Once we’ve completed a study most of us geologists feel that despite the incomplete data with which we started, our insight has overcome that basic limitation.
Ours, however, is not an exact science – and the all-too-frequent dry wildcats return us to the level of fallible mortals.
I bring this up as an introduction to my comments here on Jim Pindell’s rebuttal of my article on the origin of the Caribbean Plate ( August 2012 EXPLORER ).
Jim’s article appeared in the Seismic Project Sets Caribbean Investigation .
Whatever we serve up – Arroz a la Cubana or La Bandera Dominicana – we should check the ingredients. No one is infallible, but I feel many points in his article need correction.
Before I touch on the main ones, let me provide you with synopses of the Caribbean Pacific – first Jim’s preferred model, and then the In-situ model that I prefer.
♦ Jim’s model:
The Caribbean formed as Jurassic oceanic crust in the Pacific, thickened into an igneous “oceanic plateau” in the Cretaceous, collided with and reversed the subduction polarity of a linear intra-oceanic arc and became progressively engulfed by westward migrating North and South America (figure 1).
Diachronous interaction with the Americas curved and greatly extended the arc. Continental material along western Colombia and North America was subducted to HP/LT blueschist conditions, then resurfaced along the Greater Antilles and northern South America.
The large Maya and Chortis blocks rotated 135 degrees counterclockwise or 100 degrees clockwise, and 180 degrees counterclockwise or 80 degrees clockwise, respectively.
Plate boundary jumps defined the Yucatan and Grenada basins.
The Caribbean consists largely of Jurassic-Cretaceous oceanic and volcanic rocks.
♦ My model:
The Caribbean formed between separating North and South America, with which it shares structural fabric and stratigraphy (figure 2).
Caribbean margins carry Triassic-Cretaceous continental to marine rift deposits and carbonate platforms. Internal seismic architecture of highs flanked by wedges repeats that, seen on extended continental margins.
An overlying regional seismic marker reflects an upper Cretaceous basalt, with smooth (shallow water/continental) and rough (subaqueous) signatures.
A violent, regional Middle Eocene event generated wildflysch and emplaced major allochthons of volcanic and serpentinitic rocks, and metamorphosed marginal rocks onto the Greater Antilles and northern South America, shutting down volcanic activity.
Subsequent eastward relative Caribbean movement created eastward-migrating foredeeps progressively inverted in transpressional uplifts and invaded by their fold and thrust belts.
The Caribbean consists of extended continent (crustal thicknesses, gravity data, high silica content, ancient zircons), with volcanic rocks and serpentinized mantle.
The following are points of disagreement between the two of us.
Pindell: Today, arguments for this story are more numerous and better documented than in 1985, and none contest it.
James: Jim has the book that lists him as co-editor (GSL Special Publication 328) and several other publications (see www.kjgeology.com) that include arguments against the Pacific origin for the Caribbean.
Pindell: The paucity of arc-related tuffs in the Cretaceous Proto-Caribbean passive margin sections of Yucatán, Bahamas, northern Colombia, Venezuela and Trinidad implies allochthonous origin of the Caribbean.
James: Yucatán, the Bahamas and Trinidad lie outside the Caribbean. There are tuffs in the Turonian of Colombia’s Guajira Peninsula and Villeta Formation. Units of northern Venezuela (Villa de Cura, Tacagua, Tiara, Las Brisas, Las Mercedes), Trinidad (Toco, Sans Souci) and Tobago (Volcanic Group) include Upper Jurassic-Cretaceous volcano-sedimentary rocks formed on a passive margin.
Offshore Venezuela, Cretaceous units include volcaniclastics and basalts associated with limestones, black shales, quartz sandstones/conglomerates.
Pindell: Seismic tomography provides a measurable minimum of 1,500 kilometers of American-Caribbean relative motion.
James: Tomography in the eastern Caribbean (Van der Hilst, 1990) was necessarily derived from geographically limited stations. VdH emphasized that results “have a preliminary character and discussions and conclusions should be considered tentative.”
“The blurred image of the Atlantic lithosphere subducted below the eastern Caribbean” was “a working hypothesis” – this does not translate into a “measurable minimum of Caribbean-American Tertiary displacement.”
Pindell: Subduction and arc magmatism began along the western Costa Rica-Panama Caribbean margin in the Campanian. There is no Cenozoic Andes-type orogenesis in Costa Rica/Panamá.
James: An important tenet of the Pacific model is that subduction and arc magmatism began along Central America only when the leading edge of the Caribbean Plate encountered the Americas (in the Campanian).
In fact, arc magmatism began at least as long ago as the Albian – and probably in the Jurassic (radiolaria in volcano-sedimentary rocks on the Nicoya and Santa Elena peninsulas of Costa Rica, respectively).
Andean orogenesis is understood to involve the continent. Accreted Jurassic-Cretaceous oceanic rocks record convergence of the Pacific with Costa Rica where crustal thickness of 40-45 kilometers, seismic velocities and gravity data indicate continental foundations. Albian and Miocene quartz sands are present. High silica Tertiary ignimbrites in Costa Rica are chemically similar to continental rocks. Similar ignimbrites in Guatemala lie above Palaeozoic continental crust.
