A mathematical model for orientation data from macroscopic elliptical conical folds

An iterative least-squares technique to fit circular and elliptical conical surfaces to orientation data from folds is presented. A statistical model is used which assumes that each data point is an observation from a Fisher distribution. The mean of this distribution is assumed to lie on the curve to be fitted. Estimates of variances and covariances for the fitting parameters are calculated, and confidence intervals for the cone axis and half apical angle are estimated from variances and covariances. A normal test with null hypothesis that the cone angle is 90° determines if a conical model fits the data better than a cylindrical model. AnF test is used to determine whether an elliptical cone is a better model than a circular cone. In this fashion, macroscopic folds are classified into cylindrical, circular conical, or elliptical conical folds. Examples of these three types of fold are given. The Wynd Syncline near Jasper, Alberta is the first natural elliptical conical fold described as such.     

燕辽地区中生代构造格局的再讨论

本文中,笔者对燕辽地区的构造形迹进行了重新厘定,并将其划分成不同构造阶段的产物,提出了一些新的认识:燕辽地区范围包括"内蒙地轴"和燕山沉降带,其构造格局主要由燕辽向斜、燕辽复向斜和秦皇岛背形及其配套断裂组成,在这三个区域构造样式之下又发育次一级的褶皱构造及其配套断裂,并且后期构造对前期构造改造、叠加和利用,致使出现复杂的构造格局.海西一印支期形成了燕辽向斜及其配套断裂,"内蒙地轴"是燕辽向斜的北翼.燕辽沉降带是燕辽向斜的南翼.早中侏罗世进一步褶皱成复向斜,"内蒙地轴"南缘断裂是复向斜的轴部纵断裂并充填了上侏罗统作为裂隙充填物.中晚侏罗世开始,以秦皇岛-凌源断裂为轴面褶皱成一个背形.秦皇岛-凌源断裂的存在及其对燕山段和辽西段的影响致使燕山段和辽西段具有不同的地表出露形态.     

Structural repetition of the Hutchison Group metasediments,Eyre Peninsula,South Australia

Progressive deformation of the Palaeoproterozoic Hutchison Group metasediments, eastern Eyre Peninsula, South Australia, accompanied the development of a ～200 km‐long suture, the Kalinjala Shear Zone. High‐strain structures that developed during the Kimban Orogeny preserved in this supracrustal sequence at Sleaford Bay, southern Eyre Peninsula are: (i) KF₁ sheath folds; and (ii) KD₂ tight folds and reverse shears. Basement Archaean Sleaford Complex gneisses are interpreted to have been thrust over their cover rocks. Interleaving of individual Hutchison Group units by imbricate shears and folds are suggested to have caused an estimated strike‐normal shortening of up to 50%. However, this calculation is of less significance than the >15:1 strike‐parallel elongation of the package during KD₁. Identical structural elements reported by earlier workers are also preserved in the stratigraphic type areas of the Hutchison Group, central Eyre Peninsula. Importantly, the stratigraphic Upper and Lower Middleback Jaspilites, in the Middleback Range, are reinterpreted as one unit structurally repeated by a kilometre‐scale KF₁ sheath fold. Imbricate KD₂ shear zones are inferred to have inverted original disconformities throughout the package. In light of these observations, we suggest that the currently constructed stratigraphic succession of the Hutchison Group should be regionally re‐examined, with a special focus in those areas less affected by the imbricate structures.     

A Duplex Model for the Eastern Cape Fold Belt? Evidence from the Palaeozoic Witteberg and Bokkeveld Groups (Cape Supergroup), Near Steytlerville, South Africa

The eastern part of the Cape Fold Belt, near Steytlerville, South Africa, reveals a typical pattern of numerous, north-verging thrust faults and associated folds, interpreted as part of a large duplex structure that formed along the southern margin of Gondwana during the Late Palaeozoic. Steeply-dipping fore- and backthrusts occur in the Bokkeveld Group (middle Cape Supergroup), where strata are composed of predominantly argillaceous rocks, whereas in the more arenaceous Witteberg Group (upper Cape Supergroup) there are fewer recognizable and less closely-spaced thrusts. Open style folds characterize areas in which the Bokkeveld Group crops out, but in areas of Witteberg outcrop, folds, especially those adjacent to thrusts, are often overturned.In spite of a general absence of marker horizons, a displacement of at least 500 metres can be inferred for one prominent thrust, the Jackalsbos thrust. This fault, the northernmost in the area investigated, is probably the sole thrust in the duplex structure, linked through southward-dipping imbricates to a projected roof thrust (the Baviaanskloof thrust) cropping out immediately south of the study area.Displacements on imbricates within the duplex are difficult if not impossible to measure, but the net effect is certainly accumulative and incremental. Truncation by a roof thrust and subsequent erosional processes may explain why so few of the many thrusts so far identified in the eastern part of the fold belt can be successfully mapped, and their displacements measured. Normal and strike-slip faults, less common than thrust faults, formed during extensional tectonism related to the breakup of Gondwana, during the Mesozoic.     

Internal architecture of mass‐transport deposits in basinal carbonates: A case study from southern Italy

Submarine mass‐transport deposits represent important stratigraphic heterogeneities within slope and basinal sedimentary successions. A poor understanding of how their distribution and internal architecture affect the fluid flow migration pathway may lead to unexpected compartmentalization issues in reservoir analysis. Studies of modern carbonate mass‐transport deposits mainly focus on large seismic‐scale slope failures; however, the near‐platform basinal depositional environment often hosts mass‐transport deposits of various dimensions. The small‐scale and meso‐scale (metres to several tens of metres) carbonate mass‐transport deposits play a considerable role in distribution of sediment and therefore have an impact on the heterogeneity of the succession. In order to further constrain the geometry and internal architecture of mass‐transport deposits developed in near‐slope basinal carbonates, a structural and sedimentological analysis of sub‐seismic‐scale mass‐transport deposits has been undertaken on the eastern margin of the Apulian carbonate platform in the Gargano Promontory, south‐east Italy. These mass‐transport deposits, that locally comprise a large proportion (50 to 60%) of the base of slope to basinal sediments of the Cretaceous Maiolica Formation, typically display a vertically bipartite character, including debrites and slump deposits of varying volume ratios. A range of brittle and ductile deformation styles developed within distinct bed packages, together with the presence of both chert clasts, folded chert layers and spherical chert nodules, suggest that sediments were at different stages of lithification prior to downslope movement associated with mass‐transport deposits. This study helps elucidate the emplacement processes, frequency and character of subseismic‐scale mass‐transport deposits within the basinal carbonate environment, and thereby reduces the uncertainties in the characterization of subsurface carbonate geofluid reservoirs.