Estructura del cinturón de pliegues y cabalgamientos de Peralta, República Dominicana

Most of the geotectonic units involved in the evolution of the Northern part of the Caribbean Plate can be identified in a geological cross-section through the southern-central part of the Hispaniola Island (South of the Dominican Republic). The cross-section includes from N to S: remnants of the old Caribbean ocean (Loma Caribe Peridotites and Duarte Fm of Upper Jurassic-Lower Cretaceous age), rocks of the Primitive volcanic Island Arc (Lower Cretaceous), the Circum-Caribbean Island Arc (Upper Cretaceous to Eocene), the Peralta thrust and fold Belt and the Azua Neogene Basin. The structure consists of an imbricate thrust system with associated folds, vergent towards the South, that overthrusts the Azua Basin. The thrust system evolved in a forward propagating sequence. The first thrust sheets of the Circum-Caribbean Island Arc possibly formed in Upper Eocene times during sedimentation of the Ocoa Fm in the foredeep (area of the Peralta Belt). Ocoa Fm has a syntectonic character and is associated with the uplift of the Central Mountain Range. Thrusting continued through Oligocene times progressing towards the South. By Lower Miocene times, the Circum-Caribbean Island Arc overthrusts the Peralta Belt (Frontal Thrust of the Tireo Fm). Thrusting in the Peralta Belt continued until Plio-Pleistocene times, as indicated by the age of the rocks in the footwall to the Peralta Belt Frontal Thrust. From Miocene times thrusting was coeval with wrenching that progressively became the dominant tectonic style in the region. The Eocene-Oligocene sedimentary sequences of the Peralta Belt were deposited in a back-arc basin that was subsequently deformed during the change in subduction direction that took place in the northeastern part of the Caribbean Plate in Neogene times. The Neogene Peralta Thrust and Fold Belt may be caused by the indentation of the Beata Ridge into the Circum-Caribbean Island Arc. In this context, the eastern part of the Beata Ridge may have acted as a transform boundary separating the Los Muertos trench from the Peralta Belt. The Peralta Belt accumulated part or all the shortening laterally equivalent to that in Los Muertos accretionary prism

Trayectoria metamórfica P-T relacionada con subducción en eclogitas del Complejo de Basamento de Samaná, Cordillera Septentrional, República Dominicana

Lenses of eclogites from the Punta Balandra unit of the Samaná basement complex, Cordillera Septentrional, Dominican Republic, preserve information of the early metamorphic and tectonic history of subduction in the Greater Antilles island-arc system and their collision with the North America plate. For this reason, these rocks were investigated for reconstruction of quantitative metamorphic P-J paths based upon interpretation of meso and microfabrics, mineral assemblages and mineral chemistry, with the help of equilibrium phase diagrams calculated for specific bulk composition in the model system CKNFMASH and isopleths for selected solution end-members. The obtained subduction-related prograde evolution went from garnet-free first and garnet-bearing next lawsonite-blueschist facies to peak eclogue facies conditions at P=22-24 kbar and 7=6 1 0-625sC. The subsequent retrograde P-T path entered the epidote-blueschist (garnet-free) facies and ended within the greenschist facies, retracing the prograde evolution at low-P (i.e. indicates cooling during the exhumation of the rocks). Thus, Samaná basement complex eclogites and hydrated equivalents formed in a intraoceanic subduction zone in which WSW/W-migrating Atlantic lithosphere was subducted beneath the Caribbean plate. Thrust stacking of different tectonic slices can provide the geological framework for exhumation and cooling due to continuous cool underplating and explains the preservation of high-pressure assemblages

Geological, geophysical and geochemical structure of a fault zone developed in granitic rocks: Implications for fault zone modeling in 3-D

The structure of a fault zone developed in granitic rocks can be established on the basis of the spatial variability of geological, geophysical and geochemical parameters. In the North Fault of the Mina Ratones area (SW Iberian Massif, Spain), fault rocks along two studied traverses (SR-2 and SR-3 boreholes) exhibit systematic changes in mineralogy, geochemistry, fabrics and microstructures that are related to brittle deformation and alteration of granite to form cataclasite and subsequent gouge. The spatial distribution and intensity of these changes suggest a North Fault morphology that is consistent with the fault-core/damage-zone model proposed by Chester et al. (1993) to describe a fault zone architecture. North Fault damage zone thickness can be defined by the development of mechanically related mesoscopic faults and joints, that produce a Fracture Index (FI)>10. High FI values are spatially correlated with relative low seismic velocity zones (VP<5 km/s and VS<2.5 km/s in the well-logs), more probably related to a high concentration of fractures and geochemical alteration produced by meteoric water-granite interaction along fault surfaces. This correlation is the base of a geostatistical model proposed in the final part of this study to image the fault zone architecture of a granitic massif

Paleostress evolution during the exhumation of high-p marbles, Samaná Complex, northern Hispaniola.

