Sediment provenance variations during contrasting Mesozoic-early Cenozoic tectonic regimes of the northern Peruvian Andes and Santiago-Marañón foreland basin

CHAPTER

Sediment provenance variations during contrasting Mesozoic-early Cenozoic tectonic regimes of the northern Peruvian Andes and Santiago-Marañón foreland basin

10

Sarah W.M. George*, Brian K. Horton*,†, Lily J. Jackson*,†, Federico Moreno‡, Victor Carlotto§, Carmala N. Garzione‡ Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, TX, United States⁎ Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, Austin, TX, United States† Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY, United States‡ Universidad Nacional San Antonio Abad del Cusco, Cusco, Peru§

1 ­Introduction Andean-type convergent plate margins display spatiotemporal variations in the magnitude and timing of deformation and sedimentary basin evolution (Kley et al., 1999; Alemán and Ramos, 2000; Ramos, 2009, 2010; Horton, 2018a,b). Deviations in tectonic regime along active continental margins (shortening, extension, or neutral conditions) may be governed by the degree of mechanical coupling between the overriding and subducting plates (Horton and Fuentes, 2016; Horton, 2018b) or cyclical phases of orogenic buildup and collapse (DeCelles et al., 2009; DeCelles and Graham, 2015). Sedimentary basins register such shifts in tectonic regime through changes in basin architectures, facies distributions, accumulation rates, and sediment routing patterns that reflect sourcing from the magmatic arc, fold-thrust belt, or craton. Basins along the >7000 km long Andean orogenic belt faithfully record a complex history involving substantial alongstrike differences in deformation and basin genesis prior to widespread Andean orogenesis (Mpodozis and Ramos, 1990; McGroder et al., 2015; Horton and Fuentes, 2016; Perez et al., 2016a,b; Spikings et al., 2016). The Mesozoic–Cenozoic tectonic history of Peru involved extensional, compressional, and strike-slip episodes of imprecise age and duration with competing sediment sources feeding basins through time. The depositional age and provenance records of the following stratigraphic units are critical to accurate reconstructions of deformation and basin evolution. Mixed clastic and volcanic deposits of the Triassic Mitu Group are interpreted as a basin system linked to extension in a backarc basin, intracontinental rift, Andean Tectonics. https://doi.org/10.1016/B978-0-12-816009-1.00012-5 © 2019 Elsevier Inc. All rights reserved.

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Chapter 10  Mesozoic to early Cenozoic provenance and tectonic regimes

or orogenic collapse setting (Vivier et al., 1976; Noble and Silberman, 1978; Dalmayrac et al., 1980; Kontak et  al., 1990; Carlotto, 1998; Sempere et  al., 2002; Rosas et  al., 2007; Ramos, 2009; Carlotto et al., 2010; Reitsma, 2012; McGroder et al., 2015; Spikings et al., 2016). Shallow marine carbonates of the Upper Triassic–Lower Jurassic Pucará Group are attributed to accumulation in either a regional extensional basin or a postextensional thermal sag basin (Wine et al., 2002; Jacques, 2003; Rosas et al., 2007; Bump et al., 2008; Guimas et al., 2010). Nonmarine clastic deposits of the Jurassic Sarayaquillo Formation are variably interpreted to represent extension, postextensional thermal subsidence, or shortening (Balkwill et al., 1995; Mathalone and Montoya, 1995; Marksteiner and Aleman, 1997; Bump et al., 2008). Finally, Cretaceous–Paleogene deposits are proposed to span the transition from an unclear tectonic regime to initial Andean shortening (Steinmann, 1929; Noblet et al., 1996; Baca-Alvarez, 2004; Chacaltana et al., 2011; Horton, 2018a; Hurtado et al., 2018). Integrated constraints on depositional age, provenance, and stratigraphic architecture provide a means of identifying critical shifts in basin evolution that can be tentatively linked to larger changes in tectonic regime. An important Mesozoic–early Cenozoic record of sediment accumulation and basin evolution is preserved near the transition between the central and northern Andes at 4–6°S. In retroarc regions of northern Peru, pre-Andean and Andean sedimentary fill is exposed in the Bagua Basin, Eastern Cordillera, Subandean Zone, and Santiago-Marañón foreland basin (Figs. 1 and 2). We present new detrital zircon U-Pb geochronological results for metasedimentary bedrock (6 samples), igneous bedrock (5 samples), and sedimentary basin fill (16 samples; Fig. 3) coupled with additional provenance data, facies distributions, and structural constraints in order to: (1) demonstrate highly variable sources in time and space, including the South American craton in the east, local and distal Andean basement sources, and magmatic arc sources in the west; and (2) provide new maximum depositional age constraints that refine previous chronostratigraphic control on various sedimentary and metasedimentary units. While bedrock samples provide an important characterization of Neoproterozoic–early Paleozoic basement, results from Mesozoic–lower Cenozoic sedimentary rocks provide a framework to evaluate key elements of conflicting models of basin evolution for this segment of the Andes.

2 ­Geologic context The northern Peruvian Andes constitute a ~150–220 km wide orogenic belt above a zone of modern flat-slab subduction (Hampel, 2002). Along the Pacific coast, a broad ~100 km wide forearc plain includes Cretaceous to Quaternary strata of the lowland Sechura Basin and onshore portion of the Talara Basin (Figs. 1 and 2; Higley, 2004). Seismic profiles show Cretaceous–Paleogene strata capping Paleozoic basement (Higley, 2004; Espurt et al., 2018). The eastern edge of the Sechura Basin is bounded by the Olmos Fault System, a series of north-striking, east-dipping reverse faults that mark the western edge of the Western Cordillera (Carlotto et al., 2009). In the northernmost Peruvian Andes, the Western Cordillera comprises two segments. In the west, Paleozoic metasedimentary rocks include the Olmos Complex, Salas Formation, and Rio Seco Group variably covered by Cenozoic volcanic rocks (Figs. 2 and 4; Carlotto et al., 2009; Jaimes et al., 2011). In the east, Jurassic–Cretaceous sedimentary and volcaniclastic rocks are involved in the east-vergent Marañón fold-thrust belt, which is bound to the east by the Huancabamba thrust fault (Fig. 2; Carlotto et al., 2009; Scherrenberg et al., 2012). The Western Cordillera also hosts a wide range of igneous rocks related to arc magmatism during long-lived Jurassic to Neogene subduction.

2 ­ Geologic context

271

FIG. 1 Digital elevation model (DEM) of the northern Peruvian and Ecuadorian Andes and adjacent basins.

