Dating of Detrital Zircons from Fluvial Terraces on the Blue Ridge Escarpment: Implications for the Evolution of a Passive Margin

Home , Escarpment

DATING OF DETRITAL ZIRCONS FROM FLUVIAL TERRACES ON THE BLUE RIDGE ESCARPMENT: IMPLICATIONS FOR THE EVOLUTION OF A PASSIVE MARGIN

Jasmine J. White

A thesis submitted to the faculty at the University of North Carolina at Chapel Hill in partial fulfillment of the requirements for the degree of Master of Science in the Department of Geological Sciences

Chapel Hill 2019

Approved by:

Kevin G. Stewart

Drew S. Coleman

Tamlin M. Pavelsky

ii ABSTRACT

Jasmine J. White: DATING OF DETRITAL ZIRCONS FROM FLUVIAL TERRACES ON THE BLUE RIDGE ESCARPMENT: IMPLICATIONS FOR THE EVOLUTION OF A PASSIVE MARGIN (Under the direction of Kevin G. Stewart)

The mechanisms suggested for creating the Blue Ridge escarpment range from Triassic rift-flank uplift in the Coastal Plain to a series of normal faults at the base of the escarpment.

Geomorphological processes on top of the escarpment, such as stream capture, have led previous workers to infer that the escarpment originated to the southeast of its present-day location. There are, however, unexplained fluvial terraces on top of the Eastern Continental Divide along the escarpment that contains rounded quartz-cobbles. The provenance of the fluvial terraces has yet to be explained but could provide evidence of the pre-rift/pre-uplift topography. I analyze detrital zircons from the terraces to reveal the provenance and ancient stream-flow directions. In this study, I present evidence that the zircons in the ancient terrace deposits were deposited by west- flowing streams that originated in the Piedmont. I use these ages to further constrain the origin of the Blue Ridge escarpment.

iii ACKNOWLEDGEMENTS

There are many people I would like to thank for contributing to the completion of this thesis.

This project was supported by the Martin, Butler, and Jesse Davis Trust, administered by the

Department of Geological Sciences. I want to thank the University of North Carolina at Chapel

Hill’s Department of Geological Sciences for this opportunity and access to the funds mentioned above that allowed me to conduct my research. First, I would like to thank Kevin Stewart, my advisor. Kevin, you are an incredible educator and encourager. Thank you for giving me the chance to grow and improve in my research, helping me along the way as I learned and refined the tools of independent work. I would like to thank my committee members, Drew Coleman and Tamlin

Pavelsky, for their advice and encouragement as I completed my research and prepared for my defense. A special thanks to Deborah Harris, your patience, kindness, and the laughs that we shared are much appreciated.

I would also like to thank my fiancé, Jacob Parker, for always lending an ear for me to talk out my ideas and keeping me focused. Your patience, wisdom, and support as I fulfilled my graduate school dreams is much appreciated. To my sister Raven, thank you for always checking on me and reminding me not to take life too seriously. Mom and Dad, thank you for instilling in me the drive and perseverance that, without a doubt, carried me through the past two years. You guys taught me the value of education and this degree, is for you. Dad, thank you for introducing me into the world of geology all those years ago and helping me be the best geologist I could be!

TABLE OF CONTENTS

LIST OF TABLES ...... viii

LIST OF FIGURES ...... ix

LIST OF ABBREVIATIONS ...... x

INTRODUCTION ...... 1

GEOLOGIC SETTING...... 3

The Blue Ridge Escarpment ...... 3

METHODOLOGY ...... 5

Terrace Locations...... 5

Characteristics of Fluvial Terraces ...... 6

Detrital Zircon Selection ...... 6

DETRITAL ZIRCON AGES ...... 8

INFERRED PROVENANCE OF FLUVIAL TERRACES ...... 9

Grenville-age Zircons ...... 9

Neoproterozoic Zircons ...... 9

Paleozoic Zircons ...... 10

Provenance ...... 11

v INFERRED ORIGIN OF THE BLUE RIDGE ESCARPMENT ...... 12

180 Ma ...... 12

180 Ma to 12 Ma ...... 13

12 Ma to Present-Day ...... 14

Evidence of Cenozoic Uplift...... 16

Mantle Processes ...... 17

Support for Fault Rejuvenation ...... 18

CONCLUSION ...... 19

FIGURES ...... 21

APPENDIX 1: CATHODOLUMINESCENCE OF DETRITAL ZIRCONS IN BRT 1...... 39

APPENDIX 2: CATHODOLUMINESCENCE OF DETRITAL ZIRCONS IN BRT 3...... 40

APPENDIX 3: CATHODOLUMINESCENCE OF DETRITAL ZIRCONS IN BRT 7...... 41

APPENDIX 4: SIMPLIFIED MODEL OF SURFACE EROSION FROM 180 MA TO PRESENT DAY, BEGINNING WITH FILLED TRIASSIC BASINS AND WATER FLOWING TO THE NORTHWEST. CLOSURE DEPTHS OF AHE ISOTHERMS ARE REPRESENTED BY DASHED LINES AT A UNIFORM CLOSURE DEPTH OF 2.5 KM (BASED ON A 60 C ISOTHERM AND 20C/KM GRADIENT, MCKEON ET AL. 2014, SUGGATE ET AL. 1998). SURFACE ELEVATIONS BEGIN AT ~4.7 KM AND ERODE LATERALLY AT 23 M MY-1. AT 12 MA, 1 KM OF UPLIFT OCCURRED ~50 KM FROM THE MODERN BRE, FOLLOWED BY A RETREAT OF 4.5 KM MY-1. THE SOURCE FOR ZIRCON AGES ANALYZED IN THIS THESIS IS LOST AT ~10 MA. B) EXPECTED AHE COOLING AGES REPRESENTED IN THIS MODEL (BLACK) COMPARED TO KNOWN AGES FROM SPOTILA ET AL. 2004 AND BANK ET AL. 2001 (RED)...... 42

APPENDIX 5: SIMPLIFIED MODEL OF A BLUE RIDGE ESCARPMENT FORMED BY RIFT-FLANK UPLIFT IN THE TRIASSIC, ACCORDING TO SPOTILA ET AL. 2004...... 43

vi REFERENCES ...... 44

vii LIST OF TABLES

Table 1. U/Pb geochronologic analysis of Neoproterozoic detrital zircons from BRT 1………..35

Table 2. U/Pb geochronologic analysis of Neoproterozoic detrital zircons from BRT 3………..36

Table 3. U/Pb geochronologic analysis of detrital zircons < 400 Ma from BRT 3…..……….....37

viii LIST OF FIGURES

Figure 1. DEM of the Blue Ridge Escarpment’s abrupt change in elevation from the Piedmont to the Blue Ridge upland…………………………………..…...21

Figure 2. Gravel and sandy matrix from analyzed Blue Ridge Terrace (BRT) sites shown in Figure 3…………………………………………………………..22

Figure 3. Fluvial terrace locations in relation to important topographic features………………..23

Figure 4. Cathodoluminescence (CL) images of magmatic zircons from BRT 1, 3 and 7 displaying oscillatory zoning (3a, 7a, 7b, 7d) and various grain shapes………………………………………………………...23

Figure 5. Histogram and probability density of zircon ages in BRT 1…………………….…….24

Figure 6. Histogram and probability density of zircon ages in BRT 3………………………….25

Figure 7. Histogram and probability density of zircon ages in BRT 7………………….……….26

Figure 8. Igneous bodies throughout Virginia, North Carolina, South Carolina and Georgia…..27

