Torres Syncline Research Articles

Geology of the cretaceous Paraná Magmatic Province at the central portion of Torres Syncline, Southern Brazil

ABSTRACT Geological maps of tropical areas demand special challenges due to weathering conditions, mainly where relief has low variability. Most part of Paraná Magmatic Province is included in this scenario. An exception to this situation is the southeastern portion of the province, in the state of Rio Grande do Sul, Brazil. The stratigraphic disposition of volcanic units and the characteristics of erosion of each one caused the exposition of four identified plateaus, in which outcrop, from bottom to top: (1) basalts from Torres/Vale do Sol formations; (2) Caxias do Sul dacite; (3) Barros Cassal sequence; (4) Santa Maria rhyolite. The contacts were traced using SRTM images (through altitude and slope maps), integrating previously published semi- to detailed maps and geochemical database from our group. The resulting map (1:250,000) contributes to the volcano-stratigraphic knowledge of this huge province, and allows us to suggest the review of silicic units, initially called Palmas Formation.

Lithosphere of South American intracratonic basins: Electromagnetic and potential field data reveal cratons, terranes, and sutures

A magnetotelluric survey comprising 18 broadband stations disposed along a 450 km-long profile was carried out at the transition between the Chaco-Paraná (CPB) and the Paraná (PB) intracratonic basins in northeastern Argentina. Three-dimensional inversions of the responses show that the CPB and southern PB lithospheres are resistive (~103 Ω m) down to 120 km, but with distinct crustal and upper mantle electrical properties. Also, Bouguer gravity and density anomalies are positive at CPB, whereas they are negative at PB. We associate the CPB lithosphere with the Paleoproterozoic Rio Tebicuary craton and the southern PB lithosphere with an ancient and buried piece of craton, the Southern Paraná craton. Geochemical data of mantle xenoliths from the Cenozoic alkaline/carbonatitic province within the Rio Tebicuary craton suggest a subcontinental lithospheric mantle affected by metasomatic processes, which explains its lower resistivity (reaching values as low as 300 Ω m) and higher density (#Mg = 0.87). In contrast, the Southern Paraná craton is more resistive (>103 Ω m) and less dense, suggesting a de-hydrated, depleted, and thicker craton. These cratons are separated by a crustal conductor (15 to 20 km depth; 1–10 Ω m) that we interpret as a southward continuation of a linear anomaly (Paraná Axial Anomaly) defined in former induction studies within the PB in Brazil. Hence, we redefined the trace of this conductive lineament: instead of bending towards the Torres Syncline, it continues inside the CPB. We propose the lineament to be an Early Neoproterozoic suture zone that controlled the location of maximum subsidence in the intracratonic basins during the Paleozoic. In the Early Cretaceous, the Paraná Axial Anomaly was the site of maximum extrusion and deposition of Serra Geral basalts. This anomaly separates compositionally distinct cratonic lithospheres along its path. Melting of this heterogeneous and enriched mantle created the Paraná igneous province.

Thermal and depositional history of Early-Permian Rio Bonito Formation of southern Paraná Basin – Brazil

The Paraná Basin is a large volcano-sedimentary basin with a complex depositional history from Ordovician to the Cretaceous period. During Permian significant amounts of organic-rich beds accumulated within the Rio Bonito and Irati formations representing deposition in coastal (deltaic and barrier lagoon) and shallow marine environments, respectively. In this work, the thermal history of these beds is examined from 16 vitrinite reflectance analyses of coal beds (Rio Bonito Formation), integrated with previous data on coal rank, and 21 apatite fission-track (AFT) analyses, collected from 17 well profiles in the south of the Paraná Basin. Coal samples showed three different maturity levels. Two with reflectances of 0.4–0.6 and 0.6–0.8%Rr formed by the natural burial history of the basin (the latter is restricted to the Torres Syncline area). In contrast, the third group reached much higher values (1.0–5.0%Rr) related to intrusive igneous rocks. The AFT data show thermal consistency with the reflectance values. The sandstone samples related to the two lower reflectance patterns were partially reset, preserving AFT ages older than the stratigraphic age of the bed. Otherwise, the samples severely affected by the magmatism have Cretaceous and Early Cenozoic ages, but some were much younger than the last magmatic event, implying deep burial before cooling onset. The mean track length distribution of the partially reset samples is short due to the inheritance of tracks preserved from basement cooling history. However, the population of fully reset samples gives crucial information on the maximum temperature reached and the age when cooling started. At these conditions, around 1.0 to 3.0 km of the Paraná Basin sequences would have been removed from the surface of the studied areas by erosion since Cretaceous. Regional variations in the amounts of removed sections were controlled by the structural framework, mostly regional NW-SE and NE-SW oriented structures.

