The Yellowstone-Bighorn Research Association (YBRA) is a nonprofit research and teaching organization chartered in the state of Montana in 1936. YBRA maintains a field station south of Red Lodge, Montana, at the foot of the Beartooth Mountains at the NW corner of the Bighorn Basin. The YBRA Field Station has been host to a wide variety of primarily geological field courses and research exercises, including a YBRA-sponsored Summer Course in Geologic Field Methods , offered initially by Princeton University and subsequently by the University of Pennsylvania and the University of Houston. Enrollments in that course vary from year to year, an experience shared by other field-course programs. The YBRA field station does not depend exclusively on field-course enrollment; by diversifying its client base, YBRA has been able to operate effectively through high-amplitude variations in enrollment in traditional courses in field geology.

Early Cretaceous lawsonite eclogites and related high-pressure rocks occur as tectonic inclusions within serpentinite mélange south of the Motagua fault zone, Guatemala. Petrologic and microtextural analyses of mafic high-pressure rocks reveal three metamorphic stages linked to several deformational textures. The prograde stage represents an incipient eclogitization and is preserved in prograde garnet, along with an older S 1 –S 2 foliation. The prograde assemblage is garnet ( X Mg = ∼0.22) + omphacite (∼52 mol% jadeite) or jadeite (∼83 mol % jadeite) + lawsonite + chlorite + rutile + quartz ± phengite (3.6 Si p.f.u.); some rocks also have ilmenite and rare ferro-glaucophane. Lawsonite in garnet of some lawsonite eclogites contains rare pumpellyite inclusions. The presence of synmetamorphic brittle deformation, inclusions of pumpellyite, Fe 2+ -Mg distribution coefficients between omphacite inclusions and adjacent garnet with Ln( K D ) = 2.7–4.5, and garnet-clinopyroxene-phengite thermobarometry suggest that eclogitization initiated at temperature ( T ) = ∼300 °C and pressure ( P ) > 1.1 GPa, and continued to T = ∼480 °C and P = ∼2.6 GPa. In contrast, the retrograde eclogite-facies assemblage is characterized by reversely zoned garnet rims and omphacite ± glaucophane + lawsonite + rutile + quartz ± phengite (3.5 Si p.f.u.) along the S 3 foliation. Garnet-phengite-clinopyroxene thermobarometry yields P = ∼1.8 GPa and T = ∼400 °C. The youngest, blueschist-facies assemblage (glauco-phane + lawsonite + chlorite + titanite + quartz ± phengite) locally replaces earlier mineral assemblages along S 4 crenulations. The inferred prograde P - T trajectory lies near a geotherm of ∼5 °C km −1 , comparable to the calculated thermal and petrologic structure of the NE Japan subduction zone. These petrologic characteristics indicate: (1) the basalt-eclogite transformation may occur at T = ∼300 °C in cold subduction zones, (2) glaucophane-bearing prograde assemblages are rare during incipient eclogitization in cold subduction zones, and (3) the chlorite-consuming reactions that form Fe-Mg-Mn garnet are more effective than the lawsonite-consuming reaction that forms a grossular component. At depths of ∼100 km in cold subduction zones, dehydration embrittlement may be caused by such chlorite-consuming reactions.

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The American margins of the Caribbean comprise basins and accreted terranes recording a polyphase tectonic history. Plate kinematic models and reconstructions back to the Jurassic show that Mesozoic separation of the Americas produced passive margins that were overridden diachronously from west to east by allochthonous Caribbean plate–related arc and oceanic complexes. P-T-t and structural data, sedimentary provenance, and basin-subsidence studies constrain this history. Caribbean lithosphere is Pacific-derived and was engulfed between the Americas during their westward drift as the Atlantic Ocean opened. This began ca. 120 Ma with development of a west-dipping Benioff zone between Central America and the northern Andes, now marked by the Guatemalan and Cuban sutures in North America and by the northern Colombian and Venezuelan “sutures” of South America, persisting today as the Lesser Antilles subduction zone. Most Caribbean high-pressure metamorphic complexes originated at this subduction zone, which probably formed by arc-polarity reversal at an earlier west-facing Inter-American Arc and was probably caused by westward acceleration of the Americas. The mainly 90 Ma Caribbean basalts were extruded onto preexisting Caribbean crust ∼30 m.y. later and are not causally linked to the reversal. The Great Caribbean Arc originated at this trench and evolved up to the present, acquiring the shape of the preexisting Proto-Caribbean Seaway. The uplift and cooling history of arc and forearc terranes, and history of basin opening and subsidence, can be tied to stages of Caribbean plate motion in a coherent, internally consistent regional model that provides the basis for further studies.

