DOCSLIB.ORG

Stress History of Folding and Cleavage Development, Baraboo Syncline, Wisconsin

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Stress History of Folding and Cleavage Development, Baraboo Syncline, Wisconsin Stress history of folding and cleavage development, Baraboo syncline, Wisconsin I.W.D. DALZIEL* Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 10964 G. L. STIREWALT Department of Geology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27514 ABSTRACT arch as inliers in the flat-lying lower Subsequent reinterpretation of his data by Paleozoic sedimentary rocks of the midcon- Christie and Raleigh (1959), and by other Analysis of quartz subfabric elements tinent region (Fig. 1). The largest and best workers, in the light of recent experimental was undertaken in specimens of Precam- known of these inliers occurs in Columbia evidence (Friedman, 1964; Carter and brian Baraboo Quartzite collected around and Sauk Counties, south-central Wiscon- Friedman, 1965; Dalziel, 1969; Dalziel and the doubly plunging, asymmetric Baraboo sin, where Precambrian rocks composed Dott, 1970), has resulted in general agree- syncline. Principal stress axes determined mainly of Baraboo Quartzite form an elon- ment that the quartz subfabric bears some from quartz deformation lamellae in speci- gate ring of hills known as the Baraboo genetic relationship to the fold. Riley's "a" mens from the hinge zone of the fold are cr1 Ranges. The quartzite is certainly more fabric axes lie in a plane approximately (greatest compressive principal stress) and than 1,200 m thick and rests stratigraphi- perpendicular to the hinge line of the fold, <j2 in the plane of the bedding and cr3 (least cally upon poorly exposed silicic volcanic although there is no consistent relationship compressive principal stress) normal to rocks. Precambrian metasedimentary rock between these a axes and bedding or cleav- bedding. In the closures, crj is essentially units overlying the Baraboo Quartzite have age (Dalziel and Dott, 1970, Fig. 8b). The a perpendicular to the hinge line. On the been recorded in mining records (Weidman, axes are clearly axes of least compressive limbs of the fold, cr, is mostly parallel to the 1904; Leith, 1935; Schmidt, 1951; Dalziel principal stress (erg), as we shall discuss hinge line of the fold and cr3 is nearly verti- and Dott, 1970) but have not been posi- later. Hence, it seemed worthwhile to con- cal. At the eastern end of the northern limb tively identified in outcrop. duct a detailed study of the quartz subfabric where the fold is tightest, cr3 is again verti- The entire Precambrian succession has in the Baraboo Quartzite and utilize the in- cal, but o~i is horizontal north-south and been folded into a complex doubly plunging formation from recent experimental evi- nearly perpendicular to vertical east- asymmetric syncline with an axial surface dence. Also, most studies of the stress his- striking bedding and to the hinge line. striking approximately east-northeast and tory of natural folds have been undertaken Gliding flow features in minerals of most dipping steeply north-northwest. The on open, high-level structures lacking folded layered rocks have reflected early northern limb is essentially vertical, and the well-developed cleavage (Hansen and Borg, layer-parallel compression (cr,) oriented southern limb dips gently northward (Fig. 1962; Carter and Friedman, 1965; Scott perpendicular to the hinge line of the fold. 2). Radiometric dates indicate that the se- and others, 1965; Friedman and Stearns, Prior to our new data from the Baraboo quence is of late Precambrian age and was 1971), and the Baraboo syncline provided syncline only macrofractures, tension gash deformed and metamorphosed during an an opportunity to undertake a paleostress bands, and (in one case) twin lamellae in orogenic event 1,200 to 1,500 m.y. ago study on a fold with well-developed cleav- age. calcite have shown a1 parallel to hinge lines (Dott and Dalziel, 1972). of folds or perpendicular to bedding. The The work of C. R. Van Hise, C. K. Leith, two latter stress configurations are theoreti- W. J. Mead, and other geologists of the METHODS OF INVESTIGATION cally characteristic of stresses associated "Wisconsin School" in the latter part of the with a later stage in the history of folding. nineteenth and early part of the twentieth Specimens were collected (Fig. 1) from Hence, the detrital quartz grains on the centuries made the structures in the de- the Baraboo syncline for the present mi- limbs of the Baraboo syncline apparently formed Baraboo metasedimentary rocks crofabric study to provide information contain deformation lamellae produced known to structural geologists throughout from positions around the fold considered mainly by later stage stresses rather than the world. An account of the history of to be critical with regard to the stress his- earlier in the fold history, when cr1 was geologic research in the Baraboo district is tory of the fold (Friedman