ArticlePDF Available

Abstract

Reactivation of existing faults whose normal lies in the σ1′σ3′ plane of a stress field with effective principal compressive stresses σ1′ >σ2′ >σ3′ is considered for the simplest frictional failure criterion, τ = μσn′ = μ(σn − P), where τ and σn are respectively the shear and normal stresses to the existing fault, P is the fluid pressure and μ is the static friction. For a plane oriented at θ to σ1′, the stress ratio for reactivation is (σ1′/σ3′) = (1 + μ cot θ)/(1 − μ tan θ). This ratio has a minimum positive value at the optimum angle for reactivation given by (1/μ) but reaches infinity when θ = 2θ∗, beyond which σ3′ < 0 is a necessary condition for reactivation. An important consequence is that for typical rock friction coefficients, it is unlikely that normal faults will be reactivated as high-angle reverse faults or thrusts as low-angle normal faults, unless the effective least principal stress is tensile.
Journal of Structural Geology.
Vol. 7. No. 6, pp. 751 to 754, 1985 0191-8141/85 $03.00 + 0.00
Printed in Great Britain © 1985 Pergamon Press Ltd.
Brevia
SHORT NOTES
A note on fault reactivation
RICHARD H. SIBSON
Department of Geological Sciences, University of California, Santa Barbara, California 93106, U.S.A.
(Received
13
November
1984;
accepted in revised form
22
January
1985)
Abstract--Reactivation of existing faults whose normal lies in the ~ri~r ~ plane of a stress field with effective
principal compressive stresses cr'~ > or;_ > cr~ is considered for the simplest frictional failure criterion, r = ~cr', =
tz(~r, - P), where ~- and or, are respectively the shear and normal stresses to the existing fault, P is the fluid pressure
and # is the static friction. For a plane oriented at 0 to gl, the stress ratio for reactivation is (crl/cr~) =
(1 + ~ cot 0)/(1 - ~ tan 0). This ratio has a minimum positive value at the optimum angle for reactivation given
by 0* = ½ tan -h (1/~) but reaches infinity when 0 = 20", beyond which cr~ < 0 is a necessary condition for
reactivation, An important consequence is that for typical rock friction coefficients, it is unlikely that normal
faults will be reactivated as high-angle reverse faults or thrusts as low-angle normal faults, unless the effective
least principal stress is tensile.
IT IS now widely recognized that much intracontinental
deformation within the frictional seismogenic regime,
which commonly extends to depths of 10-15 km (Sibson
1983), is accommodated by the reactivation of existing
discontinuities rather than by the creation of new faults
(McKenzie 1972, Sykes 1978). This is especially true of
collision belts. Given our general lack of quantitative
knowledge concerning the triaxial stress state at depth,
full three-dimensional analysis of the conditions for
frictional reactivation (Bott 1959, McKenzie 1969,
Jaeger & Cook 1979) is rarely practicable, though it has
been done successfully on occasion (Raleigh
et al.
1972).
However, in view of recent suggestions that many high-
angle reverse faults and low-angle normal faults have
developed by reactivation of normal and thrust faults
respectively (e.g. Jackson 1980, Winslow 1981, Brun &
Choukroune 1983, Smith & Bruhn 1984), it is instructive
to consider certain limitations imposed by the simplest
two-dimensional analysis for frictional reactivation of a
cohesionless fault.
CONDITIONS FOR REACTIVATION
Consider a triaxial stress state with principal compres-
sive stresses 0.~ > 0" 2 > 0"3 containing a cohesionless plane
lying at an angle, 0, to 0.t, with its normal contained in
the 0.10.3 plane (Fig. 1). If a fluid pressure, P, is present,
the effective principal stresses (Hubbert & Rubey 1959)
are
0.] = (0"1 - P) > 0"~ = (ere - P) > 0.~ = (0" 3 - P).
(1)
Byerlee (1978) has shown that nearly all rocks share the
same frictional properties with a failure criterion which
may be adequately approximated by Amonton's Law
r =/~.o-~ =/z(o-, - P), (2)
where r and 0.. are, respectively, the shear and normal
stresses to the plane, and the coefficient of friction, tz is
c. 0.75 (Sibson 1983).
In terms of the effective principal stresses, equation
(2) may be rewritten
(0.~ - o'i) sin 20 =/z[(0.~ + o-i) - (0.~ - o'i) cos 20] (3)
which reduces to
R = (0.{/o'i) = (1 +/z cot 0)/(1 -/x tan 0). (4)
15
R
lO
5
=5
-lO
~u : 0,75
[
I
I
r , , I I i~ i , ,__J
e'3o 21
"3 x
Fig. 1. Stress ratio required for frictional reactivation, R = (or'i/cry),
vs reactivation angle, 0, for a static frictional coefficient, ~z = 0.75,
751
752 R.H. SmsoN
9O
6O
i , I
I
)
0 , , , , I I 'I' i
R* 4
3
2
1
I I I I I ! I I I
0 0.5 1.0
Xl
Fig. 2. Variation of optimum reactivation angle, O*, 20" and minimum
positive stress ratio for reactivation, R*, with frictional coefficient,/z.
