Timing and mechanisms for the generation and modification of the anomalous topography of the Borborema Province, northeastern Brazil
Introduction
The Borborema Province, a large cratonic shield area in northeastern Brazil (Fig. 1), underwent significant uplift during and after Cretaceous times, as indicated by many lines of evidence, most notably the shallow-water Albian limestones in the Araripe Basin (western Borborema Province) at present elevations of 700–800 m above sea level.
The main topographic expression of this uplift is the Borborema Plateau, which shows maximum elevations on the order of 1200 m above sea level and encompasses almost the whole northeastern shield of Brazil. The Borborema Plateau shows a regional axis oriented in the NE–SW direction. It is surrounded by marginal lowlands (the “Sertaneja Depression” and the coastal cuestas) (Fig. 2). Within the Borborema Plateau area, there are several mesas partially covered by the sediments of the Serra do Martins Formation, a sequence of continental sandstones and conglomerates of presumed Paleogene age. The occurrence of these sediments at high elevations also has been viewed as evidence for Cenozoic uplift affecting at least the eastern Borborema Province and possibly beyond. However, the sediments of the Serra dos Martins Formation are non-fossiliferous and the stratigraphic age has been inferred from fission-track analysis (Morais Neto et al., in press) and indirect relationships with Cenozoic volcanics (Menezes et al., 2003, Jardim de Sá et al., 2005).
During the last three decades, many hypotheses have been formulated to explain the morphological evolution of the Borborema Province, in particular, and the timing and causative mechanisms for the regional topography of northeastern Brazil, in general. These studies, mostly based on morphoclimatic arguments, interpreted the development of the Borborema Province using regional correlations of distinct (and undated) peneplained surfaces. Using these correlations, many authors assigned a range of ages for the planation surfaces and their “correlative” sediments, leading to many redundant, controversial, and sometimes inconclusive correlations. Rather than to rely on non-fossiliferous, continental deposits to constrain the timing of uplift, we used apatite fission-track analysis (AFTA) on samples from the Borborema Plateau region and surrounding areas to provide a quantitative constraint on the timing of various cooling (or paleothermal) events and thus insights into the timing of formation and denudation of the topography of northeastern Brazil.
In regions such as the Borborema Plateau, where large periods of geological time are not represented in the geological record, thermal history tools are unusually powerful in studying the nature of events during time intervals unrepresented in the preserved rock record. Constraints from thermal histories provide a unique opportunity to better understand important aspects of the Phanerozoic tectonic evolution of a region that is presently dominated by only Precambrian terrain. In this paper, we present the results from 14 apatite fission-track analysis, which provide new quantitative constraints on the thermal history of this region by constraining the timing of cooling, such as caused by uplift and/or topographic denudation, from the thermal history of individual samples. In addition, we evaluate models for the generation of regional, permanent continental topography, and discuss possible mechanisms responsible for the development of the present-day topography of the Borborema Province in terms of post-rift thermal events related to the breakup of Gondwana, tectonics and climate changes.
The origin of the positive long wavelength topography of northeastern Brazil remains enigmatic, both in terms of its timing and causative mechanism (Hegarty et al., 2004). Topographic relief distributed over many 100s of kilometers can be generated via a number of first-order processes, such as thin- and thick-skinned folding and thrusting of the crust, mantle plume activity, lithosphere delamination, and magmatic underplating of the crust. While extension can generate flanking flexural topography, the distribution tends to be spatially limited, with asymmetric flank width generally being between 100 and 200 km. These mechanisms generate permanent topography, which is in contrast to the transient topography engendered by solely thermal processes. Magmatic underplating, by effectively thickening the crust, leads to permanent topography coeval with the underplating (Brodie and White, 1995).
Recent studies by Davis and Kusznir (2004) and Kusznir and Karner (2007) have shown that depth-dependent lithospheric thinning, in which stretching of the lower crust and lithosphere mantle greatly exceeds that of the upper crust, a characteristic of many non-volcanic and volcanic rifted continental margins, can be modeled by an upwelling divergent flow model of continental extension. Such a model successfully predicts both observed depth-dependent lithosphere stretching and mantle exhumation at rifted continental margins. In this model, extension produces an outward flow of asthenosphere material towards the hinterland, thickening the adjacent continental lithosphere at distances beyond the region of localized rifting and lithospheric thinning; the immediate response to lithosphere thickening in the hinterland is to decrease the lithosphere geothermal gradient. Modeling shows that the subsequent re-equilibration of the continental hinterland lithosphere geotherm generates uplift of 400–600 m over a timescale controlled by the lithosphere thermal re-equilibration time-constant of 60–100 m.y. Interfluve topography would be further amplified by isostatic rebound in response to topographic erosion. The lateral wavelength of uplift is of the order of 400–600 km wavelength. This upwelling divergent flow model of continental extension, possibly amplified by magmatic underplating when mantle plumes exist in the region (e.g., active rifting and volcanic margins), presents a promising mechanism to help explain regional post-breakup uplift over timescales of 50–100 m.y. in the Brazilian hinterland, in particular, and adjacent to passive continental margins, in general.