and Sowers, parallel to bedding and perpendicular to the given by Dalziel and Dott (1970). The pres- 1970) and to test the reproducibility of the hinge line. The closely spaced cracks form- ent study combines microscopic analysis of results at any locality, particularly with re- ing the cleavage in the quartzite possibly the Baraboo Quartzite by Stirewalt with the gard to the position of specimens within a were controlled in orientation by relaxation field structural study of the mesoscopic fab- single bed of quartzite and their proximity following the early layer-parallel compres- ric by Dalziel. to the phyllitic layers (Table 1). Not all of sion. Key words: structural geology, folds, the specimens collected contained sufficient cleavage, structural analysis, petrofabrics, OBJECTIVES quartz grains with deformation lamellae for Precambrian. analysis. Those specimens with few lamel- The microscopic fabric of the Baraboo lae were rich in phyllosilicate. INTRODUCTION Quartzite was studied by Riley (1947), who From most of his specimens Riley (1947) concluded that it appeared to bear no rela- cut only a single thin section. In a few cases Precambrian basement rocks are locally tionship to either the mesoscopic or the he did have the control of a second section exposed along the axis of the Wisconsin macroscopic fabric. Riley's interpretation perpendicular to the first, thereby appar- was based upon the early "fabric" ap- ently justifying the use of single sections be- proach to the understanding of deforma- * Also of Department of Geological Sciences, Colum- cause the same fabric axes were obtained bia University, New York, New York 10027. tion lamellae (Ingerson and Tuttle, 1945). from each of the two sections. At least two Geological Society of America Bulletin, v. 86, p. 1671-1690, 17 figs., December 1975, Doc. no. 51206. 1671 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/12/1671/3433516/i0016-7606-86-12-1671.pdf by guest on 29 September 2021 R.4E. R.5E. R.6E. R.7E. R.8E. T.12N. T.12N. T.11N. T.11N. T.10N R.5E. R.6E. R.7E. Figure 1. Pre-Paleozoic outcrop map of the Baraboo district, Wisconsin (after Dalziel and Dott, 1970), showing specimen locations. Inset illustrates the regional setting of the Baraboo district. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/12/1671/3433516/i0016-7606-86-12-1671.pdf by guest on 29 September 2021 N. Range S. Range 1,000 m BARABOO SYNCLINE PCs PCdk B/».. Q and P. 1,000 ft "•"SEA LEVEL 1 km N. Range S. Range BARABOO SYNCLINE =1 B' T + + t +granite+ + + + + + + Figure 2. Geologic cross sections of the Baraboo syncline (after Dalziel and Dott, 1970). Slate, pCf = Freedom Formation, pCdk = Dake Quartzite, pCro = Rowley Creek Slate, P Section locations are shown in Figure 1. pCr = rhyolite, pCb = Baraboo Quartzite, pCs = Seeley Paleozoic sedimentary rocks, Q = Quaternary deposits. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/12/1671/3433516/i0016-7606-86-12-1671.pdf by guest on 29 September 2021 1674 DALZIEL AND STIREWALT mutually perpendicular thin sections were faces and appears to reflect migration of The other mesoscopic fabric elements examined from each of the specimens col- iron-rich solutions through permeable sed- listed in Table 2 are less widespread and lected for the present study. In the case of iments prior to their final lithification. occur only in the less competent phyllitic one specimen each from the North and The most prominent mesoscopic struc- layers from which insufficient data could be South Ranges and one each from the west- tural elements imposed on the Baraboo obtained for stress analysis. These meso- ern and eastern hinge zones of the fold, Quartzite are a cleavage and its intersection scopic fabric elements are important in con- three mutually perpendicular sections were with bedding. The cleavage in the quartzite sidering the significance of the stress pattern examined. Where two sections were cut, layers (S/) is at right angles to bedding, and interpreted from the quartz microfabric one was taken perpendicular to the hinge therefore it is nearly horizontal on the verti- analysis and will be discussed later. For the line of the fold and one parallel to the bed- cal north limb (Fig. 4A) and steeply south present it is sufficient to state that, although ding. The third sections, therefore, were cut dipping on the gently north dipping south field relationships clearly indicate that one parallel to the hinge line of the fold and limb (Fig. 4B). It is refracted in the phyllitic structure postdates another, there is no perpendicular to bedding. layers so as to dip northward at a moderate reason to suppose that the various meso- angle on both limbs (Figs. 4A, 4B), approx- scopic structures in the Baraboo Quartzite MESOSCOPIC STRUCTURES imately parallel to the axial surface of the are unrelated to the evolution of the fold. Bedding/cleavage intersections parallel Baraboo syncline. With the possible excep- Mesoscopic fabric elements of the the hinge line of the fold.
