Tectonic Folding Research Articles

Broken Beds, but better science; using multiple hypotheses to interpret geological data

AbstractIn this study, we propose a multiple hypotheses approach to improve interpretations of limited remotely sensed datasets, such as sparsely exposed outcrops, subsurface datasets, or planetary objects using semi‐quantitative scoring and ranking of observable features. This method is demonstrated using an outcrop example from the Broken Beds of the Upper Jurassic–Lower Cretaceous Purbeck Limestone Group exposed along Britain's Jurassic Coast. Four published hypotheses regarding their origin are refined, represented in matrix form, scored and ranked based on carefully selected outcrop features. Semi‐quantitative scoring utilises knowledge of likely processes governing the occurrence of a range of features, some of which might be ignored or down‐played to favour a single hypothesis. Furthermore, by integrating expertise from different sub‐disciplines (e.g. basin analysis, sedimentology, diagenesis), we also consider the combined evidence of multiple features. This new method results in an interpretation that favours a multi‐process origin for the Broken Beds due to evaporite dissolution, overpressure release and tectonic folding, with identified uncertainty, all useful to guide further data collection.

Arrested versus active silica diagenesis reaction boundaries—A review of seismic diagnostic criteria

AbstractThis paper evaluates previously proposed diagnostic criteria that can be used to determine whether or not there is active migration of the opal‐A to opal‐CT transition zone (TZA/CT). The criteria are based on the interpretation of 2D and 3D seismic surveys and are therefore geometrical. They involve an assessment of the relationship of the TZA/CT with polygonal fault systems, differential compaction structures and tectonic folds. The most robust evidence for an inactive ‘reaction front’ between opal‐A and opal‐CT bearing sediments is the discordance of the TZA/CT relative to present‐day isotherms. Any of these may be persuasive as diagnostic criteria for the upward arrest of the diagenetic transformation at a regional scale, but actual truncation of the TZA/CT at the modern seabed is definitive for arrested diagenesis. This study argues that diagenetic assessment based solely on a single criterion independently is not reliable as an indicator for the current state of a silica transition. As a conclusion, the analysed seismic/structural criteria should be synthesised to provide a more credible interpretation for silica diagenesis. The use of modern 2D and 3D seismic data for the reconstruction of the diagenetic history of opaline silica bearing sediments offers a new approach to the study of silica diagenesis at a regional scale.

Quaternary Reelfoot Fault Deformation in the Obion River Valley, Tennessee, USA

AbstractBlind reverse faults are challenging to detect, and earthquake records can be elusive because deep fault slip does not break the surface along readily recognized scarps. The blind Reelfoot fault in the New Madrid seismic zone in the central United States has been the subject of extensive prior investigation; however, the extent of slip at the southern portion of the fault remains unconstrained. In this study, we use lidar to map terraces and lacustrine landforms in the Obion River valley and investigate apparent broad folding resulting from slip on the buried Reelfoot fault. We compare remote surface mapping results with three auger boreholes in the ∼24 ka Finley terrace and interpret apparent warping as due to tectonic folding and not stratigraphic thickening. We combine our results with historical records of coseismic lake formation that indicate surface deformation dammed the Obion River in the 1812 CE earthquake. Older terraces (deposited at least 35–55 ka) record progressive fold scarps ≥1, ≥2, and ≥8 m high indicating a long record of earthquakes predating the existing paleoseismic record. Broad, distributed folding above the Reelfoot fault into the Obion River valley is consistent with a deep active fault tip along the southern reaches of the fault. Our analyses indicate the entire length of the fault (≥70 km) is capable of rupture and is more consistent with longer rupture scenarios.