The marble of the Samaná complex presents a widespread foliation formed during its exhumation following a general decompressive strain path from high pressure (2.0>P>0.7 GPa) and low temperature (350 MPa during deformation. In contrast, mean flow stress during grain-boundary migration is estimated in |σ1-σ3| 110 MPa Ma-1). All of these data suggest that exhumation always occurred near the brittle-ductile regime of deformation. © 2017, Instituto Geologico y Minero de Espana. All rights reserved.El trabajo ha sido financiado por los proyectos de investigación CGL2010-14890 y CGL2011-23628, auspiciados por el plan nacional I+D+i del Ministerio de Economía y Competitividad del Gobierno de España.Peer reviewe

Metamorfismo y estructura de la Formación Maimón y los Complejos Duarte y Río Verde, Cordillera Central Dominicana: implicaciones en la estructura y la evolución del primitivo Arco Isla Caribeño

The mainly metabasaltic pre-Aptian/Albian basement of the Median Belt of Hispaniola includes the Duarte Complex, the Río Verde Complex and the Maimón-Los Ranchos Formations. In base to mineral assemblages present in metabasic rocks and P-T conditions estimated from thermobarometry, the Río Verde Complex is divided into four metamorphic zones and Zone IV is further subdivided into two. The metamorphic grade increase upward in the structural sequence, from prehnite-pumpellyte facies (Zone I), through greens-chist facies (Zones II and III) and amphibolite facies (Zone IVa), to upper amphibolite facies (Zone IVb), only restricted to Cpx-bearing amphibolites just below the contact with the overlying Loma Caribe Peridotite. The metamorphic field gradient is inverse and of low-P type. The P-T paths documented for Zones IVa and IVb of the Río Verde Complex involve a two-stage prograde evolution: a first event of near isobaric heating in the low-pressure field, typical of sub-ophiolite metamorphic sole rocks and characterized by critical high-grade assemblages; and a second event marked by a medium-pressure overprint of the first-stage metamorphic assemblages following a high-P gradient. These P-T paths are interpreted to result from intra-oceanic thrusting during the closure of a back-arc basin related with the Primitive Caribbean Island Arc and the onset of subduction of arc units in the Aptian/Albian time, which formed the high-pressure metamorphic overprint. The heating and development of an inverted metamorphic gradient in the sub-ophiolite Río Verde Complex, can be genetically related with the hanginwall emplacement of the hot peridotitic slice and the conductive heat transfer downward. The studied sector of the Duarte Complex (metamorphosed oceanic plateau) is divided into three metamorphic zones. Their distribution suggest that there is a temperature increase westward and downward in the structural sequence, from the upper greenschist facies (zone A), through Ep-amphibolite and amphibolite facies (zone B), to upper amphibolite transitional to lower granulite facies (zone C). The metamorphic field gradient is normal and the mid-P type (25-30º C/km). The lowest structural levels of the complex are occupied by the gabro-norites of the La Jautía batholith, which formed in ductile shear zones Grt+Opx-bearing granulites metamorphic assemblages. The 89 Ma date obtained from foliated tonalites (U-Pb in zircons) established an Upper Cretaceous age for the main ductile shearing deformation. However, the deformation is very heterogeneous in the complex, existing regionally wide metamorphic sectors without related foliation development. Therefore, the prograde metamorphism of the Duarte Complex is interpreted to result from moderated thickening of a previously thick oceanic crust, due to the great accumulation of plateau-basalts (30 km). The post-thermal peak P-T paths suggest the unloading and cooling of the complex, during the continuous retrograde development of deformative and mylonitic non-coaxial Sp fabrics (84,6±0,5 Ma; 40 Ar/39 Ar cooling age in syn-Sp muscovite). In summary, the metamorphic rocks of the pre-Aptian/Albian basement units of the Median Belt record different stages in the history of the acretion-obduction of the Duarte plateau with the Caribbean Primitive Island Arc. All these tectonothermal events pre-date the final arc-continent collision between the Caribbean island arc and the Bahamas platform during the Late Cretaceous

