The Tierra Colorada area sits along the northern limit of the Xolapa Complex, where it is juxtaposed against the Mixteco (Paleozoic) and Guerrero (Mesozoic) terranes of southern Mexico, just north of Acapulco. This paper presents combined structural and geochronological data from Tierra Colorada area that show evidence of four deformational events and several episodes of arc magmatism during Mesozoic and Cenozoic time. The oldest magmatism is represented by ca. 165 Ma granitoids and was followed by intrusion of the foliated El Pozuelo granite (129 ± 0.5 Ma; concordant U-Pb zircon analysis). This intrusion postdates D 1 metamorphism and migmatization in the Xolapa Complex. The next magmatic episode is represented by the peraluminous, foliated El Salitre granite (55.3 ± 3.3 Ma; mineral-whole-rock Rb-Sr isochron) and the protomylonitic Las Piñas I-type granite (54.2 ± 5.8 Ma; lower intercept U-Pb zircon). Las Piñas granite is characterized by D 2 ductile fabric with normal, top-tothe north-northwest sense of shear, deformed at 45-50 Ma (Rb-Sr and K-Ar ages). The ca. 34 Ma undeformed granites correspond to the last intrusive pulse in the area, postdating both D 3 south-southwest-verging thrusting of the Cretaceous Morelos Formation over sheared granites and Lower Cretaceous volcanic rocks, and open folding during D 4. These four pulses of subduction-related magmatism in the Tierra Colorada area indicate a regular northeastward subduction at the Mesoamerican trench since Jurassic time, and alternate with contractile and/or extensional tectonic events. The gap in magmatic activity ca. 90-100 Ma roughly coincides with deposition of platformal limestones of the Morelos Formation during the middle Cretaceous. The stable conditions during deposition of the Morelos Formation may have resulted from a combination of backarc extension and development of a passive margin during the Early-middle Cretaceous, which postdated the accretion of an exotic block, either the Guerrero terrane or the Chortís block. Following the Laramide orogeny in southern Mexico (roughly during the Late Cretaceous) the Paleocene-Miocene tectonic evolution in the Tierra Colorada area involved an alternation of magmatic pulses with extensional and contractile events. This was the result of a combination of several factors, including the geometry of the subducted slab, convergence rate, stress transmission between the subducting and overlying plates, and the rate of subduction erosion.
Offshore of the Pacific side of Costa Rica, the Caribbean plate converges with the subducting Cocos plate along the Middle America Trench. The tectonics of both plates, from the Cocos Ridge to the Nicoya Peninsula, were studied with swathmapping, magnetic anomalies, and samples. Three morphological domains on the Cocos plate were defined by mapping. The broadly arched Cocos Ridge forms the southeastern domain. Adjacent to the northwest flank of Cocos Ridge is a domain where seamounts and their aprons cover about 40% of the ocean floor. Farther northwest, a sharp juncture in the oceanic crust separates the seamount domain from a deep sea plain. These three contrasting oceanic seafloor morphologies are mimicked in the morphology of the Pacific continental margin of Costa Rica. Opposite the subducting Cocos Ridge are a broad continental shelf and Osa Peninsula, which are attributed to large-scale domal uplift. Where the seamount domain has been subducted, a rugged continental slope has developed, including 55-km-long furrows trending parallel to the Cocos-Caribbean interplate convergence direction. We propose that the furrows represent paths of disruption produced by subducting seamounts. Where the smooth deep sea plain has been subducted, a well-organized accretionary prism covered by slope deposits forms a relatively smooth morphology. The Costa Rican margin illustrates the effects of subducting seafloor morphology on the continental margin structure and morphology.
Lawsonite eclogite (metabasalt and metadolerite) and associated metasedimentary rocks in a serpentinite me´lange from an area just south of the Motagua fault zone (SMFZ), Guatemala, represent excellent natural records of the forearc slab-mantle interface. Pseudosection modelling of pristine lawsonite eclogite reproduces the observed predominant mineral assemblages, and garnet compositional isopleths intersect within the phase fields, yielding a prograde P-T path that evolves from 20 kbar, 470°C (M1) to 25 kbar, 520°C (M2). The dominant penetrative foliation within the eclogite blocks is defined by minerals developed during the prograde evolution, and the associated deformation, therefore, took place during subduction. Thermometry using Raman spectra of carbonaceous material in metasedimentary rocks associated with the SMFZ eclogites gives estimates of peak-T of $520°C. Barometry using Raman spectroscopy shows unfractured quartz inclusions in garnet rims retain overpressures of up to $10 kbar, implying these inclusions were trapped at conditions just below the quartz ⁄ coesite transition, in agreement with the results of phase equilibrium analysis. Additional growth of Ca-rich garnet indicates initial isothermal decompression to 20 kbar (M3) followed by hydration and substantial cooling to the lawsonite-blueschist facies (M4). Further decompression of the hydrated eclogite blocks to the pumpellyite-actinolite facies (3-5 kbar, 230-250°C) is associated with dehydration and veining (M5). The presence of eclogite as m-to 10 m-sized blocks in a serpentinite matrix, lack of widespread deformation developed during exhumation and derived prograde P-T path associated with substantial dehydration of metabasites within the antigorite stability field suggest that the SMFZ eclogites represent the uppermost part of the forearc slab crust sampled by an ascending serpentinite diapir in an active, moderate-T subduction zone.
Precambrian and Palaeozoic basements are present in southern Mexico and Central America, where several crustal blocks are recognized by their different geological record, and juxtaposed along lateral faults. Pre-Mesozoic reconstructions must take into account the nature of such crustal blocks, their geological history, age and petrology. Some of those crustal blocks are currently located between southernmost north America (the Maya Block) and Central America (Chortís Block).To better understand the geology of these crustal blocks, and to establish comparisons between their geological history, we performed U-Pb dating of both igneous and metasedimentary key units cropping out in central and western Guatemala. In the Altos Cuchumatanes (Maya Block) granites yield both Permian (269 + 29 Ma) and Early Devonian (391 + 7.
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