Mayte Bulnes

Episodic folding inferred from syntectonic carbonate sedimentation: the Santaren anticline, Bahamas foreland

Abstract Sedimentation coeval with growth of the Santaren anticline provides an excellent opportunity to study the relationships between sedimentation and anticline uplift through time. The Santaren anticline is a kilometre-scale, NW–SE trending fold offshore of Cuba, in the Bahamas foreland of the Cuban fold and thrust belt. The growth strata associated with this anticline consist of a thick package of carbonate sediments that were deposited without major interruptions from Neogene (and perhaps before) to present day. The excellent seismic resolution and age control of a number of seismic horizons within the growth strata allowed us to define 25 growth beds, each of them representing between 0.1 and 3.2 Ma. An analysis of the thickness of these beds allowed us to determine accurate quantitative values of cumulative decompacted thickness and crestal structural relief at the time of their deposition. In addition, for the same periods, sedimentation and fold uplift rates were calculated. Moreover, some information on relationships between sedimentation and fold uplift rates was inferred from the growth stratal geometry. Growth beds that overlap the fold crest and thin over it indicate that sedimentation rates outpaced fold growth rates during their deposition. Some overlapping beds have constant thickness indicating that no fold uplift occurred during their sedimentation. The rest of the growth beds exhibit onlap/offlap geometries that do not indicate a unique sedimentation/fold uplift rate relationship. Only in those cases in which the geometry of the underlying bed at the end of its deposition is known is it possible to infer a specific sedimentation/fold uplift rate relationship. As a result of this analysis, we have been able to (1) illustrate that the geometry of the growth strata associated with the Santaren anticline results from competition between sedimentation and tectonic fold uplift, (2) document the episodic and non-steady nature of fold growth, and (3) show that short-term rates (at the scale of hundreds of thousands years) provide much insight into the interplay between sedimentation and tectonic fold uplift that control the growth stratal patterns.

Kinematic evolution and fracture prediction of the Valle Morado structure inferred from 3-D seismic data, Salta province, northwest Argentina

The Valle Morado structure, located in northwest Argentina, is an approximately 7-km-long and 4-km-wide north-northeast–south-southwest–trending anticline cut by faults that involve Paleozoic basement and Mesozoic–Cenozoic sedimentary cover. Geological interpretation of a three-dimensional seismic survey that covers the structure, plus a well and two-dimensional regional-scale seismic lines, indicates that the Valle Morado is an Andean pop-up structure formed by foreland-directed faults, resulting from reverse reactivation of Cretaceous extensional faults dipping westward, and newly formed hinterland-directed reverse faults dipping eastward. The orientation of the preexisting extensional faults is nearly perpendicular to the Andean tectonic transport direction in the northern part of the structure and oblique toward the south, resulting in almost pure contraction in the north and transpression southward.Curvature analysis performed on two interpreted reservoir seismic horizons suggests that the maximum density of open fractures is likely to occur along the anticline axis in the northern portion of the fold and along the northeast-southwest–striking foreland-directed, reactivated faults to the south. The direction of the open fractures interpreted from ultrasonic borehole image data along the well coincide with that of the open fractures predicted from the curvature analysis.

Estimating the detachment depth in cross sections involving detachment folds

This paper evaluates four different methods that have been proposed for the estimation of the detachment depth beneath detachment folds. Guidelines are presented in order to use the most suitable method in a particular region deformed by detachment folds. These guidelines are constructed considering the assumptions of each method, the influence of different parameters on the estimation of the detachment depth (folding mechanisms, cross-sectional area variations, position and orientation of the regional datum, number of stratigraphic horizons, bed length and thickness, dip and position of the bounding lines, initial thickness of the ductile unit, single anticlines or fold trains) and the available data (ductile unit thickness, regional datum, detachment depth). Moreover, a new, simple method based on two previous methods is presented. The advantages of this new method are that the calculations are extremely simple and it uses information from more than one stratigraphic horizon; therefore, it does not depend strongly on the accuracy of the data at one level. The precision of the proposed methods is tested by their application to a number of natural and experimental single anticlines and fold trains. It appears that better predictions are obtained when analysing a complete detachment fold train than when dealing with a single detachment anticline. The reason might be that ductile material flow beneath the folds along the cross-sectional plane is taken into account when dealing with a long-enough cross section. We also suggest some hypotheses concerning what the shape and dimensions of the detachment folds indicate about the detachment depth in those cases where insufficient data are available to apply the methods proposed.