Farther east, the Eastern Cordillera and Subandean Zone are composed of Mesozoic–Cenozoic sedimentary rocks that record a complex pre-Andean history and subsequent transition to Andean shortening and foreland basin development. The Eastern Cordillera is defined by a broad (~30 km wide) northwesttrending belt of Cenozoic strata (Bagua Basin) and a roughly homoclinal panel of west-dipping Mesozoic strata (Fig. 2). The eastern edge of the Eastern Cordillera constitutes a major NE-vergent structure known as the Almendro-Jumbilla Fault Zone, which places Neoproterozoic–early Paleozoic basement of the Marañón Complex onto Mesozoic strata of the Subandean Zone (Guimas et al., 2010). This ~300 km long reverse fault zone separates the Eastern Cordillera from the Subandean Zone and has been attributed to reactivation of Triassic extensional faults (Guimas et al., 2010; Rodríguez and Cueva, 2010; Chacaltana et al., 2012).

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FIG. 2 Surface geology from the 1:1,000,000 geological map of Peru (INGEMMET, 1999) with modified placement of Almendro Jumbilla Fault from Guimas et al. (2010). Sedimentary samples are shown in stratigraphic order on Fig. 3.

In the study area, the Subandean Zone is a thin-skinned fold-thrust belt involving Triassic to Neogene strata of the Santiago Basin at 4–6°S (Ham and Herrera, 1963; Megard, 1984). In the southern portion of the Santiago Basin, west-vergent faults inferred in structural reconstructions and seismic profiles have been linked to the reactivation of east-dipping normal faults of Triassic age (e.g., Chacaltana et al., 2012; Macellari and Hermoza, 2009; Rocha et al., 2008). The eastern limit of the structurally disturbed Santiago Basin, which also marks the western edge of the Marañón Basin, is defined by a structural zone consisting of the Campanquiz Anticline, flanking west-vergent Teniente Pinglo backthrust, and east-vergent Borja thrust (Fig. 2; Chacaltana et al., 2012). In the Campanquiz Anticline, the Jurassic Sarayaquillo Formation represents the deepest structural level exposed (Chacaltana et al., 2012). In this study, we refer to a single Santiago-Marañón foreland basin, recognizing that the Santiago Basin has been structurally disrupted by Subandean fold-thrust structures but shares a Mesozoic– Cenozoic stratigraphic continuity with the largely undeformed Marañón Basin to the east. Where appropriate, we restrict some descriptions to the disrupted Santiago Basin. We examine two transects: (1) an Eastern Cordillera transect that spans the eastern flank of Bagua Basin to the Almendro-Jumbilla Fault; and (2) a Subandean to foreland transect across the proximal western segments of the SantiagoMarañón foreland basin (Figs. 2 and 3).

3 ­Stratigraphic framework Neoproterozoic–early Paleozoic crystalline rocks of the Marañón and Olmos Complexes form the mechanical basement in northern Peru and are comprised of schists, gneisses, and intrusive rocks (Chacaltana et al., 2011). The Mesozoic–Paleogene succession includes ~5 km of alternating nonmarine and marine clastic deposits with subordinate carbonate strata (Figs. 3 and 4). Although the SantiagoMarañón foreland basin shares a similar stratigraphic framework across a broad region, nomenclature diverges for Cretaceous and younger deposits in the Eastern Cordillera (Figs. 3 and 4).

FIG. 3 Schematic composite stratigraphic sections through the Eastern Cordillera and the Santiago Basin between 5 and 6°S. When formation names differ between locations, formation names are given by SB for Santiago Basin, and EC for Eastern Cordillera. Constructed from field observations, map patterns from INGEMMET (2011), and thicknesses from Chacaltana et al. (2011, 2012).

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Red and purple nonmarine strata of the Triassic Mitu Group unconformably overlie basement rocks of the Marañón Complex (Figs.  3 and 4). The ~1000 m thick Mitu Group in the Eastern Cordillera contains alternating sandstones, conglomerates, and minor siltstones (Fig. 3; Chacaltana et al., 2011; Spikings et al., 2016). Facies and stratigraphic thicknesses show significant lateral variability (Reitsma, 2012; Perez et al., 2016a). Based on the original age constraints for the underlying Copacabana Group and overlying Pucará Group, a Permian–Triassic age was defined for the Mitu Group in southern Peru (Vivier et al., 1976; Dalmayrac et al., 1980; Kontak et al., 1990; Carlotto, 1998; Sempere et al., 2002; Mišković et al., 2009). However, new isotopic age constraints suggest the Mitu Group spans most of the Triassic (Carlotto et al., 2010; Reitsma, 2012; Spikings et al., 2016; Perez et al., 2016a; this study). The ~600 m thick Triassic–Jurassic Pucará Group overlaps the Mitu Group across the Eastern Cordillera transect and the Santiago-Marañón basin farther east (Figs. 3 and 4). In the Eastern Cordillera of northern Peru, the marine Pucará Group comprises the Chambará and Aramachay Formations (Chacaltana et al., 2011). Whereas the Chambará Formation is composed of limestones and dolomites, the Aramachay Formation is dominated by black shales and limestones (Rosas et al., 2007; Carlotto et al., 2009; Chacaltana et al., 2011). In the Western Cordillera, the Pucará Group includes the La Leche and Sávila Formations. The La Leche Formation is dominated by limestones and the Sávila Formation comprises black shales with intercalated volcanic ash horizons (Jaimes et al., 2011). Traditionally, the Pucará Group has been assigned to the Upper Triassic (Norian) to Lower Jurassic (Toarcian) based on biostratigraphic relationships (Stanley, 1994; Rosas et al., 2007). The nonmarine Sarayaquillo Formation overlies the Pucará Group in the Eastern Cordillera and Santiago Basin (Figs. 3 and 4; Chacaltana et al., 2011). While thermal modeling suggests ~1200 m of depositional thickness, preservation in the Santiago Basin is sporadic with Sarayaquillo thicknesses ranging between 0 and 600 m (Wine et al., 2002). Red clastic rocks of the Sarayaquillo Formation are composed of sandstones, siltstones, and conglomerates (Fig. 3). Based on limited plant fossils and age constraints for overlying and underlying units, a presumed Upper Jurassic age has been assigned to the Sarayaquillo Formation (Chacaltana et al., 2011). Recent radiometric age constraints from basins farther to the south confirm a Middle to Upper Jurassic age (Erlich et al., 2018). The 1000–3000 m thick Oyotún Formation is restricted to the Western Cordillera where it is comprised of pyroclastic andesites and dacites, breccias, and minor intercalated sandstones and siltstones (Fig. 4). A limited floral assemblage yields Lower Jurassic to Lower Cretaceous ages for the Oyotún Formation (Reyes and Caldas, 1987). The relative stratigraphic position is poorly characterized and is thought to interfinger with the Sarayaquillo Formation or occur between the Pucará Group and Sarayaquillo Formation (Sánchez et  al., 1996; Wine et  al., 2001). Similarly, the ~200 m thick Tinajones Formation is restricted to the Western Cordillera (Western Peruvian Trough) and, depending on location, either overlies or interfingers with the uppermost Oyotún Formation (Sánchez et al., 1996; Wine et al., 2001). The Tinajones Formation is composed of tuffs, sandstones, shales, and minor conglomerates. An unconformity separates the Lower Cretaceous Goyllarisquizga (~600 m) and Oriente (~150 m) Groups from underlying formations (e.g., Benavides-Cáceres, 1999; Chacaltana et al., 2011; Pfiffner and Gonzalez, 2013; Caputo, 2014). Regionally, Lower Cretaceous deposits overlie variable subcrop units, including Paleozoic metamorphic rocks and Triassic to Middle Jurassic sedimentary rocks (Fig. 2). The Goyllarisquizga Group crops out in the Western and Eastern Cordilleras and is equivalent

