LAWSONITE VORTICITY AND SUBDUCTION KINEMATICS (SIVRIHISAR, TURKEY)

Documenting deformation in high-pressure (HP) rocks of exhumed subduction complexes is critical for understanding the relationship of metamorphic reactions to seismicity and release of water to the mantle wedge in active subduction zones. In addition, the rapid exhumation of HP rocks from depths of >50 km poses the question of the mechanisms that enable rapid, large-magnitude transport of HP rocks to shallower depths. Evidence for HP deformation within subduction complexes is typically overprinted by low-P deformation, heating, and hydration reactions, complicating efforts to interpret processes that operated at depth. The exceptional preservation of HP rocks near Sivrihisar, Turkey, including lawsonite in eclogite and blueschist, allows evaluation of HP deformation and exhumation using kinematic analysis of a HP index mineral.

In the Sivrihisar Massif, layering and foliation typically dip gently to moderately, and steepen toward a fault contact with a metamorphosed ultramafic/mafic complex. The strong foliation carries a prominent oblique stretching lineation that is oriented E-NE. Marble displays columnar aragonite pseudomorphs that are systematically inclined relative to foliation, consistent with top-to-E shear. All rock types in the massif show top-to-E or NE sense of shear that developed at HP conditions; the planar and linear fabrics formed in uniform normal/dextral oblique shear across the terrain.

Kinematic vorticity analysis of lawsonite can be used to document the relative components of pure and simple shear in a subduction complex at HP, the strain that accompanies and accommodates exhumation, and the mineral fabrics and anisotropies that develop in HP rocks. For this analysis, ten samples of lawsonite-bearing blueschist and eclogite were collected from the NW part of the massif, including 2 samples from the bounding fault zone. We evaluated kinematic vorticity (Wm pure shear = 0, Wm simple shear = 1) using the shape fabric of lawsonite crystals in blueschist layers and meter-scale eclogite pods. Textural relations indicate that lawsonite behaved as a rigid grain in both glaucophane- and omphacite-dominated rocks. EBSD data show that the (010) lawsonite crystallographic axes correspond to the orientation of the presumed vorticity axis; i.e., thin sections preferentially contain the short and long axes, represented by (100) and (001), respectively, and vorticity parameters derived from 2D analysis are valid proxies of 3D rotation.

Pods record dominant simple shear (Wm > 0.8) in their eclogitic core and at pod margins partially transformed to blueschist; map-scale blueschist layers record a large component of pure shear (Wm = 0.4-0.6), consistent with oblique extrusion at depth, and local simple shear (Wm = 0.8) at the fault contact with serpentinite. A combination of oblique unroofing from beneath a weak serpentinized hanging wall and pure shear extrusion in the subduction channel accounts for rapid exhumation along a low geothermal gradient and the rare preservation of lawsonite eclogite during exhumation. Footwall rocks were not translated rigidly upward but were deformed by a large component of pure shear that was capable of accelerating exhumation by squeezing the HP rocks out of the subduction zone.

If material is pinned at some depth, the pure shear component may add a significant upward motion to the translation of material achieved by simple shear. This effect can be quantified using plane strain analytical modeling. For vorticity values obtained on blueschist layers (Wm = 0.4-0.6), the amount of pure shear extrusion has the effect of multiplying the simple shear translation several times. This component of strain adds significantly to the upward trajectory of HP rocks.

In addition, the transformation from (omphacite) eclogite to (glaucophane) blueschist during ascent results in significant weakening of the subduction channel. Flow of blueschist around meter-scale rigid eclogite pods reflects this weakening. Therefore, a positive feedback likely develops between decompression and weakening of the subduction channel. The channel is rapidly exhumed by translation from beneath the serpentinized overlying plate and by pure shear extrusion in a dextral-normal shear system, perhaps assisted by aggressive erosion at the Earth’s surface, preserving lawsonite blueschist and eclogite.

Lawsonite vorticity analysis is a powerful new method to investigate deformation and exhumation mechanisms in subduction complexes. Results of this and other studies of HP and ultrahigh-P terrains show that material weakening during decompression may be the origin of strain localization of major discontinuities and patterns of vorticity partitioning that favor rapid exhumation. This type of exhumation may be necessary for the preservation of some HP mineral assemblage at the Earth’s surface.