Fracture-aperture size—frequency, spatial distribution, and growth processes in strata-bounded and non-strata-bounded fractures, Cambrian Mesón Group, NW Argentina

Journal of Structural Geology 54 (2013) 54e71

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Fracture-aperture sizedfrequency, spatial distribution, and growth processes in strata-bounded and non-strata-bounded fractures, Cambrian Mesón Group, NW Argentina J.N. Hooker a, c, S.E. Laubach a, *, R. Marrett b a b c

Bureau of Economic Geology, The University of Texas at Austin, Austin, TX 78713, USA Department of Geological Sciences, The University of Texas at Austin, Austin, TX 78750, USA Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, UK

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 January 2013 Received in revised form 13 June 2013 Accepted 24 June 2013 Available online 6 July 2013

In Cambrian Mesón Group, NW Argentina, small faults and three opening-mode fracture sets defined by orientation and cement texture (Sets 1e3) formed sequentially in sandstone that most likely had constant mechanical properties throughout deformation. Yet the opening-mode sets display contrasting fracture-aperture-size distributions, spacing patterns, and tendency to be bed bounded. Set 1 fractures are quartz-filled or -lined opening-mode fractures with crack-seal texture, having a wide range of opening-displacement (kinematic aperture) sizes; they are irregularly spaced and non-strata-bounded fractures. Set 1 macro and microfracture-opening-displacement sizes are well described by a power law with slope
0.8. Set 2 fractures are microscopic, mostly quartz filled and have characteristic aperture sizes, are probably not bed bounded and have either a near-random or clustered spatial distribution. Set 3 fractures are quartz-lined, opening-mode fractures with extensive open pore space, having a narrow (characteristic) opening-displacement size distribution; they are regularly spaced and stratabounded. Differences between Sets 1 and 3 can be accounted for by quartz deposition resisting fracture reopening to a greater extent for Set 1 during repeated, episodic growth, where crack-seal texture is present in fracture-spanning quartz. In contrast Set 3 fractures are nearly barren with only trace-cement deposits that did not resist opening. Power-law opening-displacement size distributions may be favored in cases where fracture growth is unequally partitioned amongst variably cemented fractures, whereas a characteristic size is favored where growth is unaffected by cementation. Results imply that thermal history and diagenesis are important for fracture-size-distribution patterning. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Diagenesis Crack seal Fractal Fracture Height classification Microfracture Vein Spacing Stratigraphy

1. Introduction Size and size distribution can profoundly influence how fractures transmit and store fluids, and size attributes are therefore of interest in many practical applications (National Academy of Sciences, 1996; Guerriero et al., 2013). Opening-mode fractures vary markedly in displacement, length and height (i.e., size), in some instances over many orders of magnitude within a given array (Marrett et al., 1999; Odling et al., 1999; Gillespie et al., 2001). Fracture-size distributions may be centered on a specific value, with progressively fewer smaller or larger fractures presentda characteristic (or size-restricted) size distribution (Priest and Hudson, 1976; Dershowitz and Einstein, 1988; Gillespie et al.,

* Corresponding author. Fax: þ1 512 471 0140. E-mail address: [email protected] (S.E. Laubach). 0191-8141/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jsg.2013.06.011

1993; Hooker et al., 2012). Or fractures may follow distributions that lack characteristic size scales, such as power-law size distributions (Wong et al., 1989; Marrett et al., 1999; Odling et al., 1999; Gillespie et al., 2001; Ortega et al., 2006; Hooker et al., 2011). These disparate fracture-size and size-distribution patterns result from growth governed by rock properties, flaws, and loading and environmental conditions (Lawn and Wilshaw, 1975). In bedded sedimentary rocks, size patterns can be influenced by rock and fracture mechanics properties, dimensions (thickness), and interface properties of strata (Hobbs, 1967; Narr and Suppe, 1991; Gross and Engelder, 1995; Odling et al., 1999; Bai and Pollard, 2000; Gillespie et al., 2001; Supak et al., 2006; Tang et al., 2008). Other factors may be important in basinal settings (>3 km depth; ca. 80  C or more) in which environmental conditions such as hot, reactive fluids can promote or resist growth. Crack-seal texture in veins (Ramsay, 1980) and in some fractures in sedimentary rocks (Laubach et al., 2004) shows that cement can accumulate while

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fractures are growing. The potential for fracture growth and cement accumulation to interact and modify how fracture sizes evolve is just beginning to be explored (Olson et al., 2009; Laubach et al., 2010; Hooker et al., 2012). Here, for fracture sets formed within the same rockdquartzarenite beds of the Cambrian Mesón Group, NW Argentinadwe describe fracture-cement texture, size distributions, spacing, and tendency for fractures to be confined to beds. Hostrock mechanical properties most likely varied little during deformation (sandstones were thoroughly quartz cemented prior to fracture growth), but the fractures were subject to differing quartzcement accumulation rates and amounts. Fractures of Set 1 grew under conditions of extensive quartz accumulation. Set 1 fractures have a wider opening-displacement size and spacing range and are less likely to be stratabounded than are fractures of Set 3 that formed under conditions of trace-quartz accumulation. This difference suggests that accumulation affected partitioning of opening among growing fractures. Contrasts in opening-displacement size distribution, spacing, and height in a given rock may reflect thermal pathway (burial and exhumation history). 2. Geologic setting Huasamayo, Perchel, and Yacoraite Canyons, NW Argentina, contain outcrops of fractured Cambrian Mesón Group sandstone (Figs. 1 and 2). In Perchel and Yacoraite Canyons, folds and faults are prominent, whereas Huasamayo Canyon contains exposures of beds having uniform homoclinal dip (Fig. 2). In Huasamayo Canyon, the lowest rocks exposed are upper Proterozoic to lower Paleozoic Puncoviscana Formation sandstones, mostly turbidite deposits that underwent low-grade metamorphism and exhumation before deposition of overlying sediments (Turner, 1960; Aceñolaza et al., 1999; Aceñolaza, 2007). Unconformably overlying Puncoviscana Formation sandstones are hundreds of m of red and pink, wellsorted, medium to fine sandstone and siltstone and laminated silty mudstone of the Middle Cambrian Mesón Group (Turner, 1963, 1970; Kumpa and Sanchez, 1988; Such et al., 2007). Beds are typically several cm to a few dm thick. Tabular and trough crossbeds and lenticular to wavy and flaser beds are common. Mature mineralogy, bedforms, and local Skolithos burrows indicate deposition in a marginal marine setting (tide-dominated shelf) (Turner, 1970). These quartzarenites are thoroughly indurated with quartz cement (little or no porosity). Conformably overlying the Mesón Group are the restricted fluvial and tide-dominated-estuarine to open-marine deposits of the Upper CambrianeMiddle Ordovician Santa Victoria Group (Buatois and Mángano, 2003) (Fig. 1). Huasamayo Canyon was the focus of study, with additional sampling and observations in Perchel and Yacoraite Canyons and regional reconnaissance. Huasamayo Canyon has some advantages for fracture study owing to the simple, uniform mineralogy and distinct sandstone beds of the Mesón Group, which allow comparison of fracture attributes of different groups of fractures in the same or similar beds. Microstructures are readily imaged, and the homoclinal dip of Mesón Group rocks indicates that fractures are within a single structural domain. In Huasamayo Canyon beds strike NNE and dip moderately (25e 30 ) to the west, consistent with regional Andean fold patterns (Fig. 3). Several tectonic events are recognized in the Andes of northern Argentina, including Late Ordovician and Late Devoniane early Carboniferous folding, rifting during the Early Cretaceous to Eocene, and Andean mountain building and thrusting from the late Eocene (mostly MioceneePliocene) (Mon and Salfity, 1995). The absence of major unconformity between Cambrian and Ordovician sequences suggests that rocks in Huasamayo Canyon were not strongly affected by Ordovician orogenesis (Mon and Hongn, 1991;

Fig. 1. Regional geologic map and stratigraphic column near Tilcara, NW Argentina, modified from Gonzalez et al. (2004). Study locations for each fracture set H ¼ Huasamayo Canyon (main study area); P ¼ Perchel Canyon; Y ¼ Yacoraite Canyon. Samples are from sandstones of Campanario Formation, Mesón Group, of Turner (1963) and are clean sandstone facies type III of Kumpa and Sanchez (1988). Sampled beds above lithic-rich conglomerates locally found lower in section near basal Mesón Group contact. Contour lines in m. Inset, location of field area in NW Argentina.

