Thermochronology of the South American platform in the state of São Paulo, Brazil, through apatite fission tracks

The fission-track method (FTM) in apatite was applied to 45 samples collected in the Serra da Mantiqueira (Mantiqueira mountain range), the Serra do Mar (Mar mountain range), regions next to these mountain ranges and the coastal region between Ubatuba and Santos in the State of São Paulo, Brazil, to study the thermochronology of the South American Platform in southeast Brazil and its influence on Santos and Campos basins. The data presented in this work complement the previously presented data on the same region (Tello Saenz et al., 2003. J. S. Am. Earth Sci. 15, 765-774) with 31 new samples analyzed. The weighted mean of the corrected ages from high Mantiqueira (around 1000 m), (121 +/- 6) Ma, coincides with the South Atlantic opening. The fact that its thermal history starts at a relatively low temperature (similar to 80 degrees C) suggests that the age of similar to 120 Ma would be the formation age of Serra da Mantiqueira due to a rapid pulse, in which tracks had no time to be retained at the closure temperature, that is similar to 120 degrees C. The Serra do Mar presents a more complicated thermal history, with several reactivations indicated by the changes in the slope of its cooling curve. The thermal histories obtained in the regions next to these mountain ranges are compatible with the results mentioned above. The Santos Basin has unconformities that agree with changes in the slope thermal histories of the studied region. (c) 2005 Elsevier Ltd. All rights reserved.

A PC compatible Brazilian software for obtaining thermal histories using apatite fission track analysis

In this work a software developed in the Instituto de Fisica Gleb Wataghin, IFGW, UNICAMP, Campinas, SP, Brazil for obtaining thermal histories using apatite fission track analysis is presented. This software works in Microsoft-Windows environment. It will be freely disposable in the web site of the Departamento de Raios Cosmicos, IFGW, UNICAMP. Thermal histories obtained through this software are compared with those deduced using Monte Trax the software compatible with Apple Macintosh developed by Gallagher. (C) 2001 Elsevier B.V. Ltd. All rights reserved.

The thermal stability of fission tracks in the 612 nist glass standard at room temperature

Two wafers of the NIST (formerly NBS) glass standard SRM 612 recently irradiated have been compared to the pre-irradiated wafers RT3 and RT4 of glass SRM 962-7, stored for 9 years at 5°C, and SRM 962, stored for 20 years at room temperature. Track area densities on internal surfaces of the glass as well as track size measurements suggest that (1) the old SRM 962 and the more recent SRM 962a calibrations are consistent and (2) annealing of the fission tracks in the pre-irradiated wafers is negligible. This last experimental result enables a direct comparison of contemporary and previous fission track age calibrations. © 1995.

Recognition of Cretaceous, Paleocene, and Neogene tectonic reactivation through apatite fission-track analysis in Precambrian areas of southeast Brazil: Association with the opening of the south Atlantic Ocean

Apatite fission-track analysis was used for the determination of thermal histories and ages in Precambrian areas of southeast Brazil. Together with geological and geomorphologic information, these ages enable us to quantify the thermal histories and timing of Mesozoic and Cenozoic epirogenic and tectonic processes. The collected samples are from different geomorphologic blocks: the high Mantiqueira mountain range (HMMR) with altitude above 1000 m, the low Mantiqueira mountain range (LMMR) under 1000 m, the Serra do Mar mountain range (SMMR), the Jundiá and Atlantic Plateaus, and the coastline, all of which have distinct thermal histories. During the Aptian (∼120 Ma), there was an uplift of the HMMR, coincident with opening of the south Atlantic Ocean. Its thermal history indicates heating (from ∼60 to∼80 °C) until the Paleocene, when rocks currently exposed in the LMMR reached temperatures of ∼100 °C. In this period, the Serra do Mar rift system and the Japi erosion surface were formed. The relief records the latter. During the Late Cretaceous, the SMMR was uplifted and probably linked to its origin; in the Tertiary, it experienced heating from ∼60 to ∼90 °C, then cooling that extends to the present. The SMMR, LMMR, and HMMR were reactivated mainly in the Paleocene, and the coastline during the Paleogene. These processes are reflected in the sedimentary sequences and discordances of the interior and continental margin basins. © 2002 Elsevier Science Ltd. All rights reserved.

