Selected Presentation from the INSTAAR Monday Noon Seminar Series.

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1 Selected Presentation from the INSTAAR Monday Noon Seminar Series. Institute of Arctic and Alpine Research, University of Colorado at Boulder. This seminar presentation has been posted to the internet to foster communication with the science community and the public. Most of the INSTAAR presentations were originally given in PowerPoint format; they were converted to Adobe PDF for posting. You may need to install the free Adobe Acrobat Reader to view these files. These presentations are "works in progress". They are not peer reviewed. They should not be referenced for any kind of publication. Contact the author for proper references and additional information before any use, even for unpublished works such as your own presentations. LICENSING AGREEMENT. Free use of these presentations is limited to a nonprofit educational or private non-commercial context and requires that you contact the author, give credit to the author, and display the copyright notice. All rights to reproduce these presentations are retained by the copyright owner. Images remain the property of the copyright holder. By accessing these presentations, you are consenting to our licensing agreement. 31 Mar Eric Leonard, Geology Department, Colorado College. eleonard@coloradocollege.edu "Neogene tectonic uplift, erosion and isostasy on the Colorado Piedmont." Seminar given at INSTAAR, University of Colorado. Copyright 2003 Eric Leonard. All Rights Reserved. Leonard presentation (1.1 Mb PDF).

2 Selected Presentation from the INSTAAR Monday Noon Seminar Series. Institute of Arctic and Alpine Research, University of Colorado at Boulder Mar. Eric Leonard, Geology Department, Colorado College. "Neogene tectonic uplift, erosion and isostasy on the Colorado Piedmont." Seminar given at INSTAAR, University of Colorado. Copyright 2003 Eric Leonard. All Rights Reserved. Leonard presentation (1.1 Mb PDF). Abstract The Colorado piedmont region was tilted and broadly warped during the Late Cenozoic. This deformation involved a complex interplay of tectonic forcing, river erosion, and isostatic response to erosion. Modeled erosional isostasy closely replicates the observed pattern of deformation, but accounts for only about half its magnitude. The remainder reflects tectonic rock uplift that increases southward across the piedmont, likely reflecting proximity to the northward-propagating Rio Grande Rift. This differential uplift triggered differential erosion, concentrated on southern piedmont river systems, particularly the Arkansas River. Differential erosion led in turn to differential isostatic rock uplift focused on the Arkansas drainage. Covariation of tectonic uplift, erosion, and isostatic compensation across the piedmont reflects a strong positive

3 feedback between uplift-induced erosion and erosion-induced isostasy, which has progressed to the point that isostatic uplift is approximately equal to the initial tectonic forcing.

4 LATE CENOZOIC TECTONIC UPLIFT, EROSION, AND ISOSTASY ON THE COLORADO PIEDMONT Eric Leonard Department of Geology Colorado College

5 Southern Rockies and Western Great Plains

7 Models of Post-Laramide Evolution of Front Range /Piedmont Topography Classical Model (Epis and Chapin, 1975) -- Differential uplift of mountains with respect to plains along reactivated range-front faults. Modern mountain front is a product of this faulting. Alvarado Ridge Model (Eaton, 1986) -- Regional tectonic rock uplift centered on Southern Rockies but involving both mountains and high plains. Modern mountain front forms by differential erosion. Climate-driven Model (Gregory and Chase, 1992, 1994) -- No post-laramide tectonic uplift of mountains or high plains. Neogene incision driven by climate change. Modern mountain front forms by differential erosion.

9 Structure Contours on the Rocky Mountain Surface on Rampart Range and on the Late Eocene Unconformity on the Piedmont (Leonard and Langford, 1994)

10 Modern dip of the Late Eocene Unconformity and Paleoflow Direction in Overlying Castle Rock Conglomerate (Morse, 1985) Modern dip direction Mean X-bed dip Paleochannel trend

11 Evidence of Post-Laramide Tilting of the Front Range and Piedmont Northward post-formational tilt of late Eocene unconformity on piedmont and Rocky Mountain Surface on the Front Range (Leonard and Langford, 1994) Southward truncation of latest Eocene/Oligocene rocks below the middle Miocene sub-ogallala unconformity (Kelley and Chapin, 1995) Multiple post-eocene changes in piedmont drainage directions (Steven et al., 1997) Warping of middle Miocene basal Ogallala surface across the Arkansas River valley (Steven et al., 1997; Leonard 2002)

12 Warping of the Basal Ogallala Surface on the Piedmont

13 Possible Causes of Post-Laramide Tilting/Warping of Front Range and Piedmont Tectonic rock uplift, possibly as a far field effect of Rio Grande Rifting or the Alvarado Ridge of Eaton (Leonard and Langford, 1994; Steven et al., 1997) Differential erosionally driven isostatic rock uplift due to differential erosion by piedmont river systems

14 Research Questions Can post-laramide deformation of the Colorado piedmont be explained as a flexural isostatic response to differential erosion? More generally, to what extent was deformation driven by tectonic forcing, to what extent by geomorphic forcing? Does the deformation of the piedmont provide evidence for feedback relationships between rock uplift and erosion (Molnar and England, 1990)?

