Brazilian Passive Margin Research Articles

Extensional exhumation of cratons: insights from the Early Cretaceous Rio Negro–Juruena belt (Amazonian Craton, Colombia)

Abstract. This study presents results from apatite fission track (AFT) thermochronology to investigate the thermal history and exhumation dynamics of the Rio Negro–Juruena basement, situated within the western Guiana Shield of the Amazonian Craton. AFT dating and associated thermal history modeling in South America has largely been restricted to the plate's margins (e.g., Andean active margin, Brazilian passive margin, and others). Our paper reports on low-temperature thermochronological data from the internal part of the western Guiana Shield for the first time. This area is part of a vast cratonic lithosphere that is generally thought to be stable and little influenced by Mesozoic and Cenozoic tectonics. Our data, however, show AFT central ages ranging from 79.1 ± 3.2 to 177.1 ± 14.8 Ma, with mean confined track lengths of ca. 12 µm. Contrary to what might be expected of stable cratonic shields, inverse thermal history modeling indicates a rapid basement cooling event in the early Cretaceous. This cooling is interpreted as a significant exhumation event of the basement that was likely driven by the coeval extensional tectonics associated with back-arc rifts in the Llanos and Putumayo–Oriente–Maranon basins. The extensional tectonics facilitated both basement uplift and subsidence of the adjoining basins, increasing erosional dynamics and consequent exhumation of the basement rocks. The tectonic setting shifted in the late Cretaceous from extensional to contractional, resulting in reduced subsidence of the basins and consequential diminishing cooling rates of the Guiana Shield basement. Throughout the Cenozoic, only gradual, slow subsidence occurred in the study area due to regional flexure linked to the Andean orogeny. Comparative analysis with low-temperature thermochronology data from other west Gondwana cratonic segments highlights that exhumation episodes are highly controlled by tectonic inheritance, lithospheric strength, and proximity to rift zones. This study underscores the complex interplay between tectonic events and the response of cratonic lithosphere over geological timescales and highlights extensional settings as an important geological context for craton exhumation.

Cenozoic tectonic reactivation and its implications for landscape transience in southeastern Brazil

AbstractIntraplate settings far from tectonic plate boundaries are normally thought to be tectonically inactive regions in which the landscape evolves via the slow downcutting of fluvial systems. When identified, tectonic reactivations are thought to be of too low an amplitude and not cause any change in the landscape. However, in the absence of datable markers of tectonic deformation, the landscape can be used as a passive marker of intraplate tectonic activity. In this study, we investigate the northern boundary of the Ponta Grossa Arch in the southeastern portion of the Brazilian passive margin. We mapped and studied the spatial and temporal (relative) relationships between brittle structures and geomorphic markers of base level changes. Drainage anomalies (knickpoints and high ksn values) are correlated with structural/tectonic lineaments in which we recognize strike‐slip and transtensional faults with right‐lateral and left‐lateral kinematics. Through mapping and analysis of structural data, we constrained the underlying structural framework. The fault data demonstrate the counterclockwise rotation of the maximum stress field, which moved from the NE–SW orientation (at the Cretaceous‐Paleogene boundary) to N‐S to reach the current WNW‐ESE position. Together, the data suggest that, though subtle and low‐magnitude, the evolution of intraplate stress field forms a dynamic forcing of intracontinental landscape change.

The influence of cenozoic magmatism on drainage rearrangement processes of the northeast sector of the Borborema Highlands, northeastern Brazil

The diversity of structures and tectonic regimes in the Borborema Province caused the various morphostructural compartments to follow different evolutionary pathways throughout and after the fragmentation of Gondwana. In the northern Borborema Highlands, post-rift evolution occurred in a differentiated way, and the present study aims to understand how this occurred. For this, a set of morphological (geomorphological mapping, mapping of drainage anomalies, and extraction of relief lineaments) and morphometrics indexes (hypsometric curves, Basin Asymmetry Factor, Hack Index, Concavity Index, and χ Index) have been applied to drainage watersheds and low-order channels. The results obtained allowed us to propose a post-rift evolutionary model. To the north, the Highlands presents a recent divide resulting from a series of Cenozoic tectonomagmatic reactivations that promoted river rearrangement processes, such as drainage direction reversal and relief inversion—the current conditions of semiaridity and the absence of epigeny cause this drainage divide to remain stationary. The results diverge from the idea that only high relief margins were subject to greater tectonic control. Despite its modest altitudes, the transforming equatorial margin of Northeastern Brazil exhibits a significant recurrence of morphotectonic reactivation events, operating at different space-time scales. The area is also one of the sectors of the Brazilian passive margin with the highest record of modern seismicity.

