The geometry, kinematics, and timing of deformation along the southern segment of the Paposo fault zone, Atacama fault system, northern Chile

Journal Pre-proof The geometry, kinematics, and timing of deformation along the southern segment of the Paposo fault zone, Atacama fault system, northern Chile Rachel Ruthven, John Singleton, Nikki Seymour, Rodrigo Gomila, Gloria Arancibia, Daniel F. Stockli, John Ridley, Jerry Magloughlin PII:

S0895-9811(19)30203-2

DOI:

https://doi.org/10.1016/j.jsames.2019.102355

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SAMES 102355

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Journal of South American Earth Sciences

Received Date: 3 May 2019 Revised Date:

5 September 2019

Accepted Date: 6 September 2019

Please cite this article as: Ruthven, R., Singleton, J., Seymour, N., Gomila, R., Arancibia, G., Stockli, D.F., Ridley, J., Magloughlin, J., The geometry, kinematics, and timing of deformation along the southern segment of the Paposo fault zone, Atacama fault system, northern Chile, Journal of South American Earth Sciences (2019), doi: https://doi.org/10.1016/j.jsames.2019.102355. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

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The geometry, kinematics, and timing of deformation along the southern segment of the Paposo fault zone, Atacama fault system, northern Chile Rachel Ruthven1*, John Singleton1, Nikki Seymour1, Rodrigo Gomila2, Gloria Arancibia2, Daniel F. Stockli3, John Ridley1, and Jerry Magloughlin1 1

Colorado State University, 2Pontificia Universidad Católica de Chile, 3University of Texas at Austin *corresponding author: [email protected]

The Paposo fault zone is a major brittle-ductile strand of the Atacama fault system (AFS),

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which records sinistral shear associated with Cretaceous oblique subduction beneath northern

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Chile. New detailed geologic mapping, macro- and microstructural data, and zircon

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geo/thermochronology reveal insight into the structural evolution of the southern portion of the

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Paposo fault. The core of the Paposo fault is defined by a ~50 m-thick zone of illite-rich gouge

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that dips steeply ESE and juxtaposes fractured but unfoliated Early Jurassic tonalite west of the

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fault against mylonitic Late Jurassic to Early Cretaceous granitoids east of the fault. The

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mylonite zone east of the fault is ~0.7–1 km thick and includes a ~100–500 m-thick band of

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hydrothermally-altered ultramylonite derived from Latest Jurassic (146.5 ± 1.5 Ma) to Early

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Cretaceous (138.5 ± 1.6 Ma) granodiorite. West of this ultramylonite zone, a ~150–350 m-thick

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zone of younger tonalite (136.0 ± 1.8 Ma) parallels the Paposo fault and grades from

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protomylonite to mylonite <10 m from the gouge zone on its western margin. Mylonitic

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foliations dip steeply to moderately SE with lineations that typically plunge ~10–30° SW. Most

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SE-dipping mylonitic fabrics record oblique sinistral-reverse shear that is consistent with the

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overall pattern of small-scale brittle faults and S-C-C’ fabrics in the Paposo fault gouge zone.

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However, in several parts of the hydrothermally-altered mylonite zone, symmetric

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microstructures and S>L tectonite fabrics most likely record a significant component of zone-

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normal flattening. The upper age limit of deformation is constrained to be younger than the Late

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Jurassic/Early Cretaceous granodiorite. Hydrothermal alteration and development of high-strain

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zones in the Late Jurassic/Early Cretaceous granodiorite are locally associated with mafic dikes

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that do not cut the younger protomylonitic tonalite, indicating that most of the hydrothermal

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alteration and mylonitic strain occurred between ~139 Ma and 136 Ma. The Paposo fault gouge

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zone formed between 150–200°C based on clay mineralogy and the illite Kübler index. The

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timing of gouge formation most likely overlaps with cooling below ~180–190°C recorded by a

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zircon (U-Th)/He date of 116.6 ± 6.2 Ma from the Late Jurassic/Early Cretaceous granodiorite.

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Together these data constrain brittle and ductile deformation to the Early Cretaceous, similar to

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the age of deformation along other segments of the AFS and coeval with co-spatial arc

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magmatism. Regionally, the Paposo segment of the AFS is arcuate, trending NNW-SSE in the

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northern end and NNE-SSW in the southern end. Previous studies of fault strands along the

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northern portion of the Paposo segment document sinistral transtension, whereas oblique

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sinistral-reverse shear and local coaxial flattening record sinistral transpression along the

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southern portion of the Paposo fault. We propose that transtension and transpression along the

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AFS are controlled by the arcuate geometry, and both are compatible with sinistral simple shear

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along the N-S-trending magmatic arc.

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1. Introduction

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Strike-slip fault systems play a critical role in accommodating oblique convergence

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between oceanic and continental lithosphere. Relative plate motion across most subduction

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margins is <75° from the plate boundary (Jarrard 1986; Woodcock, 1986), and this oblique

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convergence is commonly partitioned into underthrusting of the slab and lateral transport of the

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overriding plate (Fitch, 1972; Beck, 1983). Examples of active trench-parallel strike-slip faults

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include the Sumatran fault system (e.g., McCarthy and Elders, 1997; Sieh and Natawidjaja,

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2000), the Philippine fault (e.g., Barrier et al., 1991; Quebral et al., 1996), the Liquiñe-Ofqui

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fault zone in southern Chile (e.g., Cembrano et al. 1996, 2000), and the Median Tectonic Line in

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southwest Japan (e.g., Tabei et al., 2003; Sato et al., 2015). In the geologic record, large-scale

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translations of forearc blocks and exotic terranes are typically attributed to displacement on

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trench-parallel strike-slip faults (Jarrard, 1986; Avé Lallemant and Oldow, 1988; Beck, 1991). In

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some regions these strike-slip faults are localized along magmatic arcs, suggesting an important

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relationship between deformation and magmatism (Beck 1983; Glazner, 1991; White and

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Harlow, 1993; Saint Blanquat et al., 1998). A significant amount of research has focused on the

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dynamics of oblique subduction, yet important uncertainty remains surrounding the structural

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geology of major strike-slip systems in these tectonic settings. In particular, the deformation

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histories of most intra-arc strike-slip faults are not well understood, and the temporal and spatial

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relationships between strike-slip deformation and magmatism are commonly unclear.

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Reconstructing the structural evolution of intra-arc strike-slip faults is essential for understanding

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how oblique convergence is partitioned above subduction zones.

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The ~1,000-km long Atacama fault system, which is located within the Early Cretaceous

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magmatic arc in the Coastal Cordillera of northern Chile, is considered a classic example of an

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intra-arc strike-slip system that accommodated oblique convergence. The trench-parallel

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Atacama fault system (AFS) records sinistral shear during Mesozoic oblique subduction of the

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Phoenix plate beneath the South American plate (Fig. 1) (Hervé, 1987a; Scheuber and

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Andriessen, 1990; Brown et al., 1993). Traditionally the AFS has been divided into three arcuate

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segments (from north to south): the Salar del Carmen, Paposo, and El Salado segments (e.g.,

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Naranjo, 1987; Brown et al., 1993; Fig. 1a). The AFS follows the trace of the Early Cretaceous

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arc between Iquique (20.5°S) and La Serena (30°S), Chile, suggesting deformation was localized

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along the weaker magmatic arc (Brown et al., 1993; Scheuber and Gonzalez, 1999; González,

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1999). Mylonitization along the AFS is typically thought to have occurred primarily in

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synkinematic plutonic rocks (Dallmeyer et al., 1996; Grocott and Taylor, 2002), suggesting that

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high temperatures associated with pluton emplacement prompted shear zone development.

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Grocott and Taylor (2002) interpret the elongate, tabular Early Cretaceous plutons along the AFS

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to have been emplaced via a roof-uplift–floor subsidence mechanism within a transtensional

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regime.

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Scheuber and Gonzalez (1999) interpret multiple stages of Jurassic to Cretaceous

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deformation along the Coastal Cordillera between 22° and 26°S, and Scheuber and Andriessen

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(1990) discuss the timing and conditions of ductile deformation along a portion of the Paposo

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fault, the main strand in the Paposo Segment of the AFS (Fig. 1). Arc-parallel sinistral shear

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under amphibolite-facies conditions occurred during intervals in the Early to Late Jurassic

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(Scheuber and Andriessen, 1990; Scheuber et al., 1995), but sinistral shear directly associated

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with AFS most likely initiated in the Early Cretaceous (Hervé, 1987a; Scheuber and Gonzalez,

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1999; Grocott & Taylor, 2002). Timing of deformation along the AFS is loosely constrained by

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K-Ar and 40Ar/39Ar cooling dates (Hervé, 1987a; Hervé and Marinovic, 1989; Grocott et al.,

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1994; Scheuber et al., 1995; Dallmeyer et al., 1996; Scheuber and Gonzalez, 1999; Grocott and

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Taylor, 2002). Based on K-Ar dates, Hervé (1987a) interpret sinistral ductile shear along a strand

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of the Paposo segment to be bracketed between ~144 Ma and 131 Ma. Scheuber et al. (1995)

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interpret ~126–125 Ma biotite 40Ar/39Ar and Rb/Sr dates from mylonites along the northern

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Paposo segment to directly record the timing of AFS ductile shearing. Based on geo- and

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thermochronology of mylonitic plutons along the AFS near 26°30’ S, Dallmeyer et al. (1996)

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interpret AFS mylonitization to have initiated ~127–126 Ma, whereas Taylor et al. (1998) interpret field- and age relationships in the same area to record initiation of the AFS ~132 Ma. The main strand in the Paposo segment is the Paposo fault, which strikes NNE at the

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southern end and N to NNW at the northern end with numerous NW-striking splays (Fig. 1b,

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Hervé, 1987a; Scheuber and Andriessen, 1990; Cembrano et al., 2005; Veloso et al., 2015). In

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this paper, we examine the geometry and kinematics of deformation along the southern segment

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of the Paposo fault through detailed field mapping, structural data collection, and microstructural

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analyses. In addition, we present 5 zircon U-Pb dates and 2 zircon (U-Th)/He dates of intrusive

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igneous rocks to establish the timing of deformation and the relationship between deformation

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and magmatism.

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2. Geologic background

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2.1.The Paposo Fault

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The Paposo fault is a major brittle-ductile feature of the AFS that records sinistral slip

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associated with Mesozoic oblique subduction (Hervé, 1987a; Scheuber and Andriessen, 1990;

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Brown et al., 1993). Brittle deformation overprints mylonitic fabrics along the Paposo fault

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(Scheuber and Andriessen, 1990; Alvarez et al., 2016), and cumulatively the mylonitic rocks and

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brittle fault rocks comprise the Paposo fault zone. The Paposo fault was reactivated during the

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Neogene as a steep E-dipping normal fault, creating a prominent escarpment up to ~400 m high

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(Naranjo, 1987; Hervé, 1987b; Dewey and Lamb, 1992; González et al., 2006; Loveless et al.,

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2010; Alvarez et al., 2016).

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Previous studies along the Paposo segment have focused primarily on brittle faulting and ductile deformation along the northern end near Antofagasta (Cembrano et al., 2005; Mitchell

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and Faulkner, 2009; Jensen et al., 2011; Arancibia et al., 2014; Veloso et al., 2015; Gomila et al.,

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2016). Cembrano et al. (2005) and Veloso et al. (2015) document brittle sinistral transtension

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associated with splays off the northern portion of the Paposo fault (Fig. 1b). Given the likely

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southeastward convergence direction of the Phoenix plate, sinistral transtension is consistent

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with the overall NNW trend of faults in the northern portion of the Paposo segment. The

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geometry and kinematics of brittle and ductile deformation along the southern segment have not

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been studied in detail, but the dominant NNE trend of this part of the fault system is more

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compatible with sinistral transpression.

