Inverted distribution of ductile deformation in the relatively “dry” middle crust across the Woodroffe Thrust, central Australia

Abstract

Abstract. Thrust fault systems typically distribute shear strain preferentially into the hanging wall rather than the footwall. In this paper, we present a regional-scale example that does not fit this model. The Woodroffe Thrust developed due to intracontinental shortening during the Petermann Orogeny (ca. 560-520 Ma) in central Australia. It is interpreted to be at least 10 600 km long in its general E-W strike direction, with an approximate top-to-north minimum relative displacement of 60-100 km. The associated mylonite zone is most broadly developed in the footwall. The immediate hanging wall was only marginally involved in mylonitization, as can be demonstrated from the contrasting thorium signatures of the upper amphibolite facies footwall and the granulite facies hanging wall protoliths. Thermal weakening cannot account for such an inverse deformation gradient, as syn-deformational P-T estimates for the Petermann Orogeny in the hanging wall and footwall from the same 15 locality are very similar. The distribution of pseudotachylytes, which act as preferred nucleation sites for shear deformation, also cannot provide an explanation, since these are prevalent in the immediate hanging wall. The most likely reason for the inverted deformation gradient across the Woodroffe Thrust is water-assisted weakening due to the increased, but still limited, presence of aqueous fluids in the footwall. On the contrary, the presence or absence of aqueous fluids does not appear to be linked to the regional variation in mylonite thickness, which generally increases with increasing metamorphic grade. 20

The earthquake cycle in the dry lower continental crust: insights from two deeply exhumed terranes (Musgrave Ranges, Australia and Lofoten, Norway)

“…The Woodroffe Thrust juxtaposes units with distinctly different Musgravian metamorphism [ 78 , 94 , 95 , 97 , 98 , 103 ]: (i) the hanging wall Fregon Subdomain that was thoroughly dehydrated under granulite facies conditions; and (ii) the footwall Mulga Park Subdomain that only reached amphibolite facies conditions and contains more hydrous minerals. This difference in the degree of devolatilization is clearly reflected in the regional maps of thorium concentration determined by airborne gamma-ray surveys, with a distinct jump in concentration across the Woodroffe Thrust from lower values in the hanging wall to higher values in the footwall [ 63 , 93 ]. Except for the difference in Musgravian peak metamorphism, the structural and magmatic histories in the two subdomains prior to their juxtaposition are similar, which suggests that the two units represent different crustal levels of the same terrane [ 78 , 100 ].…”

“…Wex et al . [ 93 ] used the thorium concentrations across the Woodroffe Thrust to establish that the hanging-wall-derived mylonites generally represent less than 10% of the entire width of the shear zone. In general, the thickness of the Woodroffe Thrust mylonites decreases with decreasing metamorphic grade, and increasing availability of aqueous fluids, in the northward direction of thrusting.…”

“…This ‘inverted’ strain distribution for a thrust, with a large part of the ductile deformation affecting the footwall rocks, is explained by the weaker rheology of the wetter footwall compared to the drier hanging wall [ 61 , 96 ]. The decrease in thickness of the footwall mylonites toward the north can be explained by localization favoured by weakening associated with the relatively more hydrous conditions [ 104 ], although quartz piezometry indicates higher synmylonitic differential stress in the north, consistent with the decrease in temperature in the direction of thrusting [ 93 ]. Conversely, pseudotachylytes occur mainly in the hanging wall and are generally subordinate or absent in the footwall, especially in the northernmost wetter, cooler and shallower portions.…”

“…Pressures of 1.0–1.3 GPa, corresponding to sub-eclogitic facies conditions, typical of the lower continental crust, have been estimated for the south. To the north, the pressure was lower, but less well constrained [ 62 , 93 ]. Generally, dry conditions in the hanging wall (Fregon Subdomain) and southernmost exposed footwall (Mulga Park Subdomain) during the Petermann Orogeny have been inferred from the characteristic presence of kyanite and the absence of epidote within the breakdown products of the anorthite component of plagioclase [ 63 , 93 ].…”

“…In the footwall, pseudotachylyte is uncommon in the most southerly outcrops and appears to be totally absent in the north. This corresponds both to an increase in the amount of available water and to a decrease in temperature and pressure going gradually up section in the direction of thrusting within the footwall [ 62 , 93 ]. In effect, the amount of pseudotachylyte decreases going upward in the crustal section toward the depth of the typical seismogenic zone at around 15–20 km [ 108 ].…”