Geoscientist Online

In the latest contribution to our online debate, Professor Karsten M. Storetvedt* gives his perspective on the vexed question of the origin of the Caribbean Plate

Geoscientist Online 2 December 2009

In the September 2009 issue of Geoscientist, Keith James and Maria Antonieta Lorente¹ let off a bombshell against the currently repetitive plate tectonic view of the Caribbean – the Pacific origin hypothesis ^2-5 which they argue stumbles against critical facts. Instead, the authors render an in situ model – the basis of which is an original continental fragment having subsequently undergone varying degrees of attenuation and (presumably) basification^6-7 – both simpler and in closer harmony with available geological and geophysical data. In some ways, they are back to ideas on crustal oceanization which featured in pre-plate tectonic days^8-10. Adding to oceanization mechanisms, the authors explain the Caribbean tectono-magmatic picture in terms of a superposed Alpine-age left-lateral convergent shearing between North and South America – during which the two continents have remained azimuthally relatively unchanged (their Fig. 3). However, as a wide range of regional and larger scale geophysical and geological information is omitted from their re-evaluation, the arena is open for alternative non-plate tectonic solutions. It is a well-established truth in science that once previously ignored facts are taken into account many familiar observations will be looked at with fresh eyes and given new meanings. In this article, I shall shortly review the Caribbean situation within an alternative framework – Global Wrench Tectonics^11-13, a new mobile Earth representation entirely without recourse to hypothesized plate tectonic mechanisms. However, the James/Lorente proposal of the Caribbean being basically a variably thinned and foundered continental block is preconditioned also in this new tectonic portrayal.

Another mobile perspective

The possibility of crustal/lithospheric motion of the Caribbean was discussed already in the 1930’s¹⁴ – suggesting that the belts of negative gravity anomalies associated with the Lesser Antilles Arc and the Bartlett/Cayman Rift are products of a certain eastward tectonic translation of the Caribbean. Other evidence, such as the overall arcuate shape of the combined Colombian and Venezuelan basins, the curved Lesser Antilles Arc – with its tectonic front displaying (in mid-arc position) a left-lateral WNW/ESE offset (see Fig.1), the north-facing bow-shaped Panama Isthmus with its adjacent North Panama Thrust belt, etc. give the impression that South America or at least the north-western Andean region¹⁵ have undergone a certain clockwise tectonic motion. Furthermore, the boundary between the Caribbean and North America apparently runs across Guatemala and along the major left-lateral Polochic-Motagua fault system which had its peak activity in the Upper Cretaceous-Lower Tertiary¹⁶. Focal mechanism studies¹⁷ and recent GPS observations in the north-eastern Caribbean/North America tectonic boundary¹⁸ demonstrate that, relative to North America, an eastward motion of the Caribbean is still taking place. In other words, a new mobile alternative is entering the picture (Fig.1).

Both the Caribbean and Panama basins are characterized by a crustal category that is neither normal continental nor ‘classical’ oceanic – but something in between; plateaus and aseismic ridges abound. These structures are most readily understood as cases of oceanized continental crust – attenuated marginal sectors of adjacent South America. Similarly, the deep Gulf of Mexico can be regarded an attenuated/subsided fragment of North America. During the Middle-Upper Mesozoic, the evolving oceans were affected by increasing attenuation – with isostatic subsidence and basin growth. On a smaller scale, the same happened to Middle America/Gulf of Mexico. Widespread crustal loss to the mantle gave rise to acceleration in the Earth’s spin, which in turn triggered latitude-dependent lithospheric wrenching, involving mostly moderate in situ rotations of the continents. During the Alpine climax, relative rotations of the Americas turned the Caribbean region into a broad left-lateral shear zone; the main tectonic boundary, which was palaeo-equator-aligned, formed along the Motagua fault zone (in Guatemala), the Cayman Trough and the northern wing of the Puerto Rico Trench. The eastward swing of the combined South America/northern Andes produced extensive conditions along the Middle America Trench – accounting for its graben-like structures and lack of a tectonic wedge. Thick red arrows depict the principal lithospheric motions. Shown are also locations of deep margin transects drilled during DSDP legs 66 and 67. Other abbreviations are: TeR: Tehuantepec Ridge system, MoF: Motagua fault zone, GoM: Gulf of Mexico, CaT: Cayman Trough, C: Cuba, H: Hispaniola, PuT: Puerto Rico Trench, LAA: Lesser Antiles Arc, AR: Aves Ridge, VeB: Venezuela Basin, CoB: Colombia Basin, Pa: Panama, PaB: Panama Basin, Ga: Galapagos, GaR: Galapagos Rift.