Mon and Salfity, 1995). Regional rifting occurred during the Cretaceous, along with deposition of the Pirgua Formation (Kley et al., 1997, 2005; Deeken et al., 2006). The relationdif anydbetween regional orogenic events and fractures is not apparent but not an essential constraint for the purposes of this study. 3. Methods 3.1. Data collection Outcrops were selected for continuous exposure and bed surfaces at least several m in length for measuring fracture dimensions relative to bed thickness. Fractures were described and divided into sets (Table 1) on the basis of mode, orientation, and relative timing marked by crosscutting and abutting relations using standard approaches (Hancock, 1985). Fracture-opening displacement and height were measured for fractures that intersect 1-dimensional lines of observation (scanlines) (Fig. 4). Each scanline was

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Fig. 3. Lower-hemisphere, equal-area stereoplots of poles to (a) faults, (b) Set 1, and (c) Set 3 macrofracture orientations showing representative patterns. Bedding also shown (great circles). Circles and crosses for faults ¼ two conjugate fault sets. Open and filled squares for Set 3 ¼ orientations of Beds A and B fractures, respectively.

Fig. 2. Mesón Group beds and fractures. (aed) Homoclinal bed dips and openingmode fractures at high angle to beds, Huasamayo Canyon. View toward NW.

oriented at a high angle to fracture planes. Each fracture’s cumulative opening displacement, or kinematic aperture, was measured at the point where the fracture intersects the scanline. For each fracture set we collected at least one macrofracture (outcrop) scanline and one sample for microfracture analysis. Fracture heights (Fig. 5) and spacings were measured using a tape measure. On the outcrop, opening-displacement sizes were measured using a hand lens and a comparator (Ortega et al., 2006), which allowed opening-displacement size measurement to about 0.05 mm by comparing fracture width with a printed stripe of precisely determined thickness. We subdivided microfracture samples into continuous strips of thin sections by breaking the rock instead of sawing, so that no rock would be lost between thin sections (Gomez and Laubach, 2006), allowing construction of a single, continuous, microscopic scanline extending across multiple thin sections. Images were acquired using an Oxford Instruments MonoCL2 cathodoluminescence (CL)

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Table 1 Attributes of opening-mode fracture sets, Mesón Group, Huasamayo Canyon. Set

Strike

Fill/Pore space

Texture

Spacinge

Relation to bedsg

Size distributionf

Comments

1 2a 2b 3

ENEa WNW NS WNW

Quartz/Traceb Quartz/Tracec Quartz/Tracec Quartz/Extensiveb

Crack seal Massive Massive Veneer

Irregular Irregular Irregular Regular

Non-stratabound Non-stratabound Non-stratabound Stratabound

Power law Characteristic Characteristic Characteristic

Top bound

a b c d e f g

d d

P

In Huasamayo Canyon. In macrofractures (width >0.1 mm). Pore space Set 2 > Set 1. Microfractures only. Coefficient of variation, see Table 2. Opening displacement (kinematic aperture). Perpendicular to beds. P, perfectly bed-bounded.

detector system attached to an FEI XL30 scanning electron microscope (SEM) operating at 15 kV with a large spot size. At 150, a single SEM-CL image has an area of 0.45 mm2 and is 0.77 mm wide; overlapping images compose mosaic strips extending the length of a contiguous thin section. The homogeneous quartz mineralogy of indurated quartzarenite allows straightforward identification of fractures on CL images (Milliken and Laubach, 2000). Microfracture-opening-displacement sizes were measured at the point where they cross the scanline using Didger software, which allows calculation of distances between points drawn on a Cartesian coordinate system overlain on the SEM-CL images (Gomez and Laubach, 2006). 3.2. Fracture-opening-displacement size and spacing Fracture-opening-displacement-size distribution was analyzed using cumulative-frequency plots (Marrett,1996; Ortega et al., 2006). To characterize the size distribution of both microfractures and macrofractures, we plotted fracture cumulative frequency versus opening-displacement and best-fit trendlines to these data. Cumulative frequency is cumulative number (1 for the largest fracture, 2 for the second-largest, and so on) divided by scanline length. For each dataset, the best fit was selected among power law, exponential, lognormal, and normal size-distribution types (Table 2). The relative quality of fit of each equation type was assessed by comparing c2 error values between observed and calculated fracture frequencies. Biases were identified that commonly affect and distort size data, particularly at the small and large ends of distributions owing to resolution limits and incomplete or biased sampling (Baecher and

Lanney, 1978; Bonnet et al., 2001), but such biases were not considered for equation best fitting. The regularity of fracture spacing was quantified by the coefficient of variation of the population of interfracture spacings Cv ¼ s/ m, where s is the standard deviation of the population of spacings and m is the arithmetic mean (e.g., Kagan and Jackson, 1991; Gillespie et al., 1999; Supak et al., 2006) (Table 3). The greater the value of Cv, the more irregular the fracture spacing. A Cv value of 1 is expected for a Poissonian distribution of spacings and signifies spacings between randomly positioned fractures (Gillespie et al., 1999). Accordingly, fractures having more clustering than expected for randomly arranged fractures will have Cv > 1; those having less clustering than random (closer to regular spacing) will have Cv < 1 (Gillespie et al., 1999). The coefficient of variation Cv for fracture spacings is a measure of spacing regularity that does not account for fracture size. But size is important if fracture distributions have a wide size range; larger fractures impart greater strain, and if cementation is contemporaneous, larger fractures may be more porous than smaller fractures owing to their larger volume:surface area (Laubach, 2003). Strain homogeneitydhow fracture strain is distributed with respect to fracture-opening-displacement sizedcan be quantified by using cumulative apertureda running sum of apertures encountered from the beginning to the end of a scanlinedplotted versus scanline position (Kuiper, 1960). A line on this plot connecting the origin and the final cumulative aperture represents homogeneous strain. The more heterogeneous the fracture strain, whether by size variation, clustering, or both, the greater the difference between the data and the line for homogeneous strain. To

Fig. 4. Measurement protocol, fracture scanline (1D). (a) Schematic diagram. Gray areas ¼ host rock; black areas ¼ fractures. Every point along scanline interpreted as belonging to either a fracture or spacing between fractures. X-axis ¼ position along scanline; Y-axis ¼ cumulative fracture opening (i.e., nth fracture, Fn, from beginning of scanline shows cumulative opening-displacement size equal to sum of opening-displacement sizes F1 þ F2 þ F3 þ . þ Fn). Partly after Hooker et al. (2009). (b) Example of interpreted scanline, SEM-CL mosaic, Set 1 microfractures. White line segments ¼ spacings; black line segments ¼ fractures. G, grain.

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(a)

Perfect bed-bounded

Layer 1

Layer 2

Layer 3

(b)

Top bounded

Layer 1

Layer 2

Layer 3

(c)

Hierarchical

Layer 1

Layer 2

h2

h1

Layer 3

(d)

Unbounded

Layer 1

Layer 2

Layer 3 Interface

Key

Fracture

Tip Tip

Fig. 5. Schematic classification of fracture bed boundedness (element of fracture stratigraphy). (a) Perfect bed-bounded. Set 3 commonly follows this pattern. (b) Top bounded; tallest fractures (F) end at bed boundary (Lb). Smaller (shorter, narrower) fractures exist within layer. (c) Hierarchical; fracture heights bounded by layers, but range of interbedded fractured layers is present. Set 3 locally follows this pattern. (d) Unbounded; fracture heights (and lengths) independent of layers. Set 1 follows this pattern. These elements of fracture stratigraphy (Laubach et al., 2009) may also occur as mixtures.

