Eric Tohver

Significance of the Nova Brasilândia metasedimentary belt in western Brazil: Redefining the Mesoproterozoic boundary of the Amazon craton

(� 910 Ma). The NBMB marks the Mesoproterozoic limit of the SW Amazon craton. The discordance of the NBMB to the NNW structural trend of the younger Aguapeo´ belt (200 km SE of NBMB), together with marked differences between the two belts in sedimentary environment, metamorphic grade, and timing of deformation, signify that these two belts are not geologically continuous. The ‘‘Grenvillian’’ deformation recorded by the NBMB belt marks the final docking of the Amazon craton and Paragua craton within the Rodinia framework. The Aguapeo´ belt, in contrast, seems to record only limited deformation internal to the Paragua craton. INDEX TERMS: 8102 Tectonophysics: Continental contractional orogenic belts; 3660 Mineralogy and Petrology: Metamorphic petrology; 1035 Geochemistry: Geochronology; 8025 Structural Geology: Mesoscopic fabrics; 9360 Information Related to Geographic Region: South America; KEYWORDS: Rodinia, Grenville mobile belt, Amazon craton, Paragua craton, geochronology, P-T path. Citation: Tohver, E., B. van der Pluijm, K. Mezger, E. Essene, J. Scandolara, and G. Rizzotto (2004), Significance of the Nova ...

Paleogeography of the Amazon craton at 1.2 Ga: early Grenvillian collision with the Llano segment of Laurentia

A paleomagnetic, geochronologic and petrographic study was undertaken on the flat-lying gabbros and basalts of the Nova Floresta Formation of Rondo “ nia state, western Brazil in order to constrain the Mesoproterozoic paleogeography of the Amazon craton. Measurement of the anisotropy of magnetic susceptibility on the gabbroic samples reveals a flat-lying foliation with a radiating pattern of lineations, supporting the field evidence that the gabbros are part of a large, undeformed sill. Petrographic observations of oxides in the gabbros reveals two populations of magnetite grains produced during the original cooling of the sill: large, oxyexsolved titanomagnetite grains and fine-grained magnetite in igneous reaction rims. New 40 Ar/ 39 Ar age dating of biotite and plagioclase yield ages of V1.2 Ga, which represent the rapid cooling following emplacement of the mafic magma. Whole rock dating of basalt samples yields total gas ages of 1062 ˛ 3 Ma, similar to the V1.0 Ga K/Ar ages reported by previous workers. However, the strong compositional dependence of the age spectrum renders this younger whole rock age unreliable except as a minimum constraint. A single magnetic component is found in the basalts, indistinguishable from the characteristic remanence found in the gabbros that is oriented WNW and steeply upward. This magnetization is considered to be primary and was acquired during the cooling of the sill and associated lavas. A paleomagnetic pole calculated from the Nova Floresta Formation (n = 16 sites, Plat: = 24.6‡N, Plong: = 164.6‡E, A95 = 5.5‡, Q = 5), the first reported pole for the Amazon craton for the 1200^600 Ma Rodinia time period, constrains the paleogeographic position of Amazonia at V1.2 Ga. Juxtaposition of the western Amazon craton with the Llano segment of the Laurentia’s Grenville margin causes the NF pole to lie on the 1.2 Ga portion of the combined APWP for Laurentia and Greenland, which indicates that a collision with the Amazon craton could have caused the Llano deformation in early Grenvillian times. fl 2002 Elsevier Science B.V. All rights reserved.

