Fracture partitioning: Failure mode as a function of lithology in the Monterey Formation of coastal California
Michael R. Gross Department of Geology, Florida International University, Miami, Florida 33199
ABSTRACT phosphatic marl member of the Monterey Formation is shown in Figure 1. The light-colored limestone layer in the upper half of the Fracture style in the Monterey Formation of central California photo contains a series of ptygmatically folded opening-mode veins. varies from one mechanical unit to the next, with joints and The calcite veins do not offset sedimentary laminae, and hence prop- opening-mode veins common in dolostones, limestones, and diage- agated as pure opening-mode fractures. In contrast, faults appear in netic opal-CT beds, and with faults most abundant in beds rich in the mudstone unit occupying the lower portion of the photo, off- biogenic opal A and weak minerals. This dependence of failure setting the light-colored phosphatic layers. Both the limestone and mode on lithology was consistent throughout the deformation his- mudstone layers may have extended in response to the same applied tory of the Monterey Formation, and it reflects the regional Mio- remote stress conditions, yet the strain was accommodated by dis- cene transtensional regime followed by Pliocene–Holocene com- tinctly different failure processes. This example of brittle fracture pression. Furthermore, least principal stresses derived from faults partitioning, whereby opening-mode fractures and faults are con- in mudstone correspond closely to opening-mode fractures in ad- fined to separate mechanical units, is characteristic of interbedded jacent dolostones and limestones, implying that partitioning of fail- lithologies of the Monterey Formation. A similar fracture pattern ure mode among different beds occurred in response to the same was observed by Verbeek and Grout (1983) in alternating siltstone applied tectonic stress conditions. Quantitative bulk mineralogy and sandstone layers of the Uinta Formation in northwestern analyses for samples that failed during the Pliocene–Holocene tec- Colorado. tonic phase show that beds containing <9% weak minerals invari- The goal of this study is to investigate the dependence of brittle ably failed in opening mode, whereas beds containing >22% weak failure mode on rock type among the diverse lithologies of the minerals deformed by brittle faulting. Results from rocks that Monterey Formation exposed along the Santa Barbara and Santa failed in the Miocene are more scattered but show the same general Maria coastlines. If brittle failure mode was consistent among the trends, thereby establishing a link between mineralogical compo- various lithologies throughout the deformation history, then frac- sition and failure mode in the Monterey Formation. ture partitioning indeed reflects a type of genetic deformational behavior, similar to the contrast between brittle and ductile defor- INTRODUCTION
Several factors may control the spatial distribution and relative abundances of joints and faults, including proximity to fault zones (e.g., Stearns, 1972) and structural position (e.g., Hancock, 1985; Lacazette, 1991). Joints are defined herein as fractures character- ized by opening (mode I) displacements (Pollard and Aydin, 1988), whereas faults display shear (mode II and/or mode III) displace- ments. Due to excellent exposures and hydrocarbon potential, the Monterey Formation has been the focus of numerous fracture re- lated studies (e.g., Redwine, 1981; Grivetti, 1982; Belfield et al., 1983; Snyder et al., 1983; Dunham, 1987; Bartlett, 1994). Particular attention has been devoted to the effect of silica diagenesis and rock type on fracture spacing (e.g., Belfield et al., 1983; Narr and Suppe, 1991; Gross et al., 1995). This paper marks the first attempt to relate style of brittle failure (i.e., faulting as opposed to jointing) to bulk mineralogy in a mechanically layered rock such as the Monterey Formation. The concept of fracture partitioning discussed in this paper re- Figure 1. Photograph of limestone (upper portion) and mud- fers to the difference in brittle failure mode from one layer to the stone (lower portion) mechanical units in the organic phosphatic next within a given rock column subjected to an applied remote member of the Monterey Formation at Lion’s Head, California, stress. Fracture partitioning occurs in response to differences in indicating the dependence of failure mode on lithology. Folded veins failure mechanisms between layers of different lithologies, rather in the limestone reveal pure opening-mode displacement with no than in response to structural position or regional variations in dif- shear offset of sedimentary layering, whereas laminae in the mud- ferential stress. An example of fracture partitioning in the organic stone are offset by faults.
GSA Bulletin; July 1995; v. 107; no. 7; p. 779–792; 12 figures; 1 table.
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Barbara basin margin, originally oriented north-south, has rotated 90Њ clockwise since early Miocene time to assume its current east- west orientation (Hornafius, 1985; Luyendyk, 1991). The Santa Bar- bara and Santa Maria basins underwent separate Neogene rota- tional histories; the Santa Ynez River fault (Fig. 3) marks the boundary between a rapidly rotating block to the south and a non- rotating block to the north (Luyendyk et al., 1985; Hornafius, 1985), along with a sharp contrast in Tertiary stratigraphy (Sylvester and Darrow, 1979). Two major tectonic stress regimes affected Monterey Forma- tion strata in the western Transverse Ranges. A tectonic environ- ment dominated by transtension persisted throughout Miocene time as manifested by a series of north-south–trending extensional basins in the offshore Santa Maria region (Crain et al., 1985; McCulloch, 1989), implying a north-south maximum horizontal compressive
stress (SH). A change to transpression occurred during early Plio- cene time, resulting in the northeast-southwest–directed compres- sion that persists today throughout the region as documented by earthquake focal plane solutions (Yerkes, 1985), borehole break- outs (Mount and Suppe, 1992), geodetically measured convergence Figure 2. Generalized lithostratigraphic column for the (Savage et al., 1986), and active folding and faulting (e.g., Dibblee, Monterey Formation along the western Santa Barbara coastline of 1982; Yeats, 1983; Rockwell et al., 1988). The Pliocene–Holocene California (from Isaacs, 1983). transpressive regime is divided into two deformation phases along the southern flank of the Santa Ynez Mountains: a pre-Pleistocene phase characterized by gentle folding with west-northwest fold axes, mation commonly observed in the boudinage of competent beds and an approximately coaxial post-Pleistocene phase marked by in- between incompetent beds. tense shortening, folding, and rapid uplift rates (e.g., Jackson and Yeats, 1982; Olson, 1982). Namson and Davis (1988) interpret the STRATIGRAPHY AND TECTONIC HISTORY western Transverse Ranges as an actively developing fold and thrust OF THE MONTEREY FORMATION belt characterized by fault-bend or fault-propagation folding.