Load more

Recommended publications
  • Tectonic Imbrication and Foredeep Development in the Penokean
    Tectonic Imbrication and Foredeep Development in the Penokean Orogen, East-Central Minnesota An Interpretation Based on Regional Geophysics and the Results of Test-Drilling The Penokean Orogeny in Minnesota and Upper Michigan A Comparison of Structural Geology U.S. GEOLOGICAL SURVEY BULLETIN 1904-C, D AVAILABILITY OF BOOKS AND MAPS OF THE U.S. GEOLOGICAL SURVEY Instructions on ordering publications of the U.S. Geological Survey, along with prices of the last offerings, are given in the cur­ rent-year issues of the monthly catalog "New Publications of the U.S. Geological Survey." Prices of available U.S. Geological Sur­ vey publications released prior to the current year are listed in the most recent annual "Price and Availability List." Publications that are listed in various U.S. Geological Survey catalogs (see back inside cover) but not listed in the most recent annual "Price and Availability List" are no longer available. Prices of reports released to the open files are given in the listing "U.S. Geological Survey Open-File Reports," updated month­ ly, which is for sale in microfiche from the U.S. Geological Survey, Books and Open-File Reports Section, Federal Center, Box 25425, Denver, CO 80225. Reports released through the NTIS may be obtained by writing to the National Technical Information Service, U.S. Department of Commerce, Springfield, VA 22161; please include NTIS report number with inquiry. Order U.S. Geological Survey publications by mail or over the counter from the offices given below. BY MAIL OVER THE COUNTER Books Books Professional Papers, Bulletins, Water-Supply Papers, Techniques of Water-Resources Investigations, Circulars, publications of general in­ Books of the U.S.
    [Show full text]
  • Strike and Dip Refer to the Orientation Or Attitude of a Geologic Feature. The
    Name__________________________________ 89.325 – Geology for Engineers Faults, Folds, Outcrop Patterns and Geologic Maps I. Properties of Earth Materials When rocks are subjected to differential stress the resulting build-up in strain can cause deformation. Depending on the material properties the result can either be elastic deformation which can ultimately lead to the breaking of the rock material (faults) or ductile deformation which can lead to the development of folds. In this exercise we will look at the various types of deformation and how geologists use geologic maps to understand this deformation. II. Strike and Dip Strike and dip refer to the orientation or attitude of a geologic feature. The strike line of a bed, fault, or other planar feature, is a line representing the intersection of that feature with a horizontal plane. On a geologic map, this is represented with a short straight line segment oriented parallel to the strike line. Strike (or strike angle) can be given as either a quadrant compass bearing of the strike line (N25°E for example) or in terms of east or west of true north or south, a single three digit number representing the azimuth, where the lower number is usually given (where the example of N25°E would simply be 025), or the azimuth number followed by the degree sign (example of N25°E would be 025°). The dip gives the steepest angle of descent of a tilted bed or feature relative to a horizontal plane, and is given by the number (0°-90°) as well as a letter (N, S, E, W) with rough direction in which the bed is dipping.