Figure 9. AHe ages from the Blue Ridge upland and Piedmont in NC and VA (Spotila et al., 2004)……………………………………………………………...28

Figure 10. Comparison of observed (Spotila et al., 2004) and predicted AHe ages……………………………………………………………………….……...29

Figure 11.a. Average slope along the Blue Ridge Escarpment spanning from north of Marion, NC to north of Elkin, NC...……………………………………30

Figure 11. b. Average slope along the Blue Ridge Escarpment northwest of the Dan River/Danville basin………………………………………………………31

Figure 11.c. Average slope along the Blue Ridge Escarpment southwest of the Lexington, VA. b) Histogram showing the average slope for this region as 16……………………………………………………………...32

Figure 11.d. Average slope along the Blue Ridge Escarpment west of the Culpeper basin in VA. b) Histogram showing the average slope for this region as 19……………………………………………………………...33

Figure 11.e. Average slope along the Blue Ridge Escarpment north of the Marietta-Tryon Fault system………………………………………………………..34

ix LIST OF ABBREVIATIONS

AFT Apatite Fission Track

AHe Apatite U-Th/He

ALC Arizona LaserChron Center

BRE Blue Ridge Escarpment

BRT Blue Ridge Terrace

DEM Digital Elevation Model

ECD Eastern Continental Divide

KS Kolmogorov-Smirnov

LA-ICP-MS Laser Ablation Inductively Coupled Plasma Mass Spectrometry

MeI Methylene Iodide

R0 Vitrinite Reflectance

Te Effective Elastic Thickness

x INTRODUCTION

Great Escarpments are steep topographic boundaries that range from ~ 100 to 1000 meters tall and commonly separate a coastal plain from a low-relief upland (Japsen et al., 2012).

The lowland ends at the base of the escarpment where it meets a steeper slope that is commonly incised by rivers. The upland surface begins at the top of the escarpment and can continue inland for hundreds of kilometers. These topographic boundaries, like those along the coast in Sri

Lanka, Israel, southern Africa, southeast Australia, Greenland, Brazil, and eastern North

America, range in age from 4 Ma to 200 Ma. Escarpments are 10-500 km inland of the continental margins and extend up to 500 km along the coast. There are several theories of how

Great Escarpments form, including rift-flank uplift, lithospheric delamination, and other mantle processes (Spotila et al., 2004; Al-Hajri et al., 2009; Wagner et al., 2012).

Extensional during the formation of a divergent plate boundary causes rifting of the lithosphere. Unloading of the lithosphere results in the uplift of the rift flank due to isostatic rebound (Weissel and D. Karner, 1989). Researchers from earlier studies suggest that Great

Escarpments in Africa, Australia, Israel, and Sri Lanka are the result of rift-flank uplift (Persano et al., 2002; Spotila et al., 2004; Balestrieri et al., 2005). Preserved escarpments found along rifted continental passive margins can reach heights up to ~ 1000 m (Beek et al., 2002; Persano et al., 2002). However, some rifted passive margins subside over time and remain only a few hundred meters above sea level (Green et al., 2018).

1 Rift escarpments are thought to retreat inland by erosion after the cessation of rifting

(Brown et al., 2002; Spotila et al., 2004; Braun, 2018). Escarpment-retreat rates reported for

Great Escarpments range from 3 to 10 km/Ma (Persano et al., 2002; Balestrieri et al., 2005). The distance an escarpment has eroded is measured from the continental margin and ranges from tens to hundreds of kilometers. The Drakensberg Escarpment in South Africa has retreated ~25 km inland over 130 Ma (Beek et al., 2002). Despite being ~120 Ma older, the Drakensberg escarpment has only retreated 10 km farther inland than the Israeli Escarpment (Balestrieri et al.,

2005). Reported time since rifting for the southeast Australian escarpment is 85 to 100 Ma, during which it has retreated ~ 80 km from the continental margin in the Tasman Sea (Persano et al., 2002). The lack of correlation between the time of rifting and the distance of escarpment retreat makes the evolution of rift escarpments difficult to understand.

Models of rift-flank uplift that incorporate appropriate values of lithospheric flexural rigidity (effective elastic thickness < 50 km) predict that rift flanks should not remain uplifted for hundreds of millions of years (Roberts and Yielding, 1991; Watts, 2001). The improbability of rifted margins remaining uplifted for hundreds of millions of years encouraged researchers to search for an alternate origin of Great Escarpments. Other escarpments (e.g., West Greenland,

Sweden, and Norway) are suggested to have formed millions of years post-rift as a result of geodynamic processes in the mantle (Japsen et al., 2012). Seismic data from Sweden and

Norway reveal that the crust beneath the upland is less dense than the crustal root beneath the lowland (Tesauro et al., 2008; Stratford et al., 2009). Researchers suggest the uplift in these regions is related to the abrupt change in density of the crust from the lowland to upland (Mjelde et al., 2005; Tesauro et al., 2008). Japsen et al. (2012) also argued that the escarpment in West

2 Greenland is the result of post-rift processes, perhaps by anticlinal lithospheric folding during the

Cenozoic that ultimately created the escarpment.

The Blue Ridge Escarpment (BRE) in eastern North America is ~500 km inland of the continental margin in the Atlantic Ocean. Previous researchers suggested that the BRE is the result of rift-flank uplift (Spotila et al., 2004; Prince et al., 2010), base-level drop (Pazzaglia and

Gardner, 2000), faulting (White, 1950), or localized uplift associated with lower crustal flow

(Battiau-Queney, 1989). The BRE is much older (based on the end of rifting ~200 Ma) and further inland, but shares length, relief and mantle characteristics with other Great Escarpments.

The differentiating characteristics of the BRE may help discern how and when the escarpment formed.

The purpose of this paper is to weigh the viability and implications of different mechanisms that could have formed the BRE. Comparing the erosional history of other Great Escarpments to the

BRE should narrow down how it has evolved. In an attempt to constrain how the BRE has evolved, we compare the erosional history of the BRE using new geochronological data. The research presented in this thesis includes a provenance analysis of 1,500 detrital zircons from fluvial terraces perched on the crest of the BRE.

GEOLOGIC SETTING

The Blue Ridge Escarpment

The BRE is a 300m-to-600m-high, seaward-facing step in topography that separates the

Piedmont from the Blue Ridge upland (Figure 1) (Spotila et al., 2004; Prince et al., 2010). It is located ~500 km inland of the Atlantic continental margin (Hibbard et al., 2006; Bird et al.,

2007). The BRE begins in the Piedmont in northern Virginia and continues along the eastern edge of the southern Appalachians for over 500 km before ending in Georgia (Prince et al.,

3 2010). The upland extends ~50-100 km inland from the edge of the BRE to the Valley and Ridge for the majority of the escarpment’s length but narrows into a thin ridge in northern Virginia

(Figure 1) (Prince et al., 2010). The Piedmont extends ~200 km southeast from the BRE’s base, exhibiting gentle changes in slope as it approaches the Coastal Plain.

Although the BRE is along one of the world’s oldest rifted passive margins, it maintains a high-relief (McHone, 1996; Piqué and Laville, 1996). Most areas of high relief in the

Appalachian mountains coincide with outcrops of resistant lithology (Hack, 1982), but the BRE shows no correlation between the slopes of the escarpment and changes in lithology (Prince et al., 2010; Linari et al., 2017). The upland surface and the Piedmont adjacent to the escarpment are both eroding at equal rates, requiring mechanisms other than differential erosion to create the step in topography (Miller et al., 2013; Linari et al., 2017).

