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Hedberg: Geology of Middle America – the Gulf of Mexico, Yucatan, Caribbean, Grenada and Tobago Basins and Their Margins

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Synthesis of terrane stratigraphy and assembly of S-Central America

Abstract

This synthesis is the combined result of 8 PhD-theses of the University of Lausanne, one PhD of the University of Nice, Sophia Antipolis, and several Masters carried out in the past 35 years in Costa Rica, Panama and Nicaragua. Our multidisciplinary approach to the geology of these regions included detailed mapping, measurement and sampling of volcanic and sedimentary sections, sedimentologic and biostratigraphic analyses including the study of Radiolaria, Planktonic and Larger Benthic Foraminifera, as well as corals and nannofossils. Some of our team specialized in igneous whole rock and mineral geochemistry, as well as C- and Sr-stable isotope stratigraphy. The raw data on basement, arc and forearc geochemistry, sediment age and paleo-environments were interpreted to understand convergent margin processes. Globally constrained plate tectonic models of the past 250 my of the Middle American area were developed in the frame of the Stampfli-model (Flores, 2009, Flores et al. 2015, Andjic et al. 2018). Southern Central America is composed of a puzzle of oceanic terranes that extend between the North American continental slivers of Chortis s. str., forming the mountains of NW-Nicaragua, and the South American continent, represented by the Columbian Andes (Fig. 1 a). No ancient continental basement is known from the terranes of “Oceanic Central America” (Baumgartner-Mora & Baumgartner 2017). These oceanic basements include the southern half of the classical “Chortis Block” which is formed by subduction/accretion mélanges that we called Mesquito Composite Oceanic Terrane (MCOT, Baumgartner et al. 2008). Except for the active Mid-American volcanic arc and the Herradura-Cordillera de Talamanca - Fila Coste.a area, recently uplifted by the Cocos Ridge indentation (fig. 1b), the relief is generally below 1000 m from Central Nicaragua to Western Colombia. Classically, Southern Central America was subdivided into the Chortis Block interpreted as all continental to the N, and the Chorotega and Choco blocks to the SW (Dengo 1962). The Southern limit of the Chortis Block (Dengo 1985) was placed at a hypothetical fault line connecting the E-W trending main fault in the Santa Elena Peninsula with the Hess Escarpment (Case & Holcombe 1980). For some authors this supposed fault still has a major, active strike slip component (e.g. Beccaluva et al. 1999). The Chorotega and the Choco blocks are thought to be separated by the Panama Canal Fault Zone (Case 1974, Barat et al. 2014). A more actualized view of the area was proposed by Baumgartner et al. (2008) and Andjic et al (2016, 2018, submitted), presented in Figure 1b. From NW to SE and in the order of their assembly into the Central American Landbridge, we distinguish the following paleo-tectonic terranes. The Mesquito Composite Oceanic Terrane (MCOT, Baumgartner et al., 2008; Flores, 2009; Flores et al., 2015) is a composite terrane of mostly arc-, OIB- , or plateau-derived remnants embedded in a serpentinite matrix, much like serpentinite mélanges described from the Franciscan domain of Western North America (Blake, 1984). It comprises the southern half of the classical "Chortis Block". The MCOT is defined by 4 corner localities characterized by ultramafic and mafic oceanic rocks and radiolarites of Late Triassic, Jurassic and Early Cretaceous age. 1.The Siuna Serpentinite MÈlange (SSM) contains high pressure metamorphic IAT-type mafics and Middle Jurassic (Bajocian-Bathonian) radiolarites in original, sedimentary contact with arc meta-andesites. The SSM also contains Upper Jurassic black detrital chert formed in a marginal (fore-arc?) basin shortly before subduction. This northern part of the MCOT represents an arc-arc collision between an oceanic arc and the Patuca Arc located on the N-American continental margin. Subduction during Late Jurassic/earliest Cretaceous is rapidly followed by exhumation. A phengite 40Ar/39Ar -cooling age dates the exhumation of the high pressure rocks as 139 Ma (Berriasian/Valanginian). Rapid exhumation is further documented by a basal conglomerate followed by dark volcano-pelagic limestones locally containing well-preserved Late Valanginian - early Aptian Radiolaria and Planktonic Foraminifera. In places, resedimented shallow-water limestones, containing corals, rudists, algae and Larger Foraminifera, such as Orbitolina texana, are interbedded with the pelagic limestones. These sediments are intruded by andesitic dykes and overlain by/interbedded with, large andesitic flows. Kilometric exposures of massive, Aptian-Albian shallow-water limestones (Cerro Vailavas, Siuna area) occur and may represent large olistoliths that slid into a marginal (forearc?) basin. 2. The El Castillo Mélange comprises a radiolarite block which is associated with OIB-metabasalts, both tectonically embedded in serpentinite. The radiolarite yielded a diverse Rhaetian (latest Triassic) radiolarian assemblage, the oldest fossils recovered so far from oceanic S-Central America. 3. DSDP Legs 67/84 drilled off Guatemala in the Nicaragua-Guatemala forearc basement. Serpentinites, metagabbros and basalts have long been known from these sites. They have been restudied and reveal 40Ar/39Ar-dated Upper Triassic to middle Cretaceous enriched ocean island basalts and Jurassic to Lower Cretaceous depleted island arc rocks of Pacific origin (Geldmacher et al. 2008). 4.The Santa Elena Accreted Arc of N-Costa Rica (see below) together with the serpentinite outcrops near El Castillo (2) in Southern Nicaragua, represent the southernmost out-crops of the MCOT. The area between localities 1-4 is largely covered by Cenozoic to Recent arcs and arc derived sediments, but we suspect that their basement is made of oceanic/accreted terranes. Earthquake seismic studies indicate an ill-defined, shallow MOHO in this area. The MCOT covers most of Nicaragua and could extend to Honduras and Guatemala to the W (Fig. 1b) The Santa Elena Accreted Arc (SEAA) cropping out un the Santa Elena Peninsula (Fig1b, c) is interpreted as an intraoceanic arc, which was active during the Barremian-Aptian and became accreted to the MCOT in an arc-arc collision during Albian-Cenomanian times (Escuder-Viruete & Baumgartner, 2014, Escuder-Viruete et al. 2015.) The SEAA consists of an ultramafic nappe made of low grade metamorphic, serpentinized peridotites rich in supra-subduction gabbroic dykes, topped by a spectacular outcrop of the MOHO, then followed by layered and massive gabbros. A younger (pre-middle Campanian) S-vergent overthrust placed the SEAA on a serpentinite-matrix m.lange, the Santa Rosa Accretionary Complex (SRAC, Baumgartner and Denyer, 2006,; Escuder-Viruete & Baumgartner, 2014). The SRAC contains two types of internally stratigraphically coherent tectonic slivers: 1. Pliensbachian -Toarcian ribbon-radiolarites (Bandini et al. 2011) are pervasively intruded by 174 -177 Ma old (Toarcian-Aalenian) Petit-Spot-like alkaline sills (Buchs et al. 1913, Pilet et al. 2016). 2. up to 100 m thick sequences of Albian ribbon radiolariets grade upsection into arc-derived turbidites, debris flows and trench-fill megabreccias announcing the collision with the MCOT active margin. The Manzanillo Terrane (MT, Bandini et al. 2008, Andjic et al. submitted) is a plateau fragment that extruded on a Jurassic crust of the Fallaron Plate during the Aptian-Turonian (principal pulse of the CLIP) and was intruded by an OIB-like alkaline suite (~ 89 Ma, Tortugal, Alvarado et al., 1997; Sinton et al., 2009). The MT became accreted to the MCOT around the Turonian/Coniacian boundary; it is overlain by a Coniacian—Eocene overlap sequence. Prior to its accretion, it was part of the northwestern edge of the Caribbean Large Igneous Province. The accretion timing of the MT is constrained by the Coniacian (~ 89—86 Ma) base of the overlapping Loma Chumico Formation (see abstract by Andjic et al. this volume). During the Coniacian-Santonian, the Manzanillo Terrane, now part of the Chortis Block s. l., was in a forearc position that received volcanoclastic sediments from an intermediate island arc source located east of the modern Nicoya Gulf, the “Berrugate Arc”. This arc ceased by early late Campanian, before the Mid American arc initiation. The Nicoya Complex s. str. (NC) is restricted