The stress ratio for reactivation, R, is plotted against 0
for the particular case of tz = 0.75 in Fig. 1. R has a
minimum positive value,
R* = (V1 + U -~ + /z) 2 (5)
at the optimum angle for frictional reactivation given by
0* = ½ tan -I (1//z) (Sibson 1974), but increases to infinity
for 0 = 0 and 0 = 20*. For/z = 0.75, 0* = 26.5 ° with R*
= 4, and 20* = 53 °. For 0 > 20*, R < 0 which requires o-~
< 0, that is, the effective least principal stress must be
tensile. Values of R*, O* and 20* corresponding to other
values of/z are plotted in Fig. 2.
A further limitation on allowable stress states for
frictional reactivation is that they must not induce failure
of the surrounding rock either in shear or in tension. In
Fig. 3, a composite failure envelope for intact rock is
plotted together with the envelope for frictional failure
in a series of Mohr diagrams illustrating the range of
allowable stress states for reactivation. Following Brace
(1960), the failure envelope for intact rock is taken to be
approximately of parabolic Griffith form in the tensile
field with a cohesive strength, C, equal to twice the
tensile strength, To. In the compressional field the
envelope is assumed to be of the linear Coulomb form,
r = C +/zi~r',. (6)
T
?
T 0
C
/
r
o (;~
NON-OPTIMUM O( e( • e
R°( R<•
T
?
r o
o.'
NON-OPTIMUM e'~ e~ 2e t
R*(
R~-
T
r o or.'
NON-OPTIMUM 2et~ e~ 90" -m ~ R( 0
OPTIMUM @" ~' R - R*
T
?
r
o o'~
NON-OPTIMUM -2e ~ R= w
T
r
o 0".'
NON-OPTIMUM e-tgo" R--~O
Fig. 3. Allowable stress states for frictional reactivation of an existing fault (see text for discussion).
Fault reactivation 753
Hoek (1965) found that the coefficient of internal fric-
tion,/z i generally lies between 0.5 and 1.0 for rocks, so
that for convenience the failure envelopes for intact rock
and fault reactivation in the compressional field are
plotted parallel with/x~ =/~ = 0.75.
The optimum condition for reactivation with R = R*
and 0 = 0* is shown in Fig. 3(b) with the stress circle
touching the frictional failure envelope. Note that
here may have any value greater than zero for reactiva-
tion to be possible. Stress conditions for reactivation
with 0 < 0 < 0* and 0* < 0 < 20* are shown in Figs. 3(a)
and (c), respectively. Clearly, the diameter of the stress
circle is constrained by the presence of the intact rock
envelope to be not too great, so that cr~ --, 0 as 0 trends
towards 0 or 20*. At 0 = 20", R = ~, requiring tr~ = 0
(Fig. 3d). Reactivation at 0 > 20* requires cr~ < 0 with a
progressively diminishing differential stress as 0--~ 90 ° in
order to prevent failure of the intact rock (Figs. 3e & f).
DISCUSSION
In the framework of simple 'Andersonian' faulting
(Anderson 1951), principal stress trajectories are either
vertical or horizontal and the three main classes of fault,
thrust, wrench and normal, develop in homogeneous
crust in accordance with the Coulomb criterion (eqn. 6),
depending on which of the three principal stresses is
vertical. For typical values of internal friction, faults
develop at c. 30 ° to or I . Thus, ideal normal faults should
dip at c. 60 ° and thrusts at c. 30 °. If or 1 and tr 3 are
interchanged, as may occur if a former rifted continental
margin becomes involved in continental collision, or
when a former thrust belt is caught up in a zone of
distension, normal faults may potentially be reactivated
as high-angle reverse faults, and thrusts as low-angle
normal faults. However, in both cases the 0 angle for
reactivation is c. 60 ° if the stress trajectories remain
horizontal and vertical. Such a high reactivation angle
requires either a friction coefficient,/x < 0.55 (Fig. 2),
significantly lower than the usual value of 0.75, or o'~
must be tensile. In fact, Bruhn
et al.
(1982) have
demonstrated reactivation of gently dipping joints as
low-angle normal faults with 0 values of 70-80 °, implying
either ~ < 0.35 or cr~ < 0. Vein systems associated with a
normal fault reactivated in high-angle reverse mode in
North Wales unequivocally demonstrate o-~ < 0 during
reactivation (Sibson 1981).