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2.2.Previous geologic mapping

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Our detailed geologic mapping and kinematic analysis focused on an area along the

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southern part of the Paposo fault near the town of Paposo (Fig. 1b). This area was mapped at

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1:100,000 scale by Escribano et al. (2013), who interpreted the plutonic rocks on both sides of

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the Paposo fault as the Middle to Late Jurassic Matancilla Intrusive Complex. The adjacent area

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along the Paposo fault to the north was mapped at 1:100,000 scale by Alvarez et al. (2016) and

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includes a ~0.3–2 km wide zone of variably mylonitized Early Cretaceous tonalite (tonalite of

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the Remiendos Plutonic Complex) bordered by the Matancilla Intrusive Complex to the east and

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nonmylonitic Early to Middle Jurassic Yumbes tonalite across the Paposo fault to the west.

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Bedrock exposures along the southern part of the Paposo fault are significantly better than

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exposures along the central segment, where the fault core is rarely exposed, and most slopes are

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covered with colluvium. We mapped a 15 km2 area along the southern Paposo Segment of the

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AFS at 1:10,000-scale to determine the distribution and geometry of ductile fabrics

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(Supplementary File 1, Fig. 2). Most of the structural data presented in this paper are from this

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map area, except for some minor fault data collected during reconnaissance fieldwork along the

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Paposo fault <2.5 km south of the map area. We analyzed ductile and brittle fabric data using

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the Stereonet 10 and FaultKin software (Marrett and Allmendinger, 1990; Allmendinger et al.,

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2012; Cardozo and Allmendinger, 2013). The orientations of all planar structures are given as

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strike, dip and dip quadrant, whereas the orientations of all linear structures are given as

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trend/plunge.

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3. Methods

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3.1.Identification of Units and Sampling Strategy

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Our geologic map is divided into 7 different map-scale units based on mineralogy,

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texture, and deformation characteristics (Fig. 2). We assign unit names and ages based on

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correlation with units mapped by Alvarez et al. (2016) and new U-Pb zircon geochronology data

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presented below. The Paposo fault in this region has an overall fault strike of 013-014°. Units

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west of the Paposo fault include enclaves of Upper Triassic to Lower Jurassic sedimentary rocks

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of the Pan de Azucar Formation within the Early Jurassic tonalite (Escribano et al., 2013;

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Alvarez et al., 2016; Fig. 2). Units east of the fault include Late Jurassic/Early Cretaceous

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granodiorite, which has been subdivided into four units by the degree of penetrative strain and

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alteration, and Early Cretaceous tonalite (Fig. 2, Supplementary File 1). Rock units were

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identified and differentiated based on field observations, and the freshest and most representative

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outcrops of each identified lithology were sampled for zircon U-Pb geochronology and whole-

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rock geochemistry. We note that alteration is pervasive across the entire study area. For the shear

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zones discussed below, we sampled outcrops with various degrees of strain and hydrothermal

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alteration as determined by field identification of mineral assemblages in order to understand the

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potential effects of fluids in the development of the shear zones. All geochronology samples

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have a clear structural context with respect to one or more aspects of deformation and alteration

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events that have affected the area. We sampled representative outcrops of the map units for

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microstructural study (42 samples), lithology description (35 samples), geochemical analyses (14

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samples), geochronological analyses (5 samples), and one sample for illite crystallinity analysis

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(Supplementary File 2).

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3.2.Zircon U-Pb Geochronology and (U-Th)/He Thermochronology

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We analyzed 5 magmatic samples for zircon U-Pb geochronology using laser-ablation

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inductively-coupled-plasma mass spectrometry (LA-ICP-MS) to determine the crystallization

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ages of these units. Concentrations of 202Hg, 204Pb, 206Pb, 208Pb, 232Th, 235U, and 238U were

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measured on 20 to 40 zircons for each sample on either polished epoxy mounts imaged using

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cathodoluminescence (samples 17-1-P26 and 17-1-P124) (Supplementary File 3) or on

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unpolished grains mounted parallel to the c-axis on tape mounts by depth-profiling (samples 16-

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1-P31, 18-1-P73, and 18-1-P81). Raw data were corrected for down-hole and elemental

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fractionation using GJ-1 as a primary standard (Jackson et al., 2004; Elhlou et al., 2006) and

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Plesovice (Slama et al., 2008) and Pak1 (in-house TIMS data) as secondary standards. To

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identify and recover multiple growth domains within individual zircons, we carried out LA-ICP-

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MS depth profiling following procedures described by Marsh and Stockli (2015)). While all

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zircon 206Pb/238U dates are reported (Supplementary File 4), only dates with <10% discordance

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and <10% analytical error were included in the weighted mean age calculations to eliminate

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effects of significant lead loss, mineral inclusions, or inheritance and to recover a crystallization

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age for each sample. No common Pb correction was applied. Table 1 presents weighted mean

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206

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(Supplementary File 4). Weighted mean dates based on <10% or <5% discordance filters vary by

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≤0.2%.

Pb/238U dates and 2σ errors from zircons with <10% discordance and <5% discordance

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In addition, we analyzed euhedral zircons from samples 16-1-P31 and 18-1-PJ81 by (U-

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Th)/He thermochronometry, which records cooling through ~180°C (Reiners et al., 2002;

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Reiners et al., 2004; Wolfe & Stockli, 2010). Grains were analyzed using standard procedures

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described by Wolfe and Stockli (2010). Errors reported for individual zircon (U-Th)/He aliquots

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are 8% and are based on 2σ standard error estimates from long-term Fish Canyon Tuff standard

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analyses (Reiners et al., 2002, 2004). Zircon (U-Th)/He dates are reported as the mean age ± 2

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times the maximum standard error of the mean. Error is calculated in both standard (n/standard

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deviation of ages) and alternative ((1/ × ∑  ) formats, where n is the number of aliquots

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used to calculate the mean age and σxi is the individual aliquot error. All U-Pb and (U-Th)/He

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analyses were carried out UTChron Laboratories in the Jackson School of Geosciences at the

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University of Texas at Austin.

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3.3.Geochemical Analyses

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Geochemical data were collected on samples from the Late Jurassic/Early Cretaceous

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granodiorite and Early Cretaceous tonalite to characterize hydrothermal alteration. Whole rock

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sample fragments lacking weathered surfaces were powdered and analyzed with X-ray

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fluorescence (XRF) (major elements: ALS package ME-ICP06; trace elements: ALS package

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ME-MS81) at the ALS Global Geochemistry Analytical Lab in Reno, Nevada.

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3.4.Illite crystallinity Illite crystallinity was determined for the clay gouge zone to determine temperature of

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clay formation. We analyzed the gouge using X-ray diffraction (XRD) to determine the

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mineralogy and illite crystallinity, which relate to the temperature of authigenic clay formation.

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A clay-rich gouge sample was separated to 2–0.5µm and < 0.5 µm size fractions using a

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centrifuge and analyzed using XRD at the U.S. Geological Survey in Denver, Colorado.

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4. Geology, geochronology, and geochemistry of units along the Paposo fault

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4.1.Early Jurassic tonalite and Upper Triassic to Lower Jurassic Pan de Azucar Formation

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west of the Paposo fault

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The Early Jurassic tonalite dominates the western side of the Paposo fault, although many

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of the outcrops within this unit consist of younger, more resistant mafic and intermediate dikes

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and other hypabyssal intrusions. The tonalite is chloritically altered, oxidized, and pervasively

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fractured but lacks mylonitic fabrics. Most of this unit is fine- to medium-grained with a color

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index of ~8–15% from chloritized hornblende and biotite. Its composition locally approaches

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granodiorite and quartz diorite.

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A sample from the Early Jurassic tonalite (18-1-PJ81) yielded a weighted mean zircon U-

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Pb date of ~177.4 ± 1.8 Ma (Fig. 3a, Table 1). This date has a very high mean square of weighted

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deviates (MSWD) of 27 with individual concordant zircons ranging from 189 to 166 Ma,

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pointing to possible incorporation of inherited zircons as well as undetected Pb loss. However,

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there is no clear correlation between zircon date and [U], [Th], or U/Th that would allow

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differentiation between different zircon generations. While field petrologic observations or the

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lack of different zircon habits do no support multiphase emplacement of this pluton, further

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petrologic and/or textural work is needed to resolve this age dispersion. This tonalite most likely

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correlates with the Yumbes tonalite previously mapped just north of the study area (Escribano et

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al., 2013), which has a 176 ± 6 Ma K-Ar amphibole date and a 165 ± 5 Ma K-Ar biotite date

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(Hervé and Marinovic, 1989). Herein we refer to this unit as the Yumbes tonalite. Sample 18-1-

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PJ81 from this tonalite yielded a zircon (U-Th)/He date of ~104 ± 21 Ma (Table 2).

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The Pan de Azucar Formation crops out as roof pendants in the Early Jurassic Yumbes

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tonalite and hypabyssal intrusions (Fig. 2, Supplementary File 1). In the map area, this unit

238

consists of limestone, siltstone, and fine-grained sandstone that have undergone low-grade

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contact metamorphism. These sedimentary rocks lack penetrative fabrics and significant

240

recrystallization. The siliciclastic rocks are immature and rich in clay, and the limestone is

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micritic with locally abundant silt.

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4.2.Late Jurassic/Early Cretaceous granodiorite east of the Paposo fault The Late Jurassic/Early Cretaceous granodiorite is present from ~150–350 m east of the

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Paposo fault to beyond the eastern edge of the mapping area (Fig. 2, Supplementary File 1),

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where it has previously been mapped as Matancilla Intrusive Complex (Escribano et al., 2013;

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Alvarez et al., 2016). This unit has been subdivided into 4 zones from east to west: 1) unstrained

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granodiorite, 2) a dominantly unstrained granodiorite zone (~100–250 m-thick) with 1–20 cm-

248

wide discrete shear zones (Fig. 4a), 3) a dominantly mylonitized zone with discrete unstrained

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zones (~100–500 m-thick), and 4) a hydrothermally-altered zone with pervasive mylonitic to

250

ultramylonitic fabrics (~100–500 m-thick) (Fig. 4b) (Fig. 2, Supplementary File 1). The

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granodiorite ranges mineralogically from quartz-rich granodiorite to tonalite, and the two main

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mafic minerals are chloritized biotite and less abundant hornblende, which make up ~6–14% of

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the rock in unstrained samples. All mapped parts of this granodiorite complex are intruded by

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mafic dikes.

