Search

Geo- and thermochronological methods were applied to diorites from the Cordillera Central, Luzon, Philippines to understand their emplacement and exhumation history in the island arc mountains. Five zircon U–Pb ages range from 32.54 ± 0.70 to 6.11 ± 0.15 (2SE) Ma, indicating that dioritic magmas intruded the upper crust intermittently during Cenozoic magmatism. Five zircon fission-track (ZFT) pooled ages range from 35.63 ± 2.17 to 6.91 ± 0.36 (2SE) Ma and are generally comparable with the U–Pb ages at each locality. These results suggest that the diorites cooled rapidly below ~ 250–350 °C (i.e., through the partial annealing zone of the ZFT system), following their intrusion. On the other hand, two zircon and five apatite U-Th(-Sm)/He (ZHe and AHe) weighted mean ages, ranging from 11.71 ± 0.36 to 8.82 ± 0.26 and 9.21 ± 0.52 to 0.98 ± 0.09 (2SE) Ma, respectively, indicate a decrease in cooling rates at a lower temperature range, especially through the partial retention zone of the AHe system. This observation suggests that the ZFT ages reflect initial cooling of the dioritic magma, whereas the AHe ages reflect the cooling history associated with regional exhumation following the initial cooling phase. The spatial distribution of the AHe ages suggests that rapid exhumation of the Cordillera Central during the Quaternary resulted from the block-like uplift of the entire mountain range. [ABSTRACT FROM AUTHOR]

Emergent coral reef terraces in northwest Luzon Island, Philippines are studied to understand relative sea level (RSL) changes and uplift along the Manila Trench forearc region during the Late Quaternary. Coral reef sequences in Currimao, Badoc, and Badoc Island were mapped and were described based on geomorphological characteristics, elevation patterns, and ages. The lower emergent coral reef terraces, which are dated Holocene, are subdivided into three to possibly four terrace steps (TI–TIV in ascending order) with the highest terrace (TIII–TIV) rising up to ~7–9 m amsl. The well‐preserved staircase morphology and meter‐scale terrace risers possibly imply episodes of abrupt RSL changes that resulted from coseismic uplift during the mid‐Holocene. Around 6.8 kyr BP, ~3 m of rapid uplift (possibly coseismic) is inferred to account for the vertical separation of TI (6.2 kyr BP at 3.5 m amsl) and TII (6.8 kyr BP at 6.6 m amsl) in Currimao site. A subsequent episode of RSL fall occurred at 6 kyr BP, based on dated coral samples, and has emerged the lowest terraces in the study sites. New geomorphic and radiometric data suggest that the mid‐Holocene sea level highstand in northwest Luzon occurred at around 7–6 kyr BP. From the elevation and thorium‐230 age of dated terrace surfaces, we estimated Holocene uplift rates from 0.17 ± 0.12 to 0.82 ± 0.15 mm/yr across the three study sites. The estimated uplift rates of northwest Luzon are relatively lower than the uplift rates reported in similar tectonically active regions of the west Pacific such as Papua New Guinea, Taiwan, New Zealand, and Japan. The geomorphological characteristics and age of emergent coral reef terraces reported in this study offer insights on the preservation of long‐term deformation along the northwest Luzon coastal plain that is bordered by a subduction zone and upper‐plate faults. © 2018 John Wiley & Sons, Ltd. Emergent coral reef terraces and tidal notches are useful geomorphic markers in constraining RSL changes and tectonic processes along offshore and onland faults. This study reports new geomorphic and radiometric data in northwest Luzon as evidence of RSL changes and possible prehistoric earthquakes along the Manila subduction zone and its forearc region during the mid‐Holocene. This study offers new insights to the preservation of long‐term deformation along the coast that is bordered by a subduction zone and upper‐plate faults. [ABSTRACT FROM AUTHOR]