Site Effect Potential in Fond Parisien, in the East of Port-au-Prince, Haiti

In the frame of a Belgo-Haitian cooperation project (PIC 2012–2016), a study of the local seismic hazard was performed in Fond Parisien, an area located on the foothills of the “Massif de la Selle”, along the easternmost portion of the Enriquillo Plantain Garden Fault (EPGF). The H/V Spectral Ratio (HVSR) technique was applied to study the resonance frequency of the target areas and the azimuth of the wave field. The amplification factors were estimated using Standard Spectral Ratios obtained from earthquakes recorded by a temporary seismic network. Using the Multichannel Analysis of Surface Waves method, the seismic properties of the shallow layers were investigated. Then, the results were compared to local Electrical Resistivity Tomography data. These results highlight, in the central part of Fond Parisien, an E-W zone of low velocities ranging from 200 m/s to 450 m/s and low resistivities between 1 Ωm and 150 Ωm, due both to tectonic folding of the rocks and to the presence of sediment filling in the eastern part. The latter is marked, in most of its sites, by resonances at one or more frequencies ranging from 0.7 Hz to 20 Hz. Infiltration and storage of brackish water in the underground layers also contribute to the low resistivity values. With the noise HVSR data, we also evidenced a significant influence of the EPGF on the main orientation of the seismic wavefield as in the vicinity of this fault, the azimuths are parallel to the orientation of the fault. Overall, the results also show greater potential for site effects in the block formed by the sedimentary basin and strong amplification of the seismic ground motion for the sites bordering the basin to the north and west. We interpret the amplification in the north and south-west as probably originating from topographic irregularities locally coupled with sediment deposits, while in the center of the western part, the site effects could be explained by the presence of folds and related weakened and softened rocks. By the integration of several geophysical methods, we could distinguish areas where it is possible to build more safely. These zones are located in the northern part and encompass Quisqueya Park and neighboring areas as well as the village “La Source” in the southern part. In the rest of Fond Parisien, i.e., in the more central and eastern parts, buildings should be erected with caution, taking into account the nearby presence of the EPGF and the influence of fine sediments on the amplification of the seismic motion.

Influence of tectonic exhumation on porosity of Wufeng–Longmaxi shale in the Fuling gas field of the eastern Sichuan Basin, China

Thermal cracking of organic matter (OM) can enhance gas-shale porosity. However, thermally matured shales are generally characterized by low porosity with high OM content in tectonic exhumation settings. The Wufeng–Longmaxi shale in the Fuling gas field, China, was investigated via bulk analysis and scanning electron microscopy to reveal the influence of tectonic exhumation on the porosity of OM-rich shale. Thermal maturity and organic and mineralogical compositions are regionally homogeneous in the Fuling gas field. Shale porosity is positively correlated with total organic carbon (TOC) and clay mineral content in the gentle and open anticlines, respectively. The OM-hosted pores were preserved with spherical morphologies because of overpressure-associated overburden stress unloading in the gentle anticlines, whereas OM-hosted pores in the open anticlines are characterized by high aspect ratios and are regularly aligned perpendicular to lateral compression stress. A simple simulation confirmed that tectonic folding and compaction during exhumation caused the loss of porosity in the inner arcs of the open anticlines. Only pressure shadows formed by assemblages of clay mineral platelets and mechanically competent grains inhibited tectonic compaction of some OM-hosted pores in the open anticlines. Tectonic compaction of porosity generally increased the mean value of water saturation in the open anticlines. The extremely high water saturation (>60%) in the high-TOC interval (>2 wt. %) was generally associated with natural fractures, suggesting the occurrence of gas leakage.

Clustered BSRs: Evidence for gas hydrate-bearing turbidite complexes in folded regions, example from the Perdido Fold Belt, northern Gulf of Mexico

We describe previously undocumented but extensive gas hydrate accumulations in the mouth of Perdido Canyon in the northern Gulf of Mexico. The accumulations are located within central parts of structural domes (four-way closures) and are characterized by stacked, high-amplitude bottom simulating reflections (BSRs) that we call clustered BSRs. Seismic data from Perdido Canyon show two clustered BSRs associated with turbidite sequences within two dome folds formed from tectonic folding and salt diapir rise. The northwestern (NW) and southeastern (SE) clustered BSRs have aerial extents of ∼25 km2 and 50 km2, respectively. Well log data confirm gas hydrate occurs above the NW clustered BSR, within a 225 m-thick consistently high-resistivity interval that we interpret as gas hydrate in near-vertical fractures and turbidite sands. The SE dome is only drilled at the edge of the BSR; nevertheless, the well log data indicate that a 30 m-thick gas hydrate accumulation is present. Gas chromatographic logs in both domes suggest a gradual transition from predominantly microbial gas below the BSR (500–1000 m below seafloor (mbsf)) to thermogenic gas at 1000–2000 mbsf. Based on the well log data and seismic stratigraphic analysis, we find gas hydrate is concentrated in fractures in marine mud, as well as in the pores of submarine fan turbidities, where saturations reach as high as 75%. An estimate of the total gas hydrate-bound gas volume at standard temperature and pressure is between 0.04 and 0.17 trillion cubic meters (TCM) assuming average hydrate saturation of 5-20% in a ∼45 m thick turbidite sand unit above the Perdido Canyon BSR area. Measured BSR extent and gas volume estimates indicate that the NW and SE reservoirs are among the largest gas hydrate occurrences known in the Gulf of Mexico.