Restored transect across the exhumed Grenville orogen of Laurentia and Amazonia, with implications for crustal architecture

New 40 Ar/ 39 Ar analyses from a transect across the major tectonic units of the southwest Amazon craton document the heterogeneous effects of the late Mesoproterozoic collision with the Grenville margin of North America. Basement rocks of the Amazon and adjacent Paragua cratons mostly preserve pre-Grenvillian ages (older than 1.3 Ga). Localized iso- topic age resetting at 1.18-1.12 Ga is caused by Grenvillian activation of widespread, sinistral strike-slip shear zones in the Amazon basement. In the Nova Brasilandia belt between these two cratons, new 40 Ar/ 39 Ar data record cooling through 920 Ma after the granulite facies deformation of this suture zone. Regional cooling rates calculated from compiled U/Pb, 40 Ar/ 39 Ar, and Rb/Sr thermochronologic data are used to establish post- Grenvillian exhumation patterns for the southwest Amazon and the North American belt. Paleodepths calculated for 1.0 Ga along a transect of the restored 1300-km-wide belt vary from uniformly deep levels (15-30 km) exposed in North America to shallower levels (5- 15 km) observed in the southwest Amazon. We interpret this difference as reflective of a change in tectonic architecture, i.e., thrust-dominated deformation in Laurentia versus strike-slip dominated deformation in the Amazon, with a commensurate variation in crust- al thickness. This interpretation explains the widespread preservation of both pre- Grenvillian ages and collisional ages from the Amazon craton, in contrast with the more homogeneous array of cooling ages from the North American Grenville Province marking the postorogenic extensional collapse of an overthickened crust. The asymmetrical oro- genic architecture from the reconstructed Grenville belt mirrors cross sections proposed for modern orogenic belts where deep-crustal rocks are not yet exposed.

Closing the Clymene ocean and bending a Brasiliano belt: Evidence for the Cambrian formation of Gondwana, southeast Amazon craton

We report new paleomagnetic and geochronological data from Ediacaran rift-drift carbonates in the Paraguai belt at the southern end of the suture zone between the Amazon craton and the Sao Francisco and Rio de Plata cratons, South America. Early thrusting resulted in remagnetization ca. 528 ± 36 Ma or later; the mean age is established by 40 Ar/ 39 Ar encapsulation dating of mixed authigenic and detrital illite from remagnetized carbonates from the unmetamorphosed fold-thrust belt. This remagnetization overlaps with a 525 Ma Gondwana reference pole. Metamorphic illite from the slate belt yields 40 Ar/ 39 Ar ages of 496–484 Ma, the timing of peak regional metamorphism. Oroclinal bending of the Paraguai belt was caused by a 90° clockwise rotation of the east-west limb after ca. 528 Ma, probably refl ecting the irregular margin of the southeast Amazon craton. The age of the Paraguai belt overlaps with that of the Pampean orogeny farther south along the western margin of the Rio de Plata craton, suggesting a coeval closure for the Clymene ocean separating the Amazon craton from the Sao Francisco and Rio de Plata cratons.

Late Permian-Triassic magmatic evolution in the Jinshajiang orogenic belt, SW China and implications for orogenic processes following closure of the Paleo-Tethys

Growth of continental crust involves the complex interplay of subduction zone magmatism, to generate the crust, followed by stabilization through crustal thickening, magmatism and ultimately isolation from the active plate margin. The Jinshajiang orogenic belt, SW China, provides an exceptional record of continental development as the result of closure of the Paleo-Tethys seaway and ensuing collision. A compilation of U-Pb age and geochemical data for the plutonic and volcanic rocks within the southern part of the Jinshajiang orogenic belt, including new high-precision ages for the Ludian granitoid batholith of 231 to 220 Ma, enables us to explore the interaction between magmatism and orogeny in the context of the Paleo-Tethys closure and continental amalgamation. These age and geochemical constraints, in conjunction with other geologic evidence, suggest that subduction of the Paleo-Tethys ocean dominated local tectonics prior to the Triassic, creating a volcano–plutonic arc along the eastern margin of the Qamdo-Simao terrane. Following consumption of the ocean, collision zone magmatism, dated at 247 to 237 Ma, was manifested by eruption of voluminous volcanic rocks in a suture-parallel zone. Crustal anatexis was contemporaneous with the earliest phases of collision, producing high-silica rhyolites of Early Triassic age (ca. 247-246 Ma). Between 245 and 237 Ma, the local tectonic regime switched from compression to extension, probably due to strain partitioning caused by oblique convergence, which led to the development of rift-basins and extensive syn-tectonic bimodal volcanism associated with deep-water sediments. From 234 Ma to 214 Ma, the emplacement of high-K, calc-alkaline granodiorites-monzogranites occurred prior to, or during, isostatic uplift and extension, probably caused by breakoff of the subducted slab. The resultant exhumation brought deep-seated granitoid batholiths to the surface, and was contemporaneous with intrusion of ultramafic-mafic melts. Ophiolitic mélange (ca. 362-294 Ma) and collision-related magmatic suites (247-214 Ma) are unconformably overlain by a Late Triassic (229-217 Ma) conglomerate-rich sequence that represents an overlap assemblage, across the Qamdo-Simao terrane (Indochina) and Yangtze Block of South China.

