Constraints on Passive Margin Escarpment Evolution from River Basin Reorganization in Brazil by Madison M. Douglas Submitted to the Department of Earth, Atmospheric and Planetary Sciences in Partial Fulfillment of the Requirements for the Degree of Bachelor of Science in Earth, Atmospheric and Planetary Sciences at the Massachusetts Institute of Technology June 3, 2016 2016 Madison M. Douglas. All rights reserved. The author hereby grants to MIT permission to reproduce and to distribute publicly paper and electronic copies of this thesis document in whole or in part in any medium now known or hereafter created. Author Signature redacted Department of Earth, Atmospheric and Planetary Sciences May 11, 2016 Certified Signature redacted C brtifedby_ J. Taylor Perron Thesis Supervisor redacted Accepted by_AeSignature Tanja Bosak Chair, Committee on Undergraduate Program ARCHIVE8 MASSACHUSETTS INtSTITUTE OF TECHNOLOGY 1 SEP 28 2017 LIBRARIES Constraints on Passive Margin Escarpment Evolution from River Basin Reorganization in Brazil Madison M. Douglas Department of Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 2 Acknowledgements: I would like to thank the MIT geomorphology community; in particular Kimberly Huppert, Maya Stokes, Dino Bellugi, Seulgi Moon, and Paul Richardson; for providing feedback and encouragement on this project and over the past four years. I would also like to thank Lucia Silva and Nelson Fernandes for providing insights and assistance into the geology and geomorphology of Brazil, as well as published cosmogenic erosion rates for the region. Finally, I would like to thank the EAPS undergraduates and EAPS writing advisor Jane Connor, who have provided advice, good cheer and support for this thesis over PB & J sandwiches every week. 3 Table of Contents Abstract 5 1. Introduction: Evolution of Passive Margin Escarpments 6 2. Background 8 2.1 The Formation and History of the Brazilian Escarpments 8 2.1.1 Overview and Timing 8 2.1.2 Study Sites 9 2.2 Tracking the Evolution of Drainage Networks 10 2.2.1 Drainage Basin Evolution 11 2.2.2 The Stream Power Law and Chi 13 3. Methods 16 3.1 Constructing Chi Plots 16 3.2 Calculating Drainage Area Exchange 17 4. Results 19 4.1 Elevation vs. Chi Profiles 19 4.2 Comparing Chi to Cosmogenic Erosion Rates 19 4.3 The Importance of Stream Captures 22 5. Discussion 22 5.1 Constraints on Passive Margin Escarpment Evolution 22 5.2 Stream Capture and Escarpment Retreat 24 6. Conclusions 25 Appendix I: "'Be Erosion Rates 27 Appendix II: Supplementary Maps and Chi Plots of Field Sites 30 Appendix Ill: Escarpment Retreat Rates vs. Distance to Coastline 36 References 37 4 Abstract Escarpments are present on passive margins around the world, but their evolution is poorly understood. Some geologists interpret escarpments as stationary features, whereas others have argued that they are retreating inland faster than a kilometer per Myr. I investigate Brazilian escarpments by determining whether or not the river networks on either side of the escarpment are in erosional equilibrium with each other. My approach is based on the premise that rivers on opposite sides of a stationary escarpment would be eroding at the same rate (erosional equilibrium), whereas rivers on opposite sides of a mobile escarpment would be eroding at different rates (erosional disequilibrium). I use a recently developed technique called chi mapping to assess the erosional disequilibrium of river networks along the Brazilian escarpments. For comparison, I also compile erosion rates on either side of the escarpments from cosmogenic "Be measurements in the existing literature, and use these to calculate recent retreat rates of the escarpments, which fall between 4 and 40 m/Myr. I determine that chi mapping and cosmogenic erosion rates agree on the direction of escarpment movement, but disagree on the magnitudes of the retreat rates. I also estimate the percentage of drainage area exchanged by stream capture as the escarpment drainage divide moves across the landscape. Using two different estimation methods, I find that less than 40% of drainage area is exchanged by stream capture, making divide migration the dominant mechanism for drainage basin reorganization. If my estimates of recent escarpment retreat rates are representative of long-term rates, the Brazilian escarpments have retreated up to 5 km since their formation during the Cretaceous rifting event. My analysis shows that the topographic retreat of the Brazilian escarpments could have been driven by drainage basin disequilibrium resulting in divide migration. 