Geological Society of America Bulletin, Published Online on 30 July 2014 As Doi:10.1130/B30968.1

Geological Society of America Bulletin, published online on 30 July 2014 as doi:10.1130/B30968.1

Geological Society of America Bulletin

Three-dimensional (3-D) finite strain at the central Andean orocline and implications for grain-scale shortening in orogens

Nathan Eichelberger and Nadine McQuarrie

Geological Society of America Bulletin published online 30 July 2014; doi: 10.1130/B30968.1

Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geological Society of America Bulletin Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and science. This file may not be posted to any Web site, but authors may post the abstracts only of their articles on their own or their organization's Web site providing the posting includes a reference to the article's full citation. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society.

Notes

Advance online articles have been peer reviewed and accepted for publication but have not yet appeared in the paper journal (edited, typeset versions may be posted when available prior to final publication). Advance online articles are citable and establish publication priority; they are indexed by GeoRef from initial publication. Citations to Advance online articles must include the digital object identifier (DOIs) and date of initial publication.

Three-dimensional (3-D) ﬁ nite strain at the central Andean orocline and implications for grain-scale shortening in orogens

Nathan Eichelberger1,† and Nadine McQuarrie2 1Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA 2Department of Geology and Planetary Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA

ABSTRACT ~180 °C, but they are also found where tem- tral Andes, we focus on documenting strain peratures were <180 °C. The lack of distrib- recorded by competent lithologies, in this case, Three-dimensional (3-D) finite strain uted detachments in Appalachian and Sevier sandstone and quartzite. The data presented analy ses from across the central Andes are stratigraphy may have favored minor layer- here do not defi ne a full internal strain frame- used to document the contribution of grain- parallel shortening at temperatures <180 °C work, because fi nite strain was not measured in scale strain in quartzites and sandstones to rather than formation of numerous, low- weak shale-based lithologies. At the scale of a the total shortening budget. The results are offset faults as in the central Andes. Minimal single structure, fi nite strain accommodated in compared to thermal, stratigraphic, and slip on individual central Andean thrusts relatively low-viscosity layers (such as shale) strain data from other fold-and-thrust belts to would have limited footwall burial and main- can easily be greater and more variable than determine the infl uence of lithologic strength tained low deformation temperatures. strain measured in relatively competent layers. and deformation temperature on strain ac- In classical fold models involving viscosity con- commodation during orogenic evolution. In INTRODUCTION trasts between layers, fl exural fl ow in low-vis- the central Andes, 3-D best-fi t ellipsoids are cosity layers results in thickening at fold hinges inconsistently oriented relative to structural Quantifi cation of strain at the granular scale (Fig. 1A). Such structures are observed in the trends, have short axes at high angles to bed- in fold-and-thrust belts is a critical variable in central Andes (McQuarrie and Davis, 2002), ding (Z, mean plunge = 78° ± 21°), and have accurately defi ning the kinematics of orogenic suggesting that fi nite strain in shale-dominated bedding-parallel long axes (X, mean plunge = deformation. Referred to here as internal strain, lithologies may be locally high relative to the 6° ± 24°). Ellipsoid shapes are dominantly grain-scale shortening can represent a signifi cant competent quartz grain-supported layers. In oblate (X = Y > Z), indicate low natural octa- component of the overall magnitude of shorten- other geographical locations, grain-scale short-

hedral shear strains (εS = 0.03–0.19), and ing accommodated in an orogen (Mitra, 1994; ening of the entire stratigraphic succession is have axial ratios that range from 1.02:1:0.81 Yonkee and Weil, 2010; Sak et al., 2012), alter interpreted to have occurred outboard of the

(εS = 0.19) to 1.02:1:0.97 (εS = 0.03). Highly the kinematic path proposed in balanced cross active deformation front, largely preceding and

variable Rf-ϕ data (Rf = 1.0–5.0, ϕ fl uctua- sections (e.g., Mitra, 1994; Sak et al., 2012), and unrelated to localized faulting and folding (e.g., tions exceeding 100°) indicate detrital grain is predicted to be a fundamental kinematic com- Sak et al., 2012; Yonkee and Weil, 2010). In shapes may overwhelm any measurable tec- ponent of orocline development (Weil and Suss- this case, the amount of total shortening must tonic strain fabric recorded by grain geom- man, 2004; Yonkee and Weil, 2010). Globally , be the same for all units regardless of the way etry. The best-fi t ellipsoids may refl ect either there are few orogen-wide fi nite strain data in which bulk strain was accommodated (Fig. weak compaction strain, or they may be re- sets available, so it remains a poorly quantifi ed 1B). Finite strain may be relatively high in low- lated to a depositional fabric. At a minimum, aspect of fold-and-thrust belt kinematics. Where viscosity shale layers as a consequence of intergranular strain was insuffi cient to reset the measurements have been made, granular short- nal distortion (Fig. 1A), but the total shortening detrital grain fabric, and therefore grain- ening parallel to bedding (often termed layer- in both competent and incompetent lithologies scale strain in quartzites and sandstones is parallel shortening) can be as high as ~20% at a must be equal to maintain strain compatibility not a signifi cant factor in deformation. We regional scale (e.g., Yonkee and Weil, 2010; Sak (not the case illustrated in Fig. 1B). As a result, suggest that the nonstrained nature of these et al., 2012). Thus, internal strain can represent the combined faulting, folding, and as-yet- stiffer lithologies indicates a lack of regional, a substantial component of total shortening in unconstrained grain-scale shortening of com- penetrative strain in the central Andes like fold-and-thrust belts but occurs at scales below petent lithologies in Bolivia most likely repre- that quantifi ed in similar lithologies in other the resolution of balanced cross sections (e.g., sent the total shortening budget. Focusing on orogens. The regional lack of strain may be Mitra, 1994; Duebendorfer and Meyer, 2002; grain-scale shortening recorded by competent due to deformation temperatures <180 °C Yonkee, 2005; Yonkee and Weil, 2010; Sak lithologies provides the most robust estimate and the presence of fi ve ≥1-km-thick shale et al., 2012). While internal strain is a critical of early, layer-parallel shortening while mini- detachments. In the Sevier and Appalachian kinematic consideration, it is essentially unique mizing local structural complexities related to orogens, granular strain fabrics are best to a given fold-and-thrust belt and cannot be viscosity contrasts and folding mechanisms. developed where temperatures exceeded generalized based on currently available data. However, the data presented do not fully char- To determine the contribution of internal acterize the total strain recorded in the entire †E-mail: [email protected] strain to the total shortening budget in the cen- lithologic section.

GSA Bulletin; Month/Month 2014; v. 1xx; no. X/X; p. 1–26; doi: 10.1130/B30968.1; 16 ﬁ gures; 4 tables.

Eichelberger and McQuarrie

A B curved deformation paths (e.g., Arriagada et al., Initial Initial Foredeep 2008; Eichelberger et al., 2013). η < η 1 2 The possibility remains that layer-parallel Orogenic Wedge shortening at the granular scale may also accommodate signifi cant orogeny-parallel extension. Deformed To develop an accurate kinematic history for the central Andes and create viable orocline recon- Local Pinline structions, layer-parallel shortening must be accounted for. If present, restoring signifi cant Deformed layer-parallel shortening to individual thrust Tectonic Stress sheets will alter the geometry of a balanced cross 10% thickness decrease in limbs 20% LPS section and the resulting kinematic model (i.e., R > R s s Mitra, 1994). If granular strain is not accounted for, total shortening magnitudes may underesti- Total Shortening: 45% Layer Parallel Shortening mate actual values and ultimately produce inac- curacies in the kinematics of central Andean oro- Figure 1. Structural diagrams of strain accommodation in multilayered systems with differ- cline reconstructions (Kley, 1999; Hindle et al., ing viscosities. (A) Mesoscale example of differing strain accommodation between low-vis- 2005; Arriagada et al., 2008; Eichelberger et al., cosity (η1) and high-viscosity (η2) layers. Measured strains in the low-viscosity layer would 2013), crustal deformation budgets (Isacks, generally be higher than those in the competent high-viscosity layer, but the total shortening 1988; Sheffels, 1988; Kley and Monaldi, 1998), for both is the same (45%). (B) Regional-scale scenario of grain-scale strain concentra- and shortening rates (Elger et al., 2005; Barnes tion within low-viscosity layers, violating strain compatibility. Layer shading indicates rela- et al., 2008; McQuarrie et al., 2008). tive viscosity contrast as in A (η1, η2). Layer-parallel shortening (LPS) at granular scale is In other fold-and-thrust belts where grain- generally interpreted to occur in front of an advancing orogenic wedge, prior to faulting scale shortening is constrained, the strain pat- and folding. If layer-parallel shortening only occurs in low-viscosity units (η1), the lack of tern is highly variable. This is presumably due layer-parallel shortening in competent layers (η2) violates strain compatibility (shown by to the sensitivity of strain accommodation to excess competent layer length relative to incompetent layers with layer-parallel shortening). location-specifi c factors, such as mechanical Therefore, if layer-parallel shortening is a signifi cant component of the total shortening stratigraphy, deformation conditions, and sam- budget, it should be recorded in both competent and incompetent lithologies. ple position within a given fold-and-thrust belt (Sanderson, 1982; Means, 1989; Mukul and Mitra, 1998; Yonkee, 2005; Ong et al., 2007; An understanding of the magnitude and ori- 1976; Marshak, 1988; Weil and Sussman, 2004; Long et al., 2011; Sak et al., 2012). Variability in entation of grain-scale shortening in the cen- Yonkee and Weil, 2009). the magnitude of grain-scale shortening can be tral Andes is central to defi ning the kinematic In the central Andes, prior paleomagnetic partly attributed to the temperature and pressure evolution of the Bolivian orocline. Generally, (Arriagada et al., 2006; Barke et al., 2007; dependency of intragranular and grain bound- orogenic curvature is best classifi ed based on Roperch et al., 2006; Somoza et al., 1996) ary deformation mechanisms that accommodate kinematic history: In primary arcs, the curva- and structural studies (Kley, 1996; McQuarrie, fi nite strain (e.g., Mitra and Yonkee, 1985; Hirth ture was a preexisting feature; progressive arcs 2002; Müller et al., 2002; Elger et al., 2005; and Tullis, 1992, 1994; Dunlap et al., 1997; developed structural curvature concurrently McQuarrie et al., 2008) have constrained por- Stipp et al., 2002). Fortunately, for the oro- with fold-and-thrust belt activity; and sections of the central Andean displacement fi eld, gens where strain is well documented, data sets ondary arcs were initially linear, with structural but the role of internal strain has not been including thermochronology, fl uid inclusions, curvature subsequently superimposed on the quantifi ed. Paleomagnetic as well as global and illite crystallinity provide quantitative con- fold-and-thrust belt (Weil and Sussman, 2004). positioning system (GPS) data (Allmendinger straints on temperature at the stratigraphic lev- When the Bolivian orocline was fi rst charac- et al., 2005) defi ne a broad rotation pattern of els where strain is measured. By expanding the terized based on the change in topographic counterclockwise rotations in the northern limb global coverage of internal strain data to include and structural trend of the central Andes from and clockwise rotations in the south limb. The the central Andean fold-and-thrust belt, we can NW-SE in southern Peru to N-S in southern fact that this is still observed in GPS data sug- gain new insight into granular shortening at Bolivia, it was considered a secondary arc. gests that the process continues today. Overall, the orogenic scale by correlating layer-parallel However, full differentiation between these the orocline limbs have divergent displacement shortening patterns, deformation temperatures, models for any curved mountain belt requires paths that predict extension at the orocline axis and lithologies between different fold-and- combining bulk shortening and vertical-axis (Kley, 1999). There are no major extensional thrust belts with similar data. Previously, indi- rotations with fi nite strain data (e.g., Weil et al., structures in the region; rather, the transi- vidual thrust sheets have been compared based 2010; Yonkee and Weil, 2010). Where present, tion between the limb displacement directions on quantitative strain parameters (e.g., Yonkee, the pattern of mesoscale layer-parallel shorten- appears to be accommodated by strike-slip 2005; Long et al., 2011) but never with respect ing features (such as cleavage plane orienta- and transpressional faults near the apex of the to specifi c variables, such as temperature and tion) and granular-scale strain markers have central Andean bend (Dewey and Lamb, 1992; mechanical stratigraphy, which may affect been used in conjunction with structural trans- Kley, 1999; Eichelberger et al., 2013). Recent strain accommodation at the granular scale. port direction and vertical-axis rotations to dis- reconstructions of the Bolivian orocline based The central Andes provide a natural labora- tinguish between kinematic models of orocline on this body of data classify it as progressive tory for evaluating the infl uence of deforma- formation (Carey, 1955; Ries and Shackleton, arc where curvature has developed as a result of tion temperature and mechanical strength on

2 Geological Society of America Bulletin, Month/Month 2014 Geological Society of America Bulletin, published online on 30 July 2014 as doi:10.1130/B30968.1

Andean orocline strain and orogen-scale factors inﬂ uencing grain-scale shortening

layer-parallel shortening in sandstone and GEOLOGIC BACKGROUND 69°W 63°W quartzite lithologies at the grain scale. Numer- ous studies from the orocline axis region have At the Bolivian orocline axis, there are ﬁ ve indirectly constrained maximum deformation physiographic regions (W to E): the modern temperatures through thermochronology (Ben- volcanic arc at the Western Cordillera; the high- 15°S jamin et al., 1987; Gillis et al., 2006; Barnes elevation Altiplano basin; the Eastern Cordi- et al., 2006, 2008; Ege et al., 2007; McQuarrie llera; Interandean zone; and Subandes (Fig. 2; Study Area et al., 2008). In addition, the stratigraphic sec- Isacks, 1988; Kley, 1996). The strain samples tion involved in Andean deformation is well presented in this study come from the retro- documented, including regional variations and arc fold-and-thrust belt, where the majority of EC IA SA stratigraphic position of important detachments regional shortening is accommodated by thrust (Roeder and Chamberlain, 1995; Sempere, faults and folds within the Eastern Cordillera, 1995; Welsink et al., 1995; Gonzáles et al., Interandean zone, and Subandes (i.e., Kley, 1996). Determining how strain varies between 1996; McQuarrie, 2002; Müller et al., 2002). stratigraphic levels, structural packages, and In general, exhumation and deformation in the over different temperature conditions across Eastern Cordillera initiated by ca. 40 Ma and

fold-and-thrust belts can help us to evaluate the progressed eastward over time to the modern 21°S relative importance of these factors in accom- orogenic front at the Subandes–foreland basin modating grain-scale shortening. Comparisons boundary (e.g., Barnes and Ehlers, 2009). N SA of these variables between orogens offer an Regionally, both fold-and-thrust belt width EC AP opportunity to constrain the fundamental ther- and shortening estimates reach a maximum mal and mechanical controls on the way in near the axis of the central Andean orocline 200 km IA Ordovician Quaternary which strain is accommodated and recorded and decrease northward into Peru and south- Upper Paleozoic during orogenic evolution. ward toward Argentina (i.e., Isacks, 1988; Kley Mesozoic Intrusive Tertiary Salar This study presents new three-dimensional and Monaldi, 1998; McQuarrie, 2002). The (3-D) fi nite strain data from across the central rocks involved in deformation are a continuous, Figure 2. Simplifi ed geologic map of the Andean orocline axis to assess the contribution ~15-km-thick section of Ordovician through Bolivian central Andes (modifi ed from Mc- of grain-scale shortening to the total shorten- Devonian marine siliciclastic rocks with an Quarrie, 2002). Strain samples were collected ing budget. We interpret apparent internal strain intermittently preserved section of Carbonifer- from the boxed-in study area. The approxi- from best-fi t 3-D ellipsoids (Mookerjee and ous through Cretaceous nonmarine siliciclas- mate structural boundaries of the physio- Nickleach, 2011) based on two-dimensional tics and carbonates (Roeder and Chamberlain, graphic regions are indicated at the south (2-D) ellipses measured by the normalized Fry 1995; Sempere, 1994, 1995; Gonzáles et al., end of the map area. The zones shown are: φ method (Erslev, 1988). The Rf- method is also 1996). The Paleozoic section is marked by at Altiplano (AP), Eastern Cordillera (EC), used to determine the variability in individual least three upward-coarsening sequences where Inter andean zone (IA), Subandes (SA). grain shape and to document the potential infl u- weak shale grades upward into competent ence of detrital grain geometry on the best-fi t sandstone and quartzite. These are interpreted ellipses. We note that at low strains, the infl u- to be the result of major marine transgressions ening, has been observed in low-grade metamor- ence of detrital grain geometry may exceed any that deposited the stratigraphically continu- phic Ordovician strata in the Eastern Cordillera detectable tectonic strain, in which case, best-fi t ous Ordovician Capinota, Silurian Uncia, and of southern Bolivia (Müller et al., 2002). While 2-D and 3-D ellipses/ellipsoids refl ect the aver- Devonian Belen shale packages (Fig. 3; Sem- penetrative strain is localized, the majority of the age grain shape rather than a strain ellipsoid. The pere, 1995). Interspersed between these units shortening in the Bolivian Andes is accommo- 3-D ellipsoid shapes are used to quantify appar- are numerous formations characterized by shale dated by tight folding (wavelengths of 5–15 km) ε ≤ ent strain magnitude (octahedral shear strain, s) and sandstone interbedded at high frequency and thrust faults (average offset 10 km) spaced and strain state (Flinn’s k-value). There are two (e.g., Silurian Catavi, upper Devonian forma- at structural wavelengths of 30 km or less that main objectives in quantifying internal strain in tions). In fact, the Ordovician San Benito and offset the oversteepened limbs of fault propaga- the central Andes: Devonian Vila Vila Formations are the only tion folds (McQuarrie, 2002; Müller et al., 2002; (1) to defi ne the extent to which grain-scale packages of uninterrupted, competent quartzite McQuarrie et al., 2008; Eichelberger et al., shortening of competent lithologies in the cen- and sandstone deposited on a regional scale at 2013). Fold and fault spacing is generally greater tral Andes has contributed to orocline formation the orocline axis. The shale horizons result in in the Subandes and Eastern Cordillera, more and plateau evolution; and as many as seven detachments at various strati- densely spaced in the Interandean zone, and (2) to determine potential orogen-scale fac- graphic levels throughout the central Andes in densest in the western Eastern Cordillera back- tors that infl uence how grain-scale shortening Bolivia (McQuarrie, 2002; Müller et al., 2002). thrust region (McQuarrie and DeCelles, 2001). recorded in relatively competent lithologies Penetrative strain features such as pencil struc- contributes to total shortening budgets of the tures, bed-parallel dissolution cleavage, and FINITE STRAIN ANALYSIS Sevier, Appalachian, and central Andean fold- axial planar cleavage have been observed pri- and-thrust belts. marily in shale-rich stratigraphy located in the Samples for fi nite strain analysis were col- To do so, we compare stratigraphic variations western Eastern Cordillera, but are limited to lected along and between two across-strike in deformation temperature, mechanical proper- areas near thrust fault-tip lines (McQuarrie and structural transects located between 17°S and ties, and strain measured in competent litholo- Davis, 2002). Slaty cleavage, proposed to be a 18.75°S. One transect extends across the axial gies between regions. product of both pre-Andean and Andean short- region (Eichelberger et al., 2013), while the