Andean orogenic signature in the Quaternary sandy barriers of Southernmost Brazilian Passive Margin – Paradigm as a source area

Multiple source provenance of sediments and submarine fan formation are herein investigated based on Quaternary sandy barriers of the Brazilian Southernmost Coastal Plain. LA-ICP-MS dating on 1625 detrital zircons from marine and aeolian facies sands of four successive lagoon-barrier systems are analyzed. The characterization of Archean to Pleistocene zircons into a younger Andean (22 ± 4 Ma to 1 ± 1 Ma, 48 from 1625), Mesozoic to Paleozoic, and Mesoproterozoic (479 from 1622) populations suggest that the main feeding of sediments to the coastal plain occurred through the La Plata River drainage system. The significant contribution of sediments is transported from the mouth of La Plata River northward by longshore circulation (littoral drift). Minor contributions are also recognized as. a farther source associated with the Patagonia drainage and nearby source related to the Uruguay/Rio Grande do Sul Shield and the Paraná Basin, drained by the Camaquã and Jacuí rivers. The latter one is recognized by the contribution from Neoproterozoic to Early Paleozoic, and some Paleoproterozoic and Archean zircon grains. The definition of the sources of clastic sediments allows inferences about the origination of Rio Grande Fan where both the cold Falkland and the warm Brazil currents played a major role.

South Atlantic passive margin evolution: A thermochronology case study from the Rio de Janeiro-Três Rios section, SE Brazil

The southeastern Brazilian passive margin records a complex post-rift evolution, with two parallel high-elevation features formed after the opening of the South Atlantic. We applied apatite fission track (AFT) and U–Th/He (AHe) low temperature thermochronology to constrain the thermo-tectonic history of the Serra do Mar escarpment in the area of Rio de Janeiro state. New AFT central ages for basement areas collected from a N–S transect orthogonal to the shoreline between the cities of Rio de Janeiro and Três Rios, range between 98.5 ± 5.3 and 54.1 ± 4.2 Ma, with mean track lengths between 12.34 ± 0.40 and 14.63 ± 0.17 μm. Uncorrected AHe ages lie between 68.1 ± 5.9 and 60.2 ± 7.3 Ma and are consistent with AFT results. Inverse thermal history models constrained by AFT and AHe data imply the earliest cooling onset from the Barremian (Early Cretaceous), with steady rates more common for samples closer to coastal areas. Maximum depths of denudation are between 2.5 and 4.5 km. Published thermochronological data from adjacent areas combined with the new results show a seemingly simpler post-rift evolution for the area, although suggesting structural control of age distribution and exhumation.

Tectonic and stratigraphic evolution of southern Jacuípe Basin (Brazil) based on seismic sequence stratigraphy

Jacuípe Basin, located at the northeastern Brazilian passive margin, has great hydrocarbon potential, but is considered a new frontier basin due to the lack of integrated studies. The present paper aims to comprehend the history of the basin based on the interpretation of several 2D seismic lines and well data. No pre-rift strata were detected. Within the rift supersequence, which overlies the crystalline basement of the São Francisco craton, four unconformity-bounded third-order sequences (labeled Rift 1 to 4) were recognized. Rift sequences 1 and 2 have scattered depocenters, reflecting the process of synthetic and antithetic fault installation and linkage. Rift sequence 3 has a widespread record throughout the basin. Rift sequence 4 records the ending of mechanical subsidence with minor faulting, and is followed by an uplift which caused the initial continental shelf to be exposed during most of the subsequent depositional events. The drift supersequence was split in two phases or stages. The first drift stage has sedimentation confined to the continental slope due to the uplift of the shelf region, whereas during the second drift stage sedimentation surpasses the border fault and the sedimentary succession overlies directly the rift supersequence. A structural fault map was built, depicting main faulting controls and trends, basin hinge line, main depocenters, the Jacuípe External High and a volcanic plug. The paper also clarified the still controversial southern geologic limit of the basin, and closes with a suggestion of an updated chronostratigraphic chart.

