Transpressional Folds Research Articles

Early post-breakup kinematic adjustments of continental–oceanic transform fault zones: Cape Range, Coromandal and Romanche transform margin case study

Abstract A comparison of transform margins that started their evolution as continental transforms shows differences in their tectonic style, which can be attributed to the variable kinematic adjustments they underwent during the post-breakup continental-oceanic stage of their development. Three end-member examples are presented in detail. The Cape Range transform fault zone (Western Australia) retained its strike-slip character during its entire continental-oceanic stage, as documented by the transform-perpendicular system of spreading-related magnetic stripe anomalies. The Coromandal transform fault zone (Eastern India) adjusted its kinematics to a transtensional one during its continental-oceanic stage, as indicated by the transform-oblique system of magnetic stripe anomalies and extensional component of movement indicated by a narrow zone of crustal thinning. The Romanche transform fault zone (Equatorial Africa) adjusted its kinematics to transpressional, as documented by the changing geometries of magnetic stripe anomalies and transpressional folding during its continental-oceanic development stage. Based on the recognition of the aforementioned adjustments, we suggest a new categorization of transforms into (1) those that experience transpressional adjustment, (2) those that experience transtensional adjustment and (3) those that do not experience any adjustment during their continental-oceanic development stage.

Multi-temporal tectonic evolution of Capo Granitola and Sciacca foreland transcurrent faults (Sicily channel)

Joint analysis of high-penetration multi-channel and high-resolution single-channel seismic reflection profiles, calibrated by deep well boreholes, allowed a detailed reconstruction of the Late Miocene to Recent tectonic history of the Capo Granitola and Sciacca fault systems offshore southwestern Sicily. These two fault arrays are part of a regional system of transcurrent faults that dissect the foreland block in front of the Neogene Sicilian fold and thrust belt. The Capo Granitola and Sciacca faults are thought to reactivate inherited Mesozoic to Miocene normal faults developed on the northern continental margin of Africa. During Latest Miocene-Pliocene, the two ~NNE-SSW striking faults were active in left transpression, which inverted Late Miocene extensional half-grabens and created push-up ridges along both systems. Tectonic activity decreased during the Pleistocene, but transpressional folds deform Middle-Late Pleistocene sediments as well, suggesting that the two fault systems are active. The ~40 km long longitudinal amplitude profile of 1st order folds (Capo Granitola and Sciacca anticlines) shows ~15–20 km bell-shaped undulations that represents 2nd order folds. The length of these undulations together with the map pattern of faults allowed to divide the CGFS and SFS into two segments, northern and southern, respectively. Total uplift of the Sciacca Anticline is twice than the uplift of the Capo Granitola Anticline. Incremental fold growth rates decreased during time from 0.22 mm/yr (Capo Granitola Anticline) and 0.44 mm/yr (Sciacca Anticline) in the Pliocene, to 0.07 and 0.22 mm/yr, respectively, during the last ~1.8 Ma.

Sackung and enigmatic mass movement folds on a structurally-controlled mountain ridge

The recently emerged concept of “slope tectonics” suggests that mass movement processes can produce structures similar in morphological expression to that of extensional, strike-slip and compressional tectonic deformations. Further, there has long been debate about the (tectonic or gravitational) origin of lineaments found on hillslopes. Here we present cases of where geological structure and inherited tectonic structures have preconditioned the development of mass movements and creation of slope tectonic features on a monoclinal ridge (Smrk Mt.) in the Outer Western Carpathians, Czechia. Geomorphic mapping from LiDAR-derived topography, structural measurements, electrical resistivity tomography and ground penetrating radar, were used to map synthetic and antithetic scarps, thrusts, and folds, and assess their relation to geological and tectonic structures. The scarps are found to be strongly related to transverse (NNW to NNE trending) inherited normal and strike-slip faults and mainly cross-cut the topography of the mountain ridge. Some of the folds are thought to have originated from buckling or compression in the distal parts of landslides. In other places, transpressional folds have developed oblique to major lateral shear surfaces interpreted to be sackung (mass movement scarps) that have been inherited from tectonic faults. The transpressional folds developed due to localized contraction along bends in the lateral shear surface as mass movement proceeded. Ramp-flat thrust folds developed in the compressional parts of landslides, where the landslide intersects with slope parallel sackung and/or inherited fault planes inclined to the slope. Altogether, this study demonstrates a connection between kinematics of deep-seated gravitational slope deformations (DSGSDs), shallower landslides and the origin of transpressive, ramp-flat thrust, and detachment structures in rock slopes.