Geological Society of America Bulletin, Month/Month 2014 3 Geological Society of America Bulletin, published online on 30 July 2014 as doi:10.1130/B30968.1

Eichelberger and McQuarrie

Mechanical Sample Preparation Lithology1 Characterization2 Oriented samples were collected from quartz grain-supported sandstone and quartzite beds. Since pervasive tectonic foliations were not present in the central Andes, cut planes for 2-D strain analysis were oriented based on bedding orienta- Sandstone, conglomerate Synorogenic Some siltstone, shale, tion alone. For each sample, planes were cut par- Sediments and gypsum Stiff allel to dip direction (A planes), strike direction

TERTIARY (B planes), and bedding (C planes). All images were made with the in situ bedding orientation structurally restored to horizontal, accomplished by rotating the sample around the orientation of bed strike by the dip magnitude. By restoring sample orientations based on the local structure, Sandstone, siltstone, C: Soft we can determine if additional complexity in the J/K shale and gypsum J: Stiff restoration path (such as vertical-axis rotations or transpression) is required to explain differences

C Moderate Sandstone, shale, limestone in granular principal strain axis orientation and bulk shortening directions inferred from regional Iquiri Moderate Huamapampa structural trends. Thus, A and B plane images Los Monos Alternating sandstone and grain shape plots are stratigraphic-up, and Soft Colpacucho and shale C-plane images are viewed looking down on the Stiff bed surface. Three cross-polarized light (CPL) Sica Sica photomicrographs were collected at horizontal, –30°, and +30° for each plane (Fig. 5A) and Icla/Belen Shale Soft merged into a single image to highlight grain DEVONIAN boundaries and image quartz grains regardless of variable extinction angles (after Long et al., Sta. Rosa/ Sandstone Stiff 2011). Additionally, scanning electron micro- Vila Vila scope–cathodoluminescence (SEM-CL) images (Fig. 5B) were collected for side-by-side strain Alternating sandstone Tarabuco/ Moderate analyses with polarized light images. Catavi and shale

Krusillias/ Fry and Rf-ϕ Methods Shale Soft Uncia SILURIAN Llallagua Stiff Due to the prevalence of fi ne- to medium- grained sandstone and quartzite and the relative Cancaniri Diamictite and shale Soft scarcity of deformed fossiliferous, calcareous, San Benito Sandstone, quartzite Stiff or diamictite units in the central Andes, we Anzlado Metasiltstone Stiff Soft relied on the normalized Fry method (Erslev, ORD 1988) to measure fi nite strain ellipses. The Capinota Shale and phyllite Soft Fry method measures the separation distance : 3-D strain data in this study : Major detachment horizon between grain centers in a close-packed aggre- 1: From McQuarrie (2002) C: Carboniferous gate (Fig. 5C; Fry, 1979; Ramsay and Huber, 2: From McQuarrie and Davis (2002) J/K: Jurassic/Cretaceous 1983). For anticlustered aggregates, the Fry plot has a central void defi ned by the minimum dis- Figure 3. Simplifi ed stratigraphic column from McQuarrie (2002, and references therein). tance between grain centers. The elliptical shape Mechanical characterization is from McQuarrie and Davis (2002) based on rock properties of the void is related to the anisotropic distri- from Jaeger and Cook (1979). Stars represent stratigraphic levels sampled for fi nite strain bution of grain locations due a combination of analysis for this study. matrix strain, mass transfer processes, and initial grain shape (Fig. 5D; Fry, 1979; Ramsay and Huber, 1983). Normalizing the grain center other is immediately north of the bend (Fig. 4; younger than Carboniferous were either too separation distance of all grain pairs by the sum McQuarrie, 2002). Given the friability of the poorly lithifi ed to sample or disintegrated dur- of each grain-pair radii tends to produce a dense mudstone, shale, and slate lithologies inter- ing cutting and polishing. For each sample, ring around the central void that can be fi t by an spersed throughout the stratigraphy at this three 2-D grain shape analyses were made on ellipse (Erslev, 1988). Assuming that the grains latitude, sampling ultimately focused on units three orthogonal planes. A 3-D best-fi t ellipsoid had an initially isotropic, anticlustered distribu- featuring quartz grain-supported sandstone was then fi tted to these data and used to interpret tion, the best-fi t ellipse refl ects the magnitude and quartzite beds (Fig. 3). Samples from units 3-D fi nite strain. and orientation of the penetrative strain fi eld

4 Geological Society of America Bulletin, Month/Month 2014 Geological Society of America Bulletin, published online on 30 July 2014 as doi:10.1130/B30968.1

Andean orocline strain and orogen-scale factors inﬂ uencing grain-scale shortening

67°W 66°W 65°W 64°W Subandes Interandean 3-D Strain Samples Map Units ε = 0–0.05 Zone oct Quaternary

εoct = 0.05–0.10 Miocene Volcanics εoct = 0.10–0.15 ε = 0.15–0.20 Tertiary Eastern oct 17°S Ellipsoid Unconstrained Cordillera Mesozoic Partial Strain (2-D) R < 1.10 Carboniferous R = 1.10–1.15 R = 1.15–1.20 Devonian R = 1.20–1.30 Silurian Thrust Fault Ordovician

Normal Fault 18°S

N Subandes

Interandean Zone Eastern Cordillera Kilometers 0 50 100

Figure 4. Advanced Spaceborne Thermal Emission and Reﬂ ection Radiometer (ASTER) 30 m digital elevation model (DEM) with geologic mapping from McQuarrie (2002) and Eichelberger et al. (2013). Locations of strain samples presented in this study are noted. White circles represent samples where full three-dimensional (3-D) ﬁ nite strain ellipses could not be determined due to sample preparation problems.

Colored circles are ramped according to εs magnitude, with blue reﬂ ecting low strains and red reﬂ ecting higher strains. Squares indicate the location of samples with partial strain data (A, B, and/or C planes) and are gray-scale ramped based on two-dimensional (2-D) strain. For samples with two planes, the larger of two R values is used.

experienced by the sample (Fig. 5D). The ellip- 2010, 2011a, 2011b) in order to compare the and used to create a normalized Fry plot in Mat-

ticity (Rs) is the axial ratio of the long to short results against 2-D ellipses determined by the lab (Figs. 5C and 5D). A direct least-squares axial lengths and approximates the magnitude Fry method. While the Fry measures the bulk estimation of the conic ellipse equation (Fitz- ϕ φ of strain, while the inclination ( ) is the angle distribution of grain centers, the Rf- method gibbon et al., 1999; Gal, 2003) was iteratively between the long axis and bedding, and it is measures the range of individual grain shapes. applied to the individual points in the Fry plot used to characterized the type of strain recorded Samples with highly variable grain orientations to remove exterior points until only the central φ φ (Ramsay and Huber, 1983). In addition, Rf- have a range of greater than 90° (referred to as high-density ring remained. The process records analyses were performed for a subset of samples fl uctuation, F ) and are heavily infl uenced by the the best-fi t ellipse parameters for each iteration to determine the variability in grain shape orien- initial grain shape (Ri) to the point that it exceeds and stops before the remaining points drops tation in the same data used for the Fry method. any granular ellipticity related to tectonic strain below n = 100 (Fig. 5E). Planes with greater φ ϕ The Rf- method compares the ratio of the long (Rs; Ramsay and Huber, 1983). ellipticity will converge toward specifi c and to short grain dimensions (Rf) and the orienta- Rs values, whereas plots with lower eccentrici- ≈ ϕ tion of the long dimension relative to bedding 2-D Strain Measurement ties will converge to Rs 1 but variable (Fig. (φ; e.g., Ramsay and Huber, 1983). Best-fi t 2-D 5E). Normalized residual sum of squares (RSS) φ ellipses were determined from Rf- data using From the photomicrographs of each plane, was also recorded to compare ellipse fi t between the eigenvalue method (Shimamoto and Ikeda, grain centers and long and short axes were subsequent iterations. Of the fi nal two iterations, 1976) implemented in EllipseFit 2 (Vollmer, manually picked for >150 close-packed grains the ellipse with the lowest RSS was taken as the

Geological Society of America Bulletin, Month/Month 2014 5 Geological Society of America Bulletin, published online on 30 July 2014 as doi:10.1130/B30968.1

Eichelberger and McQuarrie

Figure 5. Thin-section images and Fry A B analyses of sample EC20, an Ordovician quartzite. All thin-section images are oriented parallel to bedding. (A) Plane polarized light image of quartzite with impinging grain boundaries and limited suturing. (B) Scanning electron microscope–cathodoluminescence (SEM-CL) image of the same thin section, but different region. Dark-gray areas between detrital cores are cement overgrowths. (C) Fry analysis of SEM-CL image, excluding cement overgrowths. Red 2 mm 2 μm points are user-selected grain centers, yel- 1 low lines are long axes, and blue lines are C D short axes. (D) Fry plot from SEM-CL image in C. Circled points indicate those used to achieve the best-fi t ellipse shown, where x = 0 is bedding parallel. Axial units are arbitrary and used to determine ellipse as- 0 pect ratio. (E) Output of ellipse parameters in the iterative ellipse-fi tting process used y Position to eliminate the exterior points (uncircled points in C). The fi nal two iterations have similar axial lengths and inclinations. 4 μm −1 −1 0 1 E x Position EC22-A Elliptical Fit 2000 best fi t. Uncertainties in the ellipse parameters ϕ (Rs and ) for the fi nal fi t were then determined 1000 by fi tting ellipses to 1000 bootstrap resamples of the central point ring. 0 Points in Fit (n) Short Axis Long Axis Aspect Ratio (Rs) 2 SEM-CL Imaging 1 The accuracy of using the Fry method to

measure apparent strain magnitude is depen- Axial Length 0 dent on not only meeting the initial grain cen- 50 ter distribution assumptions discussed already, but also being able to fully image the grain 0 φ ( ° ) boundaries in order to locate the grain centers. −50 A potential complicating factor is that pressure 1 2 3 4 5 6 solution and quartz cement overgrowths not Iteration visible in CPL can obscure the original grain shape and skew strain results, especially in fi ne- grained sandstones where pressure solution can values that did not systematically vary within 1σ squares approach to minimize error between be signifi cant (Houseknecht, 1988; Dunne et al., and were statistically equivalent within 2σ (Fig. the input planar ellipses (A, B, C planes) and 1990). To determine if overgrowths were pres- 6). Because there was no signifi cant difference the corresponding sectional ellipse through the ent in the central Andes, a pilot suite of samples between strain results from the two imaging best-fi t 3-D ellipsoid (Mookerjee and Nickleach, from across the study area were imaged using techniques, the remaining strain measurements 2011). The program quantifi es the difference SEM-CL in addition to standard CPL images. were made on CPL images. between the input 2-D ellipsoids and the best-fi t The SEM-CL images were collected using 3-D ellipsoid by reporting a scalar “mean error” a Gatan MiniCL detector and a XL30 SEM 3-D Strain Ellipsoid Fitting that is proportional to the goodness of fi t, with with an accelerating voltage of 25 kV at Prince- 0 indicating a perfect fi t (Mookerjee and Nick- ton University’s Imaging and Analysis Center. From the 2-D best-fi t ellipses of the A, B, and leach, 2011). Error margins for the ellipsoid Cement overgrowths were present, and a few C planes for each sample, the short axis length, shape parameters and principal axes orienta- discontinuous dissolution surfaces and veins long axis length, inclination, planar orientation, tions are estimated based on the variability of were observed, but neither occurred frequently and respective uncertainties were used to con- 1000 simulated ellipsoids fi t to randomly drawn ϕ enough to determine systematic orientations strain a best-fi t 3-D ellipsoid using Geologic input 2-D ellipse parameters (Rs, , and planar (Figs. 5A and 5B). Fry analyses of CL and CPL Programs for Mathematica (Mookerjee and orientation) from a distribution based on the ϕ images of the same thin section had Rs and Nickleach, 2011). The program employs a least- actual 2-D parameter uncertainties (Mookerjee

6 Geological Society of America Bulletin, Month/Month 2014 Geological Society of America Bulletin, published online on 30 July 2014 as doi:10.1130/B30968.1

Andean orocline strain and orogen-scale factors inﬂ uencing grain-scale shortening

1.3 and sutured grain boundaries are poorly developed, if present at all, and isolated to just a few 1.25 adjacent grains (Figs. 5A and 5B). These observations are conceptually consistent with the low 1.2 apparent strains measured in thin section. Results for 22 additional samples with partial 1.15 strain data (1–2 planes) are reported in Table 2

1.1 and located in Figure 4. In general, the 2-D Figure 6. Comparison of best- strain results are characterized by mean strain fit two-dimensional (2-D) Aspect Ratio (R) 1.05 ratios of 1.12 ± 0.05 for A planes and 1.11 ± ellipse parameters (2σ uncer- 0.05 for B planes. The 2-D ellipse long axes are tainties) for grain shape data 1 nearly horizontal with respect to bedding (θ) measured from cross-polarized in both A and B planes (mean θ is 16° ± 10° light (CPL) and scanning elec- 0.95 and 18° ± 13°, respectively). These 2-D results A B C tron microscope–cathodolumi- 100 are typical for all of the samples collected and nescence (SEM-CL) images. 80 record very low strain magnitudes. In light of Samples are spread across the the low strain magnitudes measured in both the Eastern Cordillera and Eastern 60 3-D and 2-D data, the long (X ) and short axes Cordillera back thrust. The re- 40 (Z ) may simply reﬂ ect the dimensions of pre- sults are plotted alphabetically served detrital grain shapes rather than principal 20 for each sample: strike parallel strain axis orientations. (A-cuts), dip parallel (B-cuts), 0 and bedding parallel (C-cuts). −20 Rf-ϕ versus Fry Results

−40 φ Angular Orientation (φ) A subset of samples was selected for Rf- −60 analysis based on regional distribution, relatively large strains (ε > 0.10), prolate ellipsoid −80 S shape, and X-axes oblique to subparallel to −100 EC14 EC18 EC19 EC20 EC24 EC25 EC26 EC27 bedding. In thin section, all samples had large Optical (CPL) SEM/CL variations in grain size and orientation, which φ resulted in substantial data dispersion in Rf- plots. Comparison of the best-fi t ellipse results φ and Nickleach, 2011). Ultimately, the orienta- The mean error between the input 2-D data and between the Fry and Rf- methods shows that tion of the best-fi t 3-D ellipsoid is used to infer best-fi t 3-D ellipsoids ranges from 0.01 to 0.19, both measure equivalent ellipse orientations the trend/plunge of the principal strain axes. The indicating acceptable fi ts (Table 1). Uncertain- within uncertainty (Fig. 7A), whereas best-fi t φ ellipsoid shape parameters are used to interpret ties for principal axial orientations range from Rf- ellipses have nearly equivalent, but con- ε the average magnitude of strain ( S octahedral 1° to 90° for long and intermediate axes, while sistently higher, Rs magnitudes (Fig. 7B). Since shear strain; Nadai, 1963) and type of strain the angular error for short axes is much lower the both methods have largely identical best- (Flinn’s k-value; Flinn, 1962). (1°–32°). Average apparent strain magnitudes fi t φ values, the best-fi t 3-D ellipsoids have ε for the 3-D ellipsoids range from S = 0.03 to equivalent principal axis orientations within RESULTS 0.19 with uncertainties between 0.01 and 0.05. uncertainty. A detailed example (sample IA5) φ Ellipsoid shapes span the full range from pro- is shown in Figure 8, where the Rf- best-fi t R ≥ φ Of 106 samples collected in the fi eld, 56 were late (k 1.0) to oblate (k < 1.0), with k-values exceeds the best-fi t Fry R. The Rf- data from suffi ciently indurated to provide the three planes from 0.1 to 22.3, which correspond to a range IA5 have a weak peak in point density near φ =

necessary to constrain the 3-D ellipsoid (Fig. 4). of triple-axial ratios (X:Y:Z ) from 1.23:1:0.99 0 but show a broad range of Rf values (1.0–>3.0) ε For unsuccessful samples, at least one plane ( S = 0.17 ± 0.01, k = 22.3 ± 10) to 1.02:1:0.81 and large long axis ﬂ uctuations (F >100°), con- ε (usually bedding parallel) disintegrated during ( S = 0.19 ± 0.03, k = 0.1 ± 0.1). From outcrop sistent with the wide variety of grain shapes and thin-section preparation. The 3-D ellipsoid ﬁ ts observations, there is a notable lack of per- orientations seen in the thin sections (Fig. 8). φ based on data from just two planes produced vasive foliation as well as minimal cleavage Relative to other Rf- results, data from IA5

results with unacceptably high variability in prin- development. Bedding involved in tight minor show a typical range of Rf values but have the cipal axis orientations and ellipse shape param- folds frequently featured slickenlines consistent lowest fl uctuation (C plane, F = 108°) of the eters such that no specifi c kinematic interpreta- with fl exural slip (McQuarrie and Davis, 2002) subset. Many samples show even greater scat- φ tion was feasible. However, the 56 samples with but no well-developed axial planar cleavage. ter in the data, characterized by Rf- plots with well-constrained ellipsoids represent nearly the Minor intraformational thrust faulting was also F = 160°–180° and best-fi t φ values that confl ict full stratigraphic column exposed at the orocline observed but only in close proximity to major with Fry best-fi t φ (Fig. 7). Based on the criteria core (Table 1). The notable exception is the Sub- regional fold and fault zones. At the microscale, of Ramsay and Huber (1983), such high fl uctua- andes, where samples were impossible to collect quartz grains were mantled with cement and tions indicate that initial grain ellipticities over-

or prepare due to the high degree of weathering weak undulose extinction but exhibited no visu- whelm any measurable tectonic strain (Ri > Rs). φ and minimally lithiﬁ ed nature of the Carbonifer- ally discernible preferential alignment. Micro- The standard Rf- equations for determin-

ous, Mesozoic, and Tertiary units exposed there. structural features such as dissolution surfaces ing Ri and Rs for situations where F > 90° are

Geological Society of America Bulletin, Month/Month 2014 7 Geological Society of America Bulletin, published online on 30 July 2014 as doi:10.1130/B30968.1

Eichelberger and McQuarrie ) highly sensitive to the largest and smallest Rf values and are thus easily skewed by outliers, especially here where grain shapes are so vari- ( continued able (see micrographs in Fig. 8). Here, we report φ best-fi t Rf- ellipses in Figures 6 and 7, which Z orientation are less sensitive to such outliers. Even so, the large mean Rf (with equally large uncertainties), along with visual inspection of the micrographs φ and Rf- plots (Fig. 8), gives a better indication of grain shape variability than the best-fi t parameters and 2σ confi dence intervals alone (Fig. 7).

Overall, the signiﬁ cant dispersion in grain shape Y orientation φ

may be responsible for overestimated Rf- strain (°)T (°) P Err. T P Err. values due to the variable grain sizes present in

many thin sections. In contrast, the normalized ## Fry method is less sensitive to individual grain variations, especially in situations where oro- (°) Err.

