Geologic evolution of Western Mindoro Island and the Mindoro Suture Zone, Philippines

Abstract On Mindoro Island, Philippines, two tectonostratigraphic terranes—the North Palawan and Mindoro blocks—are separated by a steeply-dipping lithospheric boundary, here called the Mindoro Suture Zone. The North Palawan block, which constitutes much of the southern end of the South China Sea, is a continental fragment that was rifted off Eurasia when the South China Basin opened in the mid Tertiary. In the region of western Mindoro and the Mindoro Straits, rocks show evidence of crustal stretching starting in upper Eocene time or earlier. Steep dips displayed by Jurassic strata in southwest Mindoro were acquired prior to the onset of extension, but unconformably overlying upper Eocene through lower Miocene rocks were deposited in an environment of active extension and basin formation, probably associated with major transcurrent faulting. Large volumes of basalt were extruded in the mid Oligocene, at about the time that sea-floor spreading began in the South China Basin. Basalts were differentially uplifted and eroded by upper Oligocene time. Detritus derived from mafic and other source terrains was shed into actively extending grabens and half-grabens, some of which were floored by basalts. Grabens in this area trend NW, oblique to regional N-S extension indicated by magnetic anomalies in the adjacent South China Basin. Field relations on southwest Mindoro indicate that basin genesis was punctuated by local pulses of compression. Timing and patterns of extension and basin evolution suggest that the North Palawan block was bounded on the east by an extensional strike-slip (transtensional) plate boundary. The Mindoro block records a history of shortening that bears no similarity to the structural evolution of the North Palawan block. The pre-upper Cretaceous Mindoro Metamorphics, which constitute the basement of the Mindoro block, underwent intense deformation prior to upper Eocene time that led to at least one transposition of layering, and was accompanied by metamorphism to greenschist facies. The unconformably overlying late(?) Eocene Lasala Formation displays evidence of a subhorizontal, N-S (present reference frame) shortening event of probable upper Paleogene age, and a subsequent deformation of uncertain nature that also involved some shortening, and may be related to the development of the Mindoro Suture Zone. The Mindoro Suture Zone is a complex, high angle structure that separates the Mindoro block from the North Palawan block. The suture was probably active at least during upper Paleogene and lower Neogene time. In west-central Mindoro, it is defined by innumerable steeply-dipping and anastomosing faults that separate tectonic slices of material derived from the North Palawan and the Mindoro blocks, and from enigmatic sources as well. Numerous large bodies of serpentinized ultramafic rock occur as fault bounded inclusions along the entire length of the suture. We interpret the Mindoro Suture Zone as an interplate strike-slip boundary, along which the Mindoro and North Palawan blocks were initially juxtaposed in mid to upper Miocene time. Subsequent latest Miocene and younger tectonism on western Mindoro has been dominated by generally west-vergent thrust faulting in the area between the Mindoro Suture Zone and the Manila Trench.