Fission track analyses of ODP Hole 129-800A samples

Four models of fission track annealing in apatite are compared with measured fission track lengths in samples from Site 800 in the East Mariana Basin, Ocean Drilling Program Leg 129, given an independently determined temperature history. The temperature history of Site 800 was calculated using a one-dimensional, compactive, conductive heat flow model assuming two end-member thermal cases: one for cooling of Jurassic ocean crust that has experienced no subsequent heating, and one for cooling of Cretaceous ocean crust. Because the samples analyzed were only shallowly buried and because the tectonic history of the area since sample deposition is simple, resolution of the temperature history is high. The maximum temperature experienced by the sampled bed is between 16°-21°C and occurs at 96 Ma; temperatures since the Cretaceous have dropped in spite of continued pelagic sediment deposition because heat flow has continued to decay exponentially and bottom-water temperatures have dropped. Fission tracks observed within apatite grains from the sampled bed are 14.6 +/- 0.1 µm (1 sigma) long. Given the proposed temperature history of the samples, one unpublished and three published models of fission track annealing predict mean track lengths from 14.8 to 15.9 µm. These models require temperatures as much as 40°C higher than the calculated paleotemperature maximum of the sampled bed to produce the same degree of track annealing. Measured and predicted values are different because annealing models are based on extrapolation of high temperature laboratory data to geologic times. The model that makes the closest prediction is based on the greatest number of experiments performed at low temperature and on an apatite having composition closest to that of the core samples

Exhumation of the Coastal Metamorphic Belt Above the Subduction‐to‐Transform Transition, in the Southeast Caribbean Plate Corner

Plate corners that transition from subduction to transform motion can result in complex deformation. The southeastern corner of the Caribbean plate is a site where active westward subduction of the oceanic South American plate transitions to transform motion along continental South America. The Northern Range (Trinidad) and Paria (Venezuela) metamorphic mountains are located directly above this eastward propagating plate transition zone. We examined the exhumation history of the Northern Range and eastern Paria using apatite fission track (AFT) and apatite and zircon (U-Th)/He (AHe and ZHe, respectively) thermochronology on 21 bedrock samples. These samples yield ages of ∼43–6 Ma (ZHe: aliquots), ∼20–4 Ma (AFT: pooled) and ∼5–2 Ma (AHe: aliquots). Along strike of the mountains, our new and published samples show a gradual eastward increase in age. Thermal modeling reveals two phases of rapid cooling and inferred exhumation that post-dates oblique collision and that migrated from west to east. We record an ∼six-fold increase in cooling and exhumation between ∼13–9 Ma in the Paria Peninsula and western Northern Range; a deceleration followed this rapid exhumation at ∼7 and 5 Ma. Synchronous with the deceleration in the west, exhumation of the eastern Northern Range increased ∼4 Ma. These post-collisional changes in exhumation constrain the inversion to east-side-up tilting of the Northern Range to ∼4 Ma. We interpret the timing and pattern of exhumation since the mid-Miocene to be consistent with the time-transgressive processes produced by an eastward propagating lithospheric subduction-transform edge propagator fault

Exhumation of the Coastal Metamorphic Belt Above the Subduction‐to‐Transform Transition, in the Southeast Caribbean Plate Corner

Plate corners that transition from subduction to transform motion can result in complex deformation. The southeastern corner of the Caribbean plate is a site where active westward subduction of the oceanic South American plate transitions to transform motion along continental South America. The Northern Range (Trinidad) and Paria (Venezuela) metamorphic mountains are located directly above this eastward propagating plate transition zone. We examined the exhumation history of the Northern Range and eastern Paria using apatite fission track (AFT) and apatite and zircon (U-Th)/He (AHe and ZHe, respectively) thermochronology on 21 bedrock samples. These samples yield ages of ∼43–6 Ma (ZHe: aliquots), ∼20–4 Ma (AFT: pooled) and ∼5–2 Ma (AHe: aliquots). Along strike of the mountains, our new and published samples show a gradual eastward increase in age. Thermal modeling reveals two phases of rapid cooling and inferred exhumation that post-dates oblique collision and that migrated from west to east. We record an ∼six-fold increase in cooling and exhumation between ∼13–9 Ma in the Paria Peninsula and western Northern Range; a deceleration followed this rapid exhumation at ∼7 and 5 Ma. Synchronous with the deceleration in the west, exhumation of the eastern Northern Range increased ∼4 Ma. These post-collisional changes in exhumation constrain the inversion to east-side-up tilting of the Northern Range to ∼4 Ma. We interpret the timing and pattern of exhumation since the mid-Miocene to be consistent with the time-transgressive processes produced by an eastward propagating lithospheric subduction-transform edge propagator fault