Transpressional folding and associated cross-fold jointing controlling the geometry of post-orogenic vein-type W-Sn mineralization: examples from Minas da Panasqueira, Portugal

The world-class W-Sn Panasqueira deposit consists of an extensive, subhorizontal vein swarm, peripheral to a late-orogenic greisen cupola. The vein swarm consists of hundreds of co-planar quartz veins that are overlapping and connected laterally over large distances. Various segmentation structures, a local zigzag geometry, and the occurrence of straight propagation paths indicate that they exploited a regional joint system. A detailed orientation analysis of the systematic joints reveals a geometrical relationship with the subvertical F2 fold generation, reflecting late-Variscan transpression. The joints are consistently orthogonal to the steeply plunging S0–S2 intersection lineation, both on the regional and the outcrop scale, and are thus defined as cross-fold or ac-joints. The joint system developed during the waning stages of the Variscan orogeny, when already uplifted to an upper-crustal level. Veining reactivated these cross-fold joints under the conditions of hydraulic overpressures and low differential stress. The consistent subperpendicular orientation of the veins relative to the non-cylindrical F2 hinge lines, also when having an inclined attitude, demonstrates that veining did not occur during far-field horizontal compression. Vein orientation is determined by local stress states variable on a meter-scale but with the minimum principal stress consistently subparallel to fold hinge lines. The conspicuous subhorizontal attitude of the Panasqueira vein swarm is thus dictated by the geometry of late-orogenic folds, which developed synchronous with oroclinal buckling of the Ibero-Armorican arc.

Structural and stratigraphic evolution of the Calico Mountains: Implications for early Miocene extension and Neogene transpression in the central Mojave Desert, California

New geologic mapping, structural data, and 40 Ar/ 39 Ar geochronology document early Miocene sedimentation and volcanism and Neogene deformation in the Calico Mountains, located in a complexly deformed region of California9s central Mojave Desert. Across most of the Calico Mountains, volcaniclastic sediments and dacitic rocks of the Pickhandle Formation accumulated rapidly between ca. 19.4 and 19 Ma. Overlying fine-grained lacustrine beds (here referred to as the Calico Member of the Barstow Formation) are bracketed between ca. 19 and 16.9 Ma, and are thus older than the type section of the Barstow Formation in the Mud Hills. Several 17.1–16.8 Ma calc-alkaline dacite domes intrude the Calico Member and represent a previously unrecognized volcanic episode in this region. In the southern Calico Mountains, the Calico fault (part of the Eastern California shear zone) forms a west-northwest–striking, transpressional restraining bend with ∼3 km of right-lateral slip and perhaps 1 km of reverse (north side up) throw distributed on two main fault strands. Part of the Calico fault appears to have originated as an early Miocene normal fault that unroofed metavolcanic basement rocks in the footwall and created a hanging-wall basin in which Pickhandle Formation strata accumulated. This extensional slip must have largely ceased prior to deposition of the Calico Member, which unconformably overlies the Pickhandle Formation north of the Calico fault and directly overlies metavolcanic rocks south of the Calico fault. Deposition of the Pickhandle Formation and at least part of the Calico Member was coeval with rapid unroofing of the central Mojave metamorphic core complex, yet extension in the Calico Mountains is minor and is overprinted by dextral faulting and transpression. Calico Member beds north of the Calico fault are intensely folded into numerous east-west–trending, upright anticlines and synclines that represent 25%–33% (up to ∼0.5 km) north-south shortening. Folds are detached along the base of the Calico Member and thrust over the Pickhandle Formation, which dips homoclinally ∼15–30°S to SE. The geometry and distribution of folds are most compatible with localized transpression between the Calico Member and the Pickhandle Formation within a positive flower structure. Transpressional folding and faulting in the Calico Mountains postdate the ca. 17 Ma dacite intrusions and appear to be largely restricted to the area along the Calico fault restraining bend.

