The : an oroclinal basin?

JOHN MILSOM* Department of 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 – Arc, Tyrrhenian Sea–Aeolian Arc, –Hellenic Arc and Pannonian Basin–Carpathian Arc, all in the western Tethys but matched in the eastern Tethys by the 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 , 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 –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 .

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 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 . 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 .MILSOM J.

Fig. 1. Oroclines and topography in 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 indicates the direction of motion of relative to Southeast , 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. has now been entirely eliminated and arc– collision has followed.

Mediterranean oroclines The Banda Sea The – 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 , 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 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 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. Charlton 1986; Daly et al. mountain belts. The necessary stresses could have 1991; Hall 2002) therefore show an embayment of been produced by rapid expansion in small areas the lying between westernmost New adjacent to the mountains, and this seems to have Guinea and the mainland of northwestern Australia been the case in all the European examples cited in the Early Cenozoic. It is also commonly accepted above, as well as in the Banda and Scotia seas. For that there was collision in the Late Oligocene or bending to occur, there must be directions in Early Miocene between the margin of Southeast which movement is subject to only minimal con- Asia and forerunners of the Australian continent straints and also directions in which it is very strictly (Smith & Silver 1991). constrained. It is therefore essential that there are, or Despite these areas of agreement, there are were, areas of oceanic crust in some, but not all, important differences in Banda Sea reconstructions 142 J. MILSOM that have implications for comparisons with the course of this collision. One specific example of Caribbean. Figure 2 is based on the 20 Ma recon- such a hypothesis supposes that, following collision, struction of Hall (2002). It shows the outer arc the orogen developed to a level of gravitational islands of Buru, Seram, Tanimbar and Timor, instability that caused collapse and rapid extension, together with terranes that now make up eastern in a manner similar to that suggested by Platt & and southeastern Sulawesi, at the margins of an Vissers (1989) for the Alboran Sea. Dispersal Indian Ocean embayment but firmly attached to (Fig. 3) would have been facilitated by subduction Australasia. However, the material subducted of the Indian Ocean, with the Banda Sea developing beneath the Banda Sea, which represents the as a back-arc basin. Banda Sea expansion was oceanic crust of the former embayment, now limited to the south by the advance of Australia sub-crops the seafloor between the outer arc and and was eventually terminated in the east by col- Australia/New Guinea, that is, it has somehow lision in the Kai region. The continuing con- reached the position shown by the dotted line in vergence of Australia on Southeast Asia is now Figure 2. There is thus a problem with this recon- creating a new orogen in eastern Indonesia. The struction that can only be solved by complicated stratigraphic and palaeogeographic arguments for transfers across the subduction zone. this scenario have been discussed by Milsom Other hypotheses avoid this problem by suppos- (2000) and by Milsom et al. (2000) and will not ing that the Miocene collision was between a south- be repeated here. east Asian marginal arc (represented by modern West Sulawesi) and a microcontinental fragment that was rifted away from the Australian margin Comparisons in the Early Jurassic and then drifted north in advance of the main continental mass (e.g. Smith In comparing the Caribbean with the Banda Sea, & Silver 1991). The emplacement of the exten- or any other oroclinal basin, note must first be sive ophiolite sheets of eastern Sulawesi above taken of its much longer geological history. This Australian-derived basement occurred in the history has been discussed by many authors, and is

Fig. 2. Reconstruction at 20 Ma of the Banda Sea area, after Hall (2002). The dotted line, which is not part of Hall’s illustration, shows the present-day surface subcrop of subducted lithosphere, as indicated by earthquake hypocentres. CARIBBEAN: AN OROCLINAL BASIN? 143