Even if the conditions described above are not met
fully, it is apparent from Fig. 3(c) that for frictional
reactivation to occur at large 0 values in preference to
the formation of a new, favourably oriented fault, o'~
must tend towards zero, implying abnormal fluid pres-
sure conditions and comparatively low differential stress
levels at the time of reactivation. Clearly, however,
listric normal faults would be more easily reactivated in
reverse mode than the ideal 'Andersonian' variant. In
contrast, flattening of thrusts with depth tends to exacer-
bate the problem if they are to undergo frictional reacti-
vation in normal slip mode. The problem is acute also in
the case of the brittle, flat-lying detachment faults
associated with regional extension in the western United
States which, in some cases at least, appear to have been
active with dips of only a few degrees (Davis
et alo
1980).
For reactivation of such faults to occur, either fluid
pressures must be high with ~ < 0, at least intermit-
tently, or the frictional coefficient must be abnormally
low, or stress trajectories must deviate markedly from
the horizontal and vertical. When fault reactivation at
high 0 values is suspected, evidence in the form of
fault-related hydraulic extension fractures (e.g. Sibson
1981) should be sought in support of the hypothesis for
an effectively tensile least principal compressive stress
accompanying reactivation.
Acknowledgements--Special thanks to Barbara John and Bill Power
for steadily eroding my air of detachment regarding low-angle normal
faults, to Art Sylvester for constructive criticism, and to Mrs. Ellie
Dzuro for typing the manuscript. This work was supported by National
Science Foundation grant number EAR83-05876.
REFERENCES
Anderson, E. M. 1951. The Dynamics of Faulting. (2nd Edn) Oliver &
Boyd, Edinburgh.
Bott, M. H. P. 1959. The mechanics of oblique-slip faulting. Geol.
Mag. 96,109-117.
Brace, W. F. 1960. An extension of the Griffith theory of fracture to
rocks. J. geophys. Res. 65, 3477-3480.
Bruhn, R. L., Yusas, M. R. & Huertas, F. 1982. Mechanics of
low-angle normal faulting: an example from Roosevelt Hot Springs
geothermal area, Utah. Tectonophysics 86,343-361.
Brun, J-P. & Choukroune, P. 1983. Normal faulting, block tilting and
d6collement in a stretched crust. Tectonics 2,345-356.
Byerlee, J. D. 1978. Friction of rocks, Pure appl. Geophys. 116,
615-626.
Davis, G. A., Anderson, J. L., Frost, E. G. & Shackelford, T. J. 1980.
Mylonitization and detachment faulting in the Whipple-Buckskin-
Rawhide Mountains terrane, south-eastern California and western
Arizona. In: Cordilleran Metamorphic Core Complexes (edited by
Crittenden, M. D., Coney, P. J. & Davis, G. H.). Mere. geol. Soc.
Am. 153, 79-129.
Hock, E. 1965. Rock fracture under static stress conditions. Natl.
Mech. Engng Res. Inst., C.S.1.R., Pretoria, Report MEG 383.
Hubbert, M. K. & Rubey, W. W. 1959. Role of fluid pressure in the
mechanics of overthrust faulting. Bull. geol. Soc. Am. 70, 115-205~
Jackson, J. A. 1980. Reactivation of basement faults and crustal
shortening in orogenic belts. Nature, Lond. 283,343-346.
Jaeger, J. C. & Cook, N. G. W. 1979. Fundamentals of Rock
Mechanics. (3rd Edn) Chapman & Hall, London.
McKenzie, D. P. 1969. The relation between fault plane solutions for
earthquakes and the directions of the principal stresses. Bull. seism.
Soc. Am. 59,591-601.
McKenzie, D. P. 1972. Active tectonics of the Mediterranean region.
Geophys. J. R. astr. Soc. 30,109-185.
Raleigh, C. B., Healy, J. H. & Bredehoeft, J, D. 1972. Faulting and
crustal stress at Rangely, Colorado. Mon. Am. Geophys. Union 16,
275-284.
Sibson, R. H. 1974. Frictional constraints on thrust, wrench and
normal faults. Nature, Lond. 249,542-544.
Sibson, R. H. 1981. Fluid flow accompanying faulting: field evidence
and models. In: Earthquake Prediction: an International Review
(edited by Simpson, D. W. & Richards, P. G.). Am. Geophys.
Union, Maurice Ewing Series 4, 593-603.
Sibson, R. H. 1983. Continental fault structure and the shallow
earthquake source. J. geol. Soc. Lond. 140,741-767.
754 R.H. SIBSON
Smith, R. B. & Bruhn, R. L. 1984. Intraplate extensional tectonics of
the eastern Basin-Range: inferences on structural style from seismic
reflection data, regional tectonics, and thermal-mechanical models
of brittle-ductile deformation. J. geophys. Res. 89, 5733-5762.
