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. 779 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 M. R. GROSS Barbara basin margin, originally oriented north-south, has rotated 908 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 ;400 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-