Subhorizontal Foliation Research Articles

Compositional control of magnetic anisotropy in the Thomson formation, east-central Minnesota

Anisotropy of magnetic low-field susceptibility (AMS) and anisotropy of anhysteretic susceptibility (AAS) of the middle Precambrian Thomson formation, east-central Minnesota, generally record structural features, but orientations and axial ratios of the susceptibility ellipsoids depend on local magnetic mineralogy. The northern structural zone of the Thomson formation is characterized by an upright, east-west slaty cleavage. The susceptibility ellipsoids plot in the flattening field of a Flinn diagram, with minimum axes normal to the cleavage. AAS ellipsoids are coaxial with AMS ellipsoids, but are generally more prolate. Anisotropy increases with mean susceptibility at one outcrop, interpreted as mixing of weakly anisotropic, paramagnetic chlorite with more strongly anisotropic magnetite. A second outcrop shows the opposite pattern: anisotropy decreases with increasing mean susceptibility, suggesting that here the magnetite may be relatively isotropic, or possibly post-deformational. The southern structural zone of the Thomson formation is characterized by a subhorizontal foliation (S 1) and an upright crenulation cleavage (S 2). Here, the magnetic susceptibility minima are coincident with poles to S 1, and maxima are parallel to the S 1S 2 intersection. Correlation between directional susceptibilities and mean susceptibility (technique of Henry and Daly, 1983) suggests that paramagnetic minima are poles to the early foliation, while (possibly interdeformational) magnetite minima record the second deformation. This interpretation, however, is not supported by the AAS fabric, by thin section observations, or by high-field measurements of paramagnetic susceptibility. Alternatively, both directional and mean susceptibilities may be controlled by syndeformational chlorite.

Structural geology of the Lardeau Group near Trout Lake, British Columbia: implications for the structural evolution of the Kootenay Arc

The Lardeau Group is a heterogeneous assemblage of lower Paleozoic eugeoclinal strata present in the Kootenay Arc in southeastern British Columbia. It is in fault contact with lower Paleozoic miogeoclinal strata for all or some of its length along a structure termed the Lardeau shear zone. The Lardeau Group was deformed prior to mid-Mississippian time, as manifested by layer-parallel faults, folds, and evidence for early greenschist-facies metamorphism. Regional constraints indicate probable Devono-Mississippian timing of orogeny, and possible juxtaposition of the Lardeau Group over miogeoclinal strata along the Lardeau shear zone at this time. Further ductile deformation during the Middle Jurassic Columbian orogeny produced large folds with subhorizontal axes, northwest-striking foliation and faults, and orogen-parallel stretching lineations. This deformation was apparently not everywhere synchronous, and may have continued through Late Jurassic time northeast of Trout Lake. This was followed by Cretaceous(?) dextral strike-slip and normal movement on the Lardeau shear zone and other parallel faults. While apparently the locus of several episodes of faulting, the Lardeau shear zone does not record the accretion of far-travelled tectonic fragments, as sedimentological evidence ties the Lardeau Group and other outboard units to the craton.

Hercynian low-pressure-high-temperature regional metamorphism and subhorizontal foliation development in the Canigou massif, Pyrenees, France—Evidence for crustal extension

A detailed study has been made of the structural and metamorphic history of part of the Hercynian low-pressure-high-temperature regional metamorphic terrane exposed in the Canigou massif, France. Microtextural relations between, and compositional zoning within, porphyroblast phases in pelitic rocks indicate that the pressure-temperature-time ( P-T-t )trajectory followed by the massif during metamorphism involved decompression under prograde and retrograde conditions (clockwise P-T-t trajectory). The geometric relations between inclusion trails within these porphyroblasts and the structures in the adjacent matrix indicate that metamorphism was synchronous with the development of the regional subhorizontal foliation (S3). The S3 foliation formed in response to noncoaxial bulk strain. The absence of a marked hiatus in sedimentation in the Pyrenean region during Hercynian metamorphism and deformation and the widespread preservation of low-grade Hercynian metasedimentary rocks suggest that low- P -high- T metamorphism occurred in an extensional tectonic environment and that extension manifested itself in asymmetric top-to-the-northwest shear during D3.

