Serial gravity-constrained cross-sections in the Central Pyrenees validating its structural style

Author(s):  
Ruth Soto ◽  
Pilar Clariana ◽  
Conxi Ayala ◽  
Antonio M. Casas-Sainz ◽  
Teresa Román-Berdiel ◽  
...  

<p>Cenozoic contractional deformation in the Central Pyrenees generated several basement thrust sheets involving Paleozoic rocks and decoupled Mesozoic and Cenozoic cover units detached on the main décollement level, the Triassic evaporites. The overall geometry and structural architecture of the chain have already been established based on numerous geological and geophysical data obtained during several decades. This work aims to validate the overall accepted geometry of the Central part of the chain by the construction of six serial cross-sections constrained by gravity data and 2.5D gravity modelling. The study area comprises the southern half of the Axial Zone between La Maladeta and Andorra-Mont Louis granites and its southern leading edge as well as the northernmost part of the South-Pyrenean Zone.</p><p>New gravity data were acquired and combined with previous existing databases to obtain Bouguer anomaly and residual anomaly maps of the study area. Six serial gravity-constrained cross sections have been built using available geological maps, previous published works, new geological and gravity data and 2.5D gravity modelling. Density values for gravity modelling were derived from 231 laboratory measurements of rock samples collected in the field from non-weathered outcrops that include all rock types outcropping in the study area. The residual anomaly map shows a good correlation between basement thrust sheets and gravity highs whereas negative anomalies seem to correspond to (1) Mesozoic basins, (2) Triassic evaporites and (3) Late Variscan igneous bodies. The 2.5D gravity modelling along the six cross sections highlights: (i) strong along-strike variations on the gravity signal due to lateral differences of the surficial and subsurface occurrence of Triassic evaporites, (ii) different geometry at depth of the Late Variscan igneous bodies outcropping in the study area and (iii) geometric lateral variations of the basement thrust sheets and their relationship with the Mesozoic-Cenozoic units.</p>

Author(s):  
P. Clariana ◽  
R. Soto ◽  
C. Ayala ◽  
A. M. Casas-Sainz ◽  
T. Román-Berdiel ◽  
...  

AbstractA new gravity survey (1164 gravity stations and 180 samples for density analysis) combined with two new geological cross sections has been carried out in a sector of the Central Pyrenees in order to improve the characterization of basement and cover architecture. From North to South, the study area comprises the southern half of the Axial Zone and the northernmost part of the South-Pyrenean Zone. New gravity data were combined with previous existing databases to obtain the Bouguer and residual anomaly maps of the study area. The two cross sections, oriented NNE–SSW, were built from field data and previous surficial and subsurface data and cross the La Maladeta plutonic complex. The residual anomaly map shows values ranging from −18 to 16 mGal and anomalies mainly oriented N120E. The two 2.5D modelled cross sections show similar observed gravity curves coinciding with similar interpreted structural architecture. Data show a gravity high oriented N120E coinciding with the Orri basement thrust sheet and an important gravity depression, with the same orientation, coinciding with the leading edge at depth of the Rialp basement thrust sheet and interpreted as linked to a large subsurface accumulation of Triassic evaporites. The volume at depth of the La Maladeta and Arties granites has been constrained through gravity modelling. This work highlights that the combination of structural geology and gravity modelling can help to determine the structural architecture of an orogen and localize accumulations of evaporites at depth.


2020 ◽  
Author(s):  
Conxi Ayala ◽  
Pilar Clariana ◽  
Ruth Soto ◽  
Joan Martí ◽  
Aina Margalef ◽  
...  

