Brecciated rock textures are common, and may be associated with in situ deformation of rock (e.g., from hydrofracturing), cataclastic deformation in tectonic shear zones, mass flow deposits such as landslides or rock falls, and various other causes. Despite the varied environments of formation, common textural characteristics may recur. For example, angular and fractured clasts are common features in brecciated units dominated by a variety of brittle processes. Fortunately, some diagnostic characteristics permit the multiple possibilities to be constrained. We here examine these characteristics in the context of the origin of the breccia units at Site 899.
Crystalline and highly indurated rocks exposed to significant stresses and fluid pressures may display extensive brittle deformation, resulting in intense in situ brecciation. Basement cores recovered at Site 897 and Site 900 (Sawyer, Whitmarsh, Klaus, et al., 1994) contain abundant brecciated zones accounting for up to 70% of a given core. Texturally, these rocks are distinguished by angular clasts of locally derived basement rock embedded in a calcite-rich matrix. Clast sizes range from microscopic to more than 10 cm in diameter, and there is a general absence of structure, either sedimentary or tectonic. Most importantly, the brecciated zones are characteristically heterogeneous, with large volumes of relatively undisturbed, inplace basement rock, surrounded by disrupted brecciated zones. The relative homogeneity of the serpentinite breccias at Site 899 and the uniformly fragmented and disrupted nature of the particles precludes the possibility that they were formed by in situ brecciation processes. However, the picture is complicated by occurrence in the largest breccia unit of clear evidence for secondary calcite veining and repeated hydrofracturing of the breccia unit after it was formed (Morgan and Milliken, this volume).
Brittle shear zones and fault gouges have been described frequently (e.g., Rutter et al, 1986; Chester and Logan, 1987), and they exhibit a range of textures and structures, controlled in large part by lithology and the mode of deformation. Serpentinite fault gouges display certain features in common with other examples (e.g., Hoogerduijn Strating and Vissers, 1994), including discrete zones of shear localization and grain size reduction, fracture arrays, and planar foliation. Serpentine, however, is very sensitive to elevated temperatures and pressures, and will deform ductilely at temperatures as low as 300°C (Evans, 1977). Under frictional heating conditions induced along shear zones, serpentine will tend to develop distinct foliation and shear fabric (e.g., Hoogerduijn Strating and Vissers, 1994). The complete absence of such textures in the serpentinite breccias at Site 899 argues strongly for low temperature, brittle deformation.
Recent submersible observations of the textures, superficial features, and processes on active submarine talus cones (Whipple and Naidoo, 1991; Juteau et al., in press) have emphasized that large talus deposits can be formed under marine conditions adjacent to submarine scarps. These deposits include "energetic rockfalls." Indeed, we suspect there may be a complete spectrum of deposits from talus accumulations, formed by mass wasting over a significant time period, to rock avalanche deposits formed by sudden catastrophic failure and the deposition of large volumes of brecciated rock. Unfortunately, the evidence required to categorize the members within this spectrum of deposits is largely lacking. Talus accumulations are often of limited lateral extent, whereas landslide deposits can be extensive and of tabular morphology. Although we have no data on the lateral extent of the three Site 899 serpentinite breccia units, we prefer to interpret them as bedded units because they are intercalated with fossiliferous sediments and span a considerable time period. We are not aware of mass wasting talus deposits in which the fragments display the crackle and jigsaw textures. Such textures characterize landslide deposits and are also found in the Site 899 breccias.
Diapiric origins for serpentinite breccias have been proposed in a variety of settings, many related to subduction zones (e.g., Fryer and Fryer, 1987), and are attributed to the buoyancy of water-saturated serpentinite. Lockwood (1972) termed these brecciated masses "serpentinite protrusions." The extrusive products, "sedimentary serpentinites," are more frequently observed and typically display a highly sheared texture, with clasts of serpentinite entrained within a foliated serpentine mud matrix (e.g., Lockwood, 197la; Carlson, 1984; Phipps and Ballotti, 1992; Fryer and Mottl, 1992). Such deposits demonstrate the very ductile flow behavior of wet, fragmented serpentinite. Superficially similar foliated, sheared serpentinite was recognized in cores from sediments above the serpentinite at Site 897 (e.g., in the interval 897C-65R-2, 90-97 cm) and also mixed with sediments at Site 899 (e.g., in the interval 899B-31R-2, 32-37 cm). All these examples, and in particular the well-described Quaternary examples recovered on ODP Leg 125 (Fryer and Mottl, 1992) display a foliated character distinct from the brittle-textured breccia units under consideration.
Bedded units in this category include a spectrum of types ranging from debris-flow deposits, commonly with a mud matrix, to rock avalanche deposits with a cataclastically generated matrix. Many deposits termed "olistostromes" probably fall into this category (Abbate et al., 1970) and this term has been widely applied to serpentinite-rich masses in the Appenines. Olistostromes often contain giant clasts and a well-defined clast/matrix dichotomy is invariably present—a feature missing in the breccia units considered here. Yarnold (1993) has clearly distinguished debris-flow deposits from rock-avalanche or landslide deposits and outlined their characteristics when formed under subaerial conditions. Our observations at Site 899 show many similarities with subaerial landslide deposits and the major features of these units are summarized below. Fortunately, the origin of some of these deposits is unambiguous—their formation was observed. Unfortunately, rock-avalanche deposits have been mainly described in terms of their large-scale features, whereas the Site 899 serpentinite breccias are, of course, only known from the description of a single vertical core, making the comparison difficult.