The seaward extension of the trans-Guatemala shear zone is uncertain, but in case the Tehuantepec Ridge system (see Fig.1) forms a major dividing line between the actual mobile ‘units’ we ought to observe marked differences in Pacific margin accumulation astride the ridge. In fact, drilling into the lower continental slope off Guatemala (on the Middle America side of this boundary) IPOD Leg 67 obtained results strikingly different from those off southwest Mexico, on the North American side, during IPOD Leg 66 (Fig.1 for locations). Like the rest of the Middle America Trench, Leg 67 area is formed of rectilinear topographic steps indicating extensional collapse of both landward and seaward slopes. It seems that south of the Tehuantepec Ridge the Caribbean and adjacent Pacific lithosphere is undergoing a moderate eastward motion (see later) producing extensive conditions along the Middle America Trench/Benioff Zone system – hence its graben-like structures and lack of sediment accretion. An eastward translation of the Caribbean would override the thinner Atlantic crust explaining the observed sub-horizontal décollement structures and slicken-sided surfaces described from the deformation front of the Lesser Antilles Arc – as per DSDP Leg 78A. In contrast to the Middle America margin, the Mexican margin at IPOD Leg 66 sites show signs of having been tectonically compressed – including a certain accretionary wedge with landward dipping reflectors. Such observations can readily be associated with tectonic shortening along the lower continental slope provided by a clockwise (westward) swing of North America (see below), reactivating the old Pacific Benioff Zone in addition to overriding the thinner NE Pacific crust (north of the Tehuantepec Ridge). Again, we see evidence for a left-lateral offset between North America and the Caribbean.

The Lower-Middle Palaeozoic fold belt along eastern North America apparently has its southward continuation across southern Mexico/Guatemala where relatively strong Appalachian tectonic and metamorphic processes occur16, 19-20. Another branch of this fold belt can be found in a presently discontinuous position, along the north-western tip of South America – following a coast-parallel sedimentary basin of great thickness and with extensive basic volcanism, folding and metamorphism ^21-23. This belt extends from Ecuador to northern Colombia, and its relatively intense compressive deformation is described as unlike the more moderate tectonic style featuring elsewhere along the South American Andes. The fact that this presumably South American branch of the Appalachian belt is markedly offset to the east, relative to its main North American branch, suggests that the Caribbean and north-western South American Andes have been subjected to a certain clockwise rotation relative to North America (see Fig. 2).

In the early evolving stages of the Atlantic, the opposite continental margins were more closely parallel than they are today. But during the late Cretaceous-early Tertiary tectonic revolution (Alpine climax) the global palaeo-lithosphere was subjected to inertia-driven torsion – involving in situ continental rotations (as shown by curved arrows) with respect to the time-equivalent equator. These motions gave rise to the southward fanning-out shapes characterizing present-day North and South Atlantic. Note the position of the palaeo-equator: it follows the present Mediterranean and runs across the Central Atlantic, before passing the Caribbean to be. In the pre-Alpine continental configuration, the Appalachian tectonic axis of North America continued southward as a linear belt along the north-western Andes (Colombia and Ecuador), but during Alpine time this tectonic continuity was disrupted. While performing a certain eastward swing, in close association with northern South America, the Caribbean basins were turned into a broad shear zone. However, Middle America micro-block behaviour of any significance is not in evidence.