Physical experiments of transpressional folding

In order to understand the process of folding in obliquely convergent settings, we formed folds within a shear box capable of creating homogeneous transpressional deformations. Folds were created in a single layer of stiff mixed plasticine and silicone that overlay a Newtonian silicone, for a variety of plate convergence angles. As small amplitude folds became visible, they were parallel to the long axis of the horizontal finite strain ellipse. With increasing deformation, the fold hinges rotated parallel with the long axis of the horizontal finite strain ellipse for all angles of convergence. This parallelism indicates that fold hinges, once formed, rotate with the horizontal strain ellipse rather than as material lines. The experiments highlight several interesting effects of transpression dynamics. The fold hinges initiate parallel to either ṡ 1 or ṡ 2 and are parallel to either S 1 or S 2 with increasing deformation. Neither infinitesimal strain (stress) nor finite strain is resolvable solely from fold geometry. Further, the net amount of contraction determined by folding across the zone was overestimated in all cases except pure contraction. This effect is obvious for the case of wrenching, where folding implies that the zone contracts if elongation parallel to the fold hinge is not considered. Therefore, attempts to balance cross-sections in transpressional zones will tend to overestimate contraction unless the wrench component of deformation is addressed. This result is validated by applying the modeling results in folding in central California adjacent to the San Andreas fault, where cross-section balancing indicates higher amounts of contraction than predicted by plate motion.

Mapping Precambrian structures in the Sahara Desert with SIR‐C/X‐SAR radar: The Neoproterozoic Keraf Suture, NE Sudan

A major N‐trending Neoproterozoic suture between composite arc terranes of the Arabian‐Nubian Shield in the east and older crust of the Nile Craton to the west is inferred to trend N‐S close to the Nile in northern Sudan. We used shuttle imaging radar (SIR) C/X synthetic aperture radar (SAR) imagery to find and map these structures in the poorly known Keraf Suture which are not apparent on visible or near IR imagery due to extensive sand cover. L band (23 cm wavelength) radar images best resolve geologic structure; the other frequencies of the SIR‐C/X‐SAR system (X and C bands) permit qualitative evaluation of the effects of surface versus subsurface backscattering. Interpretation of L band images supported by field work indicates that the Keraf Suture is ∼50 km wide and >550 km long, making it the longest basement structure recognized to date in NE Africa. The northern part of the Suture comprises ophiolitic rocks which were thrust westward over tightly folded sediments of the Nile Craton. The southern Keraf Suture is dominated by N‐ and NNW‐trending, left‐lateral strike‐slip faults that affect previously deformed passive margin sediments. Associated with these faults are NE‐trending transpressional folds and a possible transtensional basin. These structures are interpreted to be due to NW‐SE oblique collision between the Arabian‐Nubian Shield and the Nile Craton, as east and west Gondwana collided in the last 150 m.y. of Neoproterozoic time.

