Fold Growth Research Articles

Kinematic implications of consequent channels on growing folds

[1] As soon as a fold begins to grow above base level, it becomes subject to erosion. A numerical model of simultaneous fold growth and river erosion captures the early stages of channel formation and delineates the evolution of longitudinal channel profiles. This numerical exploration suggests that the patterns of channel incision into growing folds can be used to guide kinematic interpretations of fold growth. Four types of fold limb growth are modeled: limb lengthening through (1) the fold crest or (2) the fold toe, (3) limb rotation, and (4) curved fold limbs due to trishear folding. Initially, the pattern of channel incision is sensitive to the kinematics of surface deformation. Subsequently, more mature streams evolve toward similar incision patterns with deepest incision near the midpoint of the longitudinal channel profile. We compare these model predictions to Rough Ridge on New Zealand's South Island, where high-resolution topographic data reveal the multitude of channels incised into the flanks of these folds. Because the opposing limbs of these folds have followed distinct deformation pathways, the early sensitivity of channel incision to deformation pathway can be examined. Furthermore, the lateral propagation of the fold through time allows for an approximate space-for-time substitution, such that the evolution of these incision patterns can be examined from early through more mature stages. This analysis indicates that the west limb of Rough Ridge has grown by limb lengthening through the toe and the east limb has grown primarily by lengthening through the crest.

Geometry, kinematics and fracture pattern of the Bangestan anticline, Zagros, SW Iran

AbstractThrust-related anticlines in the Zagros Simply Folded Belt provide excellent exposed analogue structures for fractured reservoirs located in the more external sectors of the belt. In these structures it is possible to study the fracture network attributes and understand their relationships to the folding process, thus gathering fundamental information for fracture modelling in reservoirs. In this work we analyse the mesoscopic deformation pattern of the NW–SE-trending Bangestan anticline (SW Zagros, Iran) and discuss its relationship to the kinematic evolution of the hosting structure. The deformation pattern mostly includes extensional structures and pressure solution cleavages striking parallel to the fold axial trend (i.e. longitudinal), transversal extensional structures, and N–S- and E–W-striking extensional structures (oriented oblique to the fold axis). With the aid of deep wells and a transversal reflection seismic profile, we constructed a balanced cross-section of the anticline and propose a kinematic evolution pathway constrained by the mesoscopic deformation pattern. Longitudinal and transversal deformation structures developed before and/or in the very early stages of fold growth. During this stage, the Bangestan anticline grew as a set of unconnected décollement anticlines involving the Cambrian to Pliocenic sedimentary cover. In a later stage, inherited basement faults were reactivated with a right-lateral strike-slip component and the previously developed anticlines propagated laterally up to their complete linkage and thrust breakthrough. This produced the right-lateral strike-slip reactivation of longitudinal joints and the development of N–S- and E–W-striking extensional structures, which were also frequently reworked as strike-slip faults.

Lateral fold growth and linkage in the Zagros fold and thrust belt (Kurdistan, NE Iraq)

ABSTRACTThe Zagros fold and thrust belt is a seismically active orogen that has accommodated the N–S shortening between the Arabian and Eurasian plates since the Miocene. Whereas the southeast parts of the belt have been studied in detail, the northwest extent has received considerably less attention, being part of the Republic of Iraq. In this study, we investigate fold growth in the area NE of Erbil (Kurdistan, Iraq). In particular, we focus on the interaction of the transient development of drainage patterns along growing antiforms, as this directly reflects the kinematics of progressive fold growth. Detailed geomorphological studies of the Bana Bawi‐, Permam‐ and Safeen‐fold trains show that these anticlines did not develop from a single embryonic fold but by lateral linkage of several different fold segments. These segments, with length between 5 and 25 km, have been detected by mapping ancient and modern river courses; these initially cut the nose of growing folds until eventually defeated, leaving curved wind gaps behind. Depending on the alignment of the initial embryonic folds, the segments can either record a linear‐ or an en‐echelon linkage. Comparison of natural examples from the Zagros fold and thrust belt in Iraq with published numerically modelled fold growth suggests that both linear‐linkage and en‐echelon linkage are mechanically feasible and are common processes during progressive shortening and fold growth.