The Monterey Formation was originally deposited in a series of FIELD MEASUREMENTS AND ANALYSES deep basins along the California margin during the middle–late Mio- cene Epoch and consists of diverse lithologic units including dolo- A combination of fracture orientation and bulk mineralogy stone, limestone, siliceous and carbonaceous shale, mudstone, or- analyses was employed in an effort to document fracture partition- ganic-phosphatic marl, diatomite, opal-CT, and quartz chert (e.g., ing and develop a macroscopic failure model for lithologic units in Bramlette, 1946; Pisciotto and Garrison, 1981; Compton, 1991). the Monterey Formation. Orientations of mode I fractures (veins Isaacs (1981a, 1983) subdivides the Monterey Formation into five and joints) and faults were measured in exposures of the Monterey informal members based on detailed stratigraphic and geochemical Formation along the Santa Barbara and Santa Maria coastlines of analyses (Fig. 2). Typical stratigraphic thicknesses for the formation California (Fig. 3). Particular attention was focused on the organic range from ϳ400 m along the Santa Barbara coastline (Isaacs, phosphatic marl member, which consists of 5- to 60-cm-thick car- 1981a) to 1100 m in the Santa Maria basin (MacKinnon, 1989). The bonate and siliceous layers interbedded with 50- to 100-cm-thick two basins are separated by the Santa Ynez Mountains, the west- sequences of organic phosphatic mudstone (Fig. 2). The dominant ernmost extension of the Transverse Ranges. Under increasing style of brittle deformation in the dolostone, limestone, and opal-CT burial depth and temperature, siliceous beds, originally composed of layers is mode I crack propagation, in the form of either joints or diatom frustules in the form of amorphous opal A, are converted to veins. In contrast, the mudstone contains faults. Fracture partition- an intermediate opal-CT phase. Further increases in temperature ing in the marl member was documented at Arroyo Burro, Goleta, result in the dissolution of opal-CT and its reprecipitation as quartz and Elwood beaches along the Santa Barbara coastline and at Surf (Murata and Larson, 1975). Due to westward thickening of the post- and Lion’s Head on the Santa Maria coastline, where bedding at- Miocene stratigraphic section, a regional silica diagenetic gradient titudes afford excellent exposures. Fracture geometry and termina- developed along the Santa Barbara coastline. As a consequence, tions were carefully described to determine relative timing of prop- diagenetic grade increases westward; the dominant silica phase in agation as well as mechanical stratigraphy. Joints belonging to the siliceous Monterey Formation rocks is opal A in the east, opal-CT most pervasive fracture set along the Santa Maria and Santa Bar- in the central portion, and quartz in the west (Isaacs, 1981b, 1982). bara coastlines are oriented perpendicular to strike and normal to The San Andreas transform system migrated eastward across bedding (e.g., Belfield et al., 1983; Narr and Suppe, 1991; Gross, the California borderland during and after Monterey deposition, 1993b). thus despite its relative youth, the Monterey Formation was sub- In order to investigate the relationship between bulk mineral- jected to intense tectonic forces throughout its history. These forces ogy and failure mode, a sample was taken for X-ray diffraction resulted in syn- and post-depositional folding and fracturing of the analysis from each bed where fracture data were collected at 16 Monterey sequence. Paleomagnetic evidence suggests the Santa localities along the Santa Barbara and Santa Maria coastlines, span-
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Figure 3. Map of western Transverse Ranges showing sites investigated and large faults (after Sylvester and Darrow, 1979). The Santa Ynez Mountains are the western-most segment of the Transverse Ranges. Fractures were measured at Arroyo Burro (ARR), Goleta (GOL), Elwood (ELW), Naples (NAP), El Capitan (CAP), Refugio (REF), Gaviota (GAV), and Alegria (ALE) along the Santa Barbara coastline, and Jalama (JAL), Boathouse (BOA), Point Arguello (ARG), Point Pedernales (PED), Surf (SUR), Lompoc Landing (LOM), Purisima Point (PUR), and Lion’s Head (LIO) along the Santa Maria coastline. Towns include Santa Maria (SM), Lompoc (L), Buellton (B), Gaviota (G), and Santa Barbara (SB). Abbreviated faults are Santa Ynez River Fault (SYRF) and Santa Maria River Fault (SMRF). Dashed lines offshore are faults interpreted from seismic data, and the pair of arrows refers to the orientation of the maximum horizontal
principal stress (SH).
ning all five members of the Monterey Formation. The samples were Faults in Mudstone prepared and run at the laboratories of the Texaco Exploration and Production Technology Department in Houston, Texas. One part in Faults in mudstone are divided into two categories: early syn-
six weight percent Al2O3 (corundum) was added to each sample as depositional faults and late tectonic faults. Early faults in the finely an internal standard, and the samples were run on a Siemens D-5000 laminated mudstones can assume either thrust or normal fault ge- powder X-ray diffractometer from 2Њ 2 to a minimum of 50Њ 2 at ometries and typically display short trace heights as measured nor- 0.02 2 steps. Based on standards derived from the Monterey For- mal to bedding, ranging from 0.1 to 3 cm; offsets are correspondingly mation (see Gross 1993a), bulk mineralogy of the samples were minor. These early faults are confined to discrete stratigraphic in- calculated using the quantitative X-ray diffraction software package tervals above and below which the strata are undisturbed (Fig. 4A). GMQUANT developed at Pennsylvania State University. The tech- Sharp erosional truncations overlain by undisturbed laminated nique analyzes the full sample diffraction pattern to determine per- mudstone characterize the tops of faulted zones and possibly rep- cent mineral abundances (Smith et al., 1987). resent scouring events. Seilacher (1969) suggests these faults formed as a result of slumping due to local earthquakes and refers to them FAILURE MODE THROUGH TIME—EARLY VERSUS as seismites. Seismicity during Monterey deposition is plausible in LATE DEFORMATION light of the active tectonism associated with progressive develop- ment of the San Andreas transform system. Further evidence for the The dependence of failure mode on lithology is consistent syndepositional origin of the early faults is sedimentary thickening throughout the deformation history of the Monterey Formation. across normal fault surfaces (i.e., growth faulting). Early- and late-formed faults are common in the mudstone layers of Tectonic faults (i.e., faults that serve as tectonic stress indica- the organic phosphatic marl member. Likewise, early- and late- tors) in the Monterey Formation are characterized by trace heights formed mode I fractures predominate in pure dolostone, limestone, ranging from 10 to 500 cm, with typical shear displacements up to and siliceous layers. 80 cm. In most places the tectonic faults extend across the entire
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Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/107/7/779/3382245/i0016-7606-107-7-779.pdf by guest on 24 September 2021 Figure 4. (A) Early syndepositional thrust faults in a mudstone unit at Elwood (close-up of Figure 4B). Note the restriction of faults to a narrow zone several centimeters thick, above and below which sedimentary laminae remain undisturbed; (B) Faults in mudstone at Elwood. Note the difference in trace height and number of offset laminae between the tectonic fault (labeled ‘‘T’’) and the syndepo- sitional faults (labeled ‘‘S’’); (C) An apparent conjugate pair of tectonic faults in mudstone at Elwood; (D) Early-formed ptygmati- cally folded mode I veins in limestone at Lion’s Head; (E) Folded and thrusted strike-perpendicular mode I vein at Lion’s Head. The coin marks a gradational mechanical layer boundary between limestone (upper half) and mudstone (lower half). Note the greater compaction in the mudstone, manifested by more intensive crenulation of the vein; (F) Planar strike-perpendicular mode I veins that developed during late-phase deformation, exposed on a bedding plane at El Capitan.