Considering the Caribbean as a subsided and chemically transformed piece of continental crust, well advanced in the process of becoming a deep ocean, Fox and Heezen²⁴ suggested that the basin may have been uplifted in the late Cretaceous-early Tertiary and that its subsequent history has been dominated by subsidence. Crustal thinning and associated isostatic sinking may easily be explained by eclogitization processes¹³ or by the corrosive effect of super-critical hydrous fluids²⁵. The Mexican basin may serve as another demonstration of a crustal collapse structure. A variety of factors, such as its closed-in continental location, the intermediate-type crust, the relatively smooth transition of crustal thicknesses between the surrounding lands and the central deep sea basin, a very low heat flow, the two-axes shape of the deep sea plain, favour an in situ oceanization model. The deep Mexican plain can be viewed to consist of two orthogonal branches (Fig.1) – directed approximately N60ºE and N330ºW respectively. These tectonic features correspond to the directions of the ubiquitous sets of orthogonal fractures in eastern United States, for which the N60ºE striking system represents the predominant set of crustal weaknesses²⁶. These fundamental rupture systems would naturally be reflected in the shape of a subsiding basin – as indeed demonstrated for the Mexican deep plain.

The orthogonal fracture regime is of global extent; after correction for Alpine-age inertia-driven lithospheric wrenching (including in situ continental rotations, mostly moderate), it can be shown that originally these fracture/joint axes had bearings slightly clockwise from due north/east13. In comparison with the prevailing joint axes of Western Europe, the North American sets are rotated c. 30º clockwise. In fact, this azimuthal difference corresponds to the amount of relative continental rotations required to account for the palaeomagnetic polar discrepancy between Europe and North America^11-13. In this process, the already thinned (and hence mechanically fragile) North Atlantic crust underwent tectonic deformation, fault reactivation and reshaping – bringing about its southward fanning-out configuration, breaking up the mid-Atlantic rift and producing mega-shearing within the thin-crusted oceanic domains. Owing to the greater inertia of upstanding continental masses, the mega-scale fracture system off eastern US has acquired a certain southwest-bended shape (see Fig.1). We are beginning to sense a new mobile tectonic system driven by changes in Earth rotation – stripped of all plate tectonic presumptions.

In situ motions of the American continents; Caribbean consequences

DSDP Leg 10 data demonstrated the relatively rapid late Cretaceous foundering of the Gulf of Mexico deep sea plain²⁷; while recovered Albian and older deposits were consistently of shallow-water nature, younger strata became abyssal between late Albian and the Santonian. This break-down of continental crust was not an isolated Gulf of Mexico affair – it was global, notably in the oceans. Embryonic deep sea basins, which had been under way since the Middle Jurassic, reached in the Upper Cretaceous abyssal depths for the first time in Earth history. Hence, it appears that the upper mantle hydrostatic pressures and volatile content, necessary to accelerate eclogitization and associated sub-crustal delaminating processes, took most of geological time to accumulate¹³. The build-up and release of upper mantle hydrostatic pressures, causing repeated uplift and subsidence of the developing oceanic basins, were apparently in phase with the transgression-regression cyclicity affecting low-lying lands¹³. Until the late Mesozoic, the South Atlantic trans-oceanic ‘land bridges’ were still relatively unimpaired, but due to the pre-existing sets of ubiquitous parallel fractures, the evolving continental margins became closely parallel²⁸. The popular view that parallel margins is prima facie evidence of hypothesized sea floor spreading has come to rest.

The deep sea basins that formed during the Jurassic were only of limited extent, frequently giving rise to anoxic conditions and black shale formation. These basins were surrounded by a mosaic of sub-aerially exposed semi-continental regions (less affected by sub-crustal break-down processes), a physiographic situation similar to that of the Caribbean. Many oceanic fragments of former land can still be recognized by the multitude of aseismic ridges and plateaus having intermediately thick crust – in the Caribbean and elsewhere. Towards the end of the Cretaceous the modern continent-ocean physiography was approaching its present state. The Moho interface as well as upper mantle volatile- and melt-holding low-velocity layers – the asthenosphere – had reached more mature stages, and substantial crustal loss to the mantle led to jerky changes in Earth’s moments of inertia. The dynamic consequences were phases of true polar wander (distinct spatial reorientations of the globe) and an overall acceleration in the Earth’s rate of rotation. For the first time in Earth history, remaining continental masses were

1. surrounded by thin and mechanically fragile oceanic crust, and
2. a more developed asthenosphere was in place – factors that provided a tectonically unstable situation.