Geomorphic evidence for ancestral drainage patterns in the Zagros Simple Folded Zone and growth of the Iranian plateau

AbstractWe describe the geomorphology of a large (~10000 km2) internally draining region within the Zagros fold-and-thrust belt of Fars province, Iran. A series of wind gaps through fold crests and a continuous line of low-slope pixels in digital elevation models indicate the presence of an older, and now abandoned, through-going river system. We suggest, from the presence of the wind gaps, that the original through-going river system was abandoned as a direct result of fold growth. At present, through-going drainage in Fars is restricted to only two major rivers, the Kul and the Mand, which bound the margins of the internally drained region. The low gradients of the Kul and the Mand rivers are similar to those in topographic profiles drawn along the course of the abandoned drainage. The Mand and Kul rivers may be defeated in the future, causing an expansion of the internally drained region, and resulting in a profound change in the distribution of sediment and surface elevations within the Zagros. The internally draining part of the Zagros resembles the Central Iranian Plateau both in its geomorphology and in the apparently slow rates of deformation within it. We speculate that the development of internally drained basins and distribution of shortening within the range may be causally linked. The geomorphology that we describe might, therefore, record a stage in the southward expansion of the non-deforming and topographically high Central Iranian Plateau.

Geomorphic constraints on active mountain growth by the lateral propagation of fault-related folding: A case study on Yumu Shan, NE Tibet

The mountain range of the Yumu Shan in NE Tibet is the topographic expression of actively growing fault-related folds developed in the hanging walls of a system of mostly northeast-vergent blind thrusts. Here we present a Tibetan example on the geomorphic response to lateral fold propagation using both qualitative analysis of the geomorphic characteristics of this region (e.g., wind and water gaps, drainage density, and along-strike topography of the range crest) and quantitative analysis based on cosmogenic 10Be surface exposure dating and optically stimulated luminescence dating on the rates of uplift and lateral propagation of the anticline. The elevations of the past positions of successively deflected paleo-rivers show a systematic decrease toward the eastern tip of the anticline, in tandem with an eastward decrease in drainage density. Combining the age data obtained by cosmogenic 10Be and OSL dating methods with the heights of abandoned terraces, we calculated average uplift and lateral propagation rates of ∼378 mm/ka over the past ∼185 ka and ∼40 mm/yr over the past ∼150 ka, respectively. Our single-site evaluation of the relation between tear faulting and channel deflection suggests that the tear faulting occurs along weakened bedrock already dissected by the former stream. This finding is inconsistent with the conventional hypothesis that the route of the deflecting stream is controlled by the orientation of the tear-faulting zone. Topographic vertical and lateral fold growth of the Yumu Shan were probably driven by the eastward propagation of a blind thrust since the late Neogene following the initial deposition of sediments in the Hexi Corridor, which were transported from the Qilian Shan.

Evolution of fluid chemistry and fluid-flow pathways during folding and faulting: an example from Taemas, NSW, Australia

Abstract In the Taemas area, New South Wales, Australia, a swarm of hydrothermal calcite and quartz veins is hosted in upright, open to close folded limestones and shales. Overprinting relationships and vein geometries demonstrate that the vein swarm formed progressively during fold growth and associated reverse faulting. Textures preserved in veins reveal that veins formed via hundreds to thousands of individual dilation and mineral precipitation events. Bedding-parallel flexural slip during fold growth was associated with laminated vein development, and limb-parallel stretching during fold growth was associated with the formation of bedding-orthogonal extension veins. The presence of subhorizontal extension fractures and severely misoriented reverse faults imply that fluid pressures exceeded lithostatic levels, at least transiently, during the development of the vein swarm. Vein δ 18 O compositions increase upwards through the Murrumbidgee Group in response to a progressive reaction of an externally derived, upwards-flowing low-δ 18 O fluid (of probable meteoric origin) with host limestones. Vein δ 18 O and 87 Sr/ 86 Sr compositions vary spatially and temporally within the same outcrop, and within individual veins. These variations are inferred to be caused by the ascent of packages of fluid along constantly changing flow pathways caused by multiple permeability creation–destruction cycles associated with fault slip and fault sealing. Vein trace and rare earth element (REE) concentrations are more variable, probably reflecting rapid rock buffering along fluid pathways on length scales of less than 10 m. Our results indicate that fluid-flow pathways change dynamically during crustal shortening, with pathways switching between states of low and high permeability during episodic fault slip and associated fracture development. Supplementary material: Two appendices are available at http://www.geolsoc.org.uk/SUP18492 .