Subdivisión geoquímica del Arco Isla Circum-Caribeño, Cordillera Central Dominicana: Implicaciones para la formación, acrecion y crecimineto cortical en un ambiente intraoceánico

The Upper Cretaceous-Eocene Circum-Caribbean island-arc system (AICC) is a complex collage of crustal units or terranes s.l. which have formed and accreted within an intra-oceanic environment since Late Jurassic-Early Cretaceous times. In the Cordillera Central of the Dominican Republic these terranes are represented by several tectonostratigraphic units, that define a pre-Aptian-Albian deformed and metamorphosed basement, over and into which the igneous rocks of the AICC were extruded or intruded. Basement sequences can be subdivided into: (1) a plume-related Duarte terrane (plateau I); (2) a primitive arc-related Maimón-Amina terrane, which includes intra- or back-arc N-MORB units (Río Verde Complex) with a subduction geochemical imprint; and (3) the Loma Caribe peridotite terrane of mantle provenance. Two successive stages of the arc growth are superposed: an Early Cretaceous arc tholeiite stage with boninitic affinities (arc I), and a Late Cretaceous-Eocene calc-alkaline stage (AICC; arc II). The arc I stage is not recorded in the Duarte terrane but is represented in the Maimón-Amina terrane by the Río Verde Complex, the Maimón Forma-tion and the Peralvillo Norte Formation. In the Duarte terrane, the arc II growth stage include the Siete Cabezas Formation (Cenomanian- Maastrichtian, plateau II) and the fill of an intra-arc basin by the Tireo Formation (Cenomanian-Maastrichtian). In the Maimón-Amina terrane, the arc II growth is represented by the Las Lagunas and Peralvillo Sur Formations. The former lies unconformably on rudist-bearing limestone of Albian age (Hatillo Limestone). The Jautía gabbro-norite batholith and the calc-alkaline foliated and non-foliated tonalitic plutons of Late-Cretaceous-Early Eocene age intrude the rocks of the Duarte terrane except the Siete Cabezas Formation. Deformation and metamorphism of the pre-Aptian-Albian basement is a consequence of the collision of the Duarte plateau terrane with the Caribbean primitive island arc (Maimón-Amina terrane), that caused the emplacement of the Duarte terrane with a fragment of oceanic lithosphere (ophiolite). The interpretation of the geological and geochemical data support a tectonomagmatic model, which relates the compositional evolution of the intruded and extruded magmas in this segment of the AICC, with a flip in the subduction polarity under the primitive arc after the Aptian/Albian collision

Datos geocronológicos preliminares de la colisión entre el Gran Arco-Isla Caribeño y la Plataforma Continental de Bahamas en el Segmento de La Española

In parallel to structural and metamorphlc studies, geochronologlcal Investigations have been initiated in eclogites and blueschists from the Samaná Peninsula complex, northern Hispaniola, using Sm-Nd, UPb, Rb-Sr and 40Ar/39Ar methods, to constraint subduction, collision and exhumation processes in the Caribbean-North America plate boundary zone. A garnet-omphacite- whole rock Sm-Nd isochron from eclogite yields an age of 86±47 Ma (l43Nd/l44Ndinitial= 0,512894±0,000057; MSWD=0,00036). Although imprecise, we interpret this age as dating the peak of eclogitic metamorphism and allows us to calculate an initial eNd value of +7,2, which suggests a relatively depleted source for the protolith and that the rock formed in an intra-oceanic island-arc setting without significant influence from continental crust. Whole rock trace elements and REE data and preliminary Pb and Rb-Sr analyses also support this interpretation. The 40Ar/39Ar analyses of phengite mineral separates from eclogite and blueschist were made to record the cooling age during retrograde metamorphism (Tc»325-3009C). Phengites of eclogite and blueschist yields average plateau ages of 36,30±0,l 3 Ma and 33,85±0,12 Ma, respectively. These Late Eocene to Early Oligocène ages are related to the regional exhumation of the high-P basement complex and are attributed to the initial oblique collision of the Bahama Platform beneath the Hispaniola arc