Download primary and secondary geological structures

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I.
 Introduction to primary and secondary geological
structures
 Study of geological faults, folds, joints and active
faulting.
 Glinometer compass and its use, Primary and
Secondary structures, Dip and strike and their
representation
 Folds, faults, joints and unconformitiestheir
description, classification, recognition in the field and
effects on outstrop patterns.
 Criteria for the recognition of the top and bottom of
sedimentary strata.
 Preliminary knowledge of foliation, Schistosity and
lineatio elementary ideas about geosyncline
progenesis, isostasy, continental drift, Island arcs,
Structural features of India
Primary and secondary structures.
Primary: depositional contacts, cross-bedding, ripple markes,
Ropy textures in lavas, mud-cracks;
Secondary: these are the real structures we’re after- and
are due to rock regional deformation. Examples:
Folds, faults, shear zones, etc.
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Folds in Rock
A fold is a bent structure that originally was
planar, such as a sedimentary bed. Folds may
be produced by either horizontal compression
or vertical forces in the crust, just as pushing
in on opposite sides of a paper or up from
below.
Terms used to describe the parts of a fold:
limb: The two sides of a fold are called limbs.
axis: A line drawn along the points of maximum
curvature of a layer of a fold. More strictly, it is
called hinge line.
axial plane: an imaginary plane surface that
divides a fold as symmetrically as possible.
plunge: If the fold axis is not horizontal, the
angle of the axis with the horizontal plane is
called plunge.
 Terms describing a fold. (Tarbuck and Lutgents)
 Types of folds
 anticline: upfolds or arches of rock layers
 syncline: downfolds or troughs of rock layers.
 monocline: only one direction of dip prevails in a fold
system.
 symmetrical fold: the axial plane is vertical with the
limbs dipping symmetrically from the axis
 asymmetrical fold: the axial plane is tilted from the
vertical with one limb dipping more steeply than the
other.
 overturned fold: one limb is tilted beyond the vertical
 recumbent fold: this is an overturned fold "lying on
its side" so that the axial plane is nearly horizontal.
 An illustration of principle types of folds.
 Anticlines and synclines in the Calico Hills near Barstow,
California. (Hamblin and Christiansen)
 Folds of deformed sedimentary strata near Palmdale,
California. The dashed-line indicates a fault also
present. (E.J. Tarbuck)
 (Top) The San Rafael monocline, Utah. (S. Trimble)
(Bottom) Illustration of monocline consisting of bent
sedimentary beds caused by faulting in the bedrock
below.
 A recumbent fold in Precambrian rocks of the Umanak
area, Greenland. (T.C.R. Pulvertaft)
 Outcrop (map) view of folds
 Anticline: the oldest beds are in the center and the beds
become progressively younger in each direction.
Synclines: the youngest bed is in the center and the beds
get progressively older in each direction.
 Symmetrical folds have equal bed widths on opposite
sides of the axial plane, but asymmetrical folds will have
different bed widths on the opposite sides.
 For a plunging anticline, the nose (formed by the
intersection of the fold system with a horizontal plane)
points in the same direction as the plunge. For a plunging
syncline, the nose points in the direction opposite to that
of the plunge.
 The diagram shows the surface of eroded remnants of a
syncline and the characteristic core of younger rocks
flanked on both sides by older rocks dipping toward the
core. (Press and Siever)
 Anticlines and synclines. The numbers 1 through 6
indicate strata of progressively younger ages. (West, p.203)
 Symmetrical and asymmetrical anticlines and synclines.
Symmetrical folds have equal bed widths on opposite
sides of the axial plane, but asymmetrical folds will
have different bed widths on the opposite sides. (West,
p.203)
 Plunging folds. Note the nose of a plunging anticline in
outcrop points in the direction of the plunge, while the
opposite is true of plunging synclines. (Tarbuck and
Lutgents)
 A plunging anticline forms a a V-shape pattern pointing
in the direction of plunge. This example is from near St.
George, Utah. (Hamblin and Christiansen)
 Rock fractures: Joints and Faults
 Joints
 A joint is a crack along which no appreciable movement
has occurred.
 Most joints are produced when rocks are deformed by
tectonic forces, with some exceptions.
 Eroded joints, Arches National Park, Utah. (W. Clay)
 Devil’s Tower, Wyoming. The columnar joints form
when igneous rocks cool and develop shrinkage
fractures producing elongated columns.
(Thomason/Stone Images)
 Joints in granitic rocks near the top of Lembert Dome, Yosemite
National Park. The joints were enhanced by weathering. (E.J. Tarbuck)
 Faults
 A fault is a fracture with relative movement of the
rocks on both sides of it, parallel to the fracture.
 Fault terminology:
strike, dip of fault plane, hanging wall, footwall.
A fresh fault scarp after an earthquake in Nevada. (S. Marshak)
(a)
(b)
(a) Slip lineations on a fault surface. (b) Breccia, broken-up rocks along
this fault. (S. Marshak)
 Types of faults:
 dip-slip fault: normal fault, reverse fault, thrust
fault
 strike-slip fault: left lateral, right lateral
 oblique-slip fault: has both strike-slip and dip-slip
component. Note: The textbook calls it "translation
fault", which is rarely used.
 Types of faults. a) Normal faults, caused by tensional forces, result
in extension. b) Reverse faults, caused by compressional forces,
result in shortening. c) Strike-slip faults associated with shearing
forces. d) Oblique slip suggests a combination of shearing and
compression/tension. (Press and Siever)
 A normal fault. (Tarbuck and Lutgents)
 A small normal fault along the road to Kolob
Terraces just north of Toquerville, Utah.
 Normal faulting in the Basin and Range Province.
Tensional stresses elongated and fractured the crust into
numerous blocks. Movement along the fractures tilted the
blocks producing parallel mountain ranges. (Tarbuck and
Lutgents)
 The relative movement of a reverse fault
 The Keystone thrust fault of southern Nevada. Dark-colored
limestone (Cambrian) has been thrust over light-colored Jurassic
sandstone, younger by some 350 million years. (J.S. Shelton)
 Diagram for a strike-slip fault (right-lateral). Note how
the stream channels have been offset by fault
movement. (after R.L. Wesson et al.)
 Strike-slip faults are commonly expressed by a series of straight linear
ridges and troughs that can be traced for long distances. Here the San
Andreas fault in southern California offsets a drainage system.
 the geologic and seismologic challenge is to determine whether
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such faults are likely to rupture during the life of the
structure(i.e., are they "active"?) and, if so, with what
displacements, with what geometries, with what magnitudes,
and with what likelihoods
Over the years, many authors have discussed various definitions
of "active" and "inactive"
faults. Suffice it is to say that modern geologic studies, together
with vastly improved age-dating capabilities,
have demonstrated unequivocally that there are all degrees of
fault activity, and any categorization into active
and inactive features is necessarily arbitrary. In worldwide damdesign practice, repeat times on faults of
significant earthquakes of a few thousand years, or a few tens of
thousands of years, are often used to distinguish
between faults one wishes to worry about and those of no
concern.