Prelude to the ‘plano’: Assessing the contribution of Jurassic crustal thickening to growth of the Cretaceous Nevadaplano

PhD. Student, Drew Levy, from the University of Nevada-Reno received an award for his proposal which summarized below. Drew will be working with Dr. Matthew Heizler.

The New Mexico Geochronology Research Laboratory (NMGRL) is a participant in the “Awards for Geochronology Student Research” program (AGeS2 ). AGeS2 grants are funded by the National Science Foundation Earthscope program, in conjunction with the Geological Society of America, and are designed to link students with geochronology laboratories to facilitate in depth student understanding of geochronology methods with hands on experience ultimately leading to publication of new data.

Significance

Orogenic plateaus are regions of thick crust (~65 km) and high elevation (>3 km) located at convergent plate margins [1]. Investigations of modern orogenic plateaus, such as the Tibetan Plateau (Asia) and the Altiplano (South America), suggest shortening of the crust and mantle lithosphere results in plateau development [2-4]. Such a crustal thickening model can be tested at an ancient plateau (i.e. the Nevadaplano of the North American Cordillera; NAC; Fig. 1A), where exhumed crust allows analysis of thickening mechanisms [5-7]. In contrast to modern plateaus [8], the timescale for growth of the Nevadaplano is poorly constrained [9]. With new research on the timing of orogenic plateau development in the NAC, we can better understand the mechanisms and rate of crustal thickening and surface uplift. An improved understanding of plateaus impacts our understanding of (1) the feedbacks between high topography and climate [2,10], (2) seismic hazard along orogenic plateau margins [11] and (3) the relationships between intraplate magmatism and mineralization in thickened crust [12].

Early Cenozoic collapse of the NAC plateau, which extended from present British Columbia to Mexico [1], is well constrained by studies of exhumed high-grade metamorphic terranes [5,13-15]. Yet, the timing and mechanism of crustal thickening remains poorly resolved in this region [9,16]. While the Sevier fold-thrust belt marked the eastern margin of the Late Cretaceous plateau [7], the role of the Jurassic fold-thrust belt of the Sevier hinterland is less understood [17](Fig. 1A). Reexamination of the hinterland thrust belt in the eastern Basin and Range (BnR) offers the opportunity to resolve the crustal shortening contribution to, and assess the timescales of, plateau orogenesis. The BnR is the ideal location to study the structural framework of the ancient Nevadaplano, as Late Cenozoic extension exhumes lower, middle and upper crustal sections affected by Mesozoic deformation, metamorphism and magmatism [18] (Fig. 1A). Study of the Nevadaplano provides a record of crustal thickening not accessible at modern plateaus, where the framework supporting high topography remains buried.

Systematic investigation of Jurassic crustal deformation in the eastern BnR is required to determine the timescales and rates of crustal thickening during plateau formation. An improved dataset of the magnitude and timing of shortening is thus required to identify the primary mechanism of plateau thickening.

Hypothesis

Proxies for a high elevation plateau supported by thick crust in the central North American Cordillera include upper amphibolite-grade mineral assemblages in exhumed lower crustal sections [19], isotopic studies of magmatic and volcanic geochemistry [20], and paleoaltimetry studies of late Cretaceous-early Cenozoic basins in the eastern Basin and Range [21]. Direct field evidence of Cretaceous shortening is well documented in the Sevier belt of Utah [7], however the magnitude of shortening in the Sevier belt alone does not account for the thickening needed to match proxies for thickness of the Nevadaplano. My preliminary fieldwork has confirmed pre-Late Jurassic deformation in northeastern Nevada, but the magnitude and timing of deformation is not previously determined. To address these issues, I am using a combination of ⁴⁰Ar/³⁹Ar thermochronology, geologic mapping, and structural analysis to test the following: Jurassic crustal shortening in the eastern Basin and Range contributed significantly to crustal thickening of the proto-Nevadaplano. Accordingly, Cretaceous thickening there was negligible (Fig. 1B).

Proposed work

To constrain the timing and rate of Jurassic deformation in northeast Nevada, I propose new ⁴⁰Ar/³⁹Ar thermochronology of synkinematic muscovite from greenschist-amphibolite facies strata in the Toano and Pilot Peak Ranges (Fig. 1B). My target samples are at stratigraphic depths of ~8 km [17], which equates to temperatures of ~200°C at a typical geothermal gradient. Therefore, to explain their observed metamorphism, they must have experienced tectonic burial or thermal perturbations due to relatively local intrusions. These end-member models have unique predictions for the cooling history of the mica, which I will directly test with thermochronology.

