The Sierra Nevada batholith in California is defined primarily by a Mesozoic volcanoplutonic arc (Figure 1) which includes a series of granitic or granodioritic intrusions as well as volcanic rocks formed in an oblique convergent plate boundary with a right-lateral component of strike-slip motion during the Mesozoic time (Saleeby, 1981).

Figure 1. Schematic diagram showing volcanoplutonic arc tectonic setting of the Sierra Nevada batholith and its geographic location (inset). Locations of two areas referred to in this discussion are also marked.

Most of the older volcanic rocks were eroded away but the remnants of some younger volcanic rocks of Tertiary to Quaternary age can still be found particularly along the eastern Sierra Nevada (Figure 2). The data presented in this case study come from the Donner Pass (Figure 2) and Mt Abbott (Figure 3) areas (for location see inset in Figure 1).

Figure 2. Geologic map and one cross section showing the distributions of the Cretaceous intrusive rocks and the remnants of predominantly Tertiary volcanic rocks in the Donner Pass area, northwest of Lake Tahoe. Notice the presence of a series of northwesterly trending faults with large apparent vertical offsets along the eastern boundary of Sierra Nevada. From Hudson (1951). The rectangle marks the location of a detailed fracture map which will be presented later.

Figure 3. Map showing a series of nearly N-S trending left-lateral strike-slip fault zones offsetting NE to E trending aplite veins up to ~20 m. Faults with this sense of slip and orientation are widespread throughout the MT. Abbott quadrangle. From Segall and Pollard (1983).

There, the glaciated surfaces of the Mesozoic intrusive rocks provide excellent opportunities for observing joints and their infillings, veins (Figure 4a) and the products of their shearing, faults (Figure 4b). The photographs and maps in Figures 4c, and d show examples of splay fractures.

Figure 4. Joints, veins, and strike-slip faults in granitic rock of Sierra Nevada. (a) Joints filled by epidote (dark) and (b, c) sheared veins as evidenced by the offset of a pair of aplite dikes or veins (light colored bands) on a hand sample. (d) Splay joints associated with an array of left-lateral faults. The splays are located on one side of the faults but are generally associated either the tips of the strands or the end region or the whole array. The hand sample in (b) courtesy of D. D. Pollard.

The occurrences of widespread strike-slip faults (Lockwood and Moore, 1979) as well as dip-slip faults (Hudson, 1951; Burnett and Jennings, 1962) in the batholith have been known for a long time. Lockwood and Moore (1979) called the strike-slip faults as microfaults since they have offsets ranging from millimeters to approximately one hundred meters in the Mt Abbott quadrangle. There, these authors identified two sets of faults: One N to NE trending right-lateral set and another E to NE trending left-lateral set. In their words, these two sets form a 'conjugate' pattern and define an overall extension consistent with the Basin and Range extension direction. Hudson (1951) in his cross section showed hundreds of meters of apparent vertical offsets of the Tertiary volcanic rocks along northwesterly trending faults in the Donner Pass-Truckee area (Figure 2).

The above mentioned faults in the Sierra Nevada, particularly those in the Mt. Abbott quadrangle have attracted considerable attention since early 1980s primarily for the purpose of understanding the mechanics and mechanisms of jointing and development of strike-slip faults from shearing of these joints in granitic intrusions. An incomplete list of these investigators includes Segall and Pollard, 1980; Segall and Pollard, 1983; Segall, 1984; Segall et al., 1900; Martel et al., 1988; Martel, 1990; Burgmann and Pollard, 1992; Bergbauer and Martel, 1999; Pachell et al., 2003; Ritz, 2013; Ritz and Pollard, 2013; and Nevitt et al., 2014.

Based on the field relations together with the geochronology of hydrothermal mineral assemblages in fractures and the altered host rock, Segall et al. (1990) concluded that the joints and the strike-slip faults due to the shearing of the joints at the Mt. Abbott area formed between 85 and 79 Ma, soon after the host pluton was emplaced. Bergbauer and Martel (1999) supported this notion using thermoelastic models to determine potential principal stress orientations at the time of the plutons' cooling and compared them with the observed dominant joint orientations (Figure 5a, b).