There have been many hypotheses in the past that attempt to explain the origin and evolution of BRE. One of the first explanations came from (White, 1950), who proposed that the

BRE is the result of uplift created by a series of normal faults at or near the base of the escarpment. Later, Spotila et al. (2004) proposed that the BRE is a remnant of a once-larger escarpment that formed as a result of rift-flank uplift in the Triassic (~200 Ma). In the Spotila et al. (2004) model of rift-flank uplift, the BRE formed near the Deep River basin (~200 km southeast of the BRE) and reached its present-day location through parallel retreat of the escarpment and drainage divide. More recently, Wagner et al. (2012) suggested that the BRE was a result of delamination beneath the upland and lowland surfaces. Seismic data reveal that the lithosphere beneath the westernmost Inner Piedmont and upland is less dense than the lithosphere beneath the adjacent lowland area (Wagner et al., 2012). Wagner et al. (2012) and

4 Hill (2018) both consider the BRE to be a result of Cenozoic uplift of the upland after a dense crustal root detached from the upper continental crust.

These previous interpretations assume that the BRE was initially southeast of its present- day location and retreated inland (Spotila et al., 2004; Prince et al., 2010). There is no conclusive evidence that the BRE originated to the southeast. However, Prince et al. (2010) proposed evidence of escarpment retreat in the form of stream capture on top of the BRE. There are fluvial terraces on top of the Eastern Continental Divide (ECD) along the BRE. Streams flow away from the divide to the northwest and southeast, providing no information about the provenance of sediment deposited on the divide. Prince et al. (2010) proposed that streams initially flowing from the east were destroyed by the parallel retreat of the ECD as the BRE eroded to the west.

The age distribution of the potential source rocks in the Piedmont are younger than the potential source rocks in the Blue Ridge, so analyzing detrital zircons from the fluvial terraces should provide evidence of whether the sediment came from the northwest or southeast. We focus on rebuilding ancient stream-flow directions that can provide insight into the topography before the

BRE and the viability of previous hypotheses.

METHODOLOGY

Terrace Locations

The North Carolina Spatial Data database contains digital elevation models with 10 x10 meter resolution (DEM10) collected by LIDAR over the southern Appalachians. We used

DEM10 to locate terraces previously studied by Prince et al. (2010) and Bank (2001). These terraces are found in broad valleys on top of the BRE at the headwaters of small streams (Prince et al., 2010). The dry or underfed drainage systems that are currently in the valleys are not powerful enough to have produced the valley or deposit the fluvial terraces.

5 Prince et al. (2010) located 14 terraces containing rounded quartz cobbles along the BRE, from southwest Virginia to southwest North Carolina. Bank (2001) reports two other locations of fluvial terraces in southwest Virginia containing rounded cobbles, including a quartzite clast similar to the Unicoi Formation and Lynchburg Group, both of which outcrop southeast of the

BRE (Henika et al., 2000; Bank, 2001).

Characteristics of Fluvial Terraces

We are interested in fluvial terraces that are perched on the BRE and contain rounded quartz cobbles. The rounded clasts are evidence that the ECD has migrated to its present-day position, cutting the terraces off from the water source that deposited the cobbles. BRE terraces from earlier studies contain cobbles with a degree of roundness that is indicative of transport distances up to ~200 km (Prince et al. 2010).

We sampled seven terraces that are clustered along the BRE in southern Virginia and southern North Carolina and contain well-rounded to sub-angular quartz cobbles (Figure 2). The terrace locations that I studied were among those described by Prince et al. (2010) and Bank

(2001) and are labeled Blue Ridge Terrace (BRT) 1-7 (Figure 3). All of the terraces were exposed by drainage or road cuts that revealed mostly quartz cobbles and a sandy-clay matrix

(Figure 2) (Prince et al., 2010). The terraces sit on rocks from the Tugaloo terrane and were expected to contain abundant zircons (Bream et al., 2004). We filled five-gallon buckets with cobbles and unconsolidated matrix from each of the seven locations (BRT 1-7).

Detrital Zircon Selection

Large crystals (100-300m) are easier to pick and are commonly selected in detrital zircon analysis (Moecher and Samson, 2006), however the smaller zircons, which are more difficult to see, may represent a different age group. Samples were cleaned and wet sieved to

6 separate material into 250-150m, 149-63m, and 62-38m bin sizes. In an attempt to increase the variety of grain sizes during the selection process, the three fractions were each processed separately. Each sample was divided into three different density bins using a Wilfley table. A hand magnet was passed over the greatest-density sediment to remove ferromagnetic minerals.

The remaining sediment was run through the Frantz magnetic separator with a current of 1.2 amps and a 10 slope to remove unwanted minerals without losing zircons (Rosenblum and

Brownfield, 2000). Lastly, heavy-mineral separation using methylene iodide (MeI) separated minerals denser than 3.32 g/cm3 for collection and further analysis.

I handpicked the remaining crystals for zircons using transfer pipettes under a binocular microscope. Euhedral and acicular zircons were abundant in all of the samples, with smaller grain sizes exhibiting more variance in shape (Figure 4). A significant input of Grenville-age zircon from Appalachian bedrock can overwhelm zircon age distributions in a small sample size

(n=100) (Prince et al., 2010) and hide any non-Grenville-age zircon , so we picked a total of

3,000 zircons (approximately 1,000 from each BRT sample). The University of Arizona

LaserChron Center (ALC) carried out the mounting and imaging of the samples. Funding for this project allowed for the analysis of 1,500 out of the 3,000 zircons to represent the population of the BRE fluvial terraces. Backscatter electron (BSE) and cathodoluminescence (CL) (Appendix

1-3) images for each sample were used to select the sites of ablation with a 20m laser. The analysis was completed via Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-

ICP-MS) at the ALC. The ALC performed the initial data reduction and analysis using an in- house Python decoding routine and Excel spreadsheet (E2agecalc).

7 DETRITAL ZIRCON AGES

The U/Pb ages from BRT 1 are bimodal with peaks at 1100 Ma and 1400 Ma (Figure 5).

A sample size of ~500 zircons resulted in five ages that were not previously obtained in provenance analysis of BRT 1 (415 Ma, 554 Ma, 571 Ma, 845 Ma, and 2351 Ma) (Prince et al.,

2010). The 415 Ma, 554 Ma, 571 Ma, and 688 Ma zircons are euhedral to rounded with oscillatory zoning (Figure 4) and are 43%, 100%, 87%, and 99% concordant, respectively (Table

1). The Early Devonian age (415  5 Ma) is the only grain in the sample with less than 80% concordance. U/Th ratios of zircons reveal whether the result is an igneous (crystallization) age or a metamorphic age (recrystallization). Zircons that have a U/Th ratio < 10 are reported to represent the crystallization age. Igneous ages are better for provenance analysis because they represent the time at which a pluton crystallized, acting as a fingerprint that matches plutons of similar ages. The Paleozoic and Neoproterozoic zircons from BRT 1 have U/Th ratios less than five, representing a magmatic origin.

The U/Pb ages from BRT 3 are bimodal with peaks at 1100 Ma and 1400 Ma (Figure 6).

A sample size of ~500 zircons resulted in four ages that were not previously obtained in provenance analysis of BRT 3 (640 Ma, 656 Ma, 2000 Ma, and 2500 Ma) (Prince et al., 2010).

The 640 Ma and 656 Ma zircons are acicular to rounded with oscillatory zoning (Figure 4) and are 102% and 95% concordance, respectively (Table 2). Only two grains from this analysis had less than 85% concordance. The Neoproterozoic zircons from BRT 3 have U/Th ratios less than five.