<p>In the Central Pyrenees, where density contrast between the Paleozoic rocks and the intruded granitic bodies is measurable, geological cross-sections constrained with gravity data help to unravel the subsurface geometry of the granites.</p><p>With this goal in mind, during 2018 and 2019 several gravimetric surveys were carried out in the Central Pyrenees to improve the existent spatial resolution of the gravity data from the databases of the Spanish and Catalan Geological Surveys, especially in La Maladeta and Andorra Mont-Louis granites’ area. After the gravity reductions, we obtained the Bouguer gravity anomaly from which we calculated the residual gravity anomaly by subtracting a third degree polynomial which represents the regional anomaly in agreement with the geometry of the crust in this region.</p><p>The gravimetric response over La Maladeta and Andorra Mont-Louis granites is markedly dissimilar pointing out differences in the composition and geometry at depth of the two granites. La Maladeta granite shows a gravimetric zonation with small variations in its amplitude from one zone to the next, consistent with small lateral changes in its composition, predominantly granodioritic. By contrast, the Andorra Mont-Louis pluton is characterized by a relative minimum suggesting a more granitic composition.</p><p>With respect to the inferred geometry at depth, the results obtained from gravity modelling show that the La Maladeta granite displays a laccolithic shape with its basal contact deeping to the North whereas the Andorra Mont-Louis granite has a more batholitic shape. Although the emplacement age of both granites is similar (Late Carboniferous – Early Permian), their different geometry at depth suggests that either (1) their emplacement mechanisms were different or (2) the subsequent Alpine orogeny affected both granites in different ways better preserving the original geometry of the Andorra Mont-Louis granite.</p>


2020 ◽  
Author(s):  
Anastasia Kushnareva ◽  
Artem Moskalenko ◽  
Alexander Pasenko

<p>The Talas Range forms the northwest part of the Caledonides of the Northern Tian Shan. Based on differences in the structural style, metamorphism and sedimentary successions, three thrust sheets have been identified – the Uzunakhmat, Talas, and Kumyshtag thrust sheets. The Talas and Kumyshtag thrust sheets consist of Neoproterozoic-Ordovician terrigenous and carbonate rock units, whereas the Uzunakhmat thrust sheet consists of Neoproterozoic terrigenous rocks metamorphosed up to greenschist facies. The Uzunakhmat thrust sheet is separated from the Talas and Kumyshtag thrust sheets by the southwest-dipping Central Talas thrust (CTT). The dextral strike-slip Talas-Fergana Fault bounds the Uzunakhmat thrust sheet in the southwest. The main deformation events occurred in the Middle-Late Ordovician.</p><p>Structural and strain studies were done along profiles normal to the strike of folds and faults and located in the northwest and southeast parts of the Uzunakhmat thrust sheet. We also incorporate in our study structural profile in the central part of the Uzunakhmat thrust sheet, documented by Khudoley (1993) and Voytenko & Khudoley (2012).</p><p>The main strain indicators were detrital quartz grains in sandstones. Rf/φ and Normalized Fry methods were used to identify the amount of strain. Oblate ellipsoids predominate with Rxz values varying mostly from 1,6 to 2,4. Long axes of strain ellipsoids are sub-horizontal with the southeast to east-southeast trend. Similar trends have long axes of the anisotropy magnetic susceptibility ellipsoid being parallel to fold axes, cleavage-bedding intersection and mineral lineation as well as the trend of the major thrusts, including CTT.</p><p>The modern shape of the Uzunakhmat thrust sheet is similar to an elongated triangle, pinching out northwest and expanding southeast. Cross-section balancing corrected for the amount of strain shows along-strike decreasing of shortening in the southeast direction. Total shortening varies from 35% to 55% between sections located about 15 km from each other. Such significant variation in shortening corresponds to variation in structural style with much more tight folds and more numerous thrusts for cross-sections with a higher amount of shortening. However, the restored length of all cross-sections is quite similar pointing to the approximately rectangular initial shape of the Uzunakhmat thrust sheet. Our interpretation is that during the Caledonian tectonic events, the Uzunakhmat thrust sheet was displaced in the northwest direction with accompanied thrusting and folding of rock units within the thrust sheet. These deformations formed the modern shape of the thrust sheet in accordance with the amount of shortening detected by cross-section balancing. This interpretation also implies that modern erosion did not significantly affect shape of the Uzunakhmat thrust sheet formed after the Caledonian deformation.</p><p>Khudoley, A.K., 1993. Structural and strain analyses of the middle part of the Talassian Alatau ridge (Middle Asia, Kirgiystan). J. Struct. Geol. 6, 693–706.</p><p>Voytenko N.V., Khudoley A.K. Structural evolution of metamorphic rocks in the Talas Alatau, Tien Shan, Central Asia: Implication for early stages of the Talas-Ferghana Fault. // C. R. Geoscience. 2012. V. 344. P. 138–148.</p>