Sykes, L. R. 1978. Intraplate seismicity, reactivation of preexisting
zones of weakness, alkaline magmatism, and other tectonism post-
dating continental fragmentation. Rev. Geophys. Space Phys. 16.
621-687,
Winslow, M. A. 1981. Mechanism for basement shortening in the
Andean foreland fold belt of southern South America. In: Thrust
and Nappe Tectonics (edited by McClay, K. & Price, N. J.). Spec.
Publs geol. Soc. Lond. 9,513-528.
... Assuming adequate pore uid pressures, and necessary cohesion and shear strength values 33,34 , fractures optimally oriented 30° to the maximum principal stress (σ 1 ) and containing the intermediate principal stress (σ 2 ) should reactivate as shear fractures, while those oriented ~ 0° to σ 1 (and containing σ 2 ) should reactivate as tensile fractures (with fractures between these two endmember situations reactivating as hybrid fractures) 34,35 . ...
... Assuming adequate pore uid pressures, and necessary cohesion and shear strength values 33,34 , fractures optimally oriented 30° to the maximum principal stress (σ 1 ) and containing the intermediate principal stress (σ 2 ) should reactivate as shear fractures, while those oriented ~ 0° to σ 1 (and containing σ 2 ) should reactivate as tensile fractures (with fractures between these two endmember situations reactivating as hybrid fractures) 34,35 . ...
Preprint
Full-text available
Fractured rock petrophysical studies rarely use temporal constraints, thus hindering fracture-related permeability (K) and connectivity (C B ) estimation during past geofluid migration. Presenting a new conceptual approach, we use a stochastic method incorporating absolute ages to reconstruct fracture arrays back in geological time. Generating ‘grown’ discrete fracture network (DFN) models, we simulate the hydraulic behaviour of fractured rock volumes from the late-Cretaceous/Palaeocene to the Devonian, via progressive fracture back-stripping. Our study reveals that maximum bulk permeability (K 1 ) increased through time from 9.47e-14 m ² to 4.44e-13 m ² (~ 3 orders of magnitude) along with the maximum horizontal permeability orientation (K Hmax ) shifting from NE-SW to NW-SE. Similarly, C B increased (1.01 to 1.79) with fracture saturation, peaking in the mid-Cretaceous. Validating our method, timing of dated offshore hydrocarbon activity fit our results. Back-stripping of time-constrained fracture sets is therefore a powerful method to investigate and quantify the dynamic evolution of petrophysical properties through geological time.
... Conversely, there is inadequate time for asperities to deform and accommodate stress changes at higher unloading rates, leading to a rapid ε 3 increase and a relatively small Δ 3 , with a low proportion of inactive AE duration in the entire unloading stage. A generic description of unloading-induced fault activation by single weak plane theory (Jaeger 1959;Sibson 1985) is schematically shown in Fig. 15a. The normal stress σ n and shear stress τ n on the original macro fracture are expressed as follows: ...
Article
Full-text available
Fault-slip rockburst in deep mining and hard rock tunneling is primarily induced by fault activation. In this study, the evolutionary process of the unloading-induced fault activation was analyzed by true triaxial unloading tests on pre-fractured specimens, and the influence of the unloading rate of σ3 (minimum principal stress) and initial σ3 value on fault activation was investigated. Intact specimens were also tested for comparison. The activation process was studied with the aid of acoustic emission and deformation monitoring. Our results indicate that asperities on original macro fracture have sufficient time to degrade gradually at a low unloading rate, and a higher unloading rate usually leads to a smaller unloading extent of σ3 at the moment of fault activation, resulting in less deformation (Δε3\Delta \varepsilon_{3}) in the direction of σ3. Moreover, the evolution of acoustic emission (AE) hit rate was divided into inactive phase and rapid increase phase during the unloading process, and both the duration of the rapid increase phase and the proportion of rapid increase duration to whole unloading duration decrease with increasing unloading rate. The initial σ3 value is inversely related to the unloading extent of σ3 and Δε3\Delta \varepsilon_{3}, while it is proportional to the post-peak stress drop and the amount of gouge within the post-failure specimens, which indicates that a larger initial σ3 intensifies the damage of the fault. Our results also show that the σ3 value at which fault activation occurs can be predicted by the single weak plane theory during the unloading of σ3. The time-delayed failure observed in three intact and one pre-fractured specimen shows that time-delayed rockburst in deep tunnels can be induced by the unloading of σ3. The strength for pre-fractured specimens in both loading and unloading tests is well fit by the Mogi-Coulomb strength criterion, and the cohesion is smaller while the friction is greater in the unloading tests than in loading-induced fault activation. The findings contribute to a better understanding on the unloading- induced fault-slip process and the mechanism of slip rockburst.