Flow and fragmentation patterns in the silicic feeder system and related deposits in the Paraná-Etendeka Magmatic Province, São Marcos, South Brazil

Voluminous silicic volcanic products from the Paraná-Etendeka Magmatic Province are exposed in the southern Brazil. A regional synformal structure (Torres Syncline) controls a tick sequence of these units in the region of São Marcos (Rio Grande do Sul). Parallel to the axis of Torres Syncline, a NW-SE-trending zone constituted by the alignment of complex structures was interpreted as the feeding system of silicic volcanism, characterized as fissural. Through a combination of field mapping and determination of fractal dimension (FD), we have identified and described distinct intermingled morphological domains, one related to flow processes and one related to fragmentation processes. Vertical flow directions in this zone exhibit two preferential directions: NW, ranging from N272° to N 355°, and NE, varying from N20° to N85°. The systematic repetition and crosscutting relations of domains point towards a cyclicity of flow and fragmentation processes in this zone. This feeder system intrudes previous basaltic and silicic volcanic rocks, the later being composed by banded deposits, interpreted as rheoignimbrites, and proximal volcanic breccias. This set is overlapped by hybrid deposits, lava flows and, finally, massive deposits.The silicic volcanism can be summarized at an early pyroclastic stage, characterized by rheoignimbrites and proximal volcanic breccias. A transition between explosive and effusive events is suggested by the occurrence of hybrid deposits, which gradually give place to lobated lavas. Local alternation with new (possibly) rheoignimbritic deposits suggests oscillatory fragmentation processes at the conduit, related to decompression and volatiles exsolution in an effective permeable system.

Lithostratigraphy and volcanology of the Serra Geral Group, Paraná-Etendeka Igneous Province in Southern Brazil: Towards a formal stratigraphical framework

The volcanic rocks of the Lower Cretaceous Paraná-Etendeka Igneous Province, in Brazil, are grouped in the Serra Geral Group. The province can be chemically divided into low-TiO2, and high-TiO2. In southern Brazil, the low-TiO2 lava pile reaches a thickness of ~1km and is formed of heterogeneous lava packages here divided into four lava formations. Torres Formation (TF) is characterized by chemically more primitive basaltic (>5wt% MgO) compound pahoehoe flow fields; these lavas stratigraphically overly aeolian sandstones of Botucatu Formation and represent the onset of the volcanic activity. Vale do Sol Formation (VSF) groups vertically stacked sheet-like rubbly pahoehoe basaltic andesites (SiO2>51wt%; MgO<5wt%). These lavas covered the former basalts in the Torres Syncline axis and pinch out towards southwest and represent the most voluminous mafic lava flows. Dacites and rhyolites of Palmas Formation (PF) overlay VSF flows in the central and eastern outcrop area and rest directly upon TF lavas in the west. The acidic units were emplaced as lava domes and widespread tabular lava flows. Esmeralda Formation (EF) is the upper stratigraphic unit and it is formed by a basaltic pahoehoe flow field emplaced during the waning phase of volcanic activity of the low-TiO2 lava sequence. Sedimentary interbeds are preserved throughout the whole lava pile and were deposited during quiescence periods of volcanic activity, and represent important stratigraphic markers (e.g. TF-VSF contact). The newly proposed stratigraphy provides promptly recognized stratigraphic units in a regional framework of fundamental importance for future correlations and provide vital information in the understanding of how the Paraná-Etendeka Igneous Province evolved through time.

Mafic subvolcanic intrusions and their petrologic relation with the volcanism in the south hinge Torres Syncline, Paraná-Etendeka Igneous Province, southern Brazil