1990 ◽  
Vol 27 (2) ◽  
pp. 187-199 ◽  
Author(s):  
M. M. Kehlenbeck ◽  
S. P. Cheadle

In this study, gravity data from 350 new gravity stations are combined with those from 50 previously surveyed stations in a detailed Bouguer anomaly map of a portion of the Quetico and Wawa subprovinces north and west of Thunder Bay, Ontario.In general, high gravity values characterize the southern and southwestern part of the area where metavolcanic rocks of the Wawa subprovince dominate. Much of the Quetico subprovince forms a broad gravitational low, reflecting extensive exposures of gneisses, schists, and migmatites. Well-defined gravity lows are associated with several granitic intrusive bodies.Three- and [Formula: see text]-dimensional gravity models of subsurface configuration of the density contrasts, representative of major rock units, indicate a trough-like structure for the metavolcanic rocks of the Wawa subprovince. This trough-like structure is flanked by a domical feature in the granitoid rocks to the south. North of the metavolcanic rocks, a succession of low-grade greywackes and slates occupies a basinal structure. These structures form the principal subsurface elements of the Wawa subprovince in this area.The gneisses, schists, and migmatites of the Quetico subprovince form a thick, southward-dipping, wedge-shaped structure that may extend under the structures of the Wawa subprovince. This wedge-shaped structure is underlain by a model unit of greater density representative of mafic gneisses and amphibolites. The denser substratum is modelled with local abrupt changes in dip corresponding in position with the Quetico and Hawkeye Lake faults.


Geophysics ◽  
1968 ◽  
Vol 33 (5) ◽  
pp. 781-804 ◽  
Author(s):  
Alexander Malahoff ◽  
Ralph Moberly

A density model of the upper crust of Wyoming, computed on structural and petrographic evidence without prior reference to gravity data, when compared with the observed gravity field, demonstrates that: (1) Existing measurements and theory of change of sedimentary rock density with depth of burial are satisfactory for gravity interpretation for local rock types and the slower rates of sedimentation in the area. (2) Gravity stations established over sedimentary basins are generally as satisfactory as those on basement rock outcrops for depicting regional Bouguer gravity trends. (3) The Laramide deformation was mainly germanotype. The regional gradient in Bouguer gravity across Wyoming has increasingly negative values to the southwest. Local variation in the Bouguer anomaly field is related mainly to the low density Cretaceous and Cenozoic sediments of the basins, and to a lesser degree to the Laramide structural blocks. Interpretation of the gravity field indicates that the blocks are bounded by near‐vertical faults extending into the upper mantle, and that the uplifted blocks are undercompensated (i.e., rootless). In order to lead to a reasonable depiction of isostasy for the region, plots of mean Bouguer anomalies versus mean elevation must be made over 2°×2° squares, or larger, because of the variation in structure and composition of the crust.


2020 ◽  
Author(s):  
Antonio M. Casas ◽  
Pablo Calvín ◽  
Pablo Santolaria ◽  
Tania Mochales ◽  
Hmidou El-Ouardi ◽  
...  