3 ­Stratigraphic framework

275

FIG. 4 Chronostratigraphic framework preserved at ~5–6°S. Modified after Eude et al. (2015) with new age constraints presented in this paper and from Mourier et al. (1988), Naeser et al. (1991), Wine et al. (2002), Chew et al. (2007), Schaltegger et al. (2008), Carlotto et al. (2009), Mišković et al. (2009), and Chacaltana et al. (2011, 2012).

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to the Oriente Group of the Subandean Zone (Figs. 3 and 4). In the Santiago Basin, the Oriente Group comprises the Cushabatay and Raya Formations (Chacaltana et al., 2012). The lower Goyllarisquizga and the Cushabatay are composed of cross-bedded white quartzose sandstones with interbedded centimeter-­scale black to green shales. The Raya Formation and Upper Goyllarisquizga contain more shale than the lower Oriente and Goyllarisquizga Groups. The Cushabatay Formation is considered Aptian–Albian in age, equivalent to the Hollín Formation of Ecuador (Watson and Sinclair, 1927; Kummel, 1946; Tschopp, 1953; Chacaltana et al., 2012). Above the basal Cretaceous sandstone is a >1 km thick interval of carbonate rocks of the Chonta Formation in the Santiago Basin, and Chulec Formation, Pulluicana Group, and Quilquiñan Group in the Eastern Cordillera (Figs. 3 and 4; Benavides, 1956; Chacaltana et al., 2011, 2012). In the Eastern Cordillera (Eastern Peruvian Trough), Upper Cretaceous carbonates are capped by shales of the Celedín Formation. In the Santiago Basin, Upper Cretaceous deposits include white sandstones of the Vivian Formation and mixed sandstones and shales of the Cachiyacu Formation. These units are overlain by clastic red sandstones, siltstones, and conglomerates of the Fundo el Triunfo and Rentema Formations in the Bagua Basin of the Eastern Cordillera, and by the Uchpayacu, Casa Blanca, and Yahuarango Formations in the Santiago-Marañón basin. Finally, a potential hiatus or condensed section separates Paleocene rocks from Eocene–Oligocene fine-grained deposits (Pozo Formation) in the SantiagoMarañón basin (Horton, 2018a; Baby et al., 2018). Higher stratigraphic levels of the Cenozoic succession are not addressed in this study.

4 ­U-Pb geochronology 4.1  ­Methods Twenty-seven samples were analyzed using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) U-Pb geochronology, including 16 medium-grained sandstones from Triassic–Paleogene units, 6 samples from metasedimentary basement rocks, 2 samples from volcanic rocks within the Oyotún Formation, and 3 plutonic rock samples from the Western Cordillera. All samples were analyzed at the University of Arizona LaserChron Center in accordance with previously established laboratory methods (Gehrels et al., 2008; Gehrels, 2014). Samples were crushed and zircon grains separated using water table, heavy liquid, and magnetic separation techniques. Final zircon separates were poured onto epoxy mounts and polished until grain interiors were exposed. Sedimentary and metamorphic samples were imaged using backscattered electrons to identify inclusions within zircon grains and to discriminate between zircon from other mineral grains. Igneous samples were imaged with cathodoluminescence imaging to ensure analysis spots were restricted to a single growth ­domain. Inclusion-free spots were selected and analyzed with the Element2 (single collector) and the Nu (multicollector) with a laser spot size of 30 μm for all grains. Standards of known ages (FC, Sri Lanka, and R33) were analyzed after every five grains of unknown age to correct for fractionation. For sandstone samples, ~120 individual zircon grains of varying size and shape, free of cracks and inclusions, were randomly selected and analyzed. 206Pb/238U ages are reported for zircons younger than 900 Ma and 206Pb/207Pb ages are provided for grains >900 Ma (see Supplemental Data). Analyses that have >10% internal uncertainty, are >20% discordant, or >5% reverse discordant were excluded from further consideration.

4 ­U-Pb geochronology

277

Igneous ages were calculated using a weighted mean of the youngest population of grains overlapping at 1σ uncertainty (Fig. 6). Maximum depositional ages were typically calculated in the same way, except when there was a distinct division within the youngest population overlapping at 1σ uncertainty (Figs.  7 and 8; see Supplemental Data). Uncertainties for igneous ages and maximum depositional ages are reported at 2σ. Statistical comparisons of similarity between detrital samples are commonly displayed using multidimensional scaling (MDS) plots (Fig. 9), in which the similarity of age distributions among samples is represented visually by their spatial proximity (Vermeesch, 2013). Similarity is evaluated using the Kolmogorov-Smirnov test, which assesses the maximum difference between cumulative density functions of individual samples (Sundell and Saylor, 2017).

4.2 ­Potential sediment sources Diagnostic source regions in northern Peru allow for determination of sediment routing systems through time (Figs. 4 and 5). Potential sources for the Mesozoic–Paleogene succession include: (1) Precambrian cratonic provinces to the east; (2) Paleozoic metamorphic basement within the Andean regions to the west; (3) Carboniferous plutons hosted in basement rocks of the Eastern Cordillera; (4) Permian–Triassic plutons in the Eastern Cordillera of southern Peru and Ecuador; (5) Jurassic to Cenozoic arc-related magmatic rocks restricted to the Western Cordillera; and (6) recycled Mesozoic–Cenozoic deposits that were derived from a range of Precambrian, Paleozoic, Permian–Triassic, and Jurassic sources. (1) Cratonic provinces exposed to the east of the foreland basin system include the Central Amazonian (>2500 Ma), Maroni-Itacaiúnas (2200–1900 Ma), Ventuari-Tapajós (2000–1800 Ma), Rio Negro-Jurena (1800–1500 Ma), Rondonia-San Ignacio (1500–1300 Ma), and Sunsás (1300–900 Ma) provinces (Teixeira et al., 1989; Tassinari and Macambira, 1999; Cordani and Teixeira, 2007). Zircon grains yielding ages >900 Ma are considered to be originally of cratonic origin (Horton et al., 2010, 2015; Gutierrez et al., 2019), although significant reworking of cratonic basement resulted in the incorporation of >900 Ma zircons into Paleozoic–Mesozoic sedimentary and metasedimentary rocks (Fig. 5). (2) Neoproterozoic–early Paleozoic metamorphic rocks such as the Olmos Complex, Marañón Complex, Isimanchi Formation, and Chiguinda Unit crop out in northern Peru and southern Ecuador (Fig. 5). Detrital zircon U-Pb geochronological results for basement samples are generally characterized by broad 700–400 Ma and 1300–900 Ma age ranges, with younger ages more common in the west (Fig. 5; Chew et al., 2007, 2008; Cardona et al., 2009; Witt et al., 2017; this study). Although the older age group overlaps temporally with ages for the Sunsás cratonic province, the presence of 700–400 Ma grains is a distinguishing age mode for Neoproterozoic–early Paleozoic Andean basement. The ability to differentiate between cratonic and Andean basement is critical because it enables discrimination between eastern and western sources, respectively. Exposures of the Olmos Complex are restricted to the Western Cordillera. The Marañón Complex is primarily exposed in the Eastern Cordillera ~50 km southwest of the study area (Fig. 5), and is exposed locally along the Almendro-Jumbilla fault system, which marks the boundary between the Eastern Cordillera and the Subandean Zone. Paleozoic metasedimentary rocks of the Isimanchi and Chiguinda units crop out in the Eastern Cordillera of Ecuador.