    [Show full text]
  • Thesis, "Structure and Evolution of the Horse Heaven Hills in South
    AN ABSTRACT OF THE THESIS OF Michael Curtis Hagood for the Master of Science in Geology presented February 21, 1985. Title: Structure-and Evolution of the Horse Heaven Hills in South-Central Washington. APPROVED BY MEMBERS OF THE THESIS COMMITTEE: Marvin H. Beeson, Chairman Michael L. Cummings Gilbert T. Benson Stephen P. Reidel The Horse Heaven Hills uplift in south-central Washington con- sists of distinct northwest and northeast trends which merge in the lower Yakima Valley. The northwest trend is adjacent to and parallels the Rattlesnake-Wallula alignment (RAW; a part of the Olympic-Wallowa lineament). The northwest trend and northeast trend consist of aligned or en echelon anticlines and monoclines whose axes are gener- ally oriented in the direction of the trend. At the intersection, La 2 folds in the northeast trend plunge onto and are terminated by folds of the northwest trend. The crest of the Horse Heaven Hills uplift within both trends is composed of a series of asymmetric, north vergent, eroded, usually double-hinged anticlines or monoclines. Some of these "major" anti- clines and monoclines are paralleled to the immediate north by lower- relief anticlines or monoclines. All anticlines approach monoclines in geometry and often change to a monoclinal geometry along their length. In both trends, reverse faults commonly parallel the axes of folds within the tightly folded hinge zones. Tear faults cut across the northern limbs of the anticlines and monoclines and are coincident with marked changes in the wavelength of a fold or a change in the trend of a fold. Layer-parallel faults commonly exist along steeply- dipping stratigraphic contacts or zones of preferred weakness in intraflow structures.
    [Show full text]
  • Deepwater Niger Delta Fold-And-Thrust Belt Modeled As a Critical-Taper Wedge: the Influence of a Weak Detachment on Styles of Fault-Related Folds
    Deepwater Niger Delta fold-and-thrust belt modeled as a critical-taper wedge: The influence of a weak detachment on styles of fault-related folds Frank Bilotti1, Chris Guzofski1, John H. Shaw2 1 Chevron 2Harvard University GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Niger delta “outer” fold-and-thrust belt very low taper Odd fault-related folds “Ductile” thickening Forethrusts and backthrusts in close proximity GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Outline • The nature of the toe of the Niger Delta • Basics of critical-taper wedge theory • The Niger Delta outer fold-and-thrust belt is at critical taper • Model parameters and results (high basal fluid pressure) • Applicability in 3D & subsequent work • Implications of high basal fluid pressure for contractional fault-related folds GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Niger Delta Bathymetry Slope fold-and-thrust belt deepwater fold-and-thrust belt GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Fold-and-thrust belts of the Niger Delta GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Regional Geologic Setting Inner Fold and Outer Fold and Thrust belt 1 i Thrust belt Detachment fold belt Extensional Growth Faults t 3250 Lobia-1 0 k m a 0 k m . l m p 4 m . numerouscr estal crestal growthfaults growth faults numerous growthfaults ? ( 3 ma ? mudd apir (?) ? 5 m P n i e i y R it u x c : e n d her s d 5 l 1 0 5 m l l i 2 5 mud diapr (?) 2 m N m s t i 5 k m velocty sag(?) 5 k m e 2 m a .5 ma s u i basal detachment a .0 i 5 a as e veoct y ag(?) velocty sag(?) .
    [Show full text]
  • Accommodation of Penetrative Strain During Deformation Above a Ductile Décollement
    University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Earth and Atmospheric Sciences, Department Papers in the Earth and Atmospheric Sciences of 2016 Accommodation of penetrative strain during deformation above a ductile décollement Bailey A. Lathrop Caroline M. Burberry Follow this and additional works at: https://digitalcommons.unl.edu/geosciencefacpub Part of the Earth Sciences Commons This Article is brought to you for free and open access by the Earth and Atmospheric Sciences, Department of at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Papers in the Earth and Atmospheric Sciences by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Accommodation of penetrative strain during deformation above a ductile décollement Bailey A. Lathrop* and Caroline M. Burberry* DEPARTMENT OF EARTH AND ATMOSPHERIC SCIENCES, UNIVERSITY OF NEBRASKA-LINCOLN, 214 BESSEY HALL, LINCOLN, NEBRASKA 68588, USA ABSTRACT The accommodation of shortening by penetrative strain is widely considered as an important process during contraction, but the distribu- tion and magnitude of penetrative strain in a contractional system with a ductile décollement are not well understood. Penetrative strain constitutes the proportion of the total shortening across an orogen that is not accommodated by the development of macroscale structures, such as folds and thrusts. In order to create a framework for understanding penetrative strain in a brittle system above a ductile décollement, eight analog models, each with the same initial configuration, were shortened to different amounts in a deformation apparatus. Models consisted of a silicon polymer base layer overlain by three fine-grained sand layers. A grid was imprinted on the surface to track penetra- tive strain during shortening.
