Deep-seated Deformation Research Articles

Constructing a 3D structural block diagram of the Central Basin in Marmara Sea by means of bathymetric and seismic data

In this study we made a comparative interpretation of multibeam bathymetric and seismic reflection data with different resolutions and penetration properties collected in the Central Basin of the Marmara Sea. Our main objectives were (i) to investigate and compare the active tectonic deformation observed on the sea bottom and within the uppermost sedimentary layers to that of the deep-seated deformation within the limits of resolution and penetration of the available geophysical data and (ii) to build a three-dimensional (3D) block diagram of the active tectonic and buried features by means of a sliced mapping technique. In this approach, we produced slice maps of the active and buried structural features at selected depths and then combined them to form a 3D structural block diagram. Motivation for our work was to produce a 3D structural diagram to derive a more detailed image of the structural features in the Central Basin where there is no available 3D seismic data. The observations from the bathymetry and seismic data and developed 3D diagram support the presence of a through-going strike-slip fault that forms a rotational depression zone against a right-stepping strike-slip faulting causing a pull-apart basin in the Central Depression zone.

The Zagros folded belt (Fars, Iran): constraints from topography and critical wedge modelling

SUMMARY The Late Miocene tectonics of the Zagros folded belt (Fars province) has for long been related solely to folding of the cover controlled by a ductile decollement between basement and the sedimentary cover. However, geological constraints, topography analysis and seismotectonic studies reveal that basement thrusting may produce locally significant deformation in the cover. To determine how the deep-seated deformation in the basement may contribute to the overall topography we first examine the filtered large and short wavelengths of the topography. We find that the short-wavelength component of the topography (20-25 km), including the Zagros folds, is superimposed on the differential uplift at the regional scale. In other words, the regional base level of folded marker horizons remains parallel to the regional topography of interest. Modelling reveals that the salt-based wedge model, alone, is not able to reproduce the large- wavelength component of the topography of the Zagros Folded Belt. This reveals that when a thick (relatively to its overburden) layer of salt forms the basal decollement it is generally too weak and cannot support the growth of significant topography. We then test an alternative thick- skinned crustal wedge model involving the crust of the Arabian margin, which is decoupled above a viscous lower crust. This model satisfactorily reproduces the observed topography and is consistent with present-day basement thrusting, topography analyses and geological constraints. We conclude that basement-involved thickening and shortening is mechanically required to produce the shape of the Zagros Folded Belt since at least 10 Ma. Finally, the involvement of the basement provides mechanical and kinematic constraints that should be accounted for cross-sections balancing and further assessing the evolution of Zagros at crustal or lithospheric scales.

Interpretation of radon anomalies in seismotectonic and tectonic-gravitational settings: the south-eastern Crati graben (Northern Calabria, Italy)

The study area is located in the south-eastern part of the Crati valley (Northern Calabria, Italy), which is a graben bordered by N–S trending normal faults and crossed by NW–SE normal left-lateral faults. Numerous severe crustal earthquakes have affected the area in historical time. Present-day seismic activity is mainly related to the N–S faults located along the eastern border of the graben. In this area, much seismically induced deep-seated deformation has also been recognised. In the present paper, radon concentrations in soil gas have been measured and compared with (a) lithology, (b) Quaternary faults, (c) historical and instrumental seismicity, and (d) deep-seated deformation. The results highlight the following: (a) There is no evidence of a strong correlation between lithology and the radon anomalies. (b) A clear correlation between the N–S geometry of radon anomalies and the orientation of main fault systems has been recognised, except in the southernmost part of the area, where the radon concentrations are strongly affected by the superposition of the N–S and the NW–SE fault systems. (c) Epicentral zones of instrumental and historical earthquakes correspond to the highest values of radon concentrations, probably indicating recent activated fault segments. In particular, high radon values occur in the zones struck by earthquakes in 1835, 1854, and 1870. (d) Deep-seated gravitational deformation generally coincides with zones characterised by low radon concentrations. In the studied area, the anisotropic distribution of radon concentrations is congruent with the presence of neotectonic features and deep-seated gravitational phenomena. The method used in this study could profitably contribute towards either seismic risk or deep-seated gravitational deformation analyses.

