Fracturing of the Panamanian Isthmus during initial collision with South America David W. Farris1, Carlos Jaramillo2, German Bayona2,3, Sergio A. Restrepo-Moreno2,4, Camilo Montes2,3, Agustin Cardona2,3, Andres Mora5, Robert J. Speakman6, Michael D. Glascock7, and Victor Valencia8 1Florida State University, Department of Earth, Ocean, and Atmospheric Sciences, Tallahassee, Florida 32306, USA 2Smithsonian Tropical Research Institute, Unit 0948, APO AA 34002-0948, USA 3Corporación Geológica ARES, Calle 44A N. 53-96 Bogotá, Colombia 4University of Florida, Department of Geological Sciences, Gainesville, Florida 32611, USA 5Instituto Colombiano del Petroleo, Ecopetrol, Bucaramanga, Colombia 6Smithsonian Institution, Museum Conservation Institute, Washington, D.C. 20012, USA 7University of Missouri, Archaeometry Laboratory, Columbia, Missouri 65211, USA 8University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA ABSTRACT tle-wedge−derived subduction zone magmas Tectonic collision between South America and Panama began at 23–25 Ma. The collision is (Pearce and Peate, 1995). signifi cant because it ultimately led to development of the Panamanian Isthmus, which in turn Within the Canal Zone, volcanic rocks had wide-ranging oceanic, climatic, biologic, and tectonic implications. Within the Panama younger than 24 Ma range from basalt to Canal Zone, volcanic activity transitioned from hydrous mantle-wedge−derived arc magma- dacite in composition, but are signifi cantly less tism to localized extensional arc magmatism at 24 Ma, and overall marks a permanent change hydrous and exclusively tholeiitic. Rock types in arc evolution. We interpret the arc geochemical change to result from fracturing of the within the younger group are bimodal with Panama block during initial collision with South America. Fracturing of the Panama block individual units dominated by either silicic led to localized crustal extension, normal faulting, sedimentary basin formation, and exten- tuffs and welded units (Las Cascadas Forma- sional magmatism in the Canal Basin and Bocas del Toro. Synchronous with this change, both tion) or basalt to basaltic-andesite lava fl ows Panama and inboard South America experienced a broad episode of exhumation indicated by and intrusive sills (Pedro Miguel Formation). (U-Th)/He and fi ssion-track thermochronology coupled with changing geographic patterns of Hornblende and other hydrous minerals are sedimentary deposition in the Colombian Eastern Cordillera and Llanos Basin. Such observa- absent. In comparison with earlier arc rocks tions allow for construction of a new tectonic model of the South America–Panama collision, (Bas Obispo Formation and older), Miocene northern Andes uplift and Panama orocline formation. Finally, synchroneity of Panama arc Canal Zone volcanism exhibits low LILEs, chemical changes and linked uplift indicates that onset of collision and Isthmus formation higher HREEs and Ti, and a signifi cantly began earlier than commonly assumed. decreased Ta anomaly (Fig. 2A). Trace element ratios are tracers of volcanic- INTRODUCTION PANAMA ARC EVOLUTION WITHIN rock tectonic and mantle environments, and Yb The Isthmus of Panama fully separated the THE CANAL ZONE normalization allows one to see through frac- Caribbean Sea and Pacifi c Ocean by 3–3.5 Ma The Panama arc formed on the trailing edge tionation processes (Pearce and Peate, 1995; (Keigwin, 1978; O’Dea et al., 2007) and is of the Caribbean plate at ca. 75–65 Ma (Buchs Wegner et al., 2011). Welch two-sample t-tests inferred to result from collision between South et al., 2010). Wörner et al. (2009) and Wegner indicate that arc groups identifi ed in Figure 2 America and the Panama block (Trenkamp et et al. (2011) divide arc activity into a depleted have unique element ratio sets, suggesting that al., 2002; Coates et al., 2004) (Fig. 1). However, Late Cretaceous–Eocene initial episode and the mantle environment changed over time this closure date is based on evolutionary diver- an enriched Miocene arc. Modern magmatism (Table DR2). In the Panama arc, ratios such gence of marine organisms and therefore must exists only west of the Canal Zone and consists as La/Yb, Th/Yb, Hf/Yb, and Ta/Yb exhibit an be a minimum age. Other evidence on when of a <2–3 Ma adakitic suite attributed variously increase and change in slope at 24 Ma (Fig. 2B), Isthmus formation began comes from shallow- to slab melting (Defant et al., 1992), a slab with Ta/Yb being the single best discriminator ing sequences in Panamanian and Colombian window (Abratis and Wörner, 2001; Wegner of this change. In rocks younger than 24 Ma, bathyal sedimentary basins at 14.8–12.8 Ma et al., 2011), or subduction erosion (Goss and the Ta/Yb ratio is >0.1. Ba/Yb is also effective (Duque-Caro, 1990; Coates et al., 2004) and Kay, 2006). at discriminating between rocks younger than folded and thrusted Upper Miocene strata in We report that depleted-type