Josep Poblet

Bed-by-bed fold growth by kink-band migration: Sant llorenç de Morunys, eastern Pyrenees

Ahstract<rowth strata deposited over and against the flank of the Sant Llorenc de Morunys fold during its final stages of deformation have been mapped at high resolution as the basis for unraveling the kinematics of fold growth. We use restoration techniques based on normal balancing assumptions to decipher the detailed kinematic history of folding. The progressive restorations, as well as balanced forward modeling, show that the last few hundred meters of fold growth were dominated by kink-band migration of a sort that is typical of much faultrelated folding. The kink-band migration has produced complex anticlinal hinge-zone geometry, including segmented fold hinges linked by disconformities and unconformities, which has direct and detailed explanation in terms of fluctuations in deposition rate relative to curved-hinge kink-band migration rate. Large fluctuations in the convolution of non-steady sedimentation and deformation are demonstrated, although the absolute fluctuations in deformation and sedimentation are unknown. At a length scale of 100 m, kink-band migration with little or no deposition is interspersed with sedimentation with little or no deformation. At the length scale of 500 m, deposition ranges from 200% to 50% of uplift. 0 1997 Elsevier Science Ltd. All rights reserved.

Geometry and Kinematics of Single-Layer Detachment Folds

Detachment folds form above, below, or above and below bed-parallel thrusts where thrust displacement is transferred into folding. Geometric and kinematic models are developed for individual single-layer detachment folds formed above the tip line of a thrust. We analyze three different detachment fold shapes (kink bands, chevron, and box detachment folds) and consider three principal detachment fold models: (1) constant limb dip where the detachment fold grows by limb lengthening, (2) constant limb length where the detachment fold grows by limb rotation, and (3) variable limb length and variable limb dip where the detachment fold grows by both limb rotation and limb lengthening. The allowance for variable forelimb thickness and excess layer-parallel shear enables us to de elop a wide range of possible detachment fold geometry. The geometric validity fields for folds with excess parallel shear are reduced when forelimb thinning takes place. Folds formed with constant limb length are likely to lock up in the initial amplification stages, whereas larger amounts of shortening can be taken up by folds formed with variable limb length. The manner in which a fold evolves affects the fracture and porosity features of the folded strata. Therefore, determining which mechanism operated to form a specific detachment fold is of particular importance in hydrocarbon exploration. The models we present are aimed at permitting more accurate geometric and kinematic interpretations of individual detachment folds. Examples of detachment folds are analyzed to illustrate the po ential benefits and limitations of the geometric model presented.

Geometries of syntectonic sediments associated with single-layer detachment folds

Abstract Three kinematic models have been proposed to account for the geometry and kinematics of detachment folds involving a homogeneous competent layer detached over a ductile unit: Model 1 — variable limb dip-constant limb length, Model 2 — constant limb dip-variable limb length, and Model 3 — variable limb dip-variable limb length. Because the same fold shape can be generated by any of the above mechanisms, fold kinematics are best determined by the geometries of syntectonic sediments. In single-layer detachment folds, growth strata patterns are controlled by axial surface activity, limb rotation, limb lengthening, fold uplift rates, sedimentation rates and deformation mechanisms of the syntectonic sediments. Asymmetric kink folds have been modelled and compared with natural examples from the southern Spanish Pyrenees. Under conditions of high sedimentation rates, Model 1 folds produce characteristic fanning growth stratal wedges that initially onlap and then progressively overlap the detachment anticline, whereas at low sedimentation rates, anticlines with fanning growth wedges on both limbs are formed. Model 2 folds, under conditions of high syntectonic sedimentation rates, form anticlines with growth strata largely parallel to the pre-growth units and thinned over the fold crest. In contrast, at low sedimentation rates, Model 2 folds show offlapping growth strata onto both limbs. Model 3 growth folds, under conditions of high syntectonic sedimentation rates, form anticlines with intermediate features from Model 1 and Model 2; at low sedimentation rates, anticlines are formed with offlapping growth structures.

Reverse modelling of detachment folds; application to the Pico del Aguila anticline in the South Central Pyrenees (Spain)

Abstract A simple method to estimate fold-amplification and thrust-movement rates for detachment folds is documented and illustrated by its application to a symmetrical detachment fold in the Southern Pyrenees, Spain. The technique provides a complete record of the kinematic evolution of detachment folds and is based on the application of equations for detachment folds involving limb rotation. The method uses the stratal pattern of the syntectonic sediments and assumes that these growth strata were deposited horizontally, that the folds involve a homogeneous competent unit detached over a ductile horizon, and that the folds can be represented by chevronkink bands. The procedure is applicable to any detachment fold with associated growth strata that display wedge geometries (‘progressive unconformities’) indicating limb rotation through time. This method can be used for both detachment folds formed with constant limb length or variable limb length, and it can also accommodate undecompacted or decompacted growth strata.