The hypabyssal intrusions investigated in this study are located in the east-central region of the state of Rio Grande do Sul, in the south hinge of the Torres Syncline. The intrusions comprise twenty-four dikes and ten sills, intruding in ponded pahoehoe, compound pahoehoe, rubbly and acidic lava flows of the south sub-Province of the Paraná Igneous Province and the sedimentary rocks of the Botucatu, Pirambóia, Santa Maria and Rio do Rasto Formations, on the edge of the Paraná Basin. The intrusive dikes in the flows have preferred NNW-SSE direction and the intrusive dikes in the sedimentary rocks have preferred NE-SW direction. Regarding the morphology, the dikes were separated into two different groups: symmetrical and asymmetrical. The small variation in facies is characterized by fine to aphanitic equigranular rocks. The rocks were divided into two types: Silica Supersaturated Tholeiite (SST) – dikes and sills consisting of plagioclase and clinopyroxene as essential minerals, with some olivine and felsic mesostasis, predominant intergranular texture and subordinate subophitic texture; and Silica Saturated Olivine Tholeiite (SSOT) – dikes consisting mainly of plagioclase, clinopyroxene and olivine, and predominant ophitic texture. The major and trace element geochemistry allows classifying these hypabyssal bodies as basalts (SSOT), basaltic andesites and trachyandesites (TSS) of tholeiitic affinity. The mineral chemistry data and the REE behavior, combined with the LILE and HFSE patterns, similar to the flows and low-Ti basic intrusions of southern Brazil and northwestern Namibia allow suggesting that these dikes and sills were part of a feeder system of the magmatism in the Paraná-Etendeka Igneous Province. The preferred direction of the intrusive dikes in the sedimentary rocks of the Paraná Basin coincides with tectonic-magmatic lineaments related to extensional processes and faulting systems that served as vents for dike swarms parallel to the Brazilian coast, with the same direction as the Namibia coast dike swarm. This suggests that these dikes were part of the triple junction system related to the opening of the South Atlantic Ocean. The preferred directions of the intrusive dikes in the lava flows are similar to the directions of the Ponta Grossa and Rio Grande Arcs and the Torres Syncline. They may have been a part of, or been caused by one or more geotectonic cycles that originated these structures. The emplacement process of the asymmetric dikes suggests they were enclosed under the hydraulic fracture model, since they do not follow a pre-existing fracture filling pattern. The emplacement of the sills conforms to the weakness zones of the sedimentary units. Regarding the intrusive dikes in the flows, divided by lithofacies associations, also taking into account the geochemical and petrographic similarities, it is observed that these dikes are part of a supply system of the basic lava flows, stratigraphically positioned above the host lava flows.

Geochemical and Sr–Nd–Pb isotopic insight into the low-Ti basalts from southern Paraná Igneous Province, Brazil: the role of crustal contamination

ABSTRACTThe south hinge of the Torres Syncline in southernmost Brazil hosts a volcanic succession of pahoehoe and rubbly Gramado-type lavas belonging to the ~132 Ma Paraná–Etendeka Igneous Province. We evaluate the geochemical and Sr–Nd–Pb isotopic variations using local-scale stratigraphy in order to discuss the petrogenesis of lava flows in a single magma type and to estimate the role of crustal contamination and the potential contaminants involved. The geochemical and isotopic variations along the lava pile are not systematic, implying that the magma chamber could have undergone successive replenishments of basaltic magma. The process of crustal assimilation explains the high and widespread initial Sr isotopic ratios at 0.707798–0.715751 and the very low εNd at −8.36 to −5.41, with associated Pb isotopic variations (18.42 < 206Pb/204Pb < 18.86; 15.65 < 207Pb/204Pb < 15.71; 38.62 < 208Pb/204Pb < 39.37). The magmatic evolution of the SCSH and LJ lava flows begins with the storage of mafic liquids during a short period in the shallow-level magma chamber, which allowed the magma ascent with composition of olivine basalts. The continuous fractional crystallization within the magma chamber coupled with variable assimilation degrees of distinct contaminants with Palaeoproterozoic and Neoproterozoic ages, in addition to significant contribution of magma recharge, led to magma ascent with basaltic andesite composition that displays at surface a simple pahoehoe morphology. The continuous magma recharge in the magma chamber coupled with higher assimilation degree allowed the formation of basaltic andesite lavas with more contaminated isotopic signatures that exhibit rubbly morphology at the surface. Differentiation process of liquids coupled with the highest assimilation degrees of distinct contaminants during longer time in a shallow-level magma chamber, which is distinct from that where SCSH and LJ magmas have been stored, led to the formation of andesites of the Morro da Cruz section that exhibit the most contaminated isotopic signatures.