... Polyphase faulting and fault reactivation occur pervasively on the earth, especially in ore fields that have undergone intense hydrothermal activity (e.g., Sibson, 1985;Cox, 2005;Pearce et al., 2020). Fluid overpressure induced by hydrothermal activity results in the perturbation of stress fields (Currenti et al., 2017;Sibson, 2017), the reactivation of pre-existing faults (Leclère and Calais, 2019;Huang et al., 2021), and the formation of new-born faults (Chiarabba et al., 2020;Mazzini et al., 2021). ...
Article
Polyphase faulting and fault reactivation are liable to occur in hydrothermal ore fields. However, the relationship between fault reactivation and mineralization remains ambiguous, particularly in the NE-SW-striking Taoshan-Zhuguangshan uranium metallogenic belt, South China Block. Field investigations, core observations, and paleostress inversions were carried out for the Lujing and Gulonggang hydrothermal ore fields. Field investigations and previous exploration profiles show that the main ore-controlling structures are the NE-SW-striking high-angle faults. The maximum density of poles shows that these faults dip mainly to SE at high angles (70-88°). Kinematic indicators and plunges of slickenlines indicate that these NE-SW-striking high-angle faults have undergone polyphase faulting and fault reactivation, including left-lateral strike-slip shearing, normal dip-slip motion, and right-lateral strike-slip shearing. Dip angles of faults (n=667) and quartz veinlets (n=407) in sub-vertical drill cores vary mainly from 30 to 65°and 60 to 70°for the Lujing ore field and from 20 to 50°and 25 to 85°for the Gulonggang ore field, respectively. The difference suggests that hydrothermal fluids migrated preferentially along the high-angle secondary fractures of the main faults. Paleostress inversions of fault-slip data (n=375) revealed six stages of paleostress regimes, including 1) the early to middle Early Cretaceous extensional regime (subhorizontal NW-SE-trending σ3), 2) the late Early Cretaceous strike-slip regime (subhorizontal NNW-SSE-trending σ1), 3) the latest Early Cretaceous to early Late Cretaceous extensional regime (subhorizontal NW-SE-trending σ3), 4) the latest Cretaceous strike-slip regime (subhorizontal WNW-SEE-trending σ1), 5) the early Eocene strike-slip regime (subhorizontal NE-SW-trending σ1), and 6) the middle to late Eocene extensional regime (subhorizontal NE-SW-trending σ3). Field investigations, core observations, and previous metallogenic ages show that uranium mineralization in the Taoshan-Zhuguangshan uranium metallogenic belt occurred mainly along NE-SW-striking high-angle normal faults under the latest Early Cretaceous to early Late Cretaceous NW-SE extension. These high-angle normal faults were not new-born but reactivated from left-lateral strike-slip faults formed in the late Early Cretaceous strike-slip regime. NE-SW-striking polyphase faulting and fault reactivation during the Cretaceous were triggered by the subduction of the Paleo-Pacific Plate underneath the South China Block.
... These studies have revealed that preexisting faults, as one type of preexisting structures, are commonly reactivated in conjunction with other weakening mechanisms during new boundary dynamics. These include localized fluid pressure (Sibson, 1985;Collettini et al., 2006;Clemenzi et al., 2015) and the presence of phyllosilicates along the fault plane (Collettini et al., 2009;Bolognesi and Bistacchi, 2016). Therefore, investigating these preexisting faults is essential for enhancing our understanding of the basin evolution and crustal deformation process. ...
... If σ 1 is vertical we are in a normal faulting regime, while a vertical σ 2 or σ 3 represents a strike-slip or reverse faulting regime, respectively. If all principal directions are off the vertical by more than 10°, we are in a non-Andersonian regime (Hafner, 1951;Sibson, 1985;Yin and Ranalli, 1992). ...
Article
Full-text available
This study employs numerical simulations based on the limit analysis (LA) method to calculate the stress distribution in a model that includes a basal detachment, featuring the lateral termination of a generic fault under compression. We conduct 2500 2D and 500 3D simulations with varying basement and fault friction angles to analyze and classify the results into clusters representing similar failure patterns to understand the stress fields. Automatic fault detection methods are employed to identify the number and positions of fault lines in 2D and fault surfaces in 3D. Clustering approaches are utilized to group the models based on the detected failure patterns. For the 2D models, the analysis reveals three primary clusters and five transitional ones, qualitatively consistent with the critical Coulomb wedge theory and the influence of inherited structural and geometric aspects over rupture localization. In the 3D models, four different clusters portray the lateral prolongation of the inherited fault. High stress magnitudes are detected between the compressive boundary and the activated or created faults and at the root of the inherited active fault. Tension zones appear near the outcropping surface relief, while stress decreases with depth at the footwall of the created back thrusts. A statistical cluster-based stress field analysis indicates that for a given cluster, the stress field mainly conserves the same orientations, while the magnitude varies with changes in friction angles and compressive field intensity, except in failure zones where variations are sparse. Small parametric variations could lead to significantly different stress fields, while larger deviations might result in similar configurations. The comparison between 2D and 3D models shows the importance of lateral stresses and their influence on rupture patterns, distinguishing between 3D analysis and 2D cross-sections. Lastly, despite using small-scale models, stress field variations over a span of a couple of kilometers are quite large.