FIG. 5 (Upper) Map of basement samples from this study, Chew et al. (2008), Cardona et al. (2009), and Witt et al. (2017). Map modified after Cardona et al. (2010). (Middle) Composite probability density functions with histograms for all Andean basement and all sedimentary samples from 2000 to 0 Ma. (Lower) Individual basement samples at three transects: 4°S, 5°S, and 6°S.

FIG. 6 U-Pb geochronological results for igneous rocks. Plots on the left show the age distribution of the youngest grains as a probability density function (black curve) and as a histogram (gray bars). The plots on the right show the individual ages with 1σ error bars. The weighted mean ages have uncertainty at 2σ, which includes systematic uncertainty propagated in quadrature.

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(3) Carboniferous granitic to granodioritic plutons (340–285 Ma) are preserved along the Eastern Cordillera at 6–14°S (Mišković et al., 2009). These plutons represent subduction-related arc magmatism along the western margin of South America during late Paleozoic orogenesis (Dalmayrac et al., 1980; Mišković et al., 2009). Carboniferous plutons are restricted to regions west of the study area. (4) Permo–Triassic plutonic, metaplutonic, and migmatitic rocks are exposed in the Eastern Cordilleras of Peru and Ecuador (Figs. 2 and 5; Reitsma, 2012; Spikings et al., 2016). Variable interpretations have been invoked to explain the origin of these plutons, including subductionrelated arc plutonism, back-arc magmatism, or rift-related volcanism (Helwig, 1972; Vivier et al., 1976; Dalmayrac et al., 1980; Spikings et al., 2016). Regardless of emplacement mechanism, the restricted occurrence of Permo–Triassic crystalline rocks provides a tracer for sedimentary rocks derived from west of the Almendro-Jumbilla fault system. (5) Jurassic through Cenozoic (~200–10 Ma) subduction related magmatic activity generated extensive plutonic, volcanic, and volcaniclastic sources that generated syndepositional zircons. These igneous rocks exhibit a wide range of magmatic compositions (Romeuf et al., 1995; Mamani et al., 2010). We present new geochronological constraints on 5 igneous samples from northern Peru to characterize potential proximal igneous sources (Figs. 2 and 6). Two samples from volcanic flows within the Oyotún Formation in the Western Cordillera yielded crystallization ages of 195.2 ± 4.0 Ma and 160.7 ± 2.3 Ma, suggesting an Early to Middle Jurassic age for the Oyotún Formation (Fig. 6; 146035 and 146032). These samples provide the first radiometric age constraints on the Oyotún Formation. In addition, three plutonic samples within the Western Cordillera were analyzed and yielded crystallization ages of 136.6 ± 2.3 Ma, 43.9 ± 0.6 Ma, and 36.8 ± 0.8 Ma (Fig. 6; SG0816151, 146037, and 146034). Andean igneous centers shifted through time but are all concentrated within the Western Cordillera. As such, Jurassic–Cenozoic zircons reflect derivation from westerly sources. (6) A thick Mesozoic–Cenozoic mixed siliciclastic and carbonate package existed to the west and east of the study area (Fig. 4). These deposits were derived mostly from arc sources and Neoproterozoic–early Paleozoic basement (Fig. 5). The basin fill extended much farther than the modern outcrop limits, with Mesozoic deposits persisting far into the Marañón Basin to the east (e.g., Wine et al., 2002) and the Marañón fold-thrust belt to the west (Scherrenberg et al., 2012).

4.3 ­Detrital zircon results and interpretations Results from 16 Triassic–Paleogene samples from the Eastern Cordillera and Subandean Zone characterize spatiotemporal differences in Mesozoic–early Cenozoic sedimentation. Significant upsection provenance shifts occur within the Eastern Cordillera and Santiago-Marañón strata (Figs.  7 and 8). Composite plots of all detrital samples demonstrate significant contributions from Andean basement (Fig. 5), as well as Andean arc and cratonic sources.

4.3.1 ­Eastern Cordillera

Detrital zircon age spectra for lower Mesozoic strata in the Eastern Cordillera are characterized by large Ordovician to Jurassic unimodal peaks, and subordinate cosmopolitan Precambrian ages

4 ­U-Pb geochronology

Eastern Cordillera Andean arc

281

Sample:Formation (n=total analyses plotted) Maximum depositional age (n=analyses in depositional age)

Permian-Triassic igneous rocks late Neoproterozoicearly Paleozoic

Sunsás Province

Craton

145306:Cretaceous Goyllarisquizga Gr (base) (n=136) MDA: 135.7 ± 2.5 Ma (n=2)

5 0

145305:Jurassic Sarayaquillo Fm (upper) (n=116) MDA: 179.3 ± 3.5 Ma (n=14)

Number of analyses

50 0

145304:Jurassic Sarayaquillo Fm (lower) (n=126) MDA: 183.5 ± 3.2 Ma (n=13)

50 0

145301:Jurassic Sarayaquillo Fm (base) (n=93) MDA: 187.8 ± 13.2 Ma (n=1)

40 20 0

145297:Triassic Mitu Gr (upper) (n=109)

10 5 0

145291:Triassic Mitu Gr (base) (n=125) MDA: 247.5 ± 2.7 Ma (n=10)

50 0

0

200

400

600

800

1000

1200

1400

1600

1800

Age (Ma)

FIG. 7 Detrital zircon U-Pb geochronology age spectra for Triassic to Cretaceous sedimentary rocks from the Eastern Cordillera transect. Samples are arranged in stratigraphic order, and probability density functions are color coded according to depositional age. Maximum depositional ages (MDA) with 2σ uncertainties are in gray text.