Late Mesoproterozoic Deformation of SW Amazonia (Rondônia, Brazil) : Geochronological and Structural Evidence for Collision with Southern Laurentia

Proposed assembly of the Rodinia supercontinent in the late Mesoproterozoic involved the collision of the Amazon craton with some portion of the southern or eastern margin of cratonic North America. Previously reported paleomagnetic data from the SW Amazon craton suggest a paleogeographic link between “Grenvillian” deformation of the SW Amazon craton and late Mesoproterozoic tectonometamorphism in southern Laurentia. A structural, geochronological, and petrological investigation of the western Amazon basement rocks (Rondônia, Brazil) was carried out in order to document evidence of a Grenvillian collision connecting the Amazon to Laurentia. Integration of 40Ar/39Ar data and feldspar thermometry data from regionally extensive strike‐slip mylonitic shear zones (Ji‐Paraná shear zone network) indicates that deformation took place at 450°–550°C between 1.18 and 1.15 Ga. An older, ca. 1.35‐Ga event found exclusively in less‐deformed basement rocks is interpreted as recording cooling from an earlier metamorphic episode (650°–800°C indicated by feldspar thermometry) unrelated to the Grenville collision. The style of deformation in the SW Amazon craton contrasts with that observed in southern Laurentia, where extensive crustal thickening accommodated by deep‐seated thrust sheets resulted in widespread thermal resetting of isotopic systems during exhumation and postorogenic cooling. In contrast, the predominantly strike‐slip activity observed in the Amazon resulted in age resetting through strain‐induced recrystallization, not regional‐scale thermal resetting. Consequently, the ages recorded by hornblende in the SW Amazon craton are slightly older than the cooling ages preserved in southern Laurentia. Differences in structural style and geochronological record are interpreted as indicative of an exhumed, asymmetric crustal structure similar to that of modern orogens.

New Late Permian paleomagnetic data from Argentina: Refinement of the apparent polar wander path of Gondwana

The Late Paleozoic–Early Mesozoic apparent polar wander path of Gondwana is largely constructed from relatively old paleomagnetic results, many of which are considered unreliable by modern standards. Paleomagnetic results derived from sedimentary sequences, which are generally poorly dated and prone to inclination shallowing, are especially common. Here we report the results of a joint paleomagnetic-geochronologic study of a volcanic complex in central Argentina. U-Pb dating of zircons has yielded a robust age estimate of 263.0 +1.6/−2.0 Ma for the complex. Paleomagnetic analysis has revealed a pretilting (primary Permian) magnetization with dual polarities. Rock magnetic experiments have identified pseudo-single domain (titano)magnetite and hematite as the mineralogic carriers of the magnetization. Lightning-induced isothermal remagnetizations are widespread in the low-coercivity magnetic carriers. The resulting paleomagnetic pole is 80.1°S, 349.0°E, A95 = 3.3°, N = 35, and it improves a Late Permian mean pole calculated from a filtered South American paleomagnetic data set. More broadly, this new, high-quality, igneous-based paleomagnetic pole falls between the previously distinct Late Permian segments of the Laurussian and Gondwanan apparent polar wander paths, suggesting that the long-recognized disparity between these large paleomagnetic data sets may be primarily due to the inclusion of low-quality or systemically biased data.