quantify this variation, we added absolute values of the maximum and minimum deviations in cumulative aperture from the homogeneous strain line and divided that sum by the total cumulative aperture. The result is a number, V0 , between 0, perfect strain homogeneity, and 1, maximum strain heterogeneity, which is possible if all strain is manifested in a single fracture (Putz-Perrier and Sanderson, 2008). 3.2.1. Bed boundedness and fracture-opening displacement along scanlines No rigorous measures exist for the tendency of fractures to be bed bounded. One measure of the tendency for fractures to cross

bedding boundaries is the fraction of fractures that cross from a given bed to anotherdknown as vertical persistence (Petit et al., 1994; Gillespie et al., 2001; Strijker et al., 2012). As defined, vertical persistence does not distinguish between large fractures whose propagation was halted at bedding boundaries or small fractures that terminate within the bed, although not because of encountering any mechanically significant stratigraphic horizon. We label as perfect bed-bounded fractures those that span vertically from an upper to a lower bed boundary (Fig. 5). A range of fracture sizes can exist in such a fracture array, but the maximum size is limited in one dimension to match layer thickness. If fractures span a layer (from top to bottom, i.e., they are fully bed bounded), opening-displacement sizes measured by layer-parallel scanline will tend to have a narrow range, and a scanline at any given elevation in the bed should recover a narrow aperture-size range. If fractures are widest in the middle of the layer and displacement approaches zero at the fracture tips, then a scanline in the middle of the bed will capture the widest apertures, and opening displacements will be smaller if the scanline is near a bed boundary. Because a bed-perpendicular slice through the fracture population will intersect fractures at various positions along fracture lengths, the scanline will encounter an opening-displacementsize range that partly reflects this sampling effect (closer or farther from lateral fracture tips) and the length and aspect-ratio distribution of the fractures. But for tabular fractures much longer than they are tall, and randomly placed fractures (in plan view), the effect on most layer-parallel, scanline measured, openingdisplacement distributions will be minor. If fractures are not perfectly bed bounded, then the openingdisplacement-size pattern measured in cross-sectional scanline will depend on the pattern of bed boundedness, which could range between end members: top bounded, hierarchical and unbounded (Fig. 5). A top bounded height distribution occurs when most of the tallest fractures end at the same stratigraphic position (usually a mechanically significant interface), but within the layer, a wide range of smaller fracture sizes (heights) is present that do not extend to the interface (i.e., they are not bed bounded). Vertical height distribution can be said to be top bounded (in the sense of being bounded by a top “size” or height), but below the bed thickness, fractures of a wide range of heights (and apertures) can be present. This bed-bounded pattern can include power-law opening-displacement-size distributions (Ortega et al., 2006) and is the typical bed-bounded pattern for many fracture arrays that include abundant microfractures (Hooker et al., 2009). The hierarchical case occurs when a range of fracture heights closely follows a range of interbedded fractured-layer thicknesses. Fracture tips for a specific fracture height are located as specific interfaces, but arrays of differing fracture heights overlap in a nested, hierarchical fracture-stratigraphy pattern that results from different fractures being bounded by a wide range of interbedded fractured layers, giving rise to a range of vertical fracture positions. Such patterns characterize some coal-fracture patterns, in which a hierarchy of subtle rock-property differences (mechanical stratigraphy) may be marked out by a wide range of fracture heights, each of which is bed bounded (Laubach et al., 1998). A layer-parallel scanline would recover a range of aperture sizes if fractures from all height categories were included. The hierarchical case is intermediate between top bounded and unbounded. Purely hierarchical and hierarchical-top-bounded mixturesdwhere smaller unbounded fractures or microfractures are presentdare also possible. Perfect bed boundedness, in which all fractures in a specific bed span vertically from upper to lower bed boundaries, is a special case of hierarchical arrangement of bed boundedness in which there is only one scale of height-constrained fractures (one layer, one fracture height).

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Table 2 Statistics of micro and macrofracture populations, for each fracture set. Set

Faults 1 1 2 2 2 3 3

Locality/Bed

Huasamayo Perchel A B C A B

Layer thickness (cm)

Size distribution c2 errora Power law

Exponential

Normal

Log-normal

N

Average width (mm)

Average spacing (mm)

Cv

V0

Range N/A N/A 15 36 40 15 36

0.003 10.0 1.08 1.74 0.224 22.1 nd nd

2  10
4 294 37.3 0.057 0.061 2.55 nd nd

0.005 1.62  107 54.2 1.22 1.15 5.35  109 nd nd

5  10
4 56.1 13.7 0.116 0.055 0.438 nd nd

44 24 42

1.8 1.2 0.8

240.2 29.9 79.9

1.92 1.49 1.30

0.32 0.38 0.26

13 9

0.3b 0.3b

88.6 130.3

0.46 0.45

0.15 0.08

Macrofractures

Microfractures N

Average width (mm)

Average spacing (mm)

Cv

V0

416 134 64 17 155

0.033 0.030 0.005 0.004 0.003

0.3 0.8 1.5 4.9 0.8

1.98 1.87 1.53 1.01 2.38

0.39 0.48 0.31 0.34 0.43

nd, not determined. a Lowest c2 error value (best fit) for each distribution equation in bold. b Set 3 fracture widths estimated to be 0.3 mm.

The unbounded end member occurs when fracture upper and lower tips are positioned at any height in a rock mass. In other words, fracture tips are not correlated with respect to a specific bed. Such a pattern could result from fracture in an effectively homogeneous medium, in which fractures are positioned without regard to bed boundaries (no meaningful mechanical stratigraphy). 4. Results 4.1. Mesón Group sandstones Mesón Group sandstone beds are typically several cm to a few dm thick. Trough and tabular crossbeds and lenticular to wavy and flaser beds are common bedforms. Although cross-stratified units are broadly lens shaped in cross-section, over lengths encompassed by our scanlines, beds are close to tabular (uniform thickness). Beds are bounded by sandstone-on-sandstone contacts or by local finergrained laminae (<2 cm thick) marked by slight negative relief. Fine-grained interbeds are siltstones or laminated silty mudstones. All sampled beds are medium to fine quartzarenites composed almost entirely of quartz grains and cement (>90% detrital quartzdQ:92, F:5, L:3). Less-common constituents are microcline, plagioclase, composite quartz-rich lithic grains, volcanic-rock fragments, and trace muscovite and heavy minerals. Clay matrix altered to illite and authigenic chlorites are present in some beds but not in those that we analyzed for fracture patterns. Little or no primary porosity is present (<1%). Although Mesón Group sandstones locally contain quartz grains to more than 2 mm diameter in lower parts of beds, grain size in sampled beds is mostly less than 1 mm (average 0.25 mm). Intergranular volume (IGV) is low, about equal to quartz-cement volume of 6e15%. Low IGV indicates that sandstones were well compacted prior to cementation. Grain interpenetration is minimal, and locally grain-point contacts are apparent in CL images. No bed-parallel stylolites were noted. Pervasive quartz cement has a mosaic

Table 3 Coefficient of variation (for fracture spacings) and strain heterogeneity defined. Abbreviation

Attribute

Reference

Formula

Cv V0

Coefficient of variationa Strain heterogeneity

1 2

s/m P Dmax þ jDmin j= ni¼ 1 ai

References: 1. Kagan and Jackson, 1991; Supak et al., 2006; 2. Kuiper, 1960. a Among fracture spacings; s ¼ standard deviation of fracture spacings; m ¼ average fracture spacing; Dmax, maximum deviation from line of homogeneous strain; Dmin, minimum deviation from line of homogeneous strain; a, aperture; n, number of fractures.

texture in plain light and low (dark) CL response, consistent with quartz cement accumulated in a diagenetic environment. Mesón Group sandstones are extremely hard and brittle; modern cracks break across grains and cement in these diagenetic quartzites. Although we did not measure Mesón Group sandstone mechanical properties, these rocks resemble other Cambrian diagenetic quartzites that have Young’s modulus values of 30e 70 GPa (Ellis et al., 2012). Fracture- and quartz-cement textural relations provide some insight into when these mechanical properties were achieved relative to fracture. The oldest fractures cut across grains and cement. From these data we infer that all three fracture sets in Huasamayo Canyon grew after sandstones were compacted and thoroughly cemented and that mechanical properties were likely uniform throughout deformation.