Tectonic sedimentology: sediment systems deciphering global to local tectonics

AbstractTectonic sedimentology has broad appeal because it can be applied at many time and length scales, such as sedimentary consequences of: changing rates of plate creation at mid‐ocean ridges, continent–continent collisions, plateaux uplift, active plate margins, extensional rift tectonics and the birth, growth and death of individual folds and faults. Tectonic sedimentology deals with the direct influence of neotectonics on sedimentation and basin development, and with the inverse problem of reconstructing the order and development of palaeotectonic structures. This review attempts to show how the scope and importance of the subject has developed over the last decade and is divided into five themes: (i) At the largest tectonic scale, constraints on mid‐ocean ridge activity and continent–continent collisions come from: (a) precise chemical analyses of fluid inclusions in marine halite deposits dating back to ca 550 Ma; (b) Sr‐isotopic ratios of marine Phanerozoic calcitic fossils; and (c) chemical composition of Archaean and Proterozoic banded iron formations. (ii) At regional scales, constraints on the timing and extent of continental plateau uplift/incision and cratonic tilting are given by: (a) palaeoaltimetry estimates from oxygen isotope depletion in carbonate palaeosols; (b) radiometric dating of palaeowater‐table speleothems; and (c) preferential river migration. (iii) At basin scales, catchment processes are emphasized as key controls on sediment flux from tectonic uplands to sedimentary basins. The inadequacy of sediment transport algorithms based on tractive stress and flow power to describe erosion, incision and sediment transport during the most formative catchment hydrological events is emphasized. (iv) Rates of basin‐scale fault and fold growth, displacement, death and basement uplift are provided by application of facies analysis and sequence stratigraphy to accurately‐dated sediments in sedimentary basins. (v) The effects of changing climate and varying basin base‐level in modifying tectonic influences on erosion and sedimentation are examined. In such cases, experimental and numerical modelling using multi‐variate parameters of subsidence rate, sediment supply and sea‐level change can shed light on the problem. Future developments will undoubtedly improve such models, hopefully establishing full linkage and feedback between sediment/water supply and changing climate within an overall tectonic framework that may itself be changing.

Carbon capture & storage in power generation and wind energy: Flexibility and reliability issues in scenarios for Northwest Europe

The Netherlands is both heavily dependent on the success of carbon capture and storage (CCS) and renewable electricity for its decarbonisation policy. The same holds for the EU and some of its Member States like the United Kingdom. This paper discusses several scenarios for the Netherlands within the context of a liberalised and a more and more connected energy market in Northwest-Europe. The research reported here shows that CCS on new coal fired power plants does not pose a threat to the increase in the electricity production by large quantities of wind energy in the electricity system, and vice versa. For renewable energy, several additional policies are in place mainly in the form of financial support as long as these renewable options are not yet profitable. For CCS, financial support is only granted now for a selection of the first demonstration projects now planned in the EU. It is not yet clear if large scale CCS and high shares of wind energy can be incorporated in a balanced way in the electricity system. This paper shows that from a technical and economic point of view, there is room for both, and under which conditions such a two fold growth will be feasible.

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.