15 Methodology Map and project the modern position of the middle Miocene basal Ogallala surface on the piedmont Compare the basal Ogallala surface with modern topography to assess amount and distribution of postmiddle Miocene erosion and deposition Use a flexural model to evaluate isostatic response to differential geomorphic loading, assuming isostatic equilibrium conditions Compare modeled and observed deformation to evaluate relative magnitudes of geomorphic/isostatic (modeled) forcing and tectonic (residual) forcing

16 Modeled North-South Transects Across the Piedmont

17 Modern Surface Topography and Basal Ogallala Surface S Canadian Arkansas Surface Topography Basal Ogalla Surface South Platte North Platte N 5 o Polynomial Fit To Basal Ogallala Surface Altitude (m) Canadian Arkansas Polynomial Best Fit Line South Platte North Platte Post-Ogallala Loading and Unloading Canadian Arkansas Surface Loading and Unloading South Platte North Platte Distance north of Colorado-New Mexico state line (km)

18 Flexural Isostatic Model (Turcotte and Schubert, 1982; Pazzaglia and Gardner, 1994) W b (c) = W o e -c/a (cos c/a + sin c/a) W b (c) = Deflection at a point at distance c from the point of loading W o = Deflection at the point of loading a = Lithospheric flexural parameter W o = r s gd xy a3 /8D r s = Crustal sediment density D xy = Cross-sectional area of loading in cell D = Flexural rigidity of the lithosphere a = (4D/r m g) 0.25 r m = Mantle density g = Acceleration of gravity

19 Input Variables FLEXURAL RIGIDITY (D) Best estimate: Nm (Angevine and Flanagan1987; Babits 1987; Reinke 1991) Modeled range: Nm DENSITY (r) Model assumed densities: r s = 2500 kg/m 3, r m = 3300 kg/m 3 Sensitivity test density range: r s = kg/m 3, r m = kg/m3

20 Limitations and Assumptions Model Limitations One-dimensional modeling Flexural rigidity of lithosphere is uniform along a transect Lithosphere is unbroken Assumes isostatic equilibrium conditions Geologic Assumptions Basal Ogallala surface is assumed to have been initially planar and east dipping In final portion of project an original dip of the basal Ogallala surface must be assumed

21 Modern Ground Surface and Basal Ogallala Surface Along North-South Transects Altitude (m) S Canadian Arkansas South Platte Canadian Arkansas South Platte Eastern transect 103 o 08'W North Platte Middle transect 103 o 50W North Platte N x Western transect 104 o 30'W 1500 Canadian Arkansas South Platte North Platte Distance north of Colorado-New Mexico state line (km) Ground Surface Topography Basal Ogallala Surface

22 Results of Flexural Modeling Basal Ogallala Surface Modeled Flexure at Nm Modeled Flexure at Nm Modeled Flexure at Nm Modeled Flexure at Nm Altitude of Basal Ogallala Surface (m) S Arkansas Arkansas Eastern transect 103 o 08'W South Platte Middle transect 103 o 50W South Platte N Western transect 104 o 30'W Modeled isostatic uplift (m) Arkansas South Platte Distance north of Colorado-New Mexico state line (km)

23 Observed vs. Modeled Tilt of the Basal Ogallala Surface Transect Observed Tilt (m) Modeled Tilt D = Nm (m) Modeled Tilt D = Nm (m) Modeled Tilt D = Nm (m) Modeled Tilt D = Nm (m) Eastern 103 o 8'W (77%) 222 (65%) 155 (45%) 69 (20%) Central 103 o 50'W (86%) 390 (73%) 277 (52%) 140 (26%) Western 104 o 30'W (66%) 537 (60%) 411 (46%) 207 (23%)

24 Components of West-to-East Tilt of the Basal Ogallala Surface (McMillan et al., 2002,; Leonard, 2002) Original slope of basal Ogallala surface Tilting due to erosionally driven isostasy Tilting due to tectonics

25 Estimates of Original Depositional Slope of Basal Ogallala Group Sediments Paleohyrdaulic measurements (McMillan et al., 2002) Best mean gradient = 1.1 m km -1 Range used in error estimation = m km -1 Analogy to modern South Platte and Arkansas gradients (Leonard, 2002) Best mean gradient = 1.7 m km -1 Range used in error estimation = m km -1

26 Location of West-to-East Profiles

27 Components of Westto-East Tilt of the Basal Ogallala Surface W Tectonic forcing Isostasy Original Slope North Platte-South Platte interfluve (41o N) E 3.5 m/km Tectonic forcing Isostasy Original Slope South Platte-Arkansas interfluve (39o20'N) 4.8 m/km Altitude (m) 250 Tectonic forcing Isostasy Original Slope Arkansas-Canadian interfluve (37oN) Distance west of 102oW (km) 6.4 m/km

