Geology of southeast Bohol,central Philippines: Accretion and sedimentation in a marginal basin

Recent field mapping has refined our understanding of the stratigraphy and geology of southeastern Bohol, which is composed of a Cretaceous basement complex subdivided into three distinct formations. The basal unit, a metamorphic complex named the Alicia Schist, is overthrust by the Cansiwang mélange, which is, in turn, structurally overlain by the Southeast Bohol Ophiolite Complex. The entire basement complex is overlain unconformably by a ～2000 m thick sequence of Lower Miocene to Pleistocene carbonate and clastic sedimentary rocks and igneous units. Newly identified lithostratigraphic units in the area include the Cansiwang mélange, a tectonic mélange interpreted as an accretionary prism, and the Lumbog Volcaniclastic Member of the Lower Miocene Carmen Formation. The Cansiwang mélange is sandwiched between the ophiolite and the metamorphic complex, suggesting that the Alicia Schist was not formed in response to emplacement of the Southeast Bohol Ophiolite Complex. The accretionary prism beneath the ophiolite complex and the presence of boninites suggest that the Southeast Bohol Ophiolite Complex was emplaced in a forearc setting. The Southeast Bohol Ophiolite Complex formed during the Early Cretaceous in a suprasubduction zone environment related to a southeast‐facing arc (using present‐day geographical references). The accretion of this ophiolite complex was followed by a period of erosion and then later by extensive clastic and carbonate rock deposition (Carmen Formation, Sierra Bullones Limestone and Maribojoc Limestone). The Lumbog Volcaniclastic Member and Jagna Andesite document intermittent Tertiary volcanism in southeastern Bohol.     

Onland signatures of the Palawan microcontinental block and Philippine mobile belt collision and crustal growth process: A review

The collision of the Palawan microcontinental block with the Philippine mobile belt had significantly influenced the geological evolution of the Philippines. Multiple collisions involving several fragments, through space and time, resulted into the collage of terranes of varying origin exposed in this part of central Philippines. Cusping of the overriding plate, volcanic arc gap, ophiolite emplacement, incipient back-arc rifting, island rotation and tilting, raised coastal terraces, metamorphism, intrusion of igneous rocks and steepened subducted slab as seen in focal mechanism solutions are some of the manifestations of this collision. A late Early Miocene to early Middle Miocene age (20–16 Ma) is proposed for the major collision between the Palawan indenter and the Philippine mobile belt. The collision boundary is located from the northern part of Mindoro through the central mountain range swinging east of Sibuyan Island in the Romblon Island Group and finally threading along the Buruanga Peninsula and eastern side of the Antique Ophiolite Complex before exiting and connecting with the Negros Trench. The collision, through accretion and crustal thickening, has contributed to the crustal growth of the Philippine archipelago.     

Indenter-tectonics in the Philippines: Example from the Palawan Microcontinental Block - Philippine Mobile Belt Collision

Abstract. The aseismic Palawan microcontinental block is an oceanic bathymetric high that has collided with the seismically-ac-tive Philippine Mobile Belt since the Early Miocene. Consequently, tectonic microblocks immediately north (Luzon) and south (Western Visayas Block) of the collision front rotated in opposite senses. The rotation led the microblocks to onramp adjacent strike-slip faults, and converted these to subduction zones, namely, the current Manila and Negros Trenches. In addition, the collision also initiated the southward propagation of a major left-lateral strike slip fault, the Philippine Fault Zone, and the Philippine Trench, which bounds the Philippine archipelago along its eastern boundary. Based on onshore and offshore data, the Philippine Fault Zone and the East Luzon Trough - Philippine Trench appears to also propagate northward. Furthermore, the opposite direction of propagation is also noted for the Manila and Negros Trenches from the locus of the collision in the Central Philippines to their northern and southern extensions, respectively. The ages of initiation of the Manila Trench (Early Miocene), Philippine Fault Zone (Middle Miocene) and Philippine Trench (Pliocene) as encountered along a west to east transect in the Central Philippines are consistent with the collision and subsequent indentation of Palawan with the rest of the Philippine Mobile Belt.     