A unimodal peak at 420-480 Ma dominates the BRT 7 dataset (Figure 7) (Horton and

Stern, 1983; Moecher et al., 2010). There were 24 out of 495 zircons from this sample that are older than 485 Ma. This sample also includes 43 zircons that are less than 400 Ma (Table 3).

8 Infrequent ages in the dataset are 325-400 Ma, 560 Ma, 700 Ma, and 1000-1450 Ma. Unlike

BRT 1 and BRT 3 analyses, BRT 7 contained 46 out of 495 grains that were less than 80% concordant, some as low 19% . Results from this dataset include more grains that represent a metamorphic age. U/Th ratios of the 325-400 Ma zircons range from .8 to as high as 155.

However, 29 of these grains represent an igneous origin. There is not a clear correlation between size, shape, and age of zircon grains, but a large-n analysis proved necessary to access younger zircon populations.

INFERRED PROVENANCE OF FLUVIAL TERRACES

Grenville-age Zircons

The 1600-2500 Ma zircons present in BRT 1 and 3 are also reported in detrital analysis from the Western Blue Ridge, eastern Tugaloo, and Dahlonega gold belt in North Carolina,

South Carolina, Georgia, and Tennessee, as well as in Australia, Europe, South America, and

Asia (Bream et al., 2004; Voice, 2010). These ages are also preserved in the West African

Craton, potentially traveling to the northwest before the break-up of Pangea (Schofield et al.,

2006). Grenville-age zircons are also recycled and reworked into the metamorphic and meta- sedimentary rocks of the Blue Ridge and Piedmont. The abundance of Grenville-age zircons in all of the terranes within the Appalachians makes it challenging to determine provenance only using these ages.

Neoproterozoic Zircons

All of the zircons from BRT 1 and BRT 3 share similarities in U/Pb ages and high concordance. Plutons that are the same age as the 541-700 Ma zircons analyzed in BRT 1 and

BRT 3 are present ~200 km southeast of the BRE within the Carolina Terrane, requiring ~90-160 km of transport from source to sink (Figure 8). A mean transport distance of 43 to 186 km was

9 determined from clast-transport analyses of the fluvial terraces along the ECD (Prince et al.,

2010).

Voice (2010) conducted a detrital zircon analysis of the Dry Fork formation in the Dan

River/Danville basin and reported a unimodal distribution with a peak at 400-450 Ma, smaller peaks at 375 Ma, 560Ma and only four grains older than 900 Ma. The Dan River/Danville basin, which is also southeast of the BRE, serves as a closer potential source of Neoproterozoic zircons.

Although BRT 1 and BRT 3 have similar U/Pb ages and high concordance, there are differences in the 1000-1400 Ma age populations (Figure 5-6). BRT 1 contains almost equal amounts of 1000 Ma, 1200 Ma, and 1400 Ma grains whereas BRT 3 has its highest peak at 1200

Ma. To determine whether or not the sediment in these terraces came from a similar source, we investigated the distribution of the samples using a Kolmogorov-Smirnov (KS) test. The KS test uses the maximum vertical deviation between two curves to calculate a test statistic (D). The p- value from the test shows the probability of seeing a test statistic as high or higher than those observed in the samples. While small p-values (< .05) are indicative of two samples that came from a different parent source, large p-values (>.05) represent samples that are not statistically different. A KS test comparing BRT 1 and BRT 3 resulted in a test statistic of D= .07 and a p- value of .175. The two terraces have an underlying shared distribution and likely came from a similar parent source. Although there are fewer Neoproterozoic zircons in BRT 3, these results support the idea that both terraces contain sediment from the southeast.

Paleozoic Zircons

It is clear that BRT 1 and BRT 3 have different sources than BRT 7 (Figures 5-7). The peak of zircon ages at 420-480 Ma in BRT 7 is similar to the ages of the Henderson Gneiss (450

Ma) and the Caesar’s Head Granite (435 Ma), which sit immediately to the north and south of

10 the terrace (Horton and Stern, 1983; Hietpas et al., 2011). As in the case of BRT 1 and BRT 3, I will focus on the younger zircon populations for more information about terranes that sourced the terrace deposit. BRT 7 had medium sphericity and sub-angular cobbles, indicative of sediment that traveled a short distance. Plutons with similar ages to the 325-400 Ma zircons in

BRT 7 are found throughout the Tugaloo, Charlotte, King’s Mountain, Cat Square, and

Sauratown Mountain terranes (Figure 8) (Secor et al., 1986; Moecher and Samson, 2006).

The bulk of zircon ages analyzed in BRT 7 are similar to the age of the Henderson Gneiss

(450 Ma), which outcrops to the northwest of the terrace. Before making inferences about the source of sediment in BRT 7, we analyzed the underlying distributions of BRT 7 and the

Henderson Gneiss (Hietpas et al., 2011). The Henderson Gneiss sample from Hietpas et al.

(2011) contained 30 U/Pb zircon ages from 400-1200 Ma. A KS test between the two terraces resulted in a test statistic of D= .26 and a p-value of 2.2 x 10-16. Results from the test provide evidence that the underlying distribution between modern stream terraces with sediment from the

Henderson Gneiss is statistically different from BRT 7 (Hietpas et al., 2011). The difference in sample size could impact the results of the test. It should be noted that the Hietpas et al. (2011) sample contained 19 zircons older that 500 Ma and none below 420 Ma. Although the sample is smaller, it contained more Grenville-age zircons and none of the 325-400 Ma population that existed in BRT 7. These results strengthen the argument that west-flowing streams delivered the sediment to BRT 7.

Provenance

The goal of this research was to determine the provenance of fluvial terraces on top of the

BRE using detrital zircon ages. Results from the large-n analysis contained younger ages than previous detrital zircon analysis from fluvial terraces (Prince et al., 2010; Hietpas et al., 2011).

11 The abundant age populations from each BRT provided little information about the provenance of our fluvial terraces. For this reason, I focus on the less-abundant ages in BRT 1, BRT 3, and

BRT 7 to identify their origins (Figures 5-7). All of the younger zircon populations in the BRT’s are indicative of northwest streams that once delivered sediment from terranes southwest of the

BRE to the terraces (Figure 8).

INFERRED ORIGIN OF THE BLUE RIDGE ESCARPMENT

Previous interpretations of when the BRE formed and how it has evolved rely on limited apatite U/Th-He (AHe) and fission track (AFT) data, the assumptions that the escarpment has remained elevated for 200 Ma, and that west-flowing streams deposited the fluvial terraces

(Spotila et al., 2004; Prince et al., 2010; McKeon et al., 2014). Neoproterozoic zircons analyzed in this study are evidence that ancient stream-flow traveled from the Piedmont to the Blue Ridge upland. Northwest-flowing streams require a Piedmont that was initially higher than the Blue

Ridge upland.

To better quantify the geomorphic and tectonic history of the BRE we created new models based on the time of rifting, sedimentation records, and AHe ages. In this model, the origin of the BRE is interpreted to be related to Cenozoic tectonic rejuvenation in the

Appalachians.

180 Ma

Rifting began in the Atlantic Ocean ~200 million years ago, placing the BRE along one of the oldest passive margins in the world (McHone, 1996; Piqué and Laville, 1996). The extensional tectonics during the Triassic resulted in rift basins and rift-flank uplift along the coast of the Eastern United States. I propose that Triassic rift-flank uplift in the North Carolina Coastal

Plain elevated the Piedmont (~5 km) higher than the modern-day upland (~4.5 km), causing

12 streams to flow to the northwest (Appendix 4). As water flows to the northwest, Neoproterozoic zircons are eroded from plutons in the eastern Piedmont and deposited in the Dan River/Danville and Deep River basins. By the time rifting ends (~180 Ma), the Triassic basins are filled with sediment from the eastern Piedmont (Appendix 4).