Drone Geologic Mapping of an Active Sand and Gravel Quarry, Desoto County, Mississippi

Aerial drone photography of an active pit within a sand and gravel quarry in DeSoto County, Mississippi, was conducted to better understand the Upland Complex, which is a high-level Pliocene terrace of the Mississippi River. The Upland Complex is of great interest economically, as it is the primary source of sand and gravel for Memphis, Tennessee and the surrounding region. The pit dimensions were approximately 820 ft (250 m) by 655 ft (200 m) and 79-ft (24 m) deep upon completion of the mining. Eight 3-D models of the pit were made at different times to illustrate the mining progression. Oblique and horizontal stereo aerial photography of the highwalls was conducted to produce 3-D models and high-resolution photomosaics of the highwalls for geologic mapping and interpretation. The mapped highwall geology included Pliocene Mississippi River bars consisting of sand, sand and gravel, and gravel ranging in thickness from 2 ft (0.6 m) to 32.8 ft (10 m), with variable cross-bed dip directions suggesting a meandering river environment of deposition. Pleistocene loess overlies the Pliocene sediment. The highwalls also revealed northerly-striking late Pliocene or Pleistocene tectonic folding, faulting, and probable earthquake liquefaction in northwestern Mississippi, where no Pliocene or Quaternary tectonic deformation had previously been reported. This study demonstrated Drone aerial photography as a quick, low cost, and safe means to study poorly accessible open-pit mining and to help understand the geology of the lower Mississippi River Valley.

Folding during soft-sediment deformation

Abstract The detailed analysis of folding in rocks was in part pioneered by John Ramsay, and resulted in a range of techniques and criteria to define folds. Although folding of unlithified or ‘soft’ sediments is typically assumed to produce similar geometries to those in ‘hard rocks’, there has to date been little detailed analysis of such folds. The aim of this paper is therefore to investigate folds developed during soft-sediment deformation (SSD) by applying techniques established for the analysis of tectonic folds during hard-rock deformation (HRD). We use the Late Pleistocene Lisan Formation exposed around the Dead Sea as our case study, as the laminated lake sediments record intricacies of fold detail generated during seismically triggered slumping of mass transport deposits (MTDs) towards the depocentre of the basin. While it is frequently assumed that folds created during SSD are chaotic and form disharmonic structures, we provide analyses that show harmonic fold trains may form during slumping, although larger upright folds cannot be traced for significant distances and are more typically disharmonic. Our analysis also reveals a range of fold styles, with more competent detrital-rich layers displaying buckles (Class 1B), as well as upright Class 1A folds marked by thickened limbs. Class 1A buckle folds are generally considered to be created by flattening that overprints folds with an original Class 1B geometry. As thickened fold limbs are truncated by overlying erosive surfaces, the vertical flattening is considered to have occurred during the slump event. Different fold shapes may partially reflect variable flattening, depending on the original orientation of upright or recumbent folds, together with continued downslope-directed simple-shear deformation that modifies the fold geometry. Analysis of fold wavelength, amplitude and bed thickness allows us to plot strain contour maps, and indicates that beds defining slump folds display viscosity contrasts in the range of 50–250, which are similar to values estimated from folds created during HRD in metamorphic rocks. A range of refold patterns, similar to those established by John Ramsay in metamorphic rocks, are observed within slumps, and are truncated by the overlying sediments, indicating that they formed during a single progressive slump event rather than distinct ‘episodes’ of superimposed deformation. This study confirms that techniques developed for the analysis of folds created during HRD are equally applicable to those formed during SSD, and that resulting folds are generally indistinguishable from one another. Extreme caution should therefore be exercised when interpreting the origin of folds in the rock record where the palaeogeographical and tectonic contexts become increasingly uncertain, thereby leading to potential misidentification of folds created during SSD.