    [Show full text]
  • PLANE DIP and STRIKE, LINEATION PLUNGE and TREND, STRUCTURAL MEASURMENT CONVENTIONS, the BRUNTON COMPASS, FIELD BOOK, and NJGS FMS
    PLANE DIP and STRIKE, LINEATION PLUNGE and TREND, STRUCTURAL MEASURMENT CONVENTIONS, THE BRUNTON COMPASS, FIELD BOOK, and NJGS FMS The word azimuth stems from an Arabic word meaning "direction“, and means an angular measurement in a spherical coordinate system. In structural geology, we primarily deal with land navigation and directional readings on two-dimensional maps of the Earth surface, and azimuth commonly refers to incremental measures in a circular (0- 360 °) and horizontal reference frame relative to land surface. Sources: Lisle, R. J., 2004, Geological Structures and Maps, A Practical Guide, Third edition http://www.geo.utexas.edu/courses/420k/PDF_files/Brunton_Compass_09.pdf http://en.wikipedia.org/wiki/Azimuth http://en.wikipedia.org/wiki/Brunton_compass FLASH DRIVE/Rider/PDFs/Holcombe_conv_and_meas.pdf http://www.state.nj.us/dep/njgs/geodata/fmsdoc/fmsuser.htm Brunton Pocket Transit Rider Structural Geology 310 2012 GCHERMAN 1 PlanePlane DipDip andand LinearLinear PlungePlunge horizontal dddooo Dip = dddooo Bedding and other geological layers and planes that are not horizontal are said to dip. The dip is the slope of a geological surface. There are two aspects to the dip of a plane: (a) the direction of dip , which is the compass direction towards which the plane slopes; and (b) the angle of dip , which is the angle that the plane makes with a horizontal plane (Fig. 2.3). The direction of dip can be visualized as the direction in which water would flow if poured onto the plane. The angle of dip is an angle between 0 ° (for horizontal planes) and 90 ° (for vertical planes). To record the dip of a plane all that is needed are two numbers; the angle of dip followed by the direction (or azimuth) of dip, e.g.
    [Show full text]
  • Deformation of Rocks
    DeformationDeformation ofof RocksRocks Rock Deformation Large scale deformation of the Earth’s crust = Plate Tectonics Smaller scale deformation = structural geology 1 Deformation of rocks Folds and faults are geologic structures Structural geology is the study of the deformation of rocks and the effects of this movement Small-Scale Folds 2 Small-Scale Faults Deformation – Stress vs. Strain Changes in volume or shape of a rock body = strain 3 Stress The force that acts on a rock unit to change its shape and/or its volume Causes strain or deformation Types of directed Stress include Compression Tension Shear Compression Action of coincident oppositely directed forces acting towards each other 4 Tension Action of coincident oppositely directed forces acting away from each other Shear Action of coincident oppositely directed forces acting parallel to each other across a surface Right Lateral Movement Left Lateral Movement 5 Differential stress Strength • Ability of an object to resist deformation •Compressive •Capacity of a material to withstand axially directed pushing forces – when the limit of compressive strength is reached, materials are crushed •Tensile •Measures the force required to pull something such as rope, wire, or a rock to the point where it breaks 6 Strain Any change in original shape or size of an object in response to stress acting on the object Kinds of deformation Elastic vs Plastic Brittle vs Ductile 7 Elastic Deformation Temporary change in shape or size that is recovered when the deforming force is removed
    [Show full text]
  • Joints, Folds, and Faults
    Structural Geology Rocks in the Crust Are Bent, Stretched, and Broken … …by directed stresses that cause Deformation. Types of Differential Stress Tensional, Compressive, and Shear Strain is the change in shape and or volume of a rock caused by Stress. Joints, Folds, and Faults Strain occurs in 3 stages: elastic deformation, ductile deformation, brittle deformation 1 Type of Strain Dependent on … • Temperature • Confining Pressure • Rate of Strain • Presence of Water • Composition of the Rock Dip-Slip and Strike-Slip Faults Are the Most Common Types of Faults. Major Fault Types 2 Fault Block Horst and Graben BASIN AND Crustal Extension Formed the RANGE PROVINCE Basin and Range Province. • Decompression melting and high heat developed above a subducted rift zone. • Former margin of Farallon and Pacific plates. • Thickening, uplift ,and tensional stress caused normal faults. • Horst and Graben structures developed. Fold Terminology 3 Open Anticline – convex upward arch with older rocks in the center