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The Internal Structure of the San Andreas Fault at Depth from Study of the Exhumed Punchbowl and San Gabriel Faults

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The Internal Structure of the San Andreas Fault at Depth from Study of the Exhumed Punchbowl and San Gabriel Faults The Internal Structure of the San Andreas Fault at Depth From Study of the Exhumed Punchbowl and San Gabriel Faults Frederick M. Chester Center for Tectonophysics, Department of Geology & Geophysics, Texas A&M University, College Station, TX 77843, USA. Introduction The internal structure of fault zones in the upper continental crust varies considerably. There is variation along strike in the form of bends and segmentation, and with depth due to changes in metamorphic grade and fluid-rock interaction, and the associated changes in deformation mechanism. The thickness of faults increases with displacement, but many faults display evidence of slip localization in the form of tabular zones of concentrated shear bordered by a damage zone of fractured and faulted rock. Studies of the Punchbowl and San Gabriel faults of the San Andreas system in southern California have focused on defining the thickness and internal structure of mature fault zones, and indicate that large displacement faults consist of several basic structural elements (Figure 1). A zone of concentrated shear, referred to as the fault core, is often defined by reorientation and destruction of primary structures in the host rock, development of cataclastic foliations, presence of extremely comminuted material such as gouge or ultracataclasite, and pronounced mineralogic alteration. At the macroscopic scale the fault core represents the principal slip surface of the fault; almost all shear displacement across the fault is achieved in the core. Further concentration of shear displacement within the fault core is recorded by mesoscopic scale discrete slip surfaces, some of which have accommodated > 1 km of slip. A fault zone may contain a core near one or both boundaries of the damage zone, a single core centralized in the damage zone, or an anastamosing and segmented network of several cores within the damage zone. Damage zones display a greater intensity of deformation relative to the surrounding host rock, and contain features such as subsidiary faults and fractures, microfractures, folded strata, comminuted grains, neomineralization, and veins. In general, the intensity of damage increases towards the fault core, the thickness of the damage zone varies laterally, and the transition from undeformed host rock to damage zone rock often is gradational. Representative observations of the internal structure of the Punchbowl and San Gabriel fault follow. Internal Structure of Principal Faults (1) (2) (3) (4) (3) (2) (1) (1) Undeformed Host Rock (2) Damaged Host Rock (3) Cataclasite (Foliated) Fault Core } Fault Zone { (4) Ultracataclasite Layer Figure 1. Schematic diagram showing the basic structural elements of mature fault zones. Variation in structure along and between faults is common as described in text. After Chester et al (1993). The Punchbowl and San Gabriel Faults The Punchbowl and San Gabriel faults, now largely inactive, were main components of the San Andreas transform system in the central Transverse Ranges of southern California during the Miocene and Pliocene. In the San Gabriel Mountains, the Punchbowl and San Gabriel faults cut post-Paleocene sedimentary rocks and Proterozoic, Jurassic, and Cretaceous crystalline rocks of the San Gabriel basement complex. The San Gabriel fault is one of the oldest components of the modern San Andreas fault system. Miocene (12 to 5 Ma) faulting on the San Gabriel fault accounted for 42-60 km of right- lateral separation. After the late Miocene, most of the transform displacements in the central Transverse Ranges occurred 30 km northeast on the Punchbowl fault and on the presently active trace of the San Andreas fault. Total right-lateral separation on the Punchbowl fault is approximately 44 km (e.g., Dibblee, 1968). Uplift and erosion of the San Gabriel Mountains have exhumed the Punchbowl and San Gabriel faults to provide excellent exposures of the products of faulting at 2 to 5 km depth. Uplift of the San Gabriel Mountains since the Pliocene is largely a result of dip-slip motion on the northward-dipping Sierra Madre-Cucamonga thrust system and by regional arching of the Transverse Ranges. In addition, the San Gabriel basement complex was a site of high relief throughout the Miocene, so older faults in the complex may be even more deeply exhumed. Microstructures and mineral assemblages of the fault rocks from the San Gabriel and Punchbowl faults are consistent with faulting at several kilometers depth (Anderson et al., 1983; Chester et al., 1993). By analogy with nearby active faults, we assume that the Punchbowl fault was seismogenic and that the structure of the fault records the passage of numerous earthquake ruptures. Structure of the Damaged Zone The extent of damage present along the fault zones is illustrated by the density of mesoscopic scale fractures and subsidiary faults and of microfractures as a function of distance from the main ultracataclasite layer of the Punchbowl fault (Figure 2a, b). Although