Knowledge of the geologic evolution of the northern margin of South America has increased tremendously, inspired by the occurrence of huge hydrocarbon deposits. This margin originated in late Triassic time when the supercontinent of Pangea broke up and North and South America drifted apart. The passive margin accommodated a thick sequence of Jurassic to Tertiary sediments. During the latest Cretaceous to the Present, the Antilles volcanic arc, built upon the Caribbean plate, migrated southeastward and collided obliquely with South America. This collision resulted in the diachronous accretion of allochthonous terranes as well as diachronous formation of a fold and thrust belt. This belt was initiated in the west (Colombia) during the latest Cretaceous and progressively moved east and reached Trinidad only in Miocene time. In front of this thrust belt, diachronous foreland basins developed. The present paper reviews the geologic evolution of northern Venezuela and adjacent areas in the Caribbean Sea, based to a large extent on a huge amount of new data released by oil companies and data collected by universities.

The margin of northern Venezuela is a complex zone representing the orogenic events from basement formation to subsequent subduction and exhumation during transpressional collision. This boundary zone has six east-west–trending belts that each record a different segment of its development. This geologic complexity requires radiometric ages to unravel, and we herein provide 48 new ages including U-Pb (4), Rb-Sr (2), 40 Ar/ 39 Ar (24), zircon and apatite fission-track (17), and 14 C (1) ages to constrain the evolution of three of these belts. These three belts are the Cordillera de la Costa, Caucagua–El Tinaco, and Serranía del Interior belts. In the Cordillera de la Costa belt, U-Pb geochronologic data indicate portions of the basement igneous and metaigneous rocks formed in the Cambro-Ordovician (513–471 Ma). New 40 Ar/ 39 Ar data from Margarita Island indicate that some of the subduction complex was rapidly cooled and exhumed, whereas other portions indicate slower cooling. This contrasts with new 40 Ar/ 39 Ar data from the Puerto Cabello portion of the subduction complex that has Eocene to Oligocene (42–28 Ma) cooling ages. New fission-track data imply the entire Cordillera de la Costa belt from Puerto Cabello to La Guaira (∼150 km) was uplifted at the same time. In the Caucagua–El Tinaco belt, the oldest 40 Ar/ 39 Ar amphibole ages from the Tinaquillo ultramafic complex are Jurassic (190 Ma). Additional amphibole 40 Ar/ 39 Ar cooling ages are older than previously recorded in either the Tinaco or Tinaquillo complex. One amphibole 40 Ar/ 39 Ar cooling age for the Tinaco complex is similar to previous U-Pb results. New apatite fission-track results from the Serranía del Interior foreland fold and thrust belt are synchronous with exhumation in the Cordillera de la Costa belt. In addition, several zircon fission-track ages in the Serranía del Interior belt are older than their fossil ages, indicating a Cretaceous minimum provenance age for Miocene beds. Significant new findings from these geochronologic studies include (1) several igneous and metaigneous bodies that may be correlated with orogenic events in the Appalachians occur within the subduction mélange; (2) the Tinaquillo complex may record Jurassic rifting; (3) Cretaceous source rocks for the Serranía del Interior sedimentary strata; (4) exhumation of the subduction complex is segmented because two regions have significantly different cooling histories, with Margarita Island exhumed in the Cretaceous, whereas to the west, the Puerto Cabello region has widespread Paleogene cooling and exhumation ages; and (5) earthquake activity in 1812 caused uplift as recorded by exposure of Recent corals.