... We therefore consider a case of pre-existing faults of a certain orientation in relatively intact host rocks. Given the maximum and minimum effective principal stresses σ′ 1 and σ′ 3 , respectively, a condition for activation of a pre-existing fault is (Sibson, 1985): Figure 6. Admissible orientations of fault planes of the reverse polarity events under the assumption of a homogeneous stress. ...
Article
Full-text available
We apply the Matrix Profile algorithm to 100 days of continuous data starting 10 days before the 2019 M 6.4 and M 7.1 Ridgecrest earthquakes from borehole seismic station B921 near the Ridgecrest aftershock sequence. We identify many examples of reversely polarized waveforms, but focus on one particularly striking earthquake pair with strongly negatively correlated P and S waveforms at B921 and several other nearby stations. Waveform‐cross‐correlation‐based relocation of these events indicates they are at about 10 km depth and separated by only 115 m. Individual focal mechanisms are poorly resolved for these events because of the limited number of recording stations with unambiguous P polarities. However, relative P and S polarity and amplitude information can be used to constrain the likely difference in fault plane orientation between the two events to be 5–20°. We explore possible models to explain these observations, including low effective coefficients of fault friction and short‐wavelength stress heterogeneity caused by prior earthquakes. Although definitive conclusions are lacking, we favor local stress heterogeneity as being more consistent with other observations for the Ridgecrest region.
... Whereas the exact mechanisms conducting to such a transition are still debated (e.g., Gapais et al., 2021), we propose that the passage from pure compressive to transcurrent regimes is a direct response of crustal thickening. Under normal Andersonian conditions (σ vertical = σ 1 , σ 2 or σ 3 ), the development of both sub-horizontal folds (upright fold with horizontal axis) and the quartz-tourmaline vein system as observed at Boulanger deposit require specific stress conditions in which the minimum principal stress, σ 3 , is vertical (e.g., Anderson, 1951;Sibson, 1985). Assuming that the vertical stress is exclusively controlled by the thickness of the overburden rock sequence, σ 3 would significantly increase during D 1 crustal thickening event by the development of tight folds and penetrative S 1/0 . ...
Article
Throughout an earthquake sequence, perturbations to the stress field may lead to changes in the distribution of active fault orientations. Resolving such changes for narrow spatiotemporal windows requires high-quality seismicity catalogs and objective techniques to measure fault orientations. We investigate the evolution of fault orientations throughout the 2019 Ridgecrest earthquake sequence using a new seismicity catalog that captures the sequence over several years. We generate this catalog using a state-of-the-art workflow for event detection, absolute and relative relocation, and moment tensor inversion. With this catalog, we measure high resolution, time-dependent changes in the orientations of active faults using a technique from spatial statistics that quantifies anisotropic features in point processes. We evaluate the results alongside those of more standard techniques based on focal mechanisms. Near the centroid of the mainshock, we observe a substantial shift in the distribution of fault orientations, whereas to the south of the mainshock centroid, we observe only a moderate transient change in the distribution of fault orientations. Compared with results derived from focal mechanisms alone, our findings suggest a smaller background differential stress and a distinct response of the stress state to postseismic deformation.
Article
Fault reactivation of bedrock structures in active fault zones influences stress state and earthquake rupture phenomena through the introduction of weak slip surfaces that impact fault zone geometry and width. Yet, geometric relationships between modern faults and older reactivated faults are difficult to quantify in rocks that have experienced multiple deformation episodes. We used new geologic mapping, geomorphic tools, and structural modeling to quantify rock uplift and subsurface fault geometry of the central part of the Maacama Fault Zone near Ukiah, California, USA, and the surrounding area. Results suggest that the northern Mayacamas Mountains are in a tectonically driven disequilibrium, with differential rock uplift focused on the western side of the range. Steeply east-dipping fault surfaces and splays characterize the geometry of the Maacama Fault Zone. We mapped two newly identified faults to the east of the main Maacama Fault, the Cow Mountain–Mill Creek Fault, and Willow Creek Fault, which align with a moderately east-dipping cluster of microseismicity between 4–10 km depth beneath the Mayacamas Mountains. Static stress modeling on the Maacama Fault Zone and newly identified faults to the east quantify slip tendency values of 0.5–0.4, which suggests that the faults are moderately to poorly suited for slip in the modern stress field and may be weak. We infer that modern uplift is driven by oblique reverse, up-to-the-east, dip-slip motion on the reactivated Cenozoic Cow Mountain–Mill Creek and Willow Creek Faults as material is advected through a restraining bend on the Maacama Fault. This study shows that reactivated bedrock faults increase the fault zone width and introduce fault surfaces that contribute a component of vertical deformation and uplift in major strike-slip fault zones. Deformation is accommodated on an interconnected network of new and reactivated faults that delineate a complex seismic hazard.
