The Caribbean: an oroclinal basin? JOHN MILSOM* Department of Earth Sciences, University of Hong Kong, Hong Kong, China *Address for correspondence: The Camp, Gladestry, Kington HR5 3NY, UK (e-mail: [email protected]) Abstract: Tightly curved mountain belts are prominent features of global topography. Typically, these ‘oroclines’ occur in areas of regional compression but enclose basins where extension has been contemporaneous with outward directed thrusting in the orogens. Examples of such basin– orogen pairs include the Alboran Sea–Gibraltar Arc, Tyrrhenian Sea–Aeolian Arc, Aegean Sea–Hellenic Arc and Pannonian Basin–Carpathian Arc, all in the western Tethys but matched in the eastern Tethys by the Banda Sea and Outer Banda Arc. The development of the basins has been variously explained by gravitational collapse of rapidly elevated mountain blocks and by extrusion prompted by asthenopheric flows, but it is not even universally agreed that similar processes have operated in all cases. Critics have cited gross differences in volcanic activity (absent from the Gibraltar Arc, modest in the Carpathians but intense in other examples) and in the presence or absence of recognizable Wadati–Benioff Zones. The superficial similarities between the Caribbean Sea–Antilles Arcs and typical oroclinal basin–orocline pairs have recently been invoked in support of an in situ Caribbean evolutionary model, even though the disputed origins of oroclines limit their reliability as analogues. The Caribbean’s considerably greater area further emphasizes the need for caution, while the most obvious objection to identifying it as a member of the oroclinal group is its very long history. Oroclinal basins typically pass from initiation to effective stabilization in a few tens of millions of years, whereas the original Caribbean oceanic crust, which is now bounded to the east and west by active subduction zones, was probably formed in the Jurassic. Rather than invoking an overall common origin for the Caribbean and the Tethyan basins, it is more useful to look for shared causes of specific individual similarities. The impact of a rigid block might be as effective in imposing curvature on a mountain belt as rapid expansion in an adjacent area. However, it does seem that the case for the crust of the Caribbean being typical of oceanic large igneous provinces (LIPs) may have been overstated and, in the light of oroclinal analogues, that some features of the still poorly understood Beata Ridge and Lower Nicaragua Rise may be most easily explained by east–west extension promoted by the conver- gence between North and South America. The Caribbean comprises a deep, complex and which partly enclose the Po Basin, actually mainly oceanic basin that is partly enclosed by an exceed this value (Fig. 1). In every one of these intensely curved and fragmented mountain belt. cases, development of the internal basin was Comparisons with the sub-Antarctic Scotia Sea accompanied by outwards-directed thrusting in and the Indonesian Banda Sea have been so fre- the orogen. quently made as to be almost commonplace. Simi- Despite obvious similarities, it is not clear that larities can, in turn, be recognized between the the Caribbean could ever be properly regarded as Banda Sea and the Alboran, Tyrrhenian, Ligurian a member, even if an end member, of the group of and Aegean Seas. Also, and despite its location oroclinal basins. The point is important, since the within the European land mass north of the Mediter- fact that the basins listed above all developed in ranean (Fig. 1), the Pannonian Basin is widely very much their present locations with respect to regarded as conforming to the same pattern of exten- the surrounding major plates has been cited in sion within the zone of Tethyan collision (e.g. support of an in situ origin for the Caribbean and Horvath & Tari 1999). The mountain ranges encir- as evidence against the alternative, Pacific, origin cling such basins have come to be known as (James 2006). The question of the extent to which oroclines, and the internal depressions are con- comparisons with other basins can provide useful sequently referred to as oroclinal basins. The term insights into the processes involved in Caribbean ‘orocline’ seems to have been first used by Carey origin and evolution is therefore worth considering. (1955), who applied it to mountain chains exhibiting In this paper, significant features of the western at least 908 of curvature. Many are curved through Tethyan basins are mentioned where appropriate, angles approaching 1808, and the Western Alps, but the discussion focuses on the Banda Sea, as From:JAMES, K. H., LORENTE,M.A.&PINDELL, J. L. (eds) The Origin and Evolution of the Caribbean Plate. Geological Society, London, Special Publications, 328, 139–154. DOI: 10.1144/SP328.5 0305-8719/09/$15.00 # The Geological Society of London 2009. 