by us to a complex, soft-sediment deformed unit cropping out in the NW-Nicoya Peninsula (Fig. 1b,c, Bandini et al. 2008, Andjic et al, 2016, 2018, submitted). The NC is a composite Pacific plateau made of multi-phase Valanginian — earliest Campanian (~ 139—83 Ma) intrusions and extrusions of plateau-type igneous rocks which intruded into Bajocian — Santonian ribbon bedded radiolarites (~ 170—83.5 Ma; Denyer and Baumgartner, 2006, Baumgartner et al. 2008). Most of the radiolarites are older than the encasing igneous rocks, as expressed by chilled margins and hydrothermal radiolarite leaching. The NC experienced three phases of plateau volcanism (Ar/Ar ages: 139—132, 119—110 and 95—83 Ma). The youngest phase (95-83 Ma) is contemporaneous to the CLIP main pulse. The NC became accreted to the Manzanillo Terrane during the early-middle Campanian causing tectonic uplift and probably subaerial exposure of the Santa Elena, Manzanillo and Nicoya terranes. During the middle Campanaian, rudist biostrones and bioclastic limestones overlapped this tectonic high (Pons et al. 2016). Western margin of the Caribbean Large Igneous Province (CLIP). In contrast to the Jurassic-Upper Cretaceous puzzle of terranes mentioned above, the Upper Cretaceous western edge of the CLIP constitutes most of the land bridge including the coastal outcrops of Herradura (Fig, 1 b, c, Arias 2003), possibly SE-Costa Rica and Panama (Buchs et al. 2010; Barat et al. 2014). The northwestern limit between the NT and the CLIP is supposed to lie between the Nicoya Peninsula and the Herradura Promontory, roughly coinciding with the on-land projection to the SW of the Hess Escarpment (fig. 1.b). The Azuero Plateau (CLIP), dated by radiolarians as Coniacian-Santonian (Kolarski et al. 1995), is considered to represent the oldest, autochthonous basement (Lissina 2005, Buchs et al. 2009, 2010). However, the recent discovery of late Hauterivian-Barreminan radiolarians in a radiolarite pebble of a Miocene fluviatile conglomerate in the Canal Zone (Kukoc et al. 2017), suggests that Early Cretaceous and older, far-travelled plateau fragments out of Panthalassa, accreted along, or form the basement of the CLIP. Provenance constraints from the Canal conglomerate favour the occurrence of an older plateau at the base of the CLIP and the Panama Isthmus, which remains to be found in situ. Initiation of the Middle American Arc (MAA). During the late Campanian, volcanic arc activity developed on the Azuero Plateau (CLIP), following the subduction initiation along its western margin (Golfito Arc, Azuero Arc and San Blas Complex, (Fig1 b,c) Buchs et al., 2010; Barat et al., 2014). The N-Costa Rica - Azuero segment of the MAA went extinct during Late Paleocene time, due to the accretion of several Galapagos Hotspot-derived plateau fragments and oceanic islands. The MAA resumed its activity during the earliest Eocene. Accreted Pacific Seamounts and Plateau fragments (APSP). During the Paleocene-Eocene, several smaller oceanic assemblages bearing a Galapagos Hotspot signature became accreted to the W-edge of the Caribbean Plate (CLIP). These include oceanic islands/seamounts originated from the Galapagos plume tail activity ("TulÌn Fm.", Arias, 2003, and Quepos, the Osa Igneous Complex, Costa Rica, Di Marco et al., 1994;; Buchs et al. 2009, 2011 a, b), and the Azuero Accretionary Complex, Panama (Fig. 1b.;Buchs et al., 2009, 2011 a, b). Several of these accreted terranes were the site of formation of Barra Honda-type Upper Paleocene small carbonate shoals (Baumgartner-Mora & Baumgartner 2017), that formed either outboard in the Pacific on emergent oceanic islands (Quepos, "TulÌn"), or along the convergent margin as a consequence of tectonic uplift in the forearc area during accretion (Inner Osa Igneous Complex: Burica). A siliceous pelagic carbonate episode in the deeper areas documents the coeval scarcity of arc-derived detritus. APSP are the basement of the Middle-Upper Eocene to Oligocene, very thick overlap sequences in the Herradura-Quepos area (Fig. 1c). Further to the SE, these forearc sequences (Terraba) became later decolled form their basement and form the Quaternary thrust belt of the Fila CosteÒa, which resulted form the ongoing indentation of the buoyant Cocos Ridge (Fig. 1 c).