How tectonic folding influences gas hydrate formation: New Zealand’s Hikurangi subduction margin

How tectonic folding influences gas hydrate formation: New Zealand’s Hikurangi subduction margin

The Effect of Deep Burial and Folding on Sandstone Reservoirs in some Giant Gas Fields, South America

The underthrust fold belt of the Andes contains some very large gas fields in which the Lower Palaeozoic Sandstone reservoirs are buried to depths of over 4-6km. The foreland depositional environment received high amounts of metamorphic and igneous rock fragments and deep burial was accompanied by substantial tectonic folding and fracturing. The combined effects of Temperature, Pressure and time on these labile sediments have reduced typical porosities <7% and matrix permeabilities <1mD so that these reservoirs are ultra-tight and effectively unconventional. Nevertheless, the fields contain very large recoverable amounts of gas with minor liquids per well. They provide an intriguing case study that contrasts with the typical concept of a conventional petroleum sandstone prospect and pose the question how many more large gas fields fit this model? These complex structural fields required modelling by 3D-Geo using fractured and folded simulation models. The micro-porosity and micro-permeability has been investigated by Curtin University using nano-scale special core analysis in an attempt to identify where the hydrocarbons reside in these rocks, how they migrate out on production, and how best to estimate and optimize ultimate recovery? High technology characterization included TIMA-SEM to map mineralogy and texture at the nano-scale; X-ray Micro-CT analysis of 3D microstructure; NMR, ultra-low Helium porosity and permeability, Hg injection for capillary pressure; Elastic and Electrical Properties to tie the seismic and log data for the modelling. Tri-axial tests helped understand the structural and tectonic history and its relation to the burial history of the reservoirs.

Late Cretaceous to Miocene and Quaternary deformation history of the Chalk: Channels, slumps, faults, folds and glacitectonics

Late Cretaceous Chalk sedimentation history across the British Isles included (i) fault controlled uplift and subsidence in Northern Ireland and the Inner Hebrides and (ii) uplift along the lines of en echelon folds in Southern Britain and northern France. Synsedimentary slump folds and downslope displacement structures are compared with penecontemporaneous interbed slides and later tectonic folds and faults. Compressional strike-slip tectonic processes at Flamborough Head, Yorkshire, illustrate intra-Chalk slump beds in a half-graben setting. Progressive ‘growth’ of structures characterises early downslope slump folding, interbed sliding and some listric faulting. Sheet-flints replacing slide shear planes and early fractures provide evidence for early movements. Availability of open-slopes or the depth of burial under which the range of structures developed is reflected in the degree of disruption and fragmentation of chalk and flint. Fragmentation provides clues to the timing of events and origin of the Late Campanian Altachuile Breccia (Northern Ireland) and the Coniacian Hope Gap slides (Sussex). Fragmentation and formation of sheet flints together help distinguish intra-Chalk tectonics from Quaternary glacitectonic structures.The role of marl seams, high porosity chalk beds and hardgrounds on bed-sliding, décollement zones and disruption of chalk blocks from bedrock in glacitectonics is discussed. Chalk formations with marl seams develop a special style of fracturing related to early interbed sliding and pore-fluid escape structures. Marl-seams are shown to be primary sedimentary features and not the products of post depositional pressure-solution. More than any other formation the Late Santonian – Early Campanian Newhaven Chalk contains extensive sheet-flints and shows great lateral variation in thickness and lithology across the fold belts of southern England and northern France.

Diversion and morphology of submarine channels in response to regional slopes and localized salt tectonics, Levant Basin

In the Levant Basin, submarine channels are abundant around the Nile deep-sea fan (NDSF), an area which is also affected by salt tectonics related to the Messinian salt giant. Here we focus on the relationship between submarine channels and obstacles formed by salt tectonics. Initially, we use methods developed for terrestrial morphological analysis and quantify channel sinuosity, width and slope in search for consistent relationships between morphometric parameters and channel response to obstacles. However, this traditional analysis did not yield robust conclusions. Then, we apply two new morphometric parameters suggested here to express the distortion of channels by obstacles: incident angle (α), defined as the acute angle between the regionally influenced channel direction and the strike of the tectonic obstacle and diversion angle (Ω), defined as the angle between the direction of the regional bathymetric slope and the average direction of the channel. These parameters illustrate the influence of the regional-scale basin geometry and the superimposed tectonic-influenced seabed patterns, on channel development. We found hyperbolic relationships between incident angle (α) and diversion angle (Ω) in which channels flowing approximately parallel (α ≈ 0°) to tectonic folds are (obviously) not diverted; channels nearly orthogonal (α ≈ 90°) to obstacles, crosscut them right through and, again, not diverted much. In contrast, channels with a general direction diagonal to the obstacles (α ≈ 40°), are diverted by ten degrees (Ω ≈ 10°). This diversion accumulates along large distances and significantly influences the regional development of channels around the NDSF. Noteworthy, this phenomenon of channel diversion, indirectly deteriorate normal slope-sinuosity relationships known from terrestrial studies. In light of these findings, we suggest that these new parameters can be applied to other basins, where submarine channels interact with seabed obstacles.