To analyze the surface as it changes from 180 Ma to its present-day orientation, we implement AHe isotherms into our model. We assign an AHe isotherm of 60C based on the radiation damage accumulation and annealing model (RDAAM,) which predicts an effective closure temperature of 62C for apatite of 60m radius (Flowers et al., 2009). Apatite previously analyzed from the southern Appalachians have a radius of 31m to 125m (Wolf et al., 1996,

1998; Farley, 2000; Spotila et al., 2004). The size and slow cooling history of Cretaceous grains make this a realistic closure temperature for the region (Wolf et al., 1996, 1998; Flowers et al.,

2009). The present-day geothermal gradient in North Carolina is 15C/km (Spotila et al., 2004;

McKeon et al., 2014). We assign a geothermal gradient of 20C/km, assuming heat flow was higher in the Mesozoic than current regional heat-flow (Nathenson and Guffanti, 1988; McKeon et al., 2014). With these parameters and a surface temperature of ~10C, AHe closure depth corresponds to ~2.5 km (Appendix 4). We based our model on the relationship between vitrinite reflectance (R0) values of coal beds in the basins, 5 km of exhumation and a 20C/km gradient

(Suggate et al.,1998, Nielson et al. 2017) (Appendix 4).

180 Ma to 12 Ma

After the basins fill with sediment from eastern North Carolina, the topography is eroded at a rate of ~23 m/Ma down to an elevation of ~300 m (Appendix 4). Erosion of the rift-flank causes the uplifted region to subside over time (Japsen et al., 2012; Green et al., 2018).

13 Studies of escarpment evolution typically assume that they have remained uplifted since the time of rifting (Japsen et al., 2012). However, models of rift-flank uplift have shown that they typically subside as a result of erosion. Flexural rigidity, commonly quantified using effective elastic thickness (Te), represents the thickness of a flexible sheet that responds to change in loads the same way as the lithosphere and asthenosphere (Tesauro et al., 2012). Modeling sustained uplift caused by rifting along continental passive margins require Te values as large as 115 km

(ten Brink and Stern, 1992; Chéry et al., 1992). Watts (2001) global compilation of Te show values are commonly < 50 km along rifted continental margins (Kusznir et al., 1991; White,

1999). Low Te values commonly exist in a Basin and Range province or rifted areas, perhaps because of a thin lithosphere (Walcott, 1970). Models that use <50 Te values show that the uplifted rift flank subsides during subsequent post-rift cooling (Japsen et al., 2012).

12 Ma to Present-Day

Our results suggest the presence of a Cenozoic fault within the Dan River/Danville basin, approximately 55 km southeast of the BRE’s present-day location. Cenozoic uplift in the

Appalachian region activates the fault and ~1 km of uplift in the Dan River/Danville basin, forming the Blue Ridge Escarpment at ~12 Ma (Appendix 4). A Spotila et al. (2004) study displayed a young (68 Ma) AHe age ~2 km east of the Dan River/Danville basin (Figure 9). The majority of the ages between the basin and the BRE are Late Cretaceous in age. Approximately

30 km southeast of the basin, ages increase to 127 Ma. The change in ages across the Dan

River/Danville basin is indicative of a change in elevation, perhaps due to a fault. The youngest age (~90 Ma) is modeled at the fault in the Dan River/Danville basin before rising back to Early

Cretaceous (~115 Ma) east of the fault. AHe ages in the vicinity of the fault may be younger if

Cenozoic uplift is more significant than predicted. The uplift transformed the Dan River/Danville

14 basin from a deposition site to a source, supplying Neoproterozoic zircons to the BRT’s in

Virginia.

After formation, the escarpment maintains its steep morphology by as it evolves by eroding westward faster than the upland is eroded down. Escarpment studies from around the world use AHe and AFT thermochronology to study landscape evolution (Gunnell et al., 2003;

Spotila et al., 2004; Balestrieri et al., 2005). Slow and constant escarpment retreat since rifting is reflected by AHe ages that become younger closer to the base of the escarpment from the lowland (Persano et al., 2002). Spotila et al. (2004) analyzed AHe from granite and high-grade metamorphic rocks from the Blue Ridge upland and Piedmont. Early Cretaceous AHe ages are found on the upland surface and ~ 80 km southeast of the BRE (Figure 9) (Spotila et al., 2004).

AHe results from the Piedmont reveal Late Cretaceous ages west of the Dan River/Danville basin and Early Cretaceous ages to the east of the basin (Figure 9). A lack of variation in AHe ages toward the base of an escarpment represents an escarpment that rapidly retreated to its present-day location after uplift (~10 to 20 Myr) (Persano et al., 2002; Braun and Beek, 2004;

Balestrieri et al., 2005; Linari et al., 2017). The rapid retreat exposes samples more quickly than is required for the minerals to lock in different cooling ages.

Using the AHe and time of uplift as parameters, we estimate the evolution of the escarpment. Following the uplift of the BRE at 12 Ma the escarpment retreats to the northwest at

~4.5 km/Ma before reaching its present-day location. With this model, eastern-sourced zircons captured in the Dan River/Danville basin are accessible until the crest of the escarpment retreats west of the basin at ~ 10 Ma, meaning the fluvial terraces on top of the divide are ~10 Ma or younger (Appendix 4). This timespan falls within the ~25 Ma timescale of the source to sink processes suggested by (Helland-Hansen et al., 2016). It is also possible that the sediment

15 remained upstream of the terraces for 10 Ma as the escarpment eroded back and that the terraces are <1 Ma. In previous models of origin and evolution of the BRE, the escarpment erodes west of the Dan River/Danville basin at ~35 Ma (Appendix 5). This model requires the unconsolidated BRT’s to be 35 Ma, or the sediment to remain upstream of the terraces for 35

Ma.

We combine the BRE’s erosional surface, expected AHe cooling ages, and spatial correlation of geologic terranes to AHe analysis from Spotila et al. (2004) and zircon ages from this study. Results from our model simulate AHe-age patterns observed in the southern

Appalachians (Spotila et al., 2004) (Appendix 4, Figure 10). Ages are oldest on top of the upland, followed by little deviation in Early Cretaceous ages southeast of the BRE.

Evidence of Cenozoic Uplift

An increase of deposition in the basins seaward of Great Escarpments is evidence of regional uplift. Geomorphic studies in the southern Appalachians show an increase in channel incision during the Neogene (Gallen et al., 2013; Miller et al., 2013; Hill, 2018). Normalized steepness and erosion rates measured in catchments from the Appalachian region are comparable to those from tectonically active basins around the world (Kirby and Whipple, 2012; Miller et al.,

2013). As the base level drops, stream profiles react to the new topography and create knickpoints along the stream bed (Gallen et al., 2013; Miller et al., 2013; Hill, 2018).

Knickpoints in the Cullasaja River basin of western North Carolina are indicative of local tectonic rejuvenation and a >150% increase in relief since the Miocene (Gallen et al., 2013). Hill

(2018) estimates a base-level decrease of ~750 meters based on a study of knickpoints in

Santeetlah Creek in western North Carolina.