Insights into fold growth using fold-related joint patterns and mechanical stratigraphy

Despite how common folds are as structural features, along-strike fold propagation has proven elusive to document. However, if a fold grows laterally along its axis, early-formed fold-related joints may differ significantly in orientation from joints that form later. In this paper, we integrate mechanical stratigraphy with joint pattern analysis to determine relative timing of jointing. Additionally, we demonstrate that joint patterns can be related to stresses on both the top and bottom of the bed during flexure. We present joint data from eight sedimentary beds on the fold terminus at Sheep Mountain Anticline, Wyoming, USA. The joint patterns around the terminus show two distinct patterns: joints in six of the beds show a radial pattern around the terminus whereas joint patterns in the two remaining beds differ from proximal units, despite being in the same structural position. Fracture resistance calculations confirm that the beds with mis-oriented fractures are less resistant to fracturing than other units in the study, and therefore would have fractured earlier in fold growth history. We present a plate bending model that illustrates potential joint patterns around a plunging fold nose from stresses along both the top and bottom of the bed. The joint strike predictions for the area in front of the inflection line on the fold nose match the orientations in our less resistant beds, which are now positioned behind the inflection line, suggesting that the fold grew laterally. The combined analysis of fracture pattern and mechanical stratigraphy provides a new way to investigate fold evolution.

A geometric and kinematic model for double‐edge propagating thrusts involving hangingwall and footwall folding. An example from the Jaca–Pamplona Basin (Southern Pyrenees)

AbstractA new geometric and kinematic model is proposed for a particular type of fault‐related folding based on the study of a natural example developed in Palaeogene carbonate rocks from the Jaca–Pamplona Basin (Southern Pyrenees). The example consists of a hangingwall anticline related to a reverse fault with variable displacement and a gentle footwall syncline. A detailed structural analysis of the structure and a cross‐section, perpendicular to its axis and parallel to the transport direction, reveals that none of the previous published models of fault‐related folds is able to simulate its main characteristics and reproduce its geometry. The main features of the new model are: double‐edge propagating fault and folding developed in both the hangingwall and the footwall. A MATLAB‐based program was created to calculate structural parameters such as shortening, structural relief and fault slip; obtain graphs of different parameters such as shortening versus slip along the fault, shortening versus fault length, and produce sections across forward models showing the different stages of fold growth. The model presented here gives an acceptable geometrical fit to the studied natural structure and provides a reasonable evolutionary history. In addition, the results obtained using the model are similar to those measured on the cross‐section. As a final step the subsurface portion of the natural fold was completed following the constraints imposed by the model. Copyright © 2010 John Wiley & Sons, Ltd.

Control of Alluvial Sedimentation at Foreland-Basin Active Margins: A Case Study from the Northeastern Ebro Basin (Southeastern Pyrenees, Spain)