of the fold (symmetrical) Isoclinal Asymmetrical Overturned Recumbent Evolution Simple Folds of a fold into a reverse fault An eroded anticline will have older beds in the middle An eroded syncline will have younger beds in middle Outcrop patterns 4 • The Strike of a body of rock is a line representing the intersection of A layer of tilted that feature with the plane of the horizon (always measured perpendicular to the Dip). rock can be • Dip is the angle below the horizontal of a geologic feature. represented with a plane. o 30 The orientation of that plane in space is defined with Strike-and- Dip notation. Maps are two- Geologic Map Showing Topography, Lithology, and dimensional Age of Rock Units in “Map View”.
    [Show full text]
  • Growth and Erosion of Fold-And-Thrust Belts with an Application to the Aconcagua Fold-And-Thrust Belt, Argentina G
    JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, B01410, doi:10.1029/2002JB002282, 2004 Growth and erosion of fold-and-thrust belts with an application to the Aconcagua fold-and-thrust belt, Argentina G. E. Hilley1 and M. R. Strecker Institut fu¨r Geowissenschaften, Universita¨t Potsdam, Potsdam, Germany V. A. Ramos Department de Geologia, Universidad de Buenos Aires, Buenos Aires, Argentina Received 1 November 2002; revised 26 August 2003; accepted 11 September 2003; published 23 January 2004. [1] The development of topography within and erosional removal of material from an orogen exerts a primary control on its structure. We develop a model that describes the temporal development of a frontally accreting, critically growing Coulomb wedge whose topography is largely limited by bedrock fluvial incision. We present general results for arbitrary initial critical wedge geometries and investigate the temporal development of a critical wedge with no initial topography. Increasing rock erodibility and/or precipitation, decreasing mass flux accreting to the wedge front, increasing wedge sole-out depth, decreasing wedge and basal decollement overpressure, and increasing basal decollement friction lead to narrow wedges. Large power law exponent values cause the wedge geometry to quickly reach a condition in which all material accreted to the front of the wedge is removed by erosion. We apply our model to the Aconcagua fold-and-thrust belt in the central Andes of Argentina where wedge development over time is well constrained. We solve for the erosional coefficient K that is required to recreate the field-constrained wedge growth history, and these values are within the range of independently determined values in analogous rock types.
    [Show full text]
  • Controls on Structural Styles and Decoupling in Stratigraphic Sequences
    Manuscript submitted to Journal of Structural Geoloy 1 Controls on structural styles and decoupling in stratigraphic sequences 2 with double décollements during thin-skinned contractional tectonics: 3 insights from numerical modelling 4 Qingfeng Meng*1, David Hodgetts 5 School of Earth and Environmental Sciences, University of Manchester, M13 9PL, UK 6 Abstract 7 Six series of particle-based numerical experiments were performed to simulate thin-skinned 8 contractional tectonics in stratigraphic sequences with double décollements during horizontal 9 shortening. The models were assigned with varying rock competence, depth and thickness of the 10 upper décollement, which resulted in significantly different styles of deformation and decoupling 11 characteristics above and below the upper décollement. The models composed of the least 12 competent material produced distributed sinusoidal detachment folds, with many shallow 13 structures profoundly decoupled from the deep-seated folds. The models composed of a more 14 competent material are dominated by faulted, diapir-cored box folds, with minor disharmonic folds 15 developed in their limbs. Differently, the results of models composed of the most competent 16 material are characterised by localised piggyback thrusts, fault-bend folds and pop-up structures 17 with tensile fractures developed in fold hinges. Depth of the upper décollements also plays an 18 important role in controlling structural decoupling, i.e. the shallower the upper décollements, the 19 higher the degree of decoupling becomes. Thicker upper décollements can provide sufficient 20 mobile materials to fill fold cores, and contribute to the formation of secondary disharmonic folds, 21 helping enhance structural decoupling. Our modelling results are comparable to the structural 1 *Corresponding author.