The common (several “intra-oceanic” arcs) reasoning, “since Costa Rica is an intra-oceanic arc these siliceous rocks must indicate formation of new continental crust,” is false logic.
Circular reasoning also is applied to Caribbean blueschists: “They form under HP/LT conditions, hence in subduction zones, therefore blueschists in Cuba, Hispaniola and Venezuela evidence subduction.”
No blueschists are present in the Central American or Lesser Antilles arcs, active at least since the early Cretaceous. HP/LT conditions can arise in foredeeps such as Trinidad’s Columbus Basin, where up to 15,000 meters of younger Tertiary sediments lie above Mesozoic sedimentary rocks probably of similar thickness.
High pressure (compaction, source rock maturation, clay diagenesis, mineral transformation, fluid expansion, osmosis, efficient seals and tectonic pressure) and low heat flux could generate HP/LT conditions. Transpressional basin inversion returns them to the surface.
Metamorphic grade declines from west to east along the Greater Antilles and along the Venezuelan-Trinidadian Coastal-Northern Ranges. It increases toward major faults in Cuba and northern South America and grades laterally from sedimentary to metamorphic rocks.
Pindell: Sinistral segmentation of Antillean rock units on the order of 300, 400 and 300 kilometers at the Oriente, San Juan-Muertos and Anegada fault zones, respectively, are apparent and consistent with Cayman Trough opening (700-1,000 kilometers – oceanic spreading fabric).
James: Satellite imagery (Jim’s 2013 figure 1) clearly shows that Cuba, Hispaniola, Puerto Rico and the Virgin Islands are sub-aerial parts of larger, much submerged blocks separated only by narrow deeps such as the Mona Channel (extension estimated 15 kilometers) and Anegada Passage.
Discussion of Greater Antillean geology only as accreted Cretaceous oceanic and arc rocks with younger cover consistently overlooks 30 kilometers of crustal thicknesses and Cretaceous marbles (intertidal-subtidal limestones now as deep as 6,000 meters – southern extension of the Bahamas carbonate platform) dredged from northern slopes.
Continental granitoids and red beds dredged from Cayman Trough walls do not lend themselves to oceanic origins here, either. Trough opening is neither calibrated (estimates from 150-1,400 kilometers; oceanic fabric appears only in the central 300 kilometers) nor dated (Pliocene, Oligocene, Eocene or Jurassic-early Cretaceous [North-South American offset] plus Oligocene- Recent [Caribbean-Americas offset]).
Pindell: Cretaceous passive margin sections of Yucatán, the Bahamas, Colombia, Venezuela and Trinidad transform upward with east-younging diachroneity into tectonically active foreland basins with “arc-bearing” turbiditic flysch (mentioned examples include Maastrichtian Sepúr, Paleocene Molino and Eocene Cacarajicara and Amaro formations).
James: An important Circum-Caribbean coeval, middle Eocene event separates Cretaceous passive margin from diachronous foreland basin history. It emplaced allochthons up to 1,000 kilometers long, 20 kilometers wide and five kilometers thick (Cuba, Venezuela, Jamaica, Hispaniola) and shut down volcanic activity along the north and south.
Middle Eocene wildflysch is followed by a regional, middle Eocene unconformity, shallow water carbonates and a late Eocene hiatus.
Luruaco, Matatere, Guárico, Pointe-a-Pierre, Chaudiere and Lizard Springs flysch records this coeval (not diachronous) event across Colombia, Venezuela and Trinidad.
The age of Guatemala´s Sepúr Fm is Paleocene-Eocene (fossils), not Maastrichtian – older content is reworked. Colombia’s Maastrichtian (not Palaeocene) Molino Fm is mainly carbonaceous claystone, not “arc-bearing turbiditic flysch.” Cuba’s Cacarajicara and Amaro Fms are K/T boundary megabeds.
Following the Eocene event, deltaic-basinal, Eocene Misoa-Trujillo-Pauji, Oligocene La Pascua-Roblecito, and Miocene Areo/Carapita, Oficina, Retrench and Herrera sediments accumulated in foredeeps related to eastward younging, dextral transpression along northern South America, continuing today east of Trinidad.
They are not “arc-bearing.”
Pindell: There remains no hint of continental crust or salt in the Caribbean.
James: Seismic similarity of internal Caribbean structure to that imaged and drilled offshore North America (half grabens, Jurassic-Cretaceous-Tertiary fill, including salt) and replication of a drilled Gulf of Mexico seafloor salt diapir by a Caribbean “seamount” (clearly diapiric, not volcanic) are not mere hints.
Jim Pindell regards the Pacific model as the simplest. Needless to say, I disagree.
Does it matter? Well, if you are interested in oil and gas, the in-situ model relates the Caribbean to its hydrocarbon-rich neighbors.
Caribbean nations should think about that.