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mechanical layer (i.e., a rock unit characterized by a specific style of deformation), unlike early faults that are confined to thin intervals. Compare trace heights and number of offset laminae between the tectonic and syndepositional faults in Fig. 4B. Offset of preexisting tectonic features, such as bed-parallel veins, along with the absence of evidence for growth faulting imply a post-lithification origin for tectonic faults. Late faults commonly display normal fault geome- tries and divide into apparent conjugate sets (Fig. 4C), with con- sistent angular relationships both between each other and with re- spect to least principal stress derived from mode I fractures in adjacent mechanical units.
Mode I Fractures in Dolostone and Pure Siliceous Layers
Mode I veins and joints characterize the type of brittle structure most commonly observed in the microcrystalline dolostone, lime- stone, and opal-CT layers of the Monterey Formation. The early Figure 5. Alternating thick-bedded limestone and mudstone veins are folded and thrusted as shown in Figures 4D and 4E, in- units in the organic phosphatic marl member of the Monterey For- dicating compaction due both to overburden and to tectonic com- mation at Lion’s Head. The lighter-colored limestone is more re- pression. Hence, members of these vein sets display considerable sistant to weathering than the mudstone. scatter in orientation. In contrast, late veins and joints are planar, are not thrusted, and are more consistent in orientation at a given outcrop (Fig. 4F). Based on crosscutting relationships, these frac- neoformed faults, according to Angelier, 1994), and if so they do not tures postdate Miocene folds and associated faults. In fact, several represent preexisting discontinuities later reactivated by a different veins along the Santa Barbara coastline extend through dolostone stress configuration. In the geometric analysis of conjugate fault
beds that were thrust-duplicated during the Pleistocene. Further- systems, the 1 axis bisects the acute angle between the fractures, 2 more, only the late fracture sets are stained with hydrocarbons, corresponds to the line of fault intersection, and 3 bisects the ob- implying that early phase (i.e., Miocene) fracturing and vein infilling tuse angle between faults (e.g., Ramsay and Huber, 1983; Angelier, preceded hydrocarbon migration in the Monterey Formation. Late- 1994). The following section examines stress configurations first in phase fracturing (i.e., Pliocene–Holocene), on the other hand, may the Santa Maria Basin and then along the Santa Barbara coastline, have occurred in conjunction with the middle–late Pleistocene oil and then discusses evidence in favor of conjugate fault origin and migration described by Jackson and Yeats (1982) in the northern coeval mode I failure. Santa Barbara basin. Santa Maria Basin STRESS ORIENTATION ANALYSIS Lion’s Head. The dependence of failure mode on lithology was The analysis of opening-mode fractures and fault orientations documented at two stations in the vicinity of Lion’s Head in the in interbedded lithologies of the organic phosphatic member en- Santa Maria basin (Fig. 3), where silica is in the quartz phase of ables one to address two important issues. One issue is whether diagenesis. Approximately 20 m separate stations A and 3, which are mode I veins/joints and macroscopic faults are indeed related to the located along strike from each other at similar structural positions. same tectonic event. In other words, did both fracture types, which Alternating limestone and mudstone units typical of the organic are found in different lithologies, propagate coevally in response to phosphatic member at Lion’s Head are shown in Figure 5. Lime- the same applied remote stress? The second issue is whether these stone beds of station A are poorly exposed, thus only strike-per- fractures developed in conjunction with, or prior to, Pliocene– pendicular veins were observed. The three-dimensional nature of Holocene compression. the limestone mechanical units is revealed at station 3, with expo-
Mode I fractures propagate in the plane of maximum (1) and sures of strike-perpendicular (same set as at station A), strike-par- intermediate (2) principal stress and normal to the least principal allel, and bed-parallel veins. Whereas the compacted strike-perpen- stress (3). Due to the lack of polished fault surfaces in the organic dicular veins are extensively folded and thrusted, strike- and bed- phosphatic marl unit, one is unable to perform stress inversion anal- parallel veins are relatively planar, indicating these veins propagated ysis based on slickenside lineations (i.e., the techniques developed during a later phase of brittle deformation. Layer boundaries be- by Angelier, 1979; Reches, 1987; and others). The alternative is to tween limestone and mudstone beds at Lion’s Head are generally perform a geometrical stress analysis assuming a conjugate fault gradational or poorly defined, leading to variability in vein height origin and dip slip motion on mean fault planes (e.g., Ramsay and because veins do not consistently terminate at discrete boundaries. Huber, 1983). On completion the results may then be used to test Locally, the strike-perpendicular mode I veins extend into neigh- the validity of these assumptions. boring mudstone beds, where folds have shorter wavelengths and One factor suggesting a conjugate origin for macroscopic faults larger amplitudes than segments of the same veins in limestone in mudstone lithologies is that fault orientations cluster strongly into (Fig. 4E), reflecting greater compaction in the mudstone. Limestone two distinct sets on stereoplots, displaying angular relationships and mechanical units at Lion’s Head are not massive, but rather consist displacements consistent with Andersonian normal faulting. Thus, of pure calcite layers alternating with impure layers, resulting in a these faults perhaps formed under a single tectonic stress field (i.e., striped appearance (Fig. 1).
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Figure 6. Fracture orientations measured at Lion’s Head stations A and 3 along the Santa Maria coastline. (a, b) Unrotated equal-area plots of poles to mode I veins in limestone layers. (c, d) Unrotated plots of poles to tectonic faults in mudstone layers. (e, f ) Rotated plots of poles to strike-perpendicular veins in limestone, mean conjugate fault orientations in mudstone, and principal stresses derived from
faults. Note that least principal stress (3) falls within the cluster of poles to veins. Dashed great circle is bedding.