    [Show full text]
  • Deformation in the Hinge Region of a Chevron Fold, Valley and Ridge Province, Central Pennsylvania
    JournalofStructuraIGeolog3`ko{ ~,.No 2, pp 157tolt, h, 1986 (~IUI-~',I41/gc~$03(10~0(Ki Printed in Oreal Britain ~{: ]t~s¢~Pcrgam-n Press lJd Deformation in the hinge region of a chevron fold, Valley and Ridge Province, central Pennsylvania DAVID K. NARAHARA* and DAVID V. WILTSCHKO% Department of Geological Sciences, The University of Michigan, Ann Arbor, Michigan 481(19, U.S A (Received 27 November 1984: accepted in revised form 18 July 1985) Abstract--The hinge region of an asymmetrical chevron fold in sandstone, taken from the Tuscarora Formation of central Pennsylvania. U.S.A., was studied in detail in an attempt to account for the strain that produced the fold shape. The'fold hinge consists of a medium-grained quartz arenite and was deformed predominantly by brittle fracturing and minor amounts of pressure solution and intracrystalline strain. These fractures include: (1) faults, either minor offsets or major limb thrusts, (2) solitary well-healed quartz veins and (3) fibrous quartz veins which are the result of repeated fracturing and healing of grains. The fractures formed during folding as they are observed to cross-cut the authigenic cement. Deformation lamellae and in a few cases, pressure solution, occurred contemporaneously with folding. The fibrous veins appear to have formed as a result of stretching of one limb: the', cross-cut all other structures. Based upon the spatial relationships between the deformation features, we believe that a neutral surface was present during folding, separating zones of compression and extension along the inner and outer arcs, respectively. Using the strain data from the major faults, the fold can be restored back to an interlimb angle of 157°; however, the extension required for such an angle along the outer arc is much more than was actually measured.
    [Show full text]
  • Fluid History of the Sideling Hill Syncline, Hancock County, Maryland
    FLUID HISTORY OF THE SIDELING HILL SYNCLINE, HANCOCK COUNTY, MARYLAND William J. Lacek A Thesis Submitted to the Graduate College of Bowling Green State University in Partial fulfillment of the requirements for the degree of MASTER OF SCIENCE August 2015 Committee: John Farver, Advisor, Charles Onasch, Margaret Yacobucci ii ABSTRACT John Farver, Advisor Fluid inclusion microthermometry was employed to determine the fluid history of the Sideling Hill syncline in Maryland with respect to its deformation history. The syncline is unique in the region in that it preserves the youngest rocks (Mississippian) in the Valley and Ridge Province and is the easternmost exposure of Mississippian rocks in this portion of the Central Appalachians. Two types of fluid inclusions were prominent in vein quartz: CH4-rich and two-phase aqueous with the former comprising about 60% of the inclusions observed. The presence of the two fluids in inclusions that appear to be coeval indicates that the migrating fluid was a CH4- saturated aqueous brine that was trapped immiscibly as separate CH4-rich and two-phase aqueous inclusions. Cross cutting relations show that at least two generations of veins formed during deformation. Similarities in chemistry of the inclusions in the different vein generations suggests that a single fluid was present during deformation. Older veins were found to have formed at depths of at least 5 km while younger veins formed at minimum depths of 9 km. Overburden for older veins is attributed to emplacement of the North Mountain Thrust (NMT) sheet (~6 km thick). The thickness of the Alleghanian clastic wedge is calculated to be ~2.5 km in the Appalachian Plateau which accounts for most of the remaining overburden in younger veins.
    [Show full text]