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Evidence for multi-rifting in the Variscan-Alpine cycle transition: insights from the European western Southern Alps
No full textWe investigate the transition between the Paleozoic Variscan cycle and the Mesozoic-Cenozoic Alpine supercontinent cycle, both of which have played a pivotal role in shaping the central European-Mediterranean plate architecture. Two main scenarios have been proposed so far for this transition: (i) a single, long-lasting, Permo-Triassic rifting event, culminating in the opening of the Alpine Tethys, or (ii) multiple, distinct rifting events, predating the onset of the Alpine cycle. Our study focuses on the European western Southern Alps (Varese area, N. Italy), where we document the tectonic events from the early Permian to the Middle Triassic. Through a combined tectono-stratigraphic and thermochronological analysis, we identify an initial early Permian rifting stage linked to magmatic activity, followed by early-middle Permian transcurrent tectonics. This phase is truncated by a middle Permian regional-scale erosional event that marks the cessation of this tectonic phase. Subsequently, during the Middle Triassic, a second phase initiated, which we interpret as the onset of the Alpine Tethys opening. This phase likely corresponds to an early stretching stage that predates the well-documented Late Triassic crustal-thinning phase. Based on our findings, we propose that the Middle Triassic stretching phase represents the first stage of the Alpine Tethys rifting, thereby challenging the hypothesis of a continuous Permo-Triassic long-lasting extension
Transpression and the build-up of the Cordillera: The example of the Bucaramanga fault (Eastern Cordillera, Colombia)
No full textWidespread wrench tectonics have been described along the northern Andes. The Bucaramanga fault, described as sinistral strike-slip, bounds the high Santander Massif. We combine structural and thermochronological data from the central-southern portion of the fault to investigate the vertical displacement. The structural survey data show old phases of activity preserved in the host rocks along the fault trace, with the superimposition of different generations of slickenlines, and both strike-slip and dip-slip kinematics indicators. New and previous thermochronological data show that differential exhumation of the fault walls has been ongoing for the last 50 Ma. The hanging wall, the Santander Massif, records, in the central portion, decreasing exhumation rates from the early Miocene to the mid-to late Miocene and, in the southern portion, constant rates from the Late Oligocene to the Pliocene. Combining such observations, the thermochronological offset resulting from the relative motion of the two fault walls is comparable with the observed drop in elevation across the fault, suggesting that the present topography of the Santander Massif is related to vertical movement along the Bucaramanga fault. We infer that the fault has a significant Neogene reverse component, consistent with the present day horizontal global positioning system vector data, long-term exhumation rates and the structural assemblage
Constraints on the Cenozoic Deformation of the Northern Eastern Cordillera, Colombia
No full textThe Eastern Cordillera of Colombia rose to maximum elevations of >5 km during the Cenozoic by inversion of a Mesozoic rift basin. Previous studies proposed that the exhumation of the Eastern Cordillera increased from ~6 Ma to the present due to the interplay between tectonic shortening and climate. In this study, we integrate new field observations, structural data, low-temperature thermochronology, thermobarometry, and vitrinite reflectance along a section through the Tablazo, Cocuy, and Llanos regions to estimate the amount of shortening and the exhumation history. Our results indicate that shortening started as early as the latest Maastrichtian-Paleocene in the Tablazo and Cocuy regions. Exhumation migrated eastward, starting in the Paleocene in the west and continuing in the Miocene in the east. The amount and rate of exhumation peaked in the Cocuy region with values of <5 km and < 0.4 km/Ma, respectively. At the highest elevations in the Cocuy Sierra, we also found evidence of a low-pressure/high-temperature metamorphic overprint, possibly related to shallow and local magmatic intrusions that occurred in the Late Miocene. Our cross-section interpretation suggests a low amount of shortening (13%) that is mainly accommodated by high-angle inverted faults and by the frontal thrust system. The presence of shallow magmatic bodies, moderate exhumation, and low shortening raises questions about the processes (isostatic versus dynamic) that drove the topographic growth of the high Cocuy Sierra
Thermochronologic evidence for the exhumational history of the Alpi Apuane metamorphic core complex, northern Apennines, Italy
No full textThe Apennine Range is a young convergent
orogen that formed over a retreating subduction zone.
