Roy Kligfield

Dating of deformation phases using K-Ar and 40Ar/39Ar techniques: results from the Northern Apennines

Paleozoic to Oligocene metasedimentary rocks present in the Alpi Apuane region of the Northern Apennines, Italy, have been sequentially deformed during a Tertiary progressive deformation. In an attempt to date the individual deformation episodes, over 50 conventional K-Ar and 1140 Ar/39Ar incremental gas release analyses have been carried out on fine grained white micas separated from samples whose structural settings were well known. Mineralogy, X-ray diffractometry, and thin-section analyses indicate that the constituent muscovite and phengite formed under metamorphic conditions of 3–4 kbars and 300–400°C during all deformational phases. Pre-existing micas were variably crenulated during each subsequent deformational phase. Both K-Ar and 40Ar/39Ar analyses were carried out on 0.6-2μm, 2–6 μm and 6–20 μm size separates of the phengitic white mica. Although the K-Ar apparent ages range from 11 to 27 Ma and are consistent with available stratigraphic constraints, the 40Ar/39Ar age spectra display variable internal discordancy. These isotopic data indicate that: (1) both the K-Ar and 40Ar/39Ar total-gas ages decrease as the degree of crenulation increases; (2) the K-Ar and 40Ar/39Ar total-gas ages decrease as grain size decreases; (3) for each sample, characteristics of the 40Ar/39Ar age spectra depend upon grain size, with fine sizes yielding discordant patterns which systematically increase in apparent age from low to high temperature and (4) phengitic micas associated with earliest structures yield generally older ages than micas associated with later structures. The isotopic results are interpreted to indicate that the major deformation phase (D1) occurred at approximately 27 Ma with subsequent pulses ending by c. 10 Ma. These results may be combined with finite strain data to suggest that the region was deformed at strain rates between 10−15 and 10−14 s−1. A 27 Ma age indicates Mid-Oligocene initiation of plate tectonic activity in the Western Mediterranean and concomitant deformation in the Northern Apennines.

Role of shear in development of the Helvetic fold-thrust belt of Switzerland

The geometric features of the Helvetic nappes of the Alps show intense internal deformation set up predominantly by simple shear during nappe transport. The nappes are bounded by thrusts that parallel incompetent layers and ramp upward stratigraphically across competent units in the direction of tectonic transport; stratal layering is oblique to nappe boundaries. Thrust surfaces formed after ductile shear zones developed in underlying pre-Triassic gneissic basement. The broad zone of shearing narrowed down as deformation increased to form zones of intense shear and ultimately to form thrust faults. Contraction along competent layers obliquely inclined to shear surfaces produced buckle folds. The geometry of these folds was modified by further shearing as their normal limbs became aligned in the extension field of the strain ellipse for simple shear, producing normal faults.

Cross Section Restoration and Balancing as Aid to Seismic Interpretation in Extensional Terranes

Structural cross sections in extensional terranes can be improved significantly by restoration and balancing principles originally developed for fold-and-thrust belts. In rift basins and passive continental margins, use of restoration techniques in analysis of growth faults, elongate salt swells, and other extensional structures results in geologically valid seismic interpretations. Sections must be in depth (as opposed to two-way time) and oriented in the direction of material transport, and decompaction must accompany restoration. Hanging-wall deformation is characterized by different processes depending on the mechanical properties of the rocks being extended, and can be approximated by various geometric and kinematic models: antithetic faulting is analogous to antithetic shear, combined antithetic and synthetic faulting is approximated by vertical shear, domino-style extension corresponds to rigid body rotation, and flexural slip/flow is modeled by bed-length balance methods. Selection of the appropriate algorithm is crucial for proper restoration and balancing. Applications of these techniques, illustrated here using seismic data from the Gulf Coast and North Sea, include (1) evaluating and adjusting seismic interpretations, (2) projecting listric fault trajectories to depth, (3) delineating more accurately the geometry and extent of hydrocarbon reservoirs, (4) identifying new exploration leads, (5) determining the deformation history of an area, and (6) constraining the timing and direction of hydrocarbon migration and entrapment.

The correlation of magnetic anisotropy with strain in the Chelmsford Formation of the Sudbury Basin, Ontario