Benefits and limitations of different 2D algorithms used in cross-section restoration of inverted extensional faults: application to physical experiments

Abstract In recent years, 2D restoration techniques have been systematically used to restore cross-sections through inverted basins. The accuracy of these techniques, and in particular which method better restores the inverted extensional faults to previous stages, is uncertain and difficult to assess in natural examples. To address this drawback, the applicability of flexural slip and vertical/oblique slip restoration techniques, executed with section restoration software, is tested through restoration of physical experiments of inverted extensional faults to their pre-inversion stage. The experiments chosen consist of simple listric and planar faults in which: (1) the original state and the kinematic path followed by the rocks to reach the final state is known, (2) the boundary conditions are known, (3) erosion is absent, and (4) the orientation of extension and compression vectors is equal. Comparing the restored sections with their corresponding actual pre-inversion stage reveals that flexural slip is the best restoration method, whereas the combination of different slip angles method gives the worst results. The accuracy of these techniques depends, to a great extent, on the master fault geometry, the coefficient of friction along it and the amount of inversion. The best results are obtained for a physical model that consists of a listric fault with 60° dip at the top of the rigid footwall, a shallow detachment, low coefficient of friction along the fault and mild amount of inversion. Since the deformation mechanisms and the geometry of the inverted structures are non-identical in physical experiments and in natural examples, the results obtained in our study should be cautiously applied to cross-sections across natural inverted basins.

Palaeogene-Neogene/present day(?) growth folding in the Bahamian foreland of the Cuban fold and thrust belt

The Santaren Anticline constitutes the frontal termination of the Cuban fold and thrust belt within the Bahamian foreland. New well and seismic data allow us to constrain in detail the evolution of this anticline. Pre-growth and syntectonic (partly post-tectonic?) units, separated by a major unconformity, are associated with the Santaren Anticline. Their geometrical features are consistent with a detachment fold. The precise age of the beginning of fold growth remains unknown. However, the complete record of well-dated syntectonic sediments documents its kinematic evolution from Mid-Eocene to Pliocene/present day, and reveals an approximately constant and very slow growth rate from Early Miocene to Pliocene/present day. The timing of evolution of the Santaren Anticline is not consistent with previous models that postulate that deformation associated with the Cuban fold and thrust belt ended in the Eocene. Our data suggest that the most external part of the Cuban fold and thrust belt was still being deformed under a compressional regime during the late Palaeogene, Neogene and probably during the Quaternary. We propose that this folding may result from compressive stresses transmitted approximately 400 km northwards from the actual plate boundary, as a result of slow convergence between the N and S American plates.

Construction of accurate geological cross-sections along trenches, cliffs and mountain slopes using photogrammetry

This paper discusses the application of close range photogrammetry for the construction of geological cross-sections from outcrops located on trenches, cliffs and mountain slopes. Our methodology is based on stereoscopic pairs of photographs of the outcrops on which geological interpretations may be carried out directly using a digital stereo viewer. It is also possible to automatically obtain point clouds. Through control points of known coordinates taken on the field and located in the photographs, the three-dimensional model recovered is correctly geo-referenced and the residual error is minimized. Layers and tectonic structures recognized in the photographs can be easily projected in any desired direction, as, for example, in the direction of the fold axes to obtain a proper view of the geometry of the structures, or in one direction parallel to the tectonic transport vector if a restoration is demanded. The application of this methodology is shown by constructing a detailed geological cross-section at the cliffs of La Conejera Inlet (Asturias, Spain). The studied structures, involving Jurassic rocks, are located in a Permian-Mesozoic extensional basin called Asturian Basin (NW Iberian Peninsula). This basin was partially formed during the opening of the Bay of Biscay and partially inverted during a Cenozoic contraction responsible for the Pyrenees and its western prolongation along the north margin of the Iberian Peninsula.

