Strike Of Folds Research Articles

Distinguishing tectonically-and gravity-driven synsedimentary deformation structures along the Apulian platform margin (Gargano Promontory, southern Italy)

The assessment of deformation types within the slope of a carbonate platform can be complicated by the possible interaction of rooted (tectonically-induced) and superficial (gravity-driven) structures. An ideal case study to document and distinguish tectonically- and gravity-driven structures is provided by the Cretaceous slope-to-basin carbonates exposed in the Gargano Promontory, southern Italy. These carbonates formed adjacent to the Apulian platform margin, which was oriented approximately NE–SW to NW–SE along the southern and northern edges of the promontory, respectively. Slump-related folds are characterised by axial planes typically oriented either sub-parallel or at small angles to the strike of the inferred paleoslope. In fact, the strike of folds is roughly NE–SW in the southern portion of the study area, whereas it is NW–SE in the northern part. Correspondingly, gravity-driven normal and reverse faults strike sub-parallel and at acute angles to the adjacent Apulian paleoslope. Cretaceous tectonic faults in the slope-to-basin carbonates form two principal sets striking NW–SE and WNW-ESE. The former set is made up of normal faults and the latter one includes mainly oblique-slip normal faults. Neither normal nor oblique-slip normal faults show any relationship with the geometry of the paleoslope. The results obtained from this study may help the interpretation of subsurface data in those geological contexts in which the interplay of gravitational and tectonic processes is responsible for deformation.

Quantification of three‐dimensional folding using fluvial terraces: A case study from the Mushi anticline, northern margin of the Chinese Pamir

AbstractFold deformation in three dimensions involves shortening, uplift, and lateral growth. Fluvial terraces represent strain markers that have been widely applied to constrain a fold's shortening and uplift. For the lateral growth, however, the utility of fluvial terraces has been commonly ignored. Situated along northern margin of Chinese Pamir, the Mushi anticline preserves, along its northern flank, flights of passively deformed fluvial terraces that can be used to constrain three‐dimensional folding history, especially lateral growth. The Mushi anticline is a geometrically simple fault‐tip fold with a total shortening of 740 ± 110 m and rock uplift of ~1300 m. Geologic and geomorphic mapping and dGPS surveys reveal that terrace surfaces perpendicular to the fold's strike display increased rotation with age, implying the fold grows by progressive limb rotation. We use a pure‐shear fault‐tip fold model to estimate a uniform shortening rate of 1.5 + 1.3/−0.5 mm/a and a rock‐uplift rate of 2.3 + 2.1/−0.8 mm/a. Parallel to the fold's strike, longitudinal profiles of terrace surfaces also display age‐dependent increases in slopes. We present a new model to distinguish lateral growth mechanisms (lateral lengthening and/or rotation above a fixed tip). This model indicates that eastward lengthening of the Mushi anticline ceased by at least ~134 ka and its lateral growth has been dominated by rotation. Our study confirms that terrace deformation along a fold's strike not only can constrain the lateral lengthening rate but can serve to quantify the magnitude and rate of lateral rotation: attributes that are commonly difficult to define when relying on other geomorphic criteria.

Regular orientation of joints in horizontally bedded sedimentary rocks of the East European platform

The paper summarizes long-term investigations of the jointing of sedimentary rocks of the East European platform. The conclusion on the existence of two joint systems striking NE and NW that are preserved in rocks of various ages of the platform sedimentary cover is drawn from the study of the jointing orientation in Paleozoic and Mesozoic rocks and processing of measurement results with the use of an original procedure reliably detecting jointing systems. In their nature, the joints are synsedimentary. It is shown that a sediment is lithified under the influence of heterogeneities at the boundary between the underlying rock (substratum) and the sediment. Cracks start to nucleate in the process of diagenesis (sediment-to-rock transformation) at the base of the newly forming bed due to the motion of a pore fluid upward, orthogonally to the sediment-substratum boundary. Azimuthal anisotropy of the pore fluid motion is predetermined by the substratum anisotropy, giving rise to the formation of extended master cracks, which control the orientation of second-order cracks developing later. The described mechanism of inherited development of cracks is capable of explaining why fixed directions of joints persist in rocks of different ages. During the subsequent deformations accommodating the development of local structural forms, either primary (lithogenetic) cracks change their bedding elements (if they are diagonal with respect to the strike of folds) or conjugate shear cracks develop and form their own, essentially tectonic jointing.