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This granodiorite complex has zircon U-Pb weighted mean dates of 138.5 ± 1.6 Ma

256

(sample 17-1-P26) and 151.3 ± 1.7 Ma (sample 16-1-P31) (Figs. 3b and c, Table 1). There is no

257

correlation between Th/U and age for sample 17-1-P26, and the only two zircons that did not

258

plot on a linear array of [Th] versus [U] were excluded from the calculated mean age for having

259

>10% discordance and as an anomalously young outlier. In contrast with this sample, sample 16-

260

1-P31 has a MSWD of 24, and a kernel density estimation reveals a bimodal distribution of dates

261

(Fig. 3d), that persists even at <2% discordance, indicating the younger peak is not the result of

262

Pb loss. While a consistent Th/U does not allow for differentiation between the two different age

263

modes, the younger one is characterized by a relatively uniform [U], [Th]. In contrast, the older

264

dates exhibit a greater variability in [U], [Th], suggesting the possibility of inheritance and/or

265

recrystallization. Hence, we interpret the young mode as the crystallization age (146.5 ± 1.5 Ma,

266

MWSD =5.1) (Fig. 3e, Table 1). The ~147 Ma sample also yielded a (U-Th)/He zircon date of

267

116.6 ± 6.2 Ma (Table 2). Highly strained aplite sills are locally present within the

268

hydrothermally-altered zone, with one of these aplite sills giving a zircon U-Pb date of 142.4 ±

269

1.3 Ma (Fig. 3f, sample 17-1-P124, Table 1). The high MSWD for this sample (13) suggests that

270

some of these zircons may be antecrystic and inherited, in which case only the younger subset of

271

zircons (~140 Ma) may be autocrystic with the dike. We note the presence of two distinct groups

272

of zircon in cathodoluminescent imagery, however these subsets do not correlate to a difference

273

in U-Pb age (dark zircons, ~137–148 Ma; brighter zircons, ~139–152 Ma) or Th/U (dark zircons,

274

0.63–1.34; brighter zircons, 0.63–1.34) (Supplementary File 3, 4). There is also no correlation

275

between Th/U and age, and only one linear trend between [Th] and [U] is present. As such, we

276

prefer the ~142 Ma age for the aplite sill. The ~147–139 Ma range for this granodiorite complex

277

is younger than the Middle to Late Jurassic Matancilla Intrusive Complex to the north (Alvarez

278

et al., 2016), and hereafter we simply refer to this unit as the Late Jurassic/Early Cretaceous

279

granodiorite complex.

280 281

Hydrothermal alteration plays an important role in deformation of the Late Jurassic/Early Cretaceous granodiorite complex. The highest strain fabrics typically occur within bleached

282

regions of the hydrothermally-altered zone and are noticeably lighter in color than the less

283

altered granodiorite (Fig. 5a). To describe the alteration, we characterized outcrop-scale,

284

microstructural, and geochemical changes between: a) spatially associated pairs of variably

285

altered mylonitic samples and b) the least altered, unstrained granodiorite versus bleached, high-

286

strain mylonitic granodiorite (Table 3). In outcrop and thin section, the less altered areas of the

287

hydrothermally-altered zone contain more mafic minerals and appear less strained (Figs. 5a and

288

b). Compared to the less altered granodiorite, the bleached granodiorite commonly lacks mafic

289

minerals, has more alteration of feldspar to saussurite and white mica, more veinlets, and more

290

quartz segregations (Figs. 5a and c). Geochemical analyses indicate a relative loss of MgO,

291

Fe2O3, and K2O in the hydrothermally-altered zone (Table 3).

292

4.3.Early Cretaceous tonalite east of the Paposo fault

293

The Early Cretaceous tonalite forms a ~150–350 m-wide zone adjacent to the Paposo

294

fault (Fig. 2, Supplementary File 1). This unit is mostly protomylonitic with very low strain

295

zones near the contact with the hydrothermally-altered granodiorite complex along the tonalite’s

296

eastern margin. The tonalite grades from protomylonite to intensely-fractured mylonite and

297

locally ultramylonite <10 m from the Paposo gouge zone. The freshest observed sample has a

298

color index of ~25% with mafic minerals consisting mostly of hornblende with lesser amounts of

299

chloritized biotite. Secondary chlorite veins locally make up 7% of this unit. Unlike the Late

300

Jurassic/Early Cretaceous granodiorite complex, the protomylonitic tonalite is neither bleached

301

nor intruded by mafic dikes. A sample from this tonalite (18-1-PJ73) yielded a zircon U-Pb date

302

of 136.0 ± 1.8 Ma (Fig. 3g, Table 1). As with the other plutonic samples, these data do not show

303

a correlation between Th/U and date, and the distribution of [Th] versus [U] falls along a single

304

linear trend. The age and map distribution of this unit are consistent with the tonalite of the Late

305

Cretaceous Remiendos Plutonic Complex mapped by Alvarez et al. (2016) just north of the study

306

area. Herein we refer to this unit as the Remiendos tonalite. The Remiendos tonalite lacks

307

bleached high strain zones and overall is less altered than the granodiorite. To characterize the

308

alteration of this zone we compare: a very low strain, relatively unaltered tonalite near the

309

eastern contact, a more altered tonalite mylonite near the western contact, and a tonalite

310

ultramylonite within a few meters of the gouge zone. Altered samples have fewer mafic minerals

311

and more quartz segregations. The data indicate that alteration and deformation were

312

accompanied by a relative increase in SiO2 and K2O and a relative loss in MgO, Fe2O3, and CaO

313

(Table 3). These geochemical trends differ from those associated with alteration in the

314

granodiorite complex, which is consistent with the interpretation that the tonalite unit was

315

emplaced following most or all of the bleaching in the granodiorite complex.

316 317

4.4.Foliated gouge in the Paposo fault core A ~50 m-thick gouge zone defines the core of the Paposo fault. This zone contains a

318

foliation defined by scaly clay-rich zones and cataclastically-deformed zones derived from the

319

Early Jurassic Yumbes tonalite on the west side of the fault (Fig. 4d). No mylonitized clasts from

320

the east side were observed in the gouge zone, but the slivers of ultramylonite and mylonite

321

along the eastern contact with the gouge zone are locally overprinted by cataclastic deformation.

322

XRD data reveal that the gouge consists primarily of illite, quartz, plagioclase, chlorite,

323

calcite, and hematite. There is no smectite clay, suggesting temperatures were above ~150°C,

324

where ≥85% of clay should be illite due to the upper thermal stability of smectite (Abad, 2007).

325

However, this lack of smectite could also reflect the initial lack of mixed-layer clays. Analyses

326

of air-dried and ethyl-glycolated preparations of the gouge yielded similar Kübler Index (full

327

width and half-maximum peak) for illite clay: ~0.53–0.61 ∆°2θ (Supplementary File 5), which is

328

consistent with clay formation within the deep diagenetic zone near the boundary with the low

329

anchizone (Fig. 6) (Verdel et al., 2011). Both the lack of smectite and the Kübler Index indicate

330

that the gouge developed in the ~150–200°C temperature range.

331 332

5. Shear zone fabrics and kinematics

333

5.1.Mylonitic fabrics

334

All mylonitic fabrics observed in the study area are developed in the Late Jurassic/Early

335

Cretaceous granodiorite complex and the Early Cretaceous Remiendos tonalite east of the

336

Paposo fault. The mean mylonitic foliation of these plutons is 041, 63 SE (strike, dip), which

337

strikes ~20–34° clockwise of the Paposo fault (Fig. 7a). This obliquity between the average

338

foliation and the Paposo fault is consistent with sinistral shear. Mylonitic lineations consistently

339

plunge southwest, and the mean orientation is 212/18 (trend/plunge) (Fig. 7a). As outlined in the

340

previous section, the granodiorite complex is divided into 4 different zones based on deformation

341

and alteration characteristics. From east to west, these zones (and in parentheses their average

342

mylonitic foliation and lineation) are: 1) an unstrained zone lacking discrete shear zones, 2) a

343

zone consisting of <50% discrete mylonitic zones within unstrained granodiorite (foliation 044,

344

49 SE; lineation 204/24), 3) a zone consisting of >50% discrete mylonitic zones (typically <1 m

345

thick) with unstrained or low strain zones (foliation 020, 62 E; lineation 192/10), and 4) a

346

hydrothermally-altered zone that is >90% mylonitic and commonly ultramylonitic (foliation 043,

347

64 SE; lineation 218/19) (Fig. 7b). In general, mylonitic foliations in the Late Jurassic/Early

348

Cretaceous granodiorite are well developed, whereas lineations are moderately to locally poorly

349

developed (S>L tectonite fabrics), suggesting a general flattening strain. The discrete shear zones

350

are commonly localized along the margins of mafic dikes.

351

The average mylonitic foliation and lineation orientations in the Early Cretaceous

352

Remiendos tonalite are 052, 68 SE and 208/33, respectively, and the angle between the mean

353

foliation and the Paposo fault is 31–45° (Fig. 7b). These protomylonite foliations form a greater

354

angle to the Paposo fault, and the lineations plunge more steeply compared to the fabrics in the

355

Late Jurassic/Early Cretaceous granodiorite complex. However, intense postmylonitic fracturing

356

throughout this unit likely resulted in more variability of fabric orientations, and fabrics are

357

difficult to measure because exposures typically do not break along foliation planes. In general,

358

the angle between protomylonite foliations and the Paposo fault appears to decrease towards the

359

fault (Fig. 2, Supplementary File 1), and mylonitic fabrics become better developed within 10 m

360

from the gouge zone. Locally ultramylonite is present within a few meters of the gouge zone.

361 362

5.2.Shear zone kinematics Thirty-three X:Z thin sections of oriented mylonite samples were used to determine shear

363

sense and to evaluate deformation conditions. The most common microstructural shear sense

364

indicators are dynamically recrystallized oblique grain-shape fabrics in quartz and synthetic

365

shear bands at a low angle to foliation (Fig. 8). Twenty-four out of 33 thin sections record a

366

sinistral shear sense (in the map-view reference frame), 9 out of 33 do not show a preferential

367

shear sense, and 1 out of 33 has subtle dextral shear sense indicators. These results indicate that

368

the dominant sense of shear in these mylonite zones is sinistral, which is consistent with the

369

obliquity between foliation and the Paposo fault zone (Fig. 7a), and samples with no preferential

370

shear sense are consistent with coaxial strain. The overall SE-dip of mylonitic fabrics and SSW-

371

plunge of lineations indicate that sinistral shear also has a component of SE-side-up reverse

372

motion.

373

To evaluate how fabric symmetry varies across the mylonite zones, we measured the

374

orientations of quartz dynamically recrystallized grain shapes. Using ImageJ (Rasband, 1997-

375

2016) we measured the angle between >200 long axes of quartz grains and the foliation on

376

photomicrographs of 9 X:Z thin sections. In samples from the hydrothermally-altered zone in the

377

Late Jurassic/Early Cretaceous granodiorite, the maximum values of quartz grain long-axis

378

orientations have a range of 5°-19° (average 12°) counterclockwise of foliation (viewed looking

379

down), but locally grain shape fabrics lack clear asymmetry (Figs. 8c and 9a). Samples from the

380

protomylonitic Remiendos tonalite have consistent asymmetric oblique grain-shape fabrics with

381

maximum values ranging from 15°-23° (average 18°) counterclockwise of the foliation (Figs. 8d

382

and 9b).

383

In addition to sinistral shear fabrics, we also observe a minority of discrete shear zones

384

within the <50% mylonitic zone which have macroscopic dextral shear indicators, and one shear

385

zone records 30 cm of reverse displacement of a mafic dike. These dextral shear zones have an

386

average orientation of 015, 85 E and an average lineation of 194/06 (Fig. 7b), and the reverse

387

shear zone is oriented 003, 55 E with a lineation of 080/53. The dextral shears are subparallel to

388

the Paposo fault, which clearly records sinistral slip. Microscopically these dextral fabrics have a

389

high degree of symmetry, suggesting they record a large component of coaxial strain.