there is large variability in mesoscopic fracture density, there is a general decrease in density with distance to low levels, probably representative of regional deformation intensity, at approximately 30 m (Chester & Logan, 1986). The boundary between the damaged zone and the surrounding undeformed host rock is gradational and irregular. The locally high fracture densities are often associated with larger subsidiary faults (Figure 2a). Observations suggest a hierarchical structure with large subsidiary faults clustered about the Punchbowl fault, and where each subsidiary fault has a halo of smaller fractures and faults about it. Microfracture density also displays a decrease in density to relatively constant and low density at about 100 m. Similar relations exist for the San Gabriel fault (Chester et al., 1993). Subsidiary faults and fractures display preferred orientation within the damaged zones of both the San Gabriel and Punchbowl faults. In both cases, subsidiary faults form a quasi-conjugate geometry with the bisector oriented at high angles to the master fault. Microfractures in the damaged zone of the Punchbowl fault have preferred orientations nearly perpendicular to the fault surface and to the slip vector of the fault, whereas fabric outside the fault zone are nearly random (Wilson, 1999). a. Mesoscopic Fracture Density – Punchbowl Fm. b. Microfracture Density of the Punchbowl Fm. 200 70 Locations of Large Subsidiary Faults Cataclasite Fine grained 60 Medium grained 150 Coarse grained 50 100 40 50 30 DP10 DP11 20 DP15 DP6 Linear Fracture Density (#/m) 0 0.1 1 10 100 Linear Fracture Density (mf/mm) 10 0.01 0.1 1 10 100 103 104 Distance From Punchbowl Fault Ultracataclasite (m) Distance from Punchbowl Fault Ultracataclasite (m) Figure 2. Density of fabric elements in the Punchbowl Formation as a function of distance from the ultracataclasite layer of the Punchbowl fault. a) After Chester & Logan (1986). B) After Wilson (1999). Structure of the Fault Core A relative measure of the amount of shear displacement in the fault zone is provided by shear induced reorientation of pre-existing fabric elements of the host rock. Along the San Gabriel fault at Devil's Canyon, the reorientation of granite dikes is largely confined to a zone several meters thick about the central ultracataclasite layer. The fault core also may be identified on the basis of several microscopic fabric elements. Volume percent of neomineralized particles and comminuted particles increase dramatically within several meters distance of the ultracataclasite and mark the boundary of the fault core (Chester et al., 1993). Similarly, loss on ignition measured by XRF increases in the fault core, which represents the increase in synfaulting alteration products of clay and zeolite within the core. Similar relations have been documented for the Punchbowl fault core. In many cases the cataclasites in the cores of the faults display composite planar fabrics geometrically similar to S-C fabrics of ductile shear zones and R-Y-P fabrics of brittle zones (Chester & Logan, 1987; Chester et al., 1993). Microfractures in the core of the Punchbowl fault occur in preferred orientations similar to that in the damaged zone with an additional set of fractures approximately parallel to the fault. The fault parallel fractures probably are related to shearing in the fault core. Ultracataclasite Layer, Punchbowl Fault, Devil's Punchbowl, CA Cataclastic Host Rocks Leucocratic basement Melanocratic basement Medium sandstone Fine sandstone 0.5 m Ultracataclasite Dark yellowish brown Olive black 0.5 m Figure 3. One of four detailed maps made of the core of the Punchbowl fault showing structure of the ultracataclasite layer. The thick black line shows the location of the prominent fracture surface interpreted as the slip surface during final stages of faulting. Sample locations indicated by the black-outlined red polygons. After Chester & Chester (1998). Detailed maps of the ultracataclasite layer of the Punchbowl fault core (at scales of 1:10 and 1:1) show similar features at four localities spaced up to two kilometers apart (Chester & Chester, 1998; Kirschner & Chester, 1998). Most displacement on the fault occurred within a < 1-m wide zone of ultracataclasite. The boundaries between the ultracataclasite and surrounding (proto)cataclasite are extremely sharp, but not parallel or planar on the meter-scale. On the basis of color, cohesion, fracture and vein fabric, and porphyroclast lithology, two main types of ultracataclasite are distinguished in the layer: an olive-black ultracataclasite in contact with the basement, and a dark yellowish brown ultracataclasite in contact with the sandstone. The two are juxtaposed along a continuous contact that is often coincident with a single, continuous, nearly planar, prominent fracture surface (pfs) that extends the length of the ultracataclasite layer in all exposures (Figure 3). No significant mixing of the brown and black ultracataclasites occurred by offset on anastamosing shear surfaces that cut the contact or by mobilization and injection of one ultracataclasite into the other. The ultracataclasites are cohesive throughout except for thin accumulations of less cohesive, reworked ultracataclasite along the pfs.
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