An automatic method for segmentation of fission tracks in epidote crystal photomicrographs

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Evolução estrutural e termocronológica meso-cenozóica da Zona de Cisalhamento Portalegre, Nordeste do Brasil

The Portalegre shear zone (ZCPa), which is located in the Rio Grande do Norte and Paraíba states (Northeastern Brazil), is na important right-lateral, northeast-trending lineament formed during the Brazilian Orogenic Cicle). The ZCPa experienced na important brittle reactivation from the Mesozoic until the present. This reactivation led to the formation of the Gangorra, Pau dos Ferros, Coronel João Pessoa, Icozinho and Rio do Peixe basins. The reactivation northern parto f the ZCPa that marks the boundary of the Potiguar Basin is denominated Carnaubais Fault. Several fracture patterns were mapped along the ZCPa. Samples were collected in Neoproterozoic granite outcrops, along the ZCPa. These samples yielded AFT ages from 86±13 to 376±57 Ma, and the mean track length from 10.9±0.8 to 12.9±1.5 mm. Samples from the East block yielded mean ages of 103 Ma, mean track lengtn 12,1mm, and mean altitude 250m, whereas samples from West block yielded mean ages of 150 Ma, which reach 345 Ma and 220 Ma in the Pau dos Ferros and Coronel João Pessoa basins, respectively. Thermal history models were sorted out for each crustal block. Samples from West block recorded a thermal history from Carboniferous Period until the Permiano, when the block experienced gradual uplift until the Cretaceous, when it underwent downfaulting and heating until the Tertiary, and it eventually experienced a rapid uplift movement until recent times. Samples from the East block presented the same cooling and heating events, but at they occurred different times. The East block thermal record started ~140 Ma, when this block experienced cooling until ~75 Ma. Both blocks show a denundacion/erosional history more similar in the Tertiary. The AFT data indicate an important tectonic event ~140 Ma, when the West block experienced downfaulting and the East block experienced uplift. This tectonic process led to the generation of several sedimentary basins in the region, including the Potiguar basin. This tectonic event is also interpreted as a rift process caused by an E-W-trending extension. It the Tertiary, some heating events can be tentatively attributed to the macau volcanic event

Uso da termocronologia por traços de fissão em apatita no reconhecimento de áreas de recarga e análises isotópicas de 234U/238U em águas subterrâneas do aquífero Itararé no município de Americana (SP)

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Micro-Raman spectroscopy and SEM/EDX applied to improve the zircon fission track method used for dating geological formations

The zircon mineral is widely studied in geochronology. In the case of the fission track method (FTM), the age is determined by the density of fission tracks at the zircon surface, which can be observed with an optical microscope after an appropriate chemical treatment (etching). The etching must be isotropic at the zircon grain surface to be used in the FTM, which leads those zircon grains whose etching is anisotropic to be discarded. The only reason for this discarding is the nonuniform morphology of the surface grain seen by optical microscopy, that is, no further physicochemical analysis is performed. In this work, combining micro-Raman and scanning electron microscopy (SEM) to study the etching anisotropy, it was shown that zircon grains that present at least one area at the surface where the density of fission track is uniform can be used in the FTM. The micro-Raman showed characteristic spectra of the standard zircon sample either from the areas where there are tracks or from where there are not. The only difference found was in the Raman bandwidths, which were broader for the areas with higher density of fission tracks. This suggests simply a decrease in the relative percentage of the crystalline/amorphous phases at these areas. The SEM/energy dispersive spectrometry (EDX) showed that there were no significant differences in the principal chemical composition at the areas with and without fission tracks. Copyright (c) 2008 John Wiley & Sons, Ltd.

On epidote fission track dating

The use of epidote in fission track dating was abandoned since the beginning of the 1980s due to difficulties like absence of a standard etching procedure, obtainment of different closure temperatures and the percentage of the datable samples. The results become much more reproducible when restricting fission track analysis to a peculiar kind of track. We are also studying confined track length, what makes possible to obtain information about fossil track annealing. Fission tracks in epidote were successfully etched with 48% HF at 35 degrees C for 12.5 min. Dating samples by the external detector method was not possible due to problems in measuring the efficiency factor held between the number of fossil fission tracks and tracks induced in mica. Dating a sample from Brejui, RN, Brazil with the population method gave a corrected age of 510 +/- 69Ma, in agreement with published U/Th-Pb ages. From the fact that the fossil track length histogram was bimodal, we were able to infer that this sample registered a thermal episode during its history.These preliminary results indicate that epidote deserves further studies to establish whether it can be employed as a thermochronological tool. (c) 2005 Elsevier Ltd. All rights reserved.

