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 fea tures are faults and blind folds and duplexes which are not easily det ectable in this island on which 75% of the surface is covered by Pleis tocene and Holocene sediments and volcanics. A detailed study of the d rainage pattern and its discontinuities in the Agusan-Davao and the Co tabato Basins when compared with field data, radar data and subsurface structures observed on seismic documents, enables a correlation of di stant seismic lines and features existing on the basin edges. The drai nage network analysis, based on description and interpretation of drai nage anomalies and frequency, allows us to distinguish: ( 1) subhomoge nous drainage areas which correspond to areas bearing consistent geolo gical characteristics, and (2) drainage anomalies. The results have be en checked in the field and compared with the tectonic features in the mountainous areas. The extension of these structures could then be tr aced confidently further towards the flat lowlands. We point out the p resence of two distinct anomaly directions that intersect in eastern M indanao: (1) Numerous NNW anomalies in the intra arc basin of Agusan-D avao and both Pacific and Central Cordilleras appear as young features in the morphology and are due to strike-slip faults of the arc-contin ent collision. Some of the faults have proven to be reactivated Middle Miocene faults, related to the initial rifting of the Agusan-Davao Ba sin. (2) N-S anomaly directions are transpressive ridges due to the co llision except one anomaly which corresponds to the trace of the Phili ppine Fault Zone and connects with the collisional features. In contra st, the Cotabato Basin in western Mindanao lies between two terranes o f different nature. In the Cotabato Basin area, the NW to NNW anomaly directions are also present and similarly correspond to strike-slip fa ults and thrust related folds, respectively. Pull-apart basins occur i n places where sagging is evidenced. An additional set of anomalies tr ends ENE and corresponds to die active extensional field. Below die Qu aternary 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 with in die compressive and strike-slip setting of accreted belts.