Randall D. Forsythe

Pliocene near-trench magmatism in southern Chile: A possible manifestation of ridge collision

Eight shallow-level intrusions and an ophiolite complex have recently been discovered in a remote region along the Pacific coast of southern Chile known as the Taitao Peninsula. The magmatic rocks are found only 10 to 15 km from the buried extension of the Peru-Chile Trench. The ophiolite body includes serpentinized ultramaflc rocks, gabbro, dikes, and an interbedded sequence of volcanic and marine sedimentary rocks. More regionally distributed granodiorite plutons and porphyritic stocks and sills intrude pre-Late Jurassic basement as well as Tertiary marine sequences of the South American forearc. K-Ar ages of 3.5 to 4.0 Ma of the ophiolite and other intrusions suggest that these rocks represent a short-lived pulse of Pliocene magmatism. Directly seaward of the Taitao Peninsula a segment of the Chile Rise bounded by the Tres Monies and Taitao fracture zones collided with the Chilean forearc and was subducted about 2.5 to 4.0 Ma. Because of this good spatial and temporal correlation, the magmatic activity of the Taitao Peninsula is likely to be a direct result of ridge subduction.

Geochemistry of the Taitao ophiolite and near-trench intrusions from the Chile margin triple junction

Abstract The Taitao ophiolite of southern Chile lies 10 km from the buried extension of the Peru-Chile trench, and less than 50 km from the present position of the Nazca/South America/Antarctica triple junction. Plio-Pleistocene radiometric and paleontologic ages indicate its formation during ridge subduction, and an ultramafic rock, gabbro, sheeted dike, volcanic and sedimentary rock psuedostratigraphy suggests formation by typical accretionary processes for oceanic lithosphere. Yet major and trace element data show that mafic dikes and volcanic units are transitional from MORB to IAT, and there are abundant silicic volcanic units of calc-alkaline character that have high LIL element and light REE concentrations relative to oceanic plagiogranites. Sr and Nd isotopic data are consistent with that of modern oceanic suites, even though having a greater internal variability. Silicic volcanic units show the more enriched Sr and depleted Nd isotopic ratios relative to dike and gabbro samples. In addition to chemical distinctions, paleobathymetric data support a shallow water origin for some of the upper volcanic units and, assuming local compensation, suggest crustal thicknesses of continental proportion. In the vicinity of the Taitao ophiolite, and extending some 40 km landward of the plate margin, are a series of silicic stocks, sills, and plutons that were intruded into the forearc at the time of ridge collision and ophiolite generation. These calc-alkaline I-type granitoids are light REE enriched and have Sr and Nd isotopic compositions similar to those of the main volcanic chain 200 km landward. Chemically, some of the silicic intrusions are indistinguishable from volcanic units of the ophiolite. In general, major, trace, REE, and isotopic variations of both the ophiolite and the distributed intrusions are atypical of simple fractionation trends for basaltic liquids. Intermediate to silicic units lie along mixing hyperbolae between Taitao gabbro and either forearc sediment or metamorphic basement on a NdSr correlation diagram, and these two crustal components support, respectively, either a 10–25% or 5–10% assimilation. Shutdown of magmatism, and therefore probably partial melting as well, appears to occur within 40 km of the trench, roughly spanning the depth interval for the disappearance of the plagioclase-lherzolite stability field as the zone of mantle upwelling is overridden by an increasing thickness of continental lithosphere. A deeper and more landward absence of partial melt related to the subducted ridge is supported by the correlation of the shutoff and re-initiation of arc volcanism over the northern and southern trailing edges of the postulated subcontinental asthenospheric window. Here, as well as elsewhere in the circum-Pacific, the general restriction of magmatism related to ridge subduction to near-trench settings supports a shallow (0 to 15 km) shutoff mechanism for adiabatic decompressive melting and a rather abrupt return to single-phase (solid) convective rise of mantle into an evolving asthenospheric window.

Concordant late Paleozoic paleomagnetizations from the Atacama Desert: implications for tectonic models of the Chilean Andes

Abstract Paleomagnetic results are reported from three formations of late Paleozoic age from the northern Chilean Andes of the Atacama Desert. For the first time primary NRM components are resolved for Paleozoic units along the western flank of the central Andes. Pole positions are calculated for the formations, and compared with APW data for cratonic South America. These comparisons reveal that the collecting sites in the northern Domeyko and Almeida Ranges of the central Andes have undergone no paleomagnetically defined rotations or translation with respect to cratonic South America since the time of NRM acquisition, which is likely to have been in the lower parts of the Kiaman Reverse Interval. If growth of the South American lithosphere has involved accretion of exotic microplates they are either likely to be substantially older than units sampled here, or be restricted to more coastal terranes. The results, taken together with other paleomagnetic data from northern Chile and southern Peru which have showed a wide range of discordance in their declinations when compared to each other or APW data, lead to the conclusion that this region of the Andes during the Mesozoic or Cenozoic has not been affected by simple processes of clockwise oroclinal bending from Peru to Chile, nor regionally consistent patterns of block rotations.

