The Analysis of Diffuse Triple Junction Zones in Plate Tectonics and the Pirate Model of Western Caribbean Tectonics

The tectonic evolution of the Caribbean region since the late Cretaceous is of fundamental significance to plate tectonics. Heretofore, widely-accepted geological interpretations have held that the east-dipping Middle America subduction zone at the west margin of the Caribbean region and the west-dipping Lesser Antilles subduction at the east margin of the Caribbean region have remained stationary relative to one another since ca. 70 Ma or earlier. In this way, a single rigid Caribbean Plate is thought to have occupied the whole Caribbean region following the initiation of subduction at both the Middle American and Lesser Antillean trenches. Modern geographic positioning system (GPS) data and geomorphological displacements indicate that non-rigid processes have been important for the tectonic evolution of the Caribbean region. However, these neo-tectonic results have not modified the common view that the width of the Caribbean Plate has remained essentially constant throughout the Cenozoic.

The consequence of a rigid Caribbean Plate of constant width is that the rates of slab-hinge retreat or trench rollback of the two bounding subduction zones cannot have been controlled by lower plate forces such as slab pull or slab suction. This is because the subducting slabs belong to completely different lower plates at the western and eastern subduction zones. Pacific lithosphere from the ancient Farallon Plate has subducted under the western Middle America Trench, whereas Atlantic lithosphere from the modern North and South American Plates has subducted under the eastern Lesser Antilles Trench. If lower plate mechanisms were important, different proportions of slab pull or slab suction due to the different proportions and character of subducted lithosphere at the respective trenches would be expected to involve different rates of slab-hinge retreat at the bounding subduction zones and thus a change in the width of the Caribbean Plate through time. Therefore, short of an extraordinary coincidence, lower plate mechanisms do not appear to be important in determining either slab-hinge or trench retreat rates of the subduction zones bounding the Caribbean Plate.

This result is surprising because it contradicts the emerging view that global plate tectonics on Earth may be driven by a subduction-derived or top-down driving mechanism, in general. For example, the Pacific Ocean region has been shrinking in conjunction with the progressive widening of the Atlantic Ocean region since the breakup of Pangea—and this has been occurring in spite of the fact that the Pacific Ocean is rifting at a faster rate than the Atlantic Ocean. The new realization is that the partitioning of negative buoyancy forces produced by sinking slabs may explain this global tectonics most simply. Partitioning of downward slab forces would lead to both lower plate advance relative to a stationary mantle and slab foundering with associated slab-hinge retreat relative to a stationary upper plate. These dual effects of slab sinking provide simultaneous and complementary explanations for rifting within the fast-spreading Pacific and slow-spreading Atlantic realm. Such interpretations support the idea that plate tectonic systems evolve due to subduction-derived or top-down forcing, but, as just noted, the inferred existence of a long-lived Caribbean Plate of constant width is contradictory to this.

The tectonic evolution of the Caribbean region since the late Cretaceous is of fundamental importance at least in part because it may preserve a key counter-example to the subduction-driven or top-down plate tectonics inferred elsewhere. This important prospect has questionable merit at the present time, however, because the standard reconstructions of Caribbean tectonics while widely-accepted are poorly-tested. The present work attempts to address this shortcoming by identifying a common theoretical framework with which to compare the predictions of the competing models of Caribbean tectonics. Only through a consideration of the alternatives can confidence be gained for a preferred solution. In the present context, the consideration of alternative interpretations of the Caribbean rock record is prerequisite to attaining confidence that the Caribbean region is indeed globally anomalous with respect to the driving mechanisms of plate tectonics.

Despite 50 years of study, the evolution of the western Caribbean Plate region is still debated and there are three possible end-members: (1) Pacific model where the western Caribbean lithosphere is derived from the eastern Pacific and the northern and southern Caribbean Plate boundaries connect directly west to the Middle America Trench at the western Caribbean Plate boundary, (2) In-situ model where the western Caribbean lithosphere is derived from depth and the northern and southern Caribbean Plate boundaries terminate in a broad zone of extension in the western Caribbean Plate, and (3) Pirate model where the western Caribbean lithosphere is derived from the southern and northern margins of North and South America, and the northern and southern Caribbean Plate boundaries have either accommodated convergence themselves, or have curved to the north and south prior to reaching the Middle America Trench. Analysis indicates all models have been important for the evolution of the western Caribbean at different times but the Pirate model may have been dominant during the Cenozoic. The Pirate model resolves the absence of fault connections between the northern and southern boundaries of the Caribbean Plate with the Middle America Trench that are essential for the Pacific model and the > 1,100 km of net strike-slip displacements inferred across the northern and southern Caribbean margins that are unexplained by the in-situ model. In the Pirate model, North and South American material is inferred to have rotated into the trailing edge of the Caribbean Plate across the western Caribbean Plate corners.

This chapter demonstrates the need for multiple working hypotheses in the evaluation of complex tectonic zones such as the diffuse northwest Caribbean triple junction zone. Where an underlying incompatibility can be identified in the contextual major plate system, a number of nonrigid processes may arise in isolation or in some combination to stabilize the system. Recognition of the relative roles for the various processes requires explicit consideration of all of these possibilities. Characteristics of the different processes are also not always diagnostic. In this case, it is a combination of phenomena preserved in the rock record that must be interpreted together in order to discriminate between the different possibilities.