How vein sealing boosts fracture widening rates – The buckling-enhanced aperture growth mechanism for syn-tectonic veins

The paper introduces the mechanism of buckling-enhanced aperture growth for syn-tectonic veins that formed in simple-shear dominated kinematic frameworks in the middle or lower crust. Apart from the well understood concepts of fracture widening driven by effective tensile stresses, buckling-enhanced fracture aperture growth relates widening to active outward buckling of more viscous incipient cement layers precipitated as hydrothermal minerals for the pore fluid on the walls of juvenile syn-tectonic veins, driven by fracture-parallel compressive creep strain in the host rocks. Thus, the mechanism proposed here follows similar principles as tectonic folding, although important differences exist. Inspired by the structural record of low-aspect ratio veins exposed in HP/LT metamorphic rocks cropping out on south Evia island, Greece, generic numerical models are calculated to study development of buckling instabilities in such incompletely cemented veins and their impact on aperture growth rates. The models indicate (1) that aperture growth rates increase with increasing viscosity contrast between the host rocks and the cement layers, (2) an increase in the thickness of the cement layers cause acceleration of aperture growth, (3) that support of restraining forces at the vein tips offered by the host rocks against buckling of the cement layers cause fully compressive states of stress ahead the fracture tips, and (4) that fracture aperture growth is possible against fully compressive fracture-normal stresses. The buckling-enhanced vein aperture growth mechanism yields important implications for the maintenance and decay of fracture-bound permeability and for the mechanical state of the middle and lower crust in seismically active regions.

Block Slides on Extremely Weak Tectonic Clay Seams in Openly Folded Tertiary Mud-Rocks at Auckland and the Rangitikei Valley, North Island, New Zealand

Block slides have developed on extremely weak, thin clay seams of tectonic origin, parallel to bedding in gently dipping sandstones and mudstones of Tertiary age. Two areas of noted instability are investigated at Auckland and the Rangitikei valley. Dimensions range from 100 m across × 100 m long for short displacement block slides up to 4 km across × 3 km long for large landslide complexes in which block slides are a major component. Displacements of blocks range from incipient (cm) through short (30 m) to 2 or 3 km for large slides. Many of the Auckland slides are dormant but likely to move in a 2000 year return period earthquake or 100 year high intensity rain storm. At Rangitikei there are many active, younger slides. Sliding rates for active failures vary from a few cm/year to 50 m in 30 min. Host rocks are weak to very weak clayey sandstones and sandy mudstones. The seams are rich in smectite. They have polished and crushed walls, may have slickensides and some contain rounded rock fragments. Laboratory shear strength of the seams is 13 kPa cohesion and 13° friction, with a lower bound of 8° at zero cohesion. Strength is increased at the field scale by waviness, steps and splays. Continuity can be demonstrated over distances of hundreds of metres. Key investigation methods were mapping, shafts and trenches. Tectonic uplift, folding and faulting of the weak Tertiary strata and river down-cutting are perpetuating block slide development.

Lignite cleat studies from the first Middle-Polish (first Lusatian) lignite seam in central Poland