A giant deep-seated slope deformation in the Italian Alps studied by paleoseismological and morphometric techniques

This work proves the existence of a large deep-seated gravitational slope deformation (DGSD) in a hilly region of the southwestern Alps, whereas DGSD are usually linked with high relief energy in mountain environments. Moreover, we describe the usefulness of applying paleoseismological techniques by means of trench excavation to date and understand the deformation history and genesis of recent morphostructures, and we found evidence of causative relationships between DGSD and surface landslides. The studied DGSD of Mt. Croce della Tola–Mt. Scincina, Italy, is the largest of the western Alps: it is 7-km long and involves a minimum area of 16 km 2 with a volume ≥3.5 km 3 probably extending further NE beneath Lake Maggiore surface. Several parallel scarps, representing the surface expression of slip-planes, affect the upper part of the slope, whereas the lower part presents a convex profile. DGSD at Mt. Croce della Tola started in the last interglacial period (120–40 ka BP) and the following glacial phase was not able to significantly modify the general slope geometry. Post-glacial deformation also occurred, especially at Mt. Scincina, after 25 ka BP. Post-glacial and active surface landslides developed on the convex lower part of the slope, suggesting that they resulted from instability due to the new profile assumed by the slopes during the deep-seated deformation. The occurrence of Mt. Scincina DGSD with a very low topographic gradient is interpreted as an effect induced by on other large DGSD.

Finite Element Analysis of a Deep-seated Slope Deformation

A study is presented of the slow time dependent movement of the deep-seated deformation of Rosone. This rock slope, which mainly consists of gneiss, is located on the Italian side of Western Alps and has been affected by recurrent instability phenomena. Due to these, and also to the presence of various villages and man-made structures (such as a hydroelectric power plant), a significant amount of geological and geomechanical data has been collected in this area. They have led to the assumption that the slow movement develops along a deep-seated sliding surface, involving a volume of rock between 22 and 35 million cubic meters. To check these hypotheses, and to get some insight into the causes of the phenomenon, a series of non-linear, time-dependent analyses has been carried out through the finite element method. The slow movement of the rock mass, and the gradual loss of its mechanical properties with increasing deformation, has been accounted for in the calculations by means of a visco-plastic constitutive law, allowing for strain softening effects. The results of analyses lead to some conclusions on the causes of the observed movements, among which the possible influence of the groundwater pore pressure is likely to play an important role.

The Sant'Agata Feltria landslide (Marche Region, central Italy): a case of recurrent earthflow evolving from a deep-seated gravitational slope deformation

This paper examines the evolution of a large-scale earthflow located in the northern sector of the Marche Region (central Italy). In this area, mass movements of different typology are particularly frequent since geological and geomorphological conditions often favour slope instability. Among these many mass movements, the one affecting the historical village of Sant'Agata Feltria has been chosen, also because its evolution has been recorded in chronicles starting from ancient times. In fact, reactivations of the 5.4 km long earthflow crossing the village have been quoted at least 12 times starting from the 16th century, even though more ancient activations have been inferred. Historic researches and geomorphologic analyses allowed the detailed reconstruction of the evolution and the mechanism of the phenomenon, outlining its causes and delineating risk zonation. In particular, a deep-seated gravitational movement (lateral spreading) marked by trenches and ponds has been recognised at the top of the landslide. The earthflow has been interpreted as a more surficial side-effect of the deep-seated phenomenon, which constitutes its source area. It has also been highlighted that reactivations coincided with long-lasting and intense rainfalls whilst no direct connection seems to exist with earthquakes, even though the area is seismically rather active and seismic shocks could have been among the major triggering factors for the deep-seated deformation. The oldest (and probably most catastrophic) recorded event itself (1561 A.D.), took place during the first stages of a period of severe climate that involved the whole of Europe.

Deformations of the near-surface part of the Earth's crust by seismologic and geodetic data obtained on Baikal geodynamic polygons

The Tunkin geodynamic polygon is taken where the deformation tensor components approximately (10 −4) for the Earth's crust upper layers, along with the relative displacements of the blocks in the fault zone, have been determined based on the application of the theory of small deformations and on an experimental base of repeated geodetic measurements. A comparative analysis of the surface deformation, calculated by geodetic measurements with respect to the deep-seated deformations obtained from seismic foci mechanisms, is given.

Omissional faulting during Mesozoic regional contraction at Carlin Canyon, Nevada

Faults at low angles to bedding that emplace younger over older rocks are widespread in the hinterland of the Sevier orogenic belt near the border between Nevada and Utah and are attributed to horizontal extension during latest Jurassic, Cretaceous, or mid-Tertiary time. At Carlin Canyon, approximately 150 km west of the border, we document a low-angle fault of probable Mesozoic age between Upper Pennsylvanian rocks and Lower Pennsylvanian strata, a contact that previously was mapped as a locally angular segment of a regional Middle Pennsylvanian unconformity and was considered evidence for Upper Pennsylvanian orogeny (Humboldt orogeny). Loss of section in Carlin Canyon was in response to top-northwest layer-parallel shear along a backthrust during northwest/southeast regional contraction. Deformation is later than Permian and earlier than Miocene but is probably latest Jurassic to Cretaceous. Structures in the upper Paleozoic rocks of Carlin Canyon may record shallow, thin-skinned contraction in the hinterland of the Sevier orogenic belt above deep-seated metamorphism and deformation exposed nearby in the Ruby Mountains-East Humboldt Range. The fault at Carlin Canyon indicates that the Humboldt orogeny reflected primarily vertical, not horizontal, displacements.