Northern Venezuela consists of a complex series of dismembered east-west–trending deformed belts that define the southern edge of the Caribbean plate. This contribution uses petrologic and geochemical data to define the tectono-magmatic affiliation of some of the belts. These include (1) mid-oceanic-ridge basalts (MORB) included within a mélange composed primarily of passive margin sediments in the Cordillera de la Costa belt, (2) Jurassic sublithospheric mantle and MORB in the Tinaco-Tinaquillo belt, (3) island-arc tholeiites of the Villa de Cura belt, and (4) oceanic island basalts and island-arc intrusive rocks on Gran Roque island, part of the Venezuelan and Dutch Leeward Antilles. Two of the belts (Cordillera de la Costa and Villa de Cura) were metamorphosed in subduction zones then exhumed and thrust onto the margin of Venezuela. The other two belts (Tinaco-Tinaquillo and Leeward Antilles) were obducted onto the edge of Venezuela. These different units record the complex evolution of the Caribbean plate: initial rifting, which formed the proto-Caribbean seafloor (MORB) and Great Arc of the Caribbean, followed by a reversal in subduction polarity caused by the overthickened crust of the Caribbean large igneous province, followed in turn by exhumation and/or obduction of the units and their subsequent emplacement onto northern Venezuela. In the Venezuelan islands, island arc activity continued through the Tertiary.

Two metamorphic belts in northern Venezuela were metamorphosed at high pressure–low-temperature conditions in a Cretaceous subduction zone. Both belts, the Cordillera de la Costa and Villa de Cura, contain several generations of quartz ± calcite veins that formed during two stages of orogenic development. The first stage encompasses five generations of ductile deformation structures. This was followed by at least two generations of brittle deformation. Quartz veins related to the different deformations contain several types of fluid inclusions. Most are two-phase with an aqueous NaCl-H 2 O solution. Texturally early fluid inclusions show a wide range of homogenization temperatures with a different salinity associated with each deformation episode. In one of the early quartz veins from the Araya Peninsula, there are also fluid inclusions containing a three-phase, CO 2 -bearing low salinity fluid. The isochores for the Villa de Cura blueschist belt do pass through the metamorphic conditions. In contrast, the isochores calculated for two suites of veins, one near Puerto Cabello and the other on the Araya Peninsula, both indicate changes in fluid density related to either leakage and/or stretching. These are interpreted to reflect relatively steep to moderate decompression paths related to plate-boundary–parallel stretching. The calculated isochores for the latest quartz veins indicate a change in cooling rate as exhumation continued during the final brittle deformation related to thrust emplacement. Two veins, one each from the Cordillera de la Costa and Villa de Cura belts, have single phase, methane-bearing fluid inclusions. These fluids may be related to methane-rich fluids expelled from underlying units related to the Serranía del Interior fold and thrust belt or to passive margin deposits such as the La Luna formation.

The Cordillera de la Costa eclogite belt, exposed along the Caribbean coastline of Venezuela near Puerto Cabello, consists of lensoid bodies and boudins of high pressure-temperature ( P-T ) metabasite in a heterogeneous matrix of mica schist and metacarbonate rocks. The metabasite bodies consist of eclogite and its retrogression products. Data for less mobile elements indicate that protoliths ranged from normal mid-oceanic-ridge basalt (N-MORB), to enriched (E)-MORB, to cumulate gabbro. Some eclogites and their retrogression products are enriched in large ion lithophile elements (LILE). The covariations of K and Ba are evidence that these elements were most likely incorporated into phengite, which has textures that suggest it crystallized from retrograde fluids. A similar style of LILE enrichment is also documented for eclogites of the Samana Peninsula, Dominican Republic, but not in eclogites from Isla de Margarita, Venezuela. Low- T, K-metasomatized basalts from the Bermuda Rise display different K-Ba systematics than the eclogite suites, which suggests that LILE enrichment of the latter rocks was not merely inherited from altered protoliths. In contrast to the LILE-enriched eclogites, some Cordillera de la Costa belt eclogite bodies have apparently been stripped of K, Rb, Ba, and U. Some metasedimentary rocks, in an outcrop that also contains LILE-poor metabasite, also show extreme LILE depletion relative to counterparts elsewhere in the Cordillera de la Costa. In this outcrop, LILE are most conspicuously depleted in a lens of kyanite + glaucophane schist that formed at P > 20 kb, T ∼600 °C. Although the rock has Al/Si ratios, rare earth element, and high field strength element abundances comparable to shale, it contains <0.3 wt% K 2 O. Some rocks of the Cordillera de la Costa eclogite belt thus appear to record LILE expulsion, probably at the “peak” P-T conditions of P > 20 kb at T ∼600 °C, whereas others chronicle LILE enrichment during retrogression at lower P-T conditions. Some outcrops show both effects. In a few outcrops, eclogitic blocks that appear to be LILE-depleted occur in metasedimentary host rocks that are not.

