Kinematic constraints on intra‐arc shear and strain partitioning in the southern Andes between 38°S and 42°S latitude

Abstract

[1] On the basis of structural field work, fault kinematic analysis, and the analysis of digital imagery, we describe the northern part of the southern Andean intra-arc Liquiñe-Ofqui Fault Zone (LOFZ) as an SC-like fault zone system accommodating part of the Nazca-South American plate convergence obliquity. Kinematic modeling suggests that the LOFZ accommodated 124 (+24/À21) km of dextral displacement between 40°S and 42°S and 67 (+13/À11) km between 38°S and 40°S since the Pliocene. Associated vertical axis rotations are 31 ± 4°c lockwise and 9 ± 1°counterclockwise along synthetic and antithetic faults, respectively. Mean Pliocene to recent shear rates along the LOFZ decrease northward from 32 ± 6 mm/yr to 13 ± 3 mm/yr compatible with partitioning of half of the convergence obliquity into the intra-arc zone north of 40°S and complete partitioning to the south. The displacement gradient along the intra-arc zone results in margin-parallel shortening of the fore arc.

Cooling histories and deformation of plutonic rocks along the Liquiñe-Ofqui Fault Zone, Southern Chile (41°-42°15'S)

“…This inferred displacement would compare well to the inferred amount of extension associated with mid-Tertiary pull-apart basins mapped to the south of the study area (e.g., SERNAGEOMIN-BRGM, 1995 1 ; Cembrano et al, 1996). Rosenau et al (2006) argue that several brittle faults in the Main Cordillera show mutual crosscutting relationships, which-at a larger scale-have offset the main Northern-Southern trending segment of the LOFZ by linkage with a hidden Northeasterntrending fault segment beneath Lago Todos Los Santos. There, they infer a local offset of about 10 km by fault-linkage, supported by information on the paleostress fi eld obtained from brittle faults using magmatic dikes as offset markers.…”

“…Starting from Pliocene times, and during most of the Quaternary, both glaciation and volcanism have shaped the landscape of the Patagonian Andes, which is presently characterized by low ice-capped topographies with a steep relief (Mercer and Sutter, 1982;Rabassa and Clapperton, 1990;Heusser, 1990;Montgomery et al, 2001;Rosenau et al, 2006).…”

“…To the north of the CTJ, the most obvious structure is the long-lived intra-arc Liquiñe-Ofqui fault zone (LOFZ, Hervé, 1976;Cembrano et al, 1996Cembrano et al, , 2000, which has accommodated an arc-parallel component of plate motion and controlled the emplacement of intrusions of the NPB (Hervé et al, 1993Pankhurst et al, 1992Pankhurst et al, , 1999Cembrano et al, 2002). Shaded relief digital elevation modeling (Rosenau et al, 2006), regional and structural fi eld mapping (Thiele et al, 1986;SERNAGEOMIN-BRGM, 1995 1 ;Diriason et al, 1998;Arancibia et al, 1999;Lavenu and Cembrano, 1999), microstructural analyses (Cembrano et al, 1996(Cembrano et al, , 2002Arancibia et al, 1999), and Ar/Ar laser-total fusion and step-heating geochronology of synkinematic minerals (Cembrano et al, , 2002López, 2001) in deformed plutonic rocks consistently demonstrate right-lateral transpression along the LOFZ in Late Miocene to Pliocene times. This interpretation is supported by apatite and zircon fi ssion track thermochronometric data collected along the southern Andes between 41° and 47°S, indicating an episode of enhanced cooling and denudation along the LOFZ in the Late Miocene to Pliocene (Thomson et al, 2001;Thomson, 2002;Adriasola et al, 2006).…”

Resistivity distribution from mid-crustal conductor to near-surface across the 1200 km long Liquiñe-Ofqui Fault System, southern Chile

“…Initiated in Jurassic times, subduction occurred mainly under a compressional tectonic regime with an extensional interval between Eocene and Miocene times (Parda-Casas & Molnar 1987;. The recent stress pattern is characterized by maximal horizontal stress trending N60 • E and subhorizontal minimal stress (Lavenu & Cembrano 1999;Rosenau et al 2006). The trench is located 260 km west of the study area, whereas the subducted Nazca Plate lies at a depth of about 120 km below Mt.…”

Forearc uplift rates deduced from sediment cores of two coastal lakes in south-central Chile

“…This dextral strike-slip discontinuity is mainly a result of oblique plate convergence (Hervé, 1994) and collision of the Chile Rise at its southern end (Nelson et al, 1994). The LOFZ decouples the south-central Chilean forearc, which is suggested to behave as a northward gliding sliver (so-called 'Chiloé block', Fig.1) by several authors (Cembrano et al, 2002;Rosenau et al, 2006). At its northern edge, this block is bounded by the NW-striking Lanalhue fault system (Glodny et al, 2008a), which splays out from the LOFZ at ∼39°S.…”