Abstract Alluvial fans are the main geomorphological and depositional features at active margins of nonmarine foreland basins. The characteristics of these fans have already been extensively studied in relation to mountain-emergent thrust fronts. However, how an entire mountain front (from emergent to blind thrusts) controls the depositional environment, organization, and evolution of a foreland fan system is not yet well explored. These controls are discussed here through a case study of the Berga Formation, deposited in the late Eocene–early Oligocene in the eastern Ebro foreland basin, south of the Pyrenean chain. Near the village of Sant Llorenc de Morunys, an extensive facies analysis of the Berga deposits suggests the occurrence of a wide range of depositional processes such as subaerial stream flows and sheet flows, subaerial to subaqueous gravity flows, and subaqueous suspension fallout. This indicates two distinct sedimentary environments. The first is a large, coarse-grained, gently sloping, braided-stream-flow-dominated alluvial fan that periodically prograded into intermittent shallow lakes. It developed with sediments provided by a regional drainage basin extending from the outer Pyrenean thrust sheets to the axial Pyrenean zone. In the study area, the flow directions on this large fan had a significant component subparallel to the orientation of the mountain-front structures. In the fan, the deposits were organized in large lithosomes (tens of meters thick and tens of kilometers long), interpreted as single fan lobes. These lithosomes are arranged into packages (tens to hundreds of meters thick) of proximal to distal sediments. These packages are interpreted as short-term and long-term progradation–retrogradation cycles of the fan, driven by low-frequency climatic changes and/or tectonic events. The second sedimentary environment is a small, coarser-grained, stream-flow- and gravity-flow-dominated alluvial fan. It developed later, with sediments coming from a local drainage basin extending only on the outermost thrust sheets of the chain. The flow directions on this fan were roughly perpendicular to the orientation of the local mountain-front structures. Therefore, during the deposition of the Berga Formation, the southeastern Pyrenean foreland presented at least one large, long-lived alluvial fan within the basin and probably several small, short-lived alluvial fans at its edge. The large fan was a permanent element of the foreland geomorphological and depositional system. The geometry and evolution of the southeastern Pyrenean thrust belt controlled its location, size, drainage pattern, depositional environment, and long-term stratigraphic sequences at a regional scale. Additionally, the growth of foreland folds modified the local bed geometries, transport pathways, environmental features, and long-term stratigraphic trends of its deposits. In contrast, the small fans were temporary, jumping from the footwalls of one active emergent thrust front to another, as a result of local reorganizations into river networks and sediment sources. The adjoining emergent thrust front controlled the locations, sizes, drainage patterns, depositional environments, and long-term stratigraphic sequences of these small fans at a local scale. Finally, it is possible to speculate whether or not feedback interactions existed between the foreland deposition and the deformation of the mountain front. We suggest that the overall organization and evolution of the Berga fans, as well as their interactions with the deformation, can be extrapolated to many other active margins of foreland basins.

“Punishment for the Sinner”: Holocene episodic subsidence and steady tectonic motion at ancient Sybaris (Calabria, southern Italy)

Although the Calabria region in southern Italy experienced Quaternary uplift, intense Holocene subsidence is documented at ancient Sybaris, one of the most powerful among the Greek colonies collectively known as Magna Greacia. Sybaris flourished along the Ionian Sea coast of north-eastern Calabria since 2.7 ka BP and until the end of the Roman empire, when it went progressively concealed below the ground. Study of the Latest Pleistocene–Holocene evolution of this area involved analysis of the relative sea-level history for individual borehole logs, based on re-calibration of published ages and stipulation of nominal sea-level positions related to each marker. For investigation of the short-term evolution, a novel compilation of geological and archaeological sea-level markers was supported by new radiometric ages. Appraisal of the position of dated markers, when compared to a sea-level curve built on purpose for this coast using an updated glacio-hydro-isostatic model, indicates a locally intense difference in vertical motion between boreholes located away from, or close to, the ancient town. Specifically, whereas data from the basement of the archaeological layers document large subsidence, more distant sites behaved differently. A site ∼2 km to the NW of the settlement was apparently stable throughout the Holocene, and a site ∼6 km to the SE experienced uplift at ∼1.5 mm/y since 0.6 ka BP. In addition, analysis reveals temporal changes in subsidence pattern in the archaeological area. Large (5–6 mm/y) Early Holocene subsidence at Sybaris slowed to ∼1.5 mm/y during the middle part of the Holocene. The slowing-down trend continued during and after historical occupation at ∼0.8 mm/y, a value similar to the long-term (35–40 ka BP) rate established from the deepest borehole samples. These data are interpreted as suggesting that sediment compaction affected the basement of Sybaris during the Early Holocene, and progressively ceased during historical times, when a tectonic signal prevailed. During historical time, the spatial difference in vertical motion is chiefly attributed to differential growth of nearby folds documented by morpho-tectonic studies on-land and seismic profiles analysis in the offshore of Sybaris. Whereas location above a syncline caused tectonic subsidence at Sybaris, the regions to the north and south record stability or uplift that reflects transition to growing anticlines. In the short term (1–10 ka scale), these local tectonic processes prevail over the regional uplift pattern, which is expressed at the 100 ka scale or over.