The role of viscosity in the emplacement of high-temperature acidic flows of Serra Geral Formation in Torres Syncline (Rio Grande do Sul State, Brazil)

The acidic flows from Serra Geral Formation in Torres Syncline, Rio Grande do Sul, Brazil, are on the top of a volcanic sequence composed by a complex facies association of compound, simple and rubbly pahoehoebasic flows, acidic lava domes, and tabular acidic lava flows. The origin and emplacement conditions of the acidic volcanic rocks are discussed in this paper based on petrology, on calculated apatite saturation thermometry temperatures, and on estimated viscosity data. The liquidus temperatures for metaluminous rhyodacite to rhyolite samples are about 1,067.5 ± 25oC in average. The viscosity (η) values vary from 105 to 106 Pas for anhydrous conditions, suggesting the emplacement of high-temperature - low-viscosity lava flows and domes. The occurrence of acidic lava domes above simple pahoehoe flows as flow-banded vitrophyres was under low effusion rates, in spite of their high temperature and low viscosities, which are reflected in their small height. The emplacement of lava domes has continued until the eruption of rubbly pahoehoe flows and the geometry of these deposits rugged the relief. Presence of tabular acidic lava flows covering the landscape indicates that it was under high effusion rates conditions and such flows had well-insulated cooled surface crusts. The capacity to attain greater distances and overpass relief obstacles is explained not only by high effusion rates, but also by very low viscosities at the time of emplacement.

Stratigraphical framework of basaltic lavas in Torres Syncline main valley, southern Parana-Etendeka Volcanic Province

The Paraná-Etendeka Volcanic Province records the volcanism of the Early Cretaceous that precedes the fragmentation of the South-Gondwana supercontinent. Traditionally, investigations of these rocks prioritized the acquisition of geochemical and isotopic data, considering the volcanic stack as a monotonous succession of tabular flows. Torres Syncline is a tectonic structure located in southern Brazil and where the Parana-Etendeka basalts are well preserved. This work provides a detailed analysis of lithofacies and facies architecture, integrated to petrographic and geochemical data. We identified seven distinct lithofacies grouped into four facies associations related to different flow morphologies. The basaltic lava flows in the area can be divided into two contrasting units: Unit I - pahoehoe flow fields; and Unit II - simple rubbly flows. The first unit is build up by innumerous pahoehoe lava flows that cover the sandstones of Botucatu Formation. These flows occur as sheet pahoehoe, compound pahoehoe, and ponded lavas morphologies. Compound lavas are olivine-phyric basalts with intergranular pyroxenes. In ponded lavas and cores of sheet flows coarse plagioclase-phyric basalts are common. The first pahoehoe lavas are more primitive with higher contents of MgO. The emplacement of compound pahoehoe flows is related to low volume eruptions, while sheet lavas were emplaced during sustained eruptions. In contrast, Unit II is formed by thick simple rubbly lavas, characterized by a massive core and a brecciated/rubbly top. Petrographically these flows are characterized by plagioclase-phyric to aphyric basalts with high density of plagioclase crystals in the matrix. Chemically they are more differentiated lavas, and the emplacement is related to sustained high effusion rate eruptions. Both units are low TiO2 and have geochemical characteristics of Gramado magma type. The Torres Syncline main valley has a similar evolution when compared to other Large Igneous Provinces, with compound flows at the base and simple flows in the upper portions. The detailed field work allied with petrography and geochemical data are extremely important to identify heterogeneities inside the volcanic pile and allows the construction of a detailed lithostratigraphical framework.

Lithofacies analysis of basic lava flows of the Paraná igneous province in the south hinge of Torres Syncline, Southern Brazil

The Paraná igneous province records the volcanism of the earlier Cretaceous that preceded the fragmentation of the Gondwana supercontinent. Historically, investigations of these rocks prioritized the acquisition of geochemical and isotopic data, considering the volcanic pile as a monotonous succession of tabular flows. This work provides a detailed analysis of the emplacement conditions of these basic volcanic rocks, applying the facies analysis method integrated to petrographic and geochemical data. The Torres Syncline is a NW–SE tectonic structure, located in southern Brazil, where a thick sequence of the Paraná–Etendeka volcanic rocks is well preserved. This study was performed in the south hinge of the syncline, where the basaltic lava flows are divided into three lithofacies associations: early compound pahoehoe, early simple pahoehoe and late simple rubbly. The first lavas that erupted were more primitive compound pahoehoe flow fields composed of olivine basalts with higher MgO contents and covered the sandstones of the Botucatu Formation. The emplacement of compound pahoehoe flow fields is possibly related to intermittent low effusion rates, whereas the emplacement of simple pahoehoe is related to sustained low effusion rates with continuous supply. The thick simple rubbly lavas are associated with high effusion rates and were formed during the main phase of volcanism in the area. The absence of paleosoils between the lavas and lithofacies associations suggests that the successive emplacement of the lava flows occurred in a relatively short time gap. Geochemically, the lithofacies associations are low-TiO2 and belong to Gramado magma type. The lavas of the south hinge of the Torres Syncline have a similar evolution when compared to other Continental Basaltic Provinces with earlier compound flows at the base and thicker simple flows in the upper portions.