φ §§ genic strain is low. In the end, the Rf- results indicate that the apparent strains measured by X orientation (°) P

both methods are heavily inﬂ uenced by the †† detrital geometry. T–+

Axial Orientations

The in situ strain ellipsoid axial orientations k error from across the orocline region show consider-

able variability in orientation (Fig. 9). Neither ** k

the X axes nor the Z axes show preferential Flinn’s alignment along a speciﬁ c trend. Stereonet 8.9.2 (Allmendinger et al., 2013; Cardozo and –+

Allmendinger, 2013) was used to calculate error S the Bingham axial distributions (Fisher et al., ε 1987) and Kamb contours (Kamb, 1959) for the regional data set in order to determine mean ori- # S entations and point concentration. For the Bing- ε ham axial distribution, the ﬁ rst eigenvector was § interpreted to reﬂ ect the mean axial orientation, error and the third eigenvector was used to locate ori- Mean entations with the lowest point density. The in (3-D) STRAIN RESULTS THREE-DIMENSIONAL 1. TABLE situ axes show broad variations in orientation, ZYX but generally the X axes weakly cluster around low plunges, and the Z axes weakly cluster at

higher plunges (Fig. 9A). The X axes have the Axial length lowest point density around higher plunges, while the reverse is true for the Z axes (eigen- † vector 3 in Fig. 9A). The dispersion in axial plunge for both sets of axes is greatly reduced

by restoring them to bedding-horizontal (rota- T* Unit Sect. tion around bedding strike by dip magnitude), (°) as indicated by greater eigenvalues for the mean Dip axial orientations (eigenvector 1, Fig. 9B) as

well as a decreased eigenvector α error ellipse. (°) 95 Strike In this reference frame, ellipsoid axes with P = P 0° are parallel to bedding and those with = Lat (°S) 90° are perpendicular to bedding. The restored X axes are generally subparallel to bedding (°W)

(P < 45°) and have a mean trend/plunge and Long minimum/maximum error ellipse of 098/04° ± 8°/11°. The restored Z axes are dominantly bed- SA2 64.26 18.15 144 60 90 C 1.08 1 0.98 0.01 0.07 0.00 0.00 4.0 1.4 0.7 128 5 1 220 21 2 24 69 2 IA13 65.23 17.64 26 28 72 Sc 1.12 1 0.90 0.10 0.15 0.01 0.01 1.1 0.3 0.2 163 15 2 59 44 2 268 42 2 Subandes SA1 64.31 18.12 32IA1 21 64.63IA4 90IA5 18.09 CIA6 112 64.87 64.89 1.23 35 17.75 64.88 17.73 108 72 1IA10 17.83 298 17IA11 146 C 0.99 23 64.97IA12 72 19 0.04 1.08 65.05 72 18.20 Osb 65.10 72 17.77 172 0.17 1 1.05 Oa 17.70 Dv 23 35 1.09 0.01 183 0.84 1 1.08 58 72 0.07 0.01 1 19 0.85 1 72 22.3 C 72 0.18 0.93 0.03 Osb 0.93 1.13 Sc 9.8 0.14 0.01 1.02 0.16 0.05 1.08 1 7.7 0.11 0.00 1 0.00 0.11 1 0.97 0.01 0.4 0.02 185 0.97 0.00 0.19 0.89 0.00 0.13 0.0 18 0.3 0.02 0.05 0.11 1.2 0.03 0.0 0.0 1 1.0 0.13 0.01 0.2 0.01 0.3 32 0.0 0.00 0.01 296 0.01 0.2 0.00 0.8 10 61 4.3 47 0.5 358 0.6 3.6 269 2 2 10 0.2 6 0.1 1.8 12 0.2 50 125 80 0.1 4 33 300 256 14 152 37 163 29 256 178 28 2 2 10 19 63 2 51 5 10 267 1 4 33 327 40 164 73 254 62 16 52 91 65 1 3 11 4 26 77 3 2 348 3 4 155 38 354 56 71 7 3 1 ID IA16IA17 65.13IA18 65.18IA19 17.75 65.15 17.82 65.14 30 17.97 18.04 2 149 47 189 15 67 72 41 72 72 Sc 72 1.05 Dv Sc Sc 1.05 1.06 1 1.05 1 1 0.94 1 0.86 0.19 0.90 0.95 0.04 0.11 0.08 0.06 0.14 0.12 0.01 0.07 0.00 0.01 0.00 0.00 0.00 0.01 0.8 0.00 0.3 0.5 0.2 0.9 0.0 0.3 0.2 0.1 0.0 0.2 308 0.1 175 32 53 218 18 10 5 16 6 4 1 55 271 144 26 318 17 3 32 5 6 4 2 177 41 248 47 105 65 79 54 4 2 1 2 SA3SA4 67.14Interandean zone 67.16 15.40IA2 15.42 98IA3 83 42 64.73 28 64.79 18.08 55IA7 17.79 55 147 DuIA8 317 25 CIA9 1.03 64.91 61 72 64.98 1.03 1 17.90 72 64.99 17.89 D 117 1 0.86 17.96 Sc 149 18 1.05 162 0.07 0.86 1.07 30 72 40 1 0.01 0.14 1 72 Sc 72 0.84 0.13 Dv 0.02 0.94 1.10 Dv 0.03 1.02 0.00 0.03 0.08 1 1.13 0.17 0.00 1 0.09 0.2 0.89 1 0.00 0.2 0.97 0.01 1.4 0.04 0.93 0.00 0.18 0.0 0.01 0.1 0.15 0.07 0.3 0.03 0.1 1.0 0.00 103 0.14 0.0 0.04 0.5 145 0.05 0.01 33 0.00 0.0 0.3 23 0.01 0.8 32 0.6 44 121 2.3 1.8 7 0.7 211 31 0.2 0.6 1 0.2 26 243 0.4 2 90 32 4 16 55 113 307 3 332 7 38 211 31 11 82 46 87 7 2 4 2 182 9 156 4 61 203 42 200 13 1 24 82 1 57 90 83 357 2 1 261 3 48 295 49 20 32 59 2 ding-oblique to -perpendicular (P > 45°) with a IA14IA15 65.22 65.19 17.67 17.72 292 187IA20 25IA21 75 72 65.89 72 65.90 17.23 Sc 17.17 Dv 163 1.02 282 1.14 22 1 38 1 55 0.95 55 Osb 0.98 Osb 0.06 1.08 0.11 1.02 0.05 1 0.12 1 0.00 0.93 0.01 0.85 0.00 0.04 0.00 0.04 0.10 0.4 0.14 6.0 0.01 0.2 0.00 1.4 0.02 0.2 0.01 1.1 1.1 80 0.1 251 0.4 0.4 1 0.7 9 0.0 6 264 2 116 19 170 39 353 43 53 7 13 167 10 6 228 20 154 25 341 46 35 12 83 10 33 341 2 62 41 15 6

8 Geological Society of America Bulletin, Month/Month 2014 Geological Society of America Bulletin, published online on 30 July 2014 as doi:10.1130/B30968.1

Andean orocline strain and orogen-scale factors inﬂ uencing grain-scale shortening

mean regional trend/plunge and error ellipse of 346/81° ± 8°/11°. Note that the Bingham axial distributions for both the in situ and restored principal axis orientations overlap within uncer-

Z orientation tainty (compare Figs. 9A and 9B). Samples from the Subandes and Interan- dean zone represent Ordovician through Car- boniferous stratigraphy (Fig. 3) from a wide range of structural positions within the orocline axis (Fig. 4). The in situ Interandean zone and Subandes strain ellipsoid orientations show no

Y orientation discernible alignment with respect to regional

T (°)T (°) P Err. T P Err. structural trends or sample orientation (Fig. 10A). Restoring the samples to bedding-hori-

## zontal shows that the X axes have low plunges and lie largely within the bedding plane (mean axial orientation T: 084°, P: 3°), while the Z axes (°) Err.

§§ are nearly perpendicular to bedding (mean axial orientation T: 342, P: 82°; Fig. 9B). The angu- X orientation (°) P †† ) lar errors associated with the X-axis orientations T–+ are signiﬁ cantly higher (between ±10° and 90°)

). compared to the Z axes (maximum ±30°; Fig. YZ continued 10C). Ellipsoids projected onto the balanced < R XY 2011). cross section have bedding-perpendicular Z axes k error and bedding-parallel X axes regardless of thrust sheet location or position with respect to fold

** axes and adjacent faults (Fig. 11). These ellip- k

Flinn’s Flinn’s soid orientations are equally consistent with a sedimentary fabric or a weak, vertical compres- –+ sive stress ﬁ eld related to compaction. Regard-

error less, this indicates that regionally signiﬁ cant, S ε granular shortening was not accommodated in the Subandes or Interandean zone. Eastern Cordillera strain measurements cover # S k < 1.0 represents oblate ellipsoids ( R ε Ordovician through Devonian stratigraphy (Fig. 3). As is the case in the Interandean zone § ), and

YZ and Subandes, in situ Eastern Cordillera ellip- error Mean > R soid axial orientations show no alignment with XY sample orientation or regional structural trend ZYX

TABLE 1. THREE-DIMENSIONAL (3-D) STRAIN RESULTS ( (3-D) STRAIN RESULTS THREE-DIMENSIONAL 1. TABLE (Fig. 12A). Restoring the samples to bedding- horizontal results in best-ﬁ t ellipsoids with X axes that are largely within the bedding plane

Axial length (mean axial orientation T: 100°, P: 1°, ±9/±16) and Z axes that range from oblique to perpendicular to bedding (mean axial orientation T: † 352°, P: 81°, ±9/±16; Fig. 12B). The best-ﬁ t X and Y axial orientations have large uncertainties

T* Unit relative to Z axes (Table 1; Fig. 11C) due to the Sect. similar X and Y axial lengths in oblate ellipsoids ; k > 1.0 represents prolate ellipsoids ( R (°) Dip (Flynn’s k < 1.0, X = Y > Z ). This results in large YZ X and Y axial orientation variations during the to R (°)

XY Monte Carlo error estimation process and pro- Strike R duces a best-ﬁ t ellipsoid with X and Y axial

Lat uncertainties that describe a common plane (°S) (Mookerjee and Nickleach, 2011). Regardless, the strain ellipsoids from the Eastern Cordi- k is the ratio of (°W)

Long llera are characterized by Z axes that range from oblique to perpendicular to bedding and Y and is the octahedral shear strain, or average natural represented by 3-D ellipsoid. S T—trend. P—plunge. Err.—Angular error.

Unit abbreviations are as follows: C—Carboniferous, Du—Upper Devonian, Dv—Devonian Vila Vila Formation, Sc—Silurian Catavi Formation, Sl—Silurian Llallagua Formation, Su—Silurian Uncia Formation, Sc—Silurian Catavi Vila Unit abbreviations are as follows: C—Carboniferous, Du—Upper Devonian, Dv—Devonian Vila t 3-D ellipsoid (see Mookerjee and Nickleach, t between two-dimensional (2-D) ellipses and best-ﬁ Mean error is the total misﬁ ε X axes that are coplanar within uncertainty but † § # †† §§ ## **Flinn’s **Flinn’s *Sect. T: section trend. T: *Sect. EC2EC3EC4 65.58 65.46 17.67 65.34 17.49 112 17.91 287 10 97 54 55 10 55 Osb Osb 55 1.10 1.05 Osb 1 1.09 1 0.89 1 0.94 0.07 0.14 0.89 0.15 0.06 0.07 0.01 0.15 0.00 0.01 0.02 0.00 0.8 0.01 0.8 0.3 0.8 0.1 0.2 0.3 0.1 275 0.1 167 6 42 89 25 3 5 4 182 61 21 201 17 3 39 5 4 19 315 336 68 43 41 2 3 2 Osb—Ordovician San Benito Formation, Oa—Ordovician Anzaldo Formation. Osb—Ordovician San Benito Formation, Oa—Ordovician Eastern Cordillera EC1 65.47 17.64 159 1 55 Osb 1.07 1 0.93 0.10 0.10 0.01 0.00 0.9 0.3 0.2 25 13 4 118 15 5 256 70 3 ID EC5EC6EC7 65.38EC8 65.21 17.98EC9 65.18 18.24 337EC10 65.25 18.24 225EC11 65.20 40 65.23 18.36 167 12 65.24 18.42 182 55 18.42 12 190 72 18.43 188 Osb 55 72 162 62 1.04 Su 15 72 Osb 68 72 1.08 Osb 1 1.06 72 1.06 72 Sc Osb 1 0.91 1 Osb 1.11 1.02 1 0.06 0.93 1.06 0.87 1 1 0.88 0.08 0.09 0.11 1 0.06 0.97 0.96 0.11 0.00 0.14 0.90 0.03 0.08 0.13 0.00 0.01 0.10 0.02 0.10 0.04 0.01 0.03 0.4 0.00 0.11 0.00 0.02 0.00 1.1 0.00 0.0 0.4 0.01 0.01 0.5 0.1 0.01 0.4 0.1 3.6 0.6 0.1 1.1 0.5 0.0 109 0.0 0.9 0.0 252 0.0 292 3.1 5 0.5 279 0.5 21 22 89 124 106 6 21 274 38 32 28 200 7 3 356 33 197 90 11 31 90 13 189 89 220 32 23 38 184 2 37 352 2 134 90 4 80 78 7 37 51 90 64 349 4 81 14 299 40 5 43 84 58 26 85 20 1 2 EC12EC13 65.26EC14 65.28EC15 18.29 65.34EC16 18.27 310 65.45EC17 18.20 331 65.49 33 18.09 312 65.51 52 18.02 312 55 55 18.06 55 31 Osb 66 55 69 Osb 1.12 14 55 Osb 1.06 16 1 55 1.06 Sc 1 55 1.14 Dv 0.88 1 Osb 0.90 1.06 0.06 1.04 1 0.91 0.06 1 0.17 0.08 1 0.94 0.12 0.90 0.05 0.01 0.11 0.85 0.01 0.05 0.00 0.14 0.03 0.01 0.00 0.12 0.00 0.15 0.8 0.02 0.5 0.00 0.05 0.01 0.8 0.6 0.7 0.01 0.01 2.4 0.0 0.2 0.0 0.5 0.2 0.0 0.6 120 162 0.0 2.2 0.1 19 265 0.5 42 0.1 20 86 4 6 301 56 18 7 221 7 7 262 28 85 174 10 21 87 20 20 355 8 207 146 18 10 0 27 84 6 3 55 5 21 87 209 46 6 70 45 275 78 7 2 62 81 9 2 3 EC18EC19 65.53EC20 65.53EC21 18.07 65.87EC22 18.07 288 65.86EC23 17.81 297 66.31 56EC24 17.50 140 66.35 51 17.28 55 65.91 85 29 17.58 162 55 Osb 18.11 125 38 55 Osb 35 1.12 128 Osb 32 55 1.10 55 39 1 1.03 Osb 55 1 Oa 1.02 55 0.94 1 O 1.01 0.92 Osb 0.06 1 0.90 1.15 1.06 0.04 1 0.13 0.05 0.81 1 1 0.13 0.85 0.13 0.03 0.10 0.07 0.99 0.91 0.01 0.19 0.00 0.01 0.10 0.05 0.13 0.00 0.03 1.8 0.00 0.12 0.11 0.04 1.2 0.02 0.4 0.3 0.00 0.01 0.04 0.0 0.1 0.5 0.4 0.05 0.01 0.0 0.4 0.2 10.2 0.0 228 0.7 0.1 119 0.1 0.0 19 0.6 0.0 70 28 9.2 319 0.0 20 278 3 17 25 235 115 19 132 33 34 3 23 71 17 1 339 50 12 23 37 60 187 10 18 21 325 33 2 25 2 272 34 12 71 43 177 64 60 49 184 35 12 79 92 7 132 57 206 3 71 78 4 47 4 9 13 EC27EC28 67.00 68.94 18.39 15.18 347 264 36 57 86 55 Sl Ou 1.18 1.08 1 1 0.94 0.92 0.07 0.06 0.16 0.12 0.00 0.01 0.04 0.01 3.1 0.9 0.9 0.7 0.0 0.0 303 251 2 19 64 11 200 143 79 42 20 10 360 33 42 11 61 7 EC25EC26 66.05 66.49 18.03 18.45 322 158 70 69 55 55 Oa Sl 1.11 1.06 1 1 0.94parallel 0.94 0.08 0.05 0.12 to 0.08bedding 0.00 0.00 0.01 0.00 1.8(Fig. 1.0 1.9 13). 0.7 0.0 The 0.0 285 Eastern 132 8 2 28Cor- 52 23 41 43 34 25 49 187 225 46 56 14 16

Geological Society of America Bulletin, Month/Month 2014 9 Geological Society of America Bulletin, published online on 30 July 2014 as doi:10.1130/B30968.1