(Fig. 7). The Triassic Mitu Group, previously considered to be of Permian–Triassic age, sits unconformably on Marañón basement and yields an Early–Middle Triassic maximum depositional age of 247.5 ± 2.7 Ma (Fig. 7; 145291). This sample, from ~10 m above the basal unconformity with the Marañón Complex, is characterized by a unimodal Permo–Triassic age peak (90% of ages) centered at ca. 265 Ma, suggesting derivation from Permo–Triassic igneous rocks to the west of the Mitu extensional basin system (Fig. 9). The upper Mitu Group yields a multimodal distribution with a sharp age peak centered at 470 Ma (~20% of ages), and another broad peak with a strong 1300–900 Ma Sunsás component, and subordinate >1300 Ma cratonic components (Fig. 7; 145297). The bimodal age peaks within the upper Mitu Group are consistent with contributions from the metamorphic basement rocks and potentially minor cratonic inputs (Figs. 5 and 9). The abrupt disappearance of

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Chapter 10  Mesozoic to early Cenozoic provenance and tectonic regimes

Santiago Basin Andean arc

Sample:Formation (n=total analyses plotted) Maximum depositional age (n=analyses in depositional age)

Permian-Triassic igneous rocks late Neoproterozoicearly Paleozoic

Sunsás Province

Craton

SG1611:Oligocene Capas Rojas Fm (n=109) MDA: 32.7 ± 0.6 Ma (n=9)

10 5 0

SG1610: Eocene Capas Rojas Fm (n=101) MDA: 39.9 ± 1.0 Ma (n=6)

10 0

SG1612: Eocene Pozo Fm (n=98)

10 5 0

SG1613:Paleocene-Eocene Yahuarango Fm (n=95)

Number of analyses

5 0

SG0802152:Paleocene Yahuarango Fm (n=100) MDA: 59.2 ± 3.2 Ma (n=1)

5 0

SG1614:Paleocene Yahuarango Fm (n=108) MDA: 67.3 ± 1.7 Ma (n=4)

5 0

SG1615:Cretaceous Cachiyacu Fm (n=100)

4 2 0

SG1616:Cretaceous Vivian Fm (n=107) 10 0

145293:Jurassic Sarayaquillo Fm (middle) (n=101)

10 5 0

145294: Jurassic Sarayaquillo Fm (n=98)

4 2 0

0

200

400

600

800

1000

Age (Ma)

1200

1400

1600

1800

FIG. 8 Detrital zircon U-Pb geochronology age spectra for Jurassic to Oligocene sedimentary rocks from the Santiago Basin. Samples are arranged in stratigraphic order, and probability density functions are color coded according to depositional age. Maximum depositional ages (MDA) with 2σ uncertainties are in gray text.

4 ­U-Pb geochronology

283

FIG. 9 (Upper) Cumulative density functions (CDF) for all sedimentary samples from 3500 to 0 Ma. The brown basement envelope was constructed with CDFs from the three transects (north, south, central) from Fig. 5. (Lower) Multidimensional scaling (MDS) plots showing relative similarity between Triassic to Oligocene detrital samples (N = 16 samples) and basement at the three transects from Fig. 5 and the composite basement age spectra (Fig. 5). Sample colors correspond to the CDF plot.

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Permo–Triassic ages and corresponding introduction of basement ages suggests either progressive unroofing of a single source or a change in sediment source. The maximum depositional age for the lower Mitu Group suggests that accumulation commenced in the Middle Triassic, which overlaps temporally with constraints on initial Mitu extension in southern Peru (Reitsma, 2012; Perez et al., 2016a,b; Spikings et  al., 2016). These results suggest coeval initiation of Mitu Group deposition along the Peruvian margin between ~14°S and 5°S, although additional constraints in central Peru are needed to confirm this idea (Reitsma, 2012; Perez et al., 2016a; Spikings et al., 2016). Coarsegrained Mitu deposition persisted until the Late Triassic when deposition of synextensional siliciclastic rocks ceased (Spikings et al., 2016). The Triassic–Jurassic Pucará Group is dominated by carbonate strata with limited detrital zircons due to relatively low siliciclastic input. For tuffs within the Aramachay Formation of the Pucará Group, Schaltegger et al. (2008) reported high precision ID-TIMS U-Pb zircon ages of 199.53 ± 0.19/0.29 Ma and 201.58 ± 0.17/0.28 Ma. New depositional age constraints presented here for underlying and overlying units support a Norian to Toarcian age for the Pucará Group (Stanley, 1994; Rosas et al., 2007; Schaltegger et al., 2008). The base of the Jurassic Sarayaquillo Formation is characterized by a large 300–200 Ma Permo– Triassic peak (~60% of ages), a 1300–900 Ma Sunsás components (~20% of ages), and a small cratonic age population from 2000 to 1300 Ma (~5% of ages) (Fig.  7; 145301). These peaks closely resemble those in the underlying Triassic Mitu strata (Fig. 9), suggesting either: (1) direct derivation from Permian–Triassic plutons and Neoproterozoic–early Paleozoic basement, or (2) recycling from the underlying Mitu Group. A single zircon yields an age of 187.8 ± 13.2 Ma, which is consistent with depositional age constraints above and below. Two additional Sarayaquillo samples (Fig.  7; 145304 and 145305) yield unimodal age peaks comprising 200–150 Ma Jurassic components and subordinate 300–200 Ma components, with maximum depositional ages of 183.5 ± 3.2 Ma and 179.3 ± 3.5 Ma, respectively. These samples show a marked decrease in Permo–Triassic populations and are dominated by Early Jurassic zircons with age peaks centered around ca. 190–185 Ma. The abundance of nearly syndepositional zircons suggests derivation from a contemporaneous magmatic arc. Coeval volcanic rocks of the Oyotún Formation (Figs. 4 and 6) in the Western Cordillera represent a potential source for upper Sarayaquillo strata. A dramatic shift occurs from the principally unimodal age distributions of the upper Sarayaquillo Formation to the broadly distributed, multimodal age distributions of the Lower Cretaceous Goyllarisquizga Formation (Figs. 7 and 9; 145306). This sample is characterized by two Precambrian peaks, including a 1300–900 Ma Sunsás peak and a 2000–1300 Ma cratonic peak, as well as the introduction of a 700–400 Ma peak and subordinate peaks between 300 and 200 Ma, 200–150 Ma and 150–100 Ma. The introduction of abundant Precambrian grains suggests primary sourcing from the South American craton to the east (Fig. 9). Younger age peaks suggest moderate axial transport. The sample yields a maximum depositional age of 135.7 ± 2.5 Ma. Our age constraints for the Sarayaquillo Formation and unconformably overlying Goyllarisquizga Formation broadly bracket a hiatus somewhere within the ~180–135 Ma timeframe. Erlich et  al. (2018) report an age of 162 ± 3 Ma as the youngest single-grain age from the Sarayaquillo, and 145 ± 2 Ma for the Goyllarisquizga Formation, implying a ~15–20 Myr hiatus.