Três Forquilhas Valley in Southern Brazil - evidence for the uplift of the volcanic plateau

Os padrões de drenagem e a encosta foram estudados no vale do rio Três Forquilhas que é situado próximo da localidade de Terra de Areia, entre o mar e a borda nordeste do planalto vulcânico, no Rio Grande do Sul. Esta área foi estudada para explicar por que a escarpa do planalto vulcânico é mais próxima ao mar nesta sua por- ção nordeste e estabelecer a história erosional desta porção da escarpa. Aqui dois rios relativamente curtos, os rios Três Forquilhas e Maquiné, seguem a Sinclinal de Torres em direção ao Atlântico. Esses rios, provavelmente, iniciaram a erodir significativamente a sinclinal e a borda do planalto a partir do Mioceno Médio a Superior, em resposta à tectônica Andina de soerguimento tectônico de colisão ao longo da maior parte da América do Sul. A erosão subsequente, provavelmente, ocorreu em pulsos e não em uma fase uniforme em ambos os divisores de águas, muito como as chuvas torrenciais atualmente, que a cada dois ou três anos “impulsionaram” a erosão local da escarpa. O estudo da sequência estratigráfica no Terciário quando da sedimentação da Bacia de Pelotas adjacente à área de estudo, nos ajudou a estabelecer esta história erosiva local (sete discordâncias entre as sucessões de depósitos, variações de espessura entre estas discordâncias e a grande escala de espessuras). O estudo de uma seção transversal do continente em direção ao litoral identificou dois paleovales em rochas do Mesozoico abaixo do nível de base dos atuais rios Três Forquilhas e Maquiné. Há um controle estrutural forte da drenagem em todas as escalas. Essas considerações sobre os processos que se desenvolvem sobre a vertente contribuem, significativamente, para a compreensão sobre a evolução da drenagem e a origem da escarpa do planalto vulcânico.

Morphological and petrographic patterns of the pahoehoe and ´a´ā flows of the Serra Geral Formation in the Torres Syncline (Rio Grande do Sul state, Brazil)

Morphological and petrographic patterns of the pahoehoe and ´a´ā flows of the Serra Geral Formation in the Torres Syncline (Rio Grande do Sul state, Brazil)

Morfologia e padrões petrográficos dos derrames pahoehoe e'a'ā da Formação Serra Geral na Sinclinal de Torres (Rio Grande do Sul)

The Serra Geral Formation volcanic sequence in Torres Syncline (Rio Grande do Sul state, Brazil) may be divided into main valley, intermediate zone, and southern hinge. The first volcanic episode along the syncline is characterized by low eruption rates ( ; 5 - 10 m³/s). The formation of 'a'ā flows occurred on a plain relief. Pahoehoe and 'a'ā flows have similar chemistry, and the difference in the flow type is related to an increase in eruption rates. The 'a'ā lavas are plagioclase and clinopyroxene phyric, or glomeroporphyritic. All present an intergranular or intersertal microcrystalline groundmass of plagioclase, clinopyroxene, and opaque minerals (<< 0.1 mm in diameter). The pahoehoe lavas differ from the 'a'ā ones in that they have a coarser-grained microcrystalline groundmass. The pahoehoe flows are microcrystalline with glomeroporphyritic and diktytaxitic textures and a plagioclase-poor matrix when compared to 'a'ā lava flows. The higher content of microlites in the 'a'ā flows is attributed to undercooling, higher rate of eruption, and degassing before and during emplacement. When it comes to regional stratigraphic correlation between the Serra Geral Formation flows, the morphological type of the lavas should be taken into account since 'a'ā flows, unlike pahoehoe, do not reach longer distances from the source.