Fig. 3. Neogene movements in the Banda Sea region, as suggested by Milsom (2000). See Figure 4 for the locations of the east-facing Sangihe Arc and the west-facing Halmahera Arc. the theme of the present volume. The summary by introduces further geological similarities, because Driscoll & Diebold (1999) is typical. They regarded there are then active continental margins underlain the original Proto-Caribbean crust as having been by well-defined subduction zones at the western formed by conventional seafloor spreading in the boundaries of both areas (Fig. 4). Inevitably, the Late Jurassic and Early Cretaceous and as having general appearance of similarity is the product of a been modified by one or more large igneous pro- number of quite separate details that vary in impor- vinces (LIPs)-style magmatic episodes that led to tance. These are considered individually below. the formation of an oceanic plateau in the Late Aspects of Caribbean geology that are not relevant Cretaceous. Subsequently, subduction zones devel- to these comparisons are not discussed. oped to both east and west, and north–south com- pression was produced by the convergence of Thrust Belts North and South America (Mu¨ller et al. 1999). There was also significant extension at times but Indonesia. Outward-directed thrusting is character- the amount of crustal thinning is poorly constrained. istic of oroclinal mountain belts, and the thrust Although many aspects of this history are contested structures of Timor, the largest island in the Outer in detail, the major disagreements are concerned Banda Arc, have been described by numerous more with where certain events took place than authors (e.g. Barber et al. 1977; Barber 1978; with when. In particular, James (2005, 2006) has Karig et al. 1987; Charlton et al. 1991a; Sawyer argued strongly against the widely accepted devel- et al. 1993; Reed et al. 1996). Expeditions to opment of the LIP in the Pacific (see, for instance, Seram have examined mirror-image structures on Pindell & Barrett 1990) and for an origin directly the opposite side of the Banda Sea (e.g. Audley- between the . Charles et al. 1979). Seismic reflection sections There is a significant difference in area as well as from around the arc, from Timor to Seram, have in age between the Banda Sea and the Caribbean, imaged foreland-directed thrusts surfacing in the and this suggests that it may be more appropriate peripheral troughs (Schlu¨ter & Fritsch 1985; to use the whole of Indonesia in comparisons Jongsma et al. 1989; Hughes et al. 1996). Folding rather than the Banda Sea alone. Doing so also occurs but is relatively minor. 144 J. MILSOM

Caribbean. The well documented thrust structures archipelago (Fig. 4a) is the Andean margin of the of northern Venezuela and the Greater Antilles Sundaland continental block, which includes islands of Cuba and Hispaniola (Iturralde-Vinent Borneo and the Malay Peninsula (Barber et al. 1998; Giunta et al. 2003, 2006) effectively replicate 2005). Sumatra is dominated by a chain of active the patterns of thrusting around the Banda Arc. volcanoes which are located above a well defined Almost equally striking are the discontinuities in Wadati–Benioff Zone (WBZ). There is a transition both the thrust orogens. Each is formed of discrete eastwards from continental crust beneath Sundaland blocks represented by the major islands, in a proper via Java, where exposed basement has been pattern consistent with stretching of an originally described as subduction me´lange (e.g. Hamilton coherent mountain range. The north coast region 1979), to the intra-oceanic arcs further east. The of South America may be previewing the Sunda– island of Sulawesi, immediately to the west of Banda arc system at some future stage in its col- the Banda Sea, is the easternmost element of the lision with Australia. Sundaland complex.

Active continental margins Caribbean. Middle America west of the Caribbean is noted for its volcanic activity, which is associated Indonesia. The large NW–SE oriented island with the WBZ that dips NE beneath the Chortı´s of Sumatra at the western end of the Indonesian Block from the Middle America Trench (Sykes &

Fig. 4. (a) Indonesia. (b) the Caribbean. The two illustrations are to the same scale. Topography in both cases is based on the GTOPO30 grids and bathymetry on the GEBCO digital atlas. The rectangle indicates the Banda Sea area, shown in more detail in the inset to Figure 1. CARIBBEAN: AN OROCLINAL BASIN? 145