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4.3.2 ­Santiago-Marañón Basin

The Santiago-Marañón Basin represents the disrupted Subandean and foreland segments of the retroarc Andean foreland basin system. The foreland basin deposits are separated from the Eastern Cordillera transect by the Almendro-Jumbilla fault zone (Fig. 2). The lowest sample is from a white, cross-bedded eolianite in the lower-middle Sarayaquillo Formation (Fig. 3; 145294). This sample yields a strong >1300 Ma cratonic population, a 1300–900 Ma population, and a 700–400 Ma peak (Fig. 8). There is an apparent upsection decrease in the cratonic population, and the middle of the Sarayaquillo Formation shows a strong Permo–Triassic peak, minor 700–400 Ma zircons, and a strong Sunsás population (Fig.  8; 145293). This age distribution could represent recycling from Mitu Group sedimentary rocks, or primary derivation from Permo–Triassic plutons and Neoproterozoic–early Paleozoic basement to the west (Fig. 9). The Cretaceous Vivian Formation shows almost exclusively ages older than 900 Ma, indicating derivation from the craton to the east (Figs. 8 and 9; SG1616). The Cretaceous Cachiyacu exhibits a strong >1300 Ma cratonic component, but also broad peaks from 700 to 400 Ma and 1300–900 Ma, consistent with Andean basement sources, and a 150–100 Ma peak derived from arc sources to the west (Figs. 8 and 9; SG1615). Upsection in the Paleocene–Eocene Yahuarango Formation, cratonic sources are replaced by a 100– 50 Ma arc source, and broad peaks from 700 to 400 Ma and 1300 to 900 Ma, indicative of Neoproterozoic– early Paleozoic basement sources (Fig.  8; SG1614). The lower Yahuarango sample yields a maximum depositional age of 67.3 ± 1.7 Ma (n = 4 grains). However, this likely does not represent true depositional age, as Kennan (2008) reports a maximum depositional age of 68.5 ± 3.18 Ma for the Vivian Formation, which is stratigraphically >400 m below the Yahuarango Formation and the Yahuarango is generally considered to be Paleogene in age (INGEMMET, 2011). Two additional Yahuarango samples show similar age spectra, with broad peaks at 1300–900 Ma and 700–400 Ma, minor Jurassic–Permian age components (Fig. 8; SG0802152 and SG1613). A single grain yields a maximum depositional age of 59.2 ± 3.2 Ma. The Eocene Pozo Formation shows dominantly 700–400 Ma and 1300–900 Ma peaks, with a minor Permo–Triassic component (Fig. 8; SG1612). Finally, two samples from the Eocene–Oligocene Capas Rojas Formation show continued derivation from 700 to 400 Ma and 1300–900 Ma sources, with an increase in Permo–Triassic and Jurassic components, and the introduction of a significant (~10%) Cenozoic arc population (Fig. 8; SG1610 and SG1611). These two Capas Rojas samples yield maximum depositional ages of 39.9 ± 1.0 and 32.7 ± 0.6 Ma, respectively.

5 ­Basin reconstruction Contrasting interpretations have been presented for the timing, duration, and nature of Mesozoic–early Cenozoic deformation and basin evolution along the western margin of South America. New insights on the age, architecture, and depositional histories of pre-Andean stratigraphic units are critical to understanding spatial and temporal trends in regional tectonics. The integration of new provenance and chronostratigraphic control from U-Pb geochronological results with regional facies distributions and structural constraints enables schematic reconstructions that illustrate the evolving basin architecture and tectonic regimes from the Middle Triassic to Paleogene (Fig. 10).

Arc Active fault Sediment dispersal Unconformity

F Latest Cretaceous to Paleogene Yahuarango Formation

b

ca

an

Hu

lt

au

aF

b am

E Late Cretaceous

Upper Cretaceous undifferentiated

D Early Cretaceous

Goyllarisquizga Formation

C Early to Middle Jurassic Oyotún Formation

Sarayaquillo Formation

Marañón High

B Late Triassic to Early Jurassic Pucará Formation

A Middle to Late Triassic Mitu Group

Ten ie

nte

?

West

Western Cordillera

Eastern Cordillera

Santiago Basin

Pin

glo

Fau lt

~100 km

Marañón Basin

East

FIG. 10 Schematic cross-section reconstructions of the Triassic–Paleogene evolution in northern Peru. Large arrows show regional stress regime. (A) Middle to Late Triassic extension, sub-basin formation, and synextensional accumulations in east-dipping half graben depocenters, including the Teniente Pinglo depocenter. Sediment sources include coeval igneous rocks and Andean basement. (B) Late Triassic to Early Jurassic postrift thermal subsidence integrates previously isolated depocenters. (C) Continued Jurassic postrift subsidence with localized extension along the east flank of the Marañón High leads to topographic emergence. (D) Early Cretaceous extension primarily focused in the west, as seen in Scherrenberg et al. (2012), and expansion of the depocenter. Sediments are sourced from the craton to the east. (E) Late Cretaceous regional subsidence. (F) Latest Cretaceous–Paleogene Andean shortening and flexural basin formation, marked by a switch in sediment polarity to western sources.

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5.1 ­Middle to Late Triassic In the Middle Triassic, extensional basin development was accompanied by rapid subsidence and accumulation of coarse-grained fluvial and alluvial fan sediments of the Mitu Group (Fig.  10). Multidimensional scaling plots depicting detrital zircon U-Pb geochronological results demonstrate that Triassic deposits were variably sourced from Permo–Triassic magmatic rocks and Neoproterozoic– early Paleozoic basement rocks (Fig. 9). Within the Mitu Group, the shift from nearly syndepositional magmatic sources to basement sources (Fig. 7) suggests either progressive unroofing of a single source or a change in source region during Mitu deposition. The complete disappearance of Permo–Triassic zircons and associated shift from subarkosic to litharenitic sandstone composition (see Supplementary Data) is consistent with exhumation of basement rocks along a fault-bounded basin margin. Normal faults of Triassic age have been imaged in the subsurface of the Subandean Zone and Marañón Basin and are selectively preserved in outcrops within the Peruvian Andes (e.g., Perez et al., 2016a; Zamora and Gil, 2018). In the Santiago Basin, westward thickening Triassic depocenters were governed largely by east-dipping normal faults (Hermoza and Baby, 2008; Macellari and Hermoza, 2009). Moreover, the modern deformation front corresponds with a Triassic normal fault, the Teniente Pinglo Fault, which controlled Triassic basin evolution and was reactivated during Cenozoic shortening (Macellari and Hermoza, 2009).