Sandstone Detrital Modes, Central Utah Foreland Region: Stratigraphic Record of Cretaceous-Paleogene Tectonic Evolution

ABSTRACT Within a sedimentary basin, the analysis of sandstone petrofacies defined by detrital modes of framework grains is a powerful means to detect areal or temporal changes in provenance related to evolving paleotectonics and paleogeography. In the central Utah foreland, the regional tectonic transition from thin-skinned Sevier thrusting during the Late Cretaceous to deep-seated Laramide deformation during the Paleogene is recorded by four contrasting petrofacies in uppermost Cretaceous and Paleogene sandstones of fluvial and deltaic facies. Coeval subaqueous environments were marine during the Cretaceous and lacustrine during the Paleogene. The Cretaceous quartzose petrofacies (Qm93, F1, Lt6) of mainly Campanian age was derived from sedime tary sources in the active Sevier thrust belt, and transported into a broad adjacent foreland basin by transverse paleocurrents. The heterogeneous Cretaceous quartzolithic petrofacies (Qm40-70, F5-20, Lt25-40) of uppermost Campanian age was derived from the Sevier orogenic belt during the waning phases of Sevier tectonism, and was distributed by longitudinal paleocurrents within a foreland basin whose floor had begun to be deformed by incipient Laramide tectonism. The Paleogene feldspathic petrofacies (Qm50, F35, Lt15) in Paleocene and Eocene strata is composed of arkosic detritus that was derived principally from basement rocks then exposed in the extensive San Luis uplift of Laramide age in south-central Colorado. The feldspathic sand was transported to the Laramide Uinta Basin and vicinity in Utah by a trunk river which flowed generally northwestward between monoclinal Laramide uplifts of the Colorado Plateau. The Paleogene quartzolithic petrofacies (Qm75, F2, Lt23) of dominantly Eocene age was derived mainly from sedimentary sources in the Laramide Uinta uplift to the north, but also in part from similar sources in the dormant Sevier thrust belt to the west. In the Rocky Mountain and intermountain regions, more widespread and systematic use of petrofacies analysis, in combination with standard paleocurrent and lithofacies analyses, could lead to better definition of the timing of Laramide uplift and improved understandi g of the evolution of complex Laramide paleogeography.

Structure and Petrology of the Alpine-type Peridotite at Burro Mountain, California, U.S.A.

