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FAULTING AND
FRACTURING
Contemporary interpretation of rock faulting is still strongly
influenced by the work of Coulomb's (1773) as introduced to the geological
literature by Anderson
(1905). Yet, these widely accepted works are far from being complete and I
explored two central issues that remained unsolved. The first issue is
faulting processes under three-dimensional strain. While 3-D strain is the
general situation in the field and experiments, faulting theories use plane-
strain assumptions. I presented the first experimental observations of fault
patterns in granites, sandstone and limestones, deformed under
three-dimensional strain (polyaxial loading). Further, following Taylor (1938), I
derived a model which is based on the assumption that slip along faults
accommodates the applied strain field. The model prediction fit well the
experimental observations and explains some anomalies of fault patterns in
the field. The second issue is fault evolution: nucleation and propagation of
a single fault and the development of fault systems; these topics became the
center of my faulting research during the last ten years. My early studies
included clay cake experiments in which I examined the growth mechanisms of
individual faults and the interaction of adjacent faults. The experiments
indicated new relations between geometry of fault patterns and host material
rheology. I used these results to explain the structure of pull-apart basins
along the Dead Sea transform and the
properties of fragmented rocks. Later, as part of the analysis of triaxial
fault experiments of Westerly granite, I
developed a model for nucleation and propagation of faults in brittle rocks.
The model is based on the interaction among tensile microcracks that develop
in a self-organized pattern. This model predicts that faults should propagate
in their own plane, making an angle of 20-30 degrees with the axis of maximum
compression (in agreement with thousands of field and laboratory
observations). The model provides the first physical explanation for the
'internal friction', the empirical parameter of Coulomb criterion.
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SEISMIC
HAZARD AND NEOTECTONICS
I have studied the seismic and tectonic activity of the last few
millennia along the Dead Sea transform. This
work included paleoseismic analysis of the Jericho
segment during the last 2000 years, Holocene faulting in the northwestern
part of the Dead Sea basin, and Holocene
activity of the Elat segment. The later study included underwater mapping of
active faults by using a submersible. In a second stage, we measured in-situ
stresses around the Dead Sea transform
including determination of the stresses from caliper logs of oil boreholes,
logging with a Televiewer system, and hydrofracturing in geotechnical wells.
The in-situ stress measurements generated some stimuton nature of current
faulting processes in Israel
and adjacent areas. Currently, I am involved in thanalysis of earthquakes in
non-linear viscoelastic crust. We investhe cycle of great earthquakes along
the San Andreas fault for a crust with nonlinear, depth-dependeviscoela.The
mpredictions fit geodetic observations in California and indthe in-situ
viscosity of crustal rocks is 3-6 orders of magnitude less then the viscosity
determined in the laboratory.
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CRUSTAL
STRESS FIELDS
Knowing the crustal stress field is essential for regional and
mechanical structural analyses. My studies of this topic dealt with field
measurements, development of numerical methods and mechanical synthesis. We
determined the first detailed map of the tectonic paleo-stresses in the
Sinai-Israel plate, I have recently measured in-situ stresses around the Dead Sea transform. These studies defined paleo- and
current structural domains in Israel
and refined the regional tectonic history. I developed a new stress inversion
method for accurate determination of the complete stress tensor from fault
slip data and microearthquakes. The method that incorporates friction along
small faults, provides a tool for regional analysis and strength evaluation
of the crust.
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EMPLACEMENT
OF VOLCANIC INTRUSIONS
In recent years I became engaged in studies of dike and laccolith
emplacement. Our field work and analyses departed from the crustal idealized
models of uniform elastic host rocks; they include layered sequences of
sedimentary rocks, depth-dependent distribution of tectonic stresses,
depth-dependence variations of rock rheology, and fractured basement rocks.
These analyses led to new mechanisms for dike segmentation, dike segment
rotations, segment termination and flow directions within dikes. We also
explored the effect of preexisting fractures on dike orientations and we
developed the first model for dike emplacement into 3-D fractured basement.
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MECHANICS
OF ASYMMETRIC FOLDING
Asymmetric folds range in size from small kink-bands through monoclines
to lithosphere bending; structures of all these scales were studiesd by me. I
started with the mechanisms of kinking in well laminated rocks, and showed in
experiments and theory the vital effect of shear stress parallel to layering
on the growth and stability of kink-bands. I then studied monoclines in the
field and experiments and developed a comprehensive model for their
development. The model incorporates three mechanisms: buckling, draping and
kinking. The relative dominance of each mechanism depends on the nature of
the tectonic loading and the rheology of the deformed rocks. This model was
also used in the interpretation of dome above laccoliths, the lithosphere
flexures of the Arabian plate, and the surface deformation associated with
some large recent earthquakes occurred along blind faults in California.
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LOCALIZATION,
NUCLEATION AND GROWTH OF FAULTS
We will explore two general approaches for fault nucleation. One
approach suggests that faults nucleate as a tiny initial nucleus surrouneded
by a small process zone and grows when a specified stress state is exceeded
at fault tips; this approach is similar to Barnblatt-Dugdale concept for
tensile fractures. The second approach, based on distributed damage model,
assumes that faults nucleate when critical mechanical conditions formulated
in terms of rock damage cause damage localization in a narrow zone. Investigating
these processes is the objective of the present work that will explore
several aspects of faulting.
