Nicholas B. Woodward

Including strain data in balanced cross-sections

Abstract Almost all published balanced cross-sections include cleaved and/or strained rocks and may therefore violate the requirement that bed-lengths and formation areas are preserved between undeformed and deformed states. Knowledge of distortions, presence and type of cleavage, and overprinting relations in poly-deformed segments are necessary to define area changes related to volume loss, strike elongation, and poly-deformation. Work in the Southern and Central Appalachians has shown variation in structural complexity and changing deformation styles both along and across the thrust-belt. This necessitates separate treatment of individual sheets during balancing. Regions of little or no interstratal slip (pinning points) should be chosen for each thrust sheet and each fold strain in the sheet. In external sheets, where deformation intensity increases towards thrusts, pin-points should be back-limb inflection points. In intermediate and internal sheets where cleavage is approximately coeval with folding, pin-points should be at fold hinges. Where cleavage and strain are heterogeneously developed throughout a stratigraphic sequence, a line-length balance using massive relatively uncleaved beds (e.g. Knox Group, Southern Appalachians) can be used as a basis for restoring the section. Unstraining of deformed layers, given strain data and cleavage trajectories for individual thrust sheets, provides line lengths and formation areas which can be fitted to the key-bed of the undeformed section. Cleavages which partially or fully postdate folding indicate continued shortening in the thrust wedge, and modify earlier pinning points in upper sheets. These cleavages, and poly-deformed segments, are difficult to restore as strain data are generally scarce in the metamorphosed internal thrust sheets.

Structural deformation of the Leoville Chondrite

Foliations defined by alignment of elongated chondrules have been noted previously in chondrites, but none displays this effect so well as Leoville (CV3). The shapes of Leoville chondrules were produced by deformation in situ, as indicated by inclusions and clasts with similar shapes and preferred orientations to those of chondrules. Similarities in the aspect ratios of apparent strain ellipses measured for chondrules alone (1.9 and 2.0 by several methods) and for the whole meteorite (2.0) indicate either that Leoville deformed homogeneously or that it deformed as a framework of touching chondrules. This amount of strain corresponds to approximately 33% uniaxial shortening, assuming constant volume. Because the strain ellipse was measured in only one orientation, this strain value is a minimum estimate for the meteorite. Lack of correlation between foliation and either shock or thermal effects argues that impact or metamorphism are unlikely to have produced this deformation. Compaction due to overburden from progressive accretion on the chondrite parent body is suggested to have been its cause. Estimates of maximum deviatoric stresses in the interiors of asteroid-sized bodies and constraints on maximum temperatures for CV3 chondrites are consistent with diffusional flow as the deformation mechanism for olivine in these chondrules. Diffusional flow is also suggested by the scarcity of observed lattice dislocations. Deformation of Leoville olivines by this mechanism at geologically reasonable strain rates appears to require higher temperatures than those believed to have been experienced by this meteorite (< 600°C). However, differences in olivine grain size, the presence of water, or a more complex deformation history might explain this discrepancy.

Structural lithic units in external orogenic zones

Abstract Changes in types of thrust fault structures are typical of external orogenic zones, and are inferred here to be related to thickness and facies changes in stratigraphie sequences. Obvious deformation style changes occur from external to intermediate thrust sheets. Similar changes in style are observed, and most easily interpreted, along-strike within individual thrust sheets. To correlate stratigraphic and structural changes, stratigraphic sections are divided into structural lithic units. The subdivisions are based on the dominant deformation styles vertically in individual cross-sections. The major variables in determining deformation style are controlling stratigraphic members within individual stratigraphic/structural units, and the stacking sequence of lithologie types into different structural units. In many areas, the critical taper of one major dominant member regionally controls where structural style changes will occur. Areal change in either internal composition or in stacking sequence causes lateral change in deformation style. Lithological changes in structural units are coincident with changes in frontal thrust ramp positions (connected by major lateral ramps), and with changes in thrust zone geometries (imbricate thrusts in one area versus a duplex zone along strike). They are also associated with changes in thrust shapes, from listric to stepped, and changes from fault-bend-fold to fault-propagation-fold structures. Controlling members may be either strong or weak. Fold dominated regions and thrust faulted regions without major shales or evaporites are usually controlled by strong controlling members. Weak controlling members are major evaporite horizons, or thick shales. A thorough understanding of stratigraphic facies realms therefore may allow predictions of types and changes in structural styles. It can also be very useful in the interpretation of less well-exposed thrust terranes.