Ewing 1965). The block is believed to be underlain, studies by White et al. (1999) for Aruba & Kerr at least in part, by continental crust (e.g. Pindell & et al. (1996) for Curac¸ao have suggested that Barrett 1990). There are transitions from continen- igneous rocks on these two islands may not be tal crust beneath Chortı´s eastwards via the quasi- subduction-related, but the island-arc character continental Nicaragua Rise, which extends out of Bonaire seems clear (Thompson et al. 2004). towards Jamaica, to the fully oceanic Caribbean More work is needed. Beardsley & Ave´ Lallemant basins, and southeastwards to the Panama intra- (2007) compiled age data on the youngest oceanic arc. The Middle America and Sumatra volcanic rocks of each of the islands in the chain trenches can be validly compared, but equating the (including the small Venezuelan islands east of to the might be Bonaire) that suggested west-to-east diachronous taking the analogy too far. termination of volcanic activity during the period from 73 to 44 Ma, and reported palaeomagnetic Volcanic arcs and structural evidence for similarly diachronous clockwise rotations through angles in excess of 908. Indonesia. The Banda Sea is bounded to the east by an active volcanic arc that consists of relatively Seismicity and seismic tomography small islands (Fig. 1, inset). There are also subsidi- ary, east–west trending lines of islands young Indonesia. The WBZ associated with the Sumatran enough for volcanic landforms to be preserved but Andean margin is generally unremarkable, but with no recorded activity. One such line is formed seismic travel-time tomography has imaged sub- by the relatively large islands of Alor and Wetar ducted lithosphere extending hundreds of kilo- and smaller centres, some of which are entirely sub- metres deeper than the deepest earthquakes merged, further to the east. A second, shorter, line (Widiyantoro & van der Hilst 1996). It is possible runs through the islands south of Seram, includ- that the lower part of the slab has detached at a ing Ambon, westwards to a small inactive volcano depth of about 200 km and is now sinking indepen- south of Buru. The cessation of activity can be dently into the mantle. explained as a consequence of the eastward The pattern in eastern Indonesia is very different. migration of the site of active subduction and, In east–west cross-sectional views the seismic additionally or alternatively, by the severance of activity beneath the Banda Sea is bounded by a the magma conduits by backthrusts. gently west-dipping line (Fig. 5a), but when There has been a similar migration of volcanic narrow north–south swathes are viewed, the activity in the Carpathians. Volcanic centres in the activity resolves into U- or V-shaped zones defining easternmost Carpathians have been active during a scoop or shoehorn-shaped surface which reaches a the last few tens of thousands of years and it maximum depth of almost 500 km beneath the would be unwise to completely rule out the possi- central part of the sea and which subcrops along bility of further eruptions (e.g. Seghedi et al. the axes of the Timor, Tanimbar and Seram 2005). This is consistent with steady eastward troughs (Hamilton 1979; Milsom 2001). Concen- migration of the Carpathian chain to eventual col- trated seismic activity beneath the region of lision with the Russian Platform. maximum curvature east of Timor may indicate incipent detachment of parts of the subducted slab, Caribbean. The volcanic line of the Lesser Antilles a process thought to have been already completed provides one of the most obvious of the resem- in the Carpathians (Milsom 2005). blances between the Caribbean and the Banda Sea. The volcanoes are relatively young in Caribbean Caribbean. Seismic activity in the Caribbean region terms, although still much older than those in the is associated principally with WBZs that dip north- Banda Arc. The oldest dated igneous rocks in the east beneath the Central American Andean margin northern Antilles, which merges with the Aves in the west and west beneath the Lesser Antilles Ridge, are Late Eocene, but in the southern Antilles, intra-oceanic arc in the east (Molnar & Sykes which are separated from the Aves Ridge by the 1969). As in Indonesia, the western WBZ is unre- Granada Basin, the oldest known igneous rocks markable. In the east, travel-time tomography has are Early Miocene (see tabulation in Pindell & been used to trace the southern end of the Lesser Barrett 1990). Antilles slab around its pronounced bend to the There does not seem to be a subduction-related west, and to define what appears to be a detachment volcanic province in the northern Caribbean that at crustal levels (VanDecar et al. 2003). Because of can be compared with that in the northern Banda the greater overall dimensions of the Caribbean, the Sea, despite the presence of the Puerto Rico contortions required of the downgoing slab are Trench, but in the south the Netherlands Antilles much less extreme than are those in eastern Indone- (Aruba, Curac¸ao and Bonaire; Fig. 4) can be seen sia. Moreover, tomography suggests that, far from as analogues of the Alor–Wetar line. Recent flattening out into a shoehorn, the Antilles slab 146 J. MILSOM

