Magmatic-tectonic effects of high thermal regime at the site of active ridge subduction: the Chile Triple Junction model

High thermal gradients are expected to be found at sites of subduction of v ery young oceanic lithosphere and more particularly at ridge-trench-trench (RTT) triple junctions, where active oceanic spreading ridges enter a subdu ction zone. Active tectonics, associated with the emplacement of two main t ypes of volcanic products, (I) MORE-type magmas, and (2) calc-alkaline acid ic magmas in the forearc, also characterize these plate junction domains. I n this context, MORE-type magmas are generally thought to derive from the b uried active spreading center subducted at shallow depths, whereas the orig in of calc-alkaline acidic magmas is more problematic. One of the best cons trained examples of ridge-trench interaction is the Chile Triple Junction ( CTJ) located southwest of the South American plate at 46 degrees 12'S, wher e the active Chile spreading center enters the subduction zone. In this are a, there is a clear correlation between the emplacement of magmatic product s and the migration of the triple junction along the active margin. The CTJ lava population is bimodal, with mafic to intermediate lavas (48-56% SiO2) and acidic lavas ranging from dacites to rhyolites (66-73% SiO2). Previous models have shown that partial melting of oceanic crust plus 10-20% of sed iments, leaving an amphibole- and plagioclase-rich residue, is the only pro cess that may account for the genesis of acidic magmas, Due to special plat e geometry in the CTJ area,a given section of the margin may be successivel y affected by the passage of several ridge segments. We emphasize that repe ated passages will lead to the development of very high thermal gradients a llowing melting of rocks of oceanic origin at temperatures of 800-900 degre esC and low pressures, corresponding to depths of 10-20 km depth only. In a ddition, the structure of the CTJ forearc domain is dominated by horizontal displacements and tilting of crustal blocks along a network of strike-slip faults. The occurrence of such a deformed domain implies that an important tectonic coupling may exist between the upper and the lower plates leading to the partitioning of the continental lithosphere and to the tectonic und erplating of very young oceanic lithosphere below the continental wedge. We assume that in the case of the CTJ, the uncommon situation of three succes sive ridge segments entering the trench at 2-3 Ma intervals only resulted i n a strong and finally long-lived thermal anomaly. This anomaly caused reme lting of underplated portions of very young, still hot oceanic lithosphere. Only particular geometrical RTT configurations are able to produce such fe atures. These include linear continental margin, short ridge segments sligh tly oblique to the trench and short transform faults. Finally, the CTJ exam ple shows that a possible scenario for the origin of calc-alkaline acidic r ocks in the near-trench region involves coeval tectonic coupling and repeat ed passage of thermal anomalies due to successive subduction of short ridge segments. Therefore, the local abundance of calc-alkaline acidic rocks, as sociated with MORE-type lavas in ancient series, could be the tracer of pla te tectonic configurations involving the subduction of short ridge segments in a relatively short duration. (C) 2000 Elsevier Science B.V. All rights reserved.