<p> Multiple constraints, including poorly known parameters, determine along-strike changes of frontal thrust structures in fold-and-thrust belts. Along the 400 km long, continuous Central Moroccan Atlas belt, structural style shows significant changes, preserving similar figures of shortening. This implies the absence of large-scale vertical-axes rotations, as demonstrated by paleomagnetic studies accomplished during the development of this project. The main factors controlling thrust geometry are:</p><p>- the geometry of Triassic-Jurassic extensional basins subsequently inverted during Cenozoic compression, with especial mention to changes of cover thickness and orientation of structures</p><p>- transfer of displacement between the northern and southern thrust systems</p><p>- transfer of displacement between the basement (Paleozoic) units and the Mesozoic cover through the Upper Triassic detachment. This factor strongly determines the width of the belt in each transect, as it occurs in other basement-and-cover fold-and-thrust belts</p><p>- cover/detachment thickness ratio.</p><p>- localization and partitioning of deformation between different structures in the inner part and the borders of the massif</p><p>- amount of superposition between different cover thrust sheets, including folded thrusts</p><p>- structural style, changing from thin-skinned style to large recumbent folds along strike, probably depending on P-T conditions and cover thickness</p><p>- backthrusts related to low cover thickness/detachment thickness ratio, especially frequent in the northern Atlas thrusts</p><p>- differential shortening between sections related to layer-parallel shortening and folds associated with cleavage development in the central part of the chain</p><p>- influence of previous structures, such as individual diapirs, salt walls or igneous intrusions that modify the pre-compressional geometry of the detachment level, nucleate structures and favor buttressing. This feature can also be a source of errors in the calculation of shortening.</p><p> All these factors result in strong along-strike changes such as branching of thrust surfaces, progression of deformation towards the foreland and differential cleavage development. Influence of structures developed during the basinal/diapiric/igneous stage results in a variability of trends that varies between from less than 10° to more than 30°, what allows in some cases to distinguish between structures controlled by basinal features and newly formed thrusts.</p><p>In spite of the different techniques for cross-sections reconstruction, and in some cases, the different interpretations for the origin of structures, the shortening figures obtained along the chain are remarkably constant, on the range of 35 km, thus implying a 18 to 30% of shortening for most of the transects what attests for the reliability of the results.</p><p>Recognition and quantification of factors controlling the development of structures is the fundamental step to determine the main thrust surfaces, and the secondary backthrusts in a region where basin inversion is one of the main constraints. Structural criteria point to a dominant southward vergence and secondary northwards-directed thrusts. Minor strike-slip components were probably localized in the core of the chain. Present-day 3-D reconstruction of the Atlas is currently being done considering all these inputs as well as those obtained from merging the vast dataset obtained.</p>


1992 ◽  
Vol 29 (5) ◽  
pp. 962-971 ◽  
Author(s):  
Pierre Keating

Gravity data from the Noranda – Val d'Or region have been reprocessed: the Bouguer anomaly map, the first vertical derivative map, and an apparent density map have been used for geological interpretation of the gravity field in this region. It is found that variations in the Bouguer anomaly can be mainly explained by density structures located within the uppermost 5 km of the crust. The vertical derivative map helps to better locate some geological contacts, and the apparent density map allows the easy distinction between thin and thick batholiths. Generalized inversion was used to calculate density cross sections from the Bouguer anomaly values, and measured surface formation densities were used as constraints. Analysis of a detailed profile in the Rouyn–Noranda area shows that steep north-dipping reflectors observed in a seismic reflection survey are associated with a north-dipping density structure.


GeoArabia ◽  
2011 ◽  
Vol 16 (1) ◽  
pp. 17-42
Author(s):  
David J.W. Cooper

ABSTRACT In the central Oman Mountains, Mesozoic deepwater off-margin sediments of the Hawasina Complex were emplaced from the northeast onto the Oman continental margin during the Late Cretaceous obduction of the Semail Ophiolite. Detailed field mapping and structural investigation have shown that, in the area studied, margin-ward detachment of continental rise sediments (Hamrat Duru Group) created two major thrust units in the face of the advancing ophiolite and subduction zone wedge of sediments from more distal parts of the Hawasina Ocean. The upper unit is preserved in jabals Wahrah and Hurah as a wedge-shaped sheet, restoring to at least 60 km perpendicular to the line of emplacement but only about 500 m thick at its maximum. Its thinner leading edge (Jabal Wahrah) comprises a classic thin-skinned imbricate fan which is divided into five laterally continuous structural zones with finer-grained structures that are influenced by local stratigraphical variations in its Early Jurassic to Early Cretaceous section. The rear part of the thrust sheet (Jabal Hurah) behaved more rigidly, reflecting a thicker and more competent sedimentary sequence spanning the Early Triassic to ?mid Cretaceous. With the exception of a major duplex along its trailing edge, significant internal thrusts are rare and shortening is mostly accommodated by asymmetrical folding. This wedge was emplaced over the trailing edge of a lower thrust unit (Hammat Shulayshil), which formed through forward propagation of the Hawasina sole thrust and which was also deformed primarily through SW-directed folding with limited internal imbrication even after a translation during emplacement of at least 150 km. A second stage of thrusting after the main emplacement phase is linked to renewed locking of the lowest thrust planes in the imbricated Hawasina sediment wedge ahead of the Semail Ophiolite and late-stage motion transferring to higher structural levels closer to the ophiolite as movement of the latter gradually ceased. This resulted in out-of-sequence re-thrusting of higher thrust sheets over lower sheets along existing thrust planes. This was accompanied by the local rotation of parts of the Jabal Wahrah imbricate fan as an effect of the heterogeneous composition of the overlying thrust units, in particular the out-of-sequence emplacement of a mountain-sized thrust block of intra-oceanic reef limestone (Jabal Kawr) over the Hamrat Duru Group immediately to the east. A third phase of compression then folded and locally thrusted this re-thrust stack. The timing of this phase is not well constrained. It may represent the final effects of the Campanian emplacement; alternatively it may be tentatively linked to limited lateral motion (gravity sliding) of the thrust stack along the flanks of the Al Jabal al-Akhdar anticline during its main growth phase in the Oligocene.