Low-temperature evolution of the Morondava rift basin shoulder in western Madagascar: An apatite fission track study

[1] The evolution of the rift shoulder and the sedimentary sequence of the Morondava basin in western Madagascar was mainly influenced by a Permo-Triassic continental failed rift (Karroo rift), and the early Jurassic separation of Madagascar from Africa. Karroo deposits are restricted to a narrow corridor along the basement-basin contact and parts of this contact feature a steep escarpment. Here, apatite fission track (AFT) analysis of a series of both basement and sediment samples across the escarpment reveals the low-temperature evolution of the exhuming Precambrian basement in the rift basin shoulder and the associated thermal evolution of the sedimentary succession. Seven basement and four Karroo sediment samples yield apparent AFT ages between ∼330 and ∼215 Ma and ∼260 and ∼95 Ma, respectively. Partially annealed fission tracks and thermal modeling indicate post-depositional thermal overprinting of both basement and Karroo sediment. Rocks presently exposed in the rift shoulder indicate temperatures of >60°C associated with this reheating whereby the westernmost sample in the sedimentary plain experienced almost complete resetting of the detrital apatite grains at temperatures of about ∼90–100°C. The younging of AFT ages westward indicates activity of faults, re-activating inherited Precambrian structures during Karroo sedimentation. Furthermore, our data suggest onset of final cooling/exhumation linked to (1) the end of Madagascar's drift southward relative to Africa during the Early Cretaceous, (2) activity of the Marion hot spot and associated Late Cretaceous break-up between Madagascar and India, and (3) the collision of India with Eurasia and subsequent re-organization of spreading systems in the Indian Ocean.

Apatite fission track analytical data for samples from ODP Leg 152

We test a new approach to understanding the tectonic evolution of passive margins by using fission-track analysis on detrital apatites from sediments deposited offshore East Greenland. These apatites have not undergone postdepositional track annealing and therefore reflect provenance. The apatites preserve a component of the source rocks' thermal history that otherwise may not be retained within the present-day outcrop. Fission-track derived denudational histories from samples at Ocean Drilling Program drill sites offshore East Greenland at lat 63°N are compared with data from the onshore Singertat Complex. Previous apatite fission-track studies and geomorphic mapping of the East Greenland coast have shown that locally up to 6 km of denudation may have occurred, implying significant tectonic or magmatic activity starting as much as 30 m.y. after breakup at 56 Ma. In contrast, apatite fission-track data presented here record <2 km of Cenozoic denudation in southeast Greenland, probably driven by magmatic underplating at the time of breakup. Large-magnitude, postrift denudation of East Greenland is restricted to the area around Kangerdlugssuaq (68°N). The timing (<40-50 Ma) and magnitude are in accord with revised plume track models suggesting that the Iceland plume crossed the margin here during the late Eocene.

Projected length annealing of etched Sm-152 ion tracks in apatite

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Effects of Etching on Zircon Grains and Its Implications for the Fission Track Method

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

A tectônica meso-cenozóica de parte da borda nordeste da Bacia do Paraná: traços de fissão em apatita

The possible development of thermal events in the central portion of São Paulo state was described based on apatite fission track analysis. Using apatites of sedimentary rocks of the Paraná Basin, modeling the thermal history was made possible due to the homogeneity of the data. Every thermal history begins with a total annealing of fission tracks, related to the Serra Geral magmatism, evolving into a cooling period. In addition to cooling after the magmatism (Early Cretaceous) two other periods of cooling were also detected, registered in the Late Cretaceous/ Paleocene and Eocene, driven as much by uplift with tectonic denudation as by faulting. The nearest portion of the edge of the basin (external to the Dome of Pitanga), registered a period of warming over the Paleocene that can be attributed to the increase in the geothermal gradient. The periods of cooling have a regional and temporal relationship with the tectonic events that occurred in the southeastern Brazil and were described in the crystalline basement. The period of warming, registered in the Late Cretaceous/ Paleocene, has local occurrence and can be found only in the southern portion of the studied area.