Some Miocene lignite seams are characterized by the presence of natural discontinuities, so-called cleats. Most often they are opening-mode fractures, consisting of two orthogonal sets (face and butt), both almost perpendicular to the bedding. This paper determines distributions of cleat orientation, spacing, and aperture from the first group of the Middle-Polish (first Lusatian) lignite seams (MPLS-1), which has been exploited, inter alia, in the Jóźwin IIB and Kazimierz N opencast mines in central Poland. All observations and measurements were conducted at macroscopic scale.The cleats' mean orientations are NE–SW and NW–SE, dipping at a high angle >75°. The first set of fractures refers to the face cleats and the second one to the butt cleats. The spacing of face cleats is from 10.0 to 17.2cm (averaging 12.65cm) and the spacing of butt cleats is from 9.3 to 24.3cm (averaging 13.75cm). On the other hand, aperture measurements of these cleats range from 0.2 to 1.9cm (averaging 0.68cm) and from 0.1 to 1.5cm (averaging 0.42cm), respectively. The obtained results clearly indicate that face cleat orientations (NW–SE) are strictly parallel to the elongation of the main tectonic structures in the study area. Their origin may be explained in at least two ways, that is, by tectonic folding (the maximum principal stress, σ1, was horizontal) or by salt diapirism (the maximum principal stress, σ1, was vertical). In both cases the intermediate principal stress, σ2, was parallel to the face cleat strikes, namely NW–SE, although the well-developed butt cleats, in the context of regional geology, provide additional evidence for the creation of an orthogonal system of fractures (cleats) in the cover of salt structures. Thus, the NW–SE direction is characteristic of both the extent of the main tectonic units in central Poland and the face cleat orientation in MPLS-1.

Fold and fabric relationships in temporally and spatially evolving slump systems: A multi-cell flow model

Folds generated in ductile metamorphic terranes and within unlithified sediments affected by slumping are geometrically identical to one another, and distinguishing the origin of such folds in ancient lithified rocks is therefore challenging. Foliation is observed to lie broadly parallel to the axial planes of tectonic folds, whilst it is frequently regarded as absent in slump folds. The presence of foliation is therefore often considered as a reliable criterion for distinguishing tectonic folds from those created during slumping. To test this assertion, we have examined a series of well exposed slump folds within the late Pleistocene Lisan Formation of the Dead Sea Basin. These slumps contain a number of different foliation types, including an axial–planar grain-shape fabric and a crenulation cleavage formed via microfolding of bedding laminae. Folds also contain a spaced disjunctive foliation characterised by extensional displacements across shear fractures. This spaced foliation fans around recumbent fold hinges, with kinematics reversing across the axial plane indicating a flexural shear fold mechanism. Overall, the spaced foliation is penecontemporaneous with each individual slump where it occurs, although in detail it is pre, syn or post the local folds. The identification of foliations within undoubted slump folds indicates that the presence or absence of foliation is not in itself a robust criterion to distinguish tectonic from soft-sediment folds. Extensional shear fractures displaying a range of temporal relationships with slump folds suggests that traditional single-cell flow models, where extension is focussed at the head and contraction in the lower toe of the slump, are a gross simplification. We therefore propose a new multi-cell flow model involving coeval second-order flow cells that interact with neighbouring cells during translation of the slump.

A morphologic characterisation of the 1963 Vajont Slide, Italy, using long-range terrestrial photogrammetry

The 1963 Vajont Slide in northeast Italy is an important engineering and geological event. Although the landslide has been extensively studied, new insights can be derived by applying modern techniques such as remote sensing and numerical modelling. This paper presents the first digital terrestrial photogrammetric analyses of the failure scar, landslide deposits, and the area surrounding the failure, with a focus on the scar. We processed photogrammetric models to produce discontinuity stereonets, residual maps and profiles, and slope and aspect maps, all of which provide information on the failure scar morphology. Our analyses enabled the creation of a preliminary semi-quantitative morphologic classification of the Vajont failure scar based on the large-scale tectonic folds and step-paths that define it. The analyses and morphologic classification have implications for the kinematics, dynamics, and mechanism of the slide. Metre- and decametre-scale features affected the initiation, direction, and displacement rate of sliding. The most complexly folded and stepped areas occur close to the intersection of orthogonal synclinal features related to the Dinaric and Neoalpine deformation events. Our analyses also highlight, for the first time, the evolution of the Vajont failure scar from 1963 to the present.

Integrating 3D seismic curvature and curvature gradient attributes for fracture characterization: Methodologies and interpretational implications