4.2. Fracture description Mesón Group strata in Huasamayo, Perchel, and Yacoraite Canyons contain barren joints, quartz-lined and -filled veins (openingmode fractures) (Figs. 2, 6 and 7), and small displacement faults. The opening-mode fractures are typically at a high angle (85e90 ) to bedding (Fig. 6). Spacing of prominent fractures is close, commonly a few to tens of cm. Bed thickness defined by slight differences in grain size or rock type or by fracture height ranges from several cm to a few dm. Fractures and numerous thin beds give exposures a blocky appearance (Figs. 2 and 5). Fractures commonly are stained red or whitish yellow. If the stains reflect fluid flow, stain patterns in fractures and host rocks suggest that some fractures were conduits and others barriers (Fig. 6). At Huasamayo Canyon the Mesón Group exposure contains at least three systematic opening-mode fracture sets that contain some quartz cement. Sets have consistent orientations and crosscutting relations (Table 1). Sets 1 and 2 fractures are quartz filled, or they have trace porosity; Set 3 fractures are open and superficially appear barren (no mineral fill) but close inspection shows that many of these joints are lined by small, faceted quartz crystals (Fig. 6). Also present are numerous quartz-sealed microfractures (opening displacement <0.1 mm). Crosscutting relations and orientation patterns show that microfractures can also be divided into sets. Mesón Group strata in Perchel and Yacoraite Canyons contain closely spaced, open but quartz-lined fractures and faults associated with folds and faults, as well as arrays that resemble those in Huasamayo Canyon, although definitive relative-timing evidence is lacking for a firm correlation of sets to be made between localities. Our study focused on opening-mode fracture sets in Huasamayo Canyon.

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Fig. 6. Fractures in outcrop. (a) Fractures with only inconspicuous veneers of quartz cement and little opening displacement (1 mm). FP ¼ fracture plane; BP ¼ bedding plane. Staining on fracture and bedding planes suggests fractures served as conduits and barriers to fluid flow. (b) Quartz-lined fracture (vein) with measureable opening displacementdthis example is w1 mm wide; veins encountered as wide as 3e4 mm. FQ ¼ fracture quartz cement; Ff ¼ fracture pore space.

4.2.1. Faults A population is present within Huasamayo Canyon of small reverse faults having shallow dip relative to bedding, some of which are bed parallel (Fig. 1). These faults are not pervasive but instead are localized and sparse. Faults have mutual crosscutting relationships and intersect at angles of 60 or less, so they are probably genetically related, conjugate thrust faults. Offsets are a few mm or less, with WNWeESE-directed shortening. Slip is kinematically consistent with regional Eocene and later Andean WNW-directed shortening, folding, and thrust-fault movement (Fig. 1). Slip is manifested in aligned quartz rods on fault surfaces (Fig. 8). Most faults are dilatant and marked by tabular damage zones of finite width, containing quartz cement and broken wallrock fragments (Fig. 8). Damage zones include opening- and mixed-mode quartz veins having a wide range of apertures. Within narrow, mm-scale fault zones of brittle deformation, cataclasis and grain-contact stylolites are prominent (Fig. 8). The transition from undamaged host rock to damaged fault rock is gradational over distances of up to a few mm and is marked by a color transition from red to pink host rock to white fault rock (Fig. 8). Fault rocks contain minor (>1%) porosity marked by

Fig. 7. (a) Set 3 fractures in outcrop. (b) Interpretation; bed-bounded fractures. Bedding planes ¼ green (L); fractures ¼ brown (F). Sampled beds A, B indicated. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

irregular to tabular pores lined by faceted quartz crystals. Porosity increases slightly toward the centers of fault damage zones (Fig. 8). Associated microfractures (width <0.1 mm) within fault damage zones are opening-mode or mixed-mode and quartz-filled. Minute pores are locally associated with organic residue near stylolites within fault damage zones (Fig. 8). Fault damage-zone width encountered along scanlines is a convenient measure of fault size. The conjugate population can be described with a single scanline encompassing faults of all sizes and both orientations. A representative fault scanline contains 44 faults within 10.65 m (Fig. 9). Fault damage-zone widths follow a lognormal size distribution (Fig. 10), with an average width of 1.8 mm. Considerable variation in spacing and strain among faults is apparent, with a coefficient of variation (Cv) value of 1.92 and V0 of 0.32. A scanline was drawn on SEM-CL images parallel to the macroscopic fault scanline; no microscopic faults were detected between macroscopic faults over the 11.9 cm observed. Faults show no tendency to be bed bounded. Small faults exist that terminate within individual beds. Larger faults may cut bedding surfaces, showing minor bedding-shortening offset. Faults locally deflect at bedding surfaces, causing bed-parallel offset, especially where bed contacts contain silt or clay laminae.

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Fig. 8. Fault attributes. (a) Fault-plane surface showing white rods of quartz cement, which track fault opening and overlap euhedral quartz-crystal faces. (b) Cross-section view of conjugate faults in layer where scanline was taken (see Fig. 9). Note diffuse boundary between faults (white) and host rock (pink). (c) SEM-CL image, note fractures, and (d) map of fault microstructure. G, grain. Fault zone comprises cataclasized and stylolitized grains and quartz cement. Porosity slightly enhanced within fault zones, to nearly 5% near centers of widest faults, compared with w1% or less within host rock. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

4.2.2. Set 1 opening-mode fractures (quartz veins) Crosscutting the faults is a population of opening-mode fractures that are filled mostly with quartz (i.e., they are veins), although some wider fractures contain trace to moderate (>50%)

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macroscopic residual pore space marked by faceted quartz crystals facing tabular pores (maximum pore width 2 mm). These Set 1 fractures have opening displacements of between 0.33 and 2.65 mmdas measured in the field. In cross-section, Set 1 fracture profiles are elliptical to tabular with sharply tapering tips, and height:width of 70e300. Fractures are perpendicular to bedding and have a narrow range of ENE strikes in Huasamayo Canyon and (locally) NNW in Perchel Canyon. Because Set 1 fractures dip steeply and strike at a high angle to the local NNE-trending fold axis, Set 1 orientation with respect to fold geometry provides no timing information. Crack-seal texture is common in Set 1 fractures, indicating repeated fracture opening and sealing during quartz precipitation (Ramsay, 1980; Hilgers and Urai, 2002; Laubach et al., 2004; Becker et al., 2010). Within fracture quartz, crack-seal texture is marked by bands of quartz cement having contrasting CL response (Fig. 11a and c), wall-rock inclusions, and linear arrays of fluid inclusions parallel to fracture walls (Fig. 12). In general, crack-seal bands are parallel to one another and to fracture walls. Crack-seal bands range in width from less than 1 to a few tens of mm. Small microfractures (<0.1 mm in width) are typically entirely sealed and composed of one or more crack-seal bands. Crack seal is common within 50e 100 mm of the walls of larger (>0.1-mm width) microfractures and macrofractures. Within the central parts of larger fractures, away from crack-seal bands at the walls, quartz cement is typically present in submillimetric euhedral blocks, with zoning apparent in CL (Fig. 11a). Where fracture pore space is preserved, it is typically among blocky euhedral cement zones (Figs. 11 and 12) within fractures greater than 0.5 mm in width. Crack-seal segments locally bridge larger fractures, in bands that overlap individual host-rock grains (Fig. 11c). Generally the wider the Set 1 fracture, the less space within is composed of crack-seal increments. Between regions of continuous crack seal at the fracture walls, Set 1 macrofractures have less than 10% crack-seal texture. The remaining space includes euhedral zoned cement with as much as 50% pore space among euhedral cement blocks. Wider macrofractures have more residual pore space. Pores are generally homogeneously distributed throughout the midregions of macrofractures and are bounded by euhedral quartz faces. Pore diameters range from microns to as much as 2 mm, amount of pore space decreasing toward fracture walls and fracture tips. A characteristic fracture width above which some porosity is preserveddthe emergent thresholddis common in many quartzcemented fracture systems in sedimentary rocks (Laubach, 2003). Parallel to Set 1 fractures is a population of quartz-filled microfractures. Microfractures resemble Set 1 macrofractures in shape, orientation, and crosscutting relations with other host-rock cement. Microfractures typically cut across grains and intergranular quartz cement with no deviation in trajectory, suggesting that the rock was cemented and indurated when fractures grew (Fig. 11). In common with larger Set 1 fractures, microfractures have little dispersion in strike (Fig. 13); 78% of microfractures strike within 25 of perpendicular to the scanline. Set 1 macrofractures and this population of microfractures are probably genetically related because all size ranges have common orientation (Fig. 13), crosscutting relations, and quartz-cement fill and texture. Because the microfractures most likely formed along with Set 1 macrofractures, we refer to them as Set 1 microfractures. A wide range of opening-displacement sizes is apparent for Set 1 microfractures, from 0.001 to 0.12 mm (Fig. 9). Set 1 fractureopening-displacement sizes were documented along one macrofracture scanline and one parallel microfracture scanline through the same rock (Fig. 4). The macrofracture scanline is 1.12 m long and intersects 24 fractures; the microfracture scanline is 132 mm long and intersects 418 fractures (Fig. 9).