Figure 5. Several sub-parallel intrusive bodies and the associated fractures in the Mt Abbott quadrangle. The inset on the right is a diagram showing the most compressive principal stress trajectories using a termoelastic model for the Lake Edison intrusion (marked as Kle). From Bergbauer and Martel (1999).

Most if not all the joints in the granodioritic rocks of Mt. Abbott quadrangle are filled by various precipitants, commonly by quartz and epidote (Figures 4a, b, c, d). Although some of the original veins are still undeformed (for example, Figure 4a), a great majority of them has been sheared. The presence of sheared joints in the same orientation as nearby fractures with only opening displacement discontinuity has been taken as evidence for opening mode fractures as the origin of the faults (Segall and Pollard, 1983). Additional evidence is the highly sheared texture of the infill in thin sections, which can be seen under a simple petrographic microscope. Passive markers in the host rock such as aplite dikes show offsets when crossed by faults (Figure 4b and c). However, the most common evidence is the presence of an opening mode joint or vein at or near the tips of a fracture at an acute angle to it (Figures 4c and d) or a younger joint set with an acute intersection angle to the older one (Figure 4d). These are called splay fractures, kink fractures, or wing cracks and more information about them can be found via the link, Splay Joints, under Joints and Structures on the homepage of this knowledgebase. It is common that splay fractures are geometrically connected to the tips of shear fractures and their segments (Figure 4d). However, a casual observer should keep in mind that every splay fracture initiates from a point at or near the tip of a shear fracture but the surface of exposure does not necessarily contain that point. Once initiated, a splay fracture can propagate in all directions and sometimes it may intersect the shear fracture well beyond it's apparent tips at a different horizon. Martel (1990) proposed that some splays in the Mt. Abbott area are related to the tip lines of slip patches along a given shear fracture or fault. However, it is likely that slip patches at the scale of the observations are related to some physical asperities and fine-scale geometric complexities along the original joints or incipient sheared joint surfaces (Ritz, 2013). Another factor influencing the intersection between a shear fracture and a splay fracture which initiated from a different shear fracture nearby (Figure 6a). In this case, there is little to constrain the termination point of a splay fracture. So, care should be taken in interpreting the initiation and termination of splay fractures between two parallel shear fractures.

Figure 6. Progressive development of faults in granodiorite in central Sierra Nevada. (a) A simple left-lateral fault zone made up of neighboring sub parallel sheared joints or small faults which are connected by a series of splay joints bounding rhombohedral blocks (highlighted in light blue). (b) A pair of simple fault zones (highlighted in dark blue) connected by three generations of splay joints dividing the slab into triangular and rectangular fragments. From Segall and Pollard (1983).

Some recent experimental and theoretical results indicate that tensile cracks in more-or-less splay fracture orientation may occur along one side of a dynamic rupture with extremely high propagation velocity (Griffith et al., 2008; Ngo et al., 2012) and along faults with rough surfaces (Dunham et al., 2011). It is possible that some of off fault splays may have been generated by the so called super shear events.

Figures 6a, b are maps showing progressive development of sheared joint-based faults in the granodioritic rocks of the Mt. Abbott quadrangle from a simple case to gradually more complex cases. Segall and Pollard (1983) and Martel et al. (1988) referred to these progressive forms as simple and compound fault zones, respectively. Both types have bounding faults which accommodate most of the slip. The compound fault zones have various generations of splays at the central parts formed by sequential shearing of the first order (initial) splays. It is possible that discontinuous echelon joints may be connected by splay joints or veins when sheared (Figure 7a). Large accumulation of opening across the stopovers between echelon sheared joints or veins are called pull-aparts (Figure 7b) which are typical structures distributed along brittle faults in highly rigid rocks.

Figure 7. (a) A fault zone with sub parallel strands and heavily populated first generation and some second generation splays within the slabs bounded by the neigjhboring sheared joint fault strands. From Martel (1990). (b) A pull apart which is filled by quartz between two left-stepping, left-lateral faults. Note that the offset of the aplite dikes matches well with the length of the pull-apart opening. Redrafted from Segall and Pollard (1980).