Article
Full-text available
Plate boundaries in continental crust are generally less sharply defined than in the oceans, with seismicity spread over broad areas. Interplate displacements appear to be largely accommodated by networks of major fault zones. A simple 2-level model for these important structures accounts for the depth distribution of most continental earthquakes, and for the observed range of faulting styles and associated rock deformation textures. The model consists of a seismogenic frictional slip regime overlying quasi-plastic mylonite belts wherein shearing is largely accommodated aseismically, due mainly to the changing response of quartz to deformation with increasing temperature. Shear resistance increases with depth to a peak value in the vicinity of the frictionaUquasi-plastic transition and then decreases rapidly. The depth to which microseismic activity extends appears inversely related to regional heat flow and can be satisfactorily modelled as the frictionaliquasi-plastic transition for different geotherms using laboratory determined flow laws for quartz-bearing rocks. Larger earthquake ruptures (M > 5.5) tend to nucleate near the base of the seismogenic regime in the region inferred to have the highest shear resistance and concentration of distortional strain energy. Consideration is also given to the depression of isotherms and seismic activity in regions of thrusting, and to the question of the downward continuation of major fault zones through the lithosphere. Decoupling of the upper crust on flat-lying shear zones may accompany higher-level dip-slip (and perhaps in some circumstances, strike-slip) faulting, being favoured by above average continental heat flow and a high quartz content in the middle or deep crust. The average level of deviatoric stress within the seismogenic regime remains an outstanding problem.
Article
Full-text available
Direct evidence that channel flow of aqueous fluids accompanies shallow crustal faulting, in some instances at least, comes from the observation of transitory surface effusions following some moderate to large earthquakes in consolidated rocks, and the textural characteristics of the hydrothermal vein systems often found associated with ancient, exhumed faults. These phenomena are examined in relation to two alternative models. In the first, the transitory post-seismic flow results from the collapse of pre-failure dilatant fractures in accordance with the dilatancy/fluid-diffusion hypothesis, so that the fault system can be regarded as a 'pump'. In the second model, the fault/fracture system functions as a 'valve' on a fluid reservoir. -Author
Article
Full-text available
In Rangely, Colorado, the magnitudes and directions of the principal stresses have been determined from hydraulic fracturing pressure data in Weber sandstone at the depth of the earthquake foci. The total principal stresses are compressive: S1 = 590 bars (8550 psi), S2 = 430 bars (6200 psi), and S3 = 315 bars (4550 psi). The direction of the maximum compressive stress, N 70°E, is consistent with the orientations of the maximum horizontal compressive stress measured by overcoring surface exposures of sandstone of the Mesa Verde formation. The earthquakes at Rangely occur on a pre-existing fault. The orientation of the fault plane and the slip direction are known from focal plane solutions. Resolving the stresses onto the fault plane in the direction of slip gives a shear stress of 80 bars (1120 psi) and a total normal stress of 350 bars (5030 psi). Laboratory data on the initiation of frictional sliding on saw cuts in samples of Weber sandstone and the computed shear and normal stresses on the fault at Rangely indicate that a pore pressure of 260 bars (3730 psi) is required to reduce the effective normal stress sufficiently to induce slip. Pore pressures in the seismically active part of the reservoir at a time of frequent earthquakes were 275 bars (4000 psi), in accord with the calculated pressure.
Article
Full-text available
Plate boundaries in continental crust are generally less sharply defined than in the oceans, with seismicity spread over broad areas. Interplate displacements appear to be largely accommodated by networks of major fault zones. A simple 2-level model for these important structures accounts for the depth distribution of most continental earthquakes, and for the observed range of faulting styles and associated rock deformation textures. The model consists of a seismogenic frictional slip regime overlying quasi-plastic mylonite belts wherein shearing is largely accommodated aseismically, due mainly to the changing response of quartz to deformation with increasing temperature. Shear resistance increases with depth to a peak value in the vicinity of the frictiona1/quasi-plastic transition and then decreases rapidly. The depth to which microseismic activity extends appears inversely related to regional heat flow and can be satisfactorily modelled as the frictional/quasi-plastic transition for different geotherms using laboratory determined flow laws for quartz-bearing rocks. Larger earthquake ruptures ( M > 5.5) tend to nucleate near the base of the seismogenic regime in the region inferred to have the highest shear resistance and concentration of distortional strain energy. Consideration is also given to the depression of isotherms and seismic activity in regions of thrusting, and to the question of the downward continuation of major fault zones through the lithosphere. Decoupling of the upper crust on flat-lying shear zones may accompany higher-level dip-slip (and perhaps in some circumstances, strike-slip) faulting, being favoured by above average continental heat flow and a high quartz content in the middle or deep crust. The average level of deviatoric stress within the seismogenic regime remains an outstanding problem.