A model for low amplitude detachment folding and syntectonic stratigraphy based on the conservation of mass equation

This paper presents a model for the structural evolution of low strain detachment folds in which rock is treated as an incompressible deformable material and its kinematics is governed by the continuity equation. The model also considers the following boundary conditions: ( i) the vertical flux across the core of the fold is described by a cosine function, ( ii) the horizontal flux due to transport along a basal detachment remains constant with depth, and ( iii) the depth of the detachment remains fixed. Upon finding analytical expressions for the velocity field of this deformation process, a model for the accumulation of growth strata and the degradation of the topography created by folding is then derived. These processes are approximated by means of the transport-diffusion equation. Solutions of these two models are in excellent agreement with geometric, and stratigraphic relations documented in fold-and-thrust belts, and analog experiments. Moreover, the model indicates that fold growth in this class of structures is a self-affine process. A compilation of detachment folds around the world indicates that these structures share a common profile. Moreover, fold amplitude and wavelength of these folds are scaled by different amounts, confirming this result. The stratigraphy obtained with the growth strata model exhibits the typical thinning, and truncation of timelines toward the core of the anticline, cross-cutting relations often observed in seismic cross-sections and field data.

Active fault, fault growth and segment linkage along the Janauri anticline (frontal foreland fold), NW Himalaya, India

The 100 km long frontal foreland fold — the Janauri anticline in NW Himalayan foothills represents a single segment formed due to inter-linking of the southern (JS1) and the northern (JS2) Janauri segments. This anticline is a product of the fault related fold growth that facilitated lateral propagation by acquiring more length and linkage of smaller segments giving rise to a single large segment. The linked portion marked by flat-uplifted surface in the central portion represents the paleo-water gap of the Sutlej River. This area is comparatively more active in terms of tectonic activity, well justified by the occurrence of fault scarps along the forelimb and backlimb of the anticline. Occurrence of active fault scarps on either side of the anticline suggests that the slip accommodated in the frontal part is partitioned between the main frontal thrust i.e. the Himalayan Frontal Thrust (HFT) and associated back-thrust. The uplift in the piedmont zone along southern portion of Janauri anticline marked by dissected younger hill range suggests fore-landward propagation of tectonic activity along newly developed Frontal Piedmont Thrust (FPT), an imbricated emergent thrust branching out from the HFT system. We suggests that this happened because the southern segment JS1 does not linked-up with the northwestern end of Chandigarh anticline segment (CS). In the northwestern end of the Janauri anticline, due to no structural asperity the tectonic activity on HFT was taken-up by two (HF1 — in the frontal part and HF2 — towards the hinterland side) newly developed parallel active faults ( Hajipur Fault) branched from the main JS2 segment. The lateral propagation and movements along HF1 and HF2 resulted in uplift of the floodplain as well as responsible for the northward shift of the Beas River. GPR and trench investigations suggest that earthquakes during the recent past were accompanied with surface rupture. OSL (optical stimulated luminescence) dates from the trench suggests occurrence of at least two events during the recent historic past, with the latest — Event II during 1500 AD (?).

Characteristics of deformation structure around the 2007 Niigata-ken Chuetsu-oki earthquake detected by multi-channel seismic reflection imaging

Abstract A multi-channel seismic reflection (MCS) survey was conducted to investigate the tectonic structure off Niigata, which caused the 2007 Niigata-ken Chuetsu-oki earthquake, using the research vessel (R/V) KAIREI of the Japan Agency for Marine-Earth Science and Technology. Based on the results of data processing and interpretation of available data, three areas are identified according to seismic characteristics. The most deformed area is located on the continental shelf near the source region of the 2007 Niigata-ken Chuetsu-oki earthquake, i.e., the area east of the Yoneyama-Ogi Uplifts. A remarkable growth of folds, including fault-related folds, and a strong reflector dipping east is identifiable by localized strain concentrations. The second area is located between the Yoneyama-Ogi Uplifts and the Jouetsu Knoll in the Toyama Trough. Although the deformation of deposits in the second area was smaller than in the first area, folds are identified. The third area is located toward the west of the Jouetsu Knoll in the Toyama Trough. No significant deformed structures developed in this area. Based on the interpretation of stratigraphy obtained in previous studies, seismic characteristics, and well data, the development of an anticline was initiated by a compression field after about 3.6 Ma. In particular, the deformation of sedimentary layers by the compression field occurred rapidly after about 1.3 Ma. Folds have grown larger toward the east after about 1.3 Ma. In addition, subsidence of about 0.2 s in sedimentary layers can be seen at the western margin of the Yoneyama-Ogi Uplifts, suggesting that tectonic movement related to reverse faulting has advanced there very recently. From aftershock distribution on a depth section on line S-2 near the hypocentral region, most hypocenters were determined to be below the strong reflector. This result suggests that fold growth has accompanied past large earthquakes, such as the 2007 Niigata-ken Chuetsu-oki earthquake.