Evolution of a nearshore and coastal macrotidal sand transport system, Queen Charlotte Islands, Canada

Abstract An extensive (40 km) coastal plain is prograding on the north coast of the Queen Charlotte Islands off the Pacific margin of Canada while the entire 120 km eastern coast of Graham Island is actively eroding. At the junction of these two systems is Rose Spit, which extends northeastward at a point of sediment transport convergence. Historical bathymetrical data and aerial photographs indicate that a spit platform extends 10 km to the northeast and at least four portions (up to 4 km in length) of the platform bank have become emergent since 1911. The area is seismically active and subject to storm-related sediment transport events superimposed on strong semi-diurnal tidal flows. The rapid emergence of the offshore banks and the net coastal changes are primarily controlled by: (1) convergence of sediment transport between the waters of Hecate Strait to the south and Dixon Entrance to the north, particularly during winter storms; and (2) localized tectonic uplift. Sediment is transported north from the shallow waters of Hecate Strait by southeasterly winter storms and east, by longshore transport, along the accreting northern coast of the Queen Charlotte Islands to converge at Rose Spit. A ‘hydraulic fence’ has formed resulting in the creation of the elongate spit platform with emergent sand bars, east of the spit. Transpressional folding has occurred in this area east of the Queen Charlotte Fault, which separates the North American and Pacific Plates. This has resulted in localized uplift since Pliocene time, complementing the accretion process.

Miocene shearing along the northern boundary of the Jalisco block and the opening of the southern Gulf of California

New geologic and structural mapping in western Mexico shows that the tectonic boundary between the Eocene–early Miocene Sierra Madre Occidental volcanic province and the Cretaceous-Paleocene Jalisco block was reactivated as a transfer zone accommodating a limited amount of lateral motion in middle and late Miocene time. Transpressional folding, left-lateral strike-slip faulting, and minor thrusting occurred between 14.5 and 11.5 Ma during initial rifting at the mouth of the Gulf of California. Inferred right-lateral transtension between ∼12 and 9 Ma took place concurrently with widespread alkaline volcanism and east-west extension at the gulf mouth and east of the Jalisco block. These events can be explained in light of the long extensional history that led to the separation of Baja California from mainland Mexico since the birth of the Magdalena microplate, about 14.5 Ma. Left-lateral shearing at the Sierra Madre Occidental–Jalisco block boundary indicates that initial (14–12 Ma) rifting at the mouth of the gulf was accomplished mainly by east-southeast motion of the block. This was caused by differential stresses applied to the North America plate at the two sides of the subducting Magdalena-Cocos plate boundary as a consequence of the waning of subduction of the Magdalena plate. Subsequent right-lateral transtension at the Sierra Madre Occidental–Jalisco block boundary and extension to the east imply a limited west-northwest motion of the block, which possibly accounts for the discrepancy between slow rifting at the mouth of the gulf and fast northwest motion of Baja California.

Middle Miocene Sandstone Reservoirs of the Penal/Barrackpore Field

The Penal/Barrackpore field was discovered in 1938 and is located in the southern subbasin of onshore Trinidad. The accumulation is one of a series of northeast-southwest trending en echelon middle Miocene anticlinal structures that was later accentuated by late Pliocene transpressional folding. Relative movement of the South American and Caribbean plates climaxed in the middle Miocene compressive tectonic event and produced an imbricate pattern of southward-facing basement-involved thrusts. Further compressive interaction between the plates in the late Pliocene produced a transpressive tectonic episode forming northwest-southeast oriented transcurrent faults, tear faults, basement thrust faults, lystric normal faults, and detached simple folds with infrequent diapiric cores. The middle Miocene Herrera and Karamat turbiditic sandstones are the primary reservoir rock in the subsurface anticline of the Penal/Barrackpore field. These turbidites were sourced from the north and deposited within the marls and clays of the Cipero Formation. Miocene and Pliocene deltaics and turbidites succeed the Cipero Formation vertically, lapping into preexisting Miocene highs. The late Pliocene transpression also coincides with the onset of oil migration along faults, diapirs, and unconformities from the Cretaceous Naparima Hill source. The Lengua Formation and the upper Forest clays are considered effective seals. Hydrocarbon trapping is structurally and stratigraphicallymore » controlled, with structure being the dominant trapping mechanism. Ultimate recoverable reserves for the field are estimated at 127.9 MMBo and 628.8 bcf. The field is presently owned and operated by the Trinidad and Tobago Oil Company Limited (TRINTOC).« less