Episodic Remagnetizations related to tectonic events and their consequences for the South America Polar Wander Path

Abstract The South American record of remagnetizations is linked to specific events of its tectonic history stretching back to Precambrian times. At the Ediacaran–Cambrian time interval (570–500 Ma), the final stages of the western Gondwana assemblage led to remagnetization of Neoproterozoic carbonates within the São Francisco–Congo Craton and at the border of the Amazon Craton, along the Araguaia–Paraguay–Pampean Belt. From the late Permian to early Triassic, the San Rafaelic orogeny and the emplacement of the Choiyoi magmatic province was responsible for widespread remagnetizations in Argentina and Uruguay. Cretaceous remagnetization has also been documented in Brazil and interpreted to result from magmatism and fault reactivations linked to the opening of the South Atlantic Ocean. We present a review of these widespread remagnetization events principally based on palaeomagnetic data and, when available, on rock magnetic and radiogenic isotope age data. This study gives an overview of the geographical distribution of the remagnetization events in South America, and provides important clues to better understand the geodynamic evolution of the South American plate at these times. In addition, magnetic mineralogy data for the different case studies presented here constrain the physical–chemical mechanisms that led to partial or total resetting of magnetic remanences in sedimentary rocks.

On the development of zones of reverse shearing in mylonitic rocks

Most shear zones contain a minor proportion of domains (recognizable from thin-section to outcrop scale) which show an opposing shear sense relative to the overall kinematic framework. In this paper, we discuss how these universal features result from the evolving deformation/strain partitioning that generally accompanies progressive general shear, principally in low metamorphic grade shear zones. Because the rheologic properties and, consequently, the contributions of the different deformation mechanisms change continuously with progressive strain, an individual domain with a predominant deformation mechanism much faster or slower than the average may depart from the general kinematics, enhancing the viscosity contrast between adjacent layers and causing locally reverse shear in the domain boundaries. Care is needed, therefore, in interpreting opposing shear senses in the same shear zone as indicating superimposed deformation events.

Comment on was there an ediacaran clymene ocean in central south america by u. g. cordani and others

The existence of Gondwana is a subject of historical interest for geologists, since it forms the basis for early scientific theories on the mobility of continents. Textbook examples of the common Gondwanan origin of the southern continents includes the Paleozoic ubiquity of Glossopteris flora and fauna such as Mesosaur, as well as the common paleoclimate history of this supercontinent. However, the facts regarding the assembly of the southern continents into Gondwana are still controversial, as the recent article by Cordani and others (2013) reminds us. The authors describe their view that the supercontinent was assembled by 600 Ma by the closure of the Goiás and Pharusian seas. This vision is opposed to that put forward recently by Trindade and others (2006) and Tohver and others (2010, 2012a) who suggest a more piecemeal assembly of the West Gondwana, where the final configuration was completed in Cambrian times after the closure of the Clymene Ocean. The Amazon-West African craton is widely considered to have formed part of the Rodinia supercontinent, sutured to Laurentia by the Grenville-aged collisional belts. The separate drift history of the central Gondwanan blocks recorded by paleomagnetism suggests that major cratons such as the São Francisco-Congo and Rio de la Plata blocks were independent of Rodinia, separated by an oceanic tract (Cordani and others, 2003; Trindade and others, 2006; Tohver and others, 2006). The major point of contention regards the timing of closure of this ocean basin, and the significance of tectonic activity that occurs in central Brazil, where the final suture between the cratonic blocks should be located. The Goiás sea is located between the Goiás magmatic arc and the São Francisco Craton, being closed at the Ediacaran (fig. 1-upper). The Clymene Ocean is located between the Amazon craton and the rest of Gondwana, being closed at the Cambrian (fig. 1-lower). The purpose of this comment is to place recent findings into the context of West Gondwana’s formation, and to highlight the facts that contradict the more traditional view defended by Cordani and others (2013). The main points raised by Cordani (i-iv in italics below) in refuting the presence of an Ediacaran to Cambrian seaway in central Brazil were: