Tectonic role of margin-parallel and margin-transverse faults during oblique subduction in the Southern Volcanic Zone of the Andes: Insights from Boundary Element Modeling

Abstract

Obliquely convergent subduction margins develop trench‐parallel faults shaping the regional architecture of orogenic belts and partitioning intraplate deformation. However, transverse faults also are common along most orogenic belts and have been largely neglected in slip partitioning analysis. Here we constrain the sense of slip and slip rates of differently oriented faults to assess whether and how transverse faults accommodate plate‐margin slip arising from oblique subduction. We implement a forward 3‐D boundary element method model of subduction at the Chilean margin evaluating the elastic response of intra‐arc faults during different stages of the Andean subduction seismic cycle (SSC). Our model results show that the margin‐parallel, NNE striking Liquiñe‐Ofqui Fault System accommodates dextral‐reverse slip during the interseismic period of the SSC, with oblique slip rates ranging between 1 and 7 mm/yr. NW striking faults exhibit sinistral‐reverse slip during the interseismic phase of the SSC, displaying a maximum oblique slip of 1.4 mm/yr. ENE striking faults display dextral strike slip, with a slip rate of 0.85 mm/yr. During the SSC coseismic phase, all modeled faults switch their kinematics: NE striking fault become sinistral, whereas NW striking faults are normal dextral. Because coseismic tensile stress changes on NW faults reach 0.6 MPa at 10–15 km depth, it is likely that they can serve as transient magma pathways during this phase of the SSC. Our model challenges the existing paradigm wherein only margin‐parallel faults account for slip partitioning: transverse faults are also capable of accommodating a significant amount of plate‐boundary slip arising from oblique convergence.

Intra‐Arc Crustal Seismicity: Seismotectonic Implications for the Southern Andes Volcanic Zone, Chile

“…In turn, a combination of strike‐slip and reverse faulting in the forearc and foreland/back‐arc regions may accommodate both margin‐parallel and margin‐orthogonal components during one interseismic stage of the subduction earthquake cycle. However, few studies (La Femina et al, ; Lange et al, ; Pérez‐Flores et al, ; Salazar et al, ; Stanton‐Yonge et al, ), which include detailed observations, yield a more complex picture regarding the way by which the margin‐parallel component accommodates within the intra‐arc. In fact, it might be entirely accommodated by margin‐oblique faults (e.g., bookshelf faulting in Central Andes volcanic arc; La Femina et al, ); or strain might distribute within variable orientations and with a wide kinematic range, as deduced from focal mechanisms in Northern Chile, with statistically dominant oblique‐slip kinematics, with substantial strike‐slip component (Salazar et al, ).…”

“…Average slip vector of subduction thrust earthquakes are indicated in blue. The expected residual margin‐parallel slip component is shown by green vectors (Stanton‐Yonge et al, ). The rectangle in white lines represent the area shown in Figures and S1.…”

“…Furthermore, tensional stress magnitudes can be larger where NE‐striking faults are close to, or intersect, master fault segments, which enhances long‐term crustal fluid pathways during interseismic tectonic loading conditions (Iturrieta et al, ). A significant portion of the bulk transpressional deformation is accommodated by slip on the intra‐arc LOFS displaying markedly slip‐rates differences from north to south along different fault segments (Iturrieta et al, ; Rosenau et al, ; Stanton‐Yonge et al, ). Calculated slip magnitudes from numerical models may be larger along pure strike‐slip fault segments, ranging from ~18 mm/year, close to southern LOFS termination (Iturrieta et al, ), down to 0.5 mm/year in the northern termination (Stanton‐Yonge et al, ; Figure ).…”

“…A significant portion of the bulk transpressional deformation is accommodated by slip on the intra‐arc LOFS displaying markedly slip‐rates differences from north to south along different fault segments (Iturrieta et al, ; Rosenau et al, ; Stanton‐Yonge et al, ). Calculated slip magnitudes from numerical models may be larger along pure strike‐slip fault segments, ranging from ~18 mm/year, close to southern LOFS termination (Iturrieta et al, ), down to 0.5 mm/year in the northern termination (Stanton‐Yonge et al, ; Figure ). Thus, as a result of highly coupled plate interfaces, the forearc sliver motion, and thermal weakness at the intra‐arc region (Cembrano et al, ; Wang et al, ), the overriding plate shows a variable degree of deformation partitioning along and across the Southern Andes (Arancibia et al, ; Lavenu & Cembrano, ; Pérez‐Flores et al, ; Rosenau et al, ).…”

“…ATFs also have been thought to control the Neogene forearc basin architecture and megathrust rupture segmentation associated with the seismic subduction cycle (Glodny et al, ; Melnick et al, ). Furthermore, deformation of the ATFs might be activated from megathrust earthquakes (Aron et al, ; Arriagada et al, ; Farías et al, ; Stanton‐Yonge et al, ), and may control crustal fluid migration within the intra‐arc region (Cembrano & Lara, ; Katz, ; Pérez‐Flores et al, ; Rivera & Cembrano, ; Roquer et al, ; Sánchez‐Alfaro et al, ; Sielfeld et al, ; Wrage et al, ).…”