I've just returned from Trinidad and Barbados in the southeast Caribbean, the first of which was the very welcoming and generous host country for the 20th Caribbean Geological Conference and Field Trips. This meeting has been convened in different Caribbean
nations every three years since the first in 1955, prior to the advent of plate tectonics.
Even then, suspicions were growing about the eastward displacement of the Caribbean relative to the Americas, accommodated by hundreds of kilometers of sinistral and dextral offset along the northern and southern flanks of the Caribbean Sea (Hess and Maxwell,
1953).
Knowing that the Atlantic Benioff zone beneath the Lesser Antilles Arc descended westward to more than 300 kilometers subsea, and that the quantitative Atlantic opening model of Bullard et al. (1965) was the shape of things to come, Wilson (1966) took the
bold and visionary step suggesting that much of today's actively migrating Caribbean lithosphere originated from the Pacific.
Wilson called it "ice-rafting," comparable to Alpine glaciers gliding along valley walls, at a time when early students of tectonics struggled to identify driving processes.
As the light of plate tectonics brightened, Molnar and Sykes (1969) assessed focal mechanisms and seismicity to define the "Caribbean Plate," providing a regional neotectonic framework for others to consider and test.
Going forward, those vying for some of the world's most exciting and instructive field areas and data sets have spent the last 46 years refining the details and reconstructing the evolution of this complex geological province. Many hundreds of geologists,
geophysicists, geochemists, plate kinematicists and dynamicists, geochronologists, geomorphologists and explorationists have scoured this region and its margins for their secrets and treasures, both academic and commercial.
Like the Mediterranean and southeast Asia/Indonesia, the Caribbean has become a classic locus for unraveling regional paleogeographic evolution.
Although there are a few who continue to question it, the collective efforts of these many geoscientists have led to numerous principles that demand a Pacific origin for the Caribbean oceanic lithosphere.
Ten of those factors are:
♦ The Greater Antilles Arc (Jamaica, Cuba, Hispaniola, Puerto Rico and probably Aves Ridge) formed by active subduction from the Barremian/Aptian to Eocene time, a period of some 80 Ma. P-T-t studies from the circum-Caribbean suture zone north of this arc
indicate that south-dipping subduction occurred over this entire time span (e.g., Garcia Casco et al., 2006; Krebs et al., 2008), suggesting at least 1,600 kilometers of subduction at the 20 millimeters-per-year minimum rate for robust arc development.
Some argue for a two-stage subduction history with a polarity reversal at about 90 Ma (Burke 1988; Kerr et al., 2003) and more than 800 kilometers of subsequent migration prior to collision with the Bahamas.
Either way, the former existence of a large, subductable ocean between the advancing arc and the Bahamas is apparent from the rocks in the arc and its suture zone.
Island arcs are not the same as continental crust: they form by subduction, and this applies to all the Antilles (Greater, Lesser and Leeward) as well as to the Panama-Costa Rica Arc. There are no valid pre-133 Ma arc-magmatic ages from any of these places,
and they should not be included in pre-Cretaceous paleogeographic reconstructions, although Jurassic reconstructions should attempt to show the origin of the oceanic crust upon which some of these arcs lie.
The rare occurrences of continental rocks in these intra-oceanic arcs (Renne et al., 1989) might have been emplaced from the start along the intra-American transform (Pindell et al., 2011), or accreted into the arc later during subduction (Garcia-Casco et
al., 2006).
Pre-Mesozoic detrital zircon grains showing up in some of these arc rocks likely derive from subducted sediment or from the ancient mantle wedges sourcing the arc magmas.
♦ After the Eocene Greater Antilles-Bahamas collision, evidence for an additional 1,000 kilometers or so of relative eastward Caribbean migration between the Americas is quite convincing.
The Cayman Trough usually is cited as an obvious recorder of Eocene ( about 50 Ma) to Recent motion (Rosencrantz et al., 1988; Leroy, 2000). Perhaps even more remarkable, however, is the eastward-youngling development of the "Tertiary Caribbean Foredeep,"
comprising the Maracaibo, Guarico and Eastern Venezuela sub-basins above the underlying Cretaceous passive margin section of Colombia, Venezuela and Trinidad. This eastwardly advancing foredeep formed by dextral oblique collision since the Paleocene at about
20 millimeters a year, similar to the opening rate of the Cayman Trough.
In addition, there is an older but similar northeastward migration of foredeep development along the North American margin, from the eastern Chortis Block (Albian) into the Sepur of southern Yucatan (Campanian-Maastrichtian) and eventually reaching the
Bahamas (Eocene).