Collision, subduction and accretion events in the Philippines: A synthesis

Abstract The Philippines preserves evidence of the superimposition of tectonic processes in ancient and present‐day collision and subduction zone complexes. The Baguio District in northern Luzon, the Palawan–Central Philippine region and the Mati–Pujada area in southeastern Mindanao resulted from events related to subduction polarity reversal leading to trench initiation, continent‐arc collision and autochthonous oceanic lithosphere emplacement, respectively. Geological data on the Baguio District in Northern Luzon reveal an Early Miocene trench initiation for the east‐dipping Manila Trench. This followed the Late Oligocene cessation of subduction along the west‐dipping proto‐East Luzon Trough. The Manila Trench initiation, which is modeled as a consequence of the counter‐clockwise rotation of Luzon, is attributed to the collision of the Palawan microcontinental block with the Philippine Mobile Belt. In the course of rotation, Luzon onramped the South China Sea crust, effectively converting the shear zone that bounded them into a subduction zone. Several collision‐related accretionary complexes (e.g. Romblon, Mindoro) are present in the Palawan–Central Philippine region. The easternmost collision zone boundary is located east of the Romblon group of islands. The Early Miocene southwestward shift of the collision boundary from Romblon to Mindoro started to end by the Pliocene. Continuous interaction between the Palawan microcontinental block and the Philippine Mobile Belt is presently taken up again along the collisional boundary east of the Romblon group of islands. The Mati–Pujada Peninsula area, on the other hand, is underlain by the Upper Cretaceous Pujada Ophiolite. This supra‐subduction zone ophiolite is capped by chert and pelagic limestones which suggests its derivation from a relatively deep marginal basin. The Pujada Ophiolite could be a part of a proto‐Molucca Sea plate. The re‐interpretation of the geology and tectonic settings of the three areas reaffirm the complex geodynamic evolution of the Philippine archipelago and addresses some of its perceived geological enigmas.     

Baguio Mineral District: An oceanic arc witness to the geological evolution of northern Luzon,Philippines

The Baguio Mineral District exposes rock formations that evince the geological and tectonic evolution of this district from a subduction‐related marginal basin to an island arc setting. Available onshore and offshore data are consistent with an Early (onset phase) to Middle (developed phase) Miocene arc polarity reversal from the east (termination of subduction along the proto‐East Luzon Trough) to the west (initiation of subduction along the Manila Trench). Geophysical modeling and geochemical data calculation showed a 30 ± 5 km crustal thickness for the mineral district. Subduction‐related multiple arc magmatism and ophiolite accretion contributed to crustal thickening. Recent information on the Oligo–Miocene Zigzag and Klondyke formations in the mineral district reveal that the marginal basin, where these rocks were deposited, has received eroded materials from adjacent terrains characterized by siliceous lithologies. Furthermore, adakitic rocks, high permeable zones and extensional zones which are exploration markers applied to identify possible mineralization targets, are prevalent in the mineral district. The geological evolution that the district had undergone mimics the evolution of island arcs worldwide in general and northern Luzon in particular.     

Geology and geochemistry of the Shuanggou ophiolite (Ailao Shan ophiolitic belt), Yunnan Province, SW China: Evidence for a slow-spreading oceanic basin origin

The early Carboniferous Shuanggou ophiolite lies in the middle segment of the Ailao Shan orogenic belt between the South China Block to the north and the Indochina Block to the south. The ophiolite consists of meta-peridotite, gabbro, diabase and basalt, capped by radiolarian-bearing siliceous rocks. No layered gabbros or sheeted dikes have been observed. The meta-peridotite underwent low degrees of partial melting, consistent with the low magma budget of this oceanic lithosphere. Whole-rock rare earth element analyses of gabbro indicate a geochemical affinity with normal mid-ocean ridge basalts, consistent with the crystallization order of plagioclase followed by clinopyroxene recognized in the gabbros. The ophiolite is believed to have formed in a small, slow-spreading oceanic basin. Collision of the Indochina Block with the South China Block in the late Paleozoic was responsible for the closure of the oceanic basin and emplacement of the ophiolite in the Ailao Shan orogenic belt.     

Characterization of the proto-Philippine Sea Plate:Evidence from the emplaced oceanic lithospheric fragments along eastern Philippines

The proto-Philippine Sea Plate(pPSP)has been proposed by several authors to account for the origin of the Mesozoic supra-subduction ophiolites along the Philippine archipelago.In this paper,a comprehensive review of the ophiolites in the eastern portion of the Philippines is undertaken.Available data on the geology,ages and geochemical signatures of the oceanic lithospheric fragments in Luzon(Isabela,Lagonoy in Camarines Norte,and Rapu-Rapu island),Central Philippines(Samar,Tacloban,Malitbog and Southeast Bohol),and eastern Mindanao(Dinagat and Pujada)are presented.Characteristics of the Halmahera Ophiolite to the south of the Philippines are also reviewed for comparison.Nearly all of the crust-mantle sequences preserved along the eastern Philippines share Early to Late Cretaceous ages.The geochemical signatures of mantle and crustal sections reflect both mid-oceanic ridge and suprasubduction signatures.Although paleomagnetic information is currently limited to the Samar Ophiolite,results indicate a near-equatorial Mesozoic supra-subduction zone origin.In general,correlation of the crust-mantle sequences along the eastern edge of the Philippines reveal that they likely are fragments of the Mesozoic pPSP.