The stratigraphy and structure of the metasedimentary rocks of the Loch Laggan-Upper Strathspey area, Inverness-shire

Synopsis The metasedimentary rocks of the Loch Laggan-Upper Strathspey area form part of the Grampian Division of the Caledonides in the Grampian Highlands. They comprise two lithostratigraphic successions, the Glenshirra and the Corrieyairack successions, which are separated by a zone of high strain, the Gairbeinn slide. Both successions consist dominantly of non-calcareous clastic metasediments. They may be correlated with similar lithostratigraphic units in the Loch Killen and the Corrieyairack Pass areas. The two successions share a similar deformational history and were both subjected to amphibolite facies metamorphism. A sub-horizontal foliation associated with an early isoclinal folding which is intensely developed in the vicinity of the Gairbeinn slide has been refolded by tight, upright, NE-SW trending folds, which have in turn been refolded by open folds with generally NW-SE trending axial traces. The absence of the Gairbeinn Pebbly Semi-psammite described from the Corrieyairack Pass area by Haselock et al. (1982) may be attributed either to tectonic excision during the sliding or to lateral facies variations.

Deformational geometry and tectonic significance of a portion of the Chilliwack Group, northwestern Cascades, Washington

The northwestern Cascades structural province can be interpreted as an accretionary complex comprising fault-bounded blocks of pre-Tertiary metamorphic rocks of diverse age and lithologic type. This paper documents the deformation in a portion of the Chilliwack Group, a unit in this complex. The Chilliwack Group is a thick sequence of volcaniclastic sedimentary rocks, calc-alkaline volcanic rocks, and limestone that is metamorphosed to low-grade blueschist facies. The rocks underwent ductile deformation during a Late Cretaceous orogenic event, producing a subhorizontal foliation and, in appropriate lithologies, subhorizontal stretching lineations that trend N20°W. Finite strain sustained by coarse clastic rocks produced RXZ values averaging 3.5. The deformation at least partially postdates the high pressure metamorphic event, based on the presence of bent and broken high-pressure mineral grains. Although early studies postulated west-vergent thrust imbrication of units in the northwest Cascades, the N20°W direction of apparent elongation in the Chilliwack Group, consistent with the direction of motion along segments of the Shuksan fault elucidated in other more recent studies, may reflect significant, highly oblique components of convergence during formation of the western North Cascades collisional orogen.

Polygenetic evolution and longitudinal transport within the Henderson mylonitic gneiss, North Carolina (southern Appalachian Piedmont)

The Henderson Gneiss of North Carolina, just east of the Brevard shear zone, displays two successive stages of mylonitization under different metamorphic conditions. Analysis of shear-sense indicators suggests a southwestward transport direction for both events. Only the second deformation appears related to the Brevard fault; the earlier deformation is more pervasive and suggests tangential displacements parallel to the Appalachian structural trend along the subhorizontal mylonitic foliation.

Detachment faulting and its relationship to older structural events on Saddle Island, River Mountains, Clark County, Nevada

On Saddle Island a detachment fault of Tertiary age separates two structurally different plates. In the lower plate, Precambrian amphibolite and gneiss are strongly overprinted by a subhorizontal mylonitic foliation that may have formed during east-directed overthrusting of Sevier age. The mylonitic foliation is in turn overprinted by a later brittle deformation expressed by the detachment fault and by two gently north-dipping zones of anastomosing faults that outline crude lenslike masses of unfractured rock. These fault zones are probably an expression of the brittle deformation that dominates during a detachment event and are themselves cut by north-south–trending, high-angle normal faults related to a second brittle deformation. Our field measurements of the orientation of fault planes and striae on these fault planes suggest that the orientation of the least principal stress direction (σ 3 ) changed during the brittle faulting event from north-northwest to northwest-southeast. Neither of these directions of extension is compatible with the strain field associated with the earlier ductile shearing responsible for the development of the mylonitic foliation.