The Cordillera de la Costa belt, exposed for at least 600 km along the EW-trending coast of Venezuela, is a subduction mélange that contains fragments (knockers) of many rock types, notably eclogite and blueschist included in a matrix of mostly mica and graphite schist. The exhumation of the eclogite occurred in three stages. During the first stage (mid-Cretaceous), buoyancy forces drove the eclogite and its enclosing low-density matrix upward along the subduction zone from ∼75 to ∼25 km depth. During the second stage (Late Cretaceous), the mélange was severely fragmented by plate boundary–parallel stretching that caused the eclogite to ascend to ∼10 km depth. During the third stage (Oligocene-Miocene), the Cordillera de la Costa belt was thrust onto the South American plate and erosion was responsible for the ultimate exhumation of the eclogite.

The Tinaquillo complex in north central Venezuela is a subhorizontal, 3-km-thick sheet consisting mostly of mylonitized harzburgitic peridotite. Along a thrust contact it overlies low-grade meta-sedimentary rocks of the Cordillera de la Costa belt. The Tinaquillo complex underlies high-grade metamorphic rocks of the Caucagua–El Tinaco belt. Based on olivine and orthopyroxene microstructures and paleothermometry, two distinct phases of deformation have been identified that occurred at different depths: coarse-grained porphyroclasts may have formed at ∼80 km depth in the asthenosphere, while fine-grained crystals (neoblasts) formed during mylonitization at ∼25–30 km depth. Gabbro sills in the complex have trace-element abundances indicating a subcontinental source; they may have formed by partial melting of the peridotite in a rising mantle diapir. Whereas, initially, rifting resulted in symmetric north-northwest–south-southeast extension (“pure shear”) above the rising diapir, kinematic analysis indicates that the mylonite formed as the result of northwestdirected “simple shear.” The timing of extension is Jurassic as indicated by several new 40 Ar/ 39 Ar age determinations. This extension may be related to the breakup of Pangea and the oblique divergence between the North and South American plates. In Tertiary time, the complex was emplaced by NS contraction as a result of oblique convergence between North and South America.

New geochemical data from the Villa de Cura blueschist belt indicate that it is a subducted (and exhumed) oceanic island-arc terrane. The majority of the metabasalts were oceanic island-arc tholeiites (7–23 wt% MgO), though more evolved tholeiites are also found. Rare earth element (REE) and immobile trace element data from the Villa de Cura belt exhibit island-arc signatures, including (1) flat to light enriched REE patterns, and (2) enrichment of large ion lithophile elements relative to high field strength elements with a strongly negative Nb anomaly. Thus, the Villa de Cura belt is similar to other Albian-Aptian age oceanic island-arc tholeiites documented throughout the Caribbean in Cuba, Hispaniola, Puerto Rico, Tobago, and Bonaire. It is not related to the Cretaceous Caribbean-Colombian oceanic plateau. From our new geochemical data and previously published metamorphic data, we propose that the Villa de Cura blue-schist sequence represents two forearc slivers of the “Great Arc of the Caribbean” that were subsequently subducted and amalgamated during exhumation.

