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Timing of deformational events in the Río San Juan complex: Implications for the tectonic controls on the exhumation of high-P rocks in the northern Caribbean subduction-accretionary prism

Abstract:

An integrated structural, petrological and geochronological study was undertaken to constrain the tectonic history and controls on the exhumation of the high-P rocks of the Río San Juan complex in the northern Caribbean subduction-accretionary wedge. In the main structural units of the complex, microtextural analyses were performed to identify the fabrics formed at peak of metamorphism in eclogite-facies conditions and during the main retrogressive event toward the low-P amphibolite or blueschist/greenschist-facies conditions.U-Pb SHRIMP dating on zircon rims (71.3±0.7Ma) coupled with 40Ar-39Ar analyses on phengite (~70-69Ma) in felsic sills placed temporal constraints on the exhumation of the Jagua Clara serpentinite-matrix mélange during the blueschist-facies stage at the early Maastrichtian. In the Cuaba unit, U-Pb TIMS zircon ages of 89.7±0.1Ma and 90.1±0.2Ma obtained for the crystallization of tonalitic/trondhjemitic melts in the lower Guaconejo and upper Jobito subunits, respectively, are similar. These ages coupled with a U-Pb SHRIMP zircon age of 87±1.8Ma obtained in a garnet amphibolite and a group of older 40Ar-39Ar cooling ages on calcic amphibole constrain the exhumation of the Guaconejo subunit from the high-P stage to the low-P stage at the ~90-83Ma time interval. Further, the age data collectively supports a genetic relationship between the distributed extensional ductile shearing, the related decompression and the local partial anatexis in the subunit, at least from the Turonian-Coniacian boundary to the early Campanian. A group of younger 40Ar-39Ar ages obtained in the mylonitized amphibolites of the basal Jobito detachment zone indicates late ductile deformation and exhumation/cooling in the late Campanian to Maastrichtian (~75-70Ma). Therefore, structural and age data established deformation partitioning and reworking of retrograde fabrics during ~20Ma in the Cuaba unit.The different exhumation rates obtained for the Jagua Clara mélange can be explained by uplift in two contrasting tectonic settings: a first stage of slow exhumation (1.4. mm/yr) in the subduction channel, largely lower than plate velocities, and a second stage of relatively fast exhumation (7.6. mm/yr) up to the surface. Therefore, the exhumation was temporally discontinuous and the velocity increase at ~. 70. Ma probably was triggered in response to the entrance of buoyant material in the subduction zone, such as a Caribeana continental ribbon or the distal part of Yucatán-Bahamas continental margin. In contrast, the exhumation path of the Guaconejo subunit is composed of a first segment from the baric peak to the low-P amphibolite stage (at 84-83. Ma) with an exhumation rate of 7.2. mm/yr, a second segment around the closure temperature of calcic-amphibole (at 82-70. Ma) with a rate of 0.4. mm/yr, and a third segment to the surface exposure (at 60. Ma) with a rate of 1.8. mm/yr. These velocity differences can be correlated with the P-T path proposed for the exhumation of the subunit, with an initial isothermal decompression from the peak in the high-P amphibolite to the eclogite-facies produced by distributed extensional shearing, followed by a relatively slow cooling at low-P in the newly acquired structural position, and the final tectonics mainly partitioned in the late Jobito detachment zone. The initial fast exhumation can be related to a major modification in the convergence conditions across the intra-oceanic subduction zone as a jump of the basal subduction thrust beyond a previously accreted arc terrane. © 2013 Elsevier B.V.