Fig. 5. Seismicity of the eastern Indonesia area. (a) Distribution of earthquake hypocentres on a cross-section viewed from the south, after Milsom (2001). Shading, on main figure and inset, indicates location of the swathe viewed in (b). (b) Cross-section along a typical north–south swathe, after Milsom (2001). The cluster of intermediate-depth shocks between 7 and 88S may be indicating incipient delamination or detachment of the subducting slab. Hypocentre locations from supporting material to Engdahl et al. (1998). dips steeply into the asthenosphere (VanDecar along both the northern and southern borders of et al. 2003). the Banda Sea, strike–slip features are not obvious in the south. To the north, however, the Sula Spur Strike–slip provinces (Fig. 1, inset) has some claim to be considered one of the most remarkable features on the Earth’s Indonesia. Although the eastward roll-back of the surface. Essentially, the spur consists of a long, Banda slab seems to require transcurrent motion narrow strip of Australasian continental crust CARIBBEAN: AN OROCLINAL BASIN? 147

(Garrard et al. 1988) that moved west (Fig. 3) and dextral displacement between these two major collided with the East Arm of Sulawesi in the Late blocks between 83 and 38 Ma and none thereafter. Miocene (Davies 1990). Such motion would have The much greater (and largely later) displace- required major left-lateral faulting along the spur’s ment at the northern (Cayman Trough) margin of southern margin and probably also right-lateral the Caribbean therefore requires there to have also faulting along its northern margin. The total dis- been large-scale shear at the opposite (southern) placements are measured in hundreds of kilometres, margin. yet the spur is in places only about a hundred kilo- The Cayman Trough clearly differs from the metres across. Tectonic processes in this region Sula Spur in almost every respect apart from its are still only poorly understood, but are evidently approximate dimensions and its similar positioning extremely complex. with respect to the ocean basin to the south, but both trough and spur seem lie at the extreme limits Caribbean. As is the case with the southern margin of the structures that might be anticipated within of the Banda Sea, the southern margin of the Carib- the plate-tectonic paradigm. bean is more notable for north–south compres- sional structures than east–west transcurrent ones, Arc-associated basins although the latter undoubtedly exist in the El Pilar fault system (Ave´ Lallemant & Sisson 2005). Indonesia. The extreme east of the Banda Sea The El Pilar has a northern counterpart in the Gua- is occupied by the Weber Basin (Fig. 7a) which, temalan Motagua–Polochic fault system (Giunta with a maximum depth of more than 7000 m, con- et al. 2006) but, as in the Banda Sea, transcurrent tains the deepest seafloor known anywhere outside displacement at the northern margin of the Carib- the subduction trenches. There is no consensus bean has been accommodated in part by a feature explanation for these great depths in what is actually (the Cayman Trough) that has no counterpart a forearc basin; the classic roll-back forearc basins anywhere else on the globe. Although the north– in the Marianas and Bonin arcs are only about half south oriented spreading centre in the trough this depth, despite having far greater along-strike is little more than 100 km long, it has produced extent, and therefore more room for subsidence. some 1100 km of oceanic crust in an extensional Moreover, roll-back cannot plausibly still be operat- episode that has lasted for much of the Cenozoic ing in the Weber area, because the Banda forearc (Leroy et al. 1996). The –South has collided with, and now rests upon, Australian America relative motions shown diagrammatically continental crust (Charlton et al. 1991b). Seismic in Figure 6 indicate only about 200 km of relative reflection surveys have clearly imaged the former