In 3D seismic interpretation, curvature is a popular attribute that depicts the geometry of seismic reflectors and has been widely used to detect faults in the subsurface; however, it provides only part of the solutions to subsurface structure analysis. This study extends the curvature algorithm to a new curvature gradient algorithm and integrates both algorithms for fracture detection using a 3D seismic test data set over Teapot Dome (Wyoming). In fractured reservoirs at Teapot Dome formed by tectonic folding and faulting, curvature helps define the crestal portion of reservoirs that is associated with strong seismic amplitude and high oil productivity. In contrast, curvature gradient helps define the regional northwest-trending and the cross-regional northeast-trending lineaments that are associated with weak seismic amplitude and low oil productivity. In concert with previous reports from image logs, cores, and outcrops, an integrated seismic curvature and curvature gradient analysis suggests that curvature might help define areas of enhanced potential to form tensile fractures, whereas curvature gradient might help define zones of enhanced potential to develop shear fractures. In fractured reservoirs at Teapot Dome where faulting and fault-related folding contribute dominantly to the formation and evolution of fractures, curvature and curvature gradient attributes can be potentially applied to differentiate fracture mode, to predict fracture intensity and orientation, to evaluate fracture volume and connectivity, and to model fracture networks.

Potencial Geoturístico do Parque Estadual da Serra do Ibitipoca, Sudeste do Estado de Minas Gerais

The Ibitipoca Mountain is a spectacularly morphostructural feature located between the Santa Rita do Ibitipoca and Lima Duarte cities, southeast Minas Gerais State, between the Parallel 21º38' and 21º44', and Meridians 43º50' and 44º00'. In this environment, was created in 1973 Ibitipoca State Park by Forest Institute of Minas Gerais State, which protects an area of 1,488 ha, with higher altitudes ranging from 1784 m, for the Morro da Lombada and 1722 m of Pico do Pião. This natural feature represents a great declivity of the Serra da Mantiqueira widely flattened with a plateau in the north portion, and a devastated south topography. In the geological context, Ibitipoca Mountain consists mainly of quartzites and coarse grain size, interspersed with layers of fine micaceous quartzites and garnet-sillimanite-biotite schist. The sediments that filled the original sedimentary basin have been metamorphosed in amphibolite facies, and deformed in a compressive tectonic regime resulting from an evolving nappe folding type, with the subsequent development of systems of faults and shear zones of high angle. The result of this tectonic event in sedimentary transitional continent-ocean, associated with erosion action, provided the development of fifteen wonderful and unique caves in quartzites, specially the Bromelias Cave with more development. The geomorphology can be summarized by the occurrence of escarpment plunging toward the river valleys and of Rio do Salto and Córrego da Mata, controlled by major tectonic folding that affected the metasedimentary rocks. The relief of the Ibitipoca Mountain present ruiniform forms, lapies and small valleys that may have been generated by processes of landslides galleries caves or doline. The rocks of PEI have a complex tectonic evolution that result in interesting geoforms. This ancient geological environment shows surprisingly exuberant flora and fauna and exotic caverns and pits that constitute potential areas to Geotourism.

Folding of granite and Cretaceous exhumation associated with regional-scale flexural slip folding and ridge subduction, Kitakami zone, northeast Japan

The early Cretaceous granitic plutons intrude the Kitakami zone, northeast Japan, whose southern and northern regions consist of forearc basin and accretionary complex rocks, respectively. All country rock of the Kitakami zone exhibit prominent pressure-solution cleavage and associated folds formed during shortening with a small component of sinistral shear, whereas most plutons show only igneous textures. The Kesengawa granite and some other plutons, however, have foliations that cut the pluton boundaries and are continuous with those observed in surrounding sedimentary rocks. We document a tectonic fold with axial planar foliation in part of the Kesengawa granite. The metamorphic minerals associated with the contact aureole and country rocks of the Kesengawa and other deformed plutons indicate an increase in metamorphic grade toward the pluton showing that deformation of the pluton and country rock took place as the plutons cooled. The Kesengawa pluton and country rocks of the southern Kitakami zone are deformed into regional scale upright folds with parasitic asymmetric folds that verge toward regional anticlinal axes whereas regional scale folds in the northern Kitakami zone are overturned and verge to the east. The Kitakami basement is not bounded by normal or reverse faults, so the style of regional exhumation does not resemble the upright or inclined extrusion noted in other regions, nor is the exhumation associated with extensional doming. Instead, the vergence of parasitic folds toward the regional fold hinges indicates flexural slip deformation at least in the late stages of exhumation and exhumation occurred in the cores of regional scale anticlines. The regional shortening of accretionary prism, forearc basin, and older forearc basement was associated with intrusion of adakitic plutons that thermally weakened the forearc basin and enhanced the deformation and exhumation. The adakitic magmatism and forearc shortening resulted from subduction of the buoyant Izanagi–Kula ridge, a regional event known as the Oshima orogeny.