The alpine-type peridotite at Burro Mountain is a partially serpentinized harzburgite-dunite body approximately 2 km in diameter. It lies in a chaotic mélange derived from the Franciscan Formation (Upper Jurassic to Upper Cretaceous) of the southern Coast Ranges of California. The peridotite is bounded on the east by a vertical fault in the Nacimiento fault zone that brings sedimentary rocks of Taliaferro's (1943b) Asuncion Group (Upper Cretaceous) into contact with the peridotite. The peridotite appears to be one of a number of tectonic lenses, having a wide range in size, that make up the mélange. These lenses include metagraywacke, metachert, greenstone, amphibolite, and blueschist, as well as ultramafic rocks, and represent a wide range of pressure-temperature environments. The outer shell of the peridotite is a sheared serpentinite zone 10–15 m thick. The peridotite was tectonically emplaced at its present level as a cold solid mass and had little effect on the mineral assemblages of the Franciscan Formation. Local development of lawsonite and aragonite in shear zones may be related to the peridotite emplacement. Foliated harzburgite forms approximately 60 per cent of the peridotite. It is a lithologically uniform rock that has an olivine: orthopyroxene ratio of approximately 75:25. Accessory clinopyroxene and chromian spinel generally make up less than 5 per cent of the harzburgite. Dunite, composed of olivine, accessory chromian spinel (< 5 per cent), and trace amounts of pyroxene, makes up approximately 40 per cent of the peridotite and occurs as dikes, sills, and irregular bodies in the harzburgite. Olivine and pyroxene show small but significant compositional variations and chromian spinel shows a large range in the cation ratio Cr/(Cr+Al+ Fe3+). The compositional variations in these minerals are related to original differences in bulk chemical composition. The following compositional ranges were determined for minerals in the harzburgite: olivine, Fo91.1−Fo91.4; orthopyroxene, En89.8−En91.1; clinopyroxene, Ca47.0Mg50.0Fe3.0−Ca48.7Mg48.2Fe3.1; chromian spinel, Cr/(Cr+Al+Fe3+) 0.37−0.55. The pyroxenes have a range in A12O3 content of 1.3−3.0 wt per cent. Olivine from dunite ranges from Fo91 to Fo92 7 and the chromian spinel has a range in the Cr/(Cr+Al+Fe3+) ratio of 0.30−0.75. Although all the dunites are lithologically similar, three distinct types are recognized on the basis of composition of coexisting olivine and chromian spinel. Structural relations between the three types of dunite suggest three periods of emplacement (possibly overlapping) of dunite into harzburgite. The evidence indicates that the dunite, and probably also the harzburgite crystallized from an ultramafic magma, probably in the upper mantle. After the magmatic episode and crystallization, the peridotite was subjected to a deep-seated plastic deformation and recrystallization. The first phase of the deformation produced a pervasive, planar structural element (S1) that crosscuts many harzburgite-dunite contacts. It is probable that some of the dunite sills were emplaced during this deformation. The foliation, S1, is defined by layers of different orthopyroxene content in harzburgite, and by discontinuous layers of chromian spinel in dunite. Flow or slip along S1 produced slip folds in harzburgite—dunite contacts with axial planes parallel to S1. At a later stage, isoclinal folds developed in S1, and the present olivine microfabric was probably formed by recrystallization in the stress field that produced the isoclinal folding. In the olivine microfabric, X tends to be perpendicular to the axial planes (S2) of the isoclinal folds and Y and Z tend to form double maxima in S2 approximately 90° apart. Mg−Fe2+ distribution between coexisting mineral pairs yields a calculated temperature of formation of approximately 1200 °C. Although this temperature is only a nominal value, it indicates that the mineral pairs equilibrated at a significantly high temperature. In view of the deformation and recrystallization, the calculated temperature possibly represents subsolidus re-equilibration of the minerals during this event. The deformation and recrystallization probably occurred shortly after crystallization while the peridotite was still at a high temperature. A later deep-seated deformation produced small scattered kink folds in S1 that tend to disrupt the major olivine microfabric. The kink folding was accompanied or followed by the development of kink bands in olivine that reflect intragranular gliding on the system T = [Okl], t = [100]. The kink bands probably formed at a minimum temperature of 1000 °C. Following the deep-seated deformation, which probably took place in the mantle, the peridotite mass was tectonically detached and moved upward to its present level in the crust. Cleavages, joints, and faults provided channels for water to pervade the peridotite and allow alteration of the primary minerals.

Analyses of Pliocene-Pleistocene Deformation and Stresses in Northern Santa Lucia Range, California

The Pliocene-Pleistocene orogeny that formed Santa Lucia Range folded its granitic and metamorphic basement as well as the overlying sedimentary rocks, which were 1−2½ miles thick in the area studied. The folds tend to flatten up-section from the basement and to become disharmonic high in the sedimentary sequence. Most are cut by reverse faults that also tend to die out up-section. Both folds and faults trend roughly parallel to basement foliations, which are older than latest Cretaceous. The folds are broad and open over granitic plutons and more closely spaced and tight (locally isoclinal) over metamorphic rocks. Within belts of metamorphic rocks lie the major reverse faults, which dip in the same direction as the metamorphic foliations. The deformation was thus directed appreciably by older rock units and structures. Basement folding involved some flexure of foliated rocks, but the principal mechanism was a gross flowage due to piecemeal slip on pre-existing fractures. In the more deformed sectors, minor faults lie about 1 foot apart and have an average displacement of 6 inches. Groove directions on 1500 minor faults measured in 43 subareas indicate systematic movement patterns. Analyzed on equal-area nets, the patterns are so like that predicted for ideal shear within a fractured-rock body that their σ 1 orientations can be approximated. These orientations are roughly accordant to petrofabric compression axes determined from calcite {0112} lamellae in 19 marbles and 26 sandstones. Most of the 81σ 1 orientations trend about normal to nearby fold axes and traces of reverse faults, while σ 2 orientations are parallel to these features and roughly horizontal. The vertical angles between σ 1 orientations and the reverse faults measure about 30 degrees. The average of all the σ 1 orientations in the basement is a line plunging 30° N. 30° E. The range trends N. 40° W. The basement rocks of the range were shortened about 12 per cent in a direction roughly normal to the range. Despite the pronounced ordering of folds along older structures, the range itself extends in nearly a straight line across major divisions of the California basement. These relationships indicate that the range was produced by deep-seated deformation, possibly a northeast-to-southwest flowage under the crust. The σ 1 orientations in the upper sandstones typically plunge parallel to bedding. This and the occurrence of disharmonic folds high in the section indicate that basement shortening caused the upper part of the sedimentary sequence to buckle more or less independently of the basement.