determine the provenance and setting of ancient arc terranes accreted to the continental crust. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Imaging P and S Attenuation in the Sacramento-San Joaquin Delta Region, Northern California

We obtain 3D Q P and Q S models for the Delta region of the Sacramento and San Joaquin rivers, a large fluvial–agricultural portion of the Great Valley located between the Sierra Nevada batholith and the San Francisco Bay–Coast Ranges region of active faulting. Path attenuation t * values have been obtained for P and S data from 124 distributed earthquakes, with a longer variable window for S based on the energy integral. We use frequency dependence with an exponent of 0.5, consistent with other studies and weakly favored by the t * S data. A regional initial model was obtained by solving for Q as a function of seismic velocity. In the final model, the Great Valley basin has low Q , with very low Q (<50) for the shallowest portion of the Delta. There is an underlying strong Q contrast to the ophiolite basement, which is thickest with highest Q under the Sacramento basin, and a change in structure is apparent across the Suisun Bay as a transition to thinner ophiolite. Moderately low Q is found in the upper crust west of the Delta region along the faults in the eastern North Bay area, whereas moderately high Q is found south of the Delta, implying potentially stronger ground motion for earthquake sources to the south. Very low Q values in the shallow crust along parts of the major fault zones may relate to sediment and abundant microfractures. In the lower crust below the San Andreas and Calaveras–Hayward–Rodgers Creek fault zones, the observed low Q is consistent with grain‐size reduction in ductile shear zones and is lowest under the San Andreas, which has large cumulative strain. Similarly, moderately low Q in the ductile lower crust of the Bay area block between the major fault zones implies a broad distributed shear zone. Online Material: Distribution of earthquake magnitude, figures of t * values, 3D initial regional velocity and Q models, station terms, and Q P inversion with 1D initial Q P model.

Trace element concentrations in porphyry copper deposits from Metaliferi Mountains, Romania: A reconnaissance study

There are fifteen Miocene intrusions with porphyry copper mineralization in the Metaliferi Mountains, Romania; most are associated with epithermal Au–Ag (±Te, Bi, Se, Ge) vein systems. The investigation of fifty samples from six deposits (Roşia Poieni, Bucium–Tarniţa, Colnic, Rovina, Valea Morii and Bolcana) indicated bulk rock mean values of 0.16–0.88ppm Au, <50ppm Mo, 0.02–0.35ppm Re, 0.06–0.6ppm In, 1.3–3.3ppm Ag, 0.23–1.17ppm Bi, 3.2–5.5ppm Se, 2.4–3.3ppm Ge, and 2.6–18ppm Te. Trace element analyses by electron microprobe suggest that part of the Au, Ag, As, Te and Se may occur as fine-particle inclusions hosted within common sulfide minerals (pyrite, chalcopyrite and bornite). Measured concentrations range from below the minimum limit of detection to as much as 1430ppm Au, 530ppm Ag, 1920ppm Te, 10,500ppm As and 2620ppm Se in pyrite, 970ppm Au, 1180ppm Ag, 1000ppm Te, 410ppm As and 420ppm Se in chalcopyrite, and 920ppm Au, 4830ppm Ag, 1470ppm Te, 4230ppm Se and 1540ppm Bi in bornite. Gold is concentrated preferentially in pyrite, while silver is consistently high (>1000ppm) in bornite. In the porphyry-type mineralization, galena forms microinclusions in pyrite, chalcopyrite and bornite, and contains up to 3750ppm Ag, 17,380ppm Se, 1570ppm Ge and 780ppm Te. Galena and pyrite from the epithermal mineralization in the Măgura–Bolcana–Troiţa system contain concentrations of Ag and Se ten times lower than in the same minerals from the porphyry copper mineralization. The present data show that Te is a significant trace element in the potassic zone of the investigated porphyry systems. The data also shows that this does not correlate with the presence or absence of ophiolitic basement, or with variation in the chemistry of the host intrusions. Tellurium enrichment in porphyry-type deposits of the Metaliferi Mountains supports their genetic relationship with the spatially associated epithermal systems.