16 Deposition of Appalachian-sourced sediment along the Atlantic coast increased by 31.3 km3/Ma during the mid-Miocene (Poag and Sevon, 1989; Pazzaglia and Brandon, 1996; Naeser et al., 2016). This rate of deposition is substantially higher than post-rift deposition rates and is possibly a response to uplift in the source terrain (Poag and Sevon, 1989). Although climate change can impact erosion rates, the existence of knickpoints and increased sediment rates imply that there has been substantial Cenozoic uplift in the southern Appalachians.

Mantle Processes

Studies that advocate for Cenozoic uplift analyze the difference in the density of crust beneath the BRE and the Piedmont (Wagner et al., 2012; Parker et al., 2013; Hill and Stewart,

2018). (Hawman, 2008) and Wagner et al. (2012) investigated crustal shortening under the Blue

Ridge mountains and discovered discontinuities within the upper mantle.

Seismic studies across the Piedmont and the Blue Ridge mountains show a crustal thickness of ~ 46 km (Pratt et al., 1988; Battiau-Queney, 1989; Wagner et al., 2012; Parker et al.,

2013). This crustal thickness is consistent until ~40 km southeast of the BRE, where they found an abrupt change in seismic velocities at depths of 46 and 60 km (Wagner et al., 2012).

They interpreted the change in velocity as a doubled Moho from tectonic wedging underneath the Laurentian lithosphere. Seismic profiles show the thinning or absence of the Moho northwest of the doubled Moho beneath the Piedmont (Wagner et al., 2012). One interpretation of the abrupt change in the density of the crust to the northwest of the eastern Inner Piedmont is delamination of a dense crustal root that uplifted the southern Appalachians (Wagner et al., 2012;

Gallen et al., 2013; Hill, 2018).

17 Support for Fault Rejuvenation

There is an abrupt change in elevation from the Piedmont to the upland west of our proposed fault in the Dan River/Danville basin. This behavior also appears west of the Culpeper basin in Virginia and north of the Marietta-Tryon fault system in South Carolina. The DEM of the BRE displays these abrupt changes, as well as the more rugged terrain in the areas without known normal faults to the southeast (Figure 1, Figure 3). The escarpment in between these features appear to start further east into the Piedmont, exhibiting a less distinct transition from lowland to upland. We propose that Cenozoic fault reactivation in the Culpeper basin, Dan

River/Danville basin and Marietta-Tryon fault system are responsible for the more distinct boundaries along the BRE. The lack of an existing fault resulted in a less abrupt and non-uniform change in elevation in the areas along the escarpment between the basins and faults.

To analyze the relationship between the topography of the BRE and normal faults to the west, I compare the slopes of different areas on the escarpment. I collected elevation and distance data by drawing transects from the Upland surface to the Piedmont on the DEM in

ArcMap. Slopes were calculated by subtracting the elevation at the base of the escarpment

(commonly in the 400-470 m range) from the elevation at the peak of the escarpment and divided by the vertical distance between the two. The average slopes are calculated from ~100 transects along the BRE.

Average slopes along the BRE are steepest ~20 km north of the Marietta-Tryon fault system (25). Slopes fall to 12 after the Marietta-Tryon fault system but rise to 21 northwest of the Dan River/Danville basin (Figure 11.a, 11.b). North of the Dan River/Danville basin average slope drops to 16 before reaching 19 west of the Culpeper basin (Figure 11.c, 11.d). Triassic basins and the Marietta-Tryon fault system are all within ~ 40 km from the base of the BRE

18 (Figure 11.e, 11.b, 11.d). Increased slopes on the BRE and the location of fluvial terraces appear to be related to the presence of Triassic basins or Mesozoic fault systems (Figure 3, Figure 11.a-

11.e).

There is a relationship between the slope of the BRE and geographic location of the basin and faults systems. The stretch of the escarpment north of the Marietta-Tryon fault system has the steepest slope, perhaps due to less erosion over a shorter retreat-distance (~20 km) since formation (Figure 11.e). The slope of the escarpment west Dan River/Danville basin is also is similar to the slope north of the Marietta-Tryon fault system. The Dan River/Danville basin fault is nearly two times farther west of the escarpment than the Mariette-Tryon fault. If how far the escarpment has retreated from its initial location does not affect how steep the escarpment has remained, perhaps slope is also related to other processes. Hill (2018) discusses how the high topography in the Appalachians does not coincide with the physiographic provinces, but cuts across the Blue Ridge in southern Virginia and into the Valley and Ridge in northern Virginia.

Delamination in the lithosphere beneath the Appalachians may cut across the physiographic provinces, influencing the high topography. If the steepness of the escarpment is also related to the abrupt change in density caused by delamination, it may explain why the escarpment west of the Culpeper basin in Virginia is less distinct.

CONCLUSION

The findings of this study are that drainage systems flowed northwest from the Piedmont to deposit fluvial terraces perched on the Blue Ridge escarpment. We used AHe age patterns and

Cambrian-Neoproterozoic zircons to constrain the evolution of the Blue Ridge escarpment.

Access to zircons found in the eastern Piedmont requires the lowland to be higher than the upland in the past. We advocate for a model of rift-flank uplift in the Triassic that elevates the

19 Piedmont and allows stream-flow to the northwest. However, we suggest the Blue Ridge escarpment formed in the Miocene as a result of delamination in the western Piedmont and eastern Blue Ridge. After the establishment of the Blue Ridge escarpment, a drainage divide formed at the crest results in parallel retreat of the escarpment and divide (Wagner et al., 2012;

Gallen et al., 2013). Topographic observations and AHe ages across the Inner Piedmont and Blue

Ridge upland are consistent with a period of rapid erosion shortly after uplift in western

Piedmont (Persano et al., 2002; Spotila et al., 2004). Acknowledging substantial rejuvenation in the post-rift topography of passive margins is an advancement in understanding their evolutionary processes. Dating the BRT’s and a cohesive AHe analysis into the North Carolina

Coastal Plain and Piedmont could provide further refinement of when the Blue Ridge escarpment formed.

20 FIGURES

Figure 1. DEM that shows the Blue Ridge Escarpment’s abrupt change in elevation from the Piedmont to the Blue Ridge upland.

Figure 2. Gravel and sandy matrix from selected Blue Ridge Terrace (BRT) sites shown in Figure 3. Gravel composition is predominantly quartz in the form of vein quartz and quartzite. All terraces have an elevation of 650 to 850 m and are within 600 m on either side of the eastern continental divide. A-C) Well-rounded cobbles ranging from high to low sphericity and a sandy- clay matrix from BRT 3. D-E) Rounded to sub-rounded, low sphericity pebbles and cobbles and sandy clay matrix of BRT1. F) sub-rounded to sub-angular ranging from high to medium sphericity from BRT 7.

Figure 3. Fluvial terrace locations in relation to important topographic features. Terraces are located along the Eastern Continental Divide (black line) at elevations of 670 m to 980 m. All terraces sit within 40 km of a Triassic rift basin (shown in grey) or Mesozoic fault systems (Marietta-Tryon graben) shown southeast of terrace 6 and 7 along the NC-SC state line.

Figure 4. Cathodoluminescence (CL) images of magmatic zircons from BRT 1, 3 and 7 displaying oscillatory zoning (3a, 7a, 7b, 7d) and various grain shapes.

Figure 5. A) Histogram and probability density of zircon ages in BRT 1 showing peaks at 1100 Ma and 1400 Ma, with grains at 554 Ma, 571 Ma, 845 Ma, and 2351 Ma. B) U/Pb Concordia plot displaying data with only one grain (415 Ma) less than 85% concordant. C) Weighted mean and Concordia plots determined with Isoplot (Ludwig, 2008).