Orientations of mode I veins in limestone beds at stations A way between mean orientations of the two fault sets. Furthermore,
and 3 are presented in lower hemisphere equal-area plots in Fig- the orientation of the least principal stress, 3, derived from the ures 6a and 6b. Poles to strike-perpendicular, strike-parallel, and faults plots within the cluster of poles to strike-perpendicular veins bed-parallel veins at station 3 plot in clusters at ϳ90Њ angles to one (Figs. 6e and 6f). another, and assume approximately orthogonal and parallel orien- Associated opening-mode fractures and faults at Lion’s Head tations with present bedding attitude. The mean pole to strike-per- appear to record Miocene deformation. First, ptygmatic folding im- pendicular veins at station A plots slightly off the great circle rep- plies that mode I veins were subjected to later deformation. Second, resenting bedding. The wide range in orientations for strike- on rotating bedding to horizontal (i.e., removing Pliocene–Holo- perpendicular veins is due in large part to their nonplanar, folded cene folding), restored strikes of the tectonic faults and strike-per- nature. Faults in mudstone lithologies at the two stations divide into pendicular veins are north-south, parallel to Miocene structures, two sets, labeled set 1 and set 2 (Figures 6c and 6d). The faults and principal stresses restore to a normal fault geometry consistent remained unfolded because they were not mineralized and, there- with Miocene deformation in the Santa Maria basin. fore, do not define a contrast in mechanical properties with the host rock material. The equal-area plots of Figures 6a–6d indicate that Santa Barbara Coastline stations A and 3 contain the same array of systematic fractures. The orientation of strike-perpendicular veins in limestone beds Arroyo Burro. As opposed to the gradual transitions observed with respect to faults in the mudstones suggests these features are at Lion’s Head, lithologic boundaries within the organic phosphatic approximately coeval products of the same stress field. The mean marl member at Arroyo Burro, Goleta, and Elwood are abrupt, orientations of strike-perpendicular veins plot approximately mid- dividing the stratigraphic section into discrete mechanical units
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Figure 7. (a, b) Equal-area plots of poles to strike-perpendicular mode I veins in dolostone and faults in mud- stone at Arroyo Burro along the Santa Barbara coastline for present and re- stored bedding attitudes, respectively. (c, d) Plots of poles to mode I veins in do- lostone and principal stresses derived from fault sets in mudstone at Arroyo Burro for present and restored bedding attitudes, respectively. Note the least
principal stress (3) falls within the cluster of poles to veins.
dominated either by mode I fracturing or faulting. Massive dolo- tical magnitudes of extension for adjacent dolostone and mudstone stone layers at Arroyo Burro contain abundant strike-perpendicular units at Arroyo Burro, despite the markedly different modes of fail- calcite veins, with a mean orientation of 234Њ/86ЊNW. The planarity ure (Gross and Engelder, 1995). The probability that equal fracture of the veins is reflected in the tight cluster of poles on equal-area strains developed under separate tectonic stress regimes is ex- plots (Fig. 7). Other vein sets were not observed in Arroyo Burro tremely low. dolostones because, in part, of limited outcrop exposure. Large tec- Goleta. A similar pattern of failure mode was observed in the tonic faults in the overlying organic mudstone group into two sets, organic phosphatic member at Goleta, although the resistant dolo- with mean orientations of 238Њ/67ЊNW and 027Њ/50ЊSE. Plunges and stone (bed 2) provides a more complete three-dimensional outcrop trends of principal stresses derived from the unrotated faults in the exposure than the massive dolostone at Arroyo Burro. The resistant
mudstone are 65Њ 204Њ for 1,23Њ047Њ for 2, and 09Њ 314Њ for 3 dolostone contains three sets of mutually perpendicular mode I cal- (Fig. 7c), with the orientation of least principal stress coinciding cite veins in strike-perpendicular, strike-parallel, and bed-parallel precisely to poles to strike-perpendicular veins found in the dolo- orientations (Fig. 8). Of the 72 veins, 69 are planar and show no
stone. The fact that maximum principal stress 1 is not orthogonal evidence of tectonic folding or vertical compaction. This geometry, to Arroyo Burro bedding attitude may indicate the beds were tilted along with the distinct angular relations to bedding (Figs. 8a and 8b)
at the time of brittle failure. For example, rotating 1 to a vertical raises the possibility that these fractures formed during Pliocene– position results in a configuration of beds dipping ϳ48Њ to the south- Holocene folding; suites of three mutually perpendicular fracture west at the time of normal faulting. Thus at failure, the beds may sets often develop in compressional tectonic environments (e.g., have been situated in a southwest-dipping panel of a fault-bend fold, Hancock, 1985; Srivastava and Engelder, 1990; Lacazette, 1991). which is consistent with both regional structural interpretations by Similar conclusions concerning the relationship between folding and Namson and Davis (1988), Shaw and Suppe (1994), and Novoa et al. fracturing along the Santa Barbara coastline were drawn by Belfield (1994) as well as timing of along-strike fracturing during fault-bend et al. (1983). Common hydrocarbon stains on planar fractures of all folding (Srivastava and Engelder, 1990). Extensional strains mea- three systematic sets and numerous hydrocarbon-filled breccia sured at Arroyo Burro further support the notion of coeval failure. zones attest to the high reservoir quality of the Monterey Forma- A detailed analysis of fracture-related strain indicates nearly iden- tion. As reported by Belfield et al. (1983), hydrocarbons are not
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Figure 8. Equal-area plots of fractures and derived stresses at Goleta along the Santa Barbara coastline. (a) Unrotated mode I veins in dolostone bed. Note the three mutually perpendicular vein sets and their relationship to bedding. (b) Mode I veins in dolostone bed on restoring bedding to horizontal. (c) Unrotated faults in mud- stone bed. (d) Principal stresses derived from faults in mudstone.
