Thermal equilibration in high pressure/low temperature rocks of fossil coastal accretionary prisms in Chile
|Location||International Geological Congress,oslo 2008|
|Author||Willner, Arne P.۱; Massonne, Hans-Joachim۱; Gerya, Taras V.۲; Gorczyk, Weronika۲; Hervé, Francisco۳|
|Holding Date||21 September 2008|
The metamorphic basement along the Chilean coast between 26°S and 55°S represents fossil deep-seated accretionary systems of Permo-Carboniferous age in the north and Mesozoic age in the south. This basement represents one of the most widely exposed accretionary systems along the circum-Pacific convergent margins and mainly consists of terrigeneous metagreywackes with some intercalated slices of oceanic crust. Such rocks were buried to deeper levels (20-40 km) and subsequently incorporated into the accretionary system by basal accretion. The metamorphic grade of these rocks is mainly related to high pressure greenschist facies extending rarely into the epidote-blueschist and albite-amphibolite facies. Rocks which were buried to shallower levels (15-20 km) experienced frontal accretion and were metamorphosed in the pumpellyite-actinolite facies. Systematic studies of maximum PT-conditions reveal a continuous decrease of the metamorphic gradient with depth and time from 15-20°C/km (pumpellyite-actinolite facies) to 10-12°C/km (epidote-blueschist facies). We relate this correlation also to different thermal reequilibration after maximum burial. This assumption was tested by two-dimensional thermomechanical numerical modeling of continental margin settings formed by subduction of an oceanic plate with rates in the range of 2 to 8 cm/yr. These numerical experiments involved extended fore-arc wedges as realized in Carboniferous to Jurassic times in Chile. We extracted synthetic PT-paths from those parts of the continental margin, where frontal or basal accretion was simulated. The corresponding particle paths follow a subhorizontal trajectory after maximum subduction, which are significantly longer in upper levels of the accretionary system with prevailing frontal accretion. Due to low exhumation velocities around 0.2-0.6 mm/yr, as derived by geochronological studies for the Carboniferous accretionary systems of Chile, a longer residence time at maximum depth within the upper levels of the accretionary system implies a longer thermal reequilibration and hence a higher metamorphic gradient for the frontal accretionary process. Forced flow as a further accretion mode appears predominantly in the numerical experiments, but has a minor representation in the Chilean coastal accretionary systems where it appears at the onset of accretion only.