140 J. MILSOM Fig. 1. Oroclines and topography in Europe and the Mediterranean. Inset: the Banda Sea, to the same scale. Topographic bases prepared in Global Mapper using the GTOPO30 global topographic grid. Bathymetries from the GEBCO digital atlas. See Figure 4 for depth scale. The dotted line defines the axis of the volcanic Inner Banda Arc. The essentially arbitrary boundary between this and the Sunda arc is conventionally placed immediately west of Alor. The arrow in the Arafura Sea indicates the direction of motion of Australia relative to Southeast Asia, after Genrich et al. (1996). CARIBBEAN: AN OROCLINAL BASIN? 141 being one of the two or three oroclinal basins most places on the far side of the orogens from the commonly cited as a Caribbean analogue, and also sources of stress. In the Carpathians at least, the the basin with which the author is most familiar. ocean has now been entirely eliminated and arc– continent collision has followed. Mediterranean oroclines The Banda Sea The Eurasia–Africa collision zone in the Mediterra- nean (Fig. 1) includes a number of basins that have Eastern Indonesia is the site of the oblique collision been formed by rapid extension within the regional between the Asian and Australasian (Australia and compressional setting (Jolivet et al. 1999; Jolivet & New Guinea) continental masses, which is a conse- Faccena 2000). In each case extension in the basin quence of the northward movement of the Indo-Aus- appears to have been accompanied by folding and tralian Plate towards an Asia that is almost stationary thrusting in the enclosing orocline, but there are in a global reference frame. The Banda Sea (Fig. 1, wide individual variations. Only in the Tyrrhenian inset) is the most important geographic element of and Ligurian seas has extension led to the creation this region, interposing as it does a number of of new oceanic crust (see Carminati et al. 1998; blocks of oceanic crust between the continental Se´ranne 1999). The Alboran and Aegean basins are masses of New Guinea to the east and Borneo–Sula- floored by highly extended continental crust (Platt wesi to the west. The sea is enclosed on three sides by & Vissers 1989; Hatzfield 1999) and this also seems a double arc. The inner arc consists of a chain of small to be true of the Pannonian basin (Horvath & Tari to medium sized volcanic islands, while the outer arc 1999), which is now infilled with thick sediments. isdominatedbythelargeislandsofTimor,Tanimbar, The relation of the Carpathian orocline to extension Kai, Seram and Buru. Igneous rocks in the outer arc appears particularly dramatic. The eastern Alps are pre-Neogene, and mainly pre-Cenozoic. were disrupted and ultimately separated into the There is no general consensus concerning the now distinct Dinarides and Carpathians by Panno- origin of the Banda Sea, but early hypotheses that nian expansion, in the course of which the Carpathian invoked the trapping of old Indian or other ocean part of the orogen seems to have been not only bent floor (e.g. Lee & McCabe 1986) have now been but detached from the Alpine chain by the opening generally discarded. Such ideas were largely of the Vienna Basin (Fig. 1). prompted by the depths of the Banda oceanic Volcanic activity in the European oroclines basins, which are more appropriate to Mesozoic varies widely, and links to subduction are in some than Cenozoic features. However, Cenozoic and, cases controversial. The Tyrrhenian and Aegean in fact, Late Cenozoic, ages are suggested by the seas are clearly back-arc to, respectively, the results of dredging and, in the case of the southern Aeolian and Hellenic arcs, but conflicting hypoth- basin, by analysis of linear magnetic anomalies eses have been presented for the Alboran Sea by (Hinschberger et al. 2001). Lonergan & White (1997) and by Platt & Vissers If the Banda Sea did not exist prior to the Late (1989). Only the Lonergan & White (1997) Cenozoic, it must have relatively recently displaced interpretation involves subduction. Volcanism may other crustal elements. The identification beneath it now have ceased in the Carpathians and, although of a scoop-shaped Wadati–Benioff Zone (Hamilton it was widespread during the Cenozoic and conti- 1979; Milsom 2001) indicates expansion over pre- nued into the Holocene (Seghedi et al. 2005), the existing oceanic crust, while the presence of Austra- past presence of a subduction zone is still disputed lasian continental fragments in the Outer Banda Arc (e.g. Knapp et al. 2005). The situation as regards to the north as well as to the south and east (Milsom the Ligurian Sea is uncertain, because of overprint- 2000 and references therein) strongly suggests that ing by more recent events (Se´ranne 1999). this subducted crust must have been part of the It is a common assumption that oroclinal curva- Indo-Australian Plate. Virtually all published tures were imposed on formerly much straighter palaeogeographies (e.g.