The Zambales Ophiolite Complex (ZOC) in the western portion of Luzon Island, Philippines represents a typical exposure of an emplaced crust-upper mantle section of an ancient lithosphere. The ZOC is divided into the Acoje and Coto Blocks based on petrological, geochemical and age disparities, thus implying diverse and complex origins for the archipelago's lithospheric sources. We used gravity and magnetic data to reveal differences in the subsurface characteristics of the two ophiolite blocks for the first time. Low Bouguer gravity (<135 mGal) and magnetic (<69 nT) anomalies characterize the ophiolitic units in the Acoje Block whereas high Bouguer gravity (>150 mGal) and magnetic (>110 nT) anomalies typify the Coto Block. Such contrasting signatures further extend at greater depths which reflect the disparities in the crustal density, the basement structure, and the depth to Moho. Petrophysical characteristics such as density, magnetic susceptibility and natural remanent magnetization (NRM) intensities correlate well with the interpreted Bouguer gravity and magnetic anomalies. Densities of gabbros and peridotites from the Acoje and Coto Blocks reveal mean values ranging from 2640 to 2810 kg m-3 and 2570 to 2690 kg m-3, respectively. Magnetic susceptibility (>4.43 × 10-3 SI) and NRM (>0.69 A m-1) data are also generally higher over the Coto Block. Both Bouguer gravity and magnetic anomaly maps reveal a prominent steep gradient that potentially marks the structural contact between Acoje and Coto Blocks. We further infer that the steep anomaly gradient validates the presence of the Lawis Fault Zone which separates the two ophiolite blocks in the Masinloc Massif. Recent field evidence from rock exposures in Coto reveals both right lateral and vertical displacements along the fault zone. [ABSTRACT FROM AUTHOR]

Abstract: Uplifted Holocene marine terraces in Pangasinan Province, westernmost Luzon Island, Philippines are reported here for the first time as geomorphic evidence of large prehistoric offshore earthquakes. We recognized up to three Holocene terrace steps. Elevations of shoreline angles are TI: 1 to 5m, TII: 2 to 7m, and TIII: 4 to 11m above mean sea level (amsl). The lateral variation in the elevation of terrace steps indicates differential uplift along the coast. The staircase topography, characterized by horizontal abrasion platforms, meter-scale terrace risers, and wave-cut notches, suggests that these geomorphic features were formed by large coseismic uplift events. Terrace morphology and radiocarbon ages further suggest that at least three coseismic coastal uplift events have occurred since the culmination of the post-glacial sea level rise ~6000years ago. We interpret TIII to be the Holocene transgressive surface (~6000 yBP) and estimated a maximum uplift rate of 1.7mm/yr which is almost the same as the Late Pleistocene uplift rate of 1.3mm/yr. We infer that proximity to offshore seismic sources has contributed to the deformation pattern of Holocene terraces and variation in the number of terrace steps. Uplift patterns of marine terraces also suggest that the magnitude and location of prehistoric earthquakes may have varied between individual coseismic uplift events. We conducted simple elastic dislocation modeling to evaluate the source parameters of the causative faults. Our preliminary models show that a landward-dipping offshore reverse fault located at the base of a steep slope separating the fore arc basin and continental shelf could be responsible for coastal uplift along the western Pangasinan peninsula. [ABSTRACT FROM AUTHOR]

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 Fe 2 O 3 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: 15–57 ppm; Cr: 231–1094 ppm; Ni: 84–484 ppm) compared to the Daram Formation samples (Co: 24–32 ppm; Cr: 234–418 ppm; Ni: 212–323 ppm). 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. [ABSTRACT FROM AUTHOR]

