Victor A. Valencia

Enhanced precision, accuracy, efficiency, and spatial resolution of U‐Pb ages by laser ablation–multicollector–inductively coupled plasma–mass spectrometry

Abstract The transition from Laramide syntectonic sedimentation of the lower Eocene Willwood Formation to the post-Laramide volcanogenic sedimentation of the middle Eocene Wapiti Formation was studied in the upper South Fork Shoshone River Valley, Wyoming. To better understand the regional age, paleogeography, and provenance of volcaniclastic sandstones in the lower stratified member of the Wapiti Formation, we sampled three units for detrital zircon U/Pb geochronology (n=241). The maximum depositional age for the sandstone units within the lower-most, middle, and upper-most units are 49.03 Ma, 49.44 Ma, and 48.99 Ma, respectively, which is consistent with previous geochronologic and paleontologic studies. These ages also are consistent with rocks deposited immediately prior to the emplacement of the Heart Mountain slide. Detrital-zircon age spectra show a transition from a mixed (recycled?) provenance, consistent with drainage from the west, composed of minor primary Eocene volcanic contributions to one dominated by primary Eocene and Archean contributions from the northern Absaroka volcanoes and the Laramide Beartooth Uplift. Thus, uplift and unroofing of the Beartooth Plateau was occurring during the deposition of the oldest members of the Wapiti Formation.

Middle Miocene closure of the Central American Seaway.

Early closing between oceans The Central American Seaway, which once separated the Panama Arc from South America, may have closed 10 million years earlier than is believed. Montes et al. report that certain minerals of Panamanian provenance began to appear in South America during the Middle Miocene, 15 to 13 million years ago (see the Perspective by Hoorn and Flantua). The presence of the minerals indicates that rivers were flowing from the Panama Arc into the shallow marine basins of northern South America. One interpretation of this finding is that large-scale ocean flow between the Atlantic and Pacific had ended by then. If this is true, then many models of paleo-ocean circulation and biotic exchange between the Americas need to be reconsidered. Science, this issue p. 226; see also p. 186 The ocean gateway that once separated South America from North America disappeared longer ago than was thought. [Also see Perspective by Hoorn and Flantua] Uranium-lead geochronology in detrital zircons and provenance analyses in eight boreholes and two surface stratigraphic sections in the northern Andes provide insight into the time of closure of the Central American Seaway. The timing of this closure has been correlated with Plio-Pleistocene global oceanographic, atmospheric, and biotic events. We found that a uniquely Panamanian Eocene detrital zircon fingerprint is pronounced in middle Miocene fluvial and shallow marine strata cropping out in the northern Andes but is absent in underlying lower Miocene and Oligocene strata. We contend that this fingerprint demonstrates a fluvial connection, and therefore the absence of an intervening seaway, between the Panama arc and South America in middle Miocene times; the Central American Seaway had vanished by that time.

Evidence for middle Eocene and younger land emergence in central Panama: Implications for Isthmus closure

The rise of the Isthmus of Panama, linked to a number of climatic, paleoceanographic, and biological events, has been studied mostly from indirect, often distal, geochemical and biotic evidence. We have upgraded existing geologic mapping in central Panama with more than 2000 field stations, over 40 petrographic analyses, and more than 30 new geochronological and thermochronological analyses. This data set suggests that the isthmus was an uninterrupted chain above sea level from late Eocene until at least late Miocene times. The basement complex of central Panama is a folded-faulted, ∼3-km-thick arc sequence, intruded by granitoid bodies and onlapped by mildly deformed upper Eocene and Oligocene strata. Six U/Pb zircon ages in the granitoids–along with published geochronological data—reveal intense late Paleocene to middle Eocene magmatism (58–39 Ma), a temporary cessation of magmatic activity between 38 and 27 Ma, and renewed magmatism between 25 and 15 Ma in a position ∼75 km south of the former magmatic axis. Thermochronological analyses in zircon (eight U-Th/He ages), and in apatite crystals (four U-Th/He ages and nine fission-track ages) obtained from a subset of 58–54 Ma granitoid bodies record a concordant Lutetian-age (47–42 Ma) cooling from ∼200 °C to ∼70 °C in ∼5 m.y., and cooling below ∼40 °C between 12 and 9 Ma. Cooling is linked to exhumation by an angular unconformity that separates the deformed basement complex below from mildly deformed, upper Eocene to Oligocene terrestrial to shallow-marine strata above. Exhumation and erosion of the basement complex are independently confirmed by lower Miocene strata that have a detrital zircon signature that closely follows the central Panama basement complex age distribution. These results greatly restrict the width and depth of the strait separating southern Central America from South America, and challenge the widely accepted notion that the Central American Seaway closed in late Pliocene time, when the ice age began.

