MIOCENE FOREBULGE DEVELOPMENT PREVIOUS TO THE BROKEN FORELAND PARTITIONING IN THE SIERRAS PAMPEANAS (ARGENTINA), SOUTHERN CENTRAL ANDES

In the south Central Andes, the “Bermejo foreland” constitutes the most outstanding retroarc basin system, developing adjacent to the Argentine Precordillera fold belt. Its synorogenic fill is represented by >10 km thick Miocene-Modern successions, and it is considered an excellent case of cratonward foredeep migration with involvement of basement in deformation during slab flattening. The timing for ideal linked foreland depozones suggests that the load-induced major flexural stage culminated at ~7 Ma. Eastward from the Bermejo foredeep, within the present Sierras Pampeanas broken foreland, no forebulge has been explicitly recorded, although slow and protracted exhumation can be suggested to imply an exposed forebulge. The lack of a previously recorded stratigraphy drove interpretations of underfilled stages for the early development of the Bermejo basin (asymmetric stage) precluding deposition within the distal foreland. This work describes and interprets a superbly exposed <0.4 km thick stack of composite paleosoils and interbedded low-gradient fluvial deposits (Los Llanos Formation) within the central Sierras Pampeanas, considered to represent a Miocene forebulge stratigraphy. It unconformably onlaps crystalline basement rocks and Permian red beds representing intermediate exposure stages, and underlies Pliocene-Modern coarse conglomerates, linked to present-day broken foreland setting. The facies associations, together to coeval normal faulting, deduced from outcrops and industry seismic lines through the western to easternmost Sierras Pampeanas, suggest forebulge tectonism synchronous with sedimentation. Even considering condensation within the forebulge, cumulative thickness can not explain the generation of accommodation space by flexural models. We propose that overlapping of positive lithosphere flexure and long-wavelength subsidence allowed accommodating of intermittent sedimentation on the forebulge prior to the foreland partitioning. Both mechanisms are consistent with development of a buried forebulge. After the initial asymmetric flexural foreland scenario the Mio-Pliocene broken foreland superposed to the ancient flexural forebulge. Hence the Modern broken foreland can be reworking early forebulge extension.