Relief evolution of the Continental Rift of Southeast Brazil revealed by in situ-produced 10Be concentrations in river-borne sediments

This study aims to quantify the denudation dynamics of the Brazilian passive margin along a segment of the Continental Rift of Southeast Brazil. The denudation rates of 30 basins that drain both horsts of the continental rift, including the mountain ranges of the Serra do Mar (seaside horst); and the Serra da Mantiqueira (continental horst); were derived from 10Be concentrations measured in sand-sized river sediment. The mean denudation rate ranges from 9.2 m Ma−1 on the plateau of the Serra do Mar to 37.1 m Ma−1 along the oceanic escarpment of the Serra do Mar. The seaward-facing scarps of both mountain ranges exhibit mean denudation rates that are approximately 1.5 times those of the inland-facing scarps. The escarpments of the horst nearer to the ocean (Serra do Mar) exhibit higher denudation rates (mean 30.2 m Ma−1) than the escarpments of the continental horst (Serra da Mantiqueira) (mean 16.5 m Ma−1). The parameters that impact these denudation rates include the catchment relief, the slope gradient, the rock and the climate. The incongruent combination of a mountainous landscape and moderate to low 10Be-based denudation rates averaging at ∼20 m Ma−1 suggests a reduction in intraplate tectonic activity beginning in the Middle Quaternary or earlier.

Changing provenance of Late Jurassic to Early Cretaceous rift-related sedimentary rocks of the South Atlantic Margin: LA-MC-ICPMS U–Pb and Lu–Hf isotopic study of detrital zircons from the Camamu Basin, Eastern Brazil

The Camamu basin is related to the breakup of Gondwana and belongs to the group of sedimentary basins that are part of the eastern Brazilian passive margin. We have determined the provenance of the precursor sediments by investigating the petrography of fluvial and eolian units from the early-rift Brotas Group and syn-rift Almada Group, coupled to in situ laser ablation MC-ICPMS U–Pb and Lu–Hf isotopic analyses of the detrital zircon grains. Detrital zircon grains from the Sergi Formation, Brotas Group, have a main population of U–Pb ages of 480–600Ma with smaller populations of ages of 900–1000Ma, 1900–2100Ma, 2500–2800Ma and 3000–3200Ma. The Almada Group zircon grains are characterized by a distribution of ages predominantly in the Paleoproterozoic and Archean. The Hf TDM model ages of the Brotas and Almada Group zircon grains range from ca. 1.1 to 4.0Ga, and the εHf(t) values vary from −31.4 to +9.6, providing evidence that some detrital zircon grains in the Camamu Basin were derived from juvenile sources but that the majority have reworked crustal sources or formed from juvenile magmatic rocks that were contaminated by older crustal material. Whole rock Sm–Nd isotope data collected previously are largely consistent with the new U–Pb, Lu–Hf zircon data but do suggest that zircon-poor detritus from the Neoproterozoic to Eopaleozoic Brasiliano Araçuaí-West Congo Belt contributed to particularly the Brotas Group sediments more than would be indicated by the zircon data alone. Thus, integrated petrographic, U–Pb, Lu–Hf zircon and Sm–Nd whole rock studies provide very important clues to the provenance history of the initial sedimentation on Camamu basin passive margin, where the early-rift phase is marked by the input of detritus from the Araçuaí-West Congo belt situated south of the basin, whereas syn-rift sedimentation received major contributions from Paleoproterozoic (Transmazonian cycle) to Archean (Jequié cycle) São Francisco craton units, located to the west.

