Emerging Data on Rodinia Rifting from Marathon Fold-Thrust Belt, Southern Laurentia Patricia W. Dickerson, Daniel F. Stockli, Richard E. Hanson & Jonathon M. Roberts GSA – Vancouver, 2014 Birth and Death of Supercontinents

Diablo Platform Ancestral Rockies-style basement-cored uplift Deformed as Pangea was born, Marathon fold-thrust belt provides new insight into the dying days of Rodinia Ouachita-Marathon thin-skinned fold and thrust belt Goetz in Goetz & Dickerson (1985) Southern Laurentia – West Texas

L–M Ordovician Strata Bearing Igneous Clasts (Roberts et al., 2010) Host strata: Shelf-derived fossiliferous limestone conglomerates within off-shelf deposits, western Ouachita- Cuyania Basin Deposits spanning ~28 Ma (Intl. Subcom. Ord. Strat., 2007) 3 formations to date: Woods Hollow (Sandbian) Ft. Peña Formation (Darriwilian) Marathon Formation (Floian)

Silicified Alkali Basalt Boulder in Limestone Cobble Conglomerate, Marathon Fm.

Rift-Related Intraplate Magmatism – Rodinia Breakup: Basalt & Trachyandesite Clasts in Fort Peña Fm Conglomerate Basalt, open blue circleTrachyandesite, blue XTrachyandesite, polarized light (Hanson, Roberts, Dickerson, & Fanning, 2012, 2013) ~706 Ma U/Pb, zircon

Geochronology - Marathon & Ft. Peña Basalts U-Pb, SHRIMP (Fanning, ANU) Marathon basalt boulder 207 Pb/ 206 Pb ages from ~1920 to ~1270 Ma Younger ages appropriate for Grenvillian basement rocks Ft. Peña basalt boulder 1 - ~1230 Ma (Grenvillian) Ft. Peña basalt boulder 2 - ~720 and ~660 Ma (Neoproterozoic)

No known outcrops of Neoproterozoic to Eocambrian lavas in the region Seismic and borehole data indicate that Grenvillian meta- morphic rocks form foundation for the Marathon Basin Boulder Source(s), Marathon Basin? PROTEROZOIC FOUNDATIONS - W. OUACHITA BASIN (modified from Karlstrom et al., 2004)

New Data from Marathon Fm: Two Magmatic Pulses during Rodinia Rifting Tuff boulders in debris flows of the Monument Spring Mbr Bentonitic layer in uppermost Marathon Fm Ma – Neoproterozoic volcanism Ma – Eocambrian volcanism U/Pb, LA-ICPMS, zircon (Stockli, UT-Austin) COEVAL WITH RIFT MAGMATIC EPISODES OF EASTERN LAURENTIA ( Roberts, 2012)

Felsic Tuff Clasts Marathon Fm, Monument Spring Mbr U/Pb, LA-ICPMS - zircon (Stockli, UT-Austin) 207 Pb/ 206 Pb ages: Ma – Grenvillian basement source Ma – Neoproterozoic rift volcanic source ~520 Ma – Eocambrian rift volcanic source

Bentonitic Layer Uppermost Marathon Fm U/Pb, LA-ICPMS – zircon (Stockli, UT-Austin) 207 Pb/ 206 Pb ages: Ma – Grenvillian basement source Ma – Neoproterozoic rift volcanic source Ma – Eocambrian rift volcanic source

South-Central Laurentia (modified from Viele, 1989) Devils River Uplift – Prior to analyses of the Marathon (M) clasts, DRU provided the sole evidence for co-located Neoproterozoic and Eocambrian magmatism in this span of the margin. M

Devils River Uplift Neoproterozoic & Eocambrian Magmatism Subsurface, 75 km ESE of Marathon fold-thrust belt exposures Rb/Sr, whole-rock age determinations on exploration well cores: 711 ± 23 Ma – metarhyolites 524 ± 32 Ma and 529 ± 31 Ma – metavolcanics within metasediments Forthcoming geochemical analyses to determine tectonic environment (modified from Nicholas and Rozendal, 1975)

Eastern Laurentia Two-Stage Rodinia Rifting – Representative Examples NEWFOUNDLAND (Cawood et al., 2001) 760 – 700 Ma felsic volcanics, dike swarms 570 – 535 Ma bimodal, basalt and rhyolite ADIRONDACKS (Cawood et al., 2001; Burton and Southworth, 2010) Ma felsic rocks of massif Ma mafic dikes CENTRAL & SOUTHERN APPALACHIANS (McCausland et al., 2011) 750 Ma dominantly felsic 615 – 590 Ma tholeiites to alkaline basalts APPALACHIANS, BLUE RIDGE (Southworth et al, 2009; Tollo et al. 2012) 760 – 680 Ma felsic volcanics and plutons 565 Ma dominantly mafic N. CAROLINA, GRANDFATHER MT. (Burton and Southworth, 2010) 760 – 700 Ma felsic volcanics 600 – 550 Ma basalt and rhyolite

Opening of Iapetus – Divergence of Laurentia and Gondwana (PLATES Program, UTIG, 2011) Two-stage rifting along E Laurentian margin at ~760 to ~700 Ma and ~570 to ~535 Ma Intraplate extension/rift geo- chemical signature for 706 Ma volcanic boulders in L-M Ordo- vician strata of Marathon Basin New 780 to 520 Ma ages suggest that S margin experienced coeval Neoproterozoic-Eocambrian rift magmatism Ma

A fragmentary record of Neoproterozoic - Eocambrian volcanism – possible within-plate extensional magmatism during rifting of south-central Laurentia Eos – the dawn of Iapetus