The Lithologic, Stratigraphic, and Structural Setting of the Giant Antamina Copper-Zinc Skarn Deposit, Ancash, Peru

Antamina, located at latitude 9° 32' S and longitude 77° 03' W in the Ancash Department of north-central Peru, is the largest known Cu-Zn skarn ore deposit. It incorporates a mineral reserve of 561 Mt, which has an average grade of 1.24 percent Cu, 1.03 percent Zn, 13.71 g/t Ag and 0.029 percent Mo, calculated at a 0.7 percent Cu equiv cutoff grade. The grandite-dominated calcic skarn formed in and around an upper Miocene porphyritic monzogranite stock emplaced into Upper Cretaceous carbonate strata that had experienced thin-skinned, northeast-verging thrusting and folding in the late Eocene Incaic orogeny. The exoskarn Cu-Zn ore is discordant to the strata of the Jumasha and overlying Celendin Formations, which comprise, respectively, massive to thick-bedded, relatively pure limestones and thin-bedded, predominantly marly limestones. The Jumasha Formation, the upper contact of which is locally defined as the top of the uppermost thick-bedded limestone or marble unit, hosts approximately three-quarters of the known exoskarn. Approximately the same fraction of the contiguous endoskarn Cu ore occurs adjacent to this formation. The overlying Celendin Formation is less extensively mineralized but, because it is widely metamorphosed to hornfels and locally converted to diopsidic skarnoid, may have inhibited the upward and outward migration of hydrothermal fluids, thereby promoting the development of the unusually large endoskarn ore zone. Ore also occurs in late hydrothermal breccias emplaced during the formation of mineralized endoskarn. The preskarn thermal metamorphic aureole around the ore deposit is expressed differently in the two host formations. Jumasha Formation limestone is coarsened and bleached to banded gray marble and locally to white marble peripheral to the intrusion and skarn. Minor scapolite occurs in dark gray bands in marble, concentrated in a discontinuous halo tens of meters wide and commonly separated from the skarn by tens of meters. Three facies of calc-hornfels are recognized in the marl beds of the Celendin Formation adjacent to the intrusion extending hundreds of meters beyond sulfide-bearing skarn: a peripheral, very fine grained, light brown phlogopitic facies; an intermediate, fine-grained, gray tremolitic facies; and a proximal, medium-grained, light green diopsidic facies. At an XCO2 of 0.1 to 0.9 and P = 100 MPa, these zones reflect temperatures increasing to circa 495°C adjacent to the intrusion. In addition, in nodular beds of the Celendin Formation that have been metamorphosed to hornfels, diagenetic calcite nodules are selectively replaced by diopside for distances of tens of meters beyond the skarn front. Such calc-silicate formation through both metamorphism and metasomatism, together with a 9 km2 cluster of Pb-Zn-Ag vein deposits, provides district-scale vectors to ore. The Antamina deposit lies on a newly recognized cross-strike structural discontinuity in the segmented Incaic Maranon thrust and fold belt, the northeast-trending Querococha arch. Southeast of the arch, Incaic folds and thrust faults strike north-northwest, but northwest of the arch they strike northerly. The plunge of fold axes concomitantly changes from south-southeast to north. Stratigraphic relationships indicate that the arch was a paleohigh, at least in the Jurassic and possibly throughout the late Paleozoic-early Mesozoic interval. The middle Miocene Carhuish pluton is exposed on the arch 30 km southwest of Antamina, whereas coeval Calipuy Supergroup volcanic units lie at similar altitudes to the north and south. Only scattered hydrothermal centers of late Miocene age are known in the Cordillera Negra, but an apparent swarm of intrusions, including the Antamina stock, occurs along the Querococha arch. Antamina is situated where the locus of changes in the strike of folds and faults and the plunge of folds steps left along the arch. At Antamina, a pair of fault-bend folds above frontal thrust ramps show approximately 500 m of dextral apparent offset across the deposit and are inferred to have been separated by a northeast-striking transfer fault or lateral ramp, itself localized by a left-stepping jog in the Valley fault, an underlying, similarly oriented transverse structure. The jog in the Valley fault is inferred to have also controlled intrusion and skarn development. This local-scale jog in the Valley fault mimics the regional step along the arch. The arch may reflect a transform segment of the originally jagged, rifted continental margin, which persisted as a transverse basement weakness. Northeast-striking, originally sinistral, basement structures affected regional-scale sedimentation and structural patterns, including articulation of the thrust and fold belt. At a local scale, they influenced lateral ramp formation and related fracture development in the overlying thrust sheets. In the proposed model, they also localized later uplift and the rapid transit of small volumes of productive melt into a shallow crustal setting, conditions favorable for formation of a giant magmatic-hydrothermal ore deposit.

Transected folds from the western part of the Bala lineament, Wales

Fold-cleavage relationships are described in the Cadair Idris area of the Lower Palaeozoic Welsh Basin, which includes the western part of the Bala Lineament. Many fold hinges are transected by the broadly contemporaneous Caledonian (Lower Devonian) cleavage. Transection angles are amongst the highest yet described from the Welsh Basin. There is a close relationship between fold strike and sense and amount of transection; N-S folds are clockwise transected, NE folds have an axial planar cleavage whilst ENE folds are anticlockwise transected. Similar relationships occur throughout the central Welsh Basin. A model of progressive deformation is described whereby weak regional transpression is modified by local factors. Viscosity contrasts are recognized as one of the most important controls on the timing of fold development and the amount of transection.

Structural Pattern of Caribbean, Comparison With Flow of Glacier: DISCUSSION

Points out discrepancies between the strike of folds, thrust faults, cross fractures, and other structural features in northeast Venezuela and the orientation of the strain ellipsoid as presented in a paper on the structure of the western Serrania del Interior by E. Rod.