Eichelberger and McQuarrie

TABLE 2. TWO-DIMENSIONAL (2-D) STRAIN DATA state for each physiographic zone (Fig. 15). Latitude Longitude Since the orocline axis is dominated by ellip- Sample (°S) (°W) Unit 2-D section θ ± 2σ R ± 2σ s soids with vertical Z axes, cases where the Z Interandean zone Bol09-145 17.88 64.6 Dv A 7 ± 13 1.10 ± 0.02 axis is not vertical are represented by squares B –22 ± 4 1.17 ± 0.02 in Figure 15. With few exceptions, oblate ellip- Bol09-307 18.07 64.92 C A 1 ± 7 1.13 ± 0.02 soids make up the majority of ellipsoid shapes. Bol09-303A 18.03 64.95 D A 5 ± 5 1.11 ± 0.02 Bol09-278 17.83 65.09 Dv A 16 ± 4 1.13 ± 0.02 This means that any apparent strain indicated B 3 ± 7 1.10 ± 0.02 by the best-fi t ellipsoids is the result of RYZ Bol09-257 17.8 65.1 Sc A –39 ± 45 1.03 ± 0.01 strains along bedding-perpendicular Z axes and Bol10-3 18.25 65.22 Sc B 41 ± 49 1.05 ± 0.02 C 2 ± 5 1.18 ± 0.02 bed-parallel Y axes. By implication, the domi- Bol09-80 17.64 65.2 Osb A 19 ± 34 1.08 ± 0.02 nance of oblate ellipsoids with long dimensions C 28 ± 18 1.06 ± 0.02 Bol09-49 17.78 65.27 Sc A 7 ± 22 1.14 ± 0.02 parallel to bedding is consistent with a detri- C –34 ± 20 1.08 ± 0.02 tal fabric that records no grain-scale orogenic Eastern Cordillera strain. There are few prolate ellipsoids, but the Bol09-322 18.17 65.13 O A 9 ± 4 1.14 ± 0.02 φ uniformly low apparent strains and high Rf- Bol09-334 18.5 65.22 Dv A 21 ± 4 1.15 ± 0.02 Bol09-396 17.6 65.3 Osb A –33 ± 30 1.05 ± 0.02 data variability indicate that even these best-fi t Bol10-38 18.12 65.42 Osb B 19 ± 20 1.04 ± 0.02 ellipsoids refl ect detrital grain shapes rather than C 14 ± 4 1.18 ± 0.03 local orogenic strain. Bol10-41 18.09 65.43 K A –33 ± 13 1.12 ± 0.02 Bol10-48 18.05 65.45 S A –17 ± 7 1.12 ± 0.02 C 2 ± 18 1.06 ± 0.03 DISCUSSION Bol10-46 18.06 65.47 K B –27 ± 15 1.06 ± 0.02 C 14 ± 22 1.04 ± 0.02 Bol11-180 18.01 65.5 Dv A –8 ± 3 1.23 ± 0.02 Strain Pattern at the Orocline Axis B –13 ± 7 1.12 ± 0.02 Bol10-63 18.12 65.55 Osb A 21 ± 3 1.27 ± 0.02 B –21 ± 7 1.09 ± 0.02 Grain shape data from competent lithologies Bol09-1 17.5 65.8 Osb B 38 ± 29 1.07 ± 0.03 in the central Andean orocline indicate uni- C –27 ± 23 1.05 ± 0.02 formly low fi nite strain magnitudes. This shows Bol09-428 17.53 66.09 Sc A –20 ± 12 1.08 ± 0.02 B 5 ± 5 1.13 ± 0.02 that grain-scale shortening of these units has not Bol11-236 17.67 66.43 Sc A –1 ± 3 1.21 ± 0.02 been a signifi cant factor in deformation of the Bol11-241 17.66 67 Sc A –12 ± 2 1.33 ± 0.02 Andean stratigraphic succession at this latitude. B –20 ± 2 1.26 ± 0.02 Bol10-178 18.09 66.15 Oa B –21 ± 7 1.11 ± 0.02 At both the outcrop and thin-section scale, there Note: Unit abbreviations are as follows: C—Carboniferous, D—Devonian, Dv—Devonian Vila Vila is a conspicuous lack of penetrative strain fab- Formation, Sc—Silurian Catavi Formation, Sl—Silurian Llallagua Formation, Su—Silurian Uncia Formation, rics commonly associated with layer-parallel Osb—Ordovician San Benito Formation, Oa—Ordovician Anzaldo Formation, O–Ordovician, S–Silurian, K– φ Cretaceous. shortening. Micrographs and Rf- data (Figs. 7 and 8) demonstrate that granular geometry and orientation are highly variable and strongly dillera ellipsoid orientations are consistent with 1.01 in the Subandes and Interandean zone and infl uenced by initial detrital grain shapes. This either a sedimentary- or compaction-related from 1.01 to 1.30 in the Eastern Cordillera (Fig. is conceptually consistent with the low apparent fabric, but, at a minimum, they do not support 14). The low apparent strain magnitudes and strains measured by the Fry method. Both meth- regionally signifi cant granular shortening. ellipsoid orientations refl ect either detrital grain ods produce 2-D ellipses with X axes approxi- shapes that do not measure orogenic strain, or, at mately parallel to bedding. φ Strain Magnitude the very most, they measure a compaction strain The variability in the Rf- data calls into ques- unrelated to orogenic deformation. tion the reliability of the best-fi t 3-D ellipsoids All of the ellipsoids showed uniformly small as a strain indicator, especially those with low ε strain magnitudes of s < 0.20 (Fig. 3; Table 1). Strain Ellipsoid Shape (Flinn’s k Value) ellipticities. Even samples with relatively larger ε ε Most samples have s ranging from 0.10 to 0.15, strains ( s = 0.10–0.20, corresponding to a triple while the larger outliers range from 0.16 to 0.19. The results presented herein show that the axial ratio range of 1.03:1:0.90–1.02:1:0.81) are ε For the entire study area, the mean s is 0.12; by strain ellipsoid pattern at the orocline axis is characterized by apparent fl attened oblate 3-D ε physiographic region, the mean s is 0.12 (n = characterized by Z axes perpendicular to bed- ellipsoids (k-value < 1.0; Fig. 15) oriented with 29) in the Eastern Cordillera, 0.11 (n = 21) in the ding and X axes parallel to bedding. We use X and Y axes parallel to bedding (Fig. 9). Only Interandean zone, and 0.13 (n = 4) in the Sub- Flinn’s k value (Flinn, 1962) to further differ- fi ve samples (SA1, IA5, IA11, IA13, IA15; andes (Table 3). Given the dominance of sub- entiate between ellipsoid geometries dominated Figs. 10 and 13) have bedding-oblique Z axes φ vertical Z axes, projecting the 3-D ellipsoids on by greater apparent strain along X axes (RXY) (P = 26°–42°), but their Rf- data (Fig. 6) show to the cross-sectional plane roughly corresponds versus those with greater strain along the Z axes some of the largest φ fl uctuations (F > 160°) and to either the X-Z or Y-Z strain plane and the X-Y (RYZ). Prolate ellipsoids have k values > 1, due angular uncertainties (both 2-D and 3-D) of the strain plane in map view. On the cross-section to greater X-axis extension than Z-axis compres- entire data set, indicative of high detrital grain ε plane, the 3-D s values correspond to 2-D axial sion (RXY > RYZ). Oblate ellipsoids have k values shape infl uence on the best-fi t ellipsoids. The ratios of RXZ/YZ = 1.03–1.23 in the Subandes < 1, due to greater X-axis extension than Z-axis bedding-parallel oblate ellipsoids may be con- ε and Interandean zone (Fig. 11) and RXZ/YZ = compression (RXY < RYZ). Best-fi t k values, s, sistent with a compaction strain fabric acquired 1.05–1.18 in the Eastern Cordillera (Fig. 13). and their respective uncertainties are plotted on during lithifi cation, but it is equally likely they

The map view RXY values range from 1.18 to Flinn diagrams to interpret the dominant strain reﬂ ect depositional grain orientations. Likewise,

10 Geological Society of America Bulletin, Month/Month 2014 Geological Society of America Bulletin, published online on 30 July 2014 as doi:10.1130/B30968.1

Andean orocline strain and orogen-scale factors inﬂ uencing grain-scale shortening

A that grains have cement overgrowths rather 80 than the sutured grain boundaries associ- 60 ated with dynamic recrystallization (Fig. 5B). The absence of microstructural evidence for 40 dynamic recrystallization (Poirier et al., 1979) in the samples presented here is consistent with 20 the low-strain-magnitude measurements. Since dynamic recrystallization is largely temperature φ 0 dependent, internal strain accumulation in the central Andes may have been limited by low- –20 temperature conditions that prevented strain accommodation by dislocation creep (i.e., –40 Hirth and Tullis, 1992). The lack of dynamic Figure 7. Comparison of Fry –60 recrystallization textures even at the base of the and Rf-ϕ best-fit two-dimen- central Andean stratigraphic section implies sional (2-D) ellipse parameters –80 deformation occurred at temperatures below and 2σ uncertainties: (A) best- the frictional-viscous transition for quartz fi t ϕ, relative to bedding (ϕ = 0) A B C 1.5 (Schmid and Handy, 1991). The temperature and (B) best-fi t axial ratios, R. B threshold estimates for grain-boundary migra- The results are plotted from tion processes range from 250 ± 20 °C to as east to west and alphabetically 1.4 high as 310 ± 30 °C (Voll, 1976; Dunlap et al., for each sample: strike parallel 1997; van Daalen et al., 1999; Stöckhert et al., (A-cuts), dip parallel (B-cuts), 1999; Stipp et al., 2002). For the southern limb 1.3 and bedding parallel (C-cuts). of the central Andean orocline, unreset zircon fi ssion-track (ZFT) ages (ca. 385–481 Ma)

f 1.2 from Ordovician, Silurian, and Devonian stra- R tigraphy (Barnes et al., 2008) limit deformation temperatures to <240 ± 10 °C (based on 1.1 the annealing temperature for zircon; Brandon et al., 1998; Fig. 16A). At the orocline axis, unreset zircon (U-Th)/He (ZHe) ages (459 Ma 1 and 475 Ma) from Upper Devonian stratigraphy in the Subandes (Eichelberger et al., 2013) further constrain deformation temperatures to SA1 IA1 IA2 IA5 IA11 IA13 IA15 EC12 EC17 EC20 EC25 EC27 <160–200 °C (Fig. 16A; Reiners et al., 2004). Based on these constraints, temperature condi- Normalized Fry Rf – φ tions were too low for signifi cant quartz crystal- plastic deformation. The lower temperatures at the orocline axis the degree of grain shape variability present in must be largely accommodated by out-of-plane and southern limb may be partly due to the the bedding-oblique prolate ellipsoid samples displacement in map-scale structures rather than amount of synorogenic sedimentation and indicates that they never accommodated suf- out-of-plane strain at a granular scale. Orogen- burial of the developing fold-and-thrust belt. ficient granular strain to attain even weak parallel displacements have been documented Tertiary synorogenic sediments preserved in anisotropic granular distributions and orienta- along strike-slip and transpressional structures Subandean synclines range from ~5–7 km in tions. Even if the ellipsoids for all the samples such as the Cochabamba and Rio Novillero fault the northern limb (Baby et al., 1995; Watts presented here merely measure detrital grain zones (Dewey and Lamb, 1992; Kley, 1999; et al., 1995) to ~2–4 km in the southern limb shapes, they rule out signifi cant granular short- Eichelberger et al., 2013). (Dunn et al., 1995; Baby et al., 1995). These ening parallel to bedding in the region. sediments are barely preserved in the Subandes One of the primary motivations for quantify- Deformation Conditions in at the orocline axis, where the modern foredeep ing grain-scale shortening in the central Andes the Central Andes containing Tertiary and younger sediments is is to assure an accurate assessment of total only 1.5–2 km (Moretti et al., 1996). Exhuma- crustal shortening and material displacement Compared to other orogens where inter- tion magnitudes estimated from thermochro- during orocline formation. The lack of granu- nal strain is well documented in competent, nology in the Bolivian Andes increase from the lar strain and layer-parallel shortening at the grain-supported rocks, the central Andes are Subandes westward to the Eastern Cordi llera, orocline axis means that out-of-plane strain is exceptional in the apparent lack of signifi - ranging from 3 km to 11 km in the north limb negligible, and shortening estimates from bal- cant grain-scale shortening. We suggest that (Barnes et al., 2006; McQuarrie et al., 2008), anced cross sections do not ignore additional both temperature and lithologic conditions 3 km to at least 6 km at the orocline core grain-scale shortening. As a result, the out-of- in the central Andes inhibited the microscale (Barnes et al., 2012; Eichelberger et al., 2013), plane motion required by the continuing evolu- deformation mechanisms that accommodate and <4 km to 7 km in the south limb (Barnes tion of the Bolivian orocline as a secondary arc layer-parallel shortening. CL images indicate et al., 2008). For the study area at the orocline

Geological Society of America Bulletin, Month/Month 2014 11 Geological Society of America Bulletin, published online on 30 July 2014 as doi:10.1130/B30968.1

Eichelberger and McQuarrie

A W E 5

4 Rf – φ R = 1.31 ± 0.8 3 φ = –22° ± 8° F = 175° Bedding R n = 151 2 Mean R = 1.63 ± 0.5 φ = 0° f Fry R = 1.12 ± 0.06 φ = –18° ± 8°

1 1 mm –90° 0° 90° φ 6 B N S 5

Rf – φ 4 R = 1.24 ± 0.06 φ = –9° ± 10° 3 F = 160° R n = 151 φ = 0° Mean Rf = 1.51 ± 0.5 2 Fry R = 1.11 ± 0.06 φ = –12° ± 10° 1 mm 1 –90° 0° 90° φ 5 C N 4 Rf – φ R = 1.27 ± 0.06 3 φ = –12° ± 10° F = 108° R n = 126 W E 2 Mean Rf = 1.48 ± 0.45 Fry R = 1.08 ± 0.08 1 mm φ = –24° ± 20° S 1 –90° 0° 90° φ

Figure 8. Example of Rf-ϕ data from the A (dip direction parallel), B (strike direction parallel), and C (bedding parallel) planes for sample IA5. The cardinal orientations of each micrograph are shown in the corners of each image. For A and B, micrographs are oriented parallel to bedding. Rf-ϕ plots are ϕ versus ln(Rf) with ϕ = 0 indicating bedding-parallel long axes. Contours are cumulative probability density, white star is best-fi t ellipse, and white circle is mean ellipse shape. Best-fi t R (axial ratio) and ϕ (long-axis angle from horizontal) with 2σ uncertainties are listed at right. F is fl uctuation of ϕ angles; n is number of grains measured. Rf-ϕ plots, contours, and best-fi t Rf-ϕ parameters were produced using EllipseFit (Vollmer, 2011a). core, even the most deeply exhumed samples exposed in the northern limb Eastern Cordi- Layer-Parallel Shortening Accommodation record minor strain. Additionally, samples llera may exhibit greater strain, since they were at Low Temperatures SA3 and SA4 come from the northern limb exhumed from greater depths and experienced Subandes, where synorogenic sediments are higher temperatures (Barnes et al., 2006, and In thrust sheets where deformation tempera- thicker than the orocline axis, yet they still indi- references therein) than rocks from the oro- tures exceeded the ~270 °C threshold for dis- cate negligible strain. It is possible that rocks cline axis where the strain data were collected. location creep and dynamic recrystallization,

12 Geological Society of America Bulletin, Month/Month 2014 Geological Society of America Bulletin, published online on 30 July 2014 as doi:10.1130/B30968.1

Andean orocline strain and orogen-scale factors inﬂ uencing grain-scale shortening

Strain Ellipsoid Axial Orientations A X Axes Z Axes 000 In-Situ Orientation 000

3 270 090 270 1 090 1 2

Figure 9. (A) In situ three-dimensional (3-D) 3 ﬁ nite strain ellipse X- and Z-axial orienta- 2 tions and (B) axial orientations after re- 180 storing bedding at the sample location to Equal Area Projection 180 Bingham Axial Distribution Bingham Axial Distribution horizontal. Gray points are axial orienta- N = 54 tions, gray lines are Kamb contours at 2σ Eigen Eigen intervals, and black numbered poles are Axis Value T/P ±min/max Axial Orientation Axis Value T/P ±min/max Bingham axial distribution eigenvectors. 1. 0.5 106/6 19°/28° Eigen Vector 1. 0.5 016/78 16°/25° Normalized eigenvalues, orientations, and 2. 0.3 198/20 2. 0.3 241/8 3. 0.2 000/69 18°/64° Mean Orientation 3. 0.2 149/8 15°/57° uncertainties are listed for each data set. The largest eigenvalue reﬂ ects the region Kamb Contour (2σ) of highest point density and is interpreted X Axes Z Axes as the mean axial orientation. The lowest B 000 Structurally Restored 000 eigenvector indicates the region of lowest Orientation point density. Plots and Bingham axial distributions were produced using Stereonet 8 (Allmendinger et al., 2013; Cardozo and All- mendinger, 2013). 3 1 270 090 270 3 090 1

2 2 180 180 Bingham Axial Distribution Bingham Axial Distribution Eigen Eigen Axis Value T/P ±min/max Axis Value T/P ±min/max 1. 0.6 098/4 8°/11° 1. 0.7 346/81 8°/11° 2. 0.3 189/12 2. 0.2 175/9 3. 0.1 350/78 8°/37° 3. 0.1 085/1 8°/37°

large coaxial strains are commonly observed in (e.g., Yonkee and Weil, 2010; Sak et al., 2012). reasonable detection, with deformation tem-

grain-supported lithologies (Rs > 1.5; Fig. 16A; The Pennsylvania salient of the Appalachian peratures of <230 °C to as low as <180 °C Yonkee, 2005; Long et al., 2011). The absence fold-and-thrust belt and the Sevier thrust belt (Table 4; Fig. 16A; Barnes et al., 2008; Eichel- of similar strain in the central Andes is easily both have greater regional layer-parallel short- berger et al., 2013). This temperature range linked to deformation temperatures <230 °C. ening than the central Andes, but deformation is comparable to the Appalachian and Sevier However, the factors responsible for limited temperatures are similar to slightly higher. In thrust belts at the shallowest levels, suggesting layer-parallel shortening in the central Andes Bhutan, minor layer-parallel shortening (~7%) the factors limiting layer-parallel shortening in and the minor contribution of grain-scale short- is limited to external thrusts where minimum the central Andes are more nuanced. In addi- ening to the total shortening budget are more deformation temperatures were deﬁ nitively tion to temperature, factors such as lithologic complex. In other fold-and-thrust belts, grain- higher, exceeding >160–200 °C based on reset composition and mechanical stratigraphy also supported lithologies record granular layer- ZHe ages (Long et al., 2011, 2012). In the cen- appear to inﬂ uence internal strain development parallel shortening at temperatures <300 °C tral Andes, strains are at or below the limit of at temperatures below ~200 °C.

Geological Society of America Bulletin, Month/Month 2014 13 Geological Society of America Bulletin, published online on 30 July 2014 as doi:10.1130/B30968.1

Eichelberger and McQuarrie

Subandean and Interandean Zone Axial Orientation A X Axes Z Axes 000 In-Situ Orientation 000 Equal Area Projection n = 25

270 090 270 090

Figure 10. Orientation of X and Z axes of ﬁ nite strain ellipsoids 180 Best Fit Axes 180 from the Subandes (SA, square) Sample Orientation and Interandean zone (IA, cir- Mean Regional Orientation cles). (A) In situ axial orientations with sample bedding orientation X Axes Z Axes (gray lines) and mean regional B 000 Structurally Restored 000 bedding orientation (dashed Orientation black line). (B) Axial data after bedding is structurally restored to horizontal by rotation around bedding trend. Numbered points are Bingham axial distribution 1 eigenvectors with the largest 270 3 090 270 1 090 eigenvector (1) interpreted to be the mean axial orientation, plotted as a star. Samples with best- ﬁ t ellipsoid axial angular errors 3 greater than 10° are indicated 2 by hollow markers. (C) Histo- grams indicate that the majority 2 SA Best Fit Axes of samples have axial errors less 180 180 than 10°. Note the difference in Mean Orientation IA Best Fit Axes Mean Orientation the error angle scale between the T: 084 P: 3 Axes w. error > 10° T: 342 P: 82 horizontal axes. Mean Axial Orientation κ = 1 α95 = 15° Kamb Contour (2σ) κ = 19 C Axial Error Angles 15 15

10 10

5 5 Measurements (#) Measurements Measurements (#) Measurements

0 0 10 20 30 40 50 60 70 80 90 0 10 20 30 X Axis Error (°) Z Axis Error (°)

14 Geological Society of America Bulletin, Month/Month 2014 Geological Society of America Bulletin, published online on 30 July 2014 as doi:10.1130/B30968.1

Andean orocline strain and orogen-scale factors inﬂ uencing grain-scale shortening

Cretaceous 5 Carboniferous Devonian 0 Silurian –5 Ordovician Basement –10Depth (km)

–15

IA18 IA15 IA16 IA10 IA17 IA8 IA7 IA2 IA1 IA3 SA1 SA2 1.06 : 1 : 0.90 1.14 : 1 : 0.98 1.05 : 1 : 0.94 1.13 : 1 : 0.97 1.05 : 1 : 0.86 1.02 : 1 : 0.97 1.10 : 1 : 0.89 1.05 : 1 : 0.84 1.08 : 1 : 0.84 1.07 : 1 : 0.94 1.23 : 1 : 0.99 1.08 : 1 : 0.98 ε = 0.07 εS = 0.12 εS = 0.12 εS = 0.08 εS = 0.11 εS = 0.14 εS = 0.03 εS = 0.15 εS = 0.17 εS = 0.18 εS = 0.09 εS = 0.17 S Rs = 1.06 Rsp = 1.13 Rsp = 1.14 Rsp = 1.03 Rsp = 1.08 Rsp = 1.20 Rsp = 1.05 Rsp = 1.17 Rsp = 1.20 Rsp = 1.23 Rsp = 1.09 Rsp = 1.03 p Rs = 1.00 Rsop = 1.06 Rsop = 0.95 Rsop = 1.00 Rsop = 1.04 Rsop = 1.04 Rsop = 1.00 Rsop = 1.00 Rsop = 1.00 Rsop = 1.06 Rsop = 1.03 Rsop = 1.12 op 0 (km) Thickness 5 10 15 20

Figure 11. Balanced cross section from Eichelberger et al. (2013) showing structural context of Subandean and Interandean zone samples. Ellipses are the projection of the three-dimensional (3-D) ellipsoid onto the cross-section plane. (Top) Deformed section shows in situ orientation of best-ﬁ t ellipsoids with long axis shown in white. Hollow markers indicate samples projected from north of the line of section where deeper stratigraphy is exposed than along the line of section. (Bottom) Structurally restored section with ellipses in restored orientation. For each sample, the following strain parameters are listed: the triple axial ratio (X:Y:Z), octahedral shear strain (εs), axial ratio in the plane of

section (RP), and axial ratio out of the plane of section (ROP).

