Sedimentary record of plate coupling and decoupling during growth of the Andes

Abstract

Geochronologic, provenance, and sediment accumulation records from the long-lived (>100 m.y.) retroarc basin at the transition from the central to southern Andes provide improved resolution to examine the duration and controls on mixed-mode deformation and an enigmatic foreland depositional hiatus. Detrital zircon U-Pb ages for the Malargüe and Neuquén basin systems of western Argentina reveal shifts in exhumation and accumulation compatible with magmatic-arc and thrust-belt sources during unsteady Cretaceous-Neogene deformation. Fully developed foreland basin conditions were only achieved during separate periods of Late Cretaceous and Neogene shortening contemporaneous with possible episodes of enhanced coupling between a westward-advancing South American plate and the subducting Nazca slab. Separating these two contractional episodes is a 20-40 m.y. phase of reduced sedimentation and unconformity development, potentially signifying a neutral to extensional mode across the retroarc hinterland to forearc region during diminished plate coupling. We propose that the Andean orogen and its foreland and forearc basins have always been sensitive to variations in subduction dynamics, such that regional shifts in slab buoyancy and subduction geometry (particularly slab dip) superimposed on plate-scale shifts in convergence have governed mechanical coupling along the plate boundary and resulting fluctuations among contractional, extensional, and neutral tectonic regimes.

Tectonic Regimes of the Central and Southern Andes: Responses to Variations in Plate Coupling During Subduction

“…Schematic block diagrams illustrating variable modes of mechanical coupling along the plate interface during orthogonal convergence in Andean‐type subduction boundaries, with associated variations in tectonic regime, magmatism, and sedimentary basin development in the overriding plate (after Heuret & Lallemand, ; Horton & Fuentes, ; Ramos, ). (a) Coupled margin with shortening and thick, coarse‐grained foreland basin fill.…”

“…The importance of this conceptual framework for the Andes concerns the existence and scope of spatially and temporally uneven phases of contrasting tectonic regimes along the plate boundary between South America and the Nazca/Antarctic Plates (e.g., Horton & Fuentes, ; Mpodozis & Cornejo, ; Mpodozis & Ramos, ; Ramos, ). Growth of the Andes by horizontal shortening and attendant thickening of the leading edge of the South American Plate is driven fundamentally by strong coupling at the interface between the downgoing and overriding plates (e.g., Dewey & Bird, ; Jordan et al, ).…”

“…A Late Cretaceous onset of Andean orogenesis is recorded by a shift to nonmarine synorogenic sedimentation in Argentina and Chile (Neuquén Group, Las Chilcas Formation, and equivalent units). Although provenance data show continued derivation from both the arc and craton (Balgord & Carrapa, ; Mescua et al, ; Tunik et al, ), a sharp increase in sediment accommodation accompanied by thrust‐related growth strata and focused exhumation demonstrate the contractional nature of the episode (Fennell et al, ; Folguera, Bottesi, et al, ; Horton & Fuentes, ; Horton et al, ). This initial foreland basin is defined by proximal clastic deposits in the Central Valley (Las Chilcas Formation; Boyce, ; Charrier et al, ) and the Principal Cordillera (Brownish‐Red Clastic Unit; Charrier et al, , ).…”

“…Map of western South America showing subduction plate boundary, oceanic aseismic ridges, zones of flat slab subduction, Andean magmatic arc, deformation front, representative stress orientations, and east‐west orogenic transects (dashed lines N, C, and S) across the central and southern Andes (after Horton & Fuentes, ; Ramos & Folguera, ).…”

“…Exploration of the structural, stratigraphic, and igneous relationships in retroarc to forearc records reveals temporal and spatial deviations in tectonic regime, alternating from the prevailing contractional mode to subordinate neutral or extensional modes. An assessment of possible driving mechanisms for this irregular and episodic behavior points to variable mechanical coupling between the subducting and overriding plates, as influenced by first‐order plate‐scale changes in convergence (e.g., Maloney et al, ; Pardo‐Casas & Molnar, ; Schellart, ; Silver et al, ; Sobolev & Babeyko, ; Somoza & Zaffarana, ) and second‐order regional cycles of plate shallowing and steepening (e.g., Haschke et al, , ; Horton & Fuentes, ; Ramos et al, , ; Ramos & Folguera, ).…”