Abstract A quantitative correlation has been established between finite strain and the anisotropy of magnetic susceptibility (AMS) in sandstones of the Precambrian Chelmsford Formation of the Sudbury Basin, Ontario. Investigations of the progressive acquisition of isothermal remanent magnetization and its destruction with reverse field and thermal demagnetization as well as microscopic examination of polished thin sections show that both pyrrhotite and magnetite are present in the sandstone. Measurements of the AMS have been carried out at 16 sites distributed throughout the Chelmsford Formation outcrop. Considerable variation in the shapes of the magnetic susceptibility ellipsoids (axes k max > k int > k min exists within each site, with the k max and k int axes contained in the cleavage planes, where developed. The magnitudes of the magnetic susceptibility axes have been quantitatively correlated with strain measurements carried out by Clendenen et al. (this issue). Two different strain to magnetics correlations have been carried out: between the site mean strain determined using deformed concretions and the mean values of the AMS at each site, and between individual specimen strain values determined at the microscopic scale and the individual specimen AMS. In both cases, separate correlations were made between strain long, intermediate and short axes and their corresponding magnetic k max , k int , and k min axes. At 10 sites, the site mean strain values were derived from strain analysis of the concretions and at an additional 6 sites the site mean strain values were derived from the AMS to strain correlation. 527 individual cores were converted to equivalent strain values using the AMS-structural correlation of individual specimens. This study extends the use of AMS methods to include strain determination in sandstones, and provides an order of magnitude increase in the available number of such data for further structural analysis.

Correlation of strain and the anisotropy of magnetic susceptibility in the Onaping Formation: evidence for a near-circular origin of the Sudbury Basin

Abstract Measurements of finite strain, the anisotropy of magnetic susceptibility, the magnetic carrier of the susceptibility, and fieldwork have been carried out in the Onaping Formation of the Sudbury Basin, Ontario. The magnetic fabric indicates that changes in the magnitude and type of magnetic susceptibility ellipsoids closely mirror the changing pattern of deformation from northwest to southeast across the Onaping Formation outcrops. A quantitative correlation has been established between finite strain measurements and the magnetic susceptibility results and this allows the number of available strain measurements to be extended by more than 500 samples. The finite strain results suggest a deformation path in which the Onaping Formation was first affected by NW-SE-directed, horizontal, layer-parallel shortening which produced an upright tectonic cleavage. Subsequently, components of overthrust, NW-directed simple shear were superimposed upon the already deformed rocks of the Onaping Formation. The result of removing the strain, according to the sequence of superimposed deformations revealed by the deformation path, suggests that the Sudbury Basin was nearly circular in shape prior to its deformation. Consequently, a major objection to the meteorite impact theory for origin of the Sudbury Structure is eliminated.

Strain analysis of a Northern Apennine shear zone using deformed marble breccias

Abstract The Alpi Apuane region of the Northern Apennines appears to have been deformed within a large-scale, low-angle shear zone with an overthrust sense of movement. The presence of mineral stretching lineations, folds progressively rotated into the X strain direction, and schistosities which intersect the nappe boundaries at small angles suggest that a component of shear strain occurred during the deformation. The strain ratios and orientations on two-dimensional sections have been determined from deformed marble breccias, reduction spots, and oncalites. Data from three or more non-perpendicular, non-principal sections have been combined to determine the finite strain ellipsoids at 33 sites within the shear zone. The finite strains have been separated into components of simple shear (γ), longitudinal strain (λ), and volume change (Δ). Algebraic expressions have been derived and graphs constructed which enable components of γ, γ and Δ, and γ and λ to be determined directly from a knowledge of strain ratio ( R ) within the shear zone and the angle (θ) between the principal strain direction and the shear zone boundary. The Alpi Apuane data indicate that neither simple shear alone, nor simple shear with volume change can satisfactorily explain the observed strains. Consideration of simple shear plus longitudinal strain leads to a general relationship in which the value of shear increases, and the values of longitudinal strain change along a SW-NE profile across the zone. Integration of the resulting shear strain-distance curves gives a minimum displacement of 4 km within the shear zone. Combination of the finite strains with the total time of deformation known from K/Ar studies leads to average strain rates from 1.4 to 9.6 × 10 −15 sec −1 . A characteristic flat-ramp-flat geometry initially formed the boundaries of what was later to develop into the overthrust shear zone, and deformation of the underlying crystalline basement is believed to have occurred by ductile shearing. Estimates of 21% crustal shortening for the region suggest that the crustal thickness prior to deformation was approximately 20 km in this part of the Northern Apennines.

Finite strain patterns and their significance in Permian rocks of the Alpes Maritimes (France)

Abstract More finite strain data has been obtained from autochthonous Permian mudstones of the Alpes Maritimes, S.E. France. These new data were computed from field measurements of green spots on all available sections, deformed mudcracks and from the quantitative correlation between magnetic susceptibility anisotropy and finite strain in these rocks. Previously published finite strain data and the new results are presented on a series of structural maps and cross-sections for the Dome de Barrot, the Tinee and Vionene region and the Roya region. As in previous studies difficulties arise in explaining the apparently variable extension parallel with the 100°, subhorizontal bedding-cleavage intersection: either this is real or there were large volume changes during the tectonic deformation. Study of quartz fibres, developed in deformed mudcracks in the Tinee valley, suggest that early in the tectonic history incremental stretching directions were parallel with the bedding—cleavage intersection, while later they were down-dip in the 100° trending cleavage. Since these Permian rocks have remained stuck to the Argentera basement they also record displacements and deformations in the basement. The early 100°, subhorizontal stretching is consistent with NW-SE dextral, strike-slip basement faulting, while later, down-dip stretching in the cleavage is consistent with contraction faults in the basement. This information and new palaeomagnetic data on the same samples are combined with recent geophysical evidence and regional tectonic studies, to provide a new precision to the tectonic history of this part of the Western Alpine External Zone.