The development had paved the way for a global geological revolution – the Alpine climax, during which also the principal Caribbean tectonic structures became entrenched.

The late Mesozoic planetary acceleration produced inertia-driven continental rotations ‘in situ’, dominated by the Coriolis Effect. For the larger continental masses the inertial rotations were only of a few tens of degrees, but these mostly modest rotations readily account for the observed discrepancies in palaeomagnetic polar wander paths^{11-13, 29}. For the first time in Earth history mobile continents were a reality. Fig. 2 shows the inferred late Cretaceous (pre-Alpine) configuration of the Atlantic region. Note that

Without any interference from between-continent mechanical interactions, Alpine tectonic rotations would be purely inertial: the northern palaeo-hemisphere would be subjected to clockwise torsion while the southern palaeo-hemisphere would undergo a corresponding counter-clockwise wrenching (relative to the palaeo-equator). It is important to note that in a global inertia system, the palaeo-equatorial belt would be particularly vulnerable to shear deformation as indeed demonstrated for the Alpine (palaeoequator-aligned) North American-Caribbean tectonic boundary.

Based on palaeomagnetic data, the smaller and hence more mobile North America


The Coriolis Effect also dominated the southern palaeo-lithosphere, but due to the relatively narrow Equatorial Atlantic, in combination with Africa’s inertia-driven counter-clockwise swing, the equatorial transect became strongly strained:


The tectonic component along the North Brazilian margin enforced a clockwise tectonic component on South America, opposing its counter-clockwise inertia-based rotation in addition to breaking up of mega scale trans-Brazilian fault zones. The resulting overall rotation of South America was only 10-15º of clockwise motion^11-12. Fig. 3a depicts late Palaeozoic to Mesozoic palaeomagnetic polar curves for South America and Africa in conjunction with the Global Polar Wander Path¹² – the latter (reference curve) is after elimination of Alpine continental motions. As can be inferred from the diagram, Africa and South America moved in opposite directions (clockwise for South America and counter clockwise for Africa), giving rise to the present southward fanning-out shape of the South Atlantic.

Diverse observations substantiate the conclusion of a minor clockwise Alpine-age rotation of South America. Diagram a) shows the Permian (P) to late Mesozoic (M) palaeomagnetic polar paths for Africa (AF) and South America (SA) in conjunction with the global reference curve (GPWP). Note that the polar paths for the two continents are located on opposite sides of the GPWP, signifying that the two land masses have moved in opposite directions – giving rise to the present southward fanning-out of the South Atlantic. The clockwise rotation of SA would produce a certain continental overriding and uplift of the southern part of the continent while such effects did not impinge upon the northern sector. Diagram b), which shows Cretaceous sea level variation for the two regions, demonstrates this difference: the northern sea level variation displays the global transgressive pattern characterizing the Upper Cretaceous, while the uplifted southern part has an opposite (regressive) trend at that time. Consistent with the rotation model, the late Cretaceous-Recent volcanic belt of southern SA (diagram c) is located at greater distances from the SE Pacific margin than corresponding volcanic activity along the northern Andes. At the southern tip of the continent, the left-lateral nature of the Magellanes Fault Zone (MF) is further evidence in favour of the rotation model. Diagram sections are based on data from Storetvedt^11-12, Macellari³⁶, and Munoz and Stern³⁹ respectively.