associated with early mode I fractures and microfaults, a relation- neotectonic SH in light of the 90Њ clockwise rotation of the Santa ship also observed at Lion’s Head. Barbara coastline from the middle Miocene to the present. Rather, The mudstone lithology of bed 3 contains only fractures with strike perpendicular veins and faults most probably represent strike- shear displacement, both early syndepositional and late tectonic parallel extensional strain associated with development of the Plio- faults. Tectonic faults group into two sets (Fig. 8c), with mean un- cene–Holocene western Tranverse Ranges fold and thrust belt. rotated orientations of 219Њ/76ЊNW and 340Њ/80ЊNE (Fig. 8d). The
orientation of the least principal stress (3) derived from tectonic Evidence for Conjugate Faulting and Coeval Mode I Failure faults in bed 3 corresponds to poles to strike-perpendicular mode I fractures in the bed 2 dolostone, a relationship also found at Arroyo Results from stress orientation analysis may now be used to Burro. address the assumption of a conjugate origin for faults in mudstones The relatively late formation of associated mode I veins and and coeval mode I failure in adjacent beds. Regarding conjugate faults along the Santa Barbara coastline is supported by the planar, origin, the least principal stress derived from fault set pairs in mud- nonfolded vein morphology. Another timing indicator is that max- stone falls precisely in the cluster of poles to strike-perpendicular
imum principal stress (1) derived from faults both for beds in their mode I fractures in adjacent lithologies at Arroyo Burro, Goleta, present folded attitude and for beds rotated to their original hori- and Lion’s Head. Such consistent behavior appears to represent a
zontal attitudes trends northeast-southwest, parallel to SH of the systematic pattern rather than mere coincidence. Furthermore, neotectonic stress field as determined from earthquake focal mech- principal stress orientations derived from fractures at Lion’s Head anisms (e.g., Corbett and Johnson, 1982), late Pleistocene and Hol- correspond to the likely Miocene tectonic stress regime, whereas ocene fault displacements (e.g., Ziony and Yerkes, 1985), and bore- stresses for Arroyo Burro and Goleta match Pliocene–Holocene hole breakouts (Mount and Suppe, 1992). If strike-perpendicular stress conditions. These results are consistent with observations of veins and faults indeed represent Miocene deformation, then cer- vein morphology, with early-formed folded veins at Lion’s Head and tainly there would be a slim chance of them coinciding with the late-formed planar veins at Arroyo Burro and Goleta. Regarding
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coeval failure, it would be unlikely for equal magnitudes of exten- mode I veins and joints, and 17 samples from beds that failed by sional strain to develop in adjacent mudstone and dolostone units faulting. Mineralogical compositions of the samples reflect the wide under different remote stress configurations. In addition, the three variability in Monterey Formation lithologies, which range from orthogonal, late-formed vein sets in the Goleta dolostone attest to pure dolostone and opal-CT rocks to marlstones rich in detrital major swings in principal stress orientations during Pliocene–Hol- minerals (Table 1). Regardless of the mineral or silica phase, rocks ocene compression, thereby narrowing the time frame in which the containing Ͼ85% of a single constituent all fail in effective tension, tectonic faults formed. Despite such a nonuniformly oriented stress implying that compositional purity promotes a mode I failure
field, the fault-derived 3 coincides precisely to 3 derived from mechanism. opening-mode fractures. If the two fracture styles indeed occurred Results from the X-ray diffraction analysis were divided into at different tectonic stages, then principal stress axes most likely two sets of data based on the timing of deformation. One group would not coincide. Finally, fault orientations along with associated represents post-diagenetic Pliocene–Holocene fracturing and con- principal stresses correspond specifically to the strike-perpendicular sists of all beds sampled along the Santa Barbara coastline as well fracture set in all cases, suggesting a genetic linkage between the two as Surf. The second data set consists of samples collected from the different modes of failure in adjacent lithologies. Thus, in light of the organic phosphatic marl sequence at Lion’s Head, where fractures above observations, the assumptions of conjugate faulting and co- most likely formed prior to diagenesis as a result of Miocene de- eval opening-mode fracturing appear valid. formation. In order to investigate the effects of diagenesis and min- eral impurities on failure mode, minerals and phases were grouped RELATIONSHIP BETWEEN BULK MINERALOGY into ‘‘mechanically strong’’ and ‘‘mechanically weak’’ categories. AND FAILURE MODE Quartz, opal-CT, dolomite, calcite, pyrite, siderite, albite, and mi- crocline were categorized as strong components, whereas the weak The observed dependence of failure mode on lithology in the constituents are opal A, kaolinite, muscovite, montmorillonite, and Monterey Formation identified in outcrop fracture surveys raises apatite. the issue of how mineralogical composition influences failure mode. In the diverse lithologies of the Monterey Formation, silica phase Santa Barbara Coastline and Surf—Pliocene–Holocene and weak impurities may play important roles in controlling failure Fracturing behavior. Therefore, an important goal is to investigate the miner- alogical criteria that may ultimately help predict brittle failure mode Although many beds rich in opal A failed by shear displace- for a given rock unit. ment, fracture partitioning in the Monterey Formation is not solely Diagenesis dramatically affects the physical properties of a function of silica phase (Fig. 9A). For example, faults developed Monterey Formation rocks (e.g., Bramlette, 1946; Murata and Lar- in beds comprised of Ͻ20% opal A where the dominant silica phase son, 1975; Pisciotto, 1981; Snyder et al., 1983). For example, major is opal-CT (samples 66 and 69). Furthermore, faulting predomi- porosity reductions accompany the two main phase transformations, nates in a bed composed of 77% opal-CT with no trace of opal A with typical porosities of 55%–70% for diatomaceous opal A rocks, (sample 89). This bed, however, contains muscovite, kaolinite, and 25%–40% for opal-CT rocks, and 10%–20% for quartz-bearing apatite, suggesting that weak minerals may indeed promote shear rocks (Isaacs, 1981b; Compton, 1991). As a consequence, elastic failure in Monterey lithologies. A plot of sample number versus the properties vary considerably among rock types, with Young’s moduli suite of weaker minerals shows a strong correlation between frac- ranging about 5, 18, and 70 GPa for highly siliceous opal A, opal-CT, ture style and lithologic composition (Fig. 9B). Without exception, and quartz bearing rocks, respectively (Gross, 1993a; Gross et al., all 57 beds composed of Ͻ9% weak minerals fail in the form of 1995). Differences in physical properties in turn affect certain as- opening-mode veins and joints. On the other hand, the five beds