Testing the influence of far-field topographic forcing on subduction initiation at a passive margin

Abstract Despite favourable gravitational instability and ridge-push, elastic and frictional forces prevent subduction initiation from arising spontaneously at passive margins. Here, we argue that forces arising from large continental topographic gradients are required to initiate subduction at passive margins. In order to test this hypothesis, we use 2D numerical models to assess the influence of the Andean Plateau on stress magnitudes and deformation patterns at the Brazilian passive margin. The numerical results indicate that “plateau-push” in this region is a necessary additional force to initiate subduction. As the SE Brazilian margin currently shows no signs of self-sustained subduction, we examined geological and geophysical data to determine if the margin is in the preliminary stages of subduction initiation. The compiled data indicate that the margin is presently undergoing tectonic inversion, which we infer as part of the continental–oceanic overthrusting stage of subduction initiation. We refer to this early subduction stage as the “Brazilian Stage”, which is characterized by > 10 km deep reverse fault seismicity at the margin, recent topographic uplift on the continental side, thick continental crust at the margin, and bulging on the oceanic side due to loading by the overthrusting continent. The combined results of the numerical simulations and passive margin analysis indicate that the SE Brazilian margin is a prototype candidate for subduction initiation.

Impact of salt-related palaeotopography on the distribution of turbidite reservoirs: Evidence from well-seismic analyses and structural restorations in the Brazilian offshore

The dynamic of gravity-driven turbidity currents is strongly influenced by the morphology of the seafloor, formed by topographic highs and lows on the margin along the depositional profile. This paper focuses on the methodology and the main results of a study on the palaeotopographic controls in a turbidite succession, with an application in an oilfield of the Brazilian offshore Campos Basin. This basin is located in the Brazilian passive margin, on which tectonics is partly controlled by halokinesis; the selected reservoirs are confined Turonian to Campanian siliciclastic turbidite systems. Six seismic-lithologic horizons of regional extension were mapped, from the Aptian salt to the Miocene as well as related faults. Four main reservoir-scale units were identified, stratigraphically comprised between two of the regional horizons, and their related surfaces were mapped. The complete ensemble of horizons and faults was used to build a multi-2D geological model. Individual surface restorations performed in every horizon allowed the structural coherence and the unfolding–unfaulting quality to be evaluated. Multi-surface restorations were then carried out in order to determine the related horizon palaeotopography of each reference depositional time. The results of the geological modeling and of the structural restorations indicate that the halokinesis-related listric faults regulated the distribution of the basal reservoirs. Additionally, at the top of the Albian carbonates, a canyon was identified, which, in association with the tectonic structures form the palaeotopographic constraints for the upper reservoir geometry. This work sheds light on the importance of structural restoration in understanding the tectonic-sedimentation interactions and the palaeotopography to the distribution of the studied turbidite reservoirs.

Pliocene-Quaternary fault control of sedimentation and coastal plain morphology in NE Brazil

Although the last major tectonic event in the Brazilian passive margin was the South America–Africa breakup during the Mesozoic, there is pervasive evidence in northeastern Brazil for pronounced faulting since the late Tertiary. The faulting was partitioned between strike-slip and normal-slip and it reactivated Precambrian shear zones as well as generating new structures. A 040–060° -trending fault set and a 300–320° -trending set have strongly influenced both the deposition of alluvial and aeolian sediments and coastal evolution. Vertical throws have attained 260m since the Pliocene, and topographic breaks resulting from cumulative late Tertiary to Quaternary slip have attained 30–40m. Fault control of sediment deposition and coastal morphology may have affected the entire South American passive margin.

Stratigraphie sismique de la marge éocène du Nord du bassin de Santos (Brésil) : relations plate-forme/ systèmes turbiditiques ; distorsion des séquences de dépôt

Abstract Because of a high siliciclastic sediment supply rate, the Eocene Brazilian passive margin shows sedimentary geometries that differ from the classical models. Two orders of sequences or cycles have been identified: 1.5–9 and 12 My. During high rates of accommodation and low sediment supply, at a 12 My time-scale, ramp morphologies occur with well developed transgressive and highstand systems tract and shelf margin wedges, at 1.5–9 My intervals, whereas turbiditic deposits are rare to non-existent. During low rates of accommodation and high sediment supply, at a 12 My time-scale, a slope morphology is built with well-expressed aggradational transgressive and lowstand systems tracts at 1.5 My intervals. Within transgressive and highstand systems tracts, turbiditic sedimentation occurs because of the increasing slope during aggradation of shelfbreak deposits.

