Jose Cembrano

The southern Andes plate boundary zone records a protracted history of bulk transpressional deformation during the Cenozoic. Transpression has been related to both oblique subduction and ridge collision. However, few structural and chronological studies of regional deformation are available to support one hypothesis or the other. The present thesis addresses along and across-strike variations in the nature and timing of plate-boundary deformation to better understand the Cenozoic tectonics of the southern Andes. A general objective was to gain insights into the nature of transpressional deformation at obliquely convergent plate margins. Five transects were mapped along the southern Andes, from 39^oS to 46^oS. The northernmost, Liquine transect (39^oS), documents ductile deformation of pre-Late Cretaceous age. Brittle deformation is represented by a regional, high angle, northeast-trending reverse fault that places greenschist facies mylonites against an undeformed Miocene granitoid. In contrast, Late Cenozoic brittle faulting of Cretaceous and Miocene plutons is well developed farther south at Reloncavi (41^oS), where contractional and strike-slip kinematics are documented. At Hornopiren (42^oS), Late Cenozoic ductile to brittle dextral strike-slip deformation along northeast striking shear zones was continuous from syntectonic pluton emplacement at 10 Ma, to low temperature, solid-state deformation at ca. 4.3 Ma. Brittle faults indicate that dextral strike-slip deformation remained active after 3 Ma. Puyuhuapi and Aysen transects (44-46^oS), document a remarkable increase in the contractional component of ductile and brittle deformation. At Puyuhuapi (44^oS), north-south trending, high-angle contractional ductile shear zones that developed from plutons, coexist with moderately dipping dextral-oblique shear zones in the wallrocks. In Aysen (45-46^o), top to the southeast, oblique thrusting predominates to the west of the Cenozoic magmatic arc, whereas dextral strike-slip shear zones develop within it. New ⁴⁰Ar-³⁹Ar data from mylonites and undeformed rocks from the five transects suggest that dextral strike-slip and contractional deformation occurred at nearly the same time but within different structural domains along and across the orogen. For instance, ⁴⁰Ar-³⁹Ar laser dating on highly strained synkinematic biotite from plutonic rocks with S-C fabrics at 42^oS documents dextral ductile shear at 4.3+0.3 Ma. Similar ages were obtained on both high strain pelitic schists with dextral strike-slip kinematics (4.4+0.3 Ma, laser on muscovite-biotite aggregates, Aysen transect, 45^oS) and on mylonitic plutonic rocks with contractional deformation (3.8+0.2 to 4.2+0.2 Ma, fine-grained, recrystallized biotite, Puyuhuapi transect). Oblique-slip, dextral reverse kinematics of uncertain age is documented at the Canal Costa shear zone (45^oS) and at the Queulat shear zone at 44^oS. Published dates for the undeformed protholiths suggest both shear zones are likely Late Miocene or Pliocene, coeval with contractional and strike-slip shear zones further north. Coeval strike-slip, oblique-slip and contractional deformation on ductile shear zones of the southern Andes suggests different degrees of along- and across-strike deformation partitioning of bulk transpressional deformation. The long-term dextral transpressional regime appears to be driven by oblique subduction. The short-term deformation is in turn controlled by ridge collision from 6 Ma to present day. This is indicated by most deformation ages and by a southward increase in the contractional component of deformation. Oblique-slip to contractional shear zones at both western and eastern margins of the Miocene belt of the Patagonian batholith define a large-scale flower structure by which deeper levels of the crust have been differentially exhumed since the Pliocene.

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Pages: 246
Supervisor: Marcos Zentilli