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The Motagua-polochic transform fault systems and their related structures in eastern Guatemala
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Structural geology of the Fuegian Andes and Magallanes fold-and-thrust belt – Tierra del Fuego Island
Full text linkA synthesis of the structural geology of the Tierra del Fuego Island, which integrates a new data set derived from field surveys and literature data of the last few years, is here presented. The main geological features of the region developed during the Mesozoic-Cenozoic Andean orogenic cycle that started in the Middle to Late Jurassic with a back-arc extension, crustal stretching and widespread volcanism, related to the break-up of Gondwanaland. An extensional fault system deriving from the mechanical and thermal subsidence led the evolution of the Rocas Verdes marginal basin, which hosts the upper Jurassic volcanoclastic rocks, the lower Cretaceous turbiditic sequences and few isolated elongated ophiolitic complexes. From the Late Cretaceous onward, the orogenic cycle of the Fuegian Andes continued with the shortening and inversion of the back-arc margin through horizontal contraction and crustal thickening. The uplift of the Cordillera, the emplacement of plutonic rocks, and the intracontinental polyphase deformation resulted from thick-skinned tectonics. The thrust system developed from its deeper roots, where the Palaeozoic basement was involved in compressional deformation, and propagated to the shallower stratigraphic levels of the northward verging Magallanes fold-and-thrust belt. The Magallanes foreland basin developed in front of the orogenic wedge that records at least four syntectonic angular unconformities from Late Cretaceous to Lower Miocene. During the Late Cretaceous Andean compression, three distinct phases of penetrative ductile deformation defined by low-greenschist facies assemblages took place, both in the basement and in the cover units. These deformations are related to a single metamorphic event with foliation development, as observed from microscopic analysis of the schist in the Ushuaia area. The first foliation S1 is preserved either as relic sericite microfolds between microlithons of the dominant S2, or as early refolded veins of recrystallized quartz. The S2 foliation is defined by oriented white mica. The crenulation of S2, which is related to D3 and occurs in most strained zones, becomes a pressure solution S3 spaced foliation, lined by opaque minerals. From the Palaeogene to the present, EW sinistral wrench tectonics affected the region as a component of the relative motion between South America and the Antarctic Peninsula. This strike-slip activity is well documented from the Carbajal valley to the Canal de Beagle region south of the Magallanes-Fagnano transform fault system. Restraining bends and overlapping step-over geometry characterize few sectors of the strike-slip faults with pop-ups, pressure ridges and uplifted slivers of crust. Releasing step-over along the transform fault system, both in on-shore and off-shore zones, formed several elongated pull-apart basins with many tens of km in length and a few km in width. The Lago Fagnano represents the main morphotectonic expression of this structural setting. A N-S geological cross-section through the Fuegian Andes synthesizes all the geological and geophysical data. The major stacks of internal thick-skinned basement involved in the thrusting are high-grade Upper Palaeozoic to Lower Tertiary metamorphic rocks. The geometry of the thrust complex is an upright, south plunging monocline of moderately tilted sedimentary cover strata, as well as related thrusts, faults and chevron folds involving the Upper Jurassic and Cretaceous rocks. The orogenic shortening of the Fuegian Andes, including the Cordillera and the Magallanes fold-and-thrust belt, reaches few hundred kilometers with a left-lateral wrenching component of many tens of meters. The Tierra del Fuego Island is characterized by low magnitude (M<3.5) and shallow crustal earthquakes. The southern part presents strong morphological evidence of Quaternary activity, related to the E-W left-lateral strike-slip faults. The actual deformation pattern presents a horizontal slip component of about 6 mm/year. Moreover, the northern sector of the Island is affected by extensional tectonics related to the normal fault systems of the eastern arms of the Magallanes Strait
Geometry and Structure of the Eastern Polochic and Motagua trasform fault systems in Eastern Guatemala
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The north-western boundaries of the Caribbean and Scotia Arcs: Structural and tectonic analogies
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Sedimentary architecture, structural setting, and Late Cenozoic depocentre migration of an asymmetric transtensional basin: Lake Izabal, eastern Guatemala