Internal geometry and growth history of a thrust-related anticline in a deep water fold belt

Although 3D seismic imaging has become a fundamental tool for the analysis of extensional fault geometries and their development, it has not been utilised to the same degree to investigate the structure and growth history of thrust faults. Here, the structural evolution of a fold and thrust belt is investigated through the analysis of an isolated fold imaged in the deep water Niger Delta. A quantitative description of thrust fault displacement through transfer zones is presented to investigate the internal structure of a complex fault-related fold, while syn-kinematic growth packages are used to chart fold growth through time, so as to also understand the early growth history. Results reveal that a number of initially separate folds coalesced along the axial trends into a single structural culmination. Numerous thrust faults with similar and opposing dip, link and transfer displacement within the structure as it changes vergence along strike. The distribution of bulk deformation along the fault-related fold is simple, systematic and similar to that observed for extensional fault systems. Deficits in fault heave, both in the form of perturbations on non-linking or aggregated profiles, and displacement minima within transfer zones, are compensated to some degree by an increase in fold-related strain. Despite this accommodation, fold amplitude or crest elevation is largely unaffected by variations in fault heave. The observations for this isolated fold should be widely applicable to fold and fault development in more complex areas.

A fluvial record of active fault‐propagation folding, Salsomaggiore anticline, northern Apennines, Italy

Fault‐propagation folds offer the potential to relate folding to underlying fault propagation and slip provided that fold kinematics can be established. Fluvial terraces have been recognized as kinematic indicators, but precise relationships between surface uplift and various folding mechanisms remain largely unexplored. This study takes advantage of a well‐preserved, progressively deformed suite of middle Pleistocene to Recent fluvial terraces above a growing fault‐propagation fold at the Apennine mountain front, northern Italy, to constrain the recent fold kinematics and place them in the context of a longer growth history gleaned from older growth strata. The geometry of straths and overlying terrace deposits defines a fixed anticlinal hinge, a rolling synclinal hinge, proxies for fault tip propagation rates, rock uplift rates, and tilting rates, and how these features and rates vary over the last ∼800 ka along ∼15 km of strike length. Field studies are augmented with DEM‐based quantitative geomorphic analyses that document catchment hypsometry, mean anticlinal hinge elevation, and long profile form. Notably, long‐term rock uplift rates (using incision as a proxy) are uniformly correlated with fault propagation and associated synclinal hinge migration, mean anticlinal hinge elevation and variations in catchment hypsometries. Channels that cross the forelimb at a high angle to the fold hinge generally have higher concavities, but channel steepness, commonly thought to reflect rock uplift, is more strongly adjusted to rock type. This study elucidates a new understanding of a complex fold growth history extending back at least 10 Ma and provides a novel demonstration of how fluvial terraces may be utilized to constrain fault‐related fold kinematics.