The Cordillera of eastern Venezuela is a south-vergent fold-thrust belt of Neogene age formed on the South American plate and its Atlantic passive margin series by right-lateral transpression during the relative eastward migration of the Caribbean plate. Together with the topographically high Serranía del Interior, the Maturín subbasin is the easternmost onshore segment of the Venezuelan Cordillera, its foothills, and its foreland. Oilfields and data coverage of the Maturín subbasin abundantly document the compound sediment wedge, its structure, and its sedimentary and tectonic evolution. In the Monagas foothills, décollements at the base of the Miocene are responsible for the formation of a complex accretionary wedge. Deeper structures in the Monagas foothills involve the Mesozoic assemblages that were emplaced by thrusting following the emplacement of the Carapita accretionary wedge. Apparent “out of sequence” relations at the surface of the Serranía del Interior and in the shallow subsurface of the Maturín subbasin are due to the interference of late deeper structures with the earlier structures of the accretionary wedge. Six alternative structural interpretations range from basement-involved to non–basement-involved décollement tectonics. These hypotheses imply varying amounts of shortening along the Serranía to foreland ranging from 9% to 66% or 15–115 km oblique component of the El Pilar fault.

Structural analysis, interpretation of seismic reflection lines, and apatite fission-track analysis in the western Serranía del Interior fold and thrust belt and in the Guárico basin of north-central Venezuela indicate that the area underwent Mesozoic and Tertiary to Recent deformation. Mesozoic deformation, related to the breakup of Pangea, resulted in the formation of the Espino graben in the southernmost portion of the Guárico basin and the formation of the Proto-Caribbean lithosphere between the diverging North and South American plates. The northern margin of Venezuela became a northward-facing passive margin. Minor normal faults formed in the Guárico basin. The most intense deformation took place in the Neogene when the Leeward Antilles volcanic island arc collided obliquely with South America. The inception of the basal fore-deep unconformity in the late Eocene–early Oligocene marks the formation of a perisutural basin on top of a buried graben system. It is coeval with minor extension and possible reactivation of Cretaceous normal faults in the Guárico basin. It marks the deepening of the foredeep. Cooling ages derived from apatite fission-tracks suggest that the obduction of the fold and thrust belt in the study area occurred in the late Oligocene through the middle Miocene. Field data and seismic interpretations suggest also that contractional deformation began during the Neogene, and specifically during the Miocene. The most surprising results of the detrital apatite fission-track study are the ages acquired in the sedimentary rocks of the easternmost part of the study area in the foreland fold and thrust belt. They indicate an Eocene thermal event. This event may be related to the Eocene NW-SE convergence of the North and South American plates that must have caused the Proto-Caribbean lithosphere to be shortened. This event is not related to the collision of the arc with South America, as the arc was far to the west during the Eocene.

The eastern Serranía del Interior foreland thrust belt in Venezuela consists of south-vergent thrusts that juxtapose Cretaceous and Paleogene passive margin units with less deformed Neogene basin strata. Apatite fission-track (AFT) ages, mainly from Cretaceous strata, are reset with distinct populations of grain ages that define two different cooling paths (CP). A number of samples have two reset ages that are apparently defined by apatite of different track retentiveness and they therefore record slightly different cooling events. CP1 has significant scatter, but populations of grain ages range from ca. 35–18 Ma, and peak ages decrease from north to south. Previous work estimated a total shortening of 115 km in the Serranía del Interior (Hung, 1997), and a two-stage model for the tectonic evolution of the eastern Serranía del Interior can be inferred. Stage 1 (45–20 Ma) involves in-sequence piggyback folding and imbricate thrusting propagating toward the south. Stage 2 (20–12 Ma) involves envelopment thrusting that doubled the thickness of the thrust sheets. Shortening within the main part of the Serranía del Interior thrust belt ceased at 12 Ma. CP2 is defined by low-retentive apatite. AFT peak ages are southward-younging between 13 and 3 Ma. Cooling ages of these low-retentive grains are only recognized in the northern part of the thrust belt near the El Pilar fault, and therefore these young cooling ages may represent reworking of the thrust belt due to transpression along the plate boundary. Deformation of the Serranía del Interior prior to Eocene and older collision of the Caribbean plate with South America is probably related to the convergence of the North and South American plates, which has been relatively constant since 50 Ma, but has been dominated by dextral transpression since late Miocene.

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