The Snake Range Décollement: An exhumed Mid‐Tertiary ductile‐brittle transition

The Snake Range décollement (SRD) in east‐central Nevada separates supracrustal rocks extended by normal faulting from ductilely deformed igneous and metamorphic rocks. A well‐known stratigraphy unaffected by earlier faulting permits analysis of both upper and lower plate strain leading to a better understanding of how vastly different rock types and deformational styles are juxtaposed along low‐angle faults in metamorphic core complexes. Middle Cambrian to Permian upper plate rocks are cut by two generations of NE trending, east directed normal faults. Both generations were initiated as high‐angle (60°) planar faults that flattened abruptly into the SRD and rotated domino style to low angles, yielding a total rotation of bedding of about 80–90°. Faulting is Tertiary in age as 35‐m.y.‐old volcanic rocks are involved and resulted in about 450–500% extension in a N55W‐;S55E direction. The SRD developed as a subhorizontal surface 6–7 km deep at the top of the Cambrian Pioche Shale. Lower plate granitic rocks and their late Precambrian‐Cambrian metamorphic country rocks were involved in progressive ductile to brittle extension at low greenschist grade, forming a penetrative subhorizontal foliation and N55–70W lineation that increases in intensity eastward and upward toward the SRD. Stretching and thinning in the lower plate is coaxial and comparable in magnitude to upper plate extension, and is interpreted as synchronous. K‐Ar ages ranging from 20 to 40 m.y. in the lower plate suggests the N. Snake Range represents a Tertiary thermal anomaly. We conclude that the SRD developed as a ductile‐brittle transition zone at 6–7 km depth. Gravity data suggests that the gently domed SRD is cut by younger Basin and Range faults, but the geology of adjacent ranges suggests that the SRD does not continue more than 60 km in any given direction. The lack of stratigraphic omission across the SRD rules out large amounts of movement on a surface that originally cut downsection, and we suggest that extensional detachment faults such as the SRD can be developed locally as boundaries between brittlely extended rocks and underlying ductile extension and intrusion.

Cumulative deformation in the barnes ice cap and implications for the development of foliation

In recent years, it has become apparent that the development of foliation and crystallographic fabric in glacier ice is strongly dependent upon cumulative deformation. However, because glaciers lack suitable strain markers and past velocity fields are unknown, the nature of the cumulative deformation is not well understood. Part of the Barnes Ice Cap, Baffin Island N.W.T., Canada has undergone little overall change in position in the past 2000 years, so a long-term steady-state velocity field can be estimated from present-day measurements in a section in which flow is essentially two-dimensional. By means of a numerical model, particle paths, isochrons, strain rates, and cumulative strains are computed for this section. There is no correspondence between the principal directions of strain rate and cumulative strain in most of the ice, the greatest divergence of about 45° being near the base of the glacier where the maximum cumulative extension direction, X, is sub-horizontal. All ice particles initially undergo a restricted amount of pure shear, and then pass gradually into a long domain of nearly simple shear. They return to a second short stage of nearly pure shear near the margin. The effects of the early stage of pure shear are never eliminated, but the cumulative strain is increasingly dominated by simple shear down-glacier. Cumulative strain magnitude, magnitude of strain rate, and age of the ice all increase rapidly with depth, and change much more slowly with distance down-glacier. We argue that foliation is formed during flow by modification of primary inhomogeneities in ice. The three most important inhomogeneities are probably sedimentary layering, crevasse infillings, and irregularities such as ice glands or lenses. During deformation, the first will behave as isochrons, the second will rotate as material planes, initially perpendicular to the isochrons, and the third will be stretched parallel to the X-direction. The numerical model indicates that the isochrons, planes representing crevasse infillings, X-direction, and particle paths are within 1° of mutual parallelism in the most distal ice. Thus a single subhorizontal foliation, dipping slightly more steeply up-glacier than the flow lines, and curving upwards near the margin would be produced in this case. This corresponds with observations.

Patterns of total strain in the crestal region of immature diapirs

Conformable domes can result from diapirism as well as cross buckling and other types of cross folding. The total-strain pattern of natural structures offers a means of discriminating between diapiric domes and nondiapiric domes. Similarly, one may distinguish between diapiric ridges and other types of antiforms.Immature diapirs in metamorphic terrains and appropriate quantitative models are characterized by crestal zones of horizontal extension. Similar zones occur in the convex-hinge regions of very competent buckles and the adjacent incompetent matrix of fold models. But these model experiments are rarely applicable to natural buckle folds in metamorphic terrains, which typically involve low competency contrasts. Where upright buckle folds can be identified by means of independent evidence, their hinge zones are generally devoid of stratiform foliation and other features due to horizontal extension. An Archean gneiss dome of the northwestern Ontario is subjected to structural analysis, and interpreted as an immature diapir on the basis of widespread subhorizontal foliation about the dome's centre. The strain pattern of the gneiss dome corresponds to that of salt diapirs in New Brunswick.