Figure 6. Histogram and probability density of zircon ages in BRT 3 showing peaks at 11oo Ma and 1400 Ma, with grains at 640 Ma, 656 Ma, 2000 Ma, and 2500 Ma. B) U/Pb Concordia plot displaying data with only two grains (997 Ma and 1364 Ma) less than 85 % concordant. C) Weighted mean and Concordia plots determined with Isoplot (Ludwig, 2008)

Figure 7. A) Histogram and probability density of zircon ages in BRT 7 showing a significant peak at 440 Ma, with grains ranging from 325 Ma to 560 Ma and eleven grains from 1006 Ma to 1433 ma. B) U/Pb Concordia plot displaying the bulk of the data is concordant and 46 out of 495 grains less than 80% concordant. C) Weighted mean and Concordia plots determined with Isoplot (Ludwig, 2008).

Figure 8. Plutons in Virginia, North Carolina, South Carolina, and Georgia that are of similar ages to detrital zircons found in fluvial terraces on top of the Blue Ridge Escarpment. Plutons age are based on zircon ages retrieved from The southern Appalachian Geochronology Database compiled by Miller (2013), Voice (2010) Global Detrital Zircon Database (GDZDb), and the USGS Geochronological Database (Zartman et al., 2003). Geologic terranes from the regional geologic map by Hibbard et al. (2006).

Figure 9. AHe ages from the Blue Ridge upland and Piedmont in NC and VA (Bank, 2001; Spotila et al., 2004)

Figure 10. Scatter plot comparing the Observed and Predicted AHe values from the Blue Ridge Upland and Piedmont.

Figure 11.a. a) Calculated slope along the Blue Ridge Escarpment spanning from north of Marion, NC to north of Elkin, NC. b) Histogram showing the average slope for this region as 12.

Figure 11.b. a) Calculated slope along the Blue Ridge Escarpment northwest of the Dan River/Danville basin. b) Histogram showing the average slope for this region as 21.

Figure 11.c. a) Calculated slope along the Blue Ridge Escarpment southwest of the Lexington, VA. b) Histogram showing the average slope for this region as 16.

Figure 11.d a) Calculated slope along the Blue Ridge Escarpment west of the Culpeper basin in VA. b) Histogram showing the average slope for this region as 19.

Figure 11.e. a) Calculated slope along the Blue Ridge Escarpment north of the Marietta-Tryon Fault system. b) Histogram showing the average slope for this region as 25.

Table 1. U-Pb geochronologic analyses of BRT 1.

Isotope ratios Apparent ages (Ma)

U/Th 207Pb* ± 206Pb* ± error 206Pb* ± 207Pb* ± Best age ± Conc Analysis 235U* (%) 238U (%) corr. 238U* (Ma) 235U (Ma) (Ma) (Ma) (%)

-BRT1 run 1 Spot 1.6 0.6488 5.2 0.0666 1.2 0.24 415.4 5 507.7 20.9 415.4 5 43.8 93

-BRT1 run 1 Spot 2.7 0.7257 1.4 0.0899 1.2 0.87 554.9 6.4 554 5.9 554.9 6.4 100.8 156 -BRT1 run 1 Spot 1.5 0.7854 1.6 0.0927 1 0.62 571.2 5.3 588.5 7 571.2 5.3 87.1 136 -BRT1 run 1 Spot 4.1 0.9726 1.6 0.1128 1 0.61 688.8 6.5 689.8 8.2 688.8 6.5 99.4 78 Table 1. U/Pb geochronologic analysis of Neoproterozoic detrital zircon from BRT 1. 2) Best age is determined from 206Pb/238U age for analyses with 206Pb/238U age <1000 Ma and from 206Pb/207Pb age for analyses with 206Pb/238Uage > 1000 Ma. 3) Concordance is based on 206Pb/238U age / 206Pb/207Pb age. Value is not reported for 206Pb/238U ages <500 Ma because of large uncertainty in 206Pb/207Pb age. 4) Analyses with 206Pb/238U age > 500 Ma and with >20% discordance (<80% concordance) are not included. 5) Analyses with 206Pb/238U age > 500 Ma and with >5% reverse discordance (<105% concordance) are not included. 6) Analyses conducted by LA-MC-ICPMS, as described by Gehrels et al. (2008).

Table 2. U-Pb geochronologic analyses of BRT 3.

Isotope ratios Apparent ages (Ma)

U/Th 207Pb* ± 206Pb* ± error 206Pb* ± 207Pb* ± Best age ± Conc Analysis 235U* (%) 238U (%) corr. 238U* (Ma) 235U (Ma) (Ma) (Ma) (%)

-BRT3 Spot 1.6 0.9059 1.7 0.1044 1.1 0.67 640.3 6.9 654.9 8.2 640.3 6.9 90.7 417 -BRT3 Spot 4.2 0.9212 1.7 0.1072 1.1 0.65 656.4 6.9 663 8.4 656.4 6.9 95.8 85

6 Table 2. U/Pb geochronologic analysis of Cryogenian detrital zircon from BRT 3. 2) Best age is determined from 206Pb/238U age for analyses with 206Pb/238U age <1000 Ma and from 206Pb/207Pb age for analyses with 206Pb/238Uage > 1000 Ma. 3) Concordance is based on 206Pb/238U age / 206Pb/207Pb age. Value is not reported for 206Pb/238U ages <500 Ma because of large uncertainty in 206Pb/207Pb age. 4) Analyses with 206Pb/238U age > 500 Ma and with >20% discordance (<80% concordance) are not included. 5) Analyses with 206Pb/238U age > 500 Ma and with >5% reverse discordance (<105% concordance) are not included. 6) Analyses conducted by LA-MC-ICPMS, as described by Gehrels et al. (2008)

Table 3. U-Pb geochronologic analyses of BRT 7. Isotope ratios Apparent ages (Ma) Analysis U/Th 207Pb* ± 206Pb* ± error 206Pb* ± 207Pb* ± Best age ± Conc 235U* (%) 238U (%) corr. 238U* (Ma) 235U (Ma) (Ma) (Ma) (%) -BRT7 run 1Spot 83 37.4 0.3852 1.0 0.0518 0.8 0.86 325.3 2.7 330.9 2.8 325.3 2.7 87.8 -BRT7 run 1Spot 125 41.2 0.3908 1.0 0.0519 0.8 0.83 326.3 2.6 334.9 2.9 326.3 2.6 82.5 -BRT7 run 1Spot 56 46.7 0.3909 1.5 0.0523 1.1 0.73 328.5 3.6 335.0 4.4 328.5 3.6 86.4 Run 2 1st set-BRT7 Spot 323 8.0 0.3874 1.4 0.0523 1.2 0.91 328.8 3.9 332.4 3.8 328.8 3.9 91.9 -BRT7 run 1Spot 193 57.9 0.3822 1.0 0.0524 0.8 0.83 329.2 2.6 328.7 2.7 329.2 2.6 101.3 Run 2 1st set-BRT7 Spot 405 46.3 0.3907 1.1 0.0524 0.9 0.83 329.5 2.8 334.9 3.0 329.5 2.8 88.6 Run 2nd set-BRT7 Spot 449 129.3 0.3881 1.0 0.0527 0.8 0.75 330.9 2.5 333.0 2.9 330.9 2.5 95.2 -BRT7 run 1Spot 35 33.5 0.4060 1.0 0.0530 0.8 0.79 333.1 2.5 346.0 2.9 333.1 2.5 76.8 37 Run 2 1st set-BRT7 Spot 412 30.3 0.3900 1.1 0.0531 0.9 0.85 333.7 3.1 334.4 3.2 333.7 3.1 98.5