390 391

5.3.Quartz crystallographic orientations We present quartz crystallographic orientations for three samples of mylonites from the

392

hydrothermally-altered zone in the granodiorite complex and one protomylonite sample from the

393

Remiendos tonalite using electron backscatter diffraction (EBSD). Quartz crystallographic

394

preferred orientations are commonly used to evaluate crystallographic slip systems, sense of

395

shear, and the coaxiality of deformation (e.g., Passchier and Trouw, 2005). We collected EBSD

396

data on an environmental scanning electron microscope at the U.S. Geological Survey

397

Microbeam Laboratory in Denver, Colorado using an accelerating voltage of 15 KeV, a working

398

distance of ~15 mm, and a 1–5 µm step size depending on the sample grain size. Quartz c-axis

399

pole figures for mylonitic samples from the hydrothermally-altered zone have maxima near the

400

finite strain Y-axis or between the Y-axis and Z-axes, which is consistent with dominant prism

401

and rhomb slip (Fig. 10a). Notably, c-axes and a-axes in these samples lack clear

402

asymmetry, suggesting they record a large component of coaxial strain (Fig. 10a). By contrast,

403

the Remiendos tonalite protomylonite sample has an asymmetric c-axis girdle and a-axes

404

distribution that are indicative of sinistral shear (Fig. 10b).

405 406

5.4.3D strain geometry associated with brittle-ductile deformation A distinct outcrop within the hydrothermally-altered zone (located at 25.0156°S,

407

70.4410°W) consists of elongate granodiorite clasts within a foliated black matrix rich in

408

tourmaline (Figs. 11 a and b). Based on the angular shapes of many clasts, the large range of

409

clast sizes, and abundance of tourmaline, we interpret this outcrop to have originated as a

410

hydrothermal breccia. The black matrix consists of quartz + feldspar + oxides + tourmaline +

411

actinolite, typically zoned by composition with interior layers of quartz + feldspar, intermediate

412

layers of tourmaline + oxides, and an external layer of actinolite. Mylonitic fabrics involving the

413

black matrix record a sinistral sense of shear, indicating that brecciation was followed by

414

mylonitization of both the granodiorite and hydrothermal material. The granodiorite clasts are

415

flattened parallel to the mylonitic foliation and elongate parallel to a weakly-developed lineation.

416

To quantify the strain geometry recorded in these clasts we measured the axial ratios and

417

orientations of 34 clasts from across the outcrop and calculated a best-fit ellipsoid from these

418

data using EllipseFit (Vollmer, 2018) (Fig. 11c). Calculated finite strain axes from these data

419

closely match those inferred from measured fabrics at the outcrop (Fig. 11d), and the best-fit

420

ellipsoid plots within the general flattening strain region, which is consistent with the S>L

421

tectonite fabric that characterizes this zone.

422 423

6. Brittle deformation along the Paposo fault Brittle faults have a much greater range of orientations than the ductile fabrics (Figs. 7

424

and 12). Most brittle faults observed along the Paposo fault record strike-slip, oblique-slip, and

425

reverse motion and are associated with Fe-oxide and chlorite ± epidote. Excluding a

426

kinematically anomalous W-dipping normal fault mapped in the southeastern part of the study

427

area, which is characterized by clay gouge rather than chlorite or epidote (Fig. 2, Supplementary

428

File 1), the average small-scale fault orientation is 020, 73 E, which is parallel to the Paposo fault

429

(Fig. 12a). Sinistral slip along N- to NE-striking planes is dominant, corresponding to SW-

430

plunging extension axes (T-axes) and NW- and SE-plunging shortening axes (P-axes) (Fig. 12b).

431

The linked Bingham kinematic axes from faults with a known slip sense corresponds to an

432

oblique sinistral-reverse fault plane solution oriented 014, 68 SE with a slip lineation rake of 28°

433

from the south (Fig. 12b). We assumed a slip sense on all striated faults with unclear kinematics,

434

based on the pattern of shortening and extension axes from faults with a known slip sense.

435

Including all these data the sinistral fault plane solution from linked Bingham kinematic axes is

436

016, 78 E with a slip lineation rake of 13° from the south (Fig 12c). This faulting regime of

437

sinistral slip with a component of SE-side up reverse slip matches the kinematic pattern from the

438

mylonites east of the Paposo fault (Figs. 7 and 12a-c).

439

The gouge zone that defines the Paposo fault core has well-developed scaly foliations

440

with prominent S, C, and C’ shear bands indicative of sinistral shear (Fig. 6a). Sparse

441

slickenlines measured on surfaces parallel to the gouge fabric are mostly associated with sinistral

442

or sinistral/SE-up slip (Fig. 12d). The mean gouge foliation is oriented 028, 73 SE, which is ~10°

443

clockwise of the overall trend of the Paposo fault (Fig. 12d). Exposures of foliated gouge that dip

444

into slopes are commonly more shallowly dipping than average, suggesting downhill creep may

445

have affected orientations. Most of our gouge measurements are from level to gently sloping

446

exposures in the central part of the map area, where gouge foliations are consistently steeply

447

dipping with a mean orientation of 024, 88 SE, and the trend of the gouge zone is well defined at

448

013–014° (Fig. 12e). The ~10° difference between gouge foliation and the fault zone trend is

449

indicative of sinistral shear across this zone. Assuming the gouge zone records simple shear, the

450

shear strain recorded by the average distributed gouge fabric is ~5.5, or ~275 m of displacement

451

across the ~50 m-wide zone. The total sinistral displacement including mylonitic shear and

452

brittle slip on discrete surfaces is likely on the order of tens of kilometers (Hervé, 1987a).

453

Altogether kinematic data from small-scale faults and the gouge zone both indicate that sinistral

454

shear with a component of reverse (E-side-up) slip was dominant along the southern Paposo fault

455

segment.

456

The W-dipping normal fault in the southeastern part of the study area (Fig. 2,

457

Supplementary File 1) may be associated with the Neogene normal-sense reactivation along the

458

Paposo fault. South of the study area we observed a small-scale E-dipping normal fault that cuts

459

the Paposo gouge zone and may also be associated with Neogene reactivation of the Paposo

460

fault. However, this slip did not rework the gouge fabrics in the study area, which still preserve

461

Cretaceous sinistral slip.

462

Epidote veins in mylonitic units east of the Paposo fault lack brecciation or evidence of

463

cataclasis, suggesting they formed as opening-mode fractures. Epidote veins measured in all

464

units east of the Paposo fault and basaltic dikes within the granodiorite complex have variable

465

orientations, but the maximum eigenvector of the pole to dikes and veins is subparallel to the

466

average stretching lineation for the shear zone (Fig. 12f). A cylindrical best fit of the poles to the

467

data suggests multiple extension directions along a plane subparallel to the Paposo fault. This

468

pattern is consistent with a general flattening strain along the Paposo fault.

469

7. Discussion

470

7.1.Geometry of map units and mylonitic fabrics

471

The southern segment of the Paposo fault consists of a steeply SE-dipping shear zone,

472

which juxtaposes a brittlely-deformed Early Jurassic Yumbes tonalite west of the fault against

473

brittlely-overprinted mylonitic Late Jurassic to Early Cretaceous granitoids east of the fault (Fig.

474

2). These mylonitic units and their subdivisions roughly parallel the NNE trend of the Paposo

475

fault. This geometric relationship suggests that development of the hydrothermally-altered high

476

strain zone and emplacement of the younger Remiendos tonalite was structurally controlled by

477

the shear zone.

478

Mylonites east of the Paposo fault record sinistral shear with a component of SE-side up

479

reverse slip, and mylonitic foliations typically strike 20–45° clockwise of the Paposo fault (Figs.

480

7a and b). The obliquity between the mylonitic foliation and the fault is consistent with overall

481

sinistral shear if the Paposo fault is viewed as a shear zone boundary. The average foliation

482

orientation in mylonites and ultramylonites is ~13° counterclockwise of the average foliation in

483

protomylonites (Fig. 7b), which is consistent with counterclockwise rotation of the finite strain

484

axes during progressive sinistral shear. In addition, approaching the Paposo fault, protomylonitic

485

fabrics within the Remiendos tonalite appear to rotate counterclockwise (Fig. 2, Supplementary

486

File 1), and fabrics are mylonitic <10 m from the margin of the gouge zone, where foliation-

487

parallel cataclasites are locally present. These relationships support the interpretation that the

488

Paposo fault zone initiated at the western margin of a steeply ESE-dipping shear zone that

489

evolved to brittle slip.

490 491

7.2.Timing of deformation Mylonitic deformation along the southern segment of the Paposo fault zone is bracketed

492

between ~147–117 Ma based on zircon U-Pb dates of the mylonitic granodiorite (146.5 ± 1.5

493

Ma) and mylonitic aplite (142.4 ± 1.3 Ma) and a zircon (U-Th)/He date of 116.6 ± 6.2 Ma from

494

the ~147 Ma granodiorite (Figs. 3c and g, Table 2). There are no zircon overgrowths in any of

495

the samples dated in this study that directly constrain the timing of ductile deformation, and

496

plutons east of the Paposo fault that are not adjacent to the shear zone do not record any

497

magmatic fabrics. However, the presence of two [Th] versus [U] trends and absence of a

498

correlation between Th/U and age in the ~147 Ma granodiorite suggest some zircon grains

499

experienced subsolidus recrystallization (Hoskin and Schaltegger, 2003), which likely occurred

500

during shearing, and the high strain recorded by the ~142 Ma aplite suggests this unit records the

501

entire duration of ductile shearing. The ~139 Ma granodiorite east of the main mylonite zone is

502

unstrained where dated but is cut by mafic dikes and is geochemically and mineralogically

503

similar to granodiorite with discrete high strain zones. The Early Cretaceous Remiendos tonalite

504

unit lacks mafic dikes and locally intense hydrothermal alteration and bleaching that characterize

505

the older granodiorite complex . The least altered and lowest strain parts of the Remiendos

506

tonalite are adjacent to the most altered and highest strain parts of the granodiorite complex (Fig.

507

2). Therefore, hydrothermal bleaching and mafic dike intrusion must have occurred between

508

emplacement of the ~139 Ma granodiorite and the ~136 Ma tonalite. Locally these dikes are

509

spatially associated with hydrothermal alteration and discrete high strain zones, suggesting

510

alteration and shear zone development were coeval with dike emplacement. Therefore, we can

511

bracket both hydrothermal alteration and the development of ultramylonitic fabrics and discrete

512

high strain zones between ~139 Ma and 136 Ma, within the interval of Early Cretaceous

513

magmatism along the southern Paposo fault (Fig. 13a). The protomylonitic fabrics in the ~136

514

Ma Remiendos tonalite must have formed after the main episode of alteration and fabric

515

development that affected the Late Jurassic/Early Cretaceous granodiorite (Fig. 13b). A

516

hornblende K/Ar age from the eastern side of the Paposo fault suggests temperatures locally

517

remained in excess of ~500°C until 131 Ma (Hervé and Marinovic, 1989).

518

Geochemical data support the interpretation that deformation and alteration of the

519

Remiendos tonalite is younger than that in the Late Jurassic/Early Cretaceous granodiorite.

520

Although alteration was accompanied by a decrease in MgO and Fe2O3 in both units, alteration

521

of the Remiendos tonalite records notable increase in K2O and decrease in CaO, whereas

522

alteration of the granodiorite complex records a decrease in K2O and no clear shift in CaO (Table

523

3). These differences suggest that compositionally different fluids were responsible for their

524

alteration, which is consistent with field relationships suggesting deformation and alteration of

525

the Remiendos tonalite postdate intense alteration and high strain fabric development in the

526

granodiorite complex.

527

The lack of penetrative strain in the Early Jurassic Yumbes tonalite most likely indicates

528

that it was at a higher structural level than the mylonitic granodiorite during deformation (which

529

is consistent with SE-side up motion across the fault zone) and/or that the Yumbes tonalite was

530

relatively cold and strong while the synkinematic Early Cretaceous plutons were hot and weak.

531

We did not observe any clasts of mylonite from the Remiendos tonalite in the gouge zone. Given

532

that the Paposo gouge zone is derived entirely from the Yumbes tonalite, the brittle Paposo fault

533

core most likely localized in the Yumbes tonalite, possibly when the hotter Remiendos tonalite

534

was still undergoing ductile deformation (Fig. 13b).

