Processes of allochthonous terrane evolution, Mindoro Island, Philippines

On Mindoro, two tectonostratigraphic terranes, the North Palawan block and the Mindoro block, are separated by a steeply dipping shear zone. The North Palawan block was rifted off southern China in the Oligocene, and comprises most of the shallow southern end of the South China Sea. The eastern part of this terrane, exposed on Mindoro, includes Jurassic and Paleogene strata, probable rift basalts of Oligocene age, and Oligo‐Miocene graben fill deposits. In the late Oligocene and early Miocene, the eastern North Palawan block underwent crustal stretching, with grabens oriented at a high angle to synchronous spreading anomalies in the South China Basin. We hypothesize that grabens formed in an overall transtensional environment near a transform boundary at the eastern margin of the opening South China Basin. The Mindoro block consists of a pre‐Upper Cretaceous low‐grade metamorphic basement, overlain by Upper Cretaceous and upper Eocene strata. The Mindoro block records several episodes of intense deformation of Mesozoic and Tertiary age, and contrasts with the North Palawan block both in terms of stratigraphy and structural evolution. The Mindoro Suture, which separates the Mindoro and North Palawan blocks, is a complex structural boundary comprising slices of diverse lithologies separated by steeply dipping, anastomosing faults. Deformed tectonic slices in the suture zone include serpentinized ultramafic rock, amphibolite, and rocks derived from both of the bounding terranes. We interpret this suture as a zone of mostly transcurrent faulting that was active through much of mid‐Tertiary time. Paleogeographic considerations for early and middle Miocene time support the premise that the west side of the central Philippine archipelago was a left‐lateral transform boundary. The Mindoro and North Palawan blocks were initially juxtaposed along the transcurrent Mindoro Suture by late Miocene time. Subsequent deformation on Mindoro includes crustal shortening associated with the southern end of the Manila Trench, which projects onshore on southwest Mindoro. The onset of shortening here probably reflects a late Miocene change in Philippine‐Eurasian plate motion. The Mindoro Suture Zone is now locked, and it acts as a rigid buttress against which strata of the North Palawan block have been structurally stacked. The Mindoro Suture Zone is similar to other terrane boundaries in the northern Philippines. The history of terrane transport in this region was apparently dominated by strike‐parallel shuffling of terranes along major transcurrent boundaries within the Philippine arc. Transcurrent motion has taken place in extensional and compressional regimes, and it is ongoing today. This may be a dominant process by which complex island arc systems are disrupted, prior to accretion onto a continental margin.