Stratigraphy and geochemistry of accreted fragments of the ancestral pacific floor in southern south America

Abstract The outer Pacific archipelago of southern Chile has allochthonous fore arc rocks that were accreted prior to the Late Jurassic along the ancestral Pacific margin of the southern South American sector of Gondwana. The fore arc rocks in the Archipielago madre de Dios (50°-50° 47′S) are divisible into three assemblages: the Tarlton Limestone, a sequence of massively bedded Upper Carboniferous to Lower Permian limestone; the Duque de York Complex, a sequence of flysch-like sedimentary rocks; and the Denaro Complex, an association of pillow basalt, bedded chert, detrital calcareous rocks, and shale. The Denaro Complex is interpreted as a sequence of the late Paleozoic ancestral Pacific floor. The Tarlton Limestone may represent a shallow water carbonate platform also built on a basaltic substrate located within the ancestral Pacific realm. The basalt units of the Denaro Complex have tholeiitic major element patterns similar to those of modern day “MOR” and “within plate” basalts. Chemical data for the Denaro Complex allow for the partial discrimination of biogenic, hydrothermal, hydrogenous, and detrital components within the sedimentary units, and facilitate comparison with modern day sediments as well as with other ophiolite sequences. Chemical data together with stratigraphic and structural field relations suggest that the evolution of the Denaro Complex and Tarlton Limestone involved seafloor spreading, hydrothermal activity, seamount volcanism, carbonate reef development, and convergence of the oceanic realm with the margin of Gondwana.

Late Paleozoic microfaunas from southernmost Chile and their relation to Gondwanaland forearc development

Newly discovered radiolaria and fusulinids of late Carboniferous–earliest Permian age document ages for off-scraped pelagic sequences within the Chilean parts of the Gondwanaland forearc. Well-preserved radiolarians are illustrated and compared with the only other known correlatives from Japan. These findings provide new insight for paleoenvironments of oceanic assemblages within the forearc as well as new constraints for its temporal development.

Destabilization of a 650 km chemical boundary layer and its bearing on the evolution of the continental crust

A mechanism for the production of a chemical change in the mantle, from primordial silicate compositions above the 650 km discontinuity to differentiated compositions below, is reviewed. Some consequences of this are the stabilization of two layer convection with a temperature contrast between the anhydrous mantle solidus and the geotherm which, at 650 km depth, is lower than any other location in the mantle. With thermal contributions from the concentration of the heat producing elements U, Th and K below the 650 km mantle boundary layer and the higher geotherms in the past, widespread or catastrophic melting may have taken place at this location. An episodic breach of this boundary layer by extensive heat and mass transport may have periodically destroyed any simple two-layer convection geometry in the mantle. Such episodic injections of mass and energy into the upper mantle from below may have been the mechanism responsible for episodes of enhanced surface tectonism and thermal activity which appear to be recorded in apparent polar wandering paths and radiometric ages of continental rocks.

Late Paleozoic noncoaxial deformationin the Green Pond outlier, New Jersey Highlands

An analysis of mesostructures in an outlier of Silurian and Devonian rocks which were folded and downfaulted within middle Proterozoic rocks of the New Jersey Highlands reveals a noncoaxial two-stage kinematic and dynamic history of deformation. Mesostructures include disjunctive slaty and spaced cleavages of two orientations and generations, slickenfiber faults, and en echelon vein arrays. Most slickenfiber faults and the predominant cleavage record northwest-southeast-directed shortening approximately normal to fold hinges. The en echelon vein arrays record a transition from northwest-southeast- to northsouth-directed shortening. East-west- to east-northeast-west-south-west- trending cleavage records a near north-south-directed shortening, and a few slickenfiber faults preserve both northwest-southeast and north-northwest-south- south-east slip directions. Crosscutting relations indicate that the direction of maximum shortening and compression rotated clock-wise from northwest- southeast (stage 1) to near north-south (stage 2). The orientations of mesostructures and their timings of development correlate with the two stages of Alleghanian deformation reported northwest of the New Jersey Highlands. The use of slickenfiber faults and en echelon vein arrays for dynamic analysis supports the notion that the two stages of noncoaxial deformation recorded by Alleghanian structures reflect a clockwise shift in the regional or tectonic stress field that operated on the New York structural recess (concave toward craton) during the late Paleozoic. Geometric relations between the mesostructures and macroscopic folds suggest that deformation of the outlier was controlled by northwest-southeast compressive block faulting in the basement during stage 1 and by reactivated left-lateral, oblique basement faulting and north- south compression during stage 2.