Fig. 6. Free-air gravity of the Caribbean, based on satellite derived grids (Sandwell & Smith 1995). U.N.R., Upper Nicaragua Rise; L.N.R., Lower Nicaragua Rise. The white traces record the motion of North America relative to South America at longitudes 75 and 858W, after Mu¨ller et al. (1999). The white numbers are ages, in Ma. The latitudinal positions of the traces have been selected so as to minimize conflicts with other information, and have no other significance. GB, Granada Basin. 148 J. MILSOM

Fig. 7. (a) Bathymetry of the Weber Basin. (b) Bathymetry of the Grenada Basin, to the same scale. Shading shows depth to basement at 6000, 8000 and 10 000 m within the basin, after Holcombe et al. (1990). Bathymetric contours from the GEBCO digital bathymetric atlas in both cases. subduction fault reaching the seafloor in the narrow converging Halmahera and Sangihe arcs would strait between Kai Besar and Kai Kecil (Milsom inevitably tend to force down the scoop-shaped et al. 1996). slab of subducted Indian Ocean lithosphere The Weber Basin depth problem is often simply beneath the Banda Sea, taking the overlying crust ignored in regional discussions, but astheno- with it. The Kai collision, far from being a spheric flow prompted by convergent tectonics problem for this explanation, is a probable necessary could provide an explanation, and an additional condition for the development of the anomalous drive for Banda Sea expansion (Fig. 8). To the depths, because it prevents the stress being relieved north, and beyond the narrow strip of continental by simple subduction zone retreat. crust represented by the Sula Spur, the Molucca A wider applicability for the mantle-flow Sea provides a rare example of arc–arc collision hypothesis is suggested by the occurrence of rather (McCaffrey et al. 1980). Asthenosphere escaping similar features within other oroclines. The Transyl- south from the collision zone between the vania Basin in the eastern loop of the Carpathians