pects of mechanical behavior. For example, Gross et al. (1995) re- containing Ͼ22% weak minerals invariably fail by faulting. Of the 74 port that joint density in chert beds is more than twice that in do- samples, 12 fall into an overlapping zone between 9% and 22% weak lomitic diatomites. Thus, changes in physical properties as a result minerals, where both types of failure occur. of diagenesis may play a role in failure mode mechanism. Field observations, however, suggest that with respect to failure Lion’s Head—Miocene Fracturing mode, pure siliceous lithologies tend to fail in opening-mode dis- placement regardless of silica phase. Beds of pure opal A diatomite Folded veins along with geometric stress analysis imply that exposed in roadcuts and quarries near Lompoc often fail by jointing. brittle failure at Lion’s Head occurred early in Monterey deforma- Likewise, late-phase mode I joints predominate in pure opal-CT tion history, prior to burial depths required for diagenesis. Although horizons along the Santa Barbara and Santa Maria coastlines. Pure rocks are presently in the quartz phase, failure most likely took place siliceous rocks in the quartz phase contain abundant post-diagenetic with silica in the form of opal A. Therefore, samples from Lion’s joints in outcrop along the Santa Maria coastline (Narr and Suppe, Head are plotted against the sum of quartz, kaolinite, muscovite, 1991) and in cores from the Point Arguello oil field (Narr, 1991). montmorillonite, and apatite, which may best represent the suite of Therefore, a second potential factor controlling failure mode is weak minerals at failure. Compared with beds that failed during compositional purity or, conversely, the amount of impurities in a late-phase deformation, the plot of Lion’s Head samples shows a rock unit. similar, albeit more scattered correlation between failure mode and mineralogy (Fig. 9C). The four beds containing Ͻ39% weak min- X-ray Diffraction Results erals failed in effective tension, whereas the five beds composed of Ͼ54% weak minerals are characterized by faulting. Of the 15 beds Mineral percentages were calculated for 89 samples from the at Lion’s Head, 6 fall within the overlap zone, a substantially larger Monterey Formation; 72 samples were taken from beds containing percentage than encountered in beds along the Santa Barbara coast-
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Sample Number Mode Quartz Calcite Pyrite Apatite Siderite Albite Microcline Kaolinite Muscovite Opal A Opal-CT Montmorillonite Protodolomite
GAV Station A, bed 1 1 I 4 0 0 0 0 3 1 0 0 0 92 0 0 Station A, bed 2 2 I 4 0 0 0 0 3 0 0 0 0 94 0 0 Station A, bed 3 3 I 5 0 0 0 0 3 0 0 0 0 92 0 0 Station A, bed 4 4 I 6 0 0 0 0 4 0 0 0 0 90 0 0 Station A, bed 5 5 I 5 0 0 0 0 2 0 0 0 0 93 0 0 Station B, bed 1, top 6 I 1 0 2 1 0 0 9 0 0 0 30 0 57 Station B, bed 1, 7I2000 0 0 0 0 0 0980 0 bottom Station B, bed 2 8 I 2 0 0 0 0 1 0 0 0 0 97 0 0 Station B, bed 3 9 I 0 18 0 0 0 0 0 0 0 0 82 0 0 Station C 10 I 0 4 0 1 0 0 9 0 0 0 0 0 86 Station D 11 I 1 22 0 0 0 0 0 0 0 0 16 0 61 Station E 12 I 5 0 0 0 0 2 0 0 0 0 93 0 0 Station F, bed 1 13 I 9 0 0 0 0 4 0 0 0 0 85 0 1 Station F, bed 2 14 I 8 0 0 0 0 6 1 0 0 0 84 0 0 Station F, bed 3 15 I 3 0 0 0 0 2 0 0 0 0 95 0 0
ALE Station A 16 I 13 0 0 0 0 11 4 0 0 0 72 0 0 Station B 17 I 21 0 0 0 0 14 6 0 0 0 57 0 2
NAP Station 1 18 I 0 9 0 1 0 0 0 0 0 0 0 0 90 Station 2 19 I 0 14 0 1 0 0 18 0 0 0 0 0 67 Station 3 20 I 2 6 0 1 0 0 3 0 0 5 21 0 62 Station 4 21 I 2 6 0 1 0 0 8 0 0 0 23 0 59 Station 5 22 I 5 14 0 0 0 2 0 0 0 7 69 0 3 Station 6 23 I 0 13 0 0 0 0 4 0 0 10 19 0 54 Station 7 24 I 2 25 0 0 0 0 1 0 1 10 60 0 1 Station 8 25 I 3 31 0 0 0 0 3 0 0 0 60 0 3 Station 9 26 I 8 50 0 0 0 2 0 0 2 20 18 0 0
CAP Station 1 27 I 0 74 0 3 0 0 0 0 4 14 0 0 5 Station 2 28 I 6 1 0 1 0 0 8 0 0 0 30 0 54 Station 3 29 I 8 33 0 0 0 2 2 0 0 0 43 0 12 Station 4 30 I 3 22 0 0 0 0 3 0 0 0 52 0 20 Station 5 31 I 2 25 0 0 0 0 8 0 0 0 18 0 47 Station 6 32 I 0 31 0 0 0 0 0 0 0 0 68 0 1 Station 7 33 I 12 0 1 0 0 0 3 0 0 12 0 0 72 Station 8 34 I 1 4 0 1 0 0 12 0 0 0 0 0 81 Station 9 35 I 0 14 0 0 0 0 13 0 0 0 0 0 72 Station 10 36 I 2 10 0 1 0 0 5 0 0 0 35 0 47 Station 11 37 I 0 16 0 0 0 0 0 0 0 0 13 0 70 Station 12 38 I 0 18 0 1 0 0 6 0 0 0 0 0 75 Station 13 39 I 0 4 0 1 0 0 6 0 0 0 11 0 78 Station 14 40 I 0 19 0 0 0 0 8 0 0 0 0 0 73 Station 15 41 I 4 11 0 0 0 0 0 0 0 0 84 0 1
GOL Station 1 42 II 13 7 0 0 0 10 3 0 3 63 0 0 0 Station 2 43 I 0 0 0 0 0 0 2 0 0 0 0 0 98 Station 3A 44 II 5 13 0 0 0 4 3 1 0 73 0 0 0 Station 3B 45 II 4 13 0 0 0 0 1 0 0 83 0 0 1 Station 4 46 II 7 35 0 2 0 4 2 0 2 47 0 0 0
ARR Station 7, M 47 II 14 0 0 4 0 7 11 0 4 61 0 0 0 Station 7, bed 1 48 I 2 7 0 0 0 0 0 0 0 0 92 0 0 Station 7, bed 2 49 I 1 16 0 0 0 0 0 0 0 0 83 0 0 Station 7, bed 3 50 I 4 13 0 0 0 0 0 0 0 0 83 0 0 Station 7, SL 1 51 I 1 0 0 1 0 0 0 0 0 0 0 0 98 Station 7, SL 2 52 I 1 0 0 1 0 0 0 0 0 0 0 0 98 Station 7, SL 3 53 I 0 0 0 1 0 0 18 0 0 0 0 0 81 Station 9, bed 1 54 I 2 6 0 0 0 0 0 0 0 0 91 0 0 Station 9, bed 2 55 I 48 0 6 0 0 0 0 0 0 0 44 0 2
ELW Station 9, bed 1 56 I 3 60 0 0 0 0 0 0 5 0 23 0 9 Station 9, bed 2 57 I 10 14 0 0 0 0 7 0 0 0 17 0 52 Station 10 58 I 0 93 0 0 0 0 0 0 7 0 0 0 0 Station 11, bed 1 59 I 2 70 0 0 0 0 0 0 0 0 28 0 0 Station 11, bed 2 60 I 1 24 0 0 0 0 0 0 0 9 66 0 0 Station 11, bed 3 61 I 1 27 0 0 0 0 0 0 0 10 62 0 0 Station 11, bed 4 62 I 2 17 0 0 0 0 0 0 0 2 79 0 0 Station 11, bed 5 63 I 2 65 0 0 0 0 0 0 0 11 22 0 0 Station 11, bed 6 64 I 2 70 0 0 0 0 0 0 0 9 20 0 0 Station 11, bed 7 65 I 0 29 0 0 0 0 0 0 0 0 71 0 0 Station 11, bed 8 66 II 1 74 0 0 0 0 1 0 0 10 13 0 0 Station 11, bed 9 67 I 1 19 0 0 0 0 0 0 0 5 75 0 0 Station 11, bed 10 68 I 0 18 0 0 0 0 0 0 0 1 81 0 0 Station 11, bed 11 69 II 12 21 0 0 0 10 2 0 2 19 31 0 2 Station 11, bed 12 70 I 2 8 0 0 0 0 0 0 0 3 87 0 0 Station 11, bed 13 71 I 1 12 0 0 0 0 0 0 0 6 81 0 0 Station 11, bed 14 72 I 1 16 0 0 0 0 0 0 0 4 80 0 0 Station 11, bed 15 73 I 0 15 0 0 0 0 0 0 0 0 84 0 0
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TABLE 1. (Continued)
Sample Number Mode Quartz Calcite Pyrite Apatite Siderite Albite Microcline Kaolinite Muscovite Opal A Opal-CT Montmorillonite Protodolomite
LIO Station A, bed 4 74 II 56 38 0 2 0 0 3 0 0 0 0 0 2 Station A, bed 5 75 I 16 84 0 0 0 0 0 0 0 0 0 0 0 Station 3, bed 1 76 I 22 67 0 0 0 0 0 0 11 0 0 0 0 Station 3, bed 3 77 I 32 59 0 0 0 0 6 0 0 0 0 0 4 Station 3, bed 4 78 I 48 32 0 0 0 0 12 0 0 0 0 0 8 Station 3, bed 5 79 I 54 41 0 0 0 0 5 0 0 0 0 0 0 Station 3, bed A 80 II 42 31 0 1 0 0 26 0 0 0 0 0 0 Station 3, bed B 81 II 58 29 0 2 0 0 0 0 0 0 0 0 11 Station 3, bed C 82 II 53 39 0 2 0 0 6 0 0 0 0 0 0 Station A, bed 1 83 I 7 70 0 7 0 0 0 0 16 0 0 0 0 Station A, bed 2 84 II 60 38 0 2 0 0 0 0 0 0 0 0 0 Station A, bed 6 85 II 37 53 0 2 0 0 9 0 0 0 0 0 0 Station A, bed 8 86 II 66 34 0 0 0 0 0 0 0 0 0 0 0 Station A, bed 10 87 II 48 33 0 4 0 0 9 0 0 0 0 0 6 Station A, bed 12 88 II 40 47 0 4 0 0 9 0 0 0 0 0 0
SUR
MU #2 89 II 7 0 0 2 0 0 5 2 5 0 77 0 2
Note: Samples collected from fracture stations along the Santa Barbara and Santa Maria coastlines. Minerals expressed in volume percent. Sample localities are shown in Figure 3 using the same abbreviations. Modes I and II refer to opening and shear displacement, respectively.
