Spatiotemporal hierarchy of fault systems and associated syn-rift stratigraphic sequences across the Santos Basin, offshore SE Brazil

Abstract

This paper uses high-resolution 2D and 3D seismic surveys tied to stratigraphic data from 155 boreholes in the mapping and hierarchization of 1551 extensional faults across the Santos Basin and its Outer High. These faults were able to control deposition in extensional basins with different ages – they are thus classified into three (3) main systems with the aim of understanding their geodynamic significance. Fault system 1 (FS1) controlled the oldest syn-rift sequence and delimits the very first half-graben basins developed during an Early Rift stage. Fault System 2 (FS2) was formed in response to crustal thinning during an Intermediate rifting stage. Fault System 3 (FS3) responded to the migration of the rift axis towards the distal margin in a Late Rift stage. Strikingly, fault reactivation occurred throughout the entirety of the proximal domain of the Santos Basin during the Late Rift Stage. This work is important as each interpreted fault system is understood to have controlled the availability of accommodation space during its peak activity, thereby shaping the inner proximal, outer proximal, and distal domains of the Santos Basin. The evolution of these fault systems also resulted in the deposition of syn-rift sequences that obey a specific order, or ranking. By correlating fault systems with their adjacent syn-rift strata, we propose 1^st-order sequence boundaries that are associated with each stage of the development of the Santos Basin, formed as a consequence of changes in thermo-mechanical conditions during the evolution of rifted margins. Identified 2^nd-order sequences are attributed to tectonic movements at the crustal-block scale. In contrast, stratigraphic sequences of the 3^rd- and 4^th-orders are associated with localised tectonic inputs. Within this framework, we use stratigraphic surfaces as structural markers across multiple observational scales. Consequently, sequence 1SEQ1 corresponds to the stretching phase, standing for the initial stages of rifting within an intraplate setting. Sequence 1SEQ2 relates to the necking stage and formation of the outer proximal domain, whereas sequence 1SEQ3 accompanied the development of the distal domain in the so-called hyperextension and continental-breakup stages. Ultimately, this work demonstrates that the tectono-stratigraphic evolution of continental margins, from the onset of rifting to continental breakup, is documented by a clear, diachronic tectono-stratigraphic record. Such a record varies along and across hierarchized fault systems that are developed at multiple scales of observation.