I plan to work with Dr. Matt Heizler at New Mexico Tech to conduct muscovite ⁴⁰Ar/³⁹Ar multi-diffusion domain modeling (MDD). His lab has been perfecting this relatively new technique, which can extract the temperature-time (T-t) path of a sample between ~425°C to ~250°C [22-24]. With this T-t curve, I will be able to assess cooling rates directly linked with timescales of thrust exhumation or conductive cooling.

In the Toano Range, I will analyze muscovite from foliated greenschist-facies strata along a vertical transect. Previous models argue these rocks were uplifted during Cretaceous shortening [16], but crosscutting relationships suggest otherwise. Five kilometers north of the proposed sample locality, a 155 Ma, undeformed pluton crosscuts equivalent strata placing a minimum age on deformation. The muscovite is foliation parallel, indicating synkinematic growth. I predict thermochronology from this locality will show rapid Middle-to-Late Jurassic cooling via thrust exhumation (Fig. 1B).

The next range east is the Pilot Range, which hosts ductilely deformed, amphibolite75 ies strata in a thrust footwall. The thrust is crosscut by 160 Ma dikes that exhibit sub-solidus deformation. Previous studies [17] suggest deformation propagated eastward from the Toano Range during the Jurassic, synchronous with intrusion of granodiorite bodies in the Pilot Range. At this field site, I have collected muscovite-biotite schist samples during a reconnaissance trip. During early summer 2019, I will return to the Pilot Range for a thorough mapping and sampling trip targeting rocks in the hanging wall and footwall sections. Regional field relationships suggest lower-plate metamorphism in the Pilot Range resulted from thrust burial, which should produce a T-t curve indicative of post-Jurassic conductive cooling (Fig. 1B).

References

1. 1. Whitney, D. L. et al. (2004) in Grocott, J. et al. Geol. Soc. Spec. Publ. 227, 167-176.
2. 2. Molnar, P. et al. (1993) Rev. Geophys. 31, 357-396.
3. 3. Zuza et al. (2018) Lithosphere.
4. 4. McQuarrie, N. (2002) GSAB 114, 950-963.
5. 5. Coney, P.J. and Harms, T. (1984) Geology 12, 550–554.
6. 6. Miller, E.L. and Gans, P.B. (1989) Geology 17, 59-62
7. 7. DeCelles, P. and Coogan, J. (2006) GSAB 118, 841-864.
8. 8. Yuan, D.Y. et al. (2013) Tectonics 32, 1358–1370.
9. 9. Ernst, W.G. (2009) Int. Geol. Rev. 51, 583-588.
10. 10. Molnar, P. et al. (2011) Ann. Rev. Ear. Plan. Sci. 38, 77-102.
11. 11. Burchfiel, B.C. et al. (2008) GSA Today 18, 4-11.
12. 12. Muntean, J.L. et al. (2011) Nature 4, 122–127.
13. 13. Lee, J. et al. (1987) in Coward, M.P. et al. Geol. Soc. Spec. Publ. 28, 267– 298.
14. 14. Wells, M.L. et al. (2000) Jour. Geophys. Res. 105, 16,303-16,327.
15. 15. Gordon, S.M. et al. (2008) Tectonics 27, TC4010.
16. 16. Camilleri, P. and Chamberlain, K. (1997) GSAB 109, 74-94.
17. 17. Miller, D.M. and Hoisch, T.D. (1995) in Miller, D.M. and Busby, C. GSA Spec. Pap. 299, 267-294.
18. 18. Colgan, J.P. and Henry, C.D. (2009) Int. Geol. Rev. 51, 920–961.
19. 19. McGrew, A.J. et al. (2000) GSAB 112, 45–60.
20. 20. Chapman, J.B. et al. (2015) Geology 43, 919–922.
21. 21. Chamberlain, C.P. et al. (2012) Amer. Jour. Sci. 312, 213-262.
22. 22. Harrison, T.M. et al. (2009) Geochim. Cosmochim. Acta 73, 1039-1051.
23. 23. Forster, M.A. and Lister, G.S. (2014) in Jourdan, F. et al. Geol. Soc. Spec. Publ. 387, 117-135.
24. 24. Long, S. P. et al.(2018) Tectonics, 37.