Dynamic of a lacustrine sedimentary system during late rifting at the Cretaceous‐Palaeocene transition: Example of the Yacoraite Formation, Salta Basin, Argentina

The architecture of lacustrine systems is the result of the complex interaction between tectonics, climate and environmental parameters, and constitute the main forcing parameters on the lake dynamics. Field analogue studies have been performed to better assess such interactions, and their impact on the facies distribution and the stratigraphic architecture of lacustrine systems. The Yacoraite Formation (Late Cretaceous/Early Palaeocene), deposited during the sag phase of the Salta rift basin in Argentina, is exposed in world-class outcrops that allowed the dynamics of this lacustrine system to be studied through facies analysis and stratigraphic evolution. On the scale of the Alemania-Met\ue1n-El Rey Basin, the Yacoraite Formation is organized with a siliciclastic-dominated margin to the west, and a carbonate-dominated margin to the east. The Yacoraite can be subdivided into four main \u2018mid-term\u2019 sequences and further subdivided into \u2018short-term\u2019 sequences recording high frequency climate fluctuations. Furthermore, the depositional profiles and identified system tracts have been grouped into two end-members at basin scale: (a) a balanced \u2018perennial\u2019 depositional system for the lower part of the Yacoraite Formation and (b) a highly alternating \u2018ephemeral\u2019 depositional system for the upper part of the Yacoraite Formation. The transition from a perennial system to an ephemeral system indicates a change in the sedimentary dynamics of the basin, which was probably linked with the Cretaceous/Tertiary boundary that induced a temporary shutdown of carbonate production and an increase in siliciclastic supply

Tectono-sedimentary evolution of the western Corinth rift (Central Greece)

International audienceThe Corinth rift (Greece) is one of the world's most active rifts. The early Plio-Pleistocene rift is preserved in the northern Peloponnese peninsula, south of the active Corinth rift. Although chronostratigraphic resolution is limited, new structural, stratigraphic and sedimentological data for an area >400 km2 record early rift evolution in three phases separated by distinct episodes of extension rate acceleration and northward fault migration associated with major erosion. Minimum total NS extension is estimated at 6.47.7 km. The earliest asymmetrical, broad rift accommodated slow extension (0.61 mm a-1) over >3 Myrs and closed to the west. North-dipping faults with throws of 10002200 m defined narrow blocks (47 km) with little footwall relief. A N-NE flowing antecedent river system infilled significant inherited relief (Lower group). In the earliest Pleistocene, significant fluvial incision coincided with a 15 km northward rift margin migration. Extension rates increased to 22.5 mm a-1. The antecedent rivers then built giant Gilbert-type fan deltas (Middle group) north into a deepening lacustrine/marine basin. N-dipping, basin margin faults accommodated throws <1500 m. Delta architecture records initiation, growth and death of this fault system over ca. 800 ka. In the Middle Pleistocene, the rift margin again migrated 5 km north. Extension rate increased to 3.44.8 mm a-1. This transition may correspond to an unconformity in offshore lithostratigraphy. Middle group deltas were uplifted and incised as new hangingwall deltas built into the Gulf (Upper group). A final increase to present-day extension rates (1116 mm a-1) probably occurred in the Holocene. Fault and fault block dimensions did not change significantly with time suggesting control by crustal rheological layering. Extension rate acceleration may be due to strain softening or to regional tectonic factors

Source-to-Sink (S2S) analysis of a lacustrine system across the K-T boundary: the Yacoraite Formation, Salta rift basin, Argentina

In this contribution, we present a source-to-sink (S2S) analysis of the Late Cretaceous to Early Cenozoic Yacoraite Formation, a typical lacustrine source rock from the Salta rift Basin (NW Argentina). The Yacoraite Formation corresponds to a mixed carbonate-siliciclastic lacustrine sedimentary system, deposited during the sag phase (post-rift) and also records the K-T boundary. An integrated source-to-sink approach was applied using sedimentary, geochronology, geochemical and isotopic datasets at basin scale (ca. 200 x 200 km), to better understand the complex interactions between production, destruction, and dilution processes that characterize the dynamic of organic-rich sediments. We used these results to discuss the high-resolution (time step ca. 0.05-1 Myr) patterns of organic carbon enrichment in a lacustrine system across the K-T boundary. We show that the Yacoraite Formation recorded major climate changes that can be documented in terms of catchment dynamic, erosion processes, carbonate accumulation trends, lacustrine dynamic and source rock quality. The background organic matter corresponds to Type I, dominated by algal growth (mean HI 600-800 mgHC/gTOC, TOC0 1-2 wt.%). The K-T boundary was the climax of a climate change initiated ca. 0.3 Myr before that induced a major change in the catchment weathering processes, which temporally corresponds to the accumulation of poor quality source rock intervals (TOC0 64 0.2 wt.% and HI &lt; 50 mgHC/gTOC) in these series. The location of the K-T boundary is highlighted by a main negative anomaly in \u3b413C of the carbonate deposits in the Yacoraite Formation, as also supported by absolute U-Pb dating of inter-fingered volcanic ashes. It was followed by a major pulse in paleo-productivity, in turn followed by a major pulse in TOC0 (10-15 wt.%) under anoxic conditions. In ca. 0.2 Myr the lacustrine dynamic and the related organic-carbon enrichment resumed to their initial setting, just prior to the preluding K-T boundary climate change. The results suggest that the Yacoraite Formation can be considered as a world-class example to illustrate how the K-T boundary is recorded in lacustrine sediments. In particular, it could be used as reference to address key questions related to cross-scale interactions, feedback loops and temporal dynamics in the sedimentary record

Source-to-Sink analysis of the Plio-Pleistocene deposits in the Suez rift (Egypt)