line. Some of the quartz detected at Lion’s Head may in fact be where is density, g is the acceleration of gravity, and h is depth. For detrital rather than diagenetic, and consequently belongs in the an overburden thickness of ϳ3 km (consistent with silica diagenesis Ϫ3 group of stiff minerals. Thus, uncertainties regarding quartz origin phase) and average density of 2500 kg m , v is ϳ75 MPa. Thus, may contribute to the overlap. A second, perhaps more important based on the assumptions outlined above, maximum principal factor is the compositional heterogeneity of beds at Lion’s Head. In stresses in the mudstone and dolostone layers at the time of failure D M contrast to sharp boundaries along the Santa Barbara coastline, both equaled ϳ75 MPa (i.e., v ϭ1 ϭ1 ϭ75 MPa). This value lithologic contacts in the organic phosphatic member at Lion’s Head of 1 anchors the right side of both Mohr’s circles of stress at 75 MPa are gradational. As a result, veins typically extend into adjacent units (Fig. 10). Failure mode envelopes for Blair Dolomite and Muddy (Fig. 4E), and limestone layers are banded, reflecting alternating Shale were selected as analogs for the two mechanical units at Ar- pure and impure horizons. The large failure mode overlap in Fig- royo Burro. ure 9C may in part reflect these overlapping lithologies. Neverthe- In order to proceed with the failure model, one must assume less, combining outcrop fracture surveys with X-ray diffraction anal- that elastic strain was the same for both mudstone and dolostone yses reveals a strong correlation between failure mode and lithology units prior to fracturing at Arroyo Burro. In other words, the uni- in the Monterey Formation. form displacement boundary conditions that exist after fracturing as measured in outcrop (Gross and Engelder, 1995) persisted prior to FAILURE MODE MODEL the onset of failure when strains were purely elastic. A survey of rock elastic properties indicates that dolostone typically displays a higher One general explanation describing the observed fracture par- Young’s modulus (E) than mudstone. Applying a simple one- titioning in the Monterey Formation incorporates linear elasticity dimensional analysis of Hooke’s Law, into the Mohr-Coulomb failure criterion. A detailed extensional strain analysis presented in Gross and Engelder (1995) reveals that D Dε d ϭ E , and (2a) uniform displacement boundary conditions prevailed during frac- ture-related extension of adjacent mudstone and dolostone units at M ϭ EMε, (2b) Arroyo Burro. Equal amounts of extensional strain were recorded d in the two units, despite the markedly different fracture styles (i.e., ε faulting in the mudstone and mode I veining in the dolostone). where d is differential stress, is elastic strain prior to failure, and Similar displacement boundary conditions must have prevailed dur- superscripts D and M refer to the dolostone and mudstone beds, D ing failure of the rock layers in Figure 1 at Lion’s Head, where the respectively. Thus, under conditions of uniform elastic strain and E M gradational contact between mudstone and limestone units does not Ͼ E , the differential stress, and consequently the Mohr circle di- permit a detachment. ameter, is greater for the dolostone layer (Fig. 10). Mode I veins and faults measured in the Monterey Formation Prior to fracturing, Mohr circles of stress resided in the stable portion of the failure envelope for both lithologies (Fig. 10a). Add- at Arroyo Burro indicate that maximum principal stress (1) was ing the effect of pore pressure would have translated the Mohr aligned subvertically and least principal stress (3) was oriented parallel to bedding strike during the Pliocene–Holocene extensional circles to the left. Breccia zones that formed explosively, bed-par- brittle deformation phase (Fig. 7). Adjacent mudstone and dolo- allel veins, and crack seal veins all attest to historical conditions of elevated fluid pressure in the Monterey Formation. One may envi- stone beds were subjected to the same overburden stress (v), which sion a scenario whereby the mudstone Mohr circle intersects the in the case of normal faulting equals 1. The overburden stress is defined as: mudstone failure envelope in the shear field, whereas the dolostone fails under conditions of effective tensile stress (Fig. 10b). By virtue v ϭ gh, (1) of the differences in y-intercept and slope of the failure envelopes,
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Figure 9. Percent mineral abundances plotted versus sample numbers showing mode of fracture dis- placement. (A) Percent opal A for samples along the Santa Barbara and Santa Maria coastlines that failed during Pliocene–Holocene deformation. (B) Percent weak minerals for samples along the Santa Barbara and Santa Maria coastlines that failed dur- ing Pliocene–Holocene deformation. (C) Percent weak minerals for samples at Lion’s Head that failed during Miocene deformation. Sample numbers and abbreviated minerals may be found in Table 1. Values on the horizontal axis have no significance other than as sample numbers in Table 1.