volcanic and 24 Ma in the Canal Zone, with younger rocks eastern Panama (Mann and Kolarsky, 1995). plutonic rocks persist until 25 Ma within the showing a distinct depletion (Fig. 2C). Con- These observations document that signifi cant central Panama Canal Zone. Older arc rocks versely, Miocene arc rocks elsewhere in Panama contraction in eastern Panama occurred since are heterogeneous and consist of plutonic and show a progressive increase in Ba/Yb and fl uid- the Middle Miocene, but do not put a fi rm limit extrusive rocks that range from calc-alkaline to mobile elements in general. on when or how the collision between South tholeiitic and basaltic to andesitic in composi- America and the Panama block initiated. We tion. These rocks are dominantly hornblende- 1GSA Data Repository item 2011297, a descrip- suggest that collision initiated at 23–25 Ma bearing (Rooney et al., 2010), have a large Ta tion of geochemical (INAA and XRF) and geochro- when South America fi rst impinged upon Pan- anomaly, exhibit relative enrichment in fl uid- nologic (U-Th/He) methods, a statistical analysis of ama arc crust as observed by distinct Panama mobile large ion lithophile elements (LILEs) the geochemical data, and data tables of the low- arc chemical changes, broad exhumation of (e.g., Cs, Rb, Ba), and have moderate heavy rare temperature thermochronology presented in the pa- per, is available online at www.geosociety.org/pubs/ the northern Andes and Panama, and extensive earth element (HREE) concentrations (Fig. 2A; ft2011.htm, or on request from editing@geosociety 1 foreland deposition in the distal Llanos Basin of Table DR1 in the GSA Data Repository ). Such .org or Documents Secretary, GSA, P.O. Box 9140, Colombia (Fig. 1). characteristics are indicative of hydrous man- Boulder, CO 80301, USA. © 2011 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, November November 2011; 2011 v. 39; no. 11; p. 1007–1010; doi:10.1130/G32237.1; 4 fi gures; Data Repository item 2011297. 1007 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/39/11/1007/3538911/1007.pdf by Smithsonian Institution Lib user on 18 September 2018 Geochemical samples 80oW Caribbean plate Paleocene-Olig. Arc A 100 Paleocene-Olig. Arc Figure 2. Instrumental neutron activation Canal Zone N. Panama Bocas del Toro deformed belt Canal Zone A Western Panama Bocas del Toro analysis (INAA) trace element geochemis- Darien Western Panama try from the Panama arc (see Table DR1). Ranges A: Averaged trace element geochemistry o Panama block 8 N 10 from different temporal and spatial groups of Panama arc rocks. Shaded red/blue fi elds Coiba Sandra Rift Plateau Extensional arc indicate the full data range for early arc and Cocos Ridge >8 Ma Llanos Basin volcanism Canal Zone, respectively. N-MORB—normal Ridge Fig. 4 Cocos Thermochronology sites 1 mid-oceanic-ridge basalt. B: La/Yb versus Panama Fracture Zone plate Malepo sample / N-MORB Mamoní batholith Ta/Yb with individual samples plotted. Re- >8–9 Ma Cerro Petaquilla gressions through the early and youngest South American Central Cordillera Post-Oligocene decrease plate Sierra Nevada S.M. arc groups intersect at Ta/Yb = 0.1. In Ca- Eastern Cordillera in subduction signal Nazca plate 500 km nal Zone rocks, this boundary is crossed at 0.1 Rb Th Ta La Sr Zr Sm Ti Dy Lu 80oW Caribbean plate (Stationary) Cs Ba Nb K Ce Nd Hf Eu Tb Yb 23–25 Ma and corresponds to a permanent BV change in arc chemistry. C: Ba/Yb versus v Volcanic Zones of Extension 100 v Bocas Canal Zone depleted Hydrous Ta/Yb with individual samples. Canal Zone v del Toro arc magmatism B arc mantle-wedge v defoN. Pan Underthrust volcanic rocks have sharply lower Ba/Yb ra- v magmatism Progressive rmed belt ama tios indicative of general large ion lithophile CaribbeanNorthward lithosphere mantle o v v v migrated 10 8 N v terranes enrichment element (LILE) depletion. D: Shervais (1982) v Zone of tectonic discrimination diagram. Canal Zone Coiba contraction 24–0 Ma Plateau Sandra Rift submarines volcanic formations (Fm.) transition from South Yb La / y=53.9x - 2.0 America 1 R2= 0.92 arc tholeiites to extensional products af- ter 25 Ma. Rocks from Bocas del Toro also Extinct 65–25 Ma Similar Panama Fracture Zone (9–0 Ma) (9–0 Zone Fracture Panama Malpelo 21 mm/yr Ridge at 8–9 Ma y=17.8x + 1.9 enrichment trends exit plot in the extensional fi eld. MORB—mid- 2 for Th/Yb and Hf/Yb ratios 0.1 R = 0.17 oceanic-ridge basalt; BAB—backarc basin; Cocos Ridge 10 Ma lithosphere v 25–27 Ma CFB—continental fl ood basalt. Subducted oceanic v reconstruction v Canal Zone C Cocos-Nazca spreading center 200 km Volcanic (210 arc km of S. American (Bas Obispo Fm.) (active until present) displacement) 1000 o C 80 W < 24 Ma Central AmericanCaribbean plate v (Stationary in Indo-Atlantic LILE depleted v hot spot reference frame) Underthrust Caribbean 100 Canal Zone v lithosphere volcanism Ba / Yb Ba / marine fossil assemblages (Kirby et al., 2008).