Stratigraphy and volcanic facies architecture of the Torres Syncline, Southern Brazil, and its role in understanding the Paraná–Etendeka Continental Flood Basalt Province

The Torres Syncline is a large structure that constitutes the eastmost outcrop of the Paraná–Etendeka CFB in South American side, and this work focuses the stratigraphy and facies architecture of the volcanic pile in the syncline. The volcanic sequence along the study area permits the division of three regions: main valley, intermediate zone and south hinge, each of them with distinct stratigraphy, which probably reflects the structural evolution of the syncline. The stratigraphy of the Torres Syncline is composed by: 1 — Botucatu palaeoerg; 2 — Basic volcanic episode I; 3 — Basic volcanic episode II, 4 — Acidic volcanic I, 5 — Basic volcanic episode III and 6 — Acidic volcanic episode II. The five volcanic episodes recognized in study area can be related to five volcanic facies architecture: compound-braided, tabular-classic, tabular/lobate escoriaceous, dome-field (acidic lavas) and tabular flows (acidic lavas). The basic episode I is composed by pahoehoe flows with a compound-braided facies architecture that covered the Botucatu palaeoerg. The basic episode II is a tabular-classic facies architecture predominantly composed by simple flows (10–20m thick) reaching the total thickness of ~500m in main valley. The acidic episode I is exposed in main valley and south hinge, and is composed by acidic lavas forming lava dome-field facies architecture with a thickness of ~150m. The basic episode III is predominantly constituted by ‘a’ā flows with tabular/lobate escoriaceous facies architecture. The acidic episode II is constituted by tabular flow volcanic facies (acidic flows) and outcrops all along the study area. The Torres Syncline constitute the eastmost on-shore exposures of the Paraná–Etendeka CFB in South American side and detailed stratigraphic, volcanological and structural studies in these area, coupled with correlation with Huab Basin (NW Namíbia, Africa) will aim the understanding of the Gondwana breakup process and the early stages of the South Atlantic margin opening.

Magnetotelluric and geomagnetic depth soundings around the Torres Syncline Hinge, southeast Paraná Basin, Brazil

A MT/GDS survey has been carried out around the NW plunging Torres Syncline in the southeastern border of the Paraná Basin, in Brazil. Induction arrows show a NW‐trending conductive anomaly nearly coincident with the hinge of the syncline and ocean effects varying as a function of the distance from the coast. 2‐D inversions of MT data along two profiles parallel to the coast and across this anomaly have distinguished electrical anomalies in the upper crust. The most prominent of these anomalies is an electrical conduit dipping to the NW from a depth of 500 m at the coast to 1000 m at a distance of about 100 km further inland. The conductor, located in the crystalline basement, can be possibly ascribed to residual metasomatic fluids concentrated along the hinge of the downfolded Torres Syncline, as a fossil record of the Early Cretaceous volcanic events that affected the Paraná Basin and underlying lithosphere.

MAPPING OF HIGH ELECTRICAL CONDUCTIVITY ON THE TORRES SYNCLINE HINGE, SOUTHEASTERN PARANÁ BASIN

A preliminary interpretation of the electrical structures beneath the SE Paraná Basin is presented based on electromagnetic geophysical soundings carried out around the Torres Syncline. GDS induction arrows show a NW-trending conductive anomaly nearly coincident with the hinge of the syncline and strong "ocean effects" observed at longer periods in sites near the coastline. 1 -D inversions of MT data at sites located inside this anomaly have distinguished a large electrical conduit, dipping to the NW from a depth of 500 m at the coast to 1000 m at a distance of about 100 km further inland. The conductor, located in the crystalline basement, could be possibly ascribed to residual metassomatic fluids concentrated along a fractured hinge of the downfolded Torres Syncline, as a fossil record of past magmatic activities in the Early Cretaceous that affected the Paraná Basin and underlying lithosphere.

Mesozoic epeirogeny at the South Atlantic margin and the Tristan hot spot

Epeirogenic structures of the eastern margin of Brazil9s Parana basin seem related to the position of the proposed Tristan hot spot in Early Cretaceous time. Several lines of evidence indicate that the hot spot was located offshore from a major structural depression, the Torres syncline, which is flanked by two structural arches about 750 km apart. Structural relief between the syncline and arches is estimated to be 4 km. Synchronism of the formative epeirogenic movements with proto-Atlantic continental rifting is indicated by the occurrence of a prominent Early Cretaceous basaltic dike swarm on the crest of one of the arches. The Torres downflexure might have originated through volcanic loading of the continental margin prior to formation of the Rio Grande Rise in the adjacent ocean basin.