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Artículo de publicación ISISin acceso a texto completoThe Carboniferous-early Permian Santo Domingo complex in coastal Chile (33.5 degrees S) preserves magmatic structures that allowed us to partially reconstruct and compare the deformation histories of two intrusive units within a mid-upper crustal zoned pluton. The oldest history is preserved in the Punta de Tralca tonalite, where microgranitoid enclaves record the emplacement and partial assimilation of mostly mafic magma into an intermediate host. Enclaves record early foliation development by a mechanical sorting and alignment of minerals during hypersolidus flow in melt-rich magma currents, followed by diffusion creep and sliding along melt-coated crystals. Structures in a weaker, tonalitic matrix record compaction, flattening, and near-solidus deformation as porous flow, aided by brittle deformation, drained residual melts. These processes produced penetrative S > L fabrics (i.e., planar more dominant that linear fabric) in an increasingly viscous, crystal-rich mush and promoted folding, fracturing, shearing, and crystal-plastic deformation as the mush approached its solidus. The deformation disrupted igneous layering and helped mobilize and concentrate melt-rich aggregates, forming diffuse patches and dikes that intruded previously deformed enclaves and matrix and aided pluton differentiation. A different deformation history is recorded by the Estero Cordoba dike, which intruded and interacted comagmatically with the Punta de Tralca tonalite. The dike records how magma flow near stiff boundaries resulted in velocity gradients that drove deformation during magma replenishment. This deformation reset inherited enclave fabrics, increased ductile stretching and winnowing, and formed linear (L > S) fabrics. This example illustrates how different styles of deformation assisted magma movement through a mid-upper crustal magma chamber and highlights the diverse origins and significance of structures generated by deformation in magmas of variable crystal-melt ratios.National Science Foundation EAR-06359

The beginning of the Laramide orogeny is a pivotal time in the geological development of the western United States, but the driving mechanism responsible for mountain building, basin formation and ore mineralization is controversial. Most prominent models suggest this event was caused by the collision of an oceanic plateau with western North America at ca. 88 Ma which caused the angle of subduction beneath the continent to shallow. This subhorizontal (flat) subduction is thought to have led to shut-down of the arc, crustal cooling, and the formation of deep, basement-involved thrust faults that penetrated far into the continental interior. In contrast to these predictions, we show that the arc experienced a magmatic surge, the lower crust was hot (835-750°C) and partially molten from 90 to 70 Ma, and cooling occurred after 75 Ma. These data contradict plateau underthrusting as the driving mechanism at 90-80 Ma; therefore, the Laramide orogeny cannot have been initiated by flat-slab subduction. We propose that the Laramide orogeny is best explained as a two-stage orogeny consisting of a syn-magmatic phase at 90-75 Ma, and a widespread mountain building phase at 75-50 Ma. Only that latter phase is linked to flat-slab subduction.

The Laramide orogeny is a pivotal time in the geological development of western North America, but its driving mechanism is controversial. Most prominent models suggest this event was caused by the collision of an oceanic plateau with the Southern California Batholith (SCB) which caused the angle of subduction beneath the continent to shallow and led to shut-down of the arc. Here, we use over 280 zircon and titanite Pb/U ages from the SCB to establish the timing and duration of magmatism, metamorphism and deformation. We show that magmatism was surging in the SCB from 90 to 70 Ma, the lower crust was hot, and cooling occurred after 75 Ma. These data contradict plateau underthrusting and flat-slab subduction as the driving mechanism for early Laramide deformation. We propose that the Laramide orogeny is a two-stage event consisting of: 1) an arc ‘flare-up’ phase in the SCB from 90-75 Ma; and 2) a widespread mountain building phase in the Laramide foreland belt from 75-50 Ma that is linked to subduction of an oceanic plateau.

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