Ouachita-Marathon Orogenic Belt Yields New Zircon U/Pb Data Revealing Two Episodes of PЄ-Є Rift Magmatism on the Buried S. Laurentian Margin
Patricia Wood Dickerson
American Geosciences Institute & Jackson School of Geosciences
Structures and strata of south-central Laurentia record supercontinent interactions from Grenvillian orogenesis and assembly of Rodinia, to its fragmentation by rifting, to the amalgamation of Pangaea. Much of the critical evidence, however, lies deeply buried beneath Ouachita thrust sheets and younger strata. Late Proterozoic–Cambrian intraplate magmatism occurred along the eastern, western, and northern margins of the Laurentian craton. Whether similar igneous activity occurred prior to the rift-drift transition along the southern margin had remained uncertain. Our emerging zircon U/Pb data from the Marathon fold-thrust belt and the Devils River Uplift reveal that there were indeed Cryogenian (700-780 Ma) and Eocambrian (600-500 Ma) episodes of intraplate magmatism in this region (Hanson et al., 2016).
Deformed as Pangaea was born, the Marathon belt thus provides new insight into the last days of Rodinia. Debris-flow deposits in Lower-Middle Ordovician sedimentary strata contain cobbles to boulders of unmetamorphosed basalts, trachyandesites and trachytes with intraplate geochemical signatures. A felsic tuff and a bentonite bed from the Marathon Fm. yielded zircons dated at 750-700 Ma (U-Pb, LA-ICPMS), consistent with the average crystallization age of ~706 Ma (U-Pb, SHRIMP, zircon) for trachytic/trachyandesitic clasts from the Ft. Peña Fm. The tuff and bentonite also contained 580- to 520-Ma grains, a suite not represented in the lavas. Detrital zircon analyses are under way to determine the source(s) of the oldest known sediments in the Marathon succession, the Upper Cambrian Dagger Flat Sandstone.
New zircon LA-ICPMS data from Devils River Uplift core samples record both formation and fragmentation of Rodinia (Rodríguez et al., 2017). Zircons from Mesoproterozoic felsic gneisses yield uniform U-Pb crystallization ages of ~1230 +/- 5 Ma [cf. protolith ages of 1238-1232 Ma for Grenvillian granitic orthogneisses in Llano Uplift]. U-Pb data for overlying Lower-Middle Cambrian metasediments/metavolcanics, exhibit pronounced peaks at 700-780 Ma and at 500-600 Ma, as in the Marathon volcanic rocks. In sum, in southern Laurentia two episodes of intraplate magmatism, coeval with those on the eastern margin, contributed to Rodinia rifting and the opening of Iapetus.
Richard E. Hanson, Jonathon M. Roberts, Patricia W. Dickerson, and C. Mark Fanning, 2016, Cryogenian intraplate magmatism along the buried southern Laurentian margin: evidence from volcanic clasts in Ordovician strata in the Marathon uplift, west Texas: Geology, v. 44, no. 7, p. 539-542. DOI:10.1130/G37889.1
Rodríguez, E., Stockli, D. F. and Dickerson, P. W., 2017, New zircon U/Pb geochron-ology from the Devils River Uplift – insights into the Neoproterozoic and early Paleozoic evolution of the southern margin of North America (abs): Geological Society of America, South-Central Section, Paper 1-2. PPT: https://gsa.confex.com/gsa/2017SC/webprogram/Paper289330.html
Patricia Wood Dickerson
Reconstructing the tectonic history of southern Laurentia is Dickerson’s research focus: seeking the diagnostic evidence for Rodinia assembly and fragmentation (West Texas, Argentine Precordillera), Pangaea amalgamation (Marathon/Solitario fold-thrust belt, Ancestral Rocky Mts.), Laramide foreland deformation (Big Bend), and Rio Grande riftng/transform faulting. She draws from those investigations in leading geological and natural history field seminars for students and professional scientists, as well as for Smithsonian groups. Pat has also served on task forces to develop scientific strategies for exploring the Moon and Mars. Research sponsors include NPS and NASA.
RODINIA ASSEMBLY & FRAGMENTATION
Hanson, R. E., Roberts, J. M., Dickerson, P. W., and Fanning, C. M., 2016, Cryogenian intraplate magmatism along the buried southern Laurentian margin: Evidence from volcanic clasts in Ordovician strata, Marathon uplift, west Texas: Geology, v. 44, no. 7, p. 539-542. doi:10.1130/G37889.1
Dickerson, P. W., 2012, The circum-Laurentian carbonate bank, the Ouachita-Cuyania Basin, and the prodigal Llanoria landmass, in J. L. Wilson and J. R. Derby, The Great American Carbonate Bank: American Association of Petroleum Geologists, Memoir 98, chapter 38, p. 959-984. PANGAEA AMALGATION &
Dickerson, P. W., 2003, Intraplate mountain building in response to continent-continent collision – the Ancestral Rocky Mountains (North America) and inferences drawn from the Tien Shan (Central Asia): Tectonophysics, v. 365, p. 129-142.
RIO GRANDE RIFT & TRANSFORMS
Dickerson, P. W., 2013, Tascotal Mesa transfer zone – an element of the Border Corridor Transform System, Rio Grande rift of West Texas and adjacent Mexico, in Hudson, M. R., and Grauch, V. J. S., New Perspectives on the Rio Grande rift: From Tectonics to Groundwater: Geological Society of America, Special Paper 494, p. 475-500.
D. F. Stockli, I. W. D. Dalziel, E. W. Collins (Jackson School of Geosciences, University of Texas, Austin); R. E. Hanson (Texas Christian University); C. M. Fanning (Australian National University); P. Bauer (New Mexico Bureau of Geology); V. J. S. Grauch (U. S. Geological Survey); B. R. Hall (Texas Parks & Wildlife Department). Education & Outreach
SMITHSONIAN INSTITUTION INSTRUCTION on Smithsonian Journeys to Patagonia, Andes (Peru, Chile), Iceland, Galápagos
NASA ASRONAUT FIELD TRAINING
Dickerson, P. W., 2004, Field geophysical training of astronauts in Taos valley – A brief synopsis: New Mexico Geological Society, 54th field conference guidebook, p. 278-281
GRADUATE, UNDERGRADUATE RESEARCH COMMITTEES (UT, TCU, SRSU) TEXTBOOK CONTRIBUTIONS
Dickerson, P. W., 2012, Hotspots, rifts, reefs, deltas, and cratonic basins – Views from space, in Roberts, D. and Bally, A. W., editors, Regional Geology and Tectonics: Principles of Geologic Analysis: Elsevier, chapter 9, p. 246-295.
Muehlberger, W. R., and Dickerson, P. W., 2012, Geological methods, in Roberts, D. G. and Bally, A. W., editors, , in Roberts, D. and Bally, A. W., editors, Regional Geology and Tectonics: Principles of Geologic Analysis: Elsevier, chapter 8, p. 217-244.
B.A. – Geology and classical archaeology (UT-Austin, 1970) Ph.D. – Geology/tectonics (UT-Austin, 1995)