535

Approximately 5 km north of the study area the Remiendos tonalite has a K-Ar biotite

536

cooling date of 129 ± 3 Ma (Hervé and Marinovic, 1989), which likely overlaps with cooling

537

through the brittle-plastic transition and approximates the onset of brittle deformation. The

538

minimum lower age limit for ductile deformation is ~117 Ma - the age when strained

539

granodiorite cooled below the zircon (U-Th)/He closure temperature of ~180-190°C (Reiners et

540

al., 2002, 2004; Wolfe and Stockli, 2010), well below the lower limit for crystal plastic

541

deformation of quartz (e.g., Sibson, 1977; Stockhert et al., 1999; Stipp et al., 2002). This ~117

542

Ma zircon (U-Th)/He date also approximates the timing of brittle gouge formation based on the

543

lack of smectite clays and the illite crystallinity Kübler index, which suggests formation in the

544

deep diagenetic zone between ~150°C and 200°C (Figs. 6b, 13c) (Verdel et al., 2011). The ~117

545

Ma date is similar to other estimates for Coastal Cordilleran exhumation (e.g. Scheuber and

546

Andriessen, 1990; Scheuber et al., 1995; Bascuñán et al., 2016), and is likely associated with

547

regional cooling due to abandonment of the Coastal Cordilleran arc.

548

The timing of deformation along the southern segment of the Paposo fault zone is slightly

549

older than the interpreted ages of AFS deformation based primarily on K/Ar and 40Ar/39Ar

550

cooling dates of mylonites (Scheuber et al., 1995; Dallmeyer et al., 1996; Taylor et al., 1998).

551

However, this age difference could be due to the occurrence of an older phase of Early

552

Cretaceous magmatism along the southern Paposo segment, which are intricately linked to

553

deformation on the AFS. The elongate shape of the Remiendos tonalite parallel to the Paposo

554

fault suggests emplacement was structurally controlled, similar to elongate synkinematic plutons

555

described along the El Salado segment near 26°30’S (Taylor et al., 1998; Grocott and Taylor,

556

2002). However, unlike the El Salado segment, pluton emplacement along the Paposo fault did

557

not occur in a transtensional regime, and we did not observe magmatic fabrics associated with

558

pluton emplacement. In addition, the Remiendos tonalite was emplaced in the hanging wall of

559

the steeply-dipping Paposo fault, which records primarily strike-slip motion, suggesting a roof-

560

uplift mechanism for pluton emplacement was not important.

561 562

7.3.Regional kinematics of the Paposo segment This study provides new constraints on the geometry and kinematics of deformation

563

along the southern Paposo segment. Minor brittle faults and gouge fabrics indicate an overall

564

kinematic regime of oblique sinistral-reverse slip along a steeply ESE-dipping fault zone. This

565

deformation resembles the overall kinematic regime in the adjacent mylonite zone, which is

566

characterized by oblique sinistral-reverse shear along a SE-dipping shear zone. In addition, a

567

discrete reverse shear zone parallel to the Paposo fault and N-S striking shears with coaxial-

568

dominated fabrics are compatible with a component of shortening across the southern portion of

569

the Paposo fault zone. Microstructures such as symmetric grain shape fabrics are common in the

570

hydrothermally-altered high strain zone (Fig. 8c), suggesting this sinistral shear zone also records

571

a component of coaxial shortening. The oblate strain geometry of the mylonitized hydrothermal

572

breccia, and the girdle distribution of poles to epidote veins are also consistent with coaxial

573

flattening and overall sinistral transpression (Fig. 12f). The steeper lineations within the

574

Remiendos tonalite compared to the older granodiorite complex (Fig. 7b) suggest that the

575

tonalite records transpression via non-coaxial dominated sinistral-reverse shear with a greater

576

reverse component than in the granodiorite complex. Transpressional strain in the

577

hydrothermally-altered granodiorite complex was partly accommodated by sinistral non-coaxial

578

shear and coaxial flattening across the shear zone. Altogether these data and observations

579

indicate a kinematic regime of sinistral transpression on the southern segment of the Paposo fault

580

zone.

581

The Paposo fault has a slightly arcuate geometry ranging from NNE-trending in the south

582

to NNW-trending in the north (Fig. 1b) and is characterized regionally by variable along-strike

583

kinematics. Brittle fault data from the northern Paposo fault system suggest sinistral transtension

584

along the subsidiary Bolfin and Jorgillo faults (Cembrano et al., 2005; Veloso et al., 2015).

585

Cembrano et al. (2005) determined that the Caleta Coloso duplex formed as a dilational jog

586

between the Bolfin and Jorgillo faults. Veloso et al. (2015) document an overall transtensional

587

sinistral regime with NW-trending compressional and NE-trending tensional principal axes. A

588

transition from transtension to transpression between the northern and southern parts of the

589

Paposo segment is consistent with the arcuate geometry of the fault system. That is, southeast-

590

directed subduction of the Phoenix plate resulted in a component of extension across NNW-

591

striking faults and a component of shortening across NNE-striking faults. These kinematic

592

relationships are also consistent with the idea that fundamentally the dominant N-S striking, arc-

593

parallel Atacama fault system is a zone of sinistral simple shear.

594 595

8. Conclusions

596

The southern segment of the Paposo fault zone is a steeply ESE-dipping fault that

597

accommodated sinistral transpression from ~139 Ma to at least ~117 Ma. Mafic dikes associated

598

with hydrothermal alteration and development of high strain fabrics in granodiorite were

599

emplaced between ~139 and ~136 Ma, and protomylonitic fabrics within the younger tonalite

600

can be bracketed from ~136 Ma to ~117 Ma. Based on XRD data and a zircon (U-Th)/He date,

601

clay gouge zone development associated with sinistral slip on the Paposo fault most likely

602

formed between 150°C and 200°C around 117 Ma. The close timing relationship between pluton

603

emplacement and age of penetrative deformation, as well as the lack of penetrative strain in the

604

Early Jurassic Yumbes tonalite suggests that ductile deformation was localized in and around

605

hot, rheologically weak syn-kinematic plutons. Deformation along the AFS may be

606

fundamentally linked to synkinematic arc magmatism in the Coastal Cordillera.

607

Evidence for sinistral transpression along the Paposo fault includes oblique sinistral-

608

reverse shear in mylonitic fabrics and brittle faults, and locally symmetric microstructures

609

suggestive of coaxial strain. Transpression and transtension along the different portions of the

610

Paposo fault are likely controlled by the arcuate geometry of the Paposo segment. Sinistral

611

transtension along the NNW-trending northern segment and sinistral transpression along the

612

NNE-trending southern segment are consistent with overall sinistral simple shear along the N-S-

613

trending AFS.

614 615

Acknowledgements

616 617 618 619 620 621 622

This project was funded by Colorado State University start-up funds and NSF grant 1822064 to J. Singleton. We thank Dave Adams and Bill Benzel for assistance with EBSD and XRD data collection, respectively, and Stewart Williams and Skyler Mavor for field assistance. We would also like to thank Lisa Stockli, Des Patterson, and Rudra Chatterjee for help with analytical work at the UTChron Laboratory. Comments by two anonymous reviewers have improved this manuscript.

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References

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Figure Captions: Figure 1: A) Simplified map of the Atacama Fault System (AFS) with an inset of the Jurassic and Early Cretaceous plate configuration during sinistral deformation along the AFS. B) The Paposo fault segment of the AFS. The study area is shown by the inset rectangle labeled Figure 2. (Modified from Cembrano et al., 2005; Brown et al., 1993; Scheuber and González, 1999). Figure 2: A) Simplified geologic map of the southern portion of the Paposo fault segment. See Supplementary File 1, for more detail. Figure 2 continued: B) Cross sections A-A’, B-B’, and CC’ from the map in A). The lines at the surface show the apparent dip of mylonitic foliation from field measurements. Table 1: Table of geochronology samples with their sample location and their weighted mean date based on <5% and <10% discordance date filters. Sample locations and dates are shown with white triangles in Figure 2a and Supplemental File 1. UTM zone 19S, WGS 84. This is a summary table of the geochronology data, and the isotopic data are listed in Supplementary File 4. Figure 3: Zircon U-Pb ages with errors bars, weighted mean age, and corresponding concordance plots filtered for less than 10% discordance for samples A) Early Jurassic Yumbes tonalite (18-1PJ81), B) granodiorite-2 (17-1-P26), C) granodiorite-1 (16-1-P31), F) aplite sill found in the hydrothermally-altered zone (17-1-P124), and G) Early Cretaceous tonalite (18-1-PJ73). The gray rectangle is the 2-σ region, assuming a single population and the hollow bars and grey ovals are dates rejected by the weighted mean algorithm. D) A kernel density estimation (KDE) plot of zircons from granodiorite-1 (16-1-P31), with a histogram of the data in green, using a bin size of 2 Myr, and the distribution in light blue, using a bandwidth of 2 Myr. This plot shows a bimodal distribution of dates. E) A weighted mean distribution of the younger population of dates from

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sample granodiorite-1 (16-1-P31). Data were processed using Iolite and Isoplot (Ludwig, 2003; Paton, et al., 2011; Petrus and Kamber, 2012) and plots were made using IsoplotR (Vermeesch, 2018). Sample locations in Figure 2 and Supplementary File 2. Table 2: Table of zircon (U-Th)/He-thermochronologic data for each aliquot from samples from the granodiorite-1 (16-1-31) and the Yumbes tonalite (18-1-PJ81). Abbreviations: 2SE = 2 times the standard error, Ft: Ft correction factor for zircon dimensions, and ESR: equivalent spherical radius. Sample locations in Supplementary File 3. Figure 4: Representative outcrop photographs of A) a discrete high strain zone in the LJ/EK granodiorite <50% mylonitic zone, B) mylonite derived from the hydrothermally-altered zone of the LJ/EK granodiorite complex (coin diameter ~27 mm), C) a protomylonite from the Early Cretaceous Remiendos tonalite unit, and D) a foliated illite-rich gouge zone along the Paposo fault. Figure 5: A) Outcrop photograph of heterogeneously altered Late Jurassic/Early Cretaceous granodiorite. The more hydrothermally-altered areas (lighter areas) are higher strain than the less hydrothermally-altered areas. B and C) Plane-polarized light photomicrographs of B) a less hydrothermally-altered sample and C) a more hydrothermally-altered sample. There is more chlorite in the less altered sample and more quartz in the more altered sample. The feldspars in C) are more altered to white mica and saussurite than feldspars in B). Abbreviations: Chlchlorite, Qtz- quartz, Plag- plagioclase. Table 3: Table of major element geochemical data from chemical analyses of powdered 5 g samples. Gray rows signify unaltered or less altered samples, and white rows are altered samples. Sample locations in Supplementary File 2. Figure 6: A) Photograph of the Paposo gouge zone with an S-C fabric and a clast from the nonmylonitic Early Jurassic Yumbes tonalite. The C fabrics are highlighted by faint blue dashed lines and the S fabrics are highlighted by faint red dashed lines. B) Kübler Index and corresponding estimated temperature of formation of the illite in the clay gouge based on XRD data (modified from Verdel et al, 2011). Figure 7: Ductile structural data from within 2 km of the Paposo fault. A) All mylonitic fabrics. The bold black plane is the mean foliation: 041, 63 SE (the plane to the maximum eigenvalue of the poles). The hollow boxes are poles to foliation planes with the average pole to foliation marked as a large hollow box. The solid boxes are lineations, and the average lineation is marked with a large solid box: 212/18. The pink line is the average Paposo fault trend in the center of the study area (013–014). B) All ductile fabrics separated into 3 groups: mylonitic and ultramylonitic fabrics (black) from the Late Jurassic/Early Cretaceous granodiorite, protomylonitic fabrics (blue) from the Early Cretaceous Remiendos tonalite, and discrete shear zones with a component of dextral shear (red). Poles to mylonitic foliations are black hollow circles, and the large hollow black circle is the mean pole to foliation, corresponding to a mean mylonitic foliation plane (bold black great circle) of 039, 60 SE. The solid black circles are mylonitic lineations, and the large solid black circle is the mean mylonitic lineation: 212/18. The blue hollow boxes are poles to protomylonitic foliations, and the large blue box is the average pole to foliation: 321/21. The

