TSK 11 Göttingen 2006 Nüchter and Stöckhert
 Fracturing and vein forma-
 tion in the middle crust - a
 record of co-seismic loading
 and post-seismic stress relax-
 ation Vortrag
 Jens-Alexander Nüchter1 Bernhard
 Stöckhert1
 Metamorphic rocks approaching the
 crustal scale brittle-ductile transition
 (BDT) during exhumation are expected
 to become increasingly affected by short
 term stress fluctuations related to seis-
 mic activity in the overlying seismogenic
 layer (schizosphere), while still resid-
 ing in a long-term viscous environment
 (plastosphere). The structural and mi-
 crostructural record of quartz veins in
 low grade – high pressure metamorphic
 rocks from southern Evia, Greece, yields
 insight into the processes and conditions
 just beneath the long-term BDT at tem-
 peratures of about 300 to 350°C, with
 switches between brittle failure and vis-
 cous flow as a function of imposed stress
 or strain rate. The following features
 are characteristic:
 1. The veins crosscut the foliation and
 all pre-existing structures;
 2. The veins have formed from tensile
 fractures, with a typical length on
 the order of 10−1 to 101 m. Vein
 orientation is uniform on the kilo-
 meter scale;
 3. Some veins branch symmetrically
 with an aperture angle of 30°, which
 is interpreted to indicate high en-
 ergy dissipation rates and crack tip
 propagation velocities approaching
 the terminal velocity similar to the
 Raleigh wave speed;
 1 Ruhr Universität Bochum, Institut für Ge-
 ologie, Mineralogie und Geophysik
 4. Fabrics of the vein quartz indicate
 that the veins formed during a sin-
 gle sealing stage by mineral precip-
 itation in open cavities;
 5. The veins show a low aspect ratio
 of about 10 to 100 and an irregular
 or characteristic lenticular shape,
 which requires distributed ductile
 deformation of the host rock;
 6. The sealing quartz crystals reveal a
 broad spectrum of microstructural
 features indicative of crystal plas-
 tic deformation at temperatures of
 about 300 to 350°C and high stress.
 7. Fluid inclusions entrapped in vein
 quartz reveal a markedly sublitho-
 static pore fluid pressure during
 crack sealing.
 Fractures propagated in a single step.
 Therefore, fluid overpressure as the only
 source of crack-driving energy is ex-
 cluded. The drop in pore-fluid pressure
 related to incremental growth would
 cause arrest of the fracture until re-
 covery. Hydraulic fracturing is there-
 fore expected to result in cyclic vein-
 ing, which is not observed in the present
 case. Opening of the fractures, com-
 mencing immediately after crack arrest,
 was controlled by ductile deformation of
 the host rock. Vein-parallel shortening
 is less than about 2%. The structural
 and microstructural record reflects an
 isothermal switch from short-term brit-
 tle failure at quasi-instantaneous load-
 ing to decelerating viscous creep with
 little strain accumulated. Individual
 veins are the result of a single sequence
 of events:
 1. A major stress peak is imposed to
 the uppermost part of the plasto-
 sphere, probably as a consequence
 1
Nüchter and Stöckhert TSK 11 Göttingen 2006
 of co-seismic loading by fault dis-
 placement in the overlying schizo-
 sphere (timescale is seconds).
 2. Fractures presumably initiate
 after a stage of enhanced stress
 corrosion with sharpening of ear-
 lier blunt flaws, resulting in a
 rise of the stress intensity factor
 KI (timescale is not certain, but
 estimated to be between seconds
 and days).
 3. Flaw sharpness rises abruptly after
 critical fracture initiation and re-
 sults in a KI peak. The fracture
 propagation velocity increases ex-
 tremely in this early stage. Dilation
 during fracturing causes an instan-
 taneous drop in pore fluid pressure;
 this implies a drop in driving force
 and consequently an arrest of the
 fractures. The time span from ini-
 tiation to arrest must be a few mil-
 liseconds.
 4. After fracture arrest, decelerating
 viscous deformation during post-
 seismic stress relaxation causes the
 opening of the fractures. Sealing of
 the fissures to become a vein takes
 place by precipitation of minerals
 from the pore fluid percolating into
 the evolving cavity. This process is
 estimated to take some time on the
 order of 100 to 104 years.
 Opening of fractures and development
 to a vein is therefore interpreted to be
 a short-term and episodic process dur-
 ing a stage of post-seismic creep. The
 record of the exhumed rocks provides
 insight into earthquake related dam-
 age in the uppermost plastosphere and
 transient crustal properties during post-
 seismic creep and stress relaxation.
 2