The proposed work integrates field observations and microstructural
analysis of faulted rocks with experimental studies. In the course of the study
we will map fault related deformation in few scales ranging from 1:500 to
50:1. We will evaluate damage using ultrasonic velocities and insitu elastic
modulus values. A series of rock mechanics experiments will establish the
relationship between damage intensity strain history and mechanical
properties. The field and laboratory results will be used to verify models of
fault nucleation by utilizing numerical simulations of damage media.
We chose block of fine grained quartz-syenitic in Gevanim dome intrusions,
Ramon as a suitable site for field work. The block contains thousands of
small and intermediate size faults that are well exposed. These faults were
mapped and the deformation around them was analyzed. We found that the
faulted block is damaged microscopicaly as is evident by lower ultrasonic
velocity, lower elastic rebound and intense microcracking. Additional mapping
of the faults geometry and the microfabric pattern will be conduced in the
course of the study. Rock mechanics experiments will establish the
relationship between damage intensity, strain history and mechanical
properties of Gevanim quarts-syenite. The results of the experimental work
will enable the mapping of rock properties and damage distribution around the
faults. This mapping is the essential tool for theoretical modeling of
faulting mechanisms. We hope that the present new approach, of damage mapping
in the field, will contribute to the understanding of faulting mechanisms.
Graduate student: Katz Oded (Ph.D.)
Research supervision: Ze'ev Reches, Vladimir Lyakhovsky & Gidon Baer
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DYNAMIC
FRACTURING OF ROCKS
Countless fractures appear in the earth's crust. Many of them grow
unstably and at high velocities as evidenced by recent earthquakes and rockbursts.
However, there is no systematic method to deduce the fracture growth velocity
from its appearance in the field. We propose to analyze the mechanisms of
unstable, dynamic fracturing in rocks by focusing on mode I, tensile
fractures (joints and dikes). The analysis includes experimental work, field
mapping of joints and dikes and numerical modeling. The planned work is based
on a recent breakthrough in fracturing experiments by Sharon and Fineberg
(1996). They established the relationships between fracture geometry,
fracture velocity and energy-release rate. Their experiments were with 2-D
plates of uniform, brittle materials, and we intend to expand this analysis
to granular, heterogeneous materials, and to explore the effects of 3-D
structures. We will develop geometric criteria for dynamic fracturing in
rocks in laboratory tests, and will apply these to resolve a set of open
questions related to joints and dikes in the field. Knowing growth velocity
of fractures is important for practical purposes like seismic hazard
evaluation, rockburst risk and geotechnical stability estimates.
Funded by BSF, 1999-2002.
Principal investigators: Reches Ze'ev, Germanovich Leonid and Fineberg
Jay
Graduate student: Sagy Amir (Ph.D.)
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FRACTURE
SYSTEMS ALONG THE DEAD
SEA BASIN
The Dead Sea basin (DSB) is a large pull-apart associated with the
105km of left-lateral slip along the Dead Sea
transform. The DSB is an elongated depression, about 150km long and up to
25km wide. The present structural analysis of the western margins of the DSB
indicates distinct strain partitioning: the horizontal component of slip is
accommodated along a subvertical fault zone inside the basinal depression,
whereas the vertical subsidence is accommodated by a belt of large normal
faults that forms the basin margins.
This belt of faults trends in a N-S direction, it is about 100km long
and 3-5km wide and its cumulative vertical displacement locally exceeds 10km.
We analyzed the relationships between faults and joints of this belt. Joints
were studied in 35 stations in which they were mapped at scale of 1:10 or
measured along scanlines. Subsurface fractures were analyzed on CAST logs in
one borehole. The regional, subvertical joints exhibit two prevailing sets,
NNE and NNW, that display ribs, plumose and branching structures. The
dominafaults ithe belt are oblique-normal with extensive zigzag segmentation.
Four sets of segments in orthorhombsymmetry are recognized here; their
orientations indicatethat they formed under 3D-strain state of vertical
maximum shortening, horizontal N-S shortening and E-W extension.strikes of
the fault sets are parallel to the strikes of the dominating joint sets, and
these relations are explained by modeling the elastic stress fields
associated with layer bending above zigzag fault systems. We also evaluated
in-situ stress state within the belt from breakout analysis and
stress-inversion of fault slip data.
The above structural features are compatible with E-W extension;
however, none of them fits the N-S left-lateral shear of the Dead Sea transform. This strain partitioning is
attributed to the following process: (1) subsidence of a "proto"
DSB due to the pull-apart motion; (2) large scale bending of the margins of
the Sinai-Israel subplate along the "proto" basin; (3) brittle
yielding of the upper crust to develop a belt of normal faults; (4)
continuous left-lateral slip and elongation of the pull-apart basin, and
semicontemporaneous vertical subsidence along the normal fault. We use this
model to evaluate the elastic thickness of the Sinai-Israel plate.
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