The relative Alpine age rotations of the Atlantic continents produced considerable shearing and metamorphism within the thin-crusted Atlantic basins. The orthogonal fracture systems of the oceans (these are not new sets of fractures but inherited ones from the original, and often still prevalent, continental basement) were strongly reactivated31. This inertia-driven shear deformation of the thin oceanic crust forms the basis of the magnetic susceptibility contrast model for interpreting marine magnetic lineations^{13, 32}. Thus, alternating bands of rocks in differing states of alteration/low temperature oxidation will be variably polarized through induction by the present geomagnetic field, giving rise to the observed parallel ‘stripes’ of magnetic highs and lows. In other words, marine-magnetic lineations are tectonic signatures basically outlining the prevailing Alpine tectonic grain and have no general link to geomagnetic polarity reversals. Due to a combination of tectonic interaction (Equatorial Atlantic) and significant transtensive conditions (Central Atlantic) mega-scale ‘E-W’ oriented transcurrent faults have offset the orthogonal ridge-parallel fracture system; later (Miocene to Recent) uplift of the mid-oceanic ridges^12-13 has sometimes reversed the direction of trans-oceanic transcurrent faulting, giving the false impression of there being a new type of oceanic fractures – called ‘transform’ faults³³. The Alpine wrenching of the oceanic lithosphere produced a ‘mid’-ocean tectonic belt, centered on a deep dislocation along which ridge uplift has taken place since the Upper Miocene^12-13. In segments of mid-ocean rifts, such as the Molloy Deep of the northern North Atlantic³⁴ and recently in a location of the Central Atlantic³⁵, the inferred crustal elimination processes have seemingly gone to completion (with the mantle exposed on the seafloor).

During the Alpine climax, the relative rotation of the Americas broke up the palaeo-equator-aligned principal tectonic boundary in the Caribbean – breaking up the left-lateral Motagua/Polochic fault system and the Cayman Trough lineaments. A major part of the palaeo-geographic re-setting in Middle America was brought forward by the larger clockwise (westward) swing of North America, producing significant transcurrent motions along the broad southern Cuba/Hispaniola fault system, the Cayman Trough/Yucatan Basin and the trans-Guatemala shear zone, with piling up of a sedimentary wedge along the North American Pacific margin (north of the Motagua fault/Tehuantepec Ridge). Accepting this mobile system, an appreciable tectonic offset between Cuba and Hispaniola is likely to have occurred. Just to the east of Cuba/Hispaniola probably represents a tectonic junction The two individually rotating mega-units – North America/adjacent Atlantic and Caribbean/South America – apparently coalesce in the deep Atlantic east of Hispaniola where the structures are ill-defined and diffuse.

The circum-Pacific Benioff Zone, which may be regarded a contraction fracture of Precambrian age, repeatedly reactivated throughout Earth’s dynamo-tectonic history, underwent in Alpine time its strongest reactivation ever. Owing to the inferred clockwise rotation of South America, the frontal margin must have overridden the south-eastern Pacific Benioff plane, producing a compressive sedimentary wedge in addition to providing a certain tectonic lifting of the southern part of the continent. In the north, however, off Ecuador, Colombia and Middle America, the eastward continental swing turned the corresponding Pacific margin into a transtensive regime – demonstrated by the absence of a regional tectonic wedge. These model-predicted differences, between the southern and northern Andean regions, are borne out by the observed distinctions in crustal structure, seismo-tectonic pattern and geological characteristics²³. While the Andes of Peru and Chile are underlain by a shallow and relatively well-defined compressive Benioff zone, focal depths in the northern sector show little regularity so definition of a uniform Benioff plane seems hardly possible there.

Macellari³⁶ compared the Cretaceous sea-level history of the northern sector (Venezuela-Colombia-Ecuador-Peru) with that of Chile-Argentina and found a marked difference in the distribution of marine strata. For the whole northern sector, he found an increasing marine inundation with maximum water depth in Turonian time – an observation that compares extremely well with the average global sea-level variation37. This suggests that any regional tectonic uplift, affecting the stand of northern South America with respect to the ocean, must have been minimal. The situation for southern South America would be quite different in that the westward moving land mass would have overridden the adjacent SE. Pacific crust, causing a certain land upheaval consistent with the observed regression for that part of the continent (Fig. 3b).

