Geoscientist Online

Geoscientist 19.11 November 2009

The “Origin of the Caribbean Plate” meeting at Sigüenza in 2006 should have been the final resting place of the “in situ” model of the Caribbean. The POP is over two decades old. It is a well established, tested model with more arguments in favour than space allows. I have outlined just five general pro-POP arguments in the box, and will respond in summary to James and Lorente’s article by challenging first their reading of the POP, and second their “in situ” interpretation of existing data.

Understanding the POP

In their provocative piece POP goes the paradigm? (Geoscientist 19.9 pp 12-14: see their Figure 2) James and Lorente argue that the "Great Arc" of the Caribbean bends and lengthens "impossibly" in Pacific Origin models. We know that the arc has a long and complex history passing through oblique extension, collision, obduction, rotation, even polarity reversal. At a glance, what James and Lorente mark as "the Arc" is not telling the full story and their objection does not consider the numerous geological processes capable of occurring to change the surface shape of the arc.

On their Figure 3, magnetic lineations are marked in the Venezuelan basin. This basin is largely covered by the Caribbean Oceanic Plateau, and these lineations do not exist in anyone else's model, POP or otherwise. Occam's razor suggests that James misinterprets, or over-interprets the data.

The authors’ description (p 13) of the POP is factually incorrect in places and seems to rely upon the “traditional” view of Burke, (1988)⁷. Many recent models (e.g. Pindell et al.^3,5) do not necessarily believe that the oceanic plateau collided with an arc, causing polarity reversal. So here it would seem that James and Lorente choose to ignore half the literature. It is vital, in proposing an alternative model to any theory, that the protagonists are up to date, in detail, with what they are fighting. Furthermore, it is now becoming established that the Caribbean Oceanic Plateau cannot be formed above a slab gap.⁸

The "in situ" model’s interpretations of seismic lines (page 14 of the printed article) prompt me to ask how a salt diapir can possibly protrude a substantial distance above the sea floor when it would surely either dissolve or spread out. Such structures are more likely seamounts. In erroneously suggesting the presence of salt in the Venezuelan basin, the conclusion they then make, that much of the basin represents continental crust, is no longer valid.

Interpretations of existing data

As for James and Lorente’s alternative interpretations of the known geology, I would make the following points.

1. Crust of 45 km thickness does not necessarily indicate continental material. Data suggests the presence of arc roots, but not necessarily continental crust, in the Caribbean plate. Average continental crust is intermediate in composition, so are mature oceanic arcs. Gravity cannot distinguish the two. Understanding geochemistry is vital in this respect. High silica does not indicate continental rocks. Crystal fractionation from a basaltic melt and crustal re-melting generate high silica contents in an intra-oceanic setting and are extremely common processes. The trace element and isotopic record from the Oceanic Plateau and arcs, through dozens upon dozens of publications shows that there is little continental material beneath the Caribbean Plate. Salt diapirs are not present.
2. The authors appear to argue for punctuated phases of extension from the Triassic through to the Oligocene. Broadly extensional tectonics lasting up to 170 Ma seems geologically unfeasible and certainly cannot account for the formation of the Oceanic Plateau. Accepting that renders much of the discussion that follows irrelevant.
3. In paragraph 4 (page 14) the authors argue that the Caribbean Plateau formed by "serpentinization of the mantle". However, in reality, we know - from geochemical and isotopic investigations of extensive land and a number of offshore samples - that it could only have formed in a short space of time by the impingement of a hot mantle plume beneath the Caribbean lithosphere. James and Lorente may argue that little of the Plateau has been drilled to work out exactly what it is. I hold my hand up; I would love to see more samples, but everything we have today fails to counter a Pacific origin model.
4. James and Lorente suggest that mid, late Cretaceous and mid Eocene unconformities are present in the Caribbean. Convergent events are not correlated across the region and I suspect their contention is a generalisation. A pause in volcanic events certainly doesn’t ring true for the Cretaceous period, the Caribbean arc successions were active throughout.
5. There is no evidence in the geochemical or isotopic record for significant continental input into the Caribbean arc(s) during their early history. Why then should continental input appear at an arbitrary point during the middle of the history of the arc, if continental material was always present in the Caribbean region? The change from island arc tholeiite to calc-alkaline compositions reflects changing sediment input and arc maturity.
6. The authors speak of back-arc spreading occurring along the Aves Ridge. The Aves Ridge is an extinct island arc - I have the geochemical data to prove it. Geophysical surveys point to thicker, arc-like crust. Hence I presume this is a mistake on the authors’ part.

James and Lorente suggest that our chemical and isotopic data need to be completely re-examined and "statistically tested". It is this point that I take particular umbrage over. Geochemists are examining and re-interpreting their data all the time, whatever model is presumed. I cannot accept the trashing of over four decades of good geological practice in the Caribbean.

I would readily admit that the Caribbean community has faced challenges regarding the exact details of the POP, challenges that are being ironed out as new information becomes available. Any model is allowed to evolve. However, James and Lorente suggest that the POP model invokes processes that are difficult to explain or test. We are explaining, we are testing. I simply do not recognise their argument, especially as the “in situ” model in itself breaks so many fundamental geological and especially geochemical concepts.

The positive upshot from the Sigüenza meeting is that there are a number of cracking articles which will appear in Special Publication 328. Research in the Caribbean has moved far beyond the objections raised by James and Lorente. Let the “in situ” model rest in peace.

References

1. Pindell, J.L., 1990, in Larue, D.K., Draper, G. (eds), Trans 12th Caribbean Geological Conference, Miami Geological Society, 1-4.
2. Pindell, J.L., 1993, in Pindell, J.L., Perkins, R.F. (eds), GCSSEPM Foundation 13th annual research conference, Gulf Coast Section, papers, 251-274.
3. Pindell, J.L., 2006, Geologica Acta, 4(1-2), 303-341.
4. Kerr, A.C. et al., 2003, AAPG Memoir, 79, 126-168.
5. Pindell, J.L. et al., 2009, Geological Society Special Publication 328, GSL.
6. van der Hilst, R., 1990. Doctoral thesis, University of Utrecht, unpublished.
7. Burke,K., 1988. Annual Reviews in Earth & Planetary Sciences, 201-230.
8. Hastie, A.R., Kerr, A.C., in final review, Earth Science Reviews.