International audienceWe present a source-to-sink (S2S) study of the Plio-Pleistocene deposits in the Suez rift (Egypt). We used stratigraphic record and quantitative geomorphology to constrain relief evolution in a rift setting from a high-resolution database at basin-scale (~300 km x 100 km) including, digital elevation model, outcrop and subsurface data. The stratigraphic architecture shows five main stages ranging from rift initiation to tectonic quiescence (Oligo-Miocene) plus a post-rift stage (Plio-Pleistocene). We quantified sediment accumulation history and analysed the relationship between catchment and sediment supply for the Plio-Pleistocene (post-rift stage). The results of the source-to-sink study for the post-rift stage were then compared to previous estimations for the main rifting stages. We show that the sediment supply dynamics of the Plio-Pleistocene deposits of the Suez rift records a renewed uplift ca. 5 Myr ago. However, we also show that a major climate shift related to the Pliocene revolution was most probably coeval to reach the magnitude of accumulation observed

Dynamics of microseismicity and its relationship with the active structures in the western Corinth Rift (Greece)

International audienceWe analyse the complete earthquake archive of the western Corinth Rift using both cross-correlations between pairs of event waveforms and accurate differential traveltimes observed at common stations, in order to identify small-scale fault structures at depth. The waveform database was generated by the dense Corinth Rift Laboratory network and includes about 205 000 events between 2000 and 2015. Half of them are accurately relocated using double-difference techniques. The novelty of this relocated catalogue is the integration of the recent westernmost earthquakes due to the extension of the network in 2010 to the western extremity of the Corinth Rift and the consideration of the whole database over more than 15 yr. The total relocated seismicity exhibits well-defined clusters at the root of the main normal faults mainly between 5 and 10 km depth in the middle of the gulf and illuminates thin active structure planes dipping north about 20° under the northern coast. Some seismicity is observed in the footwall of the main active faults, along the West and East Helike faults. We also built a multiplet database based on waveform similarity taking into account cross-correlation coefficients weighted by signal-to-noise ratios. Short-term multiplets are concentrated in the middle of the gulf along the Kamarai fault system, in a 1–2 km thick layer at 6–8 km depth, interpreted as a highly fractured geological layer. They are often associated to slow seismic migration velocities occurring in this zone during strong swarm episodes and are thus likely to be triggered by pore pressure variations. On the other hand, most long-term and regular multiplets are located deeper (7–10 km), under the northern coast, within a layer less than 0.3 km thick. They occur at the border of nearly planar structures with low seismicity rate, which we identify as fault planes, and they may be explained by aseismic slip on the fault surface around them. This supports the existence of an immature structure growing downdip towards the north at the base of the active geological layer, which possibly connects to the ductile middle crust around 15 km depth, as suggested by the occurrence of deeper events in the continuity of the 1995–fault plane. The different migration velocities (from 0.05 km d−1 to several km d−1) highlighted during the western 2014–swarms indicate that both pore pressure and creep diffusion are operating in the fault zone. The fast migrations observed in the Psathopyrgos fault zone, where a slow slip event was detected by dilatometers in 2002, compare with that for creeping faults. To the west, from spatial distribution of events, we show that the Rion–Patras fault connecting the western extremity of the Corinth Rift fault system to the Patras Rift, is dipping around 60° north–west with a rake angle of −115°. Finally, we identified two new areas within the central active zone which may correspond to large scale, locked asperities on active fault surfaces, similar in size to the main asperity broken during the 1995, MW 6.3, Aigion earthquake

Reassessment of the rifting process in the Western Corinth Rift from relocated seismicity

International audienceThe seismic activity in the western part of the Corinth Rift (Greece) over the period 2000– 2007, monitored by a dense network of three-component stations, is analysed in terms of multiplets and high precision relocation using double difference techniques. This detailed analysis provides new insights into the geometry of faults at depth, the nature and the structure of the active zone at 6–8 km depth previously interpreted as a possible detachment, and more generally into the rifting process. The seismicity exhibits a complex structure, strongly varying along the rift axis. The detailed picture of the seismic zone below the rift indicates that its shallower part (at depths of 6–8 km) is 1–1.5 km thick with a complex microstructure, and that its deeper part (at depths of 9–12 km) gently dipping to the north (10–20°) is 0.1–0.3 km thick with a microstructure consistent with the general slope of the structure. Although the nature of this seismic zone remains an open question, the presence of seismicity beneath the main active area, the strong variability of the structure along the rift over short distances and the complex microstructure of the shallower part revealed by the multiplet analysis are arguments against the hypothesis of a mature detachment under the rift: this active zone more likely represents a layer of diffuse deformation. The geometry of the mapped active faults is not well defined at depth, as no seismicity is observed between 0 and 4 km, except for the Aigion Fault rooting in the seismic layer at 6 km depth with a dip of 60°. A distinct cloud of seismicity may be associated with the antithetic Kalithea Fault, on which the 1909 Fokis earthquake (M s = 6.3) may have occurred. The link between the 1995 rupture (M s = 6.2) and the faults known at the surface has been better constrained, as the relocated seismicity favours a rupture on an offshore, blind fault dipping at 30° , rather than on the deeper part of the East Helike Fault. Consequently, the 1995 event is expected to have decreased the Coulomb stress on the East Helike Fault. To explain these seismic observations along with the geodetic observations, a new mechanical model for the rifting process in this region is proposed, involving non-elastic, mostly aseismic uniform NS opening below the rift axis, coupled with the downward and northward growth of a yet immature detachment: the reported GPS rates would mainly result from this deep, silent source, and the seismicity would reveal the detachment position, not yet connected to the ductile lower crust. In such a model, the strong fluctuations of microseismicity would result from small strain instabilities, undetected by continuous GPS and possibly related to pore pressure transients