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solid blue boxes are protomylonitic lineations, and the large solid blue box is the average lineation: 219/31. The average protomylonitic foliation plane is the bold blue great circle: 051, 69 SE. The hollow red diamonds are poles to dextral foliations, which correspond to a mean plane orientation of 015, 85 E (bold red great circle). The solid red diamonds are dextral lineations, and the average orientation marked by a large solid red diamond has an orientation of 194/06. Figure 8: Photomicrographs of mylonites with sinistral shear fabrics (in the map-view reference frame) from: A and B) samples from the hydrothermally-altered zone in the Late Cretaceous/Early Jurassic granodiorite (note oblique quartz grain shape fabric; gypsum plate inserted), and C) from the Early Cretaceous Remiendos tonalite (note oblique quartz grain shape fabric, cross-polarized light). D) Symmetric quartz grain shape fabric from the hydrothermallyaltered granodiorite (cross-polarized light). Abbreviations: Qtz - quartz, Plag - plagioclase. Figure 9: Quartz long-axis angles (red numbers) in mylonite samples from A) the hydrothermally-altered zone within the Late Jurassic/Early Cretaceous granodiorite (5 samples) and B) the Early Cretaceous Remiendos tonalite (4 samples). Angles were determined by tracing recrystallized grains using ImageJ (Rasband, 1997–2016). Sample locations in Supplementary File 2. Figure 10: Lower hemisphere pole figures for quartz [c]-axes (0001) and -axes (1120) from mylonites in the study area, with a reference frame looking down with N to NE on the left. A) Late Jurassic/Early Cretaceous hydrothermally-altered granodiorite samples 17-1-P9, 17-1-P45, and 17-1-P95. B) Early Cretaceous Remiendos tonalite protomylonitic sample 17-1-P6. Sample locations in Supplementary File 2. Figure 11: A and B) Outcrop photographs of mylonitic granodiorite clasts in a foliated tourmaline-bearing matrix. A) foliation-parallel view with angular clasts, and B) foliationperpendicular view. C) Flinn diagram of 34 strained clasts from the outcrop shown in A) and B) which plots within the flattening strain field using the Robin method (blue circle labelled with an “R”) (Robin, 2002) and the Shan method (red circle labelled with an “S”) (Shan, 2008). D) Stereoplot of the calculated strain axes using EllipseFit (Volmer, 2018) using the Shan method (red) and the Robin method (blue) and their corresponding (X-Y) foliation planes. The average foliation plane (black great circle), pole to that plane (hollow black circle), and average lineation (black circle) from the outcrop. Figure 12: Brittle structural data within 2 km of the Paposo fault. A) Minor fault planes and slickenline lineations. The bold black plane is the mean orientation: 20, 73 SE (from the maximum eigenvalue to the poles). B) Shortening axes (P axes, blue circles) and extension axes (T-axes, hollow red circles) and corresponding linked Bingham fault plane solution from all minor faults with a known sense of slip. The sinistral fault plane solution is 014, 68 SE with a slip lineation rake of 28° from the south. C) Shortening and extension axes for all faults in B and all faults with unclear kinematics, assuming a slip sense that best matches the kinematic pattern in B. The linked Bingham sinistral fault plane solution is 016, 78 E with a slip lineation rake of 13° from the south. D) All foliation measurements from the Paposo gouge zone and 4 slickenline lineations on gouge surfaces. The bold black great circle is the mean orientation: 028, 73 SE. E)

997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018

All gouge foliation measurements from the central part of the Fig. 2a map area. The bold great circle is the mean plane: 024, 88 SE, which is ~10 to 10.5° clockwise of the Paposo fault trend at this location (013-014). F) Poles to basaltic dikes (brown hollow squares) in unit KJg and epidote veins (green hollow circles) in unit Krt and KJg with a smoothed Kamb contour and a cylindrical best fit great circle (185, 85 W) to all data. The maximum eigenvalue (point 1) is oriented 191/50. Figure 13: Schematic cross sections illustrating the timing and kinematics of deformation along the Paposo fault zone. See Figure 2 for unit explanation. A) ~139 Ma to 136 Ma: intense hydrothermal alteration and mylonitic fabric development following emplacement of the Late Jurassic/Early Cretaceous granodiorite complex unit KJg4; mafic dikes are also emplaced during this interval and are locally associated with high strain zones. Mylonitic fabrics record sinistral shear, oblique sinistral-reverse shear, and/or coaxial-flattening. B) ~136 Ma –129 Ma: development of protomylonitic to mylonitic fabrics following emplacement of the Remiendos tonalite (Krt) along the western margin of the shear zone at ~136 Ma. Fabrics record sinistral shear with a component of NE-side up reverse shear. The Early Jurassic Yumbes tonalite (Jyt) lacks mylonitic fabrics and likely deformed brittlely along the incipient Paposo fault during this interval. The lower age bracket is from the K/Ar biotite age (Hervé and Marinovic, 1989) which approximates the timing of cooling below the brittle-plastic transition. C) Development of a clay gouge zone (Kgz) along the Paposo fault at ~150–200°C temperatures.

Sample

Description

18-1-PJ73 17-1-P26 17-1-P124 16-1-P31 18-1-PJ81

Remiendos tonalite Granodiorite-2 Mylonitic aplite sill Granodiorite-1 Yumbes tonalite

<5% discordance <10% discordance UTM X UTM Y # zircons Age (Ma) ±2σ (Ma) MSWD # zircons Age (Ma) ±2σ (Ma) MSWD 353936 354971 354435 354866 353526

7232069 7231061 7232464 7232484 7231583

7 10 13 8 34

135.4 138.2 142.2 146.5 177.4

2.3 1.2 1.7 1.5 1.8

5.4 1.8 18 5.1 27

15 18 19 8 40

136.0 138.5 142.4 146.5 177.9

1.8 1.6 1.3 1.5 1.7

8.9 6.2 13 5.1 28

Table 1: Table of geochronology samples with their sample location and their weighted mean date based on <5% and <10% discordance date filters. Sample locations and dates are shown with white triangles in Figure 2a and Supplemental File 1. UTM zone 19S, WGS 84. This is a summary table of the geochronology data, and the isotopic data are listed in Supplementary File 4.

Aliquot Age, Ma z161-31-1 118.9 z161-31-2 119.2 z161-31-4 112.5 z161-31-5 109.5 z161-31-6 122.8 mean: 116.6 z181-PJ81-3 110.9 z181-PJ81-5 128.5 z181-PJ81-6 99.6 z181-PJ81-7 77.3 mean: 104.1

err., Ma U (ppm) Th (ppm) 147Sm (ppm) 9.5 896 851 7.0 9.5 1036 998 9.0 9.0 1353 1546 20.2 8.8 1089 1075 8.3 9.8 1186 1470 9.0 2SE = 6.2 8.9 153 86.4 8.9 10.3 85 74.1 6.2 8.0 106 67.7 0 6.2 181 68.1 0 2SE = 21.4

[U]e 1092 1266 1710 1337 1524 173 102 122 196

He (nmol/g) mass (ug) 571 10.5 576 2.8 733 2.5 574 3.2 764 4.5 72.9 47.2 43.8 58.1

2.1 1.5 1.5 2.2

Ft 0.81 0.70 0.70 0.72 0.75

ESR 62.4 39.0 38.9 41.9 47.4

0.70 0.66 0.66 0.70

38.0 33.7 33.8 38.5

Table 2: Table of zircon (U-Th)/He-thermochronologic data for each aliquot from samples from the granodiorite-1 (16-1-31) and the Yumbes tonalite (18-1-PJ81). Abbreviations: 2SE = 2 times the standard error, Ft: Ft correction factor for zircon dimensions, and ESR: equivalent spherical radius. Sample locations in Supplementary File 3.

Late Jurassic/Early Cretaceous granodiorite complex Remiendos tonalite

Sample

Description

17-1-P26 161-P32 161-P30b 17-1-P56 17-1-P95 18-1-PJ58 18-1-PJ58b 18-1-PJ59 18-1-PJ59b 18-1-PJ60 18-1-PJ60b 18-1-PJ73 17-1-P6 18-1-PJ80

unstrained granodiorite

SiO2

65.2 unstrained granodiorite 70.0 altered mylonite 67.3 altered ultramylonite 72.8 altered ultramylonite 71.9 less altered mylonite 57.9 altered (bleached) mylonite 65.5 less altered mylonite 60.2 altered (bleached) mylonite 70.3 less altered mylonite 56.2 altered (bleached) mylonite 54.3 low strain tonalite/qtz diorite 53.9 tonalite mylonite 69.6 tonalite ultramylonite 69.7

Al2O3

Fe2O3

CaO

MgO Na2O

K2O TiO2 MnO P2O5 LOI Total % Zr (ppm)

16.6 15.9 15.3 12.5 16.1 18.7 19.9 18.7 15.7 18.7 27.7 19.2 17.0 16.1

4.54 3.24 3.57 1.32 1.06 2.96 0.54 2.37 0.61 3.28 0.37 7.41 1.91 1.96

3.32 2.25 4.80 5.60 4.26 8.87 7.18 8.20 6.92 8.91 9.68 7.92 2.99 3.22

1.70 0.89 1.39 1.63 0.86 4.53 0.69 3.70 1.22 4.33 0.38 4.72 0.90 0.69

2.69 3.64 0.56 0.20 0.70 0.51 0.26 0.71 0.27 0.68 0.41 0.60 2.01 2.60

4.36 4.34 5.41 3.75 4.09 4.44 4.61 4.51 3.17 4.67 5.70 4.23 4.75 4.38

0.49 0.39 0.91 0.51 0.34 0.72 0.86 0.77 0.38 1.26 0.78 0.75 0.30 0.32

0.08 0.11 1.37 0.05 0.09 0.82 0.04 0.33 1.49 0.03 0.16 2.09 0.04 0.07 2.00 0.06 0.12 1.54 0.01 <0.01 1.19 0.07 0.12 1.56 0.03 0.01 1.37 0.06 0.23 1.79 0.02 <0.01 1.24 0.16 0.17 1.88 0.03 0.11 1.20 0.03 0.1 1.05

100.54 101.69 101.11 100.58 101.42 100.37 100.75 100.92 100.02 100.19 100.67 101.00 100.91 100.20

234 281 625 131 216 117 133 104 205 160 739 133 160 175

Table 3: Table of major element geochemical data from chemical analyses of powdered 5 g samples. Gray rows signify unaltered or less altered samples, and white rows are altered samples. Sample locations in Supplementary File 2.