NEOTECTONIC MOVEMENTS IN NORTHEASTERN BRAZIL: IMPLICATIONS FOR A PRELIMINARY SEISMIC-HAZARD ASSESSMENT

Northeastern Brazil is located on the Brazilian passive margin within the intraplate part of South America. The region comprises a Precambrian crystalline basement deformed mainly by shear zones, overlain by Cretaceous sedimentary basins formed during the South America-Africa breakup and by Cenozoic units. There are two main sets of faults in the region, trending NE and NW. The maximum horizontal compression deduced from focal mechanisms is oblique to the NE- and the NW-trending faults, in a compressive direction which favors rightlateral and left-lateral strike-slip movement respectively. Widespread faulting from the beginning of the deposition of Barreiras Formation to the Quaternary indicates that the neotectonic period in northeastern Brazil started in the Miocene. Judging from the historical and instrumental seismicity, surface rupture and liquefaction is unlikely anywhere in the region. But the neotectonic record shows widespread evidence of liquefaction, surface faulting and coseismic uplift. Liquefaction in Quaternary alluvial sediments, for example, points to strong paleoearthquakes. The recognition of strong and pervasive seismic faulting from the Miocene to the Holocene indicates that the seismic hazard in northeastern Brazil has been underestimated.

NEOTECTONIC MOVEMENTS IN NORTHEASTERN BRAZIL: IMPLICATIONS FOR A PRELIMINARY SEISMIC-HAZARD ASSESSMENT

Northeastern Brazil is located on the Brazilian passive margin within the intraplate part of South America. The region comprises a Precambrian crystalline basement deformed mainly by shear zones, overlain by Cretaceous sedimentary basins formed during the South America-Africa breakup and by Cenozoic units. There are two main sets of faults in the region, trending NE and NW. The maximum horizontal compression deduced from focal mechanisms is oblique to the NE- and the NW-trending faults, in a compressive direction which favors rightlateral and left-lateral strike-slip movement respectively. Widespread faulting from the beginning of the deposition of Barreiras Formation to the Quaternary indicates that the neotectonic period in northeastern Brazil started in the Miocene. Judging from the historical and instrumental seismicity, surface rupture and liquefaction is unlikely anywhere in the region. But the neotectonic record shows widespread evidence of liquefaction, surface faulting and coseismic uplift. Liquefaction in Quaternary alluvial sediments, for example, points to strong paleoearthquakes. The recognition of strong and pervasive seismic faulting from the Miocene to the Holocene indicates that the seismic hazard in northeastern Brazil has been underestimated.

Seismicity and stresses in the Brazilian passive margin

Abstract Seismicity and stresses along the Brazilian continental margin show different patterns between the northeastern and southeastern regions. In the northeastern margin, earthquakes tend to occur onshore under a strike-slip regime with horizontal compression parallel to the northern coast line. In the southeastern margin, higher seismicity is observed offshore, in areas where the continental crust was highly extended during the South Atlantic rifting in the Mesozoic. Three new events in the southeast continental shelf are shown to have reverse-faulting mechanisms, from waveform modeling of short-period teleseismic P waves. Multiple depth phases (reflections from the top and bottom of the water layer and double reflections in the water layer) could be identified and better constrained the hypocenters to middle and upper crustal depths. In South America, models of intraplate stresses caused by plate boundary forces and spreading effects due to the continental/oceanic crustal transition indicate higher compressional stresses in the SE offshore area, as compared with the continental area, in agreement with the observed higher seismicity and reverse-faulting mechanisms. The combination of regional stresses, local flexural effects from thick sedimentary loads, and a presumably weaker crust from Mesozoic thinning explains the main patterns of seismicity in the northeastern and southeastern Brazilian margins.

Seismic Criteria to Recognize Potential Deep-Sea Sandstones in Brazilian Passive Margin Basins: ABSTRACT

Seismic Criteria to Recognize Potential Deep-Sea Sandstones in Brazilian Passive Margin Basins: ABSTRACT