Thermochronology, landscape evolution and denudational history of the eastern Borborema Province, northeastern Brazil.

Joao Marinho De Morais Neto (2009). Thermochronology, landscape evolution and denudational history of the eastern Borborema Province, northeastern Brazil. PhD Thesis, School of Earth Sciences, The University of Queensland.

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Author Joao Marinho De Morais Neto
Thesis Title Thermochronology, landscape evolution and denudational history of the eastern Borborema Province, northeastern Brazil.
School, Centre or Institute School of Earth Sciences
Institution The University of Queensland
Publication date 2009-01
Thesis type PhD Thesis
Supervisor Prof. Paulo Vasconcelos
Dr. Kurt Knesel
Total pages 354
Total colour pages 55
Total black and white pages 299
Subjects 04 Earth Sciences
Abstract/Summary In order to reconstruct the thermal and exhumation histories for the eastern Borborema Province, northeastern Brazil, I carried out 40Ar/39Ar thermochronology to constrain cooling in distinct tectonic blocks in the region and integrated those results with apatite fission track and (U-Th)/He data and 40Ar/39Ar results for Cenozoic volcanic units and preserved weathering profiles. In addition, I carried out measurements of cosmogenic isotope 10Be concentrations in exposed bedrocks and sediments to determine regional erosion rates and denudation patterns for the past 1.5 Ma. Combination of the thermochronological, geochronological and cosmogenic isotope data permits the establishment of a complete cooling and denudation history for the eastern Borborema Province from the late Neoproterozoic to the Present. To infer the early thermochronological history, I analysed different minerals from sixteen Precambrian basement samples to constrain cooling through high- to moderate temperature intervals – 550-450°C, 400-300°C, 350-250°C and 350-150°C – derived from the 40Ar* closure temperatures estimated for amphibole, muscovite, biotite and K-feldspar, respectively. 40Ar/39Ar cooling ages range from 575-560 Ma for amphiboles, 580-450 Ma for biotites, ca. 550 Ma for muscovite and 520-390 Ma for K-feldspars. Inversion of 39Ar data in K-feldspars, constrained by independent thermochronometers, reveals a conspicuous pattern of cooling following the Brasiliano-Pan African Orogeny. Relatively fast ascension of basement rocks characterizes the orogenic peak (~600-580 Ma), followed by a gradual relaxation during the waning stages of the thermotectonic event (ca. 530-460 Ma), implying that the exposed Brasiliano granitoids came to a shallower position in the crust (equivalent to depths < 5 km) by the late Cambrian-late Ordovician. A significantly slower cooling stage during most of the Paleozoic and Mesozoic, with cooling rates in the order of 0.5-0.3 oC/Ma, indicates that the samples remained at mid- to low-temperatures (<250-150 oC) from the mid Ordovician to the early Jurassic (460±10 Ma until 200±10 Ma). By the early Jurassic cooling accelerated, which is interpreted as the effect of regional uplift preceding Pangea’s fragmentation. Differences in the cooling patterns between adjacent terranes are negligible, indicating that no important differential movement existed between the major tectonostratigraphic domains of the eastern Borborema Province. 40Ar/39Ar K-feldspar dates from mylonites also indicate that the presumed reactivation of the Brasiliano shear zones during the Cretaceous rifting did not disturb the Ar system. Integration of fission track data from previous studies with independent geological and geochronological constraints indicates a polycyclic development for the regional relief, with at least two events of cooling/uplift recorded since the Cretaceous breakup. Post-rift exhumation of the Precambrian basement is recorded by a widespread mid Cretaceous (100-90 Ma) cooling event. A renewed stage of cooling during the late Cretaceous-early Cenozoic transition suggests increased denudation during that time, as indicated by both apatite fission track and (U-Th)/He data. On the Borborema Plateau, the sedimentary mesas of the Serra do Martins Formation, whose age is constrained as younger than Paleogene, suggest that a renewed (or continued?) episode of uplift, possibly induced by thermal upwelling related to mantle anomalies, raised the entire region sometime during the early- to mid Cenozoic. During the Eocene-Oligocene period, renewed incision promoted the removal of the sedimentary cover on the Borborema Plateau and the dissection of the adjacent lowlands. Subsequent tectonic reactivation related to several volcanic intrusions during the Oligocene-Miocene transition, possibly combined with climate changes, accelerated the denudation processes, providing the clastic sediments to the deposition of the Barreiras Formation along the coast during the early Miocene. A previously Neogene exhumation event interpreted from apatite fission track data is incompatible with the estimated original thickness of the Serra do Martins Formation, as inferred from cosmogenic data and petrographic properties. It is also incompatible with the preservation of weathering profiles and Cenozoic lava flows on the Borborema Plateau, whose 40Ar/39Ar ages indicate that those areas were already near the surface by the Oligocene-Miocene transition. Erosion rates derived from 10Be concentrations measured in 55 bedrock samples provide quantitative evidence for distinct erosional histories for the two main regional landsurfaces that characterize the Borborema physiography. Very low erosion rates (< 4 m/Ma) characterize the “Borborema” surface, particularly the highlands hosting deep weathering profiles. Moderated to high erosion rates (5-16 m/Ma) prevail in the surrounding depressions (the “Sertaneja” surface) and these erosion rates decrease with increasing distance from the escarpment that separates the two surfaces. Averaged erosion rates (> 20 m/Ma) for quartz-rich stream sediments are higher than for all crystalline and sedimentary samples and indicate that the erosion has been concentrated in the escarpment zone, whose progressive retreat towards the continent generated the interior lowlands. The fission track age distribution and our cosmogenic isotope results suggest that the active mechanism of erosion is escarpment retreat, implying that this landscape evolution process shaped the rugged topography of the eastern Borborema Province.
Keyword 40Ar/39Ar dating
thermal history
Additional Notes Pages in colour: 36, 48, 53, 55, 59, 66-67, 72, 79, 108, 127, 130, 133, 135,153-156, 159, 162, 164, 168, 172, 175-177, 179, 181-182, 184-185, 187, 189, 191, 205-206, 208, 213, 217, 227, 229, 296-301, 233, 343-350. Landscape pages A3 colour: 55 A4 colour: 48, 79, 162, 191, 217, 229. A4 b&w: 97, 109,174, 201-203, 215, 281-285, 287-291, 293-294, 303.

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