The dynamics of intra-oceanic subduction zones: A direct comparison between fossil petrological evidence (Rio San Juan Complex, Dominican Republic) and numerical simulation

Abstract

Dispersed blocks of various types of metamorphic rocks in serpentinite mélanges of the northern Dominican Republic (Hispaniola) provide fossil evidence for the dynamics of the subduction zone channel in the intra-oceanic Caribbean subduction zone system between 120 and 55 Ma. Comprehensive petrological and geochronological data on three exemplary samples of eclogite and blueschist are presented that allow a series of different but interrelated pressure–temperature–time paths to be delineated. Eclogites indicate a low P/T gradient during subduction and record conditions in the nascent stages of the subduction zone. Lu–Hf data yield 103.6 ± 2.7 Ma for peak metamorphic conditions of 23 kbar/750 °C. An anticlockwise P–T path is defined. Other blocks record the continuous cooling of the evolving subduction zone and show typical clockwise P–T-paths. Omphacite blueschists reach maximum P–T-conditions of 17–18 kbar/520 °C at 80.3 ± 1.1 Ma (Rb–Sr age data). The mature subduction zone is typified by jadeite blueschists recording very high (“cold”) P/T gradients. A Rb–Sr age of 62.1 ± 1.4 Ma dates peak metamorphic P–T conditions at 16–18 kbar/340–380 °C. The array of P–T–t data allows overall cooling rates of the subduction zone at depths of c. 60 km to be constrained at 9 °C/Ma. Cooling rates and exhumation rates (i.e., vertical component of retrograde trajectories) of the metamorphic blocks are 9–20 °C/Ma and 5–6 mm/a, respectively. The derived P–T–t array is compared with a 2-D numerical subduction-zone model published by Gerya et al. [Gerya, T.V., Stöckhert, B. and Perchuk, A.L., 2002. Exhumation of high-pressure metamorphic rocks in a subduction channel: a numerical simulation. Tectonics 142, 6-1-6-19.; 45° slab dip, 40 Ma lithosphere age, convergence rates of 10–40 mm/a], which incorporates weakening of lithospheric mantle of the hanging wall by fluids emanating from the downgoing slab, resulting in an increasingly more funnel-shaped subduction channel system with time. The numerically derived array of simulated P–T–t paths as well as the calculated rates of exhumation and cooling agree well with the P–T–t data derived from the metamorphic blocks of the Rio San Juan serpentinite mélanges when convergence rates of 15 to 25 mm/a are chosen. This value is also in accord with available paleogeographic reconstructions calling for a long-term average of 22 mm/a of orthogonal convergence. On the basis of the comparison, the onset of subduction in the Rio San Juan segment of the Caribbean Great Arc can be constrained to approximately 120 Ma. This segment was thus obviously active for more than 65 Ma. An orthogonal convergence rate of 15–25 mm/a requires that a minimum amount of 975–1625 km of oceanic crust must have been subducted. Both petrological/geochronological data and numerical simulation underscore the broad spectrum of different P–T–t paths and peak conditions recorded by material subducted at different periods of time as the subduction zone evolved and matured.