5.2 ­Late Triassic to Early Jurassic Relatively uniform postrift thermal subsidence accommodated Late Triassic–Early Jurassic deposition of carbonate rocks and black shales of the Pucará Group (Fig. 10). A record of slow regional subsidence is manifest in relatively low-energy facies, a broad ~400 km wide zone of subsidence (Rosas et al., 2007; Zamora and Gil, 2018), and stratigraphic rather than fault-bounded terminations of the Pucará Group (Macellari and Hermoza, 2009). The Pucará Group exceeded the depositional limits of the preceding Mitu Group, as demonstrated by its subsurface extent farther east and mapped westward onlap onto basement rift flanks in the Eastern Cordillera (e.g., INGEMMET, 2012a,b). Although these relationships support regional thermal subsidence as the primary mechanism for generating accommodation space, minor transtensional structures generated localized syntectonic conglomerates (e.g., Mégard, 1978; Carlotto et al., 2009). Near its top, the Pucará Group grades transitionally into sabkha deposits, above which, the basin system returns to clastic facies (Wine et al., 2001).

5.3 ­Early Jurassic to Early Cretaceous Regional backarc subsidence accommodated Jurassic deposition of the nonmarine Sarayaquillo Formation (Fig. 10). U-Pb results reveal a strong spatial contrast from unimodal age signatures in the Eastern Cordillera to diverse age signatures in the Santiago Basin, consistent with multiple source regions (Fig. 9). Detrital zircon U-Pb results indicate derivation from western sources, including the subduction-related continental volcanic arc and proximal arc facies (Oyotún Formation; Romeuf et  al., 1995; Mišković et  al., 2009), and recycling of Permo–Triassic and Neoproterozoic–early Paleozoic age populations characteristic of Triassic strata and/or the Marañón High basement rocks. The presence of zircons derived from the west suggests at least moderate topography in the Andean realm. Outcrop-scale syndepositional normal faults in the Sarayaquillo Formation and coeval breccias of the Corontachca Formation ~100 km south of the study area are consistent with extension

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along the eastern flank of the Marañón High during the Jurassic (Rodríguez et al., 2012). Jurassic exhumation is also supported by rapid cooling in the Eastern Cordillera (Laubacher and Naeser, 1994). Additionally, regional outcrop patterns suggest nondeposition or later erosion of Jurassic deposits across the Eastern Cordillera, consistent with initial topographic emergence of the so-called Marañón High during the Jurassic, possibly along a large rift flank (Mégard, 1978; Sempere et al., 2002; INGEMMET, 2012c). However, extensional faults have not been reported farther east in the northern Marañón Basin, as seismic data show a Sarayaquillo package of nearly uniform thickness (Zamora and Gil, 2018). A regional unconformity separates the uppermost Sarayaquillo from Lower Cretaceous strata, and in places the Sarayaquillo is missing entirely (Fig. 10; Wine et al., 2001, 2002; Sempere et al., 2002; Reitsma, 2012; Caputo, 2014). In the Acre Basin of Brazil, the unconformity has been bracketed ­between the Early Jurassic and Early Cretaceous (Da Cruz Cunha, 2007; Caputo, 2014). In northern Peru, constraints on maximum depositional ages (Erlich et al., 2018; this study) suggest a hiatus of ~15–20 Myr duration. This unconformity has been variably linked to extension (Rosas et al., 2007), structural reactivation, and basin inversion associated with significant plate reorganization (Zamora and Gil, 2018), or brief shortening during the Jurua Orogeny (e.g., Caputo, 2014; Hurtado et al., 2018), Nevadan Orogeny (Ham and Herrera, 1963; Wine et al., 2002; Eude et al., 2015; McGroder et al., 2015), Araucan Orogeny (e.g., Alemán and Marksteiner, 1993), and/or Vicusian Orogeny (e.g., BenavidesCáceres, 1999). Given the limited angular discordance along the Sarayaquillo-Goyllarisguizga contact in northern Peru, an additional possible explanation for the hiatus involves insufficient accommodation due to cessation of thermal subsidence and/or regional sea-level fall.

5.4 ­Early Cretaceous The Early Cretaceous commenced with the establishment of a broader, integrated basin system that exceeded the limits of the preceding Triassic–Jurassic basins (Fig.  10). Following the Late Jurassic hiatus, renewed subsidence during the Cretaceous accommodated deposition of regionally extensive quartzose sandstones (Goyllarisquizga Group and Cushabatay Formation), followed by carbonate and shale deposition until the Maastrichtian (Fig. 10). The Goyllarisquizga Formation shows a diverse suite of detrital zircon ages dominated by zircons older than 900 Ma. The abundance of Mesoproterozoic and Paleoproterozoic ages and quartzose compositions supports primary derivation from the craton to the east (Figs. 9 and 10), consistent with a westward-thickening and westward-fining (diminished sand-shale ratio) Cretaceous depocenter within this basin (Rocha et al., 2008). Younger populations may represent northward axial transport. In the west, the Goyllarisquizga Group caps basement rocks of the Marañón Complex, suggesting partial burial of the northern extent of the Marañón High. Partial burial is consistent with detrital zircon geochronological results from the northern tip of the Marañón High that show strong 1300–900 Ma and 700–400 Ma peaks, with no grains <480 Ma, suggesting local derivation from the Marañón basement (DC 5-6-21; Chew et al., 2008). The controls on Cretaceous subsidence in the Santiago-Marañón basin are unclear, but may relate to local extension (Wine et al., 2002) or far-field effects of extension in the Western Cordillera (e.g., Myers, 1975; Atherton and Webb, 1989; Benavides-Cáceres, 1999; Scherrenberg et al., 2012). Alternatively, the large regional extent of sediment accommodation is consistent with an Early Cretaceous increase in eustatic sea level (Miller et al., 2005) or perhaps geodynamic processes linked to changes in subduction parameters (Alemán and Leon, 2016).

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5.5 ­Late Cretaceous High eustatic sea-level conditions in the Late Cretaceous accommodated deposition in a broad marine basin (Fig. 10). Shales and carbonates were deposited in both the modern Eastern Cordillera (Chulec Formation, Pulluicana Group, Quilquiñan Group, and Celedín Formation) and Santiago-Marañón Basin (Chonta and Vivian Formations). Detrital zircon geochronological results demonstrate that the Vivian Formation was exclusively derived from the craton, consistent with significant marine ingressions in the Andean realm (Fig. 9). Northward axial transport within the Late Cretaceous marine basin is well documented (Pindell and Tabbutt, 1995; Jaillard et al., 2000; Gutierrez et al., 2019). Derivation from eastern sources during the Late Cretaceous is also demonstrated by recent provenance studies to the south (Hurtado et al., 2018; Erlich et al., 2018) and north (Gutierrez et al., 2019).