Fig. 8. Asthenospheric flow in eastern Indonesia, as suggested by Milsom (2001). CARIBBEAN: AN OROCLINAL BASIN? 149 is separated from the main Pannonian Basin by volcanic activity would surely eventually have the old volcanic high of the Apuseni mountains to move eastwards also, but the Weber Basin is (Horvath & Tari 1999), while the palaeogeographic probably already too deep for this transfer to be maps presented by Se´ranne (1999) suggest that accomplished by rifting of the present volcanic arc. and similarly separated the Whatever the precise history of the Lesser younger Tyrrhenian from the older Ligurian basin Antilles, it is very difficult to see the Aves Ridge (Fig. 1). There may also be an analogous drive as anything but an abandoned ‘remnant arc’ This producing the region of ductile extension recog- interpretation is supported by the maximum ages nized by Jolivet & Patriat (1999) in the southern of the igneous rocks, which are greater in the Aegean Sea. northern part of the Lesser Antilles ridge, where it merges with the Aves Ridge, than in the south Caribbean. Flows in the sub-lithospheric mantle (Pindell & Barrett 1990). The suggestion (James could well have been important in the post- 2005) that the Aves was itself a constructive plate Paleocene Caribbean also, despite the absence of margin seems incompatible with its morphology arc–arc collision. Convergence between the deep and what is known of its geology (Holcombe et al. lithospheric keels of North and South America 1990). Because of the very low magnetic latitude, would inevitably pressurize the asthenosphere magnetic anomalies in the Grenada Basin are trapped behind the subducted slabs beneath poorly defined (Bird et al. 1999), and certainly not Middle America and the Lesser Antilles. It is there- dateable, but comparison with other basins devel- fore not surprising that the Grenada Basin at the oped between active and remnant arcs (Karig eastern end of the Caribbean (Fig. 7b) appears at 1972) suggests that there may have been a series first sight to be a direct analogue of the Weber Basin. of short spreading axes, offset by transforms Closer inspection reveals important differences. and oblique to a dominantly east–west direction Not only is the Grenada Basin only about half as of extension. deep, but it occupies a back-arc rather than a forearc position. It is also almost certainly much Internal basins older. Although the actual ages are not definitely known in either case, the most commonly cited Indonesia. In addition to the Weber forearc basin, dating for the Grenada Basin places its origin in the Banda Sea includes two sub-basins that are the Paleocene–Eocene (e.g. Pindell & Barrettt more typically oceanic. The South Banda Basin is 1990), whereas the Weber Basin is probably Late clearly back-arc with respect to the present-day Neogene (Honthaas et al. 1997). This may have a Banda Arc, but the tectonic position of the North bearing on the other differences. For example, Banda Basin is less clear. Its nearest analogue the contrast in depth is much less significant if the would appear to be the Vienna Basin, which sepa- sediment fill, which is negligible in the Weber rates the Carpathians from the Eastern Alps in Basin but considerable in the Grenada Basin, is much the same way as the North Banda Basin sep- taken into account (Fig. 7b). This sediment cover arates the Outer Banda Arc from Sulawesi (Fig. 1). renders the seafloor of the Grenada Basin much Because of the absence of deep drilling in the the less rugged of the two, but the maximum depth Banda Sea, none of its sea-floor can be considered to basement in its southern part, close to the prolific securely dated. Magnetic lineations in the South sediment sources of the South American continent, Basin were interpreted by Hinschberger et al. is more than 10 km (Holcombe et al. 1990). (2001) as Late Miocene to Pliocene (6–3 Ma), Normal age-depth relationships applied to but identifications of relatively short magnetic the Grenada Basin suggest a stabilization age of anomalies in restricted basins are notoriously uncer- about 40 Ma (Holcombe et al. 1990), but the tain. Dredge hauls from the North Basin led Re´hault Weber Basin demonstrates the danger of using et al. (1994) to suggest that this had a slightly this approach in such an environment. A younger earlier, Late Miocene (c. 10 Ma), origin. The extre- age is possible, and would be consistent with the mely rugged seafloor in both basins, and the thinness apparent youth of the southern Lesser Antilles. It of the sedimentary cover, would be hard to reconcile is quite possible that the Grenada Basin originally with greater ages, even given the absence of major developed as the forearc to a volcanically active sediment sources. Aves Ridge and only became back-arc when contin- ued roll-back forced volcanic activity to transfer Caribbean. The Caribbean is often described as further east. This differs from the way in which consisting of two main sub-basins (the Colombia active arc–remnant arc pairs usually evolve by arc and Venezuela Basins; Fig. 4), each floored by splitting (e.g. Karig 1972), but is rendered plausible oceanic crust modified by massive Late Cretaceous by the situation in eastern Indonesia. Were the outpourings of basaltic lavas. The upper surface of Banda Sea able to continue its expansion eastwards, these