The inferred westward rotation of southern South America would expectedly have produced a relatively shallow detachment surface at asthenospheric level in the uppermost mantle. Hence, the deeper parts of the original Benioff Zone, a natural channel for rising magmas, would have shifted eastward relative to the continent and expressed by inland volcanism – at increasing distances southward away from the Pacific margin. The course of the principal late Cretaceous-Recent volcanic belt ^38-39, running from just north of Tierra del Fuego on the Atlantic coast to the Pacific coastal region of northern Chile, concurs with this view (Fig.3c). Thus, the volcanic axis intersects the northern Chile margin at a shallow angle (10-15º) vs. the Pacific margin, in harmony with the inferred clockwise rotation of the continent. The left-lateral nature of the major Magellanes Fault Zone, near the southern tip of S. America (Fig.3c), is other evidence supporting this view.

Alpine wrench forces displayed significant effects over a broader belt of the adjacent SE Pacific – including extensive rupturing of the Chile Ridge^12, the continent-ward widening of the Galapagos Rift, and the structurally fragmented and oval-shaped Galapagos-Panama Basin (Fig. 1), reactivating the fundamental N-S/E-W oriented fracture systems. In comparison with the E-W directed Galapagos Rift, the overall NE-SW oriented Galapagos-Panama Basin can be related to the stronger wrenching effect nearer the continent. Focal mechanism studies in the East Andean frontal fault system suggest right-lateral slip – implying that the northern Andes block is moving NNE relative to ‘stable’ South America⁴⁰. GPS studies in the northern Andes-Cocos-Carnegie region^41-42 show somewhat scattered results, but the overall velocity vector is again directed north-easterly. As there is no evidence for compression or sediment accretion along the Middle America Trench, the tectonic response to the observed crustal motion ought to be found within the Caribbean region itself – for example, by way of having produced the arcuate North Panama compressive belt. Despite the limited crustal structure information⁴³, the entire Panama-Galapagos Basin (including the relatively thick-crusted Carnegie, Cocos and Malpelo ridges) may be seen as attenuated and transformed continental crust – an in situ development not unlike that proposed for the Caribbean by James and Lorente1. In tectonic terms the north-western Andean region may then be seen as a southern extension of the mobile Caribbean block which seems to have been decoupled to some extent from its mother continent – South America.

The Colombian Andes give way northward to a split-up into Western, Central and Eastern branches – a subdivision of fault-bounded basins and uplifts near the Caribbean Sea. Pre-Tertiary structures are exposed in the uplifted ranges which display an increasing bend north-eastward, eventually turning into a single coast-parallel range along the southern Caribbean shores – to the north paralleled by the right-lateral Oca-El Pillar faults. At the eastern end of the coastal range, the basins and uplifts structure of Colombia re-appears in the Caribbean tectonic front – expressed by the Aves, Lesser Antilles and Barbados ridges, with their intervening Grenada and Tobago basins. In the context of the wrench tectonic model¹³, the principal difference between the North Colombian and the relatively thick-crusted Outer Caribbean structures is in the degree of crustal thinning and fracture density. In the general transtensive regimes of the Caribbean, provided primarily by its combined rotation with South America, basin development would be restricted to segments of the crust with the higher degree of fracturing; increasing fracture population will augment the effectiveness of mantle fluids/hydrostatic pressures in delaminating or decomposing the crust (from the bottom upwards). The arcuate shape of the Lesser Antilles Arc can be ascribed to the rotation of Caribbean/South America; similar explanations apply to the Scotia and Indonesian arcs¹³.

New Caribbean understanding – summary of events

I support the view of James and Lorente1 that the Caribbean represents thinned and (presumably) chemically modified continental crust – a phenomenon perceivably applying to all oceanic regions12-13, but in terms of tectonics my own explanation differs substantially from theirs. In my view, the attenuation processes either take the form of sub-crustal delamination through progressive eclogitization or by the strongly corrosive effect of supercritical water/hydrous fluids – processes that supposedly are most effective along predominant fault zones13. This principle is also demonstrated in Fig. 4 of James and Lorente1; their interpretation of a seismic line (Line 1293) over the Venezuela Basin depicts major fault zones in close association with maximum crustal thinning.