The Alpi Apuane massif in the northern Apennines
exposes synorogenic metamorphic rocks, and provides
information about exhumation processes associated
with accretion and retreat. (U-Th)/He and fission-track
ages on zircon and apatite are used to resolve
exhumational histories for the Apuane metamorphic
rocks and the structurally overlying, very low grade
Macigno Formation. Stratigraphic, metamorphic, and
thermochronologic data indicate that the Apuane rocks
were structurally buried to 15–30 km and 400C at
about 20 Ma. Exhumation to 240C and 9 km depth
(below sea level) occurred at 10–13 Ma. By 5 Ma the
Apuane rocks were exhumed to 70C and 2 km. The
Macigno and associated Tuscan nappe were also
structurally buried and the Macigno reached its
maximum depth of 7 km at 15 to 20 Ma.
Stratigraphic evidence indicates that the Apennine
wedge was submarine at this time. Thus we infer that
initial exhumation of the Apuane was coeval with
tectonic thickening higher in the wedge, as indicated
by synchronous structural burial of the Tuscan nappe.
From 6 to 4 Ma, thinning at shallow depth is indicated
by continued differential exhumation between the
Apuane and the Tuscan nappe at high rates. After 4
Ma, differential exhumation ceased and the Apuane
and the Tuscan nappe were exhumed at similar
rates (0.8 km/Ma), which we attribute to erosion of
the Apennines, following their emergence above
sea level
Tectonic Evolution of the Western High Atlas of Morocco: Oblique Convergence, Reactivation, and Transpression
No full textThe High Atlas of Morocco is a double-vergent mountain belt developed by Cenozoic shortening and inversion of a Triassic-Jurassic rift. The structural setting, the morphometric features, and the patterns of exhumation through time and space change remarkably both along and across the strike. Here we combine structural data with revised thermochronological data to unravel the kinematic and evolution of the western High Atlas. Our results show that the structural grain of the western High Atlas is defined by two main groups of faults, namely, thrust and oblique-slip faults, which mainly strike subparallel from W-E to NE-SW. The slip direction of the thrust structures is NNW-SSE to NW-SE oriented, and the slip direction of the oblique-slip faults is WSW-ENE to NW-SE oriented. Pieces of thermochronological and geological evidence indicate that in the last ~10 Ma the exhumation rate increased during the activity thrusts and oblique-slip faults. The coexistence of these two fault systems also suggests partitioning of deformation under a transpressive regime. In the western High Atlas, we estimate a displacement of ~12 km on the frontal thrusts and of at least ~22 km on the axial oblique-slip structures. Thrusts and oblique-slip structures together result in a total cumulative displacement of ~25 km, which represents about half of the Africa-Eurasia convergence
Slab flattening and the rise of the Eastern Cordillera, Colombia
No full textThe topographic growth of a mountain belt is commonly attributed to isostatic balance in response to crustal and lithospheric thickening. However, deeper mantle processes may also influence the topography of the Earth. Here, we discuss the role of these processes in the Eastern Cordillera (EC) of Colombia. The EC is an active, double-vergent fold and thrust belt that formed during the Cenozoic by the inversion of a Mesozoic rift, and topography there has risen up to ∼5,000 m (Cocuy Sierra). The belt is located ∼500 km away from the trench where two separate portions of the Nazca plate subduct below the South American plate. North of 5◦N, the EC rises above a flat-slab subduction region. Volcanic arc migration implies slab shallowing by ∼10 Ma and flattening up to the present-day configuration at ∼6 Ma. The occurrence of a high vP/vS anomaly and clustered seismicity below the belt at ∼160 km depth delineates the slab geometry and has been related to dehydration of the slab, suggesting the presence of a hydrated mantle wedge. We compiled thermochronologic data and inverted for the exhumation history of the chain over the last 20 Ma using the age-elevation relationship and the different closure temperatures of multiple thermochronologic systems. Results indicate that exhumation rates increased during the Plio–Pleistocene at different wavelengths and amplitudes. The small wavelength and large amplitude signals could be related to shallow crustal deformation, whereas the source of the long wavelength and moderate amplitude signal has yet to be identified. Pulses of fast exhumation are found to be concomitant with the uplift that occurred from ∼7 Ma to the present-day. Previous studies suggested that the high topography of the chain cannot be achieved solely through isostatic adjustment. The highest residual topography is centered on the highest elevations of the EC, whereas the lowest residual topography corresponds to the Magdalena Valley, following the regional slab geometry. We propose that the recent uplift and exhumation events were triggered by the transition from regular to flat-slab subduction, along with the hydration of the mantle wedge above the slab. We test the dynamic feasibility of our hypothesis with a series of numerical models for the present-day state.
Predicting the correct trends in elevation requires a flat-slab geometry, and a weak and buoyant mantle wedge
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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