QUANTIFYING THE KINEMATICS OF DETACHMENT FOLDS USING THREE-DIMENSIONAL GEOMETRY : APPLICATION TO THE MEDIANO ANTICLINE (PYRENEES, SPAIN)

The kinematics of detachment folds have been described by three different models: (1) hinge migration is responsible for fold amplification, (2) fold amplification is due to limb rotation, and (3) both hinge migration and limb rotation cause fold amplification. A numerical method is proposed to determine which of these mechanisms is responsible for the formation of natural detachment folds. This procedure consists of measuring and plotting geometric data collected from cross sections constructed across the termination of a fold where shortening dies out laterally, or in an area with a lateral shortening gradient. Assuming that observed spatial variations in fold geometry reflect temporal geometric evolution, the procedure allows determination of equations that govern the kinematics of the particular detachment fold analyzed. To validate the results obtained from the application of this technique to natural examples, they must be contrasted with other indicators of fold-amplification mechanisms such as microstructures, mesostructures, and syntectonic sediment patterns. This analysis is applied to an asymmetric growth fold, the Mediano anticline in the Southern Pyrenees of Spain, and shows that it grew due to limb rotation and minor hinge migration. These data, coupled with analysis of the growth stratal patterns using reverse and forward modeling techniques, are used to derive deformation rates and to display the kinematics of this fold.

The velocity description of deformation. paper 2: sediment geometries associated with fault-bend and fault-propagation folds

Abstract A general tectono-sedimentary forward modelling equation is used to derive two-dimensional numerical models of sediment geometries associated with developing fault-bend and fault-propagation folds. These styles of folding are described in terms of velocity models of deformation and are linked with syn-tectonic erosion, transport and sedimentation. The resultant two-dimensional numerical models simulate pre-growth and growth strata in both submarine and subaerial settings. The geometries and relationships produced by the models are broadly similar to those seen in natural examples. However, complex stratal geometries may be generated which are significantly different to those produced by previous models. Growth strata associated with fault-bend and fault-propagation folds are also compared and the distinguishing features of each mode of folding discussed. The forward models presented in this paper have predictive capabilities in terms of possible sediment geometries associated with fault-bend and fault-propagation folds and also in terms of the amount of deformation or erosion that a part of a structure may have undergone.

Kinematic evolution and structural styles of fold-and-thrust belts

Abstract Fold-and-thrust (FAT) belts occur worldwide and have long been the focus of research of structural geologists who have devised a variety of techniques to image, characterize and model their main structural features. This introductory chapter reviews the principal geological features of FAT belts formed in different settings, emphasizing aspects related to their kinematic evolution and structural styles. Despite great advances, challenges remain, particularly in the understanding of the spatial and temporal evolution (4D) of FAT belts and their controlling factors. These research efforts are being assisted by the growing availability to researchers of relatively new tools to collect field data, high quality 3D seismic data, and computer and laboratory modelling tools. This volume includes technical papers presented in the conference ‘International Meeting of Young Researchers in Structural Geology and Tectonics (YORSGET-08)’ held in Oviedo (Spain), together with other papers on the same theme. These papers deal with FAT belts in different parts of the world and cover a broad range of different aspects, from detailed structural analysis of single structures to regional issues, and from studies based on classical field structural geology to modelling.

Tectonic Evolution of the Sanga Sanga Block, Mahakam Delta, Kalimantan, Indonesia

The Sanga Sanga Block contains four large to giant hydrocarbon fields in mid-to upper Miocene deltaic sandstones of the Mahakam Delta, eastern Kalimantan (Indonesia). These fields occur in the northeast-trending Mahakam fold belt, which is characterized by long, tight, fault-bounded anticlines and broad synclines and cored by overpressured shales. Onshore sections of the fold belt are strongly deformed, uplifted, and eroded, whereas the eastern offshore sections are little deformed and buried by the progradational delta wedge. Section balancing of depth-converted seismic lines, together with scaled analog modeling, was used to develop a new tectonic model of inverted delta growth faults for the evolution of the Mahakam fold belt. Section balancing shows that the fault-bounded anticlines of the Sanga Sanga Block are formed by contractional reactivation of early delta-top extensional growth faults. The change from gravity-driven extension to regional contraction occurred at around 14 Ma. Anticlinal folds controlled local sedimentation patterns and influenced the distribution of the reservoir channel sands in the main hydrocarbon fields. Scaled analog models of progradational loading above a ductile substrate produced delta-top extensional growth faults and depobelts, together with delta-toe fold-thrust. Contraction inverted the extensional growth faults and depobelts, producing tight, fault-bounded anticlines. The results support the model of delta inversion and, thus, the most viable explanation for the geometric, kinematic, and mechanical evolution of the structures in the Sanga Sanga Block. The inverted delta model has applications to other hydrocarbon-bearing deltas around Borneo and in other contracted delta systems.

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.