Paleomagnetic data from the Americas admit the possibility of an extensional origin for the Caribbean LIP

Rubén Somoza

CONICET – Departamento de Ciencias Geológicas, FCEyN, Universidad de Buenos Aires, Argentina, somoza@gl.fcen.uba.ar

The Caribbean plate hosts one of the larger oceanic plateaus on Earth, usually referred as the Caribbean Large Igneous Province (CLIP). A debate onset early after plate tectonics was conceived about the origin and tectonic evolution of the Caribbean in general, and of the CLIP in particular. The controversy focuses on two end member models: allochtonous models propose a Mesozoic Pacific origin for the Caribbean crust (e.g., Kerr et al., 1999; Kerr & Tarney, 2005; Pindell et al., 2005, 2006) and usually attributes the CLIP to the activity of mantle plumes in the Pacific basin (e.g., Kerr et al., 1999; Kerr & Tarney, 2005). In contrast, autochtonous models propose an interamerican origin for both the Caribbean oceanic crust and the CLIP (Meschede & Frisch, 1998; James, 2006, 2007). One issue that is important to both kinds of models is the interamerican paleogeography during the Cretaceous. Unfortunately, mid-Cretaceous reconstructions are fraught with difficulties due to the lack of seafloor magnetic anomalies between the earliest Aptian (M0 anomaly) and earliest Campanian (C34y anomaly). This has resulted in the assumption that the 124-84 Ma divergence between North America and South America occurred without changes in either direction and rate, as depicted in Figure 1a by the dashed line connecting the 124 and 85 Ma positions of a test locality in northern South America with respect to a fixed North America. However, a recent analysis of Cretaceous paleomagnetic data from the Americas (Somoza & Zaffarana, 2008) suggests a different paleogeographic evolution.

Figure 1: (a) Map of the Caribbean region showing the motion of the Barranquilla locality (blue circle) with respect to North America since 135 Ma (finite rotations from Klitgord & Schouten, 1986; Nürnberg & Müller, 1991 and Müller et al., 1999). Dashed line represents the finite displacement of Barranquilla with respect to North America during times of no magnetic isochrons in the seafloor (see main text). Orange square represents the 100-Ma position of Barranquilla with respect to North America as indicated by new findings in the South Atlantic (Eagles, 2007). Note that the latter position defines a path (orange line) compatible with the paleomagnetic constraints in (b), whereas a single path connecting the 124 and 84 Ma positions (dashed line) does not. The latest Cretaceous-to-Recent convergence predicted by plate reconstructions (Müller et al., 1999) has been discussed in the context of paleomagnetic and tectonic data by Somoza (2007). (b) Red diamonds indicate Cretaceous paleolatitudes for a site in southern Yucatan [red diamond in (a)], paleolatitude values according to North American poles are from in Somoza & Zaffarana (2008) and the ca. 88-Ma North American pole from Acton & Gorgon (2005). Blue circles are Cretaceous paleolatitudes for the Barraquilla locality [blue circle in (a)] as predicted by Cretaceous South American poles (Somoza & Zaffarana, 2008). All paleolatitudes are positioned according to latitudes in map (a). Yellow bar indicates the paleomagnetically predicted timing of strong, approximately N-S divergence between the Americas in the Cretaceous. Click here or on Figure for enlargement.

The South American paleomagnetic dataset indicates that the continent experienced episodic southward motion from 135 Ma until the end of the Cretaceous, with an intervening period (125-95 Ma) of approximate stability with respect to Earth’s spin axis (Figure 1b; see also Somoza & Zaffarana, 2008). Paleomagnetism further predicts that North America experienced negligible latitudinal motion from ca. 125 Ma up to latest Cretaceous (Figure 1b, see also Somoza & Zaffarana, 2008). Then, paleomagnetism indicates that both the Americas rotated about the spin axis from 125 to 95 Ma. This is not consistent with the interamerican motion that is usually assumed, as depicted by the dashed line in Figure 1a. Indeed, the paleomagnetic constraints indicate either almost no relative motion or dominantly E-W left lateral motion between the Americas during this time interval. New findings of seafloor fabric favor the latter option, as explained below.