Figures 8 and 9 includes a series of diagrams based on the field maps some of which are presented above. 8(a) depicts initial short joint commonly adjacent to other joints. When sheared, the neighboring joints are connected by splays (Fig. 8b). Further dilation of the joints lead to thick veins and in some cases rhombohedral pull-aparts filled by quartz and epidote. Closely spaced highly overlapped or parallel joints, when sheared, are connected by a series of splays at an acute angle forming simple fault zones (Figure 9a). In an hierarchical manner, simple fault zones may be connected by various generations of splays to form compound fault zones (Figure 9b).

Figure 8. Diagrams showing closely-spaced joints (a) and their shearing (b and c). Increasing slip across the sheared joints results in linkage of the sheared segments (b) and larger apertures across the splays(c), which are commonly referred to as triangular or rhomboidal pull-aparts. From Segall and Pollard (1993).

Figure 9. (a) Idealized simple fault zones developed from shearing of closely spaced, parallel joints in granodiorite. (b) Idealized compound fault zone developed from shearing of closely spaced simple fault zones (highlighted by darker blue color) in granodiorite. From Martel (1990).

Figure 10 is a structural map of an area between the town of Truckee and Donner Pass, west of the Lake Tahoe area. In contrast to the joints in the Mt. Abbott quadrangle, most joints in this area are barren. Even though the basic mechanism of faulting is the same as shown in Figure 11, which is sequential shearing of initial joints, splaying and shearing of the splays, the map represents a more complex deformation history resulting in several sets of faults trending approximately N-S, NE-SW, NW-SE, and E-W. This case study shows that faults and related joints in granitic rocks may be very complex due perhaps to multiple stages of inversions. In this particular case, the overprinting Basin and Range type younger deformation over the Mesozoic strike-slip faulting may be responsible for such a complex fault and joint patterns.

Figure 10. Fracture and fault map based on aerial photography mapping and ground checking for the sense and magnitude of slip along some faults. Detailed map in the next figure is of the maps produced at three locations from the lower left corner of the map. Some of the complexities in the fault pattern are probably due to the Tertiary reactivation of the Mesozoic fault and fracture systems. From Aydin et al. 2002.

Figure 11. Detail map showing initial northwesterly oriented joint array which was subsequently sheared generally in a right-lateral sense producing splay fractures in N-NE orientation between adjacent sheared joints. There are a few younger enigmatic fractures in E-W orientation in the upper central part of the map indicating the changing sense of shear locally. The basic mechanism is the same as the mechanism determined at the Mt. Abboutt area. From Aydin et al. 2002.

Figure 12 includes two series of conceptual diagrams (a and b) summarizing fault formation in granitic rocks by opening mode fracturing (jointing) and shearing of the joints based on the field and laboratory models, respectively. The fundamental processes underlying these models are initial sub parallel joints dividing the rock into thin rectangular rock slabs, the rotation and shearing of which result in the formation of fault zones in various orientations. The pattern of the faults produced in the laboratory is commonly referred to as 'conjugate'. The pattern of the field based model in Figure 12 (a) is more complex. It includes multiple faults some of which may be called 'pseudo conjugate or apparent conjugate'.

Figure 12. Conceptual models of brittle fault development in granitic rocks based on the field observations (top row) and laboratory experiments (bottom row). Slightly modified from Martel et al. (1988) and Peng and Johnson (1972).

It has also been reported that pseudotachylite melts (Figure 13) occur along some faults and infills of some fractures in diagonal orientation to the faults (Griffith et al., 2008) indicating local melting associated with frictional slip with high propagation velocity (supershear) and local injection into the related splays and perhaps into other fractures not directly related to the melt generating sliding.

Figure 13. Pseudotachylyte melt along, and diagonal to, a network of faults on a glaciated surface of granitic rocks in Sierra Nevada, California. Some melt bodies were injected into the fractures in splay orientation (inset), but some others appear to be injections into nearby fractures. Photographs courtesy of D. D. Pollard. Detailed accounts of the pseudotachylyte bearing faults and the related structures are in Griffith et al. (2008).