The Geology of the Darien, Panama, and the late Miocene-Pliocene collision of the Panama arc with northwestern South America

The geology of the Darien province of eastern Panama is presented through a new geologic map and detailed biostratigraphic and paleobathymetric analysis of its Upper Cretaceous to upper Miocene sediments. The sequence of events inferred from the stratigraphie record includes the collision of the Panama arc (the southwestern margin of the Caribbean plate) and South American continent. Three tectonostratigraphic units underlie the Darien region: (1) PrecoUisional Upper Cretaceous-Eocene crystalline basement rocks of the San Bias Complex form a series of structurally complex topographic massifs along the northeastern and southwestern margins of the Darien province. These rocks formed part of a >20 m.y. submarine volcanic arc developed in a Pacific setting distant from the continental margin of northwestern South America. The northerly basement rocks are quartz diorites, granodiorites, and basaltic andésites, through dacites to rhyolites, indicating the presence of a magmatic arc. The southerly basement rocks are an accreted suite of diabase, pillow basalt, and radiolarian chert deposited at abyssal depths. PrecoUisional arc-related rocks, of Eocene to lower Miocene age, consist of 4000 m of pillow basalts and volcaniclastics, and biogenic calcareous and siliceous deepwater sediments. They consist of the Eocene'Present address: 4193, Lower Cove Run Road, Mathias, West Virginia 26812, USA; e-mail: [email protected]. Oligocène Darien Formation, the Oligocène Porcona Formation and the lower-middle Miocene Clarita Formation. PostcoUisional deposits are mostly coarseto fine-grained siliciclastic sedimentary rocks and turbiditic sandstone of upper middle to latest Miocene age. This 3000 m thick sedimentary sequence is deformed as part of a complexly folded and faulted synclinorium that forms the central Chucunaque-Tuira Basin of the Darien. The sedimentary package reveals general shallowing of the basin from bathyal to inner neritic depths during the 12.8-7.1 Ma collision of the Panama arc with South America. The sediments are divided into the upper middle Miocene Tapaliza Formation, the lower upper Miocene Tuira and Membrillo Formations, the middle upper Miocene Yaviza Formation, and the middle to upper Miocene Chucunaque Formation. The precolUsional open marine units of Late Cretaceous-middle Miocene age are separated from the overlying postcollisional sequence of middle to late Miocene age by a regional unconformity at 14.8-12.8 Ma. This unconformity marks the disappearance of radiolarians, the changeover of predominantly silica deposition from the Atlantic to the Pacific, the initiation of the upUft of the isthmus of Panama, and the onset of shallowing upward, coarser clastic deposition. This pattern is also recorded from the southern Limon Basin of Caribbean Costa Rica to the Atrato Basin of northwestern Colombia. By the middle late Miocene, neritic depths were widespread throughout the Darien region, and a regional unconformity suggests completion of the Central American arc colusión with South America by 7.1 Ma. No PUocene deposits are recorded from either the Darien or the Panama Canal Basin, and no sediments younger than 4.8 Ma have been identified in the Atrato Basin of Colombia, suggesting rapid upUft and extensive emergence of the Central American isthmus in the latest Miocene. Northward movement of the eastern segment of the Panama arc along a now quiescent Panama Canal Zone fault during EoceneOUgocene time may have dislocated the precollision arc. Since colUsion, the portion west of this fault (Chorotega Block) has remained stable, without rotation; to the east, in the Darien region, compression has been accommodated through formation of a Panama microplate with convergent boundaries to the north (North Panama deformed belt) and south (South Panama deformed belt), and suturing with South America along the Atrato Valley. Deformation within the microplate has been accommodated in the Darien province by several major left-lateral strike-slip faults that were active until the early PUocene, since when the plate has behaved rigidly.