Stress permutations: Three‐dimensional distinct element analysis accounts for a common phenomenon in brittle tectonics

Abstract

[1] Using three-dimensional (3-D) distinct element modeling, we explored a variety of simulations to characterize and interpret the stress permutations in brittle tectonics. Stress inversions of fault slip data or earthquake focal mechanisms often revealed such permutations. The main aim of our study is to produce simple, mechanically consistent 3-D models that account for these switches between the principal stress axes s 1 /s 2 or s 2 /s 3 . Even with simple boundary conditions the stress changes induced by variations in rheology are large enough to modify the local tectonic behavior and to produce permutations of principal stress axes. Rather than simple directional changes of stress axes, which exist but often remain limited, the relative variations in principal stress magnitudes are the major cause of permutations s 1 /s 2 and s 2 /s 3 . In nature, permutations being left apart, the orientations of axes often remain tightly clustered. In our experiments we adopted a ratio F = (s 2 À s 3 )/(s 1 À s 3 ) of 0.5, which makes permutations difficult (low F favors s 2 /s 3 permutations, high F favors s 1 /s 2 permutations), and we explored a variety of tectonic situations involving compression, extension, and strike slip. Our experiments indicate that the major causes of stress permutations are the heterogeneity of the brittle deformation (e.g., intact rock massifs between heavily faulted deformation zones) and the anisotropy of the mechanical properties that results from fracturing and faulting, which concur to modify the mechanical balance inside the analyzed volume and to produce stress permutations. Our models show that contrasts and anisotropy in rock properties favor stress permutations. Of major importance is the existence of relatively resistant zones at tips of the deformed zones, acting as channels where stress concentrates and switches occur. Because in nature such zones move in time and space, it is not surprising that stress permutations are so pervasive.

Extensional neotectonics around the bend of the Western/Central Alps: an overview

“…Accordingly, we suggest that the temporal switching between r1 and r2 axes took place during a single overall extensional to transtensive ''deformation phase'', with only local instabilities in space and time modifying the specific stress field (Champagnac et al 2006). Such permutations of stress axes could also be induced by rock heterogeneities and anisotropies (Hu and Angelier 2004).…”

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

“…In places, even both directions of shortening can be observed in the same outcrop with ambiguous overprinting relationships (e.g., EBA028-030, Figure 2b and Table 1) or two tensors coexist (at the geologic timescale) whose shortening and extension axes are parallel (e.g., EBA024, PAN084, Figure 2 and Table 1). We speculate that these permutations of kinematic axes are reminiscence of stress switches at the scale of the seismic cycle either due to near-field effects of coseismic stress drops [Hu and Angelier, 2004] during earthquakes nearby or due to the far-field effect of coseismic and postseismic stress changes associated with megathrust earthquakes along the NazcaSouth America plate interface [e.g., Lara et al, 2004]. Of course, other explanations may be possible.…”

Pre-seismic strain anomalies and coseismic deformation of Meinong earthquake from continuous GPS

“…In the past, 3 big historical earthquakes occurred near the Meinong event, which were 1946 Hsinhua earthquake, 2010 Jiashian earthquake, and 2012 Wutai earthquake. Interestingly, the P-axis of the Hsinhua event orients in NW direction parallel to the convergence direction of the Philippine Sea plate, however the P-axis of Jiashian, Wutai, and Meinong events trend in NE with a high angle to the convergence direction, it implies that stress permutation of common phenomenon in brittle tectonics (Hu and Angelier 2004). An alternative interpretation of NE trending of P-axis of focal mechanism is that the mountain-parallel southwestward movement dominated in Central Range and inner western Foothills could result in a NE-SW compression to reactivate the inherited NW-SE or E-W trending normal faults as a lateral ramp for these three events.…”