Evaluating the petroleum systems of the northern deep Gulf of Mexico through integrated basin analysis; an overview

Exploration and development activity has increased significantly during the past 5 years in the northern deep Gulf of Mexico. This activity has been caused by several factors, including significant discoveries in deep water (>1500-ft water depth), outstanding reservoir performance in some of these discoveries, expiration of 10-year leases originally purchased in the mid-1980s, innovative production techniques, and new federal royalty relief. Exploration and production has occurred in three general exploration subprovinces: present shelf, deep water, and the subsalt that extends from shelf into upper slope. Each subprovince consists of slightly different geology and, subsequently, different economic scenarios. This paper introduces the geologic setting for a portion of the outer shelf and upper to middle slope region in the northern Gulf of Mexico. The following eight papers demonstrate how the petroleum systems of the deep Gulf of Mexico can be analyzed by using an integrated approach. This issue of the Bulletin includes papers that describe the petroleum geology of the northern Green Canyon and Ewing Bank region: petroleum fields and discoveries, sequence stratigraphy, biostratigraphy, three-dimensional seismic stratigraphic interpretation, structural geology using restorations, interaction of salt tectonics and sedimentation, and geothermal modeling and path migration prediction.

Kinematics of large-scale asymmetric buckle folds in overthrust shear: an example from the Helvetic nappes

Kinematic analysis of asymmetric detachment folds from the Wildhorn nappe, central Helvetics, Switzerland, supports models of buckle fold formation in overthrust shear between nappe boundaries. The overall geometry shows a 2500 m thick multilayer of competent Jurassic limestones deflected into super- and subjacent less competent marls and shales by buckling above the basal decollement. Detailed investigation and measurement of the distribution, orientation, and relative timing of mesoscale structures, primarily dilatant veins and solution cleavage, allow documentation and quantification of the following kinematic development: (1) the nappe boundaries originated at a low angle (ca. 10°) to undeformed bedding; (2) local stress reorientation and buckling instabilities initiated symmetric buckle folds possibly characterized by tangential longitudinal strain; (3) the far-field stress directions imposed by the shear zone became dominant, and mesoscale structures developed asymmetrically on backlimbs and forelimbs; and (4) buckling ceased when fold interlimb angles reached 90–100°, and further shearing caused the fold limbs and axial planes to rotate into their current orientations. The total shear strain required is 2.9, equivalent to ca. 8 km of displacement of the overlying Cretaceous with respect to the base of the nappe. It is suggested that only an overthrust shear model can explain all the observations, and that other models, such as gravitational sliding and buttressing by normal faults, are incompatible with the data.

Strain studies of cleavage development in the Chelmsford Formation, Sudbury Basin, Ontario

Abstract Cleavage formation in the Precambrian Chelmsford Formation of the Sudbury Basin, Ontario, is directly related to the finite strain and deformation path. Finite strain was measured at the outcrop scale from the shapes and orientations of deformed concretions within sandstones, at the microscopic scale from the relative positions of quartz grain centers, and via quantitative correlations between the anisotropy of magnetic susceptibility and strain. Two-dimensional strains measured in different sections at each site were combined into three-dimensional strain ellipsoids at 16 sites distributed throughout the Chelmsford Formation outcrop. Using the magnetic susceptibility correlation, a further 527 individual strain ellipsoids were obtained throughout the sites. A prominent strain gradient is mirrored by the progressive development of cleavage in the Chelmsford Formation, with strain and fabric development increasing in intensity from NW to SE. At the least deformed sites, cleavage is weak or absent. The finite strain reflects a combination of vertical compaction followed by coaxial, tectonic layer-parallel shortening acting in a subhorizontal, NW-SE direction. Pressure solution is the dominant deformation mechanism responsible for the formation of cleavage. Material removed by pressure solution is redeposited as pressure fringes on quartz grains, an observation which is compatible with the form of the deformation path which indicates little effect due to volume loss in cleavage formation in these sandstones. Folds within the Chelmsford Formation indicate formation by buckling but have very low limb dips. Decomposition of the finite strain tensor into a compactional part and a layer-parallel shortening part suggests that fold initiation and amplification was preceded by about 38% layer-parallel shortening. The large number of strain ellipsoids obtained from the Chelmsford Formation provide a significant data base from which a number of important tectonic problems can be examined including the predeformational shape of the Sudbury structure.