flows is identified with the widespread B00 150 J. MILSOM seismic reflector. However, Driscoll & Diebold Buru, Buton and East Sulawesi, the gabbros and (1999) have maintained that this is a very consider- basalts may be fragments of the East Sulawesi able oversimplification and that in some areas, ophiolite and the calc-alkaline rocks could be parts notably the south-eastern part of the Venezuela of a remnant arc related to the present-day Inner Basin, the crust is actually thinner than normal Banda Arc. All of these elements are consistent oceanic. They ascribed a major role to extension with the fragmentation of a collision orogen in the in controlling basin structure but argued convin- Sulawesi region in the Mid-Cenozoic (Fig. 3). An cingly that, in the Venezuela Basin at least, this impressive sub-sea scarp suggestive of large-scale took place either during or soon after the B00 extensional faulting forms the boundary between volcanic episode. the Banda Ridges province and the North Banda The Colombia Basin is 3000–4000 m deep, Basin (R. V. Charles Darwin cruise CD30, unpub- which is shallow for an oceanic basin but unsurpris- lished data, 1990). ing given the basin’s location and history. Not only was the crust supposedly thickened by the Creta- Caribbean. In the Caribbean, the Nicaragua Rise ceous basaltic events, but the basin is infilled by may match the Indonesian Banda Ridges in frag- thick sediments derived in the main from erosion mentation and heterogeneity. This is well illustrated of the Colombian . It is actually quite hard by the free-air gravity patterns (Fig. 6), which to believe that the term ‘oceanic plateau’, so show basement structures more clearly than do widely used in describing the Caribbean, could bathymetric maps because they are less affected ever have been appropriate, since modern oceanic by the masking effect of the sedimentary cover. plateaus form very prominent bathymetric highs The picture, particularly along the lower rise, is and in many cases rise locally above sea level. one of pervasive extensional faulting. Much of this Bowland & Rosencrantz (1988) considered that, faulting and accompanying alkaline volcanism although features in the western Colombia Basin may be Miocene to Recent (see Fig. 5 in Holcombe were of plateau type, its centre was underlain by et al. 1990). The entire lower rise appears to be relatively unaltered oceanic crust. broken up into blocks that are only a few tens of The Haiti Basin is often regarded merely as a kilometres across but, despite this, the Hess Escarp- northeastern extension of the Colombia Basin but ment that separates it from the Colombia Basin is is linked to it only through a narrow gap between remarkably linear. the Beata Ridge and the Lower Nicaragua Rise The Colombia and Venezuela basins are sepa- (Fig. 6). The colinearity of the southeastern rated by the Beata Ridge, which is underlain by margin of the rise (the Hess Escarpment) with the crust that is up to 12 km thick (Holcombe et al. western margin of the ridge is very striking and 1990), and broadens out towards the SW into a it is geometrically attractive to suppose that the complex of diverging ridges. The reason for the Haiti Basin opened when transcurrent faulting relatively thick crust is uncertain. Mauffret & developed along this line. The date at which this Leroy (1997) interpreted seismic reflection data might have occurred is completely unconstrained as indicating overthrusting of the Colombia Basin but is likely to have been contemporaneous with on the Venezuela Basin, but Driscoll & Diebold the extensional faulting of the Nicaragua Rise (1999), using similar but possibly better quality discussed below. data, saw only extension in the same area. They presented a seismic section that showed the ridge Internal ridges as a highly asymmetric feature, with a very steep, fault-defined slope into the Haiti Basin to the west Indonesia. The Banda Sea sub-basins are separated but a much gentler slope into the Venezuela Basin by high-standing ridges of complex origin (Fig. 1, to the east. inset). The South Banda Basin is separated from Analogy with the Banda Ridges suggests that the partly oceanic by a region of rough the Beata Ridge may be a remnant of the original, and shallow sea floor capped by small islands. unextended, Caribbean LIP, positioned adjacent to Similarly, the Banda Ridges province between the areas in which later extension has produced mark- North and South Banda basins rises above sea edly thinner crust. Holcombe et al. (1990) suggested level only in a few low islands on which only that the changes in crustal thickness between the Recent coral limestones are exposed, but dredge Lower Nicaragua Rise, the Colombia Basin and hauls on the ridge flanks have recovered a wide the Beata Ridge indicated by seismic refraction variety of rocks, including Triassic platform carbon- surveys could have been one of the factors res- ates (Villeneuve et al. 1994), Palaeogene gabbros ponsible for the faulting that defines the various and basalts and Neogene calc-alkaline volcanics bathymetric features. It seems just as reasonable (Silver et al. 1985). The carbonates can be corre- to suggest that it was the extensional faulting that lated with Australasian crust exposed on Seram, produced the changes in crustal thickness. CARIBBEAN: AN OROCLINAL BASIN? 151