28 Components of West-to-East Tilt of the Basal Ogallala Surface PROFILES (north-to-south) TOTAL TILT (m) ASSUMED INITIAL SLOPE (m) MODELED ISOSTATIC TILT D = Nm (m) "RESIDUAL" TECTONIC TILT (m) 41 o N (interfluve) 39 o 20'N (interfluve) 38 o N (Arkansas valley) 37 o N (interfluve) (23-378) ( ) ( ) ( )

29 Late Cenozoic tectonic rock uplift increased westward and southward across the Colorado piedmont. Maximum tectonic uplift of the piedmont occurred near mountain front in the southern portion of the state. W 250 Tectonic forcing Isostasy Original Slope Tectonic forcing Isostasy Original Slope Tectonic forcing Isostasy Original Slope North Platte-South Platte interfluve (41o N) South Platte-Arkansas interfluve (39o20'N) Arkansas-Canadian interfluve (37oN) Distance west of 102oW (km) E Altitude (m)

30 Differential rock uplift has led to differential stream incision, concentrated on the southern and western regions of the piedmont. Greatest magnitude of erosion is in western portions of the Arkansas River drainage.

31 S Eastern transect 103 o 08'W N 500 Differential erosion has lead to differential flexural isostatic uplift, concentrated in the western portion of the southern piedmont, particularly along the Arkansas valley. Altitude of Sub-Ogallala Surface (m) Arkansas Arkansas Arkansas South Platte Middle transect 103 o 50W South Platte Western transect 104 o 30'W South Platte Modeled isostatic uplift (m) Distance north of Colorado-New Mexico state line (km)

32 W Tectonic forcing Isostasy North Platte-South Platte interfluve (41o N) E This differential isostatic uplift is the critical link in a positive feedback relationship between rock uplift and erosion on the piedmont. Original Slope Tectonic forcing Isostasy Original Slope Tectonic forcing South Platte-Arkansas interfluve (39o20'N) Arkansas-Canadian interfluve (37oN) Altitude (m) 1800 Isostasy Original Slope Distance west of 102oW (km)

33 Conclusions (I) Both tectonic and geomorphic forcing have played significant roles in late Cenozoic warping of the Colorado piedmont. Modeled flexural isostatic response to post middle Miocene erosion closely replicates both the pattern and wavelength of warping of the middle Miocene basal Ogallala surface, assuming a lithospheric flexural rigidity of approximately Nm. Erosionally driven flexural isostasy can account for only about half of of the post middle Miocene warping and tilt. The remainder of the deformation must reflect tectonic forcing.

34 Conclusions (II) A strong feedback relationship has developed between rock uplift and erosion on the Colorado piedmont. Differential tectonic uplift of the southern piedmont initiated processes of differential stream erosion, focused on the Arkansas River and its tributaries. This differential erosion has led, in turn, to differential isostatic rock uplift of the southern piedmont. This feedback relationship has progressed to the point that across the piedmont magnitudes of isostatic rock uplift are subequal to the magnitude of the original tectonic forcing.

35 Magnitude of Neogene Incision of Piedmont (McMillan, 2003)

39 Apatite Fission-Track Traverses on the Eastern Flank of the Colorado Rockies (data from Kelley and Chapin, in press; base map from McMillan, 2003) LP -- Longs Pk./St. Vrain River PP -- South side of Pikes Pk. WM -- Southern Wet Mts. LP PP WM

41 AFT Traverse - Longs Peak/St. Vrain Canyon ± ± ±8.5 ALTITUDE (m) ± ± ± ± ± ± ± ± ± ± DISTANCE (km) 47.5± ±3.4

42 AFT Traverse - South Side of Pikes Peak ± ALTITUDE (m) ± ± ± ± ± ± ± ± DISTANCE (km)

43 AFT Traverse -Southern Wet Mountains SO. WET MTS. AFT PROFILE ± ± ±13.2 ALTITUDE (m) ± ± ± ± ± ± DISTANCE (km)

44 Estimates of Post-Laramide Displacement on the Rampart Range Fault Colman (1985) 450m+ Jacob and Arbutus (1985) m Leonard and Langford (1994) m

45 With thanks to: Beth McMillan, University of Arkansas-Little Rock Shari Kelley, New Mexico Tech Funding provided by a Colorado College Benezet Research Grant

Selected Presentation from the INSTAAR Monday Noon Seminar Series.

Selected Presentation from the INSTAAR Monday Noon Seminar Series. Institute of Arctic and Alpine Research, University of Colorado at Boulder. http://instaar.colorado.edu http://instaar.colorado.edu/other/seminar_mon_presentations