Sevier Belt Deformation Conditions and Weber Formations demonstrate deformation tion temperatures over 250–300 °C would allow Mechanical Stratigraphy temperatures exceeded ~100–120 °C where for plastic quartz failure and probably result in layer-parallel shortening is documented in the large layer-normal fl attening strains (similar to The Wyoming salient in the Sevier fold-and- frontal thrust systems of the Sevier thrust belt Bhutan), whereas layer-parallel shortening is thrust belt of the western United States has (Fig. 16A; Burtner and Nigrini, 1994). This cor- <10% in the Darby area (Fig. 16A; Yonkee and accommodated up to 30% layer-parallel short- responds to minimum exhumation magnitudes Weil, 2010). Overall, deformation temperatures ening, depending on structural location (Crosby, comparable to the central Andes (~4–5 km), in the Sevier thrust belt were above 130 °C and 1969; Gockley, 1985; Craddock, 1992; Mitra, suggesting similar magnitudes of burial for for- range from 175 °C to 210 °C where layer-par- 1994; McNaught and Mitra, 1996; Mukul and mations where layer-parallel shortening is docu- allel shortening is most signifi cant, potentially Mitra, 1998; Yonkee, 2005; Yonkee and Weil, mented in the Sevier belt. exceeding maximum temperatures inferred for 2010). In the Sevier fold-and-thrust belt, layer- From east to west, the major thrusts of the the central Andes. parallel shortening is best defi ned in the Juras- Sevier system are the Prospect, Hogsback, Sevier thrust sheets with the highest cleavage sic Twin Creek (argillaceous micritic limestone; Absaroka, Darby, Crawford, and Meade thrusts intensity experienced elevated pressures and Imlay, 1967) and Triassic Ankareh (quartzose (Armstrong and Oriel, 1965; Royse et al., 1975; temperatures approaching ~175–210 °C due to to arkosic sandstone and mudstone; Kummel, Coogan, 1992; Yonkee et al., 1997). Illite crys- thrust fault loading along their hinterland trail- 1954) Formations (e.g., Mitra, 1994; Yonkee and tallinity data from cleavage seam materials and ing edge (Mitra and Yonkee, 1985). For exam- Weil, 2010; Weil and Yonkee, 2012). Despite thermal modeling estimated maximum tempera- ple, the Twin Creek Formation in the Crawford the apparent lithologic competency contrast tures of ~190 °C for the Twin Creek Formation thrust (buried by the older Willard and Meade between the matrix-supported Twin Creek and in the Prospect thrust, ~140 °C in the Absaroka thrusts to the west) has high cleavage inten- the grain-supported Ankareh Formations, both thrust, and ~210 °C in the Crawford thrust (Mitra sity, 10%–30% layer-parallel shortening, and formations record roughly equivalent strain and Yonkee, 1985). Fluid inclusion entrapment temperatures of ~210 °C (Fig. 16A; Mitra and orientations and magnitudes (see Yonkee and temperature estimates from the Twin Creek For- Yonkee, 1985; Yonkee and Weil, 2010). Strati- Weil, 2010). These units are directly below the mation in the Darby thrust sheet at the Absaroka graphically below the Twin Creek Formation Jurassic through Cretaceous synorogenic sedi- footwall range from 175 °C to 328 °C (Wiltschko in the same thrust, the Ankareh Formation fea- ment load deposited during Sevier deformation, et al., 2009). The lower end of this temperature tures calcite and quartz dissolution cleavage and which reached thicknesses of ~3–5 km (Yonkee range is comparable to the Crawford, Absa- 10%–30% layer-parallel shortening (Fig. 16A; et al., 1997; DeCelles and Coogan, 2006), com- roka, and Prospect thrusts (Fig. 16A), but the Yonkee and Weil, 2010). Given the presence parable to synorogenic sediment thicknesses highest fl uid inclusion temperatures probably of quartz dissolution textures in the Crawford in the central Andes. Reset AFT ages from the exceed actual deformation temperatures for the thrust, deformation temperatures of ~210 °C Jurassic Stump, Triassic Nugget, and Permian Darby thrust. As discussed already, deforma- appear to be suffi cient to activate pressure solu-

Geological Society of America Bulletin, Month/Month 2014 15 Geological Society of America Bulletin, published online on 30 July 2014 as doi:10.1130/B30968.1

Eichelberger and McQuarrie

Eastern Cordillera Axial Orientation tion in quartz. At lower temperatures, layer- parallel shortening and cleavage are less signifi - A X Axes Z Axes cant: 5%–10% for the Absaroka (~140 °C) and 000 In-Situ Orientation 000 Prospect (~180 °C) thrusts to 10% and lower for the easternmost Hogsback thrust (Fig. 16A; Equal Area Projection Mitra and Yonkee, 1985). With the exception n = 27 of the Absaroka thrust, these temperatures still exceed the likely temperature range for the central Andes (Fig. 16A). Differences in mechanical stratigraphy may 270 090 270 090 help to explain the limited nature of internal strain in the central Andes versus the Sevier fold- and-thrust belt. From a structural standpoint, the Sevier thrusts act as cohesive sheets over large distances with less internal faulting or folding than seen in the central Andes. The Sevier Best Fit Axes thrusts offset 3.5–4.5-km-thick stratigraphic 180 180 Sample Orientation packages (excluding synorogenic strata) from a Mean Regional Orientation common detachment at ~5–7 km depth and slip X Axes distances of 15–40 km at structural wavelengths B Z Axes of ~20–50 km (Dixon, 1982; Coogan, 1992; 000 Structurally Restored 000 Royse, 1993; Yonkee et al., 1997). The extent Orientation of the Sevier faults contributed to increased footwall pressures and temperatures, producing conditions amenable to pressure solution and layer-parallel shortening in the Crawford thrust (Mitra and Yonkee, 1985). By comparison, cen- 270 090 270 090 tral Andean faults generally have <10 km of slip and are spaced ≤20 km apart (Eichelberger et al., 2013). Relative to the Sevier, thrust faults in the central Andes have much lower displacements and structural wavelengths, potentially due to the presence of six thick (≥1 km) shale sequences (Fig. 16A). These mechanically weak intervals Axes Mean Orientation Mean Orientation act as detachment horizons at various strati- 180 Axes w. error > 10° 180 T: 100 P: 1 T: 352 P: 81 graphic levels, preventing slip from localizing Error = ±9 / ±16 Mean Axial Orientation Error = ±9 / ±16 along a single detachment, such as in the Sevier Kamb Contour (2σ) thrust belt (Fig. 16A). As a result, central Andean faults are more numerous but not laterally exten- C Axial Error Angles sive enough to signifi cantly increase footwall 10 20 deformation temperatures over a broad area. Composition may also be an important factor in granular shortening, especially at temperatures below ~180 °C. In particular, 5%–10% layer-parallel shortening is documented in the 5 10 Absaroka thrust, but deformation temperatures (~140 °C) were similar to the central Andes (Fig. 16A). Unlike the Absaroka thrust, spaced Measurements (#) Measurements Measurements (#) Measurements cleavage is not pervasive in the central Andes, and microtextural evidence of pressure solution 0 0 in quartz is absent. The presence of regional 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 cleavage and layer-parallel shortening at low X Axis Error (°) Z Axis Error (°) temperatures in the Absaroka thrust but not the Figure 12. Orientation of X and Z axes of fi nite strain ellipsoids from the Eastern Cordi llera. Andes may stem from the prevalence of calcar- (A) In situ axial orientations with sample bedding orientation (gray lines) and mean regional eous lithologies in the Sevier belt (Fig. 16A). bedding orientation (dashed black line). (B) Axial data after bedding is structurally restored Calcareous lithologies in Bolivia are minor and to horizontal by rotation around bedding trend. Symbology is the same as Figure 9. Samples discontinuous, whereas numerous calcareous with axial angular errors greater than 10° are indicated by hollow markers. (C) Histograms intervals that feature pervasive layer-parallel at the bottom indicate that the majority of samples have axial errors less than 10°. Note the shortening are found in the Sevier belt (Fig. difference in the error angle scale between the horizontal axes. 16A). In the Absaroka thrust, both units record

16 Geological Society of America Bulletin, Month/Month 2014 Geological Society of America Bulletin, published online on 30 July 2014 as doi:10.1130/B30968.1

Andean orocline strain and orogen-scale factors inﬂ uencing grain-scale shortening

5 0 –5 –10 EC6

Depth (km) EC5 1.08 : 1 : 0.93 –15 ε = 0.11v EC27 EC26 EC25 EC24 EC20 EC14 EC15 EC17 1.04 : 1 : 0.91 S –20 Rs = 1.12 1.18 : 1 : 0.94 1.06 : 1 : 0.94 1.11 : 1 : 0.94 1.06 : 1 : 0.91 1.03 : 1 : 0.90 1.06 : 1 : 0.91 1.14 : 1 : 0.94 1.04 : 1 : 0.85 εS = 0.09 p Rs = 0.97 Rsp = 1.10 op –25 εS = 0.16 εS = 0.08 εS = 0.12 εS = 0.11 εS = 0.10 εS = 0.11 εS = 0.14 εS = 0.15 Rs = 1.05 Rs = 1.07 Rs = 1.06 Rs = 1.11 Rs = 1.13 Rs = 1.14 Rs = 1.17 Rs = 1.18 Rsop = 1.04 –30 p p p p p p p p Rsop = 0.94 Rsop = 1.05 Rsop = 1.02 Rsop = 1.04 Rsop = 1.02 Rsop = 1.0 Rsop = 0.99 Rsop = 1.07

Thickness (km) 30

Figure 13. Balanced cross section from Eichelberger et al. (2013) showing structural context of Eastern Cordillera samples. Ellipses are the projection of the three-dimensional (3-D) ellipsoid onto the cross-section plane. (Top) Deformed section shows in situ orientation of best-ﬁ t ellipsoids with long axis shown in white. (Bottom) Structurally restored section with ellipses in restored orientation. For each sample, the following strain parameters are listed: the triple axial ratio (X:Y:Z), octahedral shear strain (εs), axial ratio in the plane of section (RP), and axial ratio out of the plane of section (ROP). Unit shading is the same as Figure 11.

comparable magnitudes of layer-parallel short- between structural domains: 20% layer-parallel Mesozoic AFT ages indicate regional Allegha- ening, but spaced cleavage is better developed in shortening in Devonian through Mississip- nian deformation temperatures of >100 °C calcareous lithologies compared to the quartz- pian rocks of the Valley and Ridge and 14% across Pennsylvania (Roden et al., 1989; Roden, ose sandstone in the Ankareh Formation (Mitra, layer-parallel shortening in Silurian through 1991; Blackmer et al., 1994), comparable to the 1994; Yonkee and Weil, 2010). For temperatures Mississippian rocks of the Appalachian Pla- central Andes, where AFT ages are regionally between 100 °C and 200 °C and pressures on teau (Nickelsen, 1963, 1983; Engelder, 1979a, reset except for the Subandes in the southern the order of 100–300 MPa, pressure solution in 1979b; Slaughter , 1982; Geiser and Engelder, limb (Barnes et al., 2008). Fluid inclusions and quartz is predicted to accommodate strain rates 1983; Gray and Mitra, 1993; Faill and Nick- vitrinite refl ectance data from West Virginia of <10–13–10–15 s–1, compared to 10–11–10–12 s–1 elsen, 1999; Sak et al., 2012). Strain measure- suggest a temperature range of 130–160 °C for for calcite (Rutter, 1976). Within the probable ments in the Pennsylvania salient region were Pennsylvanian strata in both the Appalachian ~140 °C deformation temperature range where based on crinoid ossicles in calcareous siltstone Plateau and Valley and Ridge (Reed et al., layer-parallel shortening is found in the Absa- lithologies and distorted grains (Fry method) for 2005). Thermal modeling, vitrinite refl ectance, roka thrust, predicted strain rates for both quartz grain-supported sandstone lithologies. As with and fl uid inclusion data from the Pennsylvania and calcite fall within the representative range the Sevier belt, strain results from the Appala- Valley and Ridge indicate temperature condi- for orogenic deformation (Pfi ffner and Ramsay, chians are consistent between grain-supported tions ranging from 160 °C to 217 °C between 1982; Ramsay, 2000; van der Pluijm and Mar- and matrix-supported lithologies, despite any the Devonian Catskill and the Ordovician Mar- shak, 2004). Comparable strains are measured apparent competency contrast (Sak et al., 2012). tinsburg Formations (Lacazette, 1991; Evans, in both calcareous and siliciclastic units in the Sevier belt; however, calcite is mechanically able to accommodate more internal strain by TABLE 3. STRAIN RESULTS SUMMARY pressure solution under low-pressure and low- Standard X plunge Error angle Z plunge Error angle Mean ε n deviation (°) (°) (°) (°) ε Error temperature conditions. The relative weakness S op Physiographic mean of calcareous strata in the Sevier belt may have All 0.01 54 0.04 16 12 59 16 0.01 0.04 facilitated minor internal strain and cleavage Eastern Cordillera 0.12 29 0.03 15 12 60 17 0.01 0.04 development at temperatures <180–210 °C, Interandean zone 0.01 21 0.04 16 13 57 16 0.01 0.04 Subandes 0.01 4 0.04 20 11 53 14 0.03 0.06 whereas the absence of similar lithologies in Stratigraphic mean the central Andes may have precluded cleavage Eastern Cordillera development. Ordovician 0.12 23 0.03 16 13 62 14 0.02 0.03 Silurian 0.14 5 0.03 12 12 47 25 –0.02 0.04 Appalachian Deformation Conditions Devonian 0.12 1 Subandes and Interandean zone Ordovician 0.11 5 0.05 13 16 46 16 0.00 0.04 Appalachian internal strain and layer-par- Silurian 0.11 8 0.04 13 11 61 16 0.03 0.04 allel shortening magnitudes are interpreted Devonian 0.14 7 0.08 24 12 57 16 –0.01 0.03 to be stratigraphically uniform and only vary Carboniferous 0.14 5 0.05 17 10 62 15 0.04 0.05

Geological Society of America Bulletin, Month/Month 2014 17 Geological Society of America Bulletin, published online on 30 July 2014 as doi:10.1130/B30968.1

Eichelberger and McQuarrie

67° W 66° W 65° W IA21 Vertical X Axis Vertical Y Axis 1.1 Long Axis Long Axis EC22 Intermediate Axis Short Axis 1.01 IA20 1.07 2-D Strain Data Line of Section vertical X axes EC4 1.14 Bol09-428 EC3 1.03 EC21 IA13 EC23 1.09 1.15 1.17 EC1 IA12 EC2 1.07 IA5 Bol11-241 1.12 IA14 1.05 1.16 1.02 IA4 Bol11-236 1.09 IA15 IA11 IA3 EC20 1.14 1.04 1.14 1.04 EC4 1.09 IA17 Bol09-145 EC17 1.04 IA16 IA6 1.09 1.05 1.07 EC25 EC16 IA18 IA8 1.14 1.06 EC5 1.06 1.01 IA7 Subandes 1.04 IA9 1.14

18° S 1.06 IA2 SA2 IA19 1.05 IA1 1.08 Bol10-178 EC19 1.06 EC24 1.06 EC14 1.08 1.06 1.08 EC6 EC15 1.1 SA1 1.13 IA10 1.18 EC13 1.09 1.04 EC12 EC7 1.14 1.05 EC27 EC26 1.07 EC8 1.30 1.06 EC9 N 1.03 EC11 1.06 EC10 1.06

Interandean zone Eastern Cordillera

Km 10 20 40 60 80

Figure 14. Advanced Spaceborne Thermal Emission and Refl ection Radiometer (ASTER) 30 m digital elevation model (DEM) and geologic map with three-dimensional (3-D) ellipsoids projected onto the horizontal map-view plane. Black ellipses indicate samples with vertical short (Z) axes, and white ellipses indicate samples with vertical intermediate (Y) axes. The orientations of the remaining horizontal axes are indicated by solid lines. The Rs value of the 3-D fi nite ellipsoid projected into the map-view plane is listed for each sample. The locations of two-dimensional (2-D) strain data from regions with limited 3-D data coverage are shown by circles with dashed borders. Dark-gray circles indicate 2-D fi nite ellipses with vertical X axes. Geology is from McQuarrie (2002) and Eichelberger et al. (2013).