Many secondary observations arise from the Tertiary phase of relative migration, such as the progressive increase of ash bed occurrence cored by ODP at Tiburón Rise ahead of the Caribbean Plate, and the spatial juxtaposition of dissimilar rock suites such
as those of Margarita and Eastern Venezuela.
Margarita island exposes exhumed rocks that were subducted to more than 70 kilometers in the Early Cretaceous, together with Late Cretaceous island arc granitoids, while the mainland records only passive margin sedimentation at those times with no signs
of any such tectonism.
♦ More recently, seismic tomography has given us visual confirmation of the subducted Atlantic, Cocos, Nazca and Caribbean slabs in the region, with fairly straightforward correlation between the imaged slabs and Pacific-origin Caribbean models as deciphered
from the Caribbean arcs, sutures and seismically active Benioff zones. The Atlantic and Cocos slabs lie beneath the Lesser Antilles and Central American arcs, respectively, by more than 1,000 kilometers; the Caribbean slab descends beneath Colombia/Maracaibo
by more than 700 kilometers (Masy et al., 2011); and the Proto-Caribbean slab (older part of the Atlantic) descends southward beneath Hispaniola by as much as 1,500 kilometers (van Benthem, 2013).
♦ The Caribbean islands and oceanic plate interior are dominated by thick successions of tuff-bearing strata, recording subduction-related magmatic activity, whereas the passive margin sections of the Bahamas, Yucatan and northern South America are virtually
tuff free.
Clearly, many hundreds of kilometers must have separated the evolving arcs from the passive margins, especially in the Cretaceous.
♦ An important process in the evolution of the Caribbean is that of "arc-parallel extension." During the Caribbean Plate's insertion between the Americas, the leading arc system expanded into the pre-existing space of the Proto-Caribbean Basin. This is shown
by the continuous circum-Caribbean suture belt - a legacy to the effectiveness of subduction zone roll-back (of Proto-Caribbean lithosphere), which has left no known vestiges of the former Proto-Caribbean seafloor apart from accreted packages in the circum-Caribbean
suture.
Arc lengthening has been achieved in three ways:
- One is the Eocene-Recent sinistral displacement of fragments of the Greater Antilles arc by eastward projections of the Cayman Trough as it opened - namely the Oriente, the San Juan, and the Mona Passage fault zones (Pindell and Barrett, 1990).
- A second is the Maastrichtian-Eocene oblique openings of the Yucatan (Rosencrantz, 1990; Pindell et al., 2005) and Grenada/Tobago intra-arc basins (Speed and Walker, 1991; Bird et al., 1999; Pindell et al., 1998; Pindell and Kennan, 2009), both of which
added several hundred kilometres of arc length relative to the remnant arcs of the Caribbean interior.
- A third is the creation of horst and graben, or en-echelon half-graben, structure along-strike in the arc complexes as they have undergone oblique collision with the Proto-Caribbbean margins (Pindell and Kennan, 2009).
♦ The former relative positions (relative motion history) of the Americas since the Triassic are now known with accuracies better than 200 kilometers through the Aptian, and about 50 kilometers since the Campanian. This history shows that the islands of the
Greater Antilles Arc began their subduction history (Hauterivian-Aptian) at a time when the gap between the Americas was far too small to house them spatially (see map), and when deposition along the Proto-Caribbean continental margins was entirely passive.
♦ The Chortis Block has lithologic correlation with southwest Mexico and was amalgamated into the migrating Caribbean Plate in the Campanian-Maastrichtian. The eastward migration of the site of initial subduction and arc magmatism along southwest Mexico matches
the departure of Chortis as the Cayman Trough opened.
Without this restoration of Chortis to southwest Mexico, southern Mexico would have developed a Cretaceous and Paleogene arc just like the Cordillera Occidental; instead, that arc lies on Chortis, and is now displaced to the east.
New paleomagnetic work shows significant anti-clockwise rotation of Chortis since the Eocene, by about the amount expected from comparing the trends of southwest Mexico and the northern Chortis Block.
♦ Middle Cretaceous and older Pacific fauna are known to occupy the Caribbean, in accord with the Campanian entry of the plate into the inter-American realm. Jurassic radiolarian oceanic cherts of Puerto Rico, for example, are Pacific derived, likely older
than the onset of abyssal conditions in the Proto-Caribbean seaway, and are not known in the Atlantic/Tethys.
As another example, Mitchell (2015) has identified Aptian rudists in northeast Trinidad (Toco Formation) that are otherwise only known from the Pacific margin of the Americas (e.g., Mexico).