2017 ◽  
Vol 47 (1) ◽  
pp. 3-19
Author(s):  
João Gabriel Motta ◽  
Norberto Morales ◽  
Walter Malagutti Filho

ABSTRACT: The Brasília and Ribeira fold belts have been established in south-southwestern São Francisco Craton during the Brasiliano-Pan African orogeny (0.9-0.5 Ga - Tonian to Cambrian), and played an important role in West Gondwana continent assembly. The region is given by a complex regional fold and thrust belt superposed by shearing during the orogeny late times, with superposing stress fields forming a structural interference zone. These thrust sheets encompasses assemblies from lower- to upper-crust from different major tectonic blocks (Paranapanema, São Francisco), and newly created metamorphic rocks. Re-evaluation of ground gravity datasets in a geologically constrained approach including seismology (CRUST1 model) and magnetic data (EMAG2 model) unveiled details on the deep- crust settings, and the overall geometry of the structural interference zone. The Simple Bouguer Anomaly map shows heterogeneous density distribution in the area, highlighting the presence of high-density, high metamorphic grade rocks along the Alterosa suture zone in the Socorro-Guaxupé Nappe, lying amid a series of metasedimentary thrust scales in a regional nappe system with important verticalization along regional shear zones. Forward gravity modeling favors interpretations of structural interference up North into Guaxupé Nappe. Comparison to geotectonic models shows similarities with modern accretionary belts, renewing the discussion.


Author(s):  
Kazutoshi Matsuda ◽  
Kusuo Kato ◽  
Kouki Arise ◽  
Hajime Ishii

According to the results of conventional wind tunnel tests on rectangular cross sections with side ratios of B/D = 2–8 (B: along-wind length (m), D: cross-wind length (m)), motion-induced vortex excitation was confirmed. The generation of motion-induced vortex excitation is considered to be caused by the unification of separated vortices from the leading edge and secondary vortices at the trailing edge [1]. Spring-supported test for B/D = 1.18 was conducted in a closed circuit wind tunnel (cross section: 1.8 m high×0.9 m wide) at Kyushu Institute of Technology. Vibrations were confirmed in the neighborhoods of reduced wind speeds Vr = V/fD = 2 and Vr = 8 (V: wind speed (m/s), f: natural frequency (Hz)). Because the reduced wind speed in motion-induced vortex excitation is calculated as Vr = 1.67×B/D = 1.67×1.18 = 2.0 [1], vibrations around Vr = 2 were considered to be motion-induced vortex excitation. According to the smoke flow visualization result for B/D = 1.18 which was carried out by the authors, no secondary vortices at the trailing edge were formed, although separated vortices from the leading edge were formed at the time of oscillation at the onset wind speed of motion-induced vortex excitation, where aerodynamic vibrations considered to be motion-induced vortex excitation were confirmed. It was suggested that motion-induced vortex excitation might possibly occur in the range of low wind speeds, even in the case of side ratios where secondary vortices at trailing edge were not confirmed. In this study, smoke flow visualizations were performed for ratios of B/D = 0.5–2.0 in order to find out the relation between side ratios of rectangular cross sections and secondary vortices at trailing edge in motion-induced vortex excitation. The smoke flow visualizations around the model during oscillating condition were conducted in a small-sized wind tunnel at Kyushu Institute of Technology. Experimental Reynolds number was Re = VD/v = 1.6×103. For the forced-oscillating amplitude η, the non-dimensional double amplitudes were set as 2η/D = 0.02–0.15. Spring-supported tests were also carried out in order to obtain the response characteristics of the models.


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