1995; Evans and Battles, 1999). Overall, Valley ening is documented even though the depth of thrust, we would predict large layer-normal ﬂ at- and Ridge deformation temperatures range from orogenic exposure is similar. tening strains. ~130 °C to 200 °C, but the majority of the sam- At the Allegheny Front (Valley and Ridge– In the Appalachian Plateau, vitrinite reﬂ ec- pled stratigraphy probably surpassed 160 °C, Appalachian Plateau boundary), reset K-Ar tance from Pennsylvanian strata roughly indi- exceeding deformation temperatures in the cen- ages (<400 Ma; Pierce and Armstrong, 1966) cate maximum temperatures of ~190 °C (R0 = tral Andes (Fig. 16B). The Valley and Ridge and from fault mylonite in the Ordovician Reeds- 1.2–1.4) at the Alleghany Front, diminishing to the Crawford thrust in the Sevier belt attained ville shale indicate that temperatures at this level as low as 95 °C (R0 = 0.6–1.0) at the western temperatures over ~170 °C and both accommo- exceeded at least 230 °C (Ar closure temperature edge of the Appalachian Plateau (Barker, 1988; dated 20% layer-parallel shortening (Fig. 16A; for K-feldspar; Berger and York, 1981). This Ruppert et al., 2010). Lower deformation tem- Yonkee and Weil, 2010). Thermochronologic implies that the base of the folded cover stra- peratures (100–160 °C) similar to the central exhumation constraints are limited for the Appa- tigraphy in the Appalachians may have attained Andes are estimated for the Devonian and Mis- lachians, but these deformation temperatures temperatures well above the central Andes but sissippian formations (Fig. 16B). The presence suggest exhumation magnitudes on the order of comparable to the Willard thrust in the Sevier of a detectable tectonic strain in the Appalachian at least 6–7 km (assuming 25 °C/km geothermal belt (Fig. 16A). Finite strain measurements are Plateau compared to the central Andes suggests gradient). Compared to the central Andes, this is limited for rocks below the cover stratigraphy that Appalachian Plateau deformation tempera- at the upper end of exhumation estimates for the detachment (one sample; Sak et al., 2012), but tures may have been slightly higher (closer to Eastern Cordillera, but no layer-parallel short- based on the thermal similarity to the Willard 160 °C), but as in the Sevier belt, differences in

18 Geological Society of America Bulletin, Month/Month 2014 Geological Society of America Bulletin, published online on 30 July 2014 as doi:10.1130/B30968.1

Andean orocline strain and orogen-scale factors inﬂ uencing grain-scale shortening

Subandean and Interandean Zone A 1.3 εs = 0.30 k = 5 k = 2 k = 1 k = 10

εs = 0.25

SA1 1.2 k = 0.75 εs = 0.20 (Rxy– 1) Figure 15. (A) Flinn diagram y k =

/S (R – 1) for Subandean (light gray) and x yz Interandean (dark gray) ﬁ nite IA15 = S IA9 ellipsoids. Ellipsoids with sub- xy

R IA13 k = 0.5 horizontal Z axes are indicated Prolate IA10 k = 10 ε = 0.20 by squares. White markers with IA12 s 1.1 IA20 IA7 crosses indicate ellipsoids with IA5 z IA1 angular errors >10° in all three IA3 IA6 principal axes. Shaded area sur- SA2 x IA16 IA4 k = 0.25 rounding each point indicates IA18 IA17 IA2 the error region for k value IA19 defined by 100 Monte-Carlo IA8 y k = 0.1 ellipsoid simulations per sam- IA14 SA3 IA11 SA4 IA21 x > y = z ple based on two-dimensional 1.1 1.2 1.3 (2-D) fi nite ellipse uncertainty. Oblate B Eastern Cordillera Ellipsoids with k >1 are classi- k = 0.1 ε = 0.20 1.3 s ε = 0.30 fi ed as prolate, exemplifi ed by s z k = 5 k = 2 k = 1 the theoretical ellipsoid above k = 10 the vertical axis of the Flinn x diagram. Samples with k <1 ε = 0.25 are oblate , exemplifi ed by the s ellipsoid to the right of the horizontal axis. Octahedral shear y k = 0.75 z = y > z strain contours are indicated by 1.2 ε = 0.20 the dashed lines and are at in- EC27 s y

tervals = 0.05. The k value con- /S tours are solid lines. (B) Flinn x EC23

diagram for Eastern Cordillera = S xy

R EC15 k = 0.5 ﬁ nite ellipsoids. Symbology is EC18 EC12 the same as A. EC19 1.1 EC25 EC2 EC10 EC4 EC6 EC28

EC24 EC1 EC8 k = 0.25 EC26 EC17 EC13 EC11 EC3 EC7 EC5 EC16 EC21 EC20 k = 0.1 EC9 EC24 EC22 1.1 1.2 1.3 Ryz = Sy/Sz

mechanical stratigraphy may also explain the detachment in the Appalachian Plateau (Silurian strong (Fig. 3), but they lack the layer-parallel lack of central Andean strain at temperatures Wills Creek and Salinas salt). As discussed in shortening strain fabric documented in Appala- below ~170–200 °C. relation to the Sevier belt, the central Andes fea- chian siliciclastic rocks (Sak et al., 2012) and Structurally, the Appalachian fold-and-thrust ture multiple detachment horizons but only two predicted for shallow, strong rheologies (e.g., belt is similar to the Sevier belt because it only regionally extensive quartzite and sandstone Sanderson, 1982; Yonkee, 2005). The stratig- contains two major detachments in the Valley formations thicker than 1 km (e.g., Ordovician raphy accommodating layer-parallel shortening and Ridge (Ordovician Reedsville shale and San Benito and Devonian Vila Vila; Fig. 16B). in the Appalachian Plateau consists of foreland Cambrian Waynesboro shale; Fig. 16B) and one These formations are classiﬁ ed as mechanically basin siliciclastic sediments with interspersed

Geological Society of America Bulletin, Month/Month 2014 19 Geological Society of America Bulletin, published online on 30 July 2014 as doi:10.1130/B30968.1

Eichelberger and McQuarrie

ε shale horizons that never exceed 500 m in thick- nitudes ( s < 0.20; 1.02:1:0.81). Average strain less sensitive to grain variability at low strains, ε ness (Fig. 16A). Similar central Andean silicic magnitudes range from S = 0.03 to 0.19 with these results were used to calculate best-fi t 3-D φ lithologies at similar deformation temperatures uncertainties between 0.01 and 0.05. Ellipsoid ellipsoids. Overall, the Rf- data show that most lack layer-parallel shortening, as shown here. shapes span the full range from prolate (k ≥ 1.0) strain ellipse shapes are dominated by detrital However, the Andean shale horizons feature to oblate (k < 1.0), with k values from 0.1 to 22.3 grain geometry. weak penetrative strain features localized to the that correspond to a range of triple-axial ratios (3) Best-fi t ellipsoids are dominantly oblate in ε tip lines of thrust faults (McQuarrie and Davis, (X:Y:Z ) from 1.23:1:0.99 ( S = 0.17 ± 0.01, k = shape, with Flinn’s k value < 1 (X = Y > Z ), plac- ε 2002). As suggested by comparing the Sevier 22.3 ± 10) to 1.02:1:0.81 ( S = 0.19 ± 0.03, k = ing them within the fl attening strain fi eld. The and the central Andes, the thick central Andean 0.1 ± 0.1). ellipsoid orientations are characterized by bed- φ marine shale units may have localized strain by (2) Rf- grain orientation data show angular ding-oblique to bedding-perpendicular Z axes forming regional detachments and limited strain variations in excess of 90° (F = 108°–180°) and bedding-parallel Y and X axes. The oblate accumulation in quartz-dominated lithologies and a wide range of axial ratios (Rf = 1.0–5.0). geometry, Z axes at high angles to bedding, and (Fig. 16B). The Appalachian Plateau Silurian This indicates that the initial grain shapes over- X axes within the bedding plane are equally salt detachment also localizes strain, which is whelm any measurable tectonic strain (Ri > Rs). consistent with either a detrital fabric related balanced by layer-parallel shortening and fold- Best-fi t ellipse orientations for Fry data are to sedimentation, or at most, a weak compac- φ ing in the overlying siliciclastic rocks (Sak very similar to Rf- data within uncertainty, tion strain during initial burial and lithifi cation. et al., 2012). However, the siliciclastic rocks and both are characterized by bedding-parallel At a minimum, the low strain magnitudes and remain unfaulted due to the inability of salt to long axes (φ < 30°). Ellipse axial ratios are also ellipsoid shapes preclude signifi cant bedding- φ fracture rock and form fault planes that cut up similar between methods, but best-fi t Rf- ratios parallel shortening at grain scale. As a result, section (Jackson et al., 1994). In this case, the are usually higher by ~0.10, most likely due to any potential errors in bulk-shortening estimates absence of thick shale horizons and resulting the wide variability in grain shape exhibited by for the central Andes are primarily a function mechanical homogeneity of the Appalachian most samples. Since the Fry method results are of out-of-plane motion resulting from orocline Plateau siliciclastic rocks may have favored strain accommodation by layer-parallel shortening rather than numerous, low-slip faults as seen in the central Andes. Figure 16 (on following two pages). (A) Comparative compilation of regional stratigraphy, In contrast to the Bolivian Andes, outcrop strain, and temperature constraints from the central Andes and Sevier fold-and-thrust belt. observations and thermochronology from north- The vertical axis is defi ned by an arbitrary geothermal gradient of 25 °C/km to appropri- ern Argentina document deformation tempera- ately scale the thickness of stratigraphic units. The stratigraphic columns are vertically po- tures >170 °C and pervasive pressure solution sitioned on the temperature scale based on the lowest temperature constraints available for in siliciclastic lithologies (Pearson et al., 2012). the top of the stratigraphic column. Lines connect stratigraphic units to temperatures on the Pearson et al. (2012) speculated that penetra- vertical axis at center based on deformation temperature constraints for that level. Shading tive strain in mechanically strong basement of the temperature ranges is not specifi c, but light gray generally represents ~100–200 °C; rocks helped to accommodate shortening during middle gray is ~200–300 °C, and dark gray is for >300 °C. For the central Andes, alternat- Andean deformation in Argentina. Tempera- ing light and middle gray indicates that temperatures are not constrained to have exceeded tures within the Cambrian metaturbidites and ~180 °C. Arrows indicate whether a given temperature constraint is a minimum (defor- Ordovician granitoids were suffi cient to reset mation temperatures exceeded the constraint) or a maximum (deformation temperatures AFT and partially reset ZHe Cenozoic cooling remained below the constraint). Temperature constraints in the Sevier belt are organized ages (Deeken et al., 2006; Pearson et al., 2012), by thrust sheet since maximum deformation temperature varies by structural position in indicating higher deformation temperatures the fold-and-thrust belt. The west to east stacking order and the stratigraphy carried in than in the Bolivian Andes. While internal strain each of the Sevier thrust sheets are shown at the right of the Sevier stratigraphy. For strati- is not quantifi ed for the Argentine Andes, tem- graphic levels where strain data are available, the overall magnitude of strain is reported peratures up to ~180 °F during Andean defor- as the natural octahedral shear strain (εs) in the central Andes and bed-parallel strain (RB) mation and the presence of dense cleavage are or mean strain (Rs) in the Sevier belt. For the Willard thrust, θ represents the angle be- consistent with observations from thrust sheets tween the principal stretch direction and bedding. The magnitude of out-of-plane strain is in the Sevier belt, the Appalachian Valley and reported as the natural out-of-plane strain (εOP) for the central Andes and as layer-parallel Ridge, and the Bhutanese Himalaya, where shortening (LPS) for the Sevier belt. Black arrows indicate the stratigraphic position of grain-scale shortening is on the order of 20%. detachment horizons. Central Andean lithologies are the same as Figure 3 (adapted from McQuarrie, 2002; McQuarrie and Davis, 2002). Sevier stratigraphy and structure was com- CONCLUSIONS piled from Coogan (1992), Yonkee et al. (1997), Yonkee (2005), DeCelles and Coogan (2006). Temperature constraints and references are listed in Table 4. (B) Comparative compila- This study presents new 3-D fi nite strain tion of regional stratigraphy, strain, and temperature constraints from the central Andes data distributed across the width of the Andean and the Appalachians at the Pennsylvania salient. Central Andean stratigraphy, layout, and fold-and-thrust belt and from various positions symbology are the same as A. Temperature constraints and references are listed in Table 4. throughout the ~15-km-thick stratigraphic sec- For the Appalachians, the stratigraphic section, structural groupings (Appalachian Plateau, tion involved in deformation at the axis of the Valley and Ridge), and strain data are adapted from Sak et al. (2012). Appalachian fi nite

Bolivian orocline. strain is reported for each stratigraphic level as the mean strain (RS) and the angle between Our ﬁ ndings are: the principal stretch direction (X axis) and bedding (θ). The regional layer-parallel shorten- (1) 3-D best-ﬁ t ellipsoids from across the ing values for the structural regions are shown at right. Detachments for each region are region show uniformly low apparent strain mag- indicated by black arrows.

20 Geological Society of America Bulletin, Month/Month 2014 Geological Society of America Bulletin, published online on 30 July 2014 as doi:10.1130/B30968.1

Andean orocline strain and orogen-scale factors inﬂ uencing grain-scale shortening

A Temperature/Thickness Central Andes (25°C/km) Notes: Sevier

RB = Bed parallel strain 75° Rs = mean strain (Rxy) 5 T < 100°C εs = Octahedral shear strain, mean 3-D strain

εOP = out-of-plane strain, approximates LPS 1

MESOZOIC T > 100°C 100°

εs = 5 0.14 ± 0.05 T > 100°C siltstone, RB = H: Absaroka claystone, 1.15 R < 1.05 2 B T ≈ 130°C Preuss salt – LPS LPS 1.30 ≤ 10% limestone, 5–10% CARBONIFEROUS P: LPS Twin Creek siltstone LPS 10–30% 5–10% JURASSIC

2 LPS 150° sandstone P: ~9% 21% 6 Nugget Upper T < 180°C H: R Prospect quartzose RB = B T ≈ 190°C and 1.30 < 1.10 – ε = calcareous LPS s 2 1.60 0.14 ± 0.08 mudstone, 5–10% Ankareh TRIASSIC H: LPS sandstone, LPS shale 5–10%, Icla/Belen 10–30% <5% Crawford DEVONIAN T ≈ 210°C 200° sandstone Weber PERMIAN

Sta. Rosa/ Vila Vila T < 230°C7 limestone, dolostone, minor shale, quartzite Tarabuco/ Catavi

3 250°

εs = 0.13 ± 0.04

Darby ORDOVICIAN Krusillias/ Uncia SILURIAN Crawford Thrust Crawford 175°C ≤T ≤ 325°C carbonate Absaroka Thrust -rich

300° strata Llallagua Darby/Prospect/Hogsback Thrust Darby/Prospect/Hogsback Cancaniri CAMBRIAN quartzite T = 300 – 400°C

Willard San Benito quartzite

ε = 350° s conglomerate

0.12 ± 0.03 4 Maple, Kelley Maple, Canyon R = 1.3–2 Anzlado s quartzite θ > 50° ORDOVICIAN

R = 1.5–3 slate, s diamictite, θ = 10–35° NEOPROTEROZOIC 400° greywacke R = 3–5

Capinota strong cleavage s Perry Canyon

Willard Thrust Willard θ < 10°

Figure 16.

Geological Society of America Bulletin, Month/Month 2014 21 Geological Society of America Bulletin, published online on 30 July 2014 as doi:10.1130/B30968.1

Eichelberger and McQuarrie

B Temperature/Thickness Central Andes (25°C/km) Appalachians Notes: 75° RB = bed parallel strain 5 R = mean strain (Rxy) T < 100°C AP: T> 95°C10 s εs = octahedral shear strain, mean 3-D strain 8 T > 100°C εOP = out-of-plane strain, approximates LPS MESOZOIC 100°

εs = 0.14 ± 0.05 T > 100°C5 130°C – 160°C9 CARBONIFEROUS

Mauch - Rxz = 1.19 sandstone, 150° Chunk θ = 46° 6 10 shale, siltstone Upper T < 180°C MISSISSIPPIAN VR: sandstone, Rs = T < 190°C Pocono conglomerate 1.20 – 1.23 εs = θ = 41 – 85° 0.14 ± 0.08 sandstone, LPS ≈ 14% T > 170°C Rs = shale, siltstone, Icla/Belen

1.14 – 1.25 Appalachian Plateau Catskill DEVONIAN conglomerate θ = 30 – 90° 200° 11 T ~ 200°C Rs = Trimmers - DEVONIAN 1.15 – 1.30 siltstone, shale, LPS ≈ 20% T > 225°C Rock θ = 50 – 90° Valley and Ridge Valley Sta. Rosa/ Vila Vila 7 mudstone, T < 230°C 12 Hamilton sandstone, Rs = and limestone 1.17 – 1.27 θ = 47 – 84° 1 3 Tonoloway limestone, shale Wills Creek shale Rs = 1.26

Tarabuco/ Catavi θ = 86° 250° Rs = 1.22 θ = 86° T > 230°C Clinton sandstone, ε = limestone, shale Rs = 1.14 – 1.26 s Tuscarora θ = 19 – 89° 0.13 ± 0.04 Juniata sandstone, shale 14 conglomerate Bald Eagle Krusillias/ Uncia SILURIAN Reedsville siltstone, shale, limestone Coburn Rs = 1.03 limestone, 300°

Llallagua dolostone

ORDOVICIANBellefonte SILURIAN

Cancaniri limestone, 14 Axeman dolomite, Nittany shale Stonehedge limestone

T > 370°C sandstone, San Benito Gatesburg dolomite, Warrior limestone ε = 350° Pleasent Hill limestone s CAMBRIAN sandstone, shale, 0.12 ± 0.03 Waynesboro limestone Anzlado ORDOVICIAN 400° Capinota

Figure 16 (continued).