An important inference from the paleomagnetic data is that most of the ~ 700 km of mid-Cretaceous N-S divergence between the Americas occurred between 95 and 85 Ma (Figure 1b). This may be relevant to the origin of the CLIP. There is wide agreement that most of the CLIP formed in a major magmatic pulse at 90-88 Ma and a younger ~75 Ma pulse of smaller magnitude (e.g., Sinton et al., 1998; Mauffret et al., 2001). The main magmatic pulse occurred during the period of rather fast (~7 cm/yr) interamerican divergence between 95 and 85 Ma, suggesting that the latter may have been accompanied by pervasive extensional deformation in pre-existing oceanic and/or highly attenuated continental crust (Diebold et al., 1999; James, 2007). This, in turn, may have triggered sublithosperic decompression and melting, leading to the main constructional phase of the Caribbean oceanic plateau [Ed: See also Rifting Decompression Melting page]. Note that an origin for the CLIP west of, but very close to the Americas (e.g., Meschede & Frisch, 1998) could also be possible, although this case would require the basement of the CLIP to be coupled with the Americas in order to be influenced by their divergence (e.g., a backarc basin environment). In any case, the paleomagnetic constraints suggest that it is not necessary to invoke a mantle plume to account for the CLIP.

New findings of seafloor fabric further support the paleomagnetic analysis described above. Eagles (2007) determined a 100-Ma Africa – South America reconstruction that accounts for a subtle, previously unrecognized change in the trend of South Atlantic fracture zones. This improvement allows a revision of North America – South America relative motion. The result is shown in Figure 1a, where the 100-Ma position of the South American test locality (orange square) suggests that the previously assumed 124-84 Ma, NW-SE path (dashed line in Figure 1a) is wrong. Indeed, the new reconstruction suggests dominantly eastward motion of South America with respect to North America from 124 to at least 100 Ma, followed by rapid, dominantly southward drift until 84 Ma (orange line in Figure 1a). This plate-tectonic-derived path is consistent with, and complements the paleomagnetic predictions (Figure 1b), further supporting the 95-85 Ma episode of fast interamerican divergence discussed above.

The kinematic scenario here presented is compatible with an origin of the CLIP related to plate tectonics processes. Exploring its compatibility with other geologic characteristics of the Cretaceous Caribbean is problematic, mainly because ambiguities in determining both the Cretaceous configuration of the North America – South America plate boundary and the polarity of possible subduction zones in the Greater Antilles. The latter may be illustrated by disparate opinions involving either E-NE or W-SW subduction polarity to account for the origin of the same set of Aptian-Santonian arc related rocks in the Greater Antilles (e.g., Kerr et al., 1998; 1999; White et al., 1999 vs. Pindell et al., 2005; 2006). Below are speculations on an alternative working hypothesis.

A simpler working hypothesis involves in situ evolution for the Cretaceous Caribbean. This is because plate tectonics admits the possibility of an extensional origin for the CLIP, and because in situ models are easier to test than allochtonous models. Considering the early NW-directed divergence between North America and Western Gondwana, the Early Cretaceous Caribbean region may be envisaged as comprising NW-SE extensional corridors floored by sections of thinned continental and/or oceanic lithosphere separated from each other by strike-slip and/or transform faults. Each corridor contains NE-SW extensional faults and/or spreading axes. In this way, the trend of the NW-SE faults and NE-SW extensional axes may have delineated one or several promontories/embayments in an overall E-W shaped North America - South America plate boundary. This supposed paleogeography admits the possibility that the NW-SE faults were the locus of the primitive island arc tholeites that characterize the early magmatism in the Greater Antilles (Donnelly et al., 1990). These faults passed to sinistral transpression (eventually oblique subduction) when the relative motion between the Americas changed from NW-SE to dominantly E-W and left lateral at ca. 125 Ma (Figure 1a, E-W orange path), producing calc-alkaline magmatism (Donnelly et al., 1990) and the development of HP-LT metamorphism (e.g., Pindell et al., 2005; 2006). The possible subduction of relatively young oceanic lithosphere during this stage implies that the related slabs may have fertilized the rather shallow upper mantle and then, in the post-95 Ma, fast-divergence stage (Figure 1), contributed to the generation of the LIP magmas in a similar manner as proposed by Foulger & Anderson (2005) to account for the Iceland LIP.

Acknowledgments

Comments by Keith James and Will Sager are greatly appreciated.

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