No full textLake Izabal, located in eastern Guatemala, lies in an EW-trending basin located along the transform margin between the North American and Caribbean plates. This plate boundary consists of two main left-lateral, strike-slip faults known as the Polochic and Motagua Fault System (PMFS). The basin is a 100 × 20 km lens-shaped depression in which the lake occupies its eastern half. To the north, the basin is confined by a Principal Deformation Zone (PDZ) which is recognized in this area as the most important upper-crustal branch of the Polochic Fault. The eastern part of the basin has been uplifted and dissected, exposing the basin infill along the slopes of the Montaña del Mico. Analysis of a set of multichannel seismic reflection profiles, most of them acquired within the lake, combined with well and outcrop information, allowed to outline the structure and evolution of the basin and its stratigraphic architecture. The basin is strongly asymmetrical, both in cross and along-strike directions, and is filled up by Neogene to Quaternary sediments, with the deepest side of the basin being to the north, close and parallel to the PDZ. The base of the infill is a mid-Tertiary unconformity whose differential subsidence through time brought the basin to its present structural setting. Five main evolutionary stages are recognized based on seismo-stratigraphic and structural features, which are related to the development and activity of the bordering faults. Development of the basin began in its easternmost sector. A small half graben, tilted to the south, developed in response to the activity of an Oligocene (?) or Early-Middle Miocene growth fault. In the second stage (Late Miocene), the basin tilts to the north, causing the sedimentary sequences to thicken towards the northern PDZ. This stage is dominated by oblique extension, and by progressive migration of the basin depocenter towards the west. The third stage (Early Pliocene) records a strong acceleration of the strike-slip activity which causes a rapid westward migration of the depocenter. The fourth stage (Late Pliocene), although still dominated by regional transtension, is related to the development of a transpressional event along the eastern side of the basin, as evidenced by the occurrence of folds and reverse faults parallel to the northern bend of the Polochic Fault. During the fifth stage (Quaternary), the eastern part of the basin remains relatively quiescent while the western part undergoes subsidence with localized transtension, still related to the activity in this sector of the Polochic Fault. Left-lateral movement along the Polochic Fault caused the depocenter to migrate 75–80 km at different velocities, with slip rates ranging between 2.1 and 16.3 mm/yr. This study suggests that the Polochic Fault developed in the Early-Middle Miocene in connection to the deposition of the early sediments of the Izabal Basin, even though an older age (Oligocene?) could also be possible. To our knowledge, this is one of the best examples of sedimentary architecture documenting depocentre migration due to fault movement along a transform system
Crustal structure beneath Discovery Bank in the Scotia Sea from group velocity tomography and seismic reflection data.
No full textThe Bruce, Discovery, Herdman and Jane banks, all located along the central-eastern part of the South Scotia Ridge, represent isolated topographic highs, surrounded by young oceanic crust (~5–23 Ma), whose petrological and structural nature is still the subject of speculation due to the lack of relevant data. Surface wave tomography in and around the Scotia Sea region, performed using eight broadband seismic stations and 300 events, shows that the central-eastern part of the South Scotia Ridge is characterized by negative surface wave group velocity anomalies as large as 6% in the period range from 15 s to 50 s. The spatial resolution of our data set (~300 km) makes it possible to study a specific area (centred at 61°S and 36°W) that includes Discovery Bank and appears to show dispersion characteristics similar to those found beneath the northern tip of the Antarctic Peninsula and southern South America. Surface wave dispersion curves are inverted to obtain 1-D isotropic shear wave velocity profiles that suggest a continental nature of Discovery Bank. Crustal thickness is in the range 23–28 km with a sub-Moho velocity of 4.1–4.2 km s-1. The boundaries of the negative group velocity anomalies are marked by a high level of seismic activity. The depth of the events and their large seismic moment suggest the presence of continental lithosphere. The continental-type crust of this topographic relief is supported by our interpretation of multichannel seismic reflection profiles acquired across this rise, where the observed seismic structures are interpreted as thinned and faulted continental plateau
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