22 Geological Society of America Bulletin, Month/Month 2014 Geological Society of America Bulletin, published online on 30 July 2014 as doi:10.1130/B30968.1

Andean orocline strain and orogen-scale factors inﬂ uencing grain-scale shortening

TABLE 4. SUMMARY OF STRATIGRAPHIC TEMPERATURE CONSTRAINTS Temperature constraint Number Age Stratigraphic unit Structural position Method )C°( ecnerefeR Central Andes 1 ciozoseM AS segaTFAtesernU 001< )3102(.lateregreblehciE,)8002(.latesenraB suorefinobraC AS 2 segaeHZtesernU 081< )3102(.lateregreblehciE nainoveDreppU AS Devonian Vila Vila IA 3 nairuliS CE segaTFZtesernU 032< )8002(.latesenraB naicivodrO CE,AI Sevier Prospect Darby Mesozoic– 4 Absaroka Reset AFT ages >100 Burtner and Nigrini (1994) Precambrian Meade Crawford Paris-Willard Prospect ~190 5 Jurassic Twin Creek akorasbA Illite crystallinity 031~ Mitra and Yonkee (1985) drofwarC 012~ 6 Jurassic Twin Creek Darby Fluid inclusions 175–325 Wiltschko et al. (2009) Reset Ar/Ar ages 7 ciozoretorpoeN dralliW Fluid inclusions 05–003 0 )9891(.lateeeknoY Illite crystallinity Appalachians Permian– AP 8 segaTFAteseR 001> )4991(.lateremkcalB Ordovician VR Fluid inclusions 9 Mississippian Mauch Chunk AP 061–031 )5002(.latedeeR Vitrinite refl ectance PA cefleretinirtiV ecnat 59> )0102(.latetreppuR 10 Mississippian Mauch Chunk RV ecnatcefleretinirtiV 091< )0102(.latetreppuR 11 kcoRsremmirTnainoveD RV snoisulcnidiulF 071> )5991(snavE 21 elgaEdlaBnaicivodrO RV snoisulcnidiulF 002 )1991(ettezacaL 31 noitamroFnotnerTnaicivodrO RV snoisulcnidiulF 522> )9991(selttaBdnasnavE

Reset K-Ar age >230 (TC for K-feldspar) 14 Ordovician Reedsville Alleghany Front in Tuscarora fault Pierce and Armstrong (1966) mylonite* >370 (TC for biotite) Note: Structural position abbreviations: SA—Subandes; IA—Interandean zone; EC—Eastern Cordillera; AP—Appalachians; VR—Valle and Ridge. Method abbreviations: AFT—apatite fi ssion track; ZHe—(U-Th)/He; ZFT—zircon fi ssion track. *Reported mylonite mineralogy: 50% quartz, 38% illite, 10% carbonaceous material, 2% large-grain mica, abundant fi ne-grain mica (Pierce and Armstrong, 1966).

limb rotation and orogen-parallel displacements (5) The lack of appreciable layer-parallel offset would have limited footwall burial and on transpressional or strike-slip faults. shortening recorded in central Andean sand- maintained low deformation temperatures, pre- (4) The preservation of detrital grain fab- stones and quartzites relative to other fold-and- venting signifi cant internal strain accumulation. rics and/or compaction strains emphasizes the thrust belts appears to be related to differences importance of deformation temperature in gran- in deformation temperature and mechanical ACKNOWLEDGMENTS ular-scale processes that accommodate penetra- stratigraphy. Strain, temperature, and strati- This research was supported in part by National tive strain, such as dynamic recrystallization. graphic data from the Sevier fold-and-thrust belt Science Foundation (NSF) grant EAR-0908972 and The lack of quartz deformation textures and and the Appalachians suggest that deformation Princeton University. Student fi eld assistants from previously published thermochronology data temperatures in the central Andes were insuf- Bates College (Wes Farnsworth and Keegan Runnals), Princeton University (Andrew Budnick), and Univer- indicate temperature conditions never exceeded fi cient to allow signifi cant pressure solution, sity of Pittsburgh (Lindsay Williams) were instrumen- ~240 °C and may have remained below 180 °C. as indicated by low internal strain values and tal in sample collection and strain data development. These low temperatures would have limited the minimal cleavage development at the outcrop SERGEOTECMIN of La Paz, Bolivia, provided land ability of the quartz-dominated lithologies to scale. The stratigraphy of the Bolivian orocline access credentials and sampling permits. Jamie Tito accommodate signifi cant internal strains. Large contains fi ve regional detachments, including and Sohrab Tawakoli aided in securing the required ≥ permits and made fi eld work in Bolivia logistically layer-normal fl attening strains are nonexistent in three 1-km-thick shale horizons. Compara- possible. Matty Mookerjee provided valuable assis- the central Andes but present in the Sevier thrust tively, the Sevier and Appalachians only fea- tance with geologic programs for Mathematica. Ad- sheets of the western United States and Bhuta- ture three major detachments and lack the thick ditional thanks go to Frederick Vollmer for assistance nese Himalaya where deformation temperatures continuous shale packages found in the central with EllipseFit. exceeded 230°. Layer-parallel shortening is best Andes. The dominance of mechanically weak REFERENCES CITED developed in regions of the Appalachians and stratigraphy in the Bolivian orocline may have Sevier belt where deformation temperatures localized strain along detachment horizons and Allmendinger, R.W., Smalley, R., Bevis, M., Caprio, H., and Brooks, B., 2005, Bending the Bolivian orocline reached or exceeded ~180 °C, the upper limit resulted in the high density of low-offset thrust in real time: Geology, v. 33, p. 905–908, doi:10 .1130 for central Andean deformation temperatures. faults observed at the surface. Minimal fault /G21779 .1 .

Geological Society of America Bulletin, Month/Month 2014 23 Geological Society of America Bulletin, published online on 30 July 2014 as doi:10.1130/B30968.1

Eichelberger and McQuarrie

Allmendinger, R.W., Cardozo, N.C., and Fisher, D., 2013, of Eastern Idaho and Western Wyoming: Geological regional synorogenic hydrologic structure and fl uid mi- Structural Geology Algorithms: Vectors & Tensors: Society of America Memoir 179, p. 55–82, doi: 10 gration: Geological Society of America Bulletin, v. 111, Cambridge, UK, Cambridge University Press, 289 p. .1130 /MEM179 -p55 . p. 1841–1860, doi:10 .1130 /0016 -7606 (1999)111 Armstrong, F.C., and Oriel, S.S., 1965, Tectonic development Craddock, J.P., 1992, Transpression during tectonic evolu- <1841:FIASIA>2 .3 .CO;2 . of Idaho-Wyoming thrust belt: American Association tion of the Idaho-Wyoming fold-and-thrust belt, in Faill, R.T., and Nickelsen, R.P., 1999, Appalachian moun- of Petroleum Geologists Bulletin, v. 49, p. 1847–1866. Link, P.K., Kuntz, M.A., and Piatt, L.B., eds., Regional tain section of the Valley and Ridge Province, in Arriagada, C., Roperch, P., Mpodozis, C., and Fernandez, Geology of Eastern Idaho and Western Wyoming: Geo- Schultz, C.H., ed., The Geology of Pennsylvania: Geo- R., 2006, Paleomagnetism and tectonics of the south- logical Society of America Memoir 179, p. 125–140, logical Survey of Pennsylvania Special Publication 1, ern Atacama Desert (25–28°S), northern Chile: Tec- doi: 10 .1130 /MEM179 -p125 . p. 268–285. tonics, v. 25, TC4001, doi: 10 .1029 /2005TC001923 . Crosby, G.W., 1969, Radial movements in the western Wyo- Fisher, N.I., Lewis, T.L., and Embleton, B.J., 1987, Statisti- Arriagada, C., Roperch, P., Mpodozis, C., and Cobbold, P.R., ming salient of the Cordilleran overthrust belt: Geologi- cal Analysis of Spherical Data: Cambridge, UK, Cam- 2008, Paleogene building of the Bolivian orocline: Tec- cal Society of America Bulletin, v. 80, p. 1061–1077, bridge University Press, 329 p. tonic restoration of the central Andes in 2-D map view: doi: 10 .1130 /0016 -7606 (1969)80 [1061: RMITWW]2 .0 Fitzgibbon, A., Pilu, M., and Fisher, R.B., 1999, Direct least Tectonics, v. 27, TC6014, doi:10 .1029 /2008TC002269 . .CO;2 . square fi tting of ellipses: IEEE Pattern Analysis and Baby, P., Moretti, I., Guillier, B., Limachi, R., Mendez, E., DeCelles, P.G., and Coogan, J.C., 2006, Regional structure Machine Intelligence, v. 21, p. 476–480, doi: 10 .1109 Oller, J., and Specht, M., 1995, Petroleum system of and kinematic history of the Sevier fold-and-thrust /34 .765658 . the northern and central Bolivian sub-Andean zone, in belt, central Utah: Geological Society of America Bul- Flinn, D., 1962, On folding during three-dimension progres- Tankard, A.J., Suárez S., R., and Welsink, H.J., Petro- letin, v. 118, p. 841–864, doi: 10 .1130 /B25759 .1 . sive deformation: Quarterly Journal of the Geological leum basins of South America: AAPG Memoir 62, Deeken, A., Sobel, E.R., Coutand, I., Haschke, M., Riller, U., Society of London, v. 118, p. 385–428, doi: 10 .1144 p. 445–458. and Strecker, M.R., 2006, Development of the south- /gsjgs .118 .1 .0385 . Barke, R., Lamb, S.H., and MacNiocaill, C., 2007, Late ern Eastern Cordillera, NW Argentina, constrained by Fry, N., 1979, Density distribution techniques and strained Ceno zoic bending of the Bolivian Andes: New paleo- apatite fi ssion track thermochronology: From Early length methods for determination of fi nite strains: magnetic and kinematic constraints: Journal of Cretaceous extension to middle Miocene shortening: Journal of Structural Geology, v. 1, p. 221–229, doi: 10 Geophysical Research, v. 112, B01101, doi: 10 .1029 Tectonics, v. 25, TC6003, doi:10 .1029 /2005TC001894 . .1016 /0191 -8141 (79)90041 -5 . /2006JB004372 . Dewey, J.F., and Lamb, S.H., 1992, Active tectonics of the Gal, O., 2003, Fit_ellipse: http://www .mathworks .com Barker, C.E., 1988, Geothermics of petroleum systems: Andes: Tectonophysics, v. 205, p. 79–95. /matlabcentral/fi leexchange /3215 -fi tellipse, MATLAB Implications of the stabilization of kerogen thermal Dixon, J.S., 1982, Regional structural synthesis, Wyoming Central File Exchange (accessed 14 August 2011). maturation after a geologically brief heating duration salient of western overthrust belt: American Association Geiser, P.A., and Engelder, T., 1983, The distribution of at peak temperature, in Magoon, L.B., ed., Petroleum of Petroleum Geologists Bulletin, v. 66, p. 1560–1580. layer parallel shortening fabrics in the Appalachian Systems of the United States: U.S. Geological Survey Duebendorfer, E.M., and Meyer, K.L., 2002, Penetrative foreland of New York and Pennsylvania: Evidence Bulletin 1870, p. 26–29. strain at shallow crustal levels: The role of pressure for two non-coaxial phases of the Alleghanian orog- Barnes, J.B., and Ehlers, T.A., 2009, End member models solution in accommodating regional shortening strain, eny, in Hatcher, R.D., Jr., Williams, H., and Zietz, I., for Andean plateau uplift: Earth-Science Reviews, Ventura basin, western Transverse ranges, California, eds., Contributions to the Tectonics and Geophysics v. 97, p. 105–132, doi: 10 .1016 /j .earscirev .2009 .08 .003 . in Barth, A., ed., Contributions to Crustal Evolution of of Mountain Chains: Geological Society of America Barnes, J.B., Ehlers, T.A., McQuarrie, N., O’Sullivan, P.B., the Southwestern United States: Geological Society of Memoir 158, p. 161–175. and Pelletier, J.D., 2006, Eocene to recent variations America Special Paper 365, p. 295–314, doi: 10 .1130 /0 Gillis, R.J., Horton, B.K., and Grove, M., 2006, Thermo- in erosion across the central Andean fold-thrust belt, -8137 -2365 -5 .295 . chonology, geochronology, and upper crust structure of northern Bolivia: Implications for plateau evolution: Dunlap, W.J., Hirth, G., and Teyssier, C., 1997, Thermo- the Cordillera Real: Implications for Cenozoic exhu- Earth and Planetary Science Letters, v. 248, p. 118– mechanical evolution of a ductile duplex: Tectonics, mation of the central Andean plateau: Tectonics, v. 25, 133, doi:10 .1016 /j .epsl .2006 .05 .018 . v. 16, p. 983–1000, doi: 10 .1029 /97TC00614 . TC6007, doi: 10 .1029 /2005TC001887 . Barnes, J.B., Ehlers, T.A., McQuarrie, N., O’Sullivan, P.B., Dunn, J.F., Hartshorn, K.G., and Hartshorn, P.W., 1995, Gockley, C.K., 1985, Structure and Strain Analysis of the and Tawackoli, S., 2008, Thermochronometer record of Structural styles and hydrocarbon potential of the sub- Big Elk Mountain Anticline, Caribou Mountains, central Andean plateau growth, Bolivia (19.5°S): Tec- Andean thrust belt of southern Bolivia, in Tankard, Idaho [M.S. thesis]: Boulder, Colorado, University of tonics, v. 27, TC3003, doi: 10 .1029 /2007TC002174 . A.J., Suárez S., R., and Welsink, H.J., Petroleum Ba- Colorado, 221 p. Barnes, J.B., Ehlers, T.A., Insel, N., McQuarrie, N., and sins of South America: AAPG Memoir 62, p. 523–543. González, M., Díaz-Martinez, E., and Ticlla, L., 1996, Comen- Poulsen, C.J., 2012, Linking orography, climate, and Dunne, W.M., Onasch, C.M., and Williams, R.T., 1990, The tarios sobre la estratigrafía del Silúrico y Devónico del exhumation across the central Andes: Geology, 40, doi: problem of strain-marker centers and the Fry method: norte y centro de la Cordillera Oriental y Altiplano de 10 .1130 /G33229 .1 . Journal of Structural Geology, v. 12, p. 933–938, doi: Bolivia: Simposio Sul Americano do Siluro-Devoniano, Benjamin, M.T., Johnson, N.M., and Naeser, C.W., 1987, Re- 10 .1016 /0191 -8141 (90)90067 -9 . Ponta Grossa, Brazil, p. 117–130. cent rapid uplift in the Bolivian Andes: Evidence from Ege, H., Sobel, E.R., Scheuber, E., and Jacobshagen, V., Gray, M.B., and Mitra, G., 1993, Migration of deformation fi ssion-track dating: Geology, v. 15, p. 680–683, doi: 2007, Exhumation history of the southern Altiplano pla- fronts during progressive deformation; evidence from 10 .1130 /0091 -7613 (1987)15 <680: RRUITB>2 .0 .CO;2 . teau (southern Bolivia) constrained by apatite fi ssion- detailed structural studies in the Pennsylvania anthra- Berger, G.W., and York, D., 1981, Geothermometry from track thermochronology: Tectonics, v. 26, TC1004, doi: cite region, U.S.A.: Journal of Structural Geology, 40Ar/39Ar dating experiments: Geochimica et Cosmo- 10.1029 /2005TC001869 . v. 15, p. 435–449, doi: 10 .1016 /0191 -8141 (93)90139 -2 . chimica Acta, v. 45, p. 795–811, doi: 10 .1016 /0016 Eichelberger, N., McQuarrie, N., Ehlers, T.A., Enkelmann, E., Hindle, D., Kley, J., Oncken, O., and Sobolev, S., 2005, -7037 (81)90109 -5 . Barnes, J.B., and Lease, R.O., 2013, New constraints on Crustal balance and crustal fl ux from shortening esti- Blackmer, G.C., Gomaa, I.O., and Gold, D.P., 1994, Post- the chronology, magnitude, and distribution of deforma- mates in the central Andes: Earth and Planetary Sci- Alleghanian unroofi ng history of the Appalachian ba- tion within the central Andean orocline: Tectonics, v. 32, ence Letters, v. 230, p. 113–124, doi:10 .1016 /j .epsl sin, Pennsylvania, from apatite fi ssion track analysis p. 1432–1453, doi: 10 .1002 /tect .20073 . .2004 .11 .004 . and thermal models: Tectonics, v. 13, p. 1259–1276, Elger, K., Oncken, O., and Glodny, J., 2005, Plateau-style Hirth, G., and Tullis, J., 1992, Dislocation creep regimes in doi: 10 .1029 /94TC01507 . accumulation of deformation: Southern Altiplano: Tec- quartz aggregates: Journal of Structural Geology, v. 14, Brandon, M.T., Roden-Tice, M.K., and Garver, J.I., 1998, tonics, v. 24, TC4020, doi: 10 .1029 /2004TC001675 . no. 2, p. 145–159, doi: 10 .1016 /0191 -8141 (92)90053 -Y . Late Cenozoic exhumation of the Cascadia accretion- Engelder, T., 1979a, Mechanisms for strain within the Up- Hirth, G., and Tullis, J., 1994, The brittle-plastic transition ary wedge in the Olympic Mountains, northwest Wash- per Devonian clastic sequence of the Appalachian Pla- in experimentally deformed quartz aggregates: Journal ington state: Geological Society of America Bulletin, teau, western New York: American Journal of Science, of Geophysical Research, v. 99, p. 11,731–11,748, doi: v. 110, p. 985–1009, doi: 10 .1130 /0016 -7606 (1998)110 v. 279, p. 527–542, doi: 10 .2475 /ajs .279 .5 .527 . 10 .1029 /93JB02873 . <0985: LCEOTC>2 .3 .CO;2 . Engelder, T., 1979b, The nature of deformation within the Houseknecht, D.W., 1988, Intergranular pressure solution Burtner, R.L., and Nigrini, A., 1994, Thermochronology of outer limits of the central Appalachian foreland fold in four quartzose sandstones: Journal of Sedimentary the Idaho-Wyoming thrust belt during the Sevier orog- and thrust belt in New York State: Tectonophysics, Petrology, v. 58, p. 228–246. eny: A new, calibrated, multiprocess thermal model: v. 55, p. 289–310, doi: 10 .1016 /0040 -1951 (79)90181 -1 . Imlay, R.W., 1967, Twin Creek Limestone (Jurassic) in the American Association of Petroleum Geologists Bul- Erslev, E.A., 1988, Normalized center-to-center strain analy- Western Interior of the United States: U.S. Geological letin, v. 78, p. 1586–1612. sis of packed aggregates: Journal of Structural Geology, Survey Professional Paper 540, 105 p. Cardozo, N., and Allmendinger, R.W., 2013, Spherical pro- v. 10, p. 201–209, doi: 10 .1016 /0191 -8141 (88)90117 -4 . Isacks, B.L., 1988, Uplift of the central Andean Plateau jections with OSXStereonet: Computers & Geosci- Evans, M.A., 1995, Fluid inclusions in veins from the middle and bending of the Bolivian orocline: Journal of Geo- ences, v. 51, p. 193–205, doi: 10 .1016 /j .cageo .2012 .07 Devonian shales: A record of deformation conditions physical Research, v. 93, p. 3211–3231, doi:10 .1029 .021 . and fl uid evolution in the Appalachian Plateau: Geo- /JB093iB04p03211 . Carey, S.W., 1955, The orocline concept in geotectonics: logical Society of America Bulletin, v. 107, p. 327–339, Jackson, M.P.A., Vendeville, B.C., and Schultz-Ela, D.D., Part I: Papers and Proceedings of the Royal Society of doi: 10 .1130 /0016 -7606 (1995)107 <0327: FIIVFT>2 .3 1994, Structural dynamics of salt systems: Annual Re- Tasmania, v. 89, p. 255–288. .CO;2. view of Earth and Planetary Sciences, v. 22, p. 93–117, Coogan, J.C., 1992, Structural evolution of piggyback basins Evans, M.A., and Battles, D.A., 1999, Fluid inclusion and doi: 10 .1146 /annurev .ea .22 .050194 .000521 . in the Wyoming-Idaho-Utah thrust belt, in Link, P.K., stable isotope analyses of veins from the central Appa- Jaeger, J.C., and Cook, N.G.W, 1979, Fundamentals of Rock Kuntz, M.A., and Piatt, L.B., eds., Regional Geology lachian Valley and Ridge Province: Implications for Mechanics, 3rd ed., Chapman and Hall, London, U.K.