Knowledge of when and where clastics are delivered to an evolving continental margin is essential for hydrocarbon exploration in terms of predicting the general location, timing, and the sand content and maturity of turbidite deposits. Such predictions require information about the history of topography generation, modification, and denudation. Continental landscapes are constantly in a state of flux as drainage systems evolve and interact via the process of river capture during the denudation of topography. In turn, any post-rift tectonics that modifies or generates topography will play a major role in this landscape evolution process, controlling the transport and delivery of clastics by fluvial systems to the margins. Rivers represent point sources of clastics along margins, however, their distribution, energy, and sediment content are prime functions of topography generation and distribution. Once deposition takes place, marine processes such as alongshore drift rework and redistribute the sediments along the margin. During eustatic falls, margins associated with relatively large fluvial systems will tend to be associated with significant drainage rejuvenation, river incision of the exposed shelf, canyon development, and given suitable conditions, the deposition of turbidities. A corollary of this study is to provide suggestions about when clastics from the eroding Borborema topography, as modified by the development and interaction of intracontinental drainage systems, should be delivered to the northern and northeastern Brazilian basins. While this is a local study of the Borborema Province of northeastern Brazil, the implications of this work have far reaching consequences for the stratigraphy of rifted continental margins through the development of hinterland topography.
Section snippets
Geological setting
The Precambrian Borborema Province in northeastern Brazil (Fig. 1) consists of a gigantic fan-like system of right-lateral strike-slip shear zones, juxtaposing Archean to Mesoproterozoic crystalline massifs against belts of Neoproterozoic metasediments (Van Schmus et al., 1998, Dantas et al., 1998). The Borborema strike-slip system corresponds to the west-central portion of the Pan-African/Brasiliano orogenic collage formed during the Neoproterozoic assembly of West Gondwana (600–580 Ma). The
Continental rifting and breakup
In the northeastern portion of the Borborema Province, the breakup of Western Gondwana was preceded by the outpouring of the Rio Ceará-Mirim tholeiites from the late Jurassic to Barremian (145–125 Ma; Fig. 1). South of the Potiguar Basin, this magmatism is represented by an extensive E–W-trending dike swarm (Fig. 1, Fig. 4). O'Connor and Duncan, 1990, Wilson, 1992, Lima Neto, 1998b and Oliveira (1998), for instance, argued that this igneous activity was mantle plume induced. It was further
Sampling details and background
Fission-track samples were collected along two regional transects across the Borborema Plateau (Fig. 5, Fig. 6).
Fourteen crystalline basement samples were selected for fission-track analysis, representing a broad range in rock type, age and elevation (see Table 1). They represent Precambrian igneous and metamorphic rocks from various geological domains comprising the region (Table 1). The samples were selected to provide a good spread across the study area, so as to reveal any real variation in
Basis of fission-track technology
Following evidence in the 1970s and 1980s (Gleadow et al., 1983) that the length of fission tracks in detrital grains of apatite alters in response to temperature and time, the fission-track “dating” technique was rapidly understood to be less reliable as a geochronological tool, but hugely exciting as a potential thermochronometer. Although there have been significant advances in this technology since that time, the fundamental basis of the technique remains the same. The interpretation of
Significance of the fission-track results
Over the past few years there have been a number of fission-track studies from eastern Brazil (Gallagher et al., 1994, Karner et al., 1994, Hegarty et al., 1996, Harman et al., 1998, Hackspacher et al., 2004), none of which can be easily linked to topography generation and a causative tectonic event.
For example, Harman et al. (1998) in their study of the Guaporé and São Francisco cratons show fission-track ages that range from 309–137 Ma to 260–76 Ma, respectively. They suggest that their
Conclusions
The results of 14 AFTA (apatite fission-track analysis) samples collected in two regional transects across the Borborema Plateau, northeastern Brazil, show clear evidence of higher temperatures in the past. Two episodes of cooling have been identified in Proterozoic igneous/metamorphic samples: (1) the first cooling event beginning some time between 100 and 90 Ma, and (2) a second cooling event in the Neogene between 20 and 0 Ma. Other less dominant events are also revealed in the data, as a
Acknowledgments
We thank Petrobras and Federal University of Ouro Preto (Brazil) for financial support and fieldwork logistics as part of a post-graduate program of the first author. Geotrack International (Australia) was responsible for the apatite fission-track analyses of all the Borborema samples. The analytical results, ages and length data plots can be supplied to interested groups by request to the authors. Petrobras geologists Ubiraci Soares, Otaviano Pessoa Neto and Edison Milani, as well as former
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Formerly at GEOTRACK International Pty. Ltd., Brunswick West, Victoria, Australia.