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Fig. 9. Scanline data. X-axis ¼ distance along scanline; Y-axes ¼ opening displacement and cumulative opening displacement (damage-zone width, for faults). (a) Macroscopic faults; (b) Set 1 macrofractures; (c) Set 1 microfractures; (d) macrofractures in Perchel Canyon; (e) microfractures, Perchel Canyon; (f) Set 2, Bed A; (g) Set 2, Bed A, strike filtered; (h) Set 2, Bed B; (i) Set 2, Bed B, strike filtered; (j) Set 2, Bed C; (k) Set 3, Bed A; (l) Set 3, Bed B. Opening displacements in k and l estimated; see text. Cambrian Eriboll Formation quartzarenites that resemble Mesón Group sandstones in composition and fracture style also have a roll-off at similar opening-displacement values that cannot be accounted for by censoring (Hooker et al., 2011).

The size distribution of the combined population was best fit by normalizing the cumulative number to the corresponding scanline length (Marrett et al., 1999). This size distribution is better fit by a power-law equation than by exponential, normal, or log-normal distribution (Fig. 10; Table 2). At the large-size limit of distribution, i.e., the part of the distribution representing macrofractures, the data form a concave-downward curve in logelog space, indicating a departure from power-law scaling. This departure may be either real or the effect of sampling bias; we discuss our interpretation later. But over approximately three orders of magnitude (0.001e1 mm), Set 1 fractures follow a power-law aperture-size distribution having a scaling exponent of about
0.8. The observed power law for the Set 1 opening-displacement distribution persists to the opening-displacement size where fracture porosity is widespread (the emergent threshold). In other words, the power-law relationship breaks down at the fracture size where extensive porosity is widespread. This suggests that differences in size patterns correlate with variations in degree of cement

fill. This pattern also implies that power-law extrapolation of microfractures can be used to predict the minimum spacing of open fractures. For our observed emergent threshold of 0.5 mm of opening displacement for Set 1 fractures, the microfracturefrequency power-law equation predicts an average spacing of 27 mm. This value is within a factor of two of the measured average spacing of 51 mm. Set 1 macrofractures have a Cv value of 1.49, indicating irregular spacing. V0 for macrofractures is 0.38; although strain heterogeneity has no absolute standard, V0 can be compared between fracture populations so as to assess relative strain heterogeneity. The corresponding microfractures have Cv and V0 values of 1.98 and 0.39, respectively, indicating slightly higher spacing irregularity than that of macrofractures. Similar tendencies toward clustering at different fracture-size scales reinforce the conclusion from orientation and relative timing that the large and small fractures of Set 1 are different-size fractions of a single fracture population.

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Fig. 10. Cumulative-frequency distributions of (a) fault damage-zone width and (bef) fracture-opening displacement. Open symbols ¼ measurements made using hand lens; filled symbols ¼ measurements made on SEM-CL images. Trendlines correspond to best-fit equationsdkey is inset in A. (a) Faults; (b) Set 1 macro and microfractures; (c) Set 1 macro and microfractures, Perchel Canyon; (d) Set 2, Bed A; (e) Set 2, Bed B; (f) Set 2, Bed C. Set 2 azimuth-filtered subsets not plotted because of low numbers of fractures. Set 3 fractures not plotted because width only estimated.

Although Set 1 fractures either cross bed boundaries or terminate within beds, the plan view exposure of Set 1 fractures at Huasamayo Canyon impedes quantification of vertical persistence. However, microstructural (crack-seal) textures, aperture distribution, and spatial arrangement of Set 1 fractures closely resemble those of correlative fractures exposed at Perchel Canyon (Fig. 1; Table 2) that show no tendency to stop at bedding boundaries (Fig. 14). The centers and tips of these fractures have no systematic location with respect to bedding. These fractures qualify as unbounded, according to the criteria given earlier. 4.2.3. Set 2 microfractures Another population of microfractures, Set 2, is present at two sampling localities (Fig. 1). All Set 2 fractures are microscopic and were discovered only in thin section. Set 2 is defined by size and cement texture (see later description). At sample location 1, Set 2 fractures strike NeS; at location 3, Set 2 fractures strike WNW, and in one bed they are less systematically aligned. The relative timing of the populations (Set 2a, b) having different orientations is unknown, and the “sets” may be unrelated. Here we refer to the subpopulations by strikedSets 2NeS and 2WNW. Set 2NeS cuts the faults and therefore postdates them. No definitive crosscutting relationship was observed between Set 1 fractures and those of Set 2, although Set 1 micro- and macrofractures are cut by microfractures that could correspond to Set 2. Set 2 microfractures cut across grains and intergranular quartz cement and are sealed mostly by quartz cement, except locally where the fractures cut nonquartz grains (Fig. 15). Such an arrangement typically amounts to less than 5% fracture porosity within Set 2. In contrast to Set 1 fractures, however, size dependence to fracture porosity is not apparent within Set 2, although the size range for Set 2 is much smaller. As with Set 1 fractures, the porosity-preservation pattern within Set 2 most likely stems from the tendency for quartz precipitation for a given temperature to be slower on nonquartz substrates (Lander and Walderhaug, 1999). The preservation of pore spaces among narrower Set 2 fractures suggests that these

microfractures are younger than those of more thoroughly cemented Set 1 fractures. The cement within Set 2 microfractures is generally textureless. Local branching and linking of fractures is present, but Set 2 fractures lack crack-seal texture (Fig. 15). Set 2 microfracture scanlines were constructed within three beds, Set 2WNW fractures were measured in Beds A and B, and Set 2NeS fractures were measured in Bed C (Fig. 13). Bed B microfractures are less systematically aligned, with strike maxima at 015, 155, and 175 (Fig. 13). Microfracture scanlines 9.4 and 8.5 cm in length were analyzed in Beds A and B, respectively (Table 2), where Set 3 macrofractures were also analyzed (Fig. 7). The microscanline for Bed A intersects 64 microfractures, and that for Bed B intersects 17 microfractures (Fig. 9). The microfracture sizes were analyzed in two ways: (1) collectively and (2) by filtering out microfractures outside a 30 strike window centered on a strike perpendicular to the scanline. The filtered datasets thus include only those fractures striking WNW (Fig. 13). The size distributions of all (unfiltered) microfractures in both beds are best fit by log-normal equations (Fig. 10). Within the window parallel to Set 3 strike in Bed B, 38 microfractures are present in Bed A, and only 4 aligned microfractures are presentdtoo few to define size distributions with confidence. Set 2WNW microfractures range in aperture size from 0.0009 to 0.02 mm (average 0.005) in Bed A and from 0.0007 to 0.01 mm (average 0.004) in Bed B. Because Set 2NeS fractures are strongly preferentially aligned (Fig. 13), no azimuth filtering was performed. The Bed C microscanline is 11.9 cm long and intersects 155 fractures. Set 2NeS fractures are best fit by a log-normal distribution, with a range in aperture of 0.0003e0.02 mm (average 0.003). Thus, being internally textureless, Set 2 microfractures in all beds resemble the individual crack-seal increments of Set 1 fractures in that each is generally straight bands of CL-dark-quartz cement with mm-scale widths. When all Set 2WNW microfractures of various orientations are analyzed together, spatial arrangement as documented by Cv values is 1.53 and 1.01 for Beds A and B, respectively, indicating that

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Fig. 11. SEM-CL images of Set 1 fractures. (a) Macrofracture (opening displacement w2 mm) with crack-seal texture near fracture walls and blocky, euhedral quartz cement near center. Note pores among euhedral cement blocks. (b) Interpretation of A. (c) Microfracture with crack-seal cement overlapping host-rock grain. (d) Map of C. (e) Set 1 microfractures entirely occluded by quartz cement. G, grain.