this behavior occurs despite the fact that differential stress is greater FRACTURE PARTITIONING AND MECHANICAL in the dolostone. STRATIGRAPHY Faults in the mudstone most likely formed through the coales- cence of microcracks (e.g., Scholz, 1968; Jaeger and Cook, 1979; Analysis of fractures in the Monterey Formation strongly im- Lockner et al., 1992) and thus justify use of a Coulomb analysis. In plies that mode of failure varied from one mechanical unit to the contrast, plumose morphology observed on joints throughout the next in response to the same tectonic stress conditions. Thus, the Monterey Formation suggest that mode I fractures initiated from a concept of fracture partitioning as depicted schematically in Fig- single origin point and grew as individual fractures. Thus, a fracture ure 11 applies to heterogeneous sedimentary rock masses such as mechanics criterion involving effective tensile stresses at crack tips the Monterey Formation, whereby brittle behavior may vary (e.g., Lawn and Wilshaw, 1975; Segall and Pollard, 1983) is appro- throughout the rock sequence due to differences in mechanical priate for beds with opening-mode fractures. Conditions of effective properties and composition of various units. Consequently, the me- tensile stress may have existed in the Monterey Formation as a result chanical stratigraphy of a rock sequence after deformation may con- of high pore fluid pressures combined with layer-parallel stretching. sist of unfractured layers, layers characterized by faults, and layers
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Figure 12. Fracture partitioning in the organic phosphatic member at Goleta. Note that closely spaced mode I fractures are restricted to a thin central layer, faulting characterizes brittle de- formation in the upper portion of the photograph, and layers in between remain unfractured.
predominately containing mode I fractures. One such example from the organic phosphatic marl member at Goleta beach is shown in Figure 12. Small faults appear in the upper portion of the photo- graph, whereas mode I veins are restricted to a thin layer in the Figure 10. Scenario for faulting in the mudstone mechanical center. Unfractured strata lies in between the two fractured zones. unit while opening-mode fractures develop in the dolostone unit, The dramatic differences in mechanical behavior shown in Figure 12 despite larger differential stresses in the dolostone. (a) Mudstone result from very subtle changes in composition, indicating that fail- and dolostone Mohr circles reside in stable field, hence rocks re- ure mode can be extremely sensitive to lithology. main unfractured. (b) Mohr circles translate to the left due to in- crease in pore pressure so that faults develop in mudstone and CONCLUSIONS mode I fractures develop in dolostone. Failure envelopes for Blair Dolomite and Muddy Shale were generated from data presented in Field observations, fracture orientation analysis, and bulk min- Handin (1966). Asterisk refers to effective stresses. eralogy determinations show that failure mode is indeed genetically related to lithology within interbedded units of the Monterey For- mation. Identifying the dominant fracture style for individual beds, in turn, helps define the mechanical stratigraphy for such a heter- ogeneous rock mass. Stress orientation analysis implies that faults and opening-mode fractures, restricted to separate mechanical units, formed approximately coevally in response to the same ap- plied remote stress conditions. Early ptygmatically folded veins and associated faults at Lion’s Head in the Santa Maria Basin reflect Miocene deformation, whereas planar, unfolded veins and associ- ated faults along the Santa Barbara coastline formed in conjunction with the Pliocene–Holocene development of the western Transverse Ranges fold and thrust belt. At Arroyo Burro, Goleta, and Lion’s Head, faulting in mudstone units consistently occurred in conjunc- tion with formation of a strike-perpendicular mode I fracture set, implying that the most intense regional deformation in the western Transverse Ranges is associated with strike-normal compression. Quantitative X-ray diffraction results from beds that failed dur- ing Pliocene–Holocene deformation show that rocks containing Ͻ9% weak minerals fail by mode I fracturing, whereas beds with Ͼ22% weak minerals tend to deform through faulting. Sampling Figure 11. Schematic diagram depicting the concept of fracture other units within the Monterey Formation as well as applying ad- partitioning, where deformation varies among lithologic units due ditional techniques to quantitatively assess composition will help to differences in mechanical properties. Some layers fail in opening constrain the boundaries between failure mode fields. Ultimately mode, some through faulting, and others remain unfractured. one may hope to establish mineralogical criteria that can be used to
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predict fracture style in both the Monterey Formation and other and the Ithaca Siltstone at Watkins Glen, New York [Ph.D. thesis]: University Park, Pennsyl- vania State University, 224 p. fractured rock units around the world. Lawn, B. R., and Wilshaw, T. R., 1975, Fracture of brittle solids: Cambridge, United Kingdom, Cambridge University Press, 204 p. Lockner, D. A., Moore, D. E., and Reches, Z., 1992, Microcrack interaction leading to shear fracture, ACKNOWLEDGMENTS in Tillerson, J. R., and Wawersik, W. R., eds., Proceedings of the 33rd U.S. Symposium on Rock Mechanics: Rotterdam, Netherlands, Balkema, p. 807–816. Luyendyk, B. P., 1991, A model for Neogene crustal rotations, transtension, and transpression in southern California: Geological Society of America Bulletin, v. 103, p. 1528–1536. I thank Wendy Bartlett for helpful advice and field assistance, Luyendyk, B. P., Kamerling, M. J., Terres, R. R., and Hornafius, J. S., 1985, Simple shear of southern California during Neogene time suggested by paleomagnetic declinations: Journal of Geophysi- and Frank Wind, Alfred Lacazette, and Andrew Thomas of Texaco cal Research, v. 90, p. 12454–12466. for coordinating the X-ray diffraction analyses. Discussions with MacKinnon, T. C., 1989, Petroleum geology of the Monterey Formation in the Santa Maria and Santa Barbara coastal and offshore areas, in MacKinnon, T. C., ed., Oil in the California Monterey Don Fisher, David Green, Mark Fischer, and Deane Smith are Formation: Washington, D.C., American Geophysical Union, 28th International Geological Congress Field Trip Guidebook T311, p. 11–27. greatly appreciated. Excellent reviews by Jon Olson and Earl Ver- McCulloch, D. S., 1989, Evolution of the offshore central California margin, in Winterer, E. L., beek greatly improved the manuscript. This study constitutes a por- Hussong, D. M., and Decker, R. W., eds., The eastern Pacific Ocean and Hawaii: Boulder, Colorado: Geological Society of America, The Geology of North America, v. N, p. 439–470. tion of a Ph.D. dissertation under the supervision of Terry Engelder, Mount, V. S., and Suppe, J., 1992, Present-day stress orientations adjacent to active strike-slip faults: California and Sumatra: Journal of Geophysical Research, v. 97, p. 11995–12013. to whom I am very grateful. Funding was provided by a research Murata, K. J., and Larson, R. R., 1975, Diagenesis of Miocene siliceous shales, Temblor Range, grant from Texaco to Engelder at Pennsylvania State University. California: U.S. Geological Survey Journal of Research, v. 3, p. 553–556. Namson, J., and Davis, T., 1988, Structural transect of the western Transverse Ranges, California: Implications for lithospheric kinematics and seismic risk evaluation: Geology, v. 16, p. 675–679. 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