The presence of adakites has been used as an indicator for the occurrence of gold mineralization in many mineral prospecting works. Traditionally, the unique geodynamic controls to adakite formation, particularly the high temperature gradient and other slab-melting requirements, have been taken as key elements that must be present during their formation. However, several studies have suggested alternative mechanisms. This paper presents fractional crystallization as the most viable mechanism for the generation of adakitic rocks in the Masara gold-silver mine in Eastern Mindanao, Philippines. Furthermore, this paper also argues that the occurrence of adakitic rocks does not necessarily indicate the presence of mineralization. Depending on the scale, their occurrence may be an exploration marker at a regional or district level, but at the mine-level, other more localized parameters will have to be considered. [ABSTRACT FROM AUTHOR]

Remnants of a Cretaceous lithosphere are found at the peripheries of the West Philippine Basin. These Mesozoic fragments preserve arc affinity and include the Amami Plateau, the East Halmahera Ophiolite and the ophiolites along the eastern margin of the Philippine archipelago. The eastern margin of the Philippines is composed of Early to Late Cretaceous ophiolites and ophiolitic complexes that exhibit strong subduction imprints. The early Late Cretaceous Samar Ophiolite in the central Philippines forms part of this eastern belt. Recent surveys in southern Samar revealed the presence of peridotites, gabbros, and massive and pillowed flows of the Samar Ophiolite. Major, trace and rare earth element signatures of the volcanic rock samples indicate moderate to strong subduction-related influences and formation in an island arc setting. In contrast to other similarly-aged ophiolites along the eastern margin of the archipelago, those to the north of Samar Island exhibit weak to almost mid-oceanic ridge characteristics. These differences in the geochemical signatures of the ophiolites and ophiolitic complexes along eastern Philippines require a re-evaluation of a previous model suggesting that these ophiolites originated from a single oceanic lithosphere, possibly the proto-Philippine Sea Plate. [ABSTRACT FROM AUTHOR]

This paper presents the first field, geochemical and geophysical information on the recently recognized Early Cretaceous Balud Ophiolitic Complex (BOC) in the island of Masbate in the Central Philippines. Mapping of the western limb of the island revealed that only the upper crustal section of the BOC is exposed in this area. Geochemically, the pillow basalts are characterized by transitional mid-oceanic ridge basalt-island arc tholeiitic compositions. Gravity surveys yielded low Bouguer anomaly values that are consistent with the highly dismembered nature of the BOC. Short wavelength, high amplitude magnetic anomalies registered across the study area are attributed to shallow magnetic sources. This is taken to support the model that the ophiolitic complex occurs as thin crustal slivers that are not deeply-rooted in the mantle. Comparing BOC with other ophiolites in the Central Philippines, such as those in the islands of Sibuyan, Leyte and Bohol, suggests the possibility of a common or contiguous source for similarly-aged and geochemically composed crust-mantle sequences in the region. [ABSTRACT FROM AUTHOR]

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 ± 2.7 Ma and 97.9 ± 2.8 Ma. 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. [ABSTRACT FROM AUTHOR]

Offshore and ground gravity data were utilized to estimate crustal thickness across the Central Philippines where a transition from continental to island arc terrane occurs. Significant differences in gravity anomalies were observed between the Palawan Microcontinental Block (PCB) and the Philippine Mobile Belt (PMB), two major terranes that came together through arc-continent collision. Islands of the PCB (Mindoro, Tablas, Romblon, Sibuyan and western Panay), made up of an assortment of continent-derived sedimentary and igneous rocks and slivers of ophiolitic bodies, register lower Bouguer anomalies compared to that displayed by Masbate Island in the PMB. The calculated crustal thickness of this region exhibits a complex Moho topography of non-uniform depth across the collision zone with the thickest parts (∼32 km) corresponding with ophiolitic units emplaced consequent to arc-continent collision. On the other hand, relatively thinner crust (∼21 km) within the collision zone coincides with areas surmised to have undergone attenuation following intra-arc rifting. The same characteristics are observed offshore of western Mindoro and within the Marinduque Basin, areas known to have experienced crustal thinning following regional tectonic rearrangements that triggered riftings and intra-basin openings. [ABSTRACT FROM AUTHOR]