Article
Stresses involved in shallow earthquakes and their occurrence along fault planes suggest that they occur by failure on weak planes, rather than by brittle fracture of a homogeneous material. Possible orientations of the stress tensor are examined to determine what limits fault plane solutions can place on the orientation of the greatest principal stress. For the general case of a triaxial stress, the only restriction is that this stress direction must lie in the quadrant containing P, but may be at right angles to the P direction must lie in the quadrant containing P, but may be at right angles to the P direction. Thus shallow earthquakes impose a few limitations on the orientation of the stress tensor.
Article
Rocks of the upper plate range from Precambrian to Middle Miocene in age. Ten microprobe (not so designated) analyses of 'upper- plate crystalline units' are tabulated. All but one of these are averages of 2-9 analyses; Li, Rb, Sr, and Ba are reported in p.p.m. These rocks are dominantly tholeiitic, as shown on a modified Miyashiro diagram (M.A. 76-1022). Eleven microprobe analyses of 'lower-plate crystalline units' are tabulated with Rb, Sr, and Ba in p.p.m. Seven of the tabulated analyses are averages of from 2-5 individual rock analyses. These rocks are dominantly calc-alkaline, as shown on a modified Miyashiro diagram. Aside from this petrographic information, the emphasis is on the tectonics of the terrain.A.P.
Article
Davis cites four areas of low-angle faulting in which he believes that high fluid pressures can have played no important part in the development and movement of the thrust plates, but it seems to us that the concept or some variant of it may help to explain the observed field relationships in the three out of these four areas with which we have had some first-hand experience. In the areas of the Heart Mountain thrust of Wyoming, the Muddy Mountain thrust of Nevada, and the structurally higher, crystalline thrust sheets of the Swiss Alps, field relationships which include evidence of dehydration reactions during metamorphism of evaporites and of pelitic rocks suggest that interstitial fluid pressures may have been high and thus have played an essential part in the development of the thrust faults. In the fourth area cited by Davis, that of his own studies in the Klamath Mountains of California, we have had no first-hand experience and thus are not competent to answer his criticisms.
Article
The distribution of intraplate earthquakes and of igneous rocks postdating continental rifting is summarized and placed into a plate tectonic framework for the following continental areas: eastern and central North America, Africa, Australia, Brazil, Greenland, Antarctica, Norway, Spitsbergen, India, and the margins of the Red Sea and Gulf of Aden. In continents, intraplate earthquakes tend to be concentrated along preexisting zones of weakness within areas affected by the youngest major orogenesis that predates the opening of the present oceans. Many preexisting zones of weakness (including fault zones, suture zones, failed rifts, and other tectonic boundaries), particularly those near continental margins, were reactivated during the early stages of continental separation. In contrast, intraplate shocks rarely occur within the older oceanic lithosphere or within the interiors of ancient cratonic blocks of the continents. In several places, alkaline magmatism and earthquakes extend several hundred kilometers inland from the ends of oceanic transform faults. Major preexisting zones of weakness that are oriented subparallel to the directions of relative continental separation appear to control the locations of transform faults that develop in a new ocean. In some instances, alkaline magmatism persisted along reactivated features of this type for as long as 100 m. y. after the initial stages of continental fragmentation.
Article
A mechanism for basement shortening underneath allochthonous foreland fold and thrust belts is reverse faulting along inherited normal faults. Normal faulting of basement rocks beneath much of southernmost South America indicates that up to 30–40% extension occurred before sedimentation in a subsiding foredeep. The basement is divided into blocks of similar dimensions to those in the Basin and range of Nevada, U.S.A. Post-depositional reverse motion along some of these high-angle block boundaries under the foreland can account for the 20–40 km of shortening observed in the cover rocks, without showing upthrust wedges of basement rocks. The folds in the cover rocks of the foreland fold-belt are asymmetric, open to closed with over-steepened to overturned limbs which are frequently truncated by high-angle reverse faults. These faults attain a shallower dip with depth toward a décollement plane above a basal volcanic section. Basement involvement in thrusts (0.5–1.5 km vertical displacement) is suggested where basal volcanics are involved in thrust wedges, as revealed by drilling and seismic reflection data. Block faults have been observed beneath many forelands and have always been assumed to be pre-orogenic and passive with respect to deformation of the cover. The block dimensions, fault angles, vertical displacements and other parameters from the southern Andes are inserted to calculate pre-depositional extension and syn-orogenic shortening that can occur without producing obvious basement upthrusting. The results are compared with data from the Appalachians and Canadian Rockies.