Western Mindanao Research Articles

Magmatic response to abrupt changes in geodynamic settings: Pliocene—Quaternary calc-alkaline and Nb-enriched lavas from Mindanao (Philippines)

Mindanao, the largest island in the southern Philippine archipelago, is a composite of at least two terranes; one with Eurasian affinity (western Mindanao) and the other belonging to the Philippine Mobile Belt (eastern Mindanao), of Philippine Sea plate affinity. The island is surrounded by three subduction zones that have been installed only in the past 4 m.y. Prior to this, the two terranes were separated by an ocean that disappeared continuously by subduction of its two edges beneath western and eastern Mindanao, where mostly typical arc magmatic rocks, dated at 30 Ma, 19−15 Ma, 12−11 Ma and 7−4 Ma were emplaced. The suturing of the two terranes occurred at ca. 5 Ma. Following this major structural reorganization, abrupt changes are recorded in the < 3 Ma old magmatism of the island. The emplacement of adakites, dated from 1.80 to 0.10 Ma and 0.70 Ma in eastern and western Mindanao, respectively, is thought to result from the initiation of new subductions . In western Mindanao, associated Nb-enriched basalts (NEB) and basaltic andesites (2.58−1.08 Ma) might represent either melts of mantle having interacted with adakitic liquids or of heterogenous mantle containing an OIB component. Seismic data give evidence of the presence of a detached subducted slab beneath Central Mindanao, where the recent volcanism is post-collisional. Calc-alkaline lavas, dated at 2.31−0.29 Ma, pre-date potassic volcanism dated at 0.80−0.36 Ma. Among the calc-alkaline rocks, a group of relatively magnesian basalts and basaltic andesites is also considered to be enriched in Nb and one adakitic sample has been found. The geochemical diversity of magmatic types in Mindanao is attributed to: 1. (1) the highly heterogenous character of their mantle source, which contains variable amounts of metasomatic pargasite and phlogopite, and, possibly, an additional OIB component that could contribute to Nb enrichment of NEB; 2. (2) the contribution of melts from the ubducted oceanic crust; these melts are either emplaced directly on the surface (adakites) or act as metasomatic agents leading to a Nb-enriched mantle, a probable source of NEB. Garnet and amphibole fractionation could also account for additional variations in the MREE and the HREE.

Drainage network analysis and tectonics of Mindanao, southern Philippines

Mindanao, southern Philippines is the site of an active arc-continent collision which began in the Late Pliocene. The principal tectonic features are faults and blind folds and duplexes which are not easily detectable in this island on which 75% of the surface is covered by Pleistocene and Holocene sediments and volcanics. A detailed study of the drainage pattern and its discontinuities in the Agusan-Davao and the Cotabato Basins when compared with field data, radar data and subsurface structures observed on seismic documents, enables a correlation of distant seismic lines and features existing on the basin edges. The drainage network analysis, based on description and interpretation of drainage anomalies and frequency, allows us to distinguish: (1) subhomogenous drainage areas which correspond to areas bearing consistent geological characteristics, and (2) drainage anomalies. The results have been checked in the field and compared with the tectonic features in the mountainous areas. The extension of these structures could then be traced confidently further towards the flat lowlands. We point out the presence of two distinct anomaly directions that intersect in eastern Mindanao: (1) Numerous NNW anomalies in the intra arc basin of Agusan-Davao and both Pacific and Central Cordilleras appear as young features in the morphology and are due to strike-slip faults of the arc-continent collision. Some of the faults have proven to be reactivated Middle Miocene faults, related to the initial rifting of the Agusan-Davao Basin. (2) N-S anomaly directions are transpressive ridges due to the collision except one anomaly which corresponds to the trace of the Philippine Fault Zone and connects with the collisional features. In contrast, the Cotabato Basin in western Mindanao lies between two terranes of different nature. In the Cotabato Basin area, the NW and NNW anomaly directions are also present and similarly correspond to strike-slip faults and thrust related folds, respectively. Pull-apart basins occur in places where sagging is evidenced. An additional set of anomalies trends ENE and corresponds to the active extensional field. Below the Quaternary sediments, some of the “en echelon” features corresponding to blind flat-and-ramps structures were identified. These observations provide reference examples for the evolution of basins generated within the compressive and strike-slip setting of accreted belts.

Initiation of subduction and the generation of slab melts in western and eastern Mindanao, Philippines

Adakite, found in both the eastern and western parts of Mindanao Island, Philippines, is a rare rock type, characterized by low heavy rare earth elements and Y contents together with high Sr/Y ratios, and is considered to be the result of the melting of young subducted oceanic crust, which leaves an eclogite residue. Pliocene-Quaternary adakites from western Mindanao (Zamboanga Peninsula) are probably derived from the melting of the young Miocene Sulu Sea crust, which is currently subducting beneath Zamboanga. Associated Nb-enriched basalts are thought to come from mantle metasomatized through interaction with adakitic melts. In eastern Mindanao, Pliocene-Quaternary cones and plugs of typical adakitic composition mark the trace of the Philippine fault in Surigao and north Davao. The underlying Philippine Sea crust is of Eocene age and therefore cannot melt under normal subduction thermal conditions. Thermal models indicate that melting at the start of subduction can occur. Subduction of the Philippine Sea plate began 3 to 4 Ma beneath eastern Mindanao and probably accounts for the presence of adakites along the Philippine fault.

The evolution of the geography of religious adherence in the Philippines before 1898

The introduction and acceptance of exotic religious institutions and identities greatly influenced the process of national development in the Philippines. The agents of Islam, largely unsupported by outside forces, were initially active in many parts of the archipelago after about the thirteenth century. Roman Catholic missionaries, arriving 200 years later but supported by an elaborate colonial edifice, introduced their faith through an expanding network of fixed mission centres. Distinctive and sometimes intricate geographies resulted from differing social and geographical strategies of conversion. The most general result was the division of Philippine societies into the Christian lowlands of Luzon; the central islands and northern Mindanao; the several Muslim areas of western Mindanao and Sulu; and the persisting local ethnic religious systems of the highlands.

Reconnaissance Geology and Oil Possibillities of Mindanao

Mindanao is part an island arc system which includes the Philippines and eastern Indonesia. The geologic structure of the Island is reflected by the physiography. The structural features in eastern Mindanao are northerly aligned and are strongly influenced by the active Philippine rift; those of western and central Mindanao reflect a merging of diverse tectonic and volcanic trends. The mountainous areas consist chiefly of basement and Tertiary volcanic rocks, whereas intervening lowlands are underlain by Tertiary sedimentary rocks. There are two major sedimentary basins-the Agusan-Davao trough and the Cotabato basin-which contain 6,000-15,000 feet of marine strata of geosynclinal origin. The stratigraphic succession is composed mostly of Miocene and Pliocene rocks which a e characterized by abrupt lateral and vertical lithologic changes. In addition, a thick Eocene limestone is present in the Agusan-Davao trough. There are possibilities for commercial hydrocarbon accumulation in both basins. The sedimentary section contains probable source,cap, and reservoir rocks, and potential structural and stratigrahic traps are present.