Emerging Data on Rodinia Rifting fromMarathon Fold-Thrust Belt, Southern Laurentia

Patricia W. Dickerson, Daniel F. Stockli, Richard E. Hanson & Jonathon M. Roberts

GSA – Vancouver, 2014Birth and Death of Supercontinents

Diablo PlatformAncestral Rockies-

stylebasement-cored

uplift

Deformed as Pangea was born, Marathon fold-thrust belt provides new insight into the dying

days of Rodinia

Ouachita-Marathonthin-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 circle Trachyandesite, blue X Trachyandesite, polarized light

(Hanson, Roberts,Dickerson, & Fanning, 2012, 2013)

~706 MaU/Pb, zircon

Geochronology - Marathon & Ft. Peña Basalts

U-Pb, SHRIMP (Fanning, ANU)

Marathon basalt boulder 207Pb/206Pb ages from ~1920 to ~1270 Ma Younger ages appropriate for Grenvillian basement rocksFt. Peña basalt boulder 1 - ~1230 Ma (Grenvillian)Ft. Peña basalt boulder 2 - ~720 and ~660 Ma (Neoproterozoic)

No known outcrops ofNeoproterozoic toEocambrian lavas in the region

Seismic and borehole data indicate thatGrenvillian meta- morphic rocks formfoundation for theMarathon 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

780-700 Ma – Neoproterozoic volcanism 580-520 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)

207Pb/206Pb ages: 1300-1100 Ma – Grenvillian basement source 750-700 Ma – Neoproterozoic rift volcanic source ~520 Ma – Eocambrian rift volcanic source

Bentonitic Layer Uppermost Marathon Fm

U/Pb, LA-ICPMS – zircon (Stockli, UT-Austin)

207Pb/206Pb ages: 1400-1100 Ma – Grenvillian basement source 780-750 Ma – Neoproterozoic rift volcanic source 580-520 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 UpliftNeoproterozoic & 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 – metarhyolites524 ± 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 rhyoliteADIRONDACKS (Cawood et al., 2001; Burton and Southworth, 2010) 760 - 680 Ma felsic rocks of massif 620 - 580 Ma mafic dikes CENTRAL & SOUTHERN APPALACHIANS (McCausland et al., 2011) 750 Ma dominantly felsic 615 – 590 Ma tholeiites to alkaline basaltsAPPALACHIANS, BLUE RIDGE (Southworth et al, 2009; Tollo et al. 2012) 760 – 680 Ma felsic volcanics and plutons 565 Ma dominantly maficN. 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

750 - 700 Ma

A fragmentary record ofNeoproterozoic - Eocambrian volcanism –

possible within-plate extensional magmatism during rifting of south-central Laurentia

Eos – the dawn of Iapetus