1. Tectonic Evolution of South-Central Laurentia – The West Texas Nexus Patricia Wood Dickerson University of Texas at Austin April 2013 South-Central GSA, Austin Big Bend Landsat Mosaic OR K Sullivan N view of W. TX. & Rift

2. Mesoproterozoic – Eocambrian(?) (Mosher, 2010; Grimes, 2013) 1) Intraplate rifting (1380-1327 Ma) – Granite-Rhyolite terrane Carrizo Mt. Group metaigneous and metasedimentary rocks 2) Continental margin basin development (~1250 Ma) 3)Grenvillian collision, transpression (1057-1035 Ma) Streeruwitz thrust (1000-980 Ma) 4) Franklin Mts. – Metamorphic rocks (1250 Ma) intruded by granite (1120 Ma) Granites in part coeval with Pecos Mafic Complex (1163-1073 Ma), 400 km east 5) Van Horn Ss – Synrift fan delta complex (~555 Ma?) related to Rodinia breakup and opening of Iapetus (Elston & Clough, 1993) Franklin MountainsTumbledown Mt., Van Horn area

3. Rift-Related Intraplate Magmatism – Iapetus Opening: 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

4. Opening of Iapetus – Divergence of Laurentia and Gondwana (PLATES Program, UTIG, 2011) Rifting along Laurentian margins from ~750 to ~550 Ma Intraplate extension/rift geo- chemical signature for volcanic boulders in L-M Ordovician strata of Marathon Basin 720 to 660 Ma ages consistent with Neoproterozoic-Eocambrian magmatism at source(s) of clasts 750 - 700 Ma

5. Shared History Cuyania – the Departed Laurentian Terrane

6. Parting of the Ways − Terrane Transfer (Dickerson, 2012)

7. Late Carboniferous Ancestral Rockies & Ouachita-Marathon Belt (Blakey, 2003)

8. Diablo Platform Ancestral Rockies-style basement-cored uplift Late Paleozoic Deformation in West Texas Ouachita-Marathon thin-skinned fold and thrust belt Goetz in Goetz & Dickerson (1985)

9. Basaltic phreatomag- matic volcanism, 77 Ma, Big Bend National Park − westward extension of LK Balcones intraplate igneous province (from Befus, 2008) Cretaceous-Paleogene Magmatism Red Hills intrusion(s) Presidio Co., TX Quartz monzonite Laramide, ~64 Ma (Gilmer, 2001) (Barnes et al., 1979)

10. Cretaceous-Paleogene Deformation Laramide basement-cored uplifts (Sierra del Carmen) and evaporite-cored folds (Chihuahua Tectonic Belt) Left transpressive component on major NNW- trending structures, as at Mariscal Mt. L-L fault, striae rake 40° SE

11. Thrust Fault in Mariscal Canyon (Dickerson, Muehlberger and Collins, 2010) North riverbank – revealed by 2008 flood Ksp and Kse are thrust, overturned to E Thrust is cut by N-striking normal fault; thin-bedded, overturned Ksp against massive, near-horizontal Kse

12. Paleogene Magmatism – Newly Exhumed Ignimbrite (gray, foreground) Densely welded rhyolite tuff Dips 25° SSW Discordant with respect to deformed Cretaceous strata N contact buried beneath sand and gravel; E, S & W contacts submerged Probable remnant of more extensive deposit, preserved in depression in K rocks Did it founder into a solution- collapse cavity? (common on Mariscal Mt.) Was it let down due to undercutting by the river and removal of fractured, less resistant K rock?

13. Paleogene Magmatism – Intrusions (Collins, Dickerson & Muehlberger, 2008) Glenn Spring porphyritic microgranite sill ~30.5 Ma (Ar-40/Ar-39, feldspar; Miggins in Collins et al., 2008)

14. Xenoliths Northern & Eastern Glenn Spring Sill - 4 Survey Areas (Dickerson, Muehlberger & Collins, 2007) Compositions:Quartzite Schist Marble Rare metachert Fabrics:Sheared, isoclinally folded, dynamically recrystallized Sizes:Maximum~1 m Minimum~1 mm Majority1 – 15 cm Morphology:Subangular to subrounded Distribution:From 3 or 4/sq m (3+ cm) to 9 or 10/sq m Present throughout sill, down to at least 13 m Increase in size and number toward sill center Mica schist rinds in voids – initially more numerous Marble more common away from sill margins

15. Sierra del Carmen (Coahuila, MX) GS & SdC on margin of Laurentia during Ouachita orogenesis 277 ± 2 Ma Greenschist-facies metamorphism (Carpenter, 1997) Exposed Analogues

16. Neogene-Quaternary Rio Grande Rift – Transform Complex EARTHQUAKES: Valentine 1931 (M 6.4) Alpine 1995 (M 5.7) 2012 (M 3.6) Extension with right slip (Dickerson & Muehlberger, 1994; Dickerson, 2013) Rift grabens (N) – Sunken Block Bimodal magmatism (olivine basalt dominant) Border Corridor Transform Zone basins & faults (NW) Transfer zones (W-WNW) – Tascotal Mesa TZ Rift-transform junctions – Terlingua, mafic intrusions with mantle, lower crustal xenoliths Quaternary faulting, present seismicity