5 1. Introduction: Evolution of Passive Margin Escarpments Passive margin escarpments are observed around the world, though interpretations of their evolution vary widely. Theories of escarpment formation and evolution must account for their longevity over tens of millions of years, their formation along rift zones near the coast, and the potential erosional retreat of escarpments inland. Traditionally, escarpments are thought to be formed as normal fault scarps along the continental margin during rifting events (Figure 1A; Braun & van der Beek, 2004). Then, high erosion rates along the escarpment face cause the scarp to retreat inland to the slowly eroding continental plateau. In contrast, escarpments may also form when an oceanic rifting event elevates a pre-existing drainage divide within the continent, which then incises the newly formed coastal plateau back to the stationary divide (see figure 1B; Sacek et al., 2012; Matmon et al., 2002). Both the parallel retreat and stationary divide scenarios may be influenced by post-rifting flexural responses and magmatic activity, further complicating their erosional histories (Braun & van der Beek, 2004; Gallagher & Brown, 1997). Various studies have used rock exhumation rates derived from apatite fission track (AFT) datasets, basin-averaged erosion rates from cosmogenic radionuclides (CRN), and geologic and morphometric analyses to constrain the retreat rates of escarpments. These methods generate a variety of estimated escarpment retreat rates and proposed mechanisms for retreat for the same escarpment, indicating that passive margin escarpment evolution is still poorly understood. For instance, the Great Escarpment of Australia may have experienced sustained retreat rates of up to 2 km/Myr according to the parallel retreat model of escarpment formation (Seidl et al., 1996). However, "Be erosion rates along the escarpment do not exceed -35 m/Myr, though the escarpment face is eroding more rapidly the continental interior and coastal plain, indicating the escarpment is retreating less than 35 m/Myr (Heimsath et al., 2006). Therefore morphometric and CRN erosion rates agree that the Great Escarpment of Australia is retreating, but disagree by around two orders of magnitude on the rate of retreat. A similar discrepancy occurs along the Drakensberg escarpment in South Africa, where AFT borehole records indicate higher denudation rates from -91-69 Ma than those determined by 36CI CRN studies, indicating a potential for higher escarpment retreat rates in the past (Moore & Blenkinsop, 2006; Flemming et al., 1999). Matmon et al.'s (2002) time-for-space substitutions found global escarpments to be stationary, with escarpments increasing in sinuosity with age. In contrast, "Be and 6 1 Al measurements along the Namibian escarpment indicate a slow retreat rate of 10 m/Myr, due to local river networks being close to equilibrium (Bierman & Caffee, 2001; Cockburn et al., 2000). The 6 discrepancies between AFT and CRN erosion rates may indicate that escarpments erode more rapidly early in their life, since AFT denudation rates extend from the time since rifting that the sample was last cooled below the closure temperature (typically >10 Ma), while ' 0Be erosion rates indicate slower erosion and retreat of escarpments over the last -100 ka. In summary, while independent methods have been applied to study escarpment retreat, they generally found lower retreat rates on shorter timescales and higher retreat rates on longer timescales, but do not have sufficient temporal resolution to determine the lifecycle of an escarpment (Balestrieri et al., 2000). In particular, AFT also records events such as magmatic intrusions and tectonic reactivation in addition to denudation, making it difficult to distinguish between the parallel retreat and fixed divide models of escarpment formation (Braun & van der Beek, 2004; Gallagher & Brown, 1997). However, all models of escarpment retreat include rivers as an important agent in determining the morphology and location of the escarpment by driving the movement and persistence of their drainage divides. Therefore, analysis of river networks has the potential to fill in the gaps in the timeline of escarpment retreat, since passive margin rivers record episodes of tectonic rejuvenation and disequilibrium over a long time through stream captures and knickpoints (a steepened reach of a river that typically propagates upstream as an erosional wave). In addition, models of escarpment retreat rely on bedrock rivers' erosional power, which has been studied extensively (Howard & Kerby, 1983). In this paper, I develop a new, independent method of determining whether, and in which direction, an escarpment is moving using recent advances in the analysis of bedrock river networks. I apply this method to several escarpments in Brazil and compare the results with escarpment retreat directions and rates derived from published