Fracturing of the Panamanian Isthmus during initial collision with South America

Tectonic collision between South America and Panama began at 23–25 Ma. The collision is significant because it ultimately led to development of the Panamanian Isthmus, which in turn had wide-ranging oceanic, climatic, biologic, and tectonic implications. Within the Panama Canal Zone, volcanic activity transitioned from hydrous mantle-wedge−derived arc magmatism to localized extensional arc magmatism at 24 Ma, and overall marks a permanent change in arc evolution. We interpret the arc geochemical change to result from fracturing of the Panama block during initial collision with South America. Fracturing of the Panama block led to localized crustal extension, normal faulting, sedimentary basin formation, and extensional magmatism in the Canal Basin and Bocas del Toro. Synchronous with this change, both Panama and inboard South America experienced a broad episode of exhumation indicated by (U-Th)/He and fission-track thermochronology coupled with changing geographic patterns of sedimentary deposition in the Colombian Eastern Cordillera and Llanos Basin. Such observations allow for construction of a new tectonic model of the South America–Panama collision, northern Andes uplift and Panama orocline formation. Finally, synchroneity of Panama arc chemical changes and linked uplift indicates that onset of collision and Isthmus formation began earlier than commonly assumed.

Effects of Rapid Global Warming at the Paleocene-Eocene Boundary on Neotropical Vegetation

Hot Tropical Explosion The Paleocene-Eocene Thermal Maximum (PETM), 55 million years ago, was a unique episode of rapid global warming (∼5°C), often used as an ancient analog for future global climate change. Climate alteration during the PETM has been extensively studied in the marine realm, and from a few temperate to polar terrestrial localities, but little is known about how the tropics responded to the high temperatures and high levels of CO2. Using evidence from pollen analysis, Jaramillo et al. (p. 957) show that rapid tropical forest diversification occurred during the PETM, without plant extinction or regional aridity. Unexpectedly, diversity seemed to increase at higher temperatures, contradicting previous assumptions that tropical flora will succumb if temperatures become excessive. Palynology shows that tropical forests persisted under conditions of rapid climate warming 55 million years ago. Temperatures in tropical regions are estimated to have increased by 3° to 5°C, compared with Late Paleocene values, during the Paleocene-Eocene Thermal Maximum (PETM, 56.3 million years ago) event. We investigated the tropical forest response to this rapid warming by evaluating the palynological record of three stratigraphic sections in eastern Colombia and western Venezuela. We observed a rapid and distinct increase in plant diversity and origination rates, with a set of new taxa, mostly angiosperms, added to the existing stock of low-diversity Paleocene flora. There is no evidence for enhanced aridity in the northern Neotropics. The tropical rainforest was able to persist under elevated temperatures and high levels of atmospheric carbon dioxide, in contrast to speculations that tropical ecosystems were severely compromised by heat stress.

Arc-continent collision and orocline formation: Closing of the Central American seaway

Closure of the Central American seaway was a local tectonic event with potentially global biotic and environmental repercussions. We report geochronological (six U/Pb LA-ICP-MS zircon ages) and geochemical (19 XRF and ICP-MS analyses) data from the Isthmus of Panama that allow definition of a distinctive succession of plateau sequences to subduction-related protoarc to arc volcaniclastic rocks intruded by Late Cretaceous to middle Eocene intermediate plutonic rocks (67.6 ± 1.4 Ma to 41.1 ± 0.7 Ma). Paleomagnetic analyses (24 sites, 192 cores) in this same belt reveal large counterclockwise vertical-axis rotations (70.9° ± 6.7°), and moderate clockwise rotations (between 40° ± 4.1° and 56.2° ± 11.1°) on either side of an east-west trending fault at the apex of the Isthmus (Rio Gatun Fault), consistent with Isthmus curvature. An Oligocene-Miocene arc crosscuts the older, deformed and segmented arc sequences, and shows no significant vertical-axis rotation or deformation. There are three main stages of deformation: 1) left-lateral, strike-slip offset of the arc (∼100 km), and counterclockwise vertical-axis rotation of western arc segments between 38 and 28 Ma; 2) clockwise rotation of central arc segments between 28 and 25 Ma; and 3) orocline tightening after 25 Ma. When this reconstruction is placed in a global plate tectonic framework, and published exhumation data is added, the Central American seaway disappears at 15 Ma, suggesting that by the time of northern hemisphere glaciation, deep-water circulation had long been severed in Central America.

