TSK 11 Göttingen 2006 Deckert & Ring
 Relevance of viscous flow in
 accretionary wedges Vortrag
 Hagen Deckert1 Uwe Ring2
 The orogenic wedge model (Davis et
 al. 1983; Platt 1986) marks a con-
 ceptual breakthrough in understanding
 the growth and long-term evolution of
 accretionary wedges. The characteris-
 tic rheology of subduction-related ac-
 cretionary wedges is thought to change
 from Coulomb to viscous when the
 wedge becomes thicker than ca. 15km,
 a transition that may influence the sta-
 bility and dynamics of these wedges.
 Platt (1986) proposed that viscous flow
 may trigger extensional faulting in the
 upper rear part of the wedge and Wallis
 et al. (1993) argued that viscous flow
 may cause vertical ductile thinning of
 the rear part of the wedge.
 Material fluxes control the geometric
 shape of an accretionary wedge (Bran-
 don et al. 1998; Platt 1986). Frontal
 accretion and erosion both tend to drive
 the wedge into a subcritical condition
 as the taper angle of the wedge is pro-
 gressively reduced. This leads to hori-
 zontal shortening across the wedge. If
 underplating is dominantly controlling
 the flow field in the wedge and frontal
 accretion or erosion at the rear of the
 wedge are small, the wedge is super-
 critically tapered and leading horizontal
 extension. Horizontal extension leads
 to a subhorizontal foliation and may
 eventually lead to normal faulting in
 the rear-part of the wedge. Despite
 the importance of these issues, there
 remains a paucity of detailed informa-
 1 Institut für Geowissenschaften, Johannes
 Gutenberg-Universität, Becherweg 21, 55099
 Mainz, Germany 2 Department of Geo-
 logical Sciences, University of Canterbury,
 Christchurch, New Zealand
 tion about ductile deformation and how
 viscous flow influences the stability of
 subduction-related accretionary wedges.
 Strain measurements are an instrument
 to address whether viscous flow strongly
 influences the deformation in accre-
 tionary wedges. They provide direct in-
 formation about the kinematics of an-
 cient orogenic belts. Additionally, they
 allow understanding important tectonic
 processes in subduction wedges such as
 the pattern of flow within the wedge.
 We focus on deformation analysis on
 a suite of samples from the Otago
 wedge exposed in the South Island
 of New Zealand. The Otago accre-
 tionary wedge offers a unique opportu-
 nity to study the tectonic evolution of a
 typical subduction-related accretionary
 complex. Its across-strike length of
 ca. 600 km makes it one of the largest
 exposed ancient accretionary wedges on
 Earth. Pressure and temperature esti-
 mates indicate that our samples are rep-
 resentative of deformation conditions to
 depths as great as ca. 35km. This is
 similar to maximum depths observed for
 subducting slabs beneath modern fore-
 arc highs.
 The deformation measurements show
 that the strain magnitude is generally
 small in the Otago wedge. The γoct
 values, a measure of the distortion a
 sample experienced (independent from
 the strain geometry), range from 0.34–
 3.87 for the Rf/φ strains, 1.01–4.28 for
 XTG strains across the whole suite of
 the Otago rock pile, and 0.08–0.70 for
 the absolute strains obtained from low
 metamorphic grade rocks. The Otago
 samples are characterized by consider-
 able volume strain that increases from
 the lower textural zones towards the
 high-grade interior of the wedge.
 Our strain results are inconsistent with
 1
Deckert & Ring TSK 11 Göttingen 2006
 the models which advocate supercrit-
 ically tapering of accretionary wedges
 and that supercritical tapering even-
 tually triggers normal faulting. Tak-
 ing averages of our strain measure-
 ments, a residence time in the wedge
 of 35Myr, burial depths of 30 km, coax-
 ial deformation and a depth-dependent
 rate for ductile deformation, we cal-
 culate vertically-averaged strain rates.
 Because the principal strain axes of
 the tensor average are all inclined, the
 vertical averaging changes the princi-
 pal stretches. The horizontal princi-
 pal stretch parallel to the 160°-striking
 Otago wedge becomes 0.79, that for
 across strike 0.88 and for vertical
 strain 0.44. Averaged strain rates are
 −1.44−16 s−1 for parallel-strike horizon-
 tal strain, −6.2−17 s−1 for across-strike
 horizontal strain, and −8.02−16 s−1 for
 vertical strain. The strain rates are re-
 lated to volume loss and to the effi-
 ciency with which dissolved chemicals
 are advected away. The rates are simi-
 lar to the ones calculated by Bolhar &
 Ring (2001) and Ring & Richter (2004)
 for the Franciscan wedge. These strain
 rates are orders of magnitude smaller
 than the 1−14 s−1 strain rates assumed
 by Platt (1986). Thus, our data imply
 that the Otago wedge could not shorten
 horizontally fast, and hence could not
 have steepened up its surface slope. The
 fact that shortening was accompanied
 by volume loss has another important
 and interesting consequence. Even if
 a case was envisioned in which hori-
 zontal shortening was fast enough to
 steepen up the surface slope of the
 wedge, the volume loss would not nec-
 essarily change the wedge geometry into
 a supercritical configuration triggering
 normal faulting. As a consequence of
 the slow strain rates and the high vol-
 ume loss, viscous flow probably was not
 fast enough to significantly influence the
 stability of the wedge and to form a su-
 percritically tapered wedge.
 References
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