Description:

_Journal of Structural Geology 32 (2010) 1488-1499_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Tabular fracture clusters: Dynamic fracturing produced by volatile expulsion, Sierra Nevada Batholith, California P. Riley*, B. Tikoff University of Wisconsin-Madison, 1215 W. Dayton St., Madison, WI 53706, USA Article info Article history: Received 14 April 2010 Received in revised form 1 September 2010 Accepted 18 September 2010 Available online 25 September 2010 Keywords: Tuolumne intrusive suite Dynamic fracturing Volatile expulsion Fracture Sierra Nevada batholith Abstract An undocumented type of fracture system, Tabular Fracture Clusters (TFCs), occurs in the Cathedral Peak granodiorite, Sierra Nevada Batholith, CA. TFCs are linear zones of sub-parallel, densely spaced fractures, approximately 4-40 cm wide and 3-100 m long. TFCs occur in highest density adjacent to the Johnson granite porphyry, which intruded the Cathedral Peak granodiorite. Individual fractures in TFCs exhibit only opening-mode displacement. Microstructural analysis indicates that TFC fractures contain micro-breccia of angular clasts of host rock. Fine-grained zeolite and quartz are observed along many of the fractures within TFCs, all of which are absent in the host rock. The characteristics of TFCs suggest they differ from previously documented fracture systems in the Sierra Nevada Batholith. The dense spacing, nature of the fractures, and association with miarolitic cavities provide compelling evidence that TFCs formed in response to volatile overpressure from the adjacent Johnson granite porphyry. We attribute the formation of TFCs to dynamic fracturing based on the clustered nature of extension fractures and geometric similarities to other dynamic fracturing arrays. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction The process of fracturing results in numerous fracture types that organize in different manners. Within fracture sets, shear fractures (fractures with observable shear displacement) are commonly clustered (e.g., Mitra, 1984; Little, 1996; Watterson et al., 1996), but extension fractures (fractures displaying only opening-mode displacement) are generally anti-clustered. This difference is attributed to the stress reduction shadow produced once a single extension fracture has formed (e.g., Price, 1966; Hobbs, 1967; Gross, 1993). Clusters of extensional fractures are often attributed to heterogeneities (terminations, step-overs) produced by shear fractures and faults (e.g., Cruikshank and Aydin, 1995; Myers and Aydin, 2004). In the absence of shearing, experimental evidence suggests that extensional fracture clusters can form due to high strain rates (2 × 10-3 s-1 to 7 × 10-3 s-1) through dynamic fracturing (Sagy et al., 2001; Griffith et al., 2009). Numerous experimental examples of dynamic fracturing exist (Sharon and Fineberg, 1996; Sagy et al., 2001; Griffith et al., 2009), but field examples are presently limited to shatter cones (Sagy et al., 2002, 2004) and pseudotachylyte injection from earthquake rupture (Di Toro et al., 2005). In this study, we describe a fracture system in the Cathedral Peak granodiorite, Sierra Nevada Batholith. This fracture system consists of cm-m wide clusters of densely spaced, sub-parallel extension fractures, which we term Tabular Fracture Clusters (TFCs). On a regional scale: most TFCs form in a single orientation, but locally two orthogonal sets are developed; and TFCs are spatially associated with the exposed and subsurface extent of the Johnson granite porphyry. Strain estimates from TFC arrays record elongation perpendicular (0.5-4.5°) and occasionally parallel (0.4-2.1°) to the long axis of the Johnson granite porphyry. We propose that TFCs are a result of dynamic fracturing, formed by the rapid release of overpressured volatiles, evidenced by a series of fluid-rich magmatic features associated with these structures. The intrusion of the Johnson granite porphyry provides the source of the volatiles, the rapid strain rates necessary for dynamic fracturing, and explains the array of TFC orientations. 2. Geologic setting The Tuolumne Intrusive Suite (Fig. 1), situated in the east-central Sierra Nevada Batholith, is a well-known example of a nested plutonic complex, consisting of a series of plutons that become increasingly felsic towards the intrusive suite center (Bateman, 1992). From the outermost (oldest) plutons inward, the intrusive suite consists of the Kuna Crest granodiorite (93.5 ± 0.7 Ma), the Glen Aulin tonalite (93.1 ± 0.1 Ma), the Half Dome granodiorite (92.8 ± 0.2 Ma at its exterior to 88.8 ± 0.8 Ma at its interior), the Cathedral Peak granodiorite (88.1 ± 0.2 Ma), and the Johnson granite porphyry (85.4 ± 0.1 Ma) (Coleman et al., 2004). The older plutons at the margins are predominantly granodiorite (total percent K-feldspar <20%), and progressively grade to granite in the younger plutons (total percent K-feldspar >30%). This study focuses on the two innermost plutons, the Cathedral Peak granodiorite and the Johnson granite porphyry. The Cathedral Peak granodiorite is porphyritic, containing large (4-5 cm) K-feldspar megacrysts. Megacrysts are most abundant near the contact with the Half Dome granodiorite, and decrease in both size and abundance towards its interior (Bateman and Chappell, 1979). Despite variations in megacryst size and abundance, the percent K2O is constant across the pluton, with megacrysts forming at the expense of smaller K-feldspar grains. The groundmass is similar in composition to the Half Dome granodiorite, but mafic phases are smaller and less noticeable. With increasing