Kinematic evolution and strain simulation, based on cross-section restoration, of the Maiella Mountain: an analogue for oil fields in the Apennines (Italy)

Abstract Deformation predictive methods are useful for structural analysis from the scientific and industry point of view. We apply a strain simulation technique based on the inclusion of graphical strain markers in a cross-section, and subsequent cross-section restoration and numerical processing of strain markers, to the seismic-scale Maiella Mountain anticline (Central Apennines, Italy) considered a carbonate reservoir analogue for Apennines oil fields. The procedure followed involves field mapping and structural data collection, construction of cross-sections, sequential cross-section restoration, and application of the strain simulation technique. The cross-sections presented were constructed adopting one of the various structural interpretations proposed for this structure by different authors. According to this interpretation the Maiella Mountain structure resulted from Messinian–Early Pliocene extension and subsequent Late Pliocene shortening. According to our structural model the Maiella structure is a break-thrust fold and the comparison between the present-day and the restored cross-sections yields 1.3–4.6% of extension associated with two main normal faults and 21.5–22.1% and 2.5–3.4% of shortening due to a major thrust and folding respectively. The simulation of deformation distribution shows high deformation intensity in both limbs and low deformation in the anticline crest and part of the thrust footwall.

Fold amplification history unravelled from growth strata: the Dorood anticline, NW Persian Gulf

Abstract: The origin and kinematic evolution of the Dorood growth anticline, a kilometre-scale fold located in the Iranian NW part of the Persian Gulf on the Zagros front, is discussed based on the geological interpretation of a seismic section and subsequent application of a number of techniques such as depth to detachment estimations, sequential cross-section restoration, estimations of crestal structural relief, shortening, wavelength and fold core area in various stages of fold amplification, and comparison between functions derived from the anticline analysis and functions for theoretical folds. Long-term fold growth rates indicate a slow amplification for c. 88.6 Ma punctuated by two periods of faster growth during the Late Cretaceous and from the Late Miocene to the present day. The kinematic evolution proposed involves vertical push related to reactivation of a basement fault during the oldest amplification event and fold tightening owing to buckling as a consequence of horizontal compression in the youngest event, both causing evaporite motion. Fold amplification took place by a combination of hinge migration and limb rotation.

Three-dimensional geometry of large-scale fault-propagation folds in the Cantabrian Zone, NW Iberian Peninsula

Abstract In the western part of the Cantabrian Zone, a transition occurs from a southern zone mainly dominated by thrusts to a northern zone where folds predominate. Detailed structural analysis within the ‘transitional’ zone reveals that large-scale fault-propagation folds constitute the structural framework of this area. The topography, together with variations in plunge of the fold axes, allows us to visualize their three-dimensional geometry. They exhibit a complex internal structure due to thrusting before, during, and after major folding. Thrusts are the structures that accommodate most of the shortening in deep levels, whereas the shortening is accommodated stratigraphically upward by major fault-propagation folds, imbricate thrust systems and minor structures. In general, during the initial stages of fold amplification, the backlimb of the antiforms thickened, whereas during the latter stages their forelimbs thinned. After folding, the major folds were translated and rotated due to emplacement of younger thrusts located in the foreland. Although these natural large-scale fault-propagation folds are more complex than many theoretical models proposed up to date, certain specific structural features observed in the field examples can be explained by comparing them with theoretical models.

Effects of inherited structures on inversion tectonics: Examples from the Asturian Basin (NW Iberian Peninsula) interpreted in a Computer Assisted Virtual Environment (CAVE)

Two outcrops in the Asturian Basin (northwest Iberian Peninsula) composed of Lower–Middle Jurassic alternations of limestones and marls, including black shales proven to be hydrocarbon source rocks, were analyzed from a structural point of view. In both outcrops, an inversion tectonic event of Cenozoic age caused contractional folds and faults, as well as a change in the structural style up section, superimposed on previous Mesozoic extensional structures. However, the mode and kinematic evolution of the inversion tectonics were very different; folding predominated in one of the outcrops, whereas faulting is the most important phenomenon in the other one. The characteristics of the two coastal outcrops (irregular surface, inaccessible portions, accessibility restricted to low tide periods, good quality of rock exposure and scale) led us to build virtual outcrop models using photogrammetry (structure from motion) and interpret them in a computer-assisted virtual environment (CAVE). The analysis of both field data and the virtual outcrop model–derived results (three-dimensional models and cross sections) allowed us to conclude that mechanical stratigraphy and the type, characteristics, and distribution of previous structures were the key factors that controlled the inversion tectonic mode. Awareness of their influence should help when trying to understand inversion structures in similar geological settings.