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Fig. 12. Set 1 fractures, plane-polarized light image. Note porosity (blue, P) within fracture A, slightly wider than 0.5 mm. Arrows, FIA ¼ fluid inclusion assemblage. B, microfractures readily discernible using SEM-CL (Fig. 11) mostly visible as FIA’s using light microscopy. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

microfractures are more regularly spaced than are Set 1 fractures. V0 is 0.31 and 0.34 for Beds A and B, respectively, also suggesting a more regular spacing for Set 2. Considering only those microfractures that strike WNW, Cv values for Beds A and B are 1.28 and 0.54, respectively, and V0 values are 0.25 and 0.21, respectively. Thus, filtering microfractures by azimuth reveals more regular fracture spacing in both layers. For Set 2NeS fractures, Cv is 2.38 and V0 is 0.43. Set 2NeS fractures are thus more irregularly spaced than both Sets 1 and 2WNW. The regularity of spacing of Set 2WNW fractures is intermediate between that of Sets 1 and 3, by either fracture-spacing measure used. The associated Cv near 1.0 for each population is consistent with a nearrandom fracture spatial arrangement (Gillespie et al., 1999). Set 2Ne S fractures are more clustered. Because Set 2 fractures were sampled only in thin sections taken from the middle of layers, the degree to which the fractures are bed bounded is unknown, but given the microscopic scale of these fractures they are unlikely to be bed bounded, at least on the scale of bedding that affects large fractures of Set 3. Fractured-layer thickness for Set 3 identified by the height of layer-spanning macrofractures (Set 3, next section) of Bed A is 15 cm, and of Bed B is 36 cm (Fig. 7). No clear fracture Set 3 heights are present in Bed C, but this bed is bound above and below by recessed silt laminae, approximately 40 cm apart. Measurements of Set 2c fracture length and width made from SEM-CL maps comprise entire-fracture lengths ranging from 148 to 1072 mm. Corresponding apertures range from 0.85 to 6.20 mm and show a slight positive correlation with length, though their relationship is difficult to define owing to the low number of fractures that were imaged from tip to tip. Nonetheless, extrapolation of this relationship to the length scale corresponding to the layer thickness, fractures that span the height of the layers should be 0.4 mm wide or more. Thus the observed aperture range probably represents non-stratabound fractures.

Fig. 13. Rose diagrams of microfracture strikes, (a) Set 1 and (b) Set 2. Scanline orientations ¼ Set 1, 157 (Bed C); Set 2, 020, 028, and 080 (Beds A and B). Bar and circle segment ¼ mean and 95% confidence. Strike measured only for fractures that intersect scanline, systematically undersampling fractures striking at low angle to line of observation. More strike dispersion evident in rose diagrams of fractures measured in 2D areas.

Length:width aspect ratios are between 100 and 425, consistent with published measurements of macroscopic fractures (Vermilye and Scholz, 1995; Johnson and McCaffrey, 1996). If the observed Set 2 microfractures are bed bounded, they would need to be exceptionally tall for their widths, having height:width ratios of 7500e6.7  105. 4.2.4. Set 3 quartz-lined, opening-mode fractures A third population of fractures is present within the same bedsdA and Bdfrom which Set 2WNW fracture data derive. Set 3 consists of bedding-perpendicular, opening-mode fractures with strong WNW preferred strike (Fig. 3), regular spacing, and a low range of opening-displacement sizes (no fractures as wide as 1 mm were observed). Estimated height:width is typically less than 1500:

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Fig. 14. Bed boundedness of Set 1 fractures, Perchel Canyon. (a) Photograph. F, quartzfilled fractures; L, bed interfaces. (b) Interpretation, unbounded (non-bed-bounded) fractures. Size, spatial distribution and quartz-cement textures match Set 1 fractures, Huasamayo Canyon (Figs. 9 and 10).

The typical crystal covers the fracture wall over an area close to grain size most likely because each crystal grows in crystallographic continuity with a broken grain. Most crystals are close to equidimensional (i.e., crystal heights are close to crystal widths). If the Set 3 fractures were much narrower than approximately 0.1 mm, these crystals would bridge the fracture and euhedral faces could therefore not line the fracture wallsdthey would be broken, anhedral surfaces where the fracture split. In summary, Set 3 fractures have a narrow range of widthsda few tenths of 1 mmdand no evidence of shear displacement. Set 3 fractures were measured along bedding-parallel scanlines. The 106-cm-long scanline measured along Bed A intersects 13 fractures and 12 intervening spacings (Table 2), with an average spacing of 8.8 cm (Fig. 9). The 104-cm-long scanline measured along Bed B intersects nine fractures and eight intervening spacings, with an average spacing of 13.0 cm. Set 3 macrofractures in Bed A have a Cv of 0.46; those in Bed B have a value of 0.45. Note that values are lower than those from Sets 1 and 2; Set 3 macrofractures are more regularly spaced. V0 values are approximate for these macrofractures because, although the width range is small (nearly 0.3 mm), we could not accurately measure opening-displacement values. Assigning a uniform opening displacement to each Set 3 fracture of 0.3 mm, V0 is 0.15 for Bed A and 0.08 for Bed B. All Set 3 fractures terminate at some bedding-parallel surface, including each upper and lower fracture tip. The stratigraphic surfaces at which fractures stop can be thinly laminated, silty mudstone, but in many cases the breaks are clean silt laminae (<2 cm thick). In some cases the breaks appear to be sandstone-onsandstone contacts. Most fractures in Beds A and B terminate at the upper and lower boundaries of their respective bedsdindeed, this pattern is the basis for defining each bed’s fractured-layer thickness. Within Bed A, eight of 13 fractures terminate at the upper contact; 11 of 13 terminate at the lower contact (Table 3). Within Bed B, seven of nine fractures terminate at the upper bed boundary; eight of nine terminate at the lower bed boundary. Those fractures that cross to adjacent beds each terminate at some farther bedparallel surface. 5. Discussion 5.1. Fracture timing

all widths estimated at 0.3 mm, bedding thicknesses are 150 and 360 mm, giving H:W of 500 and 1200, respectively. Set 3 fractures resemble barren joints but contain thin veneers of faceted authigenic quartz on fracture surfaces (Fig. 6). Set 3 fractures can usually be distinguished from barren joints in the field by minute faceted quartz crystals on fracture surfaces or by haloes of bleaching of red sandstone to white to yellow within a few mm of fracture walls (Fig. 6). Despite narrow apertures, almost all Set 3 fractures preserve extensive porosity, marked by euhedral quartz-crystal faces along fracture walls and the tendency for the rock to break along these faces (Fig. 6). Set 3 fracture-opening displacements are difficult to measure with precision because large Set 3 fractures are generally not sealed or bridged by quartz and the rock typically breaks along fractures. But outcrops permit the range of Set 3 apertures to be tightly constrained. Each Set 3 fracture’s opening displacement is small (<1 mm wide), including possible local widening at the outcrop surface, as fracture-bound blocks fall out of place. Using field and microscope observations we estimate the maximum width of Set 3 fractures at 0.3 mm. The minimum of the range of widths is likely near the grain-diameter scale (a few tenths of 1 mm). This estimate is based on the size of fracture-lining euhedral quartz faces (Fig. 6).

The faults and Set 1 opening-mode fractures have orientations and displacement patterns that are kinematically consistent with Andean deformation. Regionally, NEeSW and NWeSE striking fracture sets are present (Mon, 1987), and in this region. Marrett et al. (1994) and Marrett and Strecker (2000) found a record in younger rocks of NWeSE and NEeSW MioceneePliocene Andean contraction. But evidence of Huasamayo Canyon fracture timing is not definitive. Orientations could also possibly be related to Ordovician orogenesis, although there was no deep burial at this time, Cretaceous rifting, although no thrusts are known at this time, or other minor tectonic events. Although lacking a well-constrained burial curve that might allow inference of fracture timing from quartz-cement deposit volumes and fluid inclusions (Fall et al., 2012), we can nevertheless note a few relations that are relevant to fracture timing and therefore fracture with respect to burial/ exhumation. Burial during the Cambro-Ordovician cannot have been more than a few km and perhaps as little as hundreds of m given the passive margin setting and modest thickness of section (Deeken et al., 2006). Such shallow burial suggests that significant quartz cementation is later. Evidence for Ordovician deformation is found in the Puna, close to our study area but this area differs radically. Comprising thick, highly deformed flysch typically

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Fig. 15. (a) SEM-CL image of Set 2 microfracture. Fsp, feldspar. (b) Interpretation. G, grain; F, fracture; P, porosity. Note that fracture is porous where it cuts nonquartz substrate (feldspar). Same key as for Fig. 8.

