Paleomagnetic evidence of the brittle deformation of the Central Patagonian Batholith at Gastre area (Chubut Province, Argentina)

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Highlights

  • The Central Patagonian Batholith (CPB) did not registered any vertical rotations during Jurassic times.

  • The paleomagnetic data obtained from the CPB completely refutes the Jurassic Gastre Fault System.

  • The CPB block studied in the Gastre area was tilted in the NE direction due to the efforts of the Andean Orogeny.

  • There was no remagnetization of the CPB rocks in Gastre during the Jurassic Period.

Abstract

The Central Patagonian Batholith (CPB) in the Gastre area, central Patagonia, constitutes a set of two I-type Late Triassic plutonic suites, the Gastre superunit with a U-Pb zircon age of 221 ± 2 Ma, and the Lipetrén superunit with a U-Pb SHRIMP age of 215 ± 1 Ma. The source of this calc-alkaline batholith is characterized by crustal and mantle contributions, and it registers NW-SE subvertical magmatic and solid-state fabrics. These features, together with its intraplate position away from the inferred proto-Gondwana margin, have made it the focus of different tectonic interpretations. Early studies have interpreted it as the record of major dextral motions along the transcontinental NW-SE-striking Gastre fault system in Jurassic times. Later interpretations have proposed that the magmatic and tectonic foliations of the CPB were formed during a sinistral transpressive regime which was aided by Late Paleozoic widespread NW-SE subvertical structures in the North Patagonian Massif. Paleomagnetism, a unique tool to detect the presence of tectonic block rotations on vertical axes, was applied in the CPB in order to constrain the timing and type of deformation in the area. A paleomagnetic pole was obtained, which statistical parameters are: N = 45, Lat. = 81.4°S, Long. = 207°E, K = 11.5, A95 = 6.6°. Although the position of this pole does not coincide with equivalent Late Triassic poles, this position can be reconciled with the presence of a NE tilting of about 11° of the sampled block of the batholith. The tilting would have been aided by the NW-SE subvertical structures that affect the area. These paleomagnetic results rule out the possibility of vertical axis rotations from Late Triassic to present times and suggest that the ductile syn- to post-emplacement deformation of the CPB in Gastre area occurred during this period (Late Triassic), being the later brittle deformation triggered by the Andean Orogeny a possible explanation for this tilting.

Introduction

The Central Patagonian Batholith (CPB; Fig. 1) was originally defined as extensive Late Triassic calc-alkaline plutonic suite aligned in an NW-SE direction, in the central-southern sector of the North Patagonian Massif from Gastre to Pilcaniyeu (Rapela et al., 1991, 1992; Rapela and Kay, 1988). Both, the CPB and its metamorphic host rocks, are affected by a low-temperature solid-state deformation event which originated the presence of tectonic NW-SE subvertical foliations of variable intensity degree, which were originally described as “Gastre system” by Coira et al. (1975). The CPB is located in an inland position with respect to the proto-Gondwana margin, and it represents a key element in paleogeographic models of pre-Gondwana breakup, as it was emplaced during the transition of the Gonwanide (Late Paleozoic) and the Andean (Jurassic to present) orogenic cycles in South America. It has a geochemical signature intermediate between mantle and crustal magmas (Rapela et al., 1991, 1992; Rapela and Pankhurst, 1992). The Late Triassic magmas of CPB have been recently proposed by Navarrete et al. (2019) as part of an inland magmatic arc.

Early studies of the CPB in its type area interpreted it as intruded syn-kinematically within a transcontinental NW-SE dextral shear zone named “Gastre Fault System” (Rapela et al., 1991, 1992), a useful tool widely invoked to restore Patagonia to achieve a tectono-stratigraphic correlation between the Paleozoic successions from the Malvinas-Falkland islands and the Cape fold belt of South Africa (Hole et al., 2016; MacDonald et al., 2003; Marshall, 1994; Storey et al., 1999). However, mesoscale observations in the type locality of the Gastre fault were against the idea of Jurassic dextral transcontinental faults traversing the Gastre area (von Gosen and Loske, 2004; Zaffarana et al., 2010, 2014). Furthermore, the results of a paleomagnetic study from the overlying Lonco Trapial Formation indicated that no clockwise tectonic rotations took place in the Jurassic through the Gastre area (Zaffarana and Somoza, 2012). The internal structure of the CPB was mapped using anisotropy of magnetic susceptibility (AMS), a study which led to the definition of a pattern of NW-SE subvertical magmatic and tectonic fabrics, interpreted as produced by a syn-tectonic emplacement within a sinistral transpressive regime (Zaffarana et al., 2017). This work uses the same dense grid of sampling sites of that previous AMS study of the CPB in order to study its deformation from the paleomagnetic point of view. Paleomagnetism, as a unique tool able to recognize tectonic block rotations in the vertical axis, is a fundamental method to unravel kinematics of shear zones. The paleomagnetic pole calculated in this study contributes to the discussion about the timing and style of deformation of this area of the North Patagonian Massif.

Section snippets

Local geology

In its type locality in the area of Gastre, the CPB has an excellent exposure, where it is composed of two superunits: the older Gastre Superunit and the younger Lipetrén Superunit (Fig. 1). The oldest intrusive rocks are the equigranular hornblende-biotite granodiorites (EHBG) which gradually pass to the porphyritic biotite-hornblende monzogranites (PBHM, Zaffarana et al., 2014). The Gastre Superunit is also composed of stocks of equigranular biotitic monzogranites (EBM), stocks of dioritic to

Sampling method, rock magnetism and demagnetization

This work presents the paleomagnetic measurements of the same AMS sites reported by Zaffarana et al. (2017). The sampling method was designed to fit for both kinds of studies (at least five different samples were obtained from each sampling site). The collection of 1219 oriented cores belongs to 149 stations (Zaffarana et al., 2017, and references therein). From those samples, 240 were studied for paleomagnetic proposes, of which, 100 had interpretable paleomagnetic behavior and belonged to the

Discussion

In order to analyze the dynamics recorded by the remanent magnetic directions, integrated in the paleomagnetic pole shown in this work, it is necessary to compare it with other paleopoles of the same age. For this purpose, the global average poles calculated by Kent and Irving (2010) and Torsvik et al. (2012) for 210 Ma, transported to South American coordinates through the translations proposed in those works, and also, the paleomagnetic pole calculated for the Norian by Vizán et al. (2004)

Conclusions

The development of the fractures and lineaments of the so-called “Gastre Fault System” has, possibly, its origin in the fabric inherited from the Late Paleozoic, a set of structures with an NW-SE direction. These same structures would have facilitated the intrusion of the granite bodies that formed the CPB during the Late Triassic, making the fabric of these granites to be subparallel to these Paleozoic structures. The paleomagnetic pole obtained in this work was defined by rocks bearing

Acknowledgements

This work was partially funded by doctoral scholarship PICT 1074-2006 from Agencia Nacional de Promoción Científica y Tecnológica and by PIP 02828 project from Consejo Nacional de Investigaciones Científicas y Técnicas.

We would like to thank Emiliano Renda for his support during this work, to Carmen Martínez Dopico for her advice, and Leandro Gallo for his help and constructive and inspiring discussions. We dedicate this work to our colleague, mentor, and friend Rubén Somoza, without whom none

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