North–south convergence to those produced in the Banda Sea. In the Carib- bean there is good evidence for thrusting from the Indonesia. The Asian land mass has been almost north, at the Muertos Trough south of Puerto Rico, stationary in the mantle reference frame throughout as well as from the south, where an accretionary the Cenozoic, but during that period Australia has prism has developed north of the Netherlands been moving steadily northwards as part of the Antilles (e.g. Giunta et al. 2006). Indian Ocean plate. Its present-day convergence on Southeast Asia in the vicinity of Timor is esti- / 8 mated to be about 7.5 cm annum at about N15 E Conclusions (Genrich et al. 1996). Given their present rela- tive positions, it appears that Australia will strike In the preceding section, similarities between the Asia only a glancing blow, but at relatively Caribbean region and Indonesia have been iden- high speed. tified and discussed. The list is impressive, but the generally-accepted two-phase origin of most of Caribbean. The Late Mesozoic and Cenozoic the Caribbean basement, first as normal oceanic history of relative motion between North and crust and then as a volcanic oceanic plateau, South America, as estimated by Mu¨ller et al. suggests very great differences between the two (1999), is summarized in Figure 6. Beginning in areas. Clearly, arguments for specific evolutionary the latest Paleocene (c. 56 Ma), South America hypotheses that are based on supposed analogies has converged on North America at a rate that between the Caribbean and the Banda Sea (or increases from east to west but which is currently any other oroclinal basin) must be treated with about 8 mm/annum in the central region. This is extreme caution. Although the stress needed to only about a tenth of the rate of advance of Australia create an orocline from a rectilinear mountain on Asia but not very different from the Africa– chain could be produced by rapid expansion in an Europe convergence within which the oroclinal area adjacent to it, and this process seems to have basins of western Tethys have developed. Regard- created the Tethyan oroclines, it seems probable less of whether the Caribbean developed between that the impact on a long-established subduction the Americas as suggested by James (2006), or in zone of a rigid, but localized, crustal block (a ‘rigid the Pacific to the west as argued by Pindell et al. indentor’ in the terminology of Indo-Asian collision) (2006), it would have been located between North would have very similar effects. Indentor impact is and South America as they approached each other. implicit in a Pacific origin for the main Caribbean The observed structures of the Tethyan oroclines Plate, and most of the Caribbean/Indonesia suggest that east–west extension of the Caribbean similarities can be explained equally well by either would be expected during this compressional phase. process. There seem to be only a few aspects of Caribbean geology for which orocline analogies can Backthrusting provide fresh insights. One lesson that can be drawn from the Tethyan Indonesia. Continental crust has now entered the basins that might be applicable to the Caribbean is subduction zone south of Timor, and the continu- that extension is not only not prohibited by conti- ing northward movement of Australia must be nental convergence but may actually be promoted accommodated elsewhere. North-directed thrusts by it. It may therefore be appropriate to take a in the southern Banda Sea have been imaged on a second look at the development of the Caribbean number of research cruises (e.g. Silver & Reed during the Cenozoic convergence of North and 1988) and are consistent with GPS campaigns that South America. Is it possible that extension during have shown Timor moving with the Australian this period, far from being confined to the Cayman continent and converging on Seram (e.g. Genrich Trough and Granada Basin, was distributed far et al. 1996). The locus of convergence appears to more widely through the Caribbean than is currently be a thrust expressed as a narrow trough just north supposed? The free-air patterns on the Upper and, of the inner-arc island of Wetar (Breen et al. especially, the Lower Nicaragua rises are indicative 1989). The existence of a mirror-image feature of pervasive extensional faulting. The eastern end of south of Seram has been the subject of much infor- the lower rise is readily interpreted as conjugate mal speculation, but no conclusive evidence has to southeastern Hispaniola and the western flank yet been presented. of the Beata Ridge, suggesting that the Haiti Basin is an extreme consequence of this faulting, that it Caribbean. Although the rates are much slower than may be younger than the remainder of the Colombia in Indonesia, continental convergence in the Carib- Basin and that it may even be floored by Neogene bean has a history spanning tens of millions of years oceanic crust. Moreover, just as the Vienna Basin and it is unsurprising to find backthrusts analogous has been opened by the eastward migration of the 152 J. MILSOM

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