♦ The Caribbean plate interior is dominated by many kilometers of oceanic plateau basalts and diabases that were extruded and intruded at about 90-92 Ma. In contrast, this was one of the most passive periods of stratigraphic development on the Proto-Caribbean
passive margins (the time of deposition of La Luna Fm), and yet we know of virtually nothing in these margins that can be attributed to the creation of the Caribbean Plateau.
It is irrational to accept that one of the larger plume eruptions on the planet took place without any magmatic or stratigraphic implications for the adjacent passive margins; the eruptions must have occurred at distance, somewhere to the west of Colombia.
♦ Not strictly part of the Caribbean, but closely related to Caribbean evolution, is the well-known phase of near-pole seafloor spreading in the Gulf of Mexico that caused a nearly 40-degree anti-clockwise rotation of the Yucatan Block in the Late Jurassic,
the former spreading ridge of which can be observed now in gravity data (see map), unpublished aero-magnetic data and seismic mapping of the Gulf of Mexico's seafloor fabric by many exploration companies.
We should not place more significance on the realignment of selected (of many) fault sets than on the extensive mapping that has been done in the Gulf of Mexico.
I could go on, but the above points are surely sufficient to provide a basis for understanding and acknowledging the Pacific origin of the Caribbean oceanic lithosphere and its intra-oceanic arcs. The most visionary workers in the 1950s and 1960s got it
right.
As I wrote in the July 2013 EXPLORER, the Pacific origin of Caribbean oceanic lithosphere is clear, and I know of no data that contest it. James (2009) gives a different view based on certain observations and opinions, but in reality, any point of view that
does not embrace large relative migration of the Caribbean between the Americas ignores the vast wealth of data outlined above.
Also, the people generating that data do not bias their interpretations toward any particular model; the data sets speak for themselves.
What about the Caribbean as an exploration province?
According to seismic reflection and refraction data - and as sampled by DSDP several times in the Caribbean interior as well as onshore in Aruba, Curacao, Jamaica, Hispaniola, Costa Rica, Panama and Nicaragua - the majority of the Caribbean's basement is
Turonian-Coniacian basalt and diabase formed at extensional hot spot(s) or extruded onto older oceanic crust above a more regional plume head.
The interpretation by James (2009) that salt forms tall pinnacles rising well above the surrounding seafloor causes one to wonder if salt would be able to do that; surely these are submarine basaltic volcanoes that are not yet fully buried.
There is one known swath of probable Early Cretaceous normal ocean crust in the southeast corner of the Venezuelan Basin (known by refraction), but we don't know the pre-Coniacian stratigraphy there. There could be other smaller such swaths.
But apart from such zones, the Coniacian section on top of the Caribbean basalt plateau is the oldest regional source rock known, a function of an anoxic event(s) that correlates with organically rich portions of the shelfal Napo, Villeta, La Luna, Querecual,
Gautier/Naparima Hill and Canje formations spanning Ecuador to Suriname.
It makes little difference if the Caribbean's Coniacian section was deposited north of South America (fixist view) or to the northwest of South America during its insertion between the Americas (mobilist view).
However, it makes a world of difference when it comes to understanding the rest of the Caribbean's geology as noted above, because that geology makes no sense at all without acknowledging that the Americas have drifted west around a swath of Pacific lithosphere,
and that the Proto-Caribbean basin between the Americas was subducted beneath the Caribbean arcs.
Do we need more seismic?
Of course we do; there are loads of local uncertainties - but let's not push our luck and expect to see Jurassic rift-related salt inboard of the circum-Caribbean suture. There are several areas within the Caribbean where thrusting or gravitational collapse
over the Caribbean crust has produced sufficiently thick sediment wedges for hydrocarbon maturation. This is true in the North Panama Foldbelt, the San Pedro Basin/Muertos Trough, the southern Grenada and Tobago basins, and the South Caribbean Foldbelt, all
of which are potentially sourced by the Caribbean's Upper Cretaceous TOC-rich shales and pelagics.
In addition, we can learn more about the interface between Caribbean and Proto-Caribbean sedimentary assemblages in places like the Tobago Basin, the Barbados Ridge, and offshore northeastern Trinidad.
Are Pacific origin models for the Caribbean more complicated than fixist ones?
No.
The Caribbean Plate has been effectively pinned in the mantle or hot spot reference frame between the opposing Central American and Lesser Antilles Benioff zones since the Late Cretaceous, while the North and South American plates have drifted westward from
Africa, building the Andes and North American Cordillera as they progressively have forced their way over the Pacific Benioff zones.
In contrast, fixist views require that the Caribbean Plate has migrated westward with the American plates, hence little relative motion between them. If this were the case, we would expect Andean or Cordilleran types of shortening in Costa Rica, Panama and
western Chortis, too, but we do not see such shortening there.