Sample 18-1-PJ73 17-1-P26 17-1-P124 16-1-P31 18-1-PJ81

Description Remiendos tonalite Granodiorite-2 Mylonitic aplite sill Granodiorite-1 Yumbes tonalite

UTM Y 7232069 7231061 7232464 7232484 7231583

UTM X 353936 354971 354435 354866 353526

# zircons 7 10 13 8 34

<5% discordance Age (Ma) ±2σ (Ma) 135.4 2.3 138.2 1.2 142.2 1.7 146.5 1.5 177.4 1.8

MSWD 5.4 1.8 18 5.1 27

# zircons 15 18 19 8 40

<10% discordance Age (Ma) ±2σ (Ma) 136 1.8 138.5 1.6 142.4 1.3 146.5 1.5 177.9 1.7

MSWD 8.9 6.2 13 5.1 28

Table 1: Table of geochronology samples with their sample location and their weighted mean date based on <5% and <10% discordance date filters. Sample locations and dates are shown with white triangles in Figure 2a and Supplemental File 1. UTM zone 19S, WGS 84. This is a summary table of the geochronology data, and the isotopic data are listed in Supplementary File 4.

Aliquot

Age, Ma

err., Ma

U (ppm)

Th (ppm)

147Sm (ppm)

[U]e

He (nmol/g)

mass (ug)

Ft

ESR

z161-31-1 z161-31-2 z161-31-4 z161-31-5

118.9 119.2 112.5 109.5

9.5 9.5 9.0 8.8

896 1036 1353 1089

851 998 1546 1075

7.0 9.0 20.2 8.3

1092 1266 1710 1337

571 576 733 574

10.5 2.8 2.5 3.2

0.81 0.70 0.70 0.72

62.4 39.0 38.9 41.9

z161-31-6

122.8

9.8

1186

1470

9.0

1524

764

4.5

0.75

47.4

mean:

116.6

2SE = 6.2

z181-PJ81-3 z181-PJ81-5 z181-PJ81-6

110.9 128.5 99.6

8.9 10.3 8.0

153 85 106

86.4 74.1 67.7

8.9 6.2 0

173 102 122

72.9 47.2 43.8

2.1 1.5 1.5

0.70 0.66 0.66

38.0 33.7 33.8

z181-PJ81-7

77.3

6.2

181

68.1

0

196

58.1

2.2

0.70

38.5

mean:

104.1

2SE = 21.4

Table 2: Table of zircon (U-Th)/He-thermochronologic data for each aliquot from samples from the granodiorite-1 (16-1-31) and the Yumbes tonalite (18-1-PJ81). Abbreviations: 2SE = 2 times the standard error, Ft: Ft correction factor for zircon dimensions, and ESR: equivalent spherical radius. Sample locations in Supplementary File 3.

Late Jurassic/Early Cretaceous granodiorite complex Remiendos tonalite

Sample 17-1-P26 161-P32 161-P30b 17-1-P56 17-1-P95

Description

18-1-PJ58 18-1-PJ58b 18-1-PJ59 18-1-PJ59b 18-1-PJ60 18-1-PJ60b 18-1-PJ73 17-1-P6 18-1-PJ80

less altered mylonite altered (bleached) mylonite less altered mylonite altered (bleached) mylonite less altered mylonite altered (bleached) mylonite

unstrained granodiorite unstrained granodiorite altered mylonite altered ultramylonite altered ultramylonite

low strain tonalite/qtz diorite tonalite mylonite tonalite ultramylonite

SiO2 65.2 70.0 67.3 72.8 71.9

Al2O3 16.6 15.9 15.3 12.5 16.1

Fe2O3 4.54 3.24 3.57 1.32 1.06

CaO 3.32 2.25 4.80 5.60 4.26

MgO 1.70 0.89 1.39 1.63 0.86

Na2O 4.36 4.34 5.41 3.75 4.09

K2O 2.69 3.64 0.56 0.20 0.70

TiO2 0.49 0.39 0.91 0.51 0.34

MnO 0.08 0.05 0.04 0.03 0.04

P2O5 0.11 0.09 0.33 0.16 0.07

LOI 1.37 0.82 1.49 2.09 2.00

Total % 100.54 101.69 101.11 100.58 101.42

Zr (ppm) 234 281 625 131 216

57.9 65.5 60.2 70.3 56.2 54.3 53.9 69.6 69.7

18.7 19.9 18.7 15.7 18.7 27.7 19.2 17.0 16.1

2.96 0.54 2.37 0.61 3.28 0.37 7.41 1.91 1.96

8.87 7.18 8.20 6.92 8.91 9.68 7.92 2.99 3.22

4.53 0.69 3.70 1.22 4.33 0.38 4.72 0.90 0.69

4.44 4.61 4.51 3.17 4.67 5.70 4.23 4.75 4.38

0.51 0.26 0.71 0.27 0.68 0.41 0.60 2.01 2.60

0.72 0.86 0.77 0.38 1.26 0.78 0.75 0.30 0.32

0.06 0.01 0.07 0.03 0.06 0.02 0.16 0.03 0.03

0.12 <0.01 0.12 0.01 0.23 <0.01 0.17 0.11 0.1

1.54 1.19 1.56 1.37 1.79 1.24 1.88 1.20 1.05

100.37 100.75 100.92 100.02 100.19 100.67 101.00 100.91 100.20

117 133 104 205 160 739 133 160 175

Table 3: Table of major element geochemical data from chemical analyses of powdered 5 g samples. Gray rows signify unaltered or less altered samples, and white rows are altered samples. Sample locations in Supplementary File 2.

69°00’

70°00’

69°00’

Iquique

70°00’

North America Farallon Plate South America

Salar del Carmen

Phoenix Plate

23°30’

Ant ofag as

Pacific Ocean

ta

22°

Tocopilla

Jo llo

rgi Fa ult

Antofagasta

Paposo

26°

Chañaral 24°30’

lt o Fau

Copiapó

Co. Paranal

El Salado

CHILE - P

ERU - TRE

NCH

Taltal

Papos

28°

Vallenar

25°00’

A

ault Coloso F

Fault

Paposo

24°00’

n Bolfi

24°

La Serena 50 km

30°

Argentina

Figure 2

15 km

B

Figure 1: A) Simplified map of the Atacama Fault System (AFS) with an inset of the Jurassic and Early Cretaceous plate configuration during sinistral deformation along the AFS. B) The Paposo fault segment of the AFS. The study area is shown by the inset rectangle labeled Figure 2. (Modified from Cembrano et al., 2005; Brown et al., 1993; Scheuber and González, 1999).

A Late Jurassic/Early Cretaceous hydrothermally-altered granodiorite 17-1-P9

17-1-P45

17-1-P95

B Early Cretaceous Remiendos tonalite 17-1-P6

Figure 10: Lower hemisphere pole figures for quartz [c]-axes (0001) and -axes (1120) from mylonites in the study area, with a reference frame looking down with N to NE on the left. A) Late Jurassic/Early Cretaceous hydrothermally-altered granodiorite samples 17-1-P9, 17-1-P45, and 17-1-P95. B) Early Cretaceous Remiendos tonalite protomylonitic sample 17-1-P6. Sample locations in Supplementary File 2.

A

B

SW C

NE

SW

NE

N

D

5

X/Y

4 Z

3 S

2 1 1

Y

R

2

3

Y/Z

4

5

X

Figure 11: A and B) Outcrop photographs of mylonitic granodiorite clasts in a foliated tourmaline-bearing matrix. A) foliation-parallel view with angular clasts, and B) foliation-perpendicular view. C) Flinn diagram of 34 strained clasts from the outcrop shown in A) and B) which plots within the flattening strain field using the Robin method (blue circle labelled with an “R”) (Robin, 2002) and the Shan method (red circle labelled with an “S”) (Shan, 2008). D) Stereoplot of the calculated strain axes using EllipseFit (Volmer, 2018) using the Shan method (red) and the Robin method (blue) and their corresponding (X-Y) foliation planes. The average foliation plane (black great circle), pole to that plane (hollow black circle), and average lineation (black circle) from the outcrop.

A

B

N

C

N

N

2 2

1

1

3

n=90 planes n=73 lineations

D

E

N

N

n=41

3

F

~10 o

n=73

N

2

3

1

n=29 planes

n=17 planes

n=54 veins n=10 dikes Figure 12: Brittle structural data within 2 km of the Paposo fault. A) Minor fault planes and slickenline lineations. The bold black plane is the mean orientation: 20, 73 SE (from the maximum eigenvalue to the poles). B) Shortening axes (P axes, blue circles) and extension axes (T-axes, hollow red circles) and corresponding linked Bingham fault plane solution from all minor faults with a known sense of slip. The sinistral fault plane solution is 014, 68 SE with a slip lineation rake of 28° from the south. C) Shortening and extension axes for all faults in B and all faults with unclear kinematics, assuming a slip sense that best matches the kinematic pattern in B. The linked Bingham sinistral fault plane solution is 016, 78 E with a slip lineation rake of 13° from the south. D) All foliation measurements from the Paposo gouge zone and 4 slickenline lineations on gouge surfaces. The bold black great circle is the mean orientation: 028, 73 SE. E) All gouge foliation measurements from the central part of the Fig. 2a map area. The bold great circle is the mean plane: 024, 88 SE, which is ~10 to 10.5° clockwise of the Paposo fault trend at this location (013-014). F) Poles to basaltic dikes (brown hollow squares) in unit KJg and epidote veins (green hollow circles) in unit Krt and KJg with a smoothed Kamb contour and a cylindrical best fit great circle (185, 85 W) to all data. The maximum eigenvalue (point 1) is oriented 191/50.

WSW

ENE

Jyt

KJg4

KJg3

KJg1

KJg2

A ~139 Ma –136 Ma

Jyt

Krt

KJg4

KJg3

KJg2

KJg1

B ~136 Ma –129 Ma Symbology sinistral slip/shear reverse-sense shear coaxial flattening

Jyt

Krt

KJg4

KJg3

trace of mylonitic foliation

KJg1 KJg mafic2dikes Paposo fault

hydrothermal alteration

C ~117 Ma

brecciated zone

Kgz

~500 m

Figure 13: Schematic cross sections illustrating the timing and kinematics of deformation along the Paposo fault zone. See Figure 2 for unit explanation. A) ~139 Ma to 136 Ma: intense hydrothermal alteration and mylonitic fabric development following emplacement of the Late Jurassic/Early Cretaceous granodiorite complex unit KJg4; mafic dikes are also emplaced during this interval and are locally associated with high strain zones. Mylonitic fabrics record sinistral shear, oblique sinistral-reverse shear, and/or coaxial-flattening. B) ~136 Ma –129 Ma: development of protomylonitic to mylonitic fabrics following emplacement of the Remiendos tonalite (Krt) along the western margin of the shear zone at ~136 Ma. Fabrics record sinistral shear with a component of NE-side up reverse shear. The Early Jurassic Yumbes tonalite (Jyt) lacks mylonitic fabrics and likely deformed brittlely along the incipient Paposo fault during this interval. The lower age bracket is from the K/Ar biotite age (Hervé and Marinovic, 1989) which approximates the timing of cooling below the brittle-plastic transition. C) Development of a clay gouge zone (Kgz) along the Paposo fault at ~150–200°C temperatures.