Geological applicability of critical-wedge thrust-belt models

Critical-wedge models for foreland thrust belts, although popular paradigms for thrust mechanics, also predict definite progressive changes within the orogenic belt. The most important constraint on critical-wedge models is the need for deformation of the interior of the wedge when new sheets are accreted to its front. Well-dated foreland thrust belts have sequential formation of new thrust sheets in the front of the belt. Thus, they also require continued superimposed deformation within the wedge. Geologic mapping and fabric studies provide evidence of only minor later intra-wedge shortening, severely reducing the acceptability of an accreting critically tapered wedge model. To apply critically tapered wedge models to foreland fold-and-thrust belts, one must relax requirements for constant wedge strengths and basal sliding laws. The wedge will need to strengthen after initial thrust emplacement, and the fault zones will probably need to progressively weaken. It is most likely that critical-wedge models which apply well to accretionary wedges do not apply in simple ways to foreland fold-and-thrust belts.

Thrust fault geometry of the Snake River Range, Idaho and Wyoming

The Snake River Range is located at the north end of the Absaroka fault system in the Idaho-Wyoming-Utah thrust belt, and it adjoins the Teton Mountains. Nine imbricate sheets of the Absaroka system form a shingled array of overlapping thrusts. Most fault nomenclature in the range dates from early, discontinuous, reconnaissance studies wherein names were applied from the top thrust down whether or not the thrust sheets separated by this method are, or ever were, connected as mechanical blocks. Fault names are revised here to consider the lateral continuity of sheets as the major criteria for defining the block to which a name should be applied. The Absaroka thrust and St. John thrust are recognized to be equivalent different parts of a major transfer zone, using the continuity of sheets as the main criteria. The map pattern of the range is dominated by major re-entrants to the west in the traces of the St. John and Absaroka thrusts. The re-entrants in both major faults are caused by folding in their foot walls related to lateral ramping of each thrust upward to the southeast. The folding of the St. John thrust above the Indian Creek Culmination also folds the thrust sheets above it, helping to mark the emplacement of the imbricate sheets as being from top to bottom and west to east. The Absaroka system thrusts formed from 20 to 40 km west of their present position. They are folded by underlying faults of the Jackson or, farther south, the Darby system, indicating that the Absaroka system was emplaced into its present position by motion on these underlying faults. Discussions concerning mechanical interaction between the thrust belt and a foreland buttress (ancestral Teton or Targhee uplift) concentrate on intensities of deformation and changes in geometry adjacent to the proposed buttress. The individual imbricate geometries and regional changes in trend are more related to a changing stratigraphic package and the lateral thrust ramps than to geographic proximity to any proposed buttress.

Kink detachment fold in the southwest Montana fold and thrust belt

The Hossfeldt anticline in the southwest Montana thrust belt is characterized by a kink geometry and probably overlies a thrust detachment at depth. The mesofabric distribution in the limbs documents that the eastern overturned limb has undergone most of the rotation and internal deformation during folding, leaving the gently dipping western limb virtually undeformed. The anticline exhibits unique mesofabrics in its hinge region that require a pinned anticlinal hinge during its evolution. The half-wavelength of the Hossfeldt anticline-Eustis syncline pair coincides with that predicted from buckling theory, if one considers the massive carbonates of the Paleozoic section as a competent beam. Although the geometry and mesofabric distribution of the Hossfeldt anticline satisfy the geometric requirements of either a fault-propagation fold or a detachment kink fold, the buckling wavelength strongly suggests that its origin was as a kink-buckle fold above a flat detachment rather than as a fault-propagation fold above a thrust ramp.

Relationships between early Paleozoic facies patterns and structural trends in the Saltville thrust family, Tennessee Valley and Ridge, southern Appalachians

Structural style variations across the Tennessee segment of the Valley and Ridge thrust belt are correlated with two facies changes in important structural/stratigraphic units. The Middle and Upper Cambrian Conasauga Group and the Middle Ordovician Chickamauga Group change their internal stratigraphy regionally at oblique angles to the trend of this segment of the thrust belt. These stratigraphic variations correlate geographically with structural style changes within individual thrust sheets, whereas other variables, such as temperature, pressure, and magnitude of tectonic transport, do not. In the Conasauga, the east to west, limestone to shale facies transition coincides with areas where structural styles change along thrusts at the Conasauga level. The shale-dominated and carbonate-dominated facies show simple frontal imbricate thrusts, whereas in the shale/carbonate zone of interlayering, duplexes occur. In the Chickamauga, an east-west change from basinal shale to shelfal limestone controlled the regional presence of a middle-level glide zone in the paths of eastern thrusts and its absence to the west. This stratigraphic inhomogeneity also controlled a change in the Knoxville area from surficial folding in the northeast to surficial thrusting in the southwest. The terminology of structural lithic units and their subsidiary predominant and conforming members is applied in dividing the stratigraphic section into structurally significant packages.