5.6 ­Latest Cretaceous–Paleogene The latest Cretaceous marks increased accumulation and a fundamental shift in basin accommodation, paleodrainage, and sediment routing related to Andean shortening (Fig.  10). The Cachiyacu Formation shows contributions from Andean basement and arc sources to the west consistent with earliest Andean shortening and topographic growth in the west feeding clastic sediment to an embryonic foreland basin. In Ecuador, Balkwill et al. (1995) show evidence of early foreland conditions in the Late Cretaceous with thinning of the Late Cretaceous Napo shale onto localized uplifts. More regionally, a sharp increase in sediment accumulation rates is recorded by latest Cretaceous to Paleocene time during the deposition of the Tena and Tiyuyacu Formations (Balkwill et al., 1995; Baby et al., 2004; Horton, 2018a). Similarly, detrital zircon U-Pb results indicate uplift of the Eastern Cordillera of Ecuador at 75–65 Ma based on zircons that have similar exhumational and depositional ages (Martin-Gombojav and Winkler, 2008; Gutierrez et al., 2019.; Vallejo et al., 2019). Together these data support a latest Cretaceous onset of Andean shortening in northern Peru and Ecuador. Paleocene shortening and crustal loading led to the establishment of an integrated flexural foreland basin, which is manifest as a shift in the polarity of sediment dispersal, with the uplift and exhumation of Andean sources in the west (Fig. 10), as demonstrated by an increase in arc and Neoproterozoic– early Paleozoic basement populations relative to cratonic populations. Early Paleogene samples exhibit large 1300–900 Ma and 900–400 Ma age peaks typical of the Olmos and Marañón basement exposed in the Andes (Figs.5, 8, and 9). Paleocene sandstone compositional data show not only a large component of metamorphic rock fragments (see Supplemental Data) emblematic of metamorphic basement, but conglomerate clast counts for Paleocene strata reveal considerable recycling of sedimentary rocks (Fig.  3), consistent with recycling of Paleozoic–Mesozoic cover strata from the Western Cordillera (Western Peruvian Trough). Metamorphic grains from Neoproterozoic–early Paleozoic basement may have been derived from the Olmos Complex to the west in the Western Cordillera, or from outcrops of the Marañón Complex to the south. By the late Eocene, the magmatic arc was a major source of foreland detritus, perhaps reflecting a more proximal arc, or a volumetric increase in arc productivity. The Eocene marked a return to marine conditions, with a shallow marine incursion in the foreland (Hermoza et al., 2005; Chacaltana et al., 2012). The Eocene was characterized by shortening to the west in the Marañón fold-thrust belt, which progressively propagated eastward, but remained restricted to the Western Cordillera (e.g., Megard, 1984; Espurt et al., 2018).

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6 ­Conclusions (1) Detrital zircon U-Pb geochronological results provide new depositional age constraints and provenance information for the Triassic–Paleogene deposits in the northern Peruvian Andes between 5°S and 6°S. Highly variable sources through time reflect a fluctuating system characterized by: (a) Triassic extensional basin genesis with synrift, nonmarine, clastic accumulations of the Mitu Group; (b) Upper Triassic–Jurassic postextensional thermal sag deposits of the carbonate Pucará Group and siliciclastic Sarayaquillo Formation, with localized fault-induced subsidence; (c) a phase of low-energy deposits accumulated during the Cretaceous; (d) the onset of Andean shortening in the latest Cretaceous; and (e) progressive eastward advance of the fold-thrust belt and integrated foreland basin system during the Paleogene. (2) New geochronological results constrain the maximum depositional age of the basal Mitu Group at the northernmost exposures to the middle Triassic. The middle Triassic onset of Mitu deposition overlaps within uncertainty of constraints on basal Mitu deposits over 1000 km to the south (Reitsma, 2012; Perez et al., 2016a,b; Spikings et al., 2016). The Mitu Group shows initial derivation from Permo–Triassic igneous sources to the west, and then a rapid shutoff of igneous sources and a switch to local basement sources, likely reflecting exhumation along basin bounding faults. (3) During an ~60 Myr phase of postrift thermal subsidence with potential pulses of extension in the Late Triassic–Jurassic, marine carbonates of the Pucará Group and nonmarine red beds of the Sarayaquillo Formation blanketed the Eastern Cordillera, Subandean Zone, and foreland. The Sarayaquillo Formation yields maximum depositional ages from ~190 to 180 Ma, which briefly postdate the establishment of an active arc along the Peruvian Andes. Based on provenance data and regional subcrop relationships, we suggest topographic emergence of the Marañón High during the deposition of the Sarayaquillo Formation. (4) The top of the Sarayaquillo Formation is marked by a ~15–20 My hiatus. This unconformity is interpreted to represent an abandonment or bypass surface based on the irregular subcrop relationships as observed in outcrop and subsurface data. The unconformity likely reflects major plate reorganizations and transpressional reactivation of older structures. Early Cretaceous age spectra show sourcing from the craton to the east, which may reflect far-field extension, slab dynamic processes, or fluctuations in eustatic sea level. (5) In the latest Cretaceous (Maastrichtian) at ~70 Ma, there is rapid shift in sediment polarity, shown by the increase in arc populations derived from the west. Based on the shift in provenance, accompanied with a change from marine to fluvial deposition, we take this to represent the onset of upper plate shortening. Paleogene samples reflect primary derivation from the west associated with the progressive exhumation of Neoproterozoic–early Paleozoic basement (Olmos Complex), and recycling of older strata. The lack of arc-derived zircons between the late Paleocene to middle Eocene may reflect signal dilution due to rapid exhumation in the west or a waning of arc activity.

­Acknowledgments This research was supported by National Science Foundation grant EAR-1338694, and grants from the Geological Society of America, American Association of Petroleum Geologists, Society for Sedimentary Geology, and National Science Foundation Graduate Research Fellowship Program. We thank Jheiner Puclla for field assistance,

­References

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and K. Milliken, R. Erlich, N. Perez, C. Mackaman-Lofland, A. Calle, T. Capaldi, E. G. Gutierrez, C. Vallejo, M. Odlum, K. Butler, and J. Hirtz for helpful discussions. This manuscript was improved by thoughtful reviews from Kurt Sundell and an anonymous reviewer.

­Appendix: Supplementary material Supplementary material related to this chapter can be found on the accompanying CD or online at https://doi.org/10.1016/B978-0-12-816009-1.00012-5.

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­Further reading Dickinson, W.R., Suczek, C.A., 1979. Plate tectonics and sandstone compositions. Am. Assoc. Pet. Geol. Bull. DOI:0149-1423/79/B012-0003$03.00/0. Sanchez, A., 1995. Geología de los cuadrángulos de Bagua Grande, Jumbilla, Lonya Grande, Chachapoyas, Rioja, Leimebamba y Bolívar. INGEMMET. Wilson, J.J., 1963. Cretaceous stratigraphy of Central Andes of Peru. Am. Assoc. Pet. Geol. Bull. 47, 1–34.