covered in angular unconformity by Cretaceous or older rocks these rocks are likely an accretionary wedge and possibly allochthonous. The lack of angular unconformity in our area excludes significant Ordovician deformation. Quartz-filled fractures cross cut thrust faults, and the only observed thrusts affect Cretaceous and Tertiary rocks (Marrett et al., 1994). Fracture timing must be Andean. Early on (e.g., Miocene) that might have included rapid thrust loading (i.e., approximately adiabatic), but during fracture-growth exhumation probably was dominant. Given the numerous loading paths (Engelder, 1985; English, 2012; Fall et al., 2012) and low strain (Olson et al., 2009) that can lead to fracture, the protracted burial and tectonic history of Cambrian Mesón Group rocks leaves ample scope for fracture development. Although compliant sandstones can survive extensive deformation with little or no fracture (Ellis et al., 2012), Mesón Group sandstones are brittle quartzites prone to fracture. Elsewhere, similar Cambrian quartzites with similar structural histories have many more fracture sets (Laubach and Diaz-Tushman, 2009). The simplicity of the three-set opening-mode fracture pattern in the Mesón Group, within Andean-age folds, is therefore surprising. Fractures that cut across compacted grains and cement-filled intergranular volume show that all Mesón Group fracture sets formed in indurated rock. From this observation we infer that rock mechanical properties most likely varied little as fractures formed. Crosscutting relations show that some faults predate Set 2. Crackseal textures show that Set 1 fractures grew under conditions of quartz precipitation, which is rapid in sedimentary basins above 80  C (Lander and Walderhaug, 1999). Incomplete quartz fill and quartz bridges within larger fractures resemble the style of quartz accumulation in fractures found in deep sedimentary basins (Laubach et al., 2004), rather than the more complete pattern of quartz fill in metamorphic quartz veins (Ramsay, 1980). Faults and Set 1 opening-mode fractures have more complete quartz infill than do Set 3 opening-mode fractures, probably reflecting deeper

burial and longer thermal exposure. Although Set 2 microfractures are filled mostly by quartz, local pore spaces within these thin fractures imply a shorter thermal exposure than that for Set 1dall Set 1 microfractures that are as narrow as Set 2 microfractures, are sealed by quartz. Thin quartz deposits and extensive porosity in Set 3 fractures show that these fractures most likely formed at depth but experienced cooler conditions during or after they formed. Because the sets are present in the same canyon, it follows that Set 3 fractures are likely younger than Set 1 fractures, an interpretation compatible with crosscutting relations. Although differing quartz-cement volumes in similar sized fractures suggest Set 2 is older than Set 3, the relative timing of Sets 2 and 3 from crossing relations is ambiguous. Fractures of both sets are present within the same beds, but owing to the wide spacing and thin cement accumulations of Set 3 and to the low angle of strike between the two sets, no unambiguous crosscutting relationships were observed. The common location and, in some cases, overlapping orientation range of Sets 2 and 3 fractures suggest that they could have formed at or near the same time, but each population has a narrow size range with no continuity or transitional sizes and spacing characteristics also differ. Therefore, we treat these two populations separately. 5.2. Fracture patterns and diagenesis The opening-mode sets formed sequentially in the same sandstones, yet sets display contrasting fracture-cement textures, aperture-size distributions, spacing patterns, and tendencies to be bed bounded. One possible explanation for differences between the sets is that the mechanical properties of the beds or their interfaces changed between development of the sets, possibly as a result of changes in overburden load or as a result of compaction, cement precipitation, dissolution, creep (Laubach et al., 2009), or the presence of earlier fractures. Because Set 2 fractures cut across the faults, and Sets 1 and 2 are sealed or bridged by cement in

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crystallographic continuity with the host rock, the effects of early fractures altering, for example, bulk-rock modulus, are probably negligible. Although some changes in host rocks cannot be ruled out, we discount the possibility of significant rock-property change, given the uniform quartz-rich composition of these rocksdthey contain few compliant, compactible, or reactive phases; they lack extensive stylolite development; and pervasive quartz cement was emplaced prior to emplacement of the oldest fracture set. For bed bounded and unbounded fractures in a carbonate rock exposure, Gillespie et al. (2001) called on near surface, dilatant bedding plans to stop height growth for their bed bound set. Although in Mesón Group outcrops we found no evidence that open cracks exist along bed interfaces, this phenomenon cannot be ruled out for halting Set 3 height growth. If sandstones and interfaces had constant mechanical properties throughout deformation, differences between the sets might reflect differing loading conditions or differing environmental conditions, in this case manifested in different processes occurring within fractures. As noted, because the loading conditions, along with cause(s) and timing of the sets, are mostly unknown, we focus on possible environmental effects within fractures. A notable difference between sets is the presence of crack-seal texture in Set 1 fractures (Fig. 11) and the absence of this texture in Sets 2 and 3 fractures. Crack-seal texture in fracture quartz indicates that widening involved repeated reopening and breakage of quartz deposited in fractures between opening increments. Incomplete quartz-cement deposits spanning fractures and adhering to fracture walls is likely to be a kind of adhesive bond (Baljon and Robbins, 1996) and might tend to make fractures harder to reopen (Fagereng, 2011). The magnitude of this effect is unknown but probably varies with the type of cement and substrate (Gale and Holder, 2010) and the part (or proportion) of the fracture surface that is bridged by cement. Within Set 2 microfractures, quartz cement spans the fractures, but the absence of crack-seal texture suggests that the fractures did not widen further after cement precipitation. At least for each individual fracture, therefore, the cement is postkinematic (Laubach, 2003) in the sense that cement postdates opening of the host fracture, although cement may have been concurrent with the opening of nearby fractures in the same set. Here we focus on Sets 1 and 3; these fracture populations include fractures of comparable size. The presence of crack-seal bridges shows that Set 1 grew under conditions of rapid quartz accumulation relative to fracture wideningdconditions that are widespread in sedimentary basins (>3 km depth; ca. 80  C or more) (Laubach et al., 2004). Narrow fractures have fewer crack-seal increments than do wider fractures. Differences in the number of crack-seal increments between different Set 1 fractures show that certain fractures widened more times than other fractures. In other words, opening was partitioned unequally between fractures in this set. Crack-seal texture is absent in Set 3, which also has overall lower quartz amounts, and more fracture pore space for fractures of a given opening-displacement size. To make crack-seal texture, quartz needs to grow across (span) fractures between opening increments. Set 3 fractures may have widened incrementally despite lack of crack-seal texture, but if repeated widening occurred, it did not involve repeated breakage of cement bonds across the fracture width. Cement and grain breakage were limited to fracture tips. Set 3 fractures are not bridged with cement, as evidenced by continuous lining of fracture walls with euhedral crystal faces. Therefore, Set 3 fractures did not form amid sufficiently rapid quartz precipitation for bridging and repeated within fracture-cement breakage. Small fractures fill more readily than larger fractures, owing to their greater surface area:volume. Quartz-cement accumulation