Geologic evolution of the Xolapa Complex, southern Mexico: Evidence from U-Pb zircon geochronology

The Xolapa Complex of southern Mexico is composed of mid-crustal arc-related gneisses of poorly resolved ages, intruded by undeformed Cenozoic calc-alkaline plutons. Twelve undeformed and deformed tonalitic/ granodioritic samples from three transects across the Sierra Madre del Sur (Acapulco, Puerto Escondido, and Puerto Angel) were chosen for U-Pb zircon analysis. The measurements were performed on single crystals of zircons, using a multiple-collector laserablation inductively coupled plasma–mass spectrometer (MC-LA-ICP-MS). About 20–30 crystals were measured from each sample. Three gneisses and migmatites from the eastern transect (Puerto Angel), located 30–42 km from the coast yielded Grenvilleaged zircons (970–1280 Ma), suggesting that the samples represent Oaxacan basement, not deformed Xolapa Complex. The central transect (Puerto Escondido) yielded Oligocene ages (25–32 Ma) on undeformed plutons as well as mid-Mesozoic and Permian ages on gneisses. Most samples along the Puerto Escondido transect contain inherited ca. 1.1 Ga xenocrystals of zircons. The western transect (Acapulco) yielded Late Jurassic–Early Cretaceous ages (160–136 Ma) on gneisses, and Paleocene (55 Ma) and Oligocene (34 Ma) ages on undeformed plutons, with no inherited Grenville ages. The older ages and xenocrystic zircons in arc-related Xolapa Complex mirror the crustal ages found in neighboring terranes (Mixteca and Oaxaca) to the north of the Xolapa Complex, suggesting an autochthonous origin of Xolapa with respect to its neighboring northbounding terranes. The new data and previously published ages for Xolapa suggest that metamorphism and migmatization of the deformed arc rocks took place prior to the Cenozoic. Eocene and Oligocene plutons representing renewed arc-related magmatism in the area are common throughout Xolapa, and probably represent the more deeply exposed continuation of the Sierra Madre Occidental arc to the northwest. The available U-Pb data argue against the previously proposed eastward migration of magmatism between Acapulco and Puerto Angel during the Oligocene.

Detrital zircon U/Pb geochronology of southern Guerrero and western Mixteca arc successions (southern Mexico): New insights for the tectonic evolution of southwestern North America during the late Mesozoic

Late Jurassic–Cretaceous arc-related volcaniclastic rocks from the southern Guerrero and western Mixteca terranes of Mexico were analyzed by U-Pb detrital zircon geochronology (laser ablation-multicollector– inductively coupled plasma–mass spectroscopy) to place constraints on the depositional history and provenance of the rocks. Pre–Middle Jurassic basement rocks and sandstone from the Upper Cretaceous Mexcala Formation were also analyzed to defi ne the origin and provenance of the prevolcanic substratum, and the time of accretion of Guerrero composite terrane sequences. Data from the Taxco-Taxco Viejo, Teloloapan, and Arcelia assemblages indicate that the youngest (129–141 Ma) zircon fraction in each sequence was derived from local volcanic sources, whereas older populations (ca. 247–317, 365–459, 530–617, 712–878, 947–964, 1112–1188, 1350–1420, 1842– 1929, 2126–2439, and 2709–3438 Ma) show sediment infl ux from varied sources, most likely through grain recycling. The major zircon clusters in these sequences match the populations recorded in the nearby Acatlan Complex. In contrast, the Huetamo sample is dominated by Lower Cretaceous (ca. 126 Ma) zircons of local volcanic provenance, and the Zihuatanejo sample contains zircon clusters (ca. 259, ca. 579, and ca. 947–1162 Ma) comparable to major populations recorded in the underlying Arteaga Complex. A sample from the Middle Triassic– Middle Jurassic Arteaga Complex at Tzitzio contains zircon clusters (ca. 202–247, ca. 424, ca. 600, ca. 971, and ca. 2877 Ma) consistent with an ultimate derivation from both North American and South American sources. The sample from the Las Ollas suite contains comparable zircon populations (ca. 376–475, ca. 575, ca. 988–1141, and ca. 2642–2724 Ma), and it is interpreted to be part of the prevolcanic basement. In contrast, the youngest zircon cluster (ca. 105 Ma) in the Mexcala Formation coincides with the major volcanic events in the Taxco-Taxco Viejo, Teloloapan, and Arcelia assemblages, whereas the older clusters (ca. 600, ca. 953, ca. 1215, ca. 1913, and ca. 2656–2859 Ma) broadly match the major populations recorded in rocks from the Acatlan Complex. These new data combined with available geochemical and isotopic data indicate that the Taxco-Taxco Viejo arc assemblage developed on continental crust. The Acatlan Complex is the most plausible candidate. The Teloloapan and Arcelia arc assemblages were developed on oceanic crust as offshore arcs facing the Acatlan Complex. The Zihuatanejo terrane assemblages were developed on the Arteaga Complex, and evidence no infl uence from the Acatlan Complex. This suggests that these assemblages were formed farther away or in a restricted basin. The Guerrero composite and Mixteca arc successions are coeval with the Alisitos arc of northern Mexico and in part with the Nevada and Klamath ranges of the southwestern United States, and with the arc series from the Greater and Lesser Antilles and northern South America. Data indicate that during late Mesozoic time, southwestern North America was a site of intensive volcanism in a complex arc-trench system similar to that of the east Pacifi c. Our data are consistent with a diachronic accretion of the Guerrero composite terrane sequences, beginning during late Cenomanian time with the amalgamation of the Teloloapan and probably the Arcelia assemblages, and fi nishing at the end of Cretaceous time with the accretion of the Zihuatanejo terrane assemblages.