Numerical estimation of the initial hinge-line irregularity required for the development of sheath folds: A pure shear model

Non-cylindrical sheath folds often develop by the accentuation of inherent geometrical irregularities along the hinge line during progressive deformation. This paper presents an analysis of the growth of such non-cylindrical folds in pure shear as a function of a non-dimensional initial irregularity factor ζ o ( ζ o = −log 10 [ A o/ W o], where A o and W o are the amplitude and the wavelength of a sinuous hinge line, respectively). Curvature accentuation of the hinge line produces sheath-like non-cylindrical folds in passive layers only when ζ o ≤ 1. Folds with ζ o > 1 remain weakly or moderately non-cylindrical even after a large finite strain ( λ = 36). An increasing flattening strain (1 > k value > 0) component in the deformation reduces the degree of non-cylindricity. We extend the analysis into layered systems with viscosity contrasts (i.e. viscosity ratio, R), considering a strain-partitioning factor λ f (the strain ratio between the fold hinge and bulk medium). Using finite element methods, it is shown that λ f can be small even for low values of R (4–8). Consequently, strong non-cylindricity does not grow in higher-viscosity layers for ζ o = 1. We ran experiments using three-dimensional finite element models to investigate the additional effects of mechanical bending on curvature accentuation of the hinge line. These experiments also show little accentuation of hinge curvature irrespective of R, when ζ o > 1. However, for ζ o ≤ 1, hinge-line curvature accentuates moderately when R < 8. It follows from our analysis that strongly non-cylindrical folds cannot develop unless the magnitude of initial hinge-line irregularity is sufficiently large (i.e. ζ o ≪ 1). We also discuss the implications of the numerical results for pure shear models in the analysis of sheath folds in shear zones.

Cenozoic stratigraphic development in the north Chilean forearc: Implications for basin development and uplift history of the Central Andean margin

Analysis of the Cenozoic stratigraphic development of the forearc of northern Chile between 18°S and 23°30′S, allows constraints to be placed on the timing and nature of basin formation and the uplift history of the Central Andes. Chronostratigraphic charts have been constructed from 20 lithostratigraphic sections distributed throughout the forearc. Sections were taken from the Longitudinal Valley, Central Depression, Calama Basin, Salar de Atacama, Precordillera and the western flank of the Western Cordillera. Correlation and timing of events is largely based on the presence of dated volcanic horizons in all the studied sections. Three chronostratigraphic units are defined based upon the presence of regional unconformities. Deposition of the Late Eocene to Early Miocene chronostratigraphic unit (38–19 Ma) commenced across an irregular unconformity surface between ∼ 38 and 30 Ma with alluvial fan and fluvial sediments derived from the east interbedded with rhyolitic ignimbrites. Aggradation after 25 Ma resulted in development of a large broad basin over much of northern Chile that expanded eastwards through onlap onto basement. Deposition terminated around 19 Ma with the development of an angular unconformity over much, but not all of the study area. During deposition of the Early to Late Miocene chronostratigraphic unit (18–10 Ma) emergent volcanic source areas to the east provided catchments for large fluvial systems that drained westwards into endorheic ephemeral lacustrine basins. Fold growth affected sedimentation restricting accommodation space to small intra-thrust basins in the Precordillera and localised disruption and unconformity development in the Longitudinal Valley. The Late Miocene to present day chronostratigraphic unit (10–0 Ma) followed the development of a regional angular unconformity at 10 Ma. Sedimentation was restricted to a series of thrust-bounded endorheic basins in both the Central Depression and the Precordillera sourced from the east with volcanic activity limited to the periodic eruption of extensive ignimbrite sheets. Alluvial fan, fluvial and lacustrine sedimentation dominated within the endorheic basins from ∼ 8 to 3 Ma. After development of a regional unconformity at 3 Ma a change to isolated evaporite sub-basins took place in the Central Depression with small lacustrine basins developed along the flank of the Western Cordillera. The scale and grain size recorded in the sedimentary systems indicates that a substantial source area was located in the present day area of the Western Cordillera by 30 Ma and that this has persisted to the present day. This area also shed material eastwards into the Altiplano. The presence of such a topographic feature by 30 Ma suggests that a significant proportion of Andean uplift had occurred prior to the Late Miocene. This important uplift phase should be incorporated into any model of Andean uplift. The evidence from the basin-fill succession suggests that sediments accumulated in a basin developed in front of a broad monocline between 38 and 19 Ma and that a transition to a thrust-bounded foreland style basin took place after the development of the unconformity at 19 Ma.