governed by surface area and temperature provides a mechanism for fracture size- and time-dependent sealing (Laubach, 2003). In a setting of episodic fracture growth during cement accumulation, unequal and variable fracture sealing and adhesion might contribute to a tendency for differential partitioning of opening among fractures of differing size, which, models suggest, can result in power-law width distributions (Clark et al., 1995; Hooker et al., 2012). Such rule-based fracture widening models resemble models of fractureedynamic interaction and linkage, which have been specifically proposed to account for power-law fault length scaling (Cowie et al., 1993; Cladouhos and Marrett, 1996), as well as opening-mode fracture length scaling (Davy et al., 2010). An explanation for the wide range of Set 1 macrofractureopening-displacement sizes and the even wider range of openingdisplacement sizes of associated microfractures is that both groups of fractures are part of a single population that could be described by a power law. Set 1 macro and microfracture-opening-displacement sizes are well described by a power law with slope
0.8. Continuity is evident between the micro- and macrofracture cumulative-frequency trends (Fig. 10). The combined population of Set 1 micro- and macrofractures has a pronounced roll-off (concave down) segment of the distribution in the range of 0.3e3 mm opening-displacement size that could be a sampling artifact (censoring bias; Baecher and Lanney, 1978; Bonnet et al., 2001) or that could mark the upper size limit of the power-law distribution. Although the largest and least common fractures may be over- or undersampled, the type of censoring that affects fracture lengths measured in 2D (Pickering et al., 1995; Bonnet et al., 2001) does not affect opening-displacement size data from scanlines. In long Set 1 scanlines, the concave-downward roll-off is consistent with the absence of cm-scale opening-displacement or larger fractures. The roll-off therefore most likely represents the true end of power-law size distribution for these opening-mode fractures. The observed power law for Set 1 opening-displacement distribution persists to the opening-displacement size where fracture porosity is widespread (the emergent threshold), consistent with previous observations of fractures in dolostones in the Mexican Sierra Madre Oriental (Ortega et al., 2006), arkosic sandstones in the Piceance Basin (Hooker et al., 2009), and quartzarenites of the Eriboll Formation (Hooker et al., 2011). This persistence of powerlaw scaling among microfractures up to near the widespread occurrence of fracture porosity (emergent threshold) suggests that the presence or absence of fracture-spanning cement within fractures is related to differences in fracture-size patterning. 5.3. Fracture patterns and stratigraphy Sets 1 and 3, two of our three opening-mode fracture sets are consistent with a broadly recognized relationship between fracture-size scaling and bed boundedness. Namely, Set 1 fractures that are not bed bounded have a wide range of sizes that are well fit by a power-law size (length or opening displacement) distribution; Set 3 bed-bounded fractures have a narrow, characteristic size range, similar to patterns described elsewhere (Odling et al., 1999; Gillespie et al., 2001; Peacock, 2004; Weiss et al., 2006; Odonne et al., 2007). Set 2 microfractures height patterns are indeterminate but they are probably not bed bounded and have a narrow opening-displacement size range. But this is unsurprising for a group of exclusively microscopic structures. The literature also contains accounts of large fractures that are bed bounded and that show power-law aperture-size distributions. Sets of strongly stratabounded fractures in the Cretaceous Cupido Formation dolostones in NE Mexico show power-law opening-displacement scaling (Ortega et al., 2006; Hooker et al., 2012). Although these fractures are top bounded in our scheme, the power-law opening-

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displacement distributions persist among large, bedding-bounded fractures. Although the incomplete sampling afforded by core makes fracture height hierarchies challenging to document in the subsurface, the vertical termination patterns of some subsurface fractures that show power-law opening-displacement patterns suggest these arrays are also top bounded (Hooker et al., 2009; Laubach et al., 2009). The correlation between stratigraphic restrictions (or their absence) on fracture height patterns and opening-displacement size distributions in line samples is imperfect. Perfectly bed-bounded fractures will have by definition a characteristic height distribution that is controlled by layer thickness. A characteristic opening-displacement distribution is anticipated among perfectly bed-bounded fractures because in an elastic medium with restricted fracture height, fracture-opening displacement should scale with fracture height (Olson, 2003). One exception would be for crack-tip bluntingdfor example if fracture widening is accomplished by bed-boundary sliding. In such a case, aperture could scale with layer-parallel length, which could be unrestricted and scale as a power law. Other inelastic deformation processes could also permit a height-restricted fracture to widen progressively. But there is little evidence of bed-boundary sliding near opening-mode fracture tips in the Mesón Group examples. These explanations also do not account for scaling in top-bounded fracture arrays, the most common pattern for arrays that comprise macro and microfractures. Synkinematic cement is a common feature of power-law sizedistributed fracture sets, including Set 1. In contrast, fractures having a characteristic size distribution typically are barren or nearly so or feature only postkinematic cement, as is the case for Sets 2 and 3. This relationship is consistent, regardless of fracturebedding relationships. Synkinematic cement and power-law scaling are present in the unbounded veins in carbonate rocks described by Gillespie et al. (2001) and in the top-bounded fractures described by Ortega et al. (2006). Bed-bounded fractures described by Gillespie et al. (2001) are also barren (no cement) and barren fractures (joints) typically have narrow opening-displacement-size ranges. The reason that characteristic scaling behavior and perfect bed boundedness frequently correlate in outcrop may be that nonscaling fractures typically form near the surface, where cement precipitation is slow or non-existent and the mechanical effects of bedding may be significant owing to weak and possibly even dilatant bedding interfaces. Such mechanically significant interfaces are effective at arresting fracture height growth (Renshaw and Pollard, 1995; Gillespie et al., 2001). At depth, interfaces and interbedded non-fracturing beds may be strong or weak, thus both strata-bounded and non-strata-bounded arrays are possible. Strata-bounded veins (e.g., fractures in the Cupido Formation) and non-strata-bounded, characteristic-sized microfractures (Set 2) suggest that synkinematic cement precipitation, not mechanical stratigraphy, controls fracture-opening-displacement scaling patterns. Strata-bounded Set 3 fractures are regularly spaced and more widely spaced in thicker beds (Fig. 7), whereas non-strata-bounded Set 1 fractures are clustered and lack systematic spacing pattern relative to stratigraphic boundaries. Although the details of the origin of stratabound fracture spacing that is regular and proportional to bed thickness are debated and probably vary by specific geologic setting (Schöpfer et al., 2011), in general this phenomenon is explained by tensile stress relief lateral to fractures. This stress relief decays away from fractures along bedding and inhibits fractures to form up to a distance that is proportional to layer thickness. Such an explanation presupposes that the host rock is elastic. Some fractures containing crack-seal texture (similar to Set 1) have been

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shown to form over millions of years (Becker et al., 2010; Fall et al., 2012). Cement-filled fractures that evolve over time periods comparable in length to the dissipation of strain and stress shadows by creep might therefore tend to have less regular spacing. 6. Conclusions Following the formation of a sparse array of mm-displacement, low angle, conjugate reverse faults, three sets of bed-normal, opening-mode fractures formed sequentially in Mesón Group quartzarenite sandstones (Table 1). Because fractures grew after sandstones were compacted and thoroughly cemented, mechanical properties were likely uniform throughout deformation. Set 1 fractures have crack-seal texture, showing that quartz spanned fractures while they grew. Set 2 fractures are microscopic and mostly quartz filled but lack crack-seal texture. Set 3 fractures are porous and quartz-lined. Quartz that fills or even spans fractures reestablishes some cohesion and will tend to resist fracture reopening more than a completely open fracture might have. This process of fracture cohesion is more likely in Set 1 fractures, in which crack-seal quartz textures correlate with a wide range of opening-displacement sizes. Macro and microfracture-openingdisplacement sizes are well described by a power law with slope
0.8 over about two orders of magnitude. The lower law relation ends at the fracture-aperture size where widespread fracture porosity is present, marked by non-artificial, natural change in sizeefrequency that resembles censoring. In contrast, Sets 2 and 3 fractures have narrow, characteristic openingdisplacement size distributions. Quartz deposition may favor power-law opening-displacement size distributions by partitioning repeated, episodic growth into a few less-cemented fractures. Rulebased fracture-growth models suggest that this sort of statistical partitioning can produce power-law size distributions (Clark et al., 1995; Hooker et al., 2012). In addition to differing in cement textures and openingdisplacement distribution, Set 1 fractures are irregularly spaced and non-strata-bounded fractures, whereas Set 3 fractures are regularly spaced and stratabounded. Set 2 fractures are either clustered or nearly randomly positioned and probably are not stratabounded. Spatial patterns may reflect the mechanical cohesive effects of synkinematic cement precipitation and mechanical stratigraphy. Because quartz accumulation primarily reflects fracture timing and thermal exposure, burial history may therefore influence these other aspects of fracture-size patterning. Because infilling cement deposits can reduce effective (or open) length, height, and opening-displacement (width) distribution, how cement accumulates affects fracture permeability (Laubach, 2003; Philip et al., 2005; Olson et al., 2009), suggesting that critical components of the fracture system for permeability (size, degree of cement fill) have interrelated development. If cement accumulation affects partitioning of opening among growing fractures, fracture size and spacing may be sensitive to thermal history, in turn a consequence of burial and exhumation. Acknowledgments This study was funded partly by grant DE-FG02-03ER15430 from Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy, by the GDL Foundation (fieldwork), by the Fracture Research and Application Consortium and by the Geology Foundation of the Jackson School of Geosciences, The University of Texas at Austin. We are grateful to Juan Iñigo, Peter Eichhubl, and András Fall for discussions and to Joao Hippertt and Veerle Vandeginste for reviews.

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