Symbology sedimentary bedding gouge foliation mylonitic foliation vertical mylonitic foliation mylonitic lineation fault with dip & slickenline trend normal fault Paposo fault (sinistral slip) contact (dashed=approximate, dotted=concealed) zircon U-Pb date (Ma) road B-710

500 m

80 0

Jyt

0

70

C

900

KJg4

N

contour interval: 20 m

Units Qa Quaternary alluvium

600

Kgz Early Cretaceous gouge zone along the Paposo fault

146.5 ± 1.5

Krt

600

Krt Early Cretaceous Remiendos tonalite Jurassic to Early Cretaceous granodiorite, hydrothermally KJg4 Late altered and pervasively mylonitic

142.4 ± 1.3

Jyt

KJg3 Late Jurassic to Early Cretaceous granodiorite, >50% mylonitic

KJg3

C’

KJg2 Late Jurassic to Early Cretaceous granodiorite, <50% mylonitic 136.0 ± 1.8

KJg1 Late Jurassic to Early Cretaceous unstrained granodiorite

700

KJg2

7232000 Jyt Early Jurassic Yumbes tonalite 353000

Qa

0

50 TrJpa Upper Triassic to Lower Jurassic Pan de Azucar Formation

KJg1 900

B 177.4 ± 1.8 400

TrJpa

A Qa

TrJpa 200

Kgz

0

50

138.5 ± 1.6

KJg4

600

Krt

KJg3 KJg2 700

B’

KJg1 900

0

80

7230000

A’ 355000

A

Figure 2: A) Simplified geologic map of the southern portion of the Paposo fault segment. See Supplementary File 1, for more detail.

2B

A

A’

800

Elevation (m)

700

KJg3

600

400

KJg4

Krt

Kgz

500

KJg1

KJg2

Jyt

300

TrJpa

200 100 0

50

100

150

200

250

300

350

400

450

500

550

600

650

700

750

800

850

900

950

1,000

1,050

1,100

1,150

1,200

1,250

1,300

1,350

1,400

1,450

1,500

1,550

1,600

1,650

1,700

1,750

1,800

1,850

1,900

1,950

2,000

2,050

B

B’

800

Elevation (m)

700 600 500 400 300 200 100

C

0

50

100

150

200

250

300

350

400

450

500

550

600

650

700

750

800

50

100

150

200

250

300

350

400

450

500

550

600

650

700

750

800

850

900

950

1,000

1,050

1,100

1,150

1,200

900

950

1,000

1,050

1,100

1,150

1,200

1,250

1,300

1,350

1,400

1,450

1,500

1,550

1,600

1,650

1,700

1,750

1,800

1,850

1,900

1,950

2,000

2,050

2,100

2,150

1,350

1,400

1,450

1,500

1,550

1,600

1,650

1,700

1,750

1,800

1,850

1,900

1,950

2,000

2,050

2,100

2,150

2,200

2,250

2,300

2,350

2,250

2,300

2,350

Elevation (m)

700 600 500 400 300

0

850

500 m 1,250

1,300

2,200

Figure 2 continued: B) Cross sections A-A’, B-B’, and C-C’ from the map in A). The lines at the surface show the apparent dip of mylonitic foliation from field measurements.

C’

9

11 13 15 17 19 N

130 ●

13

15

17

19

E

| ||| | | | |

||

150 140

|

150 155 age [Ma]

|

160

165

1

mean = 136.0 ± 1.8 Ma (n=15) MSWD = 8.9

207

0.16 Pb/235U

0.18

3

4

N

5

6

7

8

180● 170 ● 160 ● 150 ●

0.022 0.020

125

140 ●

2

Remiendos tonalite (18-1-PJ73)

130

150 ●

0.18

mean = 146.5 ± 1.5 Ma (n=8) MSWD = 5.1

206

160 ●

0.16 Pb/235U

207

0.028

150

|| | |

145

Age (Ma) 135 140

170 ●

|

140

0.20

0.14

110●

152

11

Pb/238U 0.024 0.026

0.18 207 Pb/235U

●

130● 0.14

9 N

0

●

0.026 7

7

1

150●

206 Pb/238U 0.024

5

140 ●

142

160 ●

G

0.022

140 135

3

5

Frequency 2 3 4

170 ●

0.028

155 Age (Ma) 145 150 1

3

n=20

180 ●

Aplite sill (17-1-P124)

mean = 142.4 ± 1.3 Ma (n=19) MSWD = 13

Pb/238U 0.024 1

190 ●

0.16

●

206

0.22

0.030 Pb/238U 0.026

9 11 13 15 17 19 21 23 25 N

150 ●

Age (Ma) 144 146 148

0.20 Pb/235U

5

207

206

0.024 0.022 7

160 ●

●

120 0.18

0.028

165 160 Age (Ma) 150 155 145 140 135

F

5

170 ●

0.020

130 170 ● ●

●

0.022

180 ●

0.028 0.026

170 160

Age (Ma) 140

Pb/238U 0.030 206

190 ●

D

mean = 151.3 ± 1.7 Ma (n=24) MSWD = 24

3

mean = 138.5 ± 1.6 Ma (n=18) MSWD = 6.2

150

0.032

200 Age (Ma) 180 190

200 ●

Granodiorite-1 (16-1-P31)

1

Granodiorite-2 (17-1-P26)

210 ●

1 3 5 7 9 12 15 18 21 24 27 30 33 36 39 N

C

B

0.028

220

mean = 177.9 ± 1.7 Ma (n=40) MSWD = 28

0.026

Yumbes tonalite (18-1-PJ81)

145

A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 N

140 ● 130 ● 0.14

207

0.16 Pb/235U

0.18

Figure 3: Zircon U-Pb ages with errors bars, weighted mean age, and corresponding concordance plots filtered for less than 10% discordance for samples A) Early Jurassic Yumbes tonalite (18-1-PJ81), B) granodiorite-2 (17-1-P26), C) granodiorite-1 (16-1-P31), F) aplite sill found in the hydrothermally-altered zone (17-1-P124), and G) Early Cretaceous tonalite (18-1-PJ73). The gray rectangle is the 2-σ region, assuming a single population and the hollow bars and grey ovals are dates rejected by the weighted mean algorithm. D) A kernel density estimation (KDE) plot of zircons from granodiorite-1 (16-1-P31), with a histogram of the data in green, using a bin size of 2 Myr, and the distribution in light blue, using a bandwidth of 2 Myr. This plot shows a bimodal distribution of dates. E) A weighted mean distribution of the younger population of dates from sample granodiorite-1 (16-1-P31). Data were processed using Iolite and Isoplot (Ludwig, 2003; Paton, et al., 2011; Petrus and Kamber, 2012) and plots were made using IsoplotR (Vermeesch, 2018). Sample locations in Figure 2 and Supplementary File 2.

A

B

C

D

Figure 4: Representative outcrop photographs of A) a discrete high strain zone in the LJ/EK granodiorite <50% mylonitic zone, B) mylonite derived from the hydrothermally-altered zone of the LJ/EK granodiorite complex (coin diameter ~27 mm), C) a protomylonite from the Early Cretaceous Remiendos tonalite unit, and D) a foliated illite-rich gouge zone along the Paposo fault.

A

B Qtz

Chl

Plag

C Plag Qtz

Figure 5: A) Outcrop photograph of heterogeneously altered Late Jurassic/Early Cretaceous granodiorite. The more hydrothermally-altered areas (lighter areas) are higher strain than the less hydrothermally-altered areas. B and C) Plane-polarized light photomicrographs of B) a less hydrothermally-altered sample and C) a more hydrothermally-altered sample. There is more chlorite in the less altered sample and more quartz in the more altered sample. The feldspars in C) are more altered to white mica and saussurite than feldspars in B). Abbreviations: Chl- chlorite, Qtz- quartz, Plagplagioclase.

A

N

B

1.4 1.2 Shallow Diagenetic Zone ~100°C

Kübler Index (KI)

1.0 0.8 0.6 0.4

Deep Diagenetic Zone Paposo fault gouge

Low Anchizone High Anchizone

0.2

S

~200°C ~300°C

Epizone

0.0

Figure 6: A) Photograph of the Paposo gouge zone with an S-C fabric and a clast from the nonmylonitic Early Jurassic Yumbes tonalite. The C fabrics are highlighted by faint blue dashed lines and the S fabrics are highlighted by faint red dashed lines. B) Kübler Index and corresponding estimated temperature of formation of the illite in the clay gouge based on XRD data (modified from Verdel et al, 2011).

A

N

B

N

n= 102 mylonitic foliations n= 55 mylonitic lineations n= 31 protomylonitic foliations n= 7 protomylonitic lineations n=139 planes n= 6 dextral foliations n=66 lineations n= 4 dextral lineations Figure 7: Ductile structural data from within 2 km of the Paposo fault. A) All mylonitic fabrics. The bold black plane is the mean foliation: 041, 63 SE (the plane to the maximum eigenvalue of the poles). The hollow boxes are poles to foliation planes with the average pole to foliation marked as a large hollow box. The solid boxes are lineations, and the average lineation is marked with a large solid box: 212/18. The pink line is the average Paposo fault trend in the center of the study area (013–014). B) All ductile fabrics separated into 3 groups: mylonitic and ultramylonitic fabrics (black) from the Late Jurassic/Early Cretaceous granodiorite, protomylonitic fabrics (blue) from the Early Cretaceous Remiendos tonalite, and discrete shear zones with a component of dextral shear (red). Poles to mylonitic foliations are black hollow circles, and the large hollow black circle is the mean pole to foliation, corresponding to a mean mylonitic foliation plane (bold black great circle) of 039, 60 SE. The solid black circles are mylonitic lineations, and the large solid black circle is the mean mylonitic lineation: 212/18. The blue hollow boxes are poles to protomylonitic foliations, and the large blue box is the average pole to foliation: 321/21. The solid blue boxes are protomylonitic lineations, and the large solid blue box is the average lineation: 219/31. The average protomylonitic foliation plane is the bold blue great circle: 051, 69 SE. The hollow red diamonds are poles to dextral foliations, which correspond to a mean plane orientation of 015, 85 E (bold red great circle). The solid red diamonds are dextral lineations, and the average orientation marked by a large solid red diamond has an orientation of 194/06.

A

B

Qtz

Qtz

C

N

S

D

N

S

Qtz Qtz Plag Figure 8: Photomicrographs of mylonites with sinistral shear fabrics (in the map-view reference frame) from: A and B) samples from the hydrothermally-altered zone in the Late Cretaceous/Early Jurassic granodiorite (note oblique quartz grain shape fabric; gypsum plate inserted), and C) from the Early Cretaceous Remiendos tonalite (note oblique quartz grain shape fabric, cross-polarized light). D) Symmetric quartz grain shape fabric from the hydrothermally-altered granodiorite (cross-polarized light). Abbreviations: Qtz - quartz, Plag - plagioclase.

A Late Jurassic/Early Cretaceous granodiorite N = 204 16-1-P23a

19˚

N = 207 17-1-P9

20

20

15

15

10

10

5

N = 202 17-1-P43c

20

15˚

N = 204 17-1-P84

20

15

15

10

10

5

N = 204 17-1-P95

5

5˚

5

9˚

20 15 10

11˚

5

B Early Cretaceous Remiendos tonalite N = 203 16-1-P24

20

N = 202 17-1-P6

15

15

15˚

N = 203 17-1-P50

23˚

10 5

20

20

19˚

N = 206 18-1-PJ77

10 5

20

15

15

10

10

5

15˚

5

Figure 9: Quartz long-axis angles (red numbers) in mylonite samples from A) the hydrothermally-altered zone within the Late Jurassic/Early Cretaceous granodiorite (5 samples) and B) the Early Cretaceous Remiendos tonalite (4 samples). Angles were determined by tracing recrystallized grains using ImageJ (Rasband, 1997–2016). Sample locations in Supplementary File 2.

• • • • •

The NNE-striking Paposo fault zone records brittle-ductile sinistral transpression High-strain mylonites formed from ~139–136 Ma; minor strain continued after ~136 Ma Mylonitization was coeval with arc plutonism along the shear zone Foliated clay gouge in the Paposo fault core formed at 150–200°C around 117 Ma Transtension and transpression along the AFS are consistent with fault geometry