-BRT7 run 1Spot 6 13.0 0.3934 1.4 0.0533 1.3 0.88 334.5 4.2 336.8 4.2 334.5 4.2 94.8 -BRT7 run 1Spot 249 28.4 0.4361 1.3 0.0533 1.2 0.92 334.6 3.9 367.5 4.0 334.6 3.9 57.6 Run 2 1st set-BRT7 Spot 342 18.4 0.4036 1.0 0.0539 0.7 0.73 338.7 2.4 344.3 2.9 338.7 2.4 88.7 -BRT7 run 1Spot 118 155.9 0.3962 1.1 0.0541 0.9 0.82 339.6 2.9 338.9 3.1 339.6 2.9 101.6 -BRT7 run 1Spot 252 42.5 0.3976 0.9 0.0545 0.8 0.81 342.1 2.5 339.9 2.7 342.1 2.5 105.4 -BRT7 run 1Spot 50 6.6 0.4135 1.0 0.0552 0.9 0.88 346.4 3.1 351.4 3.1 346.4 3.1 90.1 -BRT7 run 1Spot 151 10.1 0.4269 2.0 0.0556 1.8 0.92 348.7 6.2 361.0 6.1 348.7 6.2 79.0 Run 2 1st set-BRT7 Spot 345 41.7 0.4364 2.8 0.0558 2.0 0.73 350.0 6.9 367.7 8.6 350.0 6.9 72.7 -BRT7 run 1Spot 160 8.2 0.4155 1.1 0.0560 0.9 0.87 351.0 3.2 352.8 3.2 351.0 3.2 96.2 -BRT7 run 1Spot 168 77.9 0.4022 4.8 0.0563 4.3 0.89 353.0 14.6 343.2 14.0 353.0 14.6 127.1 -BRT7 run 1Spot 60 18.0 0.4268 1.2 0.0568 1.0 0.88 356.1 3.6 360.9 3.6 356.1 3.6 90.9 Run 2 1st set-BRT7 Spot 381 8.9 0.4192 1.2 0.0569 1.0 0.82 357.0 3.5 355.5 3.7 357.0 3.5 103.3

-BRT7 run 1Spot 187 10.3 0.4402 1.3 0.0571 1.0 0.79 357.9 3.5 370.4 3.9 357.9 3.5 79.7 Run 2 1st s et-BRT7 Spot 348 1.7 0.4410 1.3 0.0576 1.2 0.89 360.8 4.1 371.0 4.0 360.8 4.1 83.0 -BRT7 run 1Spot 174 6.1 0.4404 1.2 0.0582 0.9 0.72 364.6 3.1 370.5 3.8 364.6 3.1 89.4 Run 2 1st set-BRT7 Spot 354 6.7 0.4402 1.1 0.0585 1.0 0.89 366.3 3.4 370.4 3.4 366.3 3.4 92.5 -BRT7 run 1Spot 274 5.4 0.4454 1.0 0.0588 0.8 0.84 368.1 3.0 374.1 3.1 368.1 3.0 89.4 Run 2nd set-BRT7 Spot 429 7.8 0.4342 0.9 0.0592 0.7 0.77 370.9 2.6 366.2 2.8 370.9 2.6 110.3 Run 2 1st set-BRT7 Spot 299 2.2 0.4448 1.1 0.0593 1.0 0.86 371.4 3.4 373.6 3.5 371.4 3.4 95.9 -BRT7 run 1Spot 23 4.0 0.4508 1.7 0.0598 1.4 0.80 374.3 5.1 377.8 5.5 374.3 5.1 93.6 Run 2 1st set-BRT7 Spot 383 3.9 0.4635 1.1 0.0605 1.0 0.89 378.7 3.5 386.7 3.5 378.7 3.5 87.1 Run 2nd set-BRT7 Spot 475 4.3 0.4627 1.4 0.0609 1.2 0.87 381.1 4.6 386.1 4.6 381.1 4.6 91.6 -BRT7 run 1Spot 78 6.6 0.4624 1.1 0.0609 1.0 0.87 381.1 3.7 385.9 3.6 381.1 3.7 91.9 Run 2 1st set-BRT7 Spot 343 1.7 0.4654 1.1 0.0617 0.9 0.90 385.8 3.5 388.0 3.4 385.8 3.5 96.2

38 Run 2 1st set-BRT7 Spot 315 0.8 0.4684 0.7 0.0617 0.6 0.78 386.1 2.1 390.1 2.3 386.1 2.1 93.4

-BRT7 run 1Spot 57 3.0 0.4603 1.2 0.0617 1.0 0.82 386.2 3.6 384.5 3.7 386.2 3.6 103.2 -BRT7 run 1Spot 277 2.7 0.4762 0.9 0.0619 0.7 0.73 387.2 2.5 395.5 3.0 387.2 2.5 87.2 Run 2nd set-BRT7 Spot 435 1.6 0.5238 2.5 0.0622 0.8 0.34 388.8 3.2 427.7 8.6 388.8 3.2 60.5 -BRT7 run 1Spot 116 1.2 0.5166 1.6 0.0622 0.7 0.46 389.1 2.7 422.9 5.5 389.1 2.7 63.6 -BRT7 run 1Spot 212 4.0 0.4803 1.2 0.0625 0.9 0.81 390.6 3.5 398.3 3.8 390.6 3.5 88.2 Run 2 1st set-BRT7 Spot 349 3.4 0.4716 1.1 0.0627 0.9 0.84 392.3 3.5 392.3 3.5 392.3 3.5 100.0 -BRT7 run 1Spot 244 2.9 0.5218 2.8 0.0628 1.1 0.41 392.7 4.4 426.3 9.6 392.7 4.4 64.1 Run 2nd set-BRT7 Spot 483 5.5 0.4967 1.2 0.0630 1.0 0.87 393.6 3.8 409.5 3.9 393.6 3.8 78.7 -BRT7 run 1Spot 54 4.8 0.4790 1.2 0.0630 0.9 0.79 393.8 3.5 397.4 3.9 393.8 3.5 94.1 Table 3. U/Pb geochronologic analysis of detrital zircon from BRT 7 that are younger than 400 Ma. 2) Best age is determined from 206Pb/238U age for analyses with 206Pb/238U age <1000 Ma and from 206Pb/207Pb age for analyses with 206Pb/238Uage > 1000 Ma. 3) Concordance is based on 206Pb/238U age / 206Pb/207Pb age. Value is not reported for 206Pb/238U ages <500 Ma because of large uncertainty in 206Pb/207Pb age. 4) Analyses with 206Pb/238U age > 500 Ma and with >20% discordance (<80% concordance) are not included. 5) Analyses with 206Pb/238U age > 500 Ma and with >5% reverse discordance (<105% concordance) are not included. 6) Analyses conducted by LA-MC-ICPMS, as described by Gehrels et al. (2008).

APPENDIX 1: CATHODOLUMINESCENCE OF DETRITAL ZIRCONS IN BRT 1.

APPENDIX 2: CATHODOLUMINESCENCE OF DETRITAL ZIRCONS IN BRT 3.

APPENDIX 3: CATHODOLUMINESCENCE OF DETRITAL ZIRCONS IN BRT 7.

APPENDIX 5: SIMPLIFIED MODEL OF A BLUE RIDGE ESCARPMENT FORMED BY RIFT-FLANK UPLIFT IN THE TRIASSIC, ACCORDING TO SPOTILA ET AL. 2004.

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