24 Geological Society of America Bulletin, Month/Month 2014 Geological Society of America Bulletin, published online on 30 July 2014 as doi:10.1130/B30968.1

Andean orocline strain and orogen-scale factors inﬂ uencing grain-scale shortening

Kamb, W.B., 1959, Ice petrofabric observations from Blue Mukul, M., and Mitra, G., 1998, Finite strain and strain can Association of Petroleum Geologists Memoir 62, Glacier, Washington, in relation to theory and experi- variation analysis in the Sheeprock thrust sheet; an p. 459–479. ment: Journal of Geophysical Research, v. 64, p. 1891– internal thrust sheet in the Provo salient of the Sevier Roperch, P., Sempere, T., Macedo, A., Arriagada, C., For- 1909, doi:10 .1029 /JZ064i011p01891 . fold-and-thrust belt, central Utah: Journal of Structural nari, M., Tapia, C., Garcia, M., and Laj, C., 2006, Kley, J., 1996, Transition from basement-involved to thin- Geology, v. 20, no. 4, p. 385–405, doi:10 .1016 /S0191 Counterclockwise rotation of late Eocene-Oligocene skinned thrusting in the Cordillera Oriental of south- -8141 (97)00087 -4 . fore-arc deposits in southern Peru and its signifi cance ern Bolivia: Tectonics, v. 15, p. 763–775, doi:10 .1029 Müller, J.P., Kley, J., and Jacobshagen, V., 2002, Structure for oroclinal bending in the central Andes: Tectonics, /95TC03868 . and Cenozoic kinematics of the Eastern Cordillera, v. 25, TC3010, doi: 10 .1029 /2005TC001882 . Kley, J., 1999, Geologic and geometric constraints on a kine- southern Bolivia (21°S): Tectonics, v. 21, 1037, doi:10 Royse, F., 1993, An overview of the geologic structure of matic model of the Bolivian orocline: Journal of South .1029 /2001TC001340 . the thrust belt in Wyoming, northern Utah, and eastern American Earth Sciences, v. 12, p. 221–235, doi: 10 Nadai, A., 1963, Theory of Flow and Fracture of Solids: En- Idaho, in Snoke, A.W., Stiedtmann, J.R., and Roberts, .1016 /S0895 -9811 (99)00015 -2 . gineering Societies Monographs: New York, McGraw- S.M., eds., Geology of Wyoming: Geological Survey Kley, J., and Monaldi, C.R., 1998, Tectonic shortening and Hill, 705 p. of Wyoming Memoir 5, p. 272–311. crustal thickness in the central Andes; how good is the Nickelsen, R.P., 1963, Fold patterns and continuous defor- Royse, F., Warner, M.A., and Reese, D.L., 1975, Thrust belt correlation?: Geology, v. 26, p. 723–726, doi:10 .1130 mation mechanisms of the central Pennsylvania folded structural geometry and related stratigraphic problems, /0091 -7613 (1998)026 <0723: TSACTI>2 .3 .CO;2 . Appalachians, in Cate, A.S., ed., Tectonics and Cam- Wyoming-Idaho-northern Utah, in Boylard, E.W., ed., Kummel, B., 1954, Triassic Stratigraphy of Southeastern brian-Ordovician Stratigraphy in the Central Appa- Deep Drilling Frontiers of the Central Rocky Moun- Idaho and Adjacent Areas: U.S. Geological Survey lachians of Pennsylvania: Pittsburgh, Pennsylvania, tains: Denver, Rocky Mountain Association of Geolo- Professional Paper 0254-H, p. 165–194. Pittsburgh Geological Society, Guidebook, p. 13–29. gists, p. 41–45. Lacazette, A.J., Jr., 1991, Natural Hydraulic Fracturing in Nickelsen, R.P., 1983, Aspects of Alleghanian deformation, Ruppert, L.F., Hower, J.C., Ryder, R.T., Levine, J.R., Trippi, the Bald Eagle Sandstone in Central Pennsylvania and in Nickelsen, R.P., and Cotter, E., eds., Silurian Depo- M.H., and Grady, W.C., 2010, Geologic controls on the Ithaca Siltstone at Watkins Glen, New York [Ph.D. sitional History and Alleghanian Deformation in the thermal maturity patterns in Pennsylvanian coal-bear- thesis]: University Park, Pennsylvania, Pennsylvania Pennsylvania Valley and Ridge: 49th Field Conference ing rocks in the Appalachian basin: International Jour- State University, 225 p. of Pennsylvania Geologists: Danville, Pennsylvania, nal of Coal Geology, v. 81, p. 169–181, doi: 10 .1016 /j Long, S., McQuarrie, N., Tobgay, T., and Hawthorne, J., Guidebook, Pennsylvania Geological Society, p. 29–39. .coal .2009 .12 .008 . 2011, Quantifying internal strain and deformation tem- Ong, P.F., van der Pluijm, B.A., and Van der Voo, R., 2007, Rutter, E.H., 1976, The kinetics of rock deformation by pres- perature in the eastern Himalaya, Bhutan; implications Early rotation and late folding in the Pennsylvania sure solution: Philosophical Transactions of the Royal for the evolution of strain in thrust sheets: Journal of salient (U.S. Appalachians); evidence from calcite- Society of London, ser. A, Mathematical and Physical Structural Geology, v. 33, p. 579–608, doi:10 .1016 /j twinning analysis of Paleozoic carbonates: Geological Sciences, v. 283, p. 203–219, doi: 10 .1098 /rsta .1976 .jsg .2010 .12 .011 . Society of America Bulletin, v. 119, p. 796–804, doi: .0079 . Long, S., McQuarrie, N., Tobgay, T., Coutand, I., Cooper, 10 .1130 /B26013 .1 . Sak, P.B., McQuarrie, N., Oliver, B.P., Lavdovsky, N., and F.J., Reiners, P.W., Wartho, J.A., and Hodges, K.V., Pearson, D.M., Kapp, P., Reiners, P.W., Gehrels, G.E., Jackson, M.S., 2012, Unraveling the central Appala- 2012, Variable shortening rates in the eastern Hima- Ducea, M.N., Pullen, A., Otamendi, J.E., and Alonso, chian fold-thrust belt, Pennsylvania; The power of se- layan thrust belt, Bhutan; insights from multiple ther- R.N., 2012, Major Miocene exhumation by fault-prop- quentially restored balanced cross sections for a blind mochronologic and geochronologic data sets tied to agation folding within a metamorphosed, early Paleo- fold-thrust belt: Geosphere, v. 8, p. 685–702, doi:10 kinematic reconstructions: Tectonics, v. 31, TC5004, zoic thrust belt; northwestern Argentina: Tectonics, .1130 /GES00676 .1 . doi: 10 .1029 /2012TC003155 . v. 31, TC4023, doi: 10 .1029 /2011TC003043 . Sanderson, D.J., 1982, Models of strain variation in nappes Marshak, S., 1988, Kinematics of orocline and arc formation Pfi ffner, O.A., and Ramsay, J.G., 1982, Constraints on and thrust sheets; a review: Tectonophysics, v. 88, in thin-skinned orogens: Tectonics, v. 7, no. 1, p. 73–86. geological strain rates; arguments from fi nite strain p. 201–233, doi: 10 .1016 /0040 -1951 (82)90237-2 . McNaught, M.A., and Mitra, G., 1996, The use of fi nite states of naturally deformed rocks: Journal of Geo- Schmid, S.M., and Handy, M.R., 1991, Towards a genetic strain data in constructing a retrodeformable cross- physical Research, v. 87, p. 311–321, doi:10 .1029 classifi cation of fault rocks; geological usage and tec- section of the Meade thrust sheet, southeastern Idaho, /JB087iB01p00311 . tonophysical implications, in Müller, D.W., McKenzie, U.S.A.: Journal of Structural Geology, v. 18, p. 573– Pierce, K.L., and Armstrong, R.L., 1966, Tuscarora fault; an J.A., and Weissert, H., eds., Controversies in Modern 583, doi: 10 .1016 /S0191 -8141 (96)80025 -3 . Acadian(?) bedding-plane fault in central Appalachian Geology: London, Academic Press, p. 339–361. McQuarrie, N., 2002, The kinematic history of the central Valley and Ridge Province: American Association of Sempere, T., 1994, Kimmeridgian? to Paleocene tectonic Andean fold-thrust belt, Bolivia; implications for Petroleum Geologists Bulletin, v. 50, p. 385–390. evolution of Bolivia, in Salfi ty, J.A., ed., Cretaceous building a high plateau: Geological Society of America Poirier, J.P., Guillope, M., Nicolas, A., Darot, M., and Wil- Tectonics of the Andes: Wiesbaden, Germany, Vieweg Bulletin, v. 114, p. 950–963, doi:10 .1130 /0016 -7606 laime, C., 1979, Deformation induced recrystallization Publishing, p. 168–212, doi: 10 .1007 /978 -3 -322 (2002)114 <0950: TKHOTC>2 .0 .CO;2 . of minerals: Bulletin de Mineralogie, v. 102, p. 67–74. -85472 -8_4 . McQuarrie, N., and Davis, G.H., 2002, Crossing the several Ramsay, J.G., 2000, A strained Earth, past and present: Sci- Sempere, T., 1995, Phanerozoic evolution of Bolivia and scales of strain-accomplishing mechanisms in the hinter- ence, v. 288, p. 2139–2141, doi:10 .1126 /science .288 adjacent regions, in Tankard, A.J., Suarez, R., and Wel- land of the central Andean fold-thrust belt, Bolivia: Jour- .5474 .2139 . sink, H.J., eds., Petroleum Basins of South America: nal of Structural Geology, v. 24, p. 1587–1602, doi:10 Ramsay, J.G., and Huber, M.I., 1983, Techniques of Modern American Association of Petroleum Geologists Mem- .1016 /S0191 -8141 (01)00158 -4 . Structural Geology: Strain Analysis, Volume 1: Lon- oir 62, p. 207–230. McQuarrie, N., and DeCelles, P., 2001, Geometry and struc- don, Academic Press, 307 p. Sheffels, B.M., 1988, Structural Constraints on Structural tural evolution of the central Andean backthrust belt, Reed, J.S., Spotila, J.A., Eriksson, K.A., and Bodnar, R.J., Shortening in the Bolivian Andes [Ph.D. thesis]: Bolivia: Tectonics, v. 20, p. 669–692, doi:10 .1029 2005, Burial and exhumation history of Pennsylvanian Cambridge, Massachusetts, Massachusetts Institute of /2000TC001232 . strata, central Appalachian basin; an integrated study: Technology, 167 p. McQuarrie, N., Barnes, J.B., and Ehlers, T.A., 2008, Geo- Basin Research, v. 17, p. 259–268, doi:10 .1111 /j .1365 Shimamoto, T., and Ikeda, Y., 1976, A simple algebraic metric, kinematic, and erosional history of the central -2117 .2005 .00265 .x . method for strain estimation for deformed ellipsoidal Andean plateau, Bolivia (15–17°S): Tectonics, v. 27, Reiners, P.W., Spell, T.L., Nicolescu, A., and Zanetti, K.A., objects: 1. Basic theory: Tectonophysics, v. 36, p. 315– TC3007, doi: 10 .1029 /2006TC002054 . 2004, Zircon (U-Th)/He thermochronometry; He dif- 337, doi: 10 .1016 /0040 -1951 (76)90107 -4 . Means, W.D., 1989, Stretching faults: Geology, v. 17, p. 893– fusion and comparisons with 40Ar/39Ar dating: Geochi- Slaughter, J.A., 1982, Fossil Distortion and Pressure Solution 896, doi:10 .1130 /0091 -7613 (1989)017 <0893: SF>2 .3 mica et Cosmochimica Acta, v. 68, p. 1857–1887, doi: in the Middle to Upper Devonian Clastics and Lime- .CO;2. 10 .1016 /j .gca .2003 .10 .021 . stones of the Appalachian Plateau, Central New York Mitra, G., 1994, Strain variation in thrust sheets across the Ries, A.C., and Shackleton, R.M., 1976, Patterns of strain [M.S. thesis]: Storrs, Connecticut, University of Con- Sevier fold-and-thrust belt (Idaho-Utah-Wyoming); variation in arcuate fold belts: Philosophical Transac- necticut, 320 p. implications for section restoration and wedge taper tions of the Royal Society of London, ser. A, Math- Somoza, R., Singer, S., and Coira, B., 1996, Paleomagnetism evolution: Journal of Structural Geology, v. 16, p. 585– ematical and Physical Sciences, v. 283, p. 281–288, of Upper Miocene ignimbrites at the Puna; an analysis 602, doi: 10 .1016 /0191 -8141 (94)90099 -X . doi: 10 .1098 /rsta .1976 .0085 . of vertical-axis rotations in the central Andes: Journal Mitra, G., and Yonkee, W.A., 1985, Relationship of spaced Roden, M.K., 1991, Apatite fi ssion-track thermochronology of Geophysical Research, v. 101, p. 11,387–11,400, cleavage to folds and thrusts in the Idaho-Utah-Wyo- of the southern Appalachian basin; Maryland, West doi: 10 .1029 /95JB03467 . ming thrust belt: Journal of Structural Geology, v. 7, Virginia, and Virginia: The Journal of Geology, v. 99, Stipp, M., Stuenitz, H., Heilbronner, R., and Schmid, S.M., p. 361–373, doi: 10 .1016 /0191 -8141 (85)90041 -0 . p. 41–53, doi: 10 .1086 /629472 . 2002, The eastern Tonale fault zone; a “natural labora- Mookerjee, M., and Nickleach, S., 2011, Three-dimensional Roden, M.K., Miller, D.S., Gardner, T.W., and Sevon, W.D., tory” for crystal plastic deformation of quartz over strain analysis using Mathematica: Journal of Struc- 1989, Apatite fi ssion-track thermochronology of the a temperature range from 250 to 700 °C: Journal of tural Geology, v. 33, p. 1467–1476, doi:10 .1016 /j .jsg Pennsylvania Appalachian basin: Geomorphology, Structural Geology, v. 24, p. 1861–1884, doi:10 .1016 .2011 .08 .003 . v. 2, p. 39–51, doi: 10 .1016 /0169 -555X (89)90005 -6 . /S0191 -8141 (02)00035 -4 . Moretti, I., Baby, P., Mendez, E., and Zubieta, D., 1996, Roeder, D., and Chamberlain, R.L., 1995, Structural geol- Stöckhert, B., Brix, M.R., Kleinschrodt, R., Hurford, A.J., Hydrocarbon generation in relation to thrusting in the ogy of sub-Andean fold and thrust belt in northwest- and Wirth, R., 1999, Thermochronometry and micro- Sub Andean zone from 18° to 22°S, Bolivia: Petroleum ern Bolivia, in Tankard, A.J., Suarez, R., and Welsink, structures of quartz; a comparison with experimental Geoscience, v. 2, p. 17–28. H.J., eds., Petroleum Basins of South America: Ameri- fl ow laws and predictions on the temperature of the

Geological Society of America Bulletin, Month/Month 2014 25 Geological Society of America Bulletin, published online on 30 July 2014 as doi:10.1130/B30968.1

Eichelberger and McQuarrie

brittle-plastic transition: Journal of Structural Geol- tral Andes: Earth Planetary Science Letters, v. 134, Yonkee, W.A., 2005, Strain patterns within part of the Wil- ogy, v. 21, p. 351–369, doi:10 .1016 /S0191 -8141 p. 9–21. lard thrust sheet, Idaho-Utah-Wyoming thrust belt: (98)00114 -X . Weil, A.B., and Sussman, A.J., 2004, Classifying curved oro- Journal of Structural Geology, v. 27, p. 1315–1343, van Daalen, M., Heilbronner, R., and Kunze, K., 1999, Ori- gens based on timing relationships between structural doi: 10 .1016 /j .jsg .2004 .06 .014 . entation analysis of localized shear deformation in development and vertical-axis rotations, in Sussman, Yonkee, W.A., and Weil, A.B., 2010, Reconstructing the quartz fi bres at the brittle-ductile transition: Tectono- A.J., and Weil, A.B., eds., Orogenic Curvature: Integrat- kinematic evolution of curved mountain belts; internal physics, v. 303, p. 83–107, doi:10 .1016 /S0040 -1951 ing Paleomagnetic and Structural Analyses: Geological strain patterns in the Wyoming salient, Sevier thrust (98)00264 -9 . Society of America Special Paper 383, p. 1–17. belt, U.S.A.: Geological Society of America Bulletin, van der Pluijm, B.A., and Marshak, S., 2004, Earth Struc- Weil, A.B., and Yonkee, W.A., 2012, Layer-parallel short- v. 122, p. 24–49, doi: 10 .1130 /B26484 .1 . ture: An Introduction to Structural Geology and Tec- ening across the Sevier fold-thrust belt and Laramide Yonkee, W.A., Parry, W.T., Bruhn, R.L., 1989, Thermal tonics: New York, W.W. Norton & Company, 672 p. foreland of Wyoming; spatial and temporal evolution models of thrust faulting: Constraints from fl uid-in- Voll, G., 1976, Recrystallization of quartz, biotite and feld- of a complex geodynamic system: Earth and Planetary clusion observations, Willard thrust sheet, Idaho-Utah- spars from Erstfeld to the Leventina nappe, Swiss Alps, Science Letters, v. 357–358, p. 405–420, doi: 10 .1016 Wyoming thrust belt: Geological Society of America and its geological signifi cance: Schweizerische Miner- /j .epsl .2012 .09 .021 . Bulletin, v. 101, no. 2, p. 304–313, doi: 10 .1130 /0016 alogische und Petrographische Mitteilungen, v. 56, Weil, A.B., Yonkee, A., and Sussman, A., 2010, Reconstruct- -7606 (1989)101 <0304: TMOTFC>2 .3 .CO;2 . p. 641–647. ing the kinematic evolution of curved mountain belts: Yonkee, W.A., DeCelles, P.G., and Coogan, J., 1997, Kine- Vollmer, F.W., 2010, A comparison of ellipse-fi tting tech- A paleomagnetic study of Triassic red beds from the matics and synorogenic sedimentation of the eastern niques for two and three-dimensional strain analysis, Wyoming salient, Sevier thrust belt, U.S.A.: Geo- frontal part of the Sevier orogenic wedge, northern and their implementation in an integrated computer logical Society of America Bulletin, v. 122, no. 1–2, Utah, in Link, P.K., and Kowallis, B.K., eds., Protero- program designed for fi eld-based studies: Abstract p. 3–23, doi: 10 .1130 /B26483 .1 . zoic to Recent Stratigraphy, Tectonics, and Vol- T21B-2166, Fall Meeting, American Geophysical Welsink, H.J., Franco, M.A., and Oviedo, G.C., 1995, canology, Utah, Nevada, Southern Idaho, and Central Union, San Francisco, California. Andean and pre-Andean deformation, Boomerang Mexico: Brigham Young University Studies, v. 42, Vollmer, F.W., 2011a, EllipseFit 2.0: http:// www Hills area, Bolivia, in Tankard, A.J., Suarez, R., and p. 355–380. .frederickvollmer.com /ellipsefi t/ (accessed 9 January Welsink, H.J., eds., Petroleum Basins of South Amer- 2014). ica: American Association of Petroleum Geologists SCIENCE EDITOR: CHRISTIAN KOEBERL Vollmer, F.W., 2011b, Best-fi t strain from multiple angles Memoir 62, p. 481–499. ASSOCIATE EDITOR: STEFANO MAZZOLI of shear and implementation in a computer program Wiltschko, D.V., Lambert, G.R., and Lamb, W., 2009, Condi- MANUSCRIPT RECEIVED 17 JULY 2013 for geological strain analysis: Geological Society of tions during syntectonic vein formation in the footwall REVISED MANUSCRIPT RECEIVED 14 MAY 2014 America Abstracts with Programs, v. 43, no. 1, p. 147. of the Absaroka thrust fault, Idaho-Wyoming-Utah fold MANUSCRIPT ACCEPTED 16 JUNE 2014 Watts, A.B., Lamb, S.H., Fairhead, J.D., and Dewey, J.F., and thrust belt: Journal of Structural Geology, v. 31, 1995, Lithospheric fl exure and bending of the Cen- p. 1039–1057, doi: 10 .1016 /j .jsg .2009 .03 .009 . Printed in the USA

26 Geological Society of America Bulletin, Month/Month 2014