Being located along the late Cretaceous-early Tertiary palaeo-equatorial belt, where global wrench forces would have been at their maximum, the Caribbean tectonics would expectedly be relatively complex. Based on the above considerations, the Caribbean sector is tectonically part of South America. Owing to eastward wrenching of northern South America, the regional E-W set of fundamental orthogonal fractures would be particularly disposed to Alpine reactivation – fault enlargement and associated basin development. The following Caribbean development stages may be discerned:

1. The original configuration of the American land masses was azimuthally somewhat different from today (Fig. 2). The Lower-Middle Palaeozoic Appalachian fold belt that continued to southern Mexico and Guatemala had its southward extension along the time equivalent geosyncline of Colombia and Ecuador – representing American fragments of a globe-encircling (palaeo-equator aligned) deformation zone^{12-13 .}
2. In Jurassic times worldwide crustal attenuation processes and related basin subsidence were gaining ground – including early development of smaller the scale basins of the Caribbean and Panama-Galapagos regions. The evolving depressions were bounded by pre-existing fracture zones along which occasional minor magmatism came about. In Middle-Upper Jurassic, thick sequences of tropical limestone accumulated.
3. During the Cretaceous crustal oceanization accelerated, and modern abyssal plains appeared for the first time in Earth history. By the end of that era thick Mesozoic carbonate sections had formed, but due to an incomplete crustal attenuation smaller scale Caribbean sub-basins were frequently delimited by continental ridges. The widespread late Mesozoic crustal loss to the mantle led to changes in Earth’s moment of inertia – instigating jerky alterations of its rotation. The ensuing Alpine tectonic revolution, with its inertia-driven in situ continental rotations, led to widespread tectono-magmatic activity – including the Caribbean region.
4. The late Cretaceous-early Tertiary rotations of the Americas led to their approximate present juxtaposition. The original position of Cuba, basically positioned to the north of the principal tectonic boundary, was most likely somewhere to the east (or north-east) of Hispaniola. Resulting from the continental reorientations, the South American branch of the Appalachian fold belt was disrupted from its original alignment. The eastward translation of the Caribbean gave rise to significant shear tectonics – both along the margins and internally. In transtensive locations volcanic activity became widespread. Neogene tectono-magmatic activity has followed the earlier tectonic pattern, but with much reduced intensity.
5. The eastward swing of Caribbean/South America led to significant shearing along the Polochic-Motagua fault system/Cayman Trough/Puerto Rico Trench – ending in a shallow inclined thrust front seaward of the Lesser Antilles Arc. The overriding model is consistent with work of the Meyerhoffs44 who regarded the frontal region as having been thrust eastward. Consistent with this explanation, DSDP Leg 78A unveiled the faulted nature of the late Tertiary sedimentary wedge – showing fracturing, reverse faulting and penetrative shearing at various depths in the c. 450 m section drilled. During the late Cretaceous-early Tertiary peak activity, the whole region was caught between the differential movements of the Americas triggering basaltic and granitic magmatism in numerous locations^24.
6. Blueschist belts and other HP/LT metamorphic rocks, in addition to occurrences of upper mantle material tectonically emplaced/intruded in the solid state, formed along the main tectonic ruptures^{16, 45}.
7. Being a region of relatively strong deformation, linear magnetic anomalies would have developed in thin-crusted areas with more intense shearing. Contradicting the popular isochron interpretation of such anomalies, Donnelly²⁰ submitted that the anomalies are suspiciously parallel to topographic lineaments and buried scarps of the Venezuelan Basin, and in some instances, buried scarps and magnetic anomalies coincide. In the same thread, Diebold and others⁴⁶ noted a correlation between magnetic anomalies and structural grain. In addition, along the Caribbean tectonic front the NE-striking magnetic lineations of the Venezuelan Basin are cut by the NNE-striking Grenada sequence of anomalies, indication a swing in the frontal shear fabric.

Acknowledgements

I am greatly indebted to Frank Cleveland and Rebecca Taule for illustrations and animations.

* Inst. of Geophysics, Univ. of Bergen, Bergen, Norway [email protected]
http://www.storetvedt.com/karsten/index.htm

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