The role of fault reactivation in the geomorphological evolution of coastal landforms on passive continental margins: Evidence from a tectonic estuary in southern Brazil
Graphical abstract
Introduction
Coastal landforms on passive continental margins typically occur in the flat, broad coastal plains and wide continental shelves that separate deep seawater systems from erosional hinterlands. Transition from continental to oceanic crust at depth result in major bathymetric and topographic anomalies genetically linked to a variety of coastal environments in the surface. Landforms formed due to depositional processes (e.g., beaches, barrier islands, estuaries, deltas, coastal dunes) or coastal erosion (e.g., sea cliffs, sea stacks, canyons) are often shaped by the action of waves, tides, currents and coastal rivers, which in turn are strongly influenced by changes in climate and tectonics (Burbank and Anderson, 2011; Summerfield, 1989, Summerfield, 1991). Estuaries and bays are key examples of landforms typically found on continental passive margins and intraplate settings worldwide, being mostly classified as coastal plain-type, lagoon-type and fjord-type estuaries (Perillo, 1995).
Classical definitions of passive margins consider full tectonic stability after continental break-up, with little flexural deformation due to sedimentary loading and thermal contraction (Leroy et al., 2004). While active tectonic landforms are often restricted to plate boundary settings, shaping of modern coastal landforms on passive margins is commonly attributed to non-tectonic sea-level fluctuations. By definition, tectonic-type estuaries typically result from crustal subsidence along active margin settings (e.g., San Francisco Bay, near San Andreas transform boundary) (Perillo, 1995). However, while increasing evidence of post-break-up deformation are reported on passive margins worldwide (e.g., South America, West Africa, Australia, India, Greenland, Norway, Kazakhstan) (Leroy et al., 2008; Pedoja et al., 2011; Walker et al., 2016; Campbell et al., 2015; Amborn, 2003), the extent of which Neogene to Quaternary intraplate stress fields reactivate pre-existing fault zones and exert direct influence on coastal landforms remains uncertain.
In this context, the Brazilian passive margin shows an atypical scenario: a widespread coastal progradation and relative sea-level drop of 1.5 to 3.5 m in the last 6000 years (Angulo et al., 2006) resulted in infilling of fluvial valleys and consequent vanishing of classic coastal plain-type estuaries; still, at least forty-one large tidal bays are present along the coast, representing local transgressive zones that cannot solely be explained by the ongoing rates of sea-level rise (Church et al., 2001). Although Lessa (2005) and Lessa et al. (2018) have reported evidence of local subsidence and tectonic reactivation processes near other Brazilian estuarine systems in the Quaternary, it is known that pre-existing structural frameworks and uplift alone may induce gravitational collapse of mountain fronts, escarpment dissection, watershed asymmetry and rectilinear drainage anomalies under stable tectonic conditions. Therefore, distinguishing recent tectonic landforms from inherited ones in passive margins is essential and requires integrated approaches to avoid misinterpretations.
In this context, we highlight the occurrence of recurrent seismic activity along the South American passive margin, especially in the SE-S Brazilian continental shelf (Assumpção, 1998; Assumpcao, 1992; Agurto-Detzel et al., 2017), being seismicity an important evidence of neotectonic activity in such areas. Considering that analogue coastal landforms on similar settings may exist around the world, we aim to provide three models of tectonic-estuary formation from extensional fault reactivations to support analogue findings considering their morphotectonic background and specific intraplate stress regimes.
Section snippets
Geological, structural and geomorphological contexts
The Babitonga estuary is situated in the southern Brazilian coast and surrounded by an extensive variety of rocks with a wide range of ages. Main geological units consist of Precambrian terrains, Neoproterozoic molasses basins, Serra do Mar granitic suites (Basei et al., 2009; Cury, 2009; Júnior et al., 1993, Júnior et al., 1995; Kaul and Cordani, 2000), mafic and alkaline rocks from Paraná-Etendeka LIP (Peate, 1997; Piccirillo and Melfi, 1988; Milani, 2004), Neogene sedimentary sequences,
Methods
We have provided detailed characterization of surface topography and drainage network to integrate geomorphological analysis (based on DEM-SRTM at a spatial resolution of 30 m; USGS, n.d.), morphometric indices and structural data of four main hydrographic basins: Itapocu River, Cubatão Norte River, Linguado and Saí-Mirim. Digital processing was performed using ArcGIS 10.3 and GlobalMapper v.16.0 software on raster and vector data (shapefiles) with UTM projection (WGS84 horizontal and EGM96
Results
The hypsometric map of the study area was analyzed along with four swath profiles (with WNW-ESE direction, Fig. 2) that section the four catchments. Both geomorphological domains show abrupt contrasts in altimetry and slope, the first being marked by steep, mountainous and strongly wavy reliefs that can reach up to 1500 m high upstream of the Quiriri River (Cubatão Norte catchment), and the second by smooth to slightly wavy plains (Fig. 2). Main elements consist of topographic lineaments,
Models of tectonic estuaries in passive margins
Coastal landforms on passive margins may be controlled by pre-existing fault zones reactivated due to intraplate tectonic stress and onshore uplift. In such areas, erosion may turn steep mountain-fronts into flat plains quite rapidly, especially when climate, tectonics and epeirogenic processes act together as catalysts for landscape evolution. While in arch-type margins drainage divide is located inland of the escarpment, in shoulder-type passive margins (e.g., South America) elevation
Conclusions
Recent intraplate stress fields in passive margins are commonly underestimated despite of their great potential on shaping landforms by means of (1) reactivating former gravitational structures, (2) reactivating major pre-existing lineaments and shear zones and/or (3) creating new faults, all processes depending on specific intraplate tectonic regimes. On passive margins located in tropical to sub-tropical regions, flat coastal areas are even more susceptible to land degradation due to
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We are grateful to the Neotectonics Group of the Federal University of Paraná (UFPR) for their support in the development of the research. We also thank the Geoprocessing Nucleus (NUGEO) at UFPR, the postgraduate program in Geology of UFPR, the Coordination for the Improvement of Higher Education Personnel (CAPES) for financial support and the National Council for Scientific and Technological Development (CNPq) for the Research Grant PQ-2 given o the first coauthor (Process nº 307738/2019-1).
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