Significance of Provenance Ages from the Chiapas Massif Complex (Southeastern Mexico): Redefining the Paleozoic Basement of the Maya Block and Its Evolution in a Peri-Gondwanan Realm

Medium- to high-grade metasedimentary rocks are exposed as isolated domains in mostly metaigneous crystalline rocks of the Chiapas Massif Complex (CMC), which forms the basement of the southern Maya Block, southeastern Mexico. Laser ablation multicollector inductively coupled plasma mass spectrometry U-Pb isotope analyses on inherited detrital zircon cores show two principal age distributions: (a) Group I, with its highest peak at 500–650 Ma and smaller peaks at ∼380–400 Ma, 1.0–1.2 Ga, 1.5–2.0 Ga, and 2.6–3.1 Ga, and (b) Group II, with its highest peak at 1.0–1.2 Ga, a minor peak at 1.5–1.6 Ga (or ∼1.0-Ga zircon cores only), and a lack of 500–650-Ma zircons. The cores are commonly surrounded by metamorphic or anatectic overgrowths, and some zircon ages were reset by a tectonothermal event at ∼250 Ma. The age distribution of zircon cores from Group I metasediments are similar to detrital zircon ages in Carboniferous-Permian sediments (Santa Rosa Formation) in southeastern Chiapas, which were shed mostly from continental crust dominated by Pan-African–Brasiliano orogenic cycles. Group II metasediments have different sources, principally from Grenville orogens, such as the Oaxacan Complex, and early mid-Proterozoic sources, such as the Rio Negro–Juruena Province of western Amazonia. The lowest stratigraphic unit in the CMC (the Jocote Unit) is intruded by lower Ordovician S-type granite and contains only 1.5–1.6-Ga and older detrital zircons. The protoliths of the CMC can be correlated with Paleozoic strata across the southern Maya Block, and they show parallels with the geologic history and provenance patterns of protoliths from the Acatlán Complex (Mixteca Terrane) of central southern Mexico. The geologic history of the CMC and the Maya Block can be interpreted in terms of tectonothermal events accompanying accretion of the CMC and polarity reversal during formation of the western Pangea margin.

U/Pb geochronology of Devonian and older Paleozoic beds in the southeastern Maya block, Central America: Its affinity with peri-Gondwanan terranes

Paleozoic rocks of the Maya (Yucatan) block of Central America include granitoids, rhyolitic-dacitic volcanic rocks, clastic sedimentary strata ranging from conglomerate to shale/phyllite, and minor limestone. To date, published isotopic ages, paleontologic data, and interpretations of field relations have been contradictory. For instance, U/Pb ages of zircon and monazite from granitoids yield Silurian-Devonian ages (ca. 420–405 Ma); the granitoids are bordered by metasedimentary contact aureoles, yet the bulk of their protolith has been assigned to the Pennsylvanian-Permian (Macal Formation or Santa Rosa Group). In this paper, we resolve the paradox by showing that pre–Lower Devonian beds are present in the Maya Mountains and constitute the wall rock for the intrusions. We dated 23 igneous zircons from a rhyolite interbedded with conglomerates obtaining a 406 +7 / −6 Ma (2s) median U/Pb age, which represents the time of eruption of the previously recognized volcaniclastic Bladen Formation. Furthermore, U/Pb geochronology of detrital zircon grains from a sandstone cobble, and outcrops of sandstone and quartzite yielded equivalent age spectra, indicating that they represent a separate geologic unit, which we herein define as the Baldy unit. Recognition of a distinct, pre-Devonian unit is justified because the sandstone cobble was collected from a conglomerate associated with the dated rhyolite, implying that the Baldy unit must have been deposited, lithified, exposed, and eroded before deposition of the Bladen Formation. The Baldy unit predates Silurian–Devonian magmatism because it does not contain interbedded volcanic rocks and is intruded by the dated granites. The five youngest detrital zircons of Baldy sedimentary rocks yielded ages in the 600–520 Ma range. These features imply that Baldy deposition took place some time between the Cambrian and Silurian. Baldy zircon age spectra have peaks at ca. 1.52 Ga, ca. 1.21 Ga, and ca. 1.02 Ga, consistent with provenance from Baltica or Amazonia, not Laurentia. Based on our new data and previously known geologic attributes of the Maya block, we propose that, during the Cambrian–Silurian, the block9s position was along the West Amazonia side of Gondwana9s periphery.