proximity to the Johnson granite porphyry at its interior, an increase in aplite dikes occurs in the Cathedral Peak granodiorite. Aplite dikes can be traced from the Johnson granite porphyry into the Cathedral Peak granodiorite, and are thus thought to be a product of Johnson granite porphyry intrusion (Bateman, 1992; Titus et al., 2005). The Johnson granite porphyry, a fine-grained leucogranite, is the youngest pluton in the Tuolumne Intrusive Suite, emplaced at 85.4 ± 0.1 Ma (Coleman and Glazner, 1997). Mafic phases are almost non-existent. Xenoliths of the Cathedral Peak granodiorite, ranging in size from cm to 100s-m, are abundant and display both sharp and gradational contacts (Titus et al., 2005). Miarolitic cavities, aplite dikes, and pegmatite veins are abundant throughout the Johnson granite porphyry. The fine-grained nature and presence of miarolitic cavities led to the inference that the Johnson granite porphyry was a ‘quenched’ pluton, with abundant fluids (Bateman, 1992). Bateman (1992) suggested that a decrease in pressure could lead to quenching, possibly due to a volcanic eruption. The Tuolumne Intrusive Suite was emplaced at shallow crustal levels (~1-3 kbar or ~3-10 km depth; Ague and Brimhall, 1988) during active tectonism in the Sierra Nevada magmatic arc. During intrusion of the Tuolumne Intrusive Suite, the Sierra Nevada Batholith experienced dextral transpression, causing the formation of numerous shear zones, collectively termed the Sierra Crest shear zone system (e.g., Tikoff and de Saint Blanquat, 1997). Dextral shear zones affect the eastern edge of the Tuolumne Intrusive Suite in the Cascade Lake (Tikoff et al., 2005) and Gem Lake shear zones (Greene and Schweickert, 1995). At its southern margin, the dextral Bench Canyon shear zone in the Ritter Range roof pendant is truncated by the intrusive suite, but may have influenced intrusive suite shape and emplacement (McNulty, 1995; Titus et al., 2005). 3. Tabular fracture clusters (TFCs) Fracture sets in the Sierra Nevada Batholith have received attention dating back to Mayo (1941). Large-scale systematic fracture sets, formed in the Late Cretaceous (Segall et al., 1990), were documented via aerial photographs by Lockwood and Lydon (1975) and Lockwood and Moore (1979). More recent studies of the Late Cretaceous lineaments have been focused in the southern section (i.e., south of 37°14'15" N. lat.) of the batholith (e.g., Segall and Pollard, 1983a, 1983b; Segall et al., 1990; Bergbauer and Martel, 1999; Pachell and Evans, 2002; Pachell et al., 2003; d'Alessio and Martel, 2005). The fractures in this study are variably spaced, ranging from 10s of cm-scale spacing up to 10s of m-scale spacing, and are commonly associated with a bleached zone and mineralization along the fracture plane (Segall and Pollard, 1983a; Segall et al., 1990). Studies on these fracture sets have yielded insight into jointing in cooling plutons (Bergbauer and Martel, 1999) and shear fracture initiation._ Ключевые слова: pollard, sub-parallel, tfc fracture, journal structural, miarolitic cavity, geological society, shear fracture, tabular zone, magnitude higher, spacing, geology, doi, strike-slip fault, peak granodiorite, quartz, total number, hobbs, opening-mode displacement, clustered set, cathedral peak, percent elongation, parallel, sub-surface extent, alteration, dynamic fracture, tfc emanating, structural, tfcs formed, volatile overpressure, length, oriented sub-parallel, occur, aydin, elongation, segall pollard, highest density, wnw direction, central, tuolumne intrusive, orthogonal set, dynamic fracturing, miarolitic, tectonics, cavity, elongation perpendicular, setting, geophysical, structure, miocene magmatism, sierra, magma chamber, late cretaceous, volatile, segall, dynamic, pluton, geological, riley tikoff, formed, nature, pollard segall, saint blanquat, johnson granite, extensional fracture, development, width, formation, material, regional scale, ericson, larger fracture, abbot, aligned sub-parallel, spherical, set, structural geology, sierra nevada, fracture set, peak, pollard christiansen, tfcs, direction orthogonal, bateman, direction, paterson, reches, central portion, extension fracture, feldspar, magma, observed, cathedral peak granodiorite, magmatic feature, sagy, fracture wall, zone, svenson, high, strain, intrusive suite, cm, review, dome granodiorite, cathedral, tfcs occur, large, blake, tfc margin, pattern, martel, journal structural geology, tuolumne meadows, tfc, journal geophysical, tuolumne, host rock, fineberg, alteration zone, batholith, granite porphyry, extension, increasing length, potassium feldspar, journal, ?eld, bulletin, feature, fracturing, intrusive, fault, shear zone, spatial association, tabular, press, glazner, fracture mechanics, long axis, long, johnson granite porphyry, granite, location, perpendicular, nne, magmatic, rapid, dike, field photograph, biotite grain, mechanics, joint, undocumented type, nne-striking tfcs, shear, porphyry, elsevier, fracture, coleman, ruprecht, single fracture, pegmatite, tikoff journal, tectonophysics, area, field, density, interior, rapid release, wa, dense fracturing, observed tfcs, granodiorite, orientation, california, spherical alteration, angular clast, gross, mt, gravity titus, adjacent, atkinson, rapid loading, eld, tfcs exhibit, large fracture, nevada, grain, rock, nevada batholith, fracture density, di toro, freund, magmatic structure, sub-parallel nature, riley, bleached interior, evolution, strike, range, johnson, geologic map, spherical structure, pegmatite dike, granitic rock, tikoff, suite, tfc strike, society, mayo