Mesozoic to Tertiary evolution of the southwestern proto-Pacific Gondwana margin

Abstract

The Andean margin of South America formed part of the proto-Pacific margin of Gondwana, and has been the site of long-lived subduction. The tectonic evolution of the margin has been influenced by the breakup of that supercontinent and interactions with the subduction zone and the subducting proto-Pacific plates. Since Late Jurassic times the margin has experienced two distinct tectonic regimes. The first was principally controlled by extension associated with Gondwana dispersal, resulting in the development of extensional basins along the length of the margin. The second regime, principally controlled by convergence with minor episodes of extension, initiated at different times along the margin and continues today. During this second period the margin experienced major uplift, leading particularly to the development of the Altiplano-Puna Plateau system, as well as numerous fold and thrust belts. The role of subduction along the proto-Pacific margin in contributing to the development of these events is quantitatively examined in the framework of a recently developed kinematic plate model. The absolute velocity of the overriding South American plate, the convergence velocity between the downgoing oceanic slab and the South American plate, and the age of the subducting slab are resolved along the margin back to 170 Ma to determine correlations between these parameters and deformation along the Andean margin. Any single parameter examined in isolation did not produce a correlation with deformation in the overriding plate, indicating that more complex interactions among several factors are required to produce the observed pattern of deformation. The development of extensional basins floored by mafic/oceanic crust was associated with periods when the motion of the South American plate was directed away from the trench in conjunction with the subduction of oceanic crust older than c.50 Myr. Contraction events, such as the development of fold and thrust belts and of the Altiplano-Puna Plateau, were associated with convergence velocities greater than 4 cm/yr. The absolute motion of South America was directed trenchward during these events, although the magnitude of motion was very small. The development of extensional basins that did not progress to the formation of mafic crust was accompanied by diverse conditions along the subduction zone, as were large magmatic fluxes leading to batholith emplacement, indicating that these events are not primarily controlled by any of the parameters investigated. In addition, no correlations could be established related to the structural style of fold and thrust belts.

Whereas plate scale processes control the tectonic regime along the margin, other mechanisms that act within the overall context of the imposed plate setting can affect deformation at more regional or local scales. The Rocas Verdes Basin in southern Patagonia is an unusual setting as it is one of only two extensional basins floored by ocean crust in a string of Mesozoic extensi onal ensialic basins developed along the Andean margin. The Rocas Verdes Basin is interpreted to have undergone different evolutional histories along the length of the basin. Negative (i.e. extensional) convergence rates were present in the southern portion of the basin during opening, while positive convergence rates affected the north, are implicated as contributing factors resulting in different magnitudes of extension.

The Cordillera Darwin metamorphic complex in southernmost Patagonia exposes amphibolite facies kyanite-staurolite metapelitic schist, the highest grade metamorphic rocks in the Andean chain south of Ecuador. Closure of the Rocas Verdes Basin in this area resulted in continental underthrusting beneath the arc, resulting in the amphibolite facies metamorphism. Thrust-controlled exhumation terminated metamorphism. Garnet in metapelitic schist has patchy textures where regions of clear grossular-rich garnet with fine-grained S1 inclusion trails are cut by turbid regions of spessartine-pyrope-rich garnet containing! inclusi ons of biotite, muscovite, plagioclase, and quartz that align with S2 in the matrix. Aqueous micro-inclusions in the turbid regions suggest that garnet recrystallization to form the patchy textures was facilitated by fluid ingress; recrystallization appears contemporaneous with the growth of matrix kyanite and staurolite. Pseudosection modelling in Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) describes a P-T path for rocks of the Cordillera Darwin of nearly isothermal decompression from 12 to 9 kbar at T≈620°C. Along this path, garnet mode is predicted to decrease from a maximum of c.5% to <1%, driving garnet dissolution. In situion-probe U-Th-Pb dating of S2 matrix monazite, texturally associated with the growth of S2 kyanite and staurolite, shows that exhumation of the Cordillera Darwin was underway by 72.6 ± 1.1 Ma. Later granitic intrusions produced contact aureoles including sillimanite bearing migmatites in the adjacent rocks at P≈6 kbar.

The Seno Otway region lies midway between the comparatively well studied regions of Ultima Esperanza in the north, and Cordillera Darwin in the south. The area includes rocks that reflect the two distinct tectonic regimes of the South American margin: there is a complete record from the development of the Jurassic – Late Cretaceous Rocas Verdes marginal basin to the Cretaceous Magallanes foreland basin succession. Sedimentation into the Rocas Verdes Basin consisted of the hemipelagic mudstones and fine-grained sandstones of the Zapata and Canal Bertrand formations. The formations are mudstone dominated with an increasing abundance of sand in the upper portions recording the onset of contraction and basin closure. Thrusting that obducted portions of the mafic marginal basin floor onto the South American continent was contemporary with the development of the Magallanes foreland basin. Sedimentation into this basin included repeated fining upward sequences of in terbedded mudstone and sandstone turbidites called the Latorre forma tion, and the overlying Escarpada Formation, composed of interbedded sandstone, mudstone, and conglomerate. Emergent portions of the Rocas Verdes Basin and Late Jurassic volcanic rock, exposed through thrusting, were eroded to form parts of the Late Cretaceous sedimentary units. The Magallanes fold and thrust belt involved the metamorphic Paleozoic basement in this region. Folding styles are tight to isoclinal near the main obduction thrusts and broaden to become close to open toward the foreland. An accompanying axial planar cleavage is fine-scaled and pervasive in regions of tight folding and is only weakly developed toward the foreland. Metamorphic grade in the region does not exceed greenschist facies. Deformation in portions of the Latorre and Escarpada formations is low enough to allow for the preservation of original sedimentary features, including various paleocurrent indicators. These indicators record the presence of multiple submarine fans in the Magallanes forelan d basin in the Late Cretaceous.

The Andean margin of South America is commonly considered the type example of an ocean – continent convergent margin. Geological variability along its length provides a unique opportunity to validate macroscopic models for lithospheric weakening leading to the formation and destruction of marginal basins. During the Early Mesozoic most of the Andean margin lay below sea level as a series of extensional basins formed above a subduction zone. Cretaceous compression related to intraplate shortening resulted in basin inversion, with highly variable thickening and uplift. Crustal thickening in the area now known as Cordillera Darwin resulted in high-grade metamorphism from tectonic processes commonly associated with regions of continent – continent convergence. Such diversity in the Rocas Verdes Basin thus provides valuable insights into the evolution of a margin perhaps too commonly regarded as uniform and coherent. Basin development and inversion reflect related but distinct extensional and contractional regimes along the length of the Andean margin, and events in the history of the basin are demonstrably linked with plate-scale processes.