HR: 17:00h
AN: T14A-05    [Abstracts]
TI: Stress generation and hierarchical fracturing in reactive systems
AU: * Jamtveit, B
EM: bjorn.jamtveit@geo.uio.no
AF: PGP, University of Oslo, P.O.Box 1048 Blindern, Oslo, N-0316, Norway
AU: Iyer, K
EM: k.h.iyer@fys.uio.no
AF: PGP, University of Oslo, P.O.Box 1048 Blindern, Oslo, N-0316, Norway
AU: Royne, A
EM: anja.royne@fys.uio.no
AF: PGP, University of Oslo, P.O.Box 1048 Blindern, Oslo, N-0316, Norway
AU: Malthe-Sorenssen, A
EM: malthe@fys.uio.no
AF: PGP, University of Oslo, P.O.Box 1048 Blindern, Oslo, N-0316, Norway
AU: Mathiesen, J
EM: joachim.mathiesen@fys.uio.no>
AF: PGP, University of Oslo, P.O.Box 1048 Blindern, Oslo, N-0316, Norway
AU: Feder, J
EM: feder@fys.uio.no
AF: PGP, University of Oslo, P.O.Box 1048 Blindern, Oslo, N-0316, Norway
AB: Hierarchical fracture patterns are the result of a slowly driven fracturing process that successively divides the rocks into smaller domains. In quasi-2D systems, such fracture patterns are characterized by four sided domains, and T-junctions where new fractures stop at right angles to pre-existing fractures. We describe fracturing of mm to dm thick enstatite layers in a dunite matrix from the Leka ophiolite complex in Norway. The fracturing process is driven by expansion of the dunite matrix during serpentinization. The cumulative distributions of fracture lengths show a scaling behavior that lies between a log – normal and power law (fractal) distribution. This is consistent with a simple fragmentation model in which domains are divided according to a ‘top hat' distribution of new fracture positions within unfractured domains. Reaction-assisted hierarchical fracturing is also likely to be responsible for other (3-D) structures commonly observed in serpentinized ultramafic rocks, including the mesh-textures observed in individual olivine grains, and the high abundance of rectangular domains at a wide range of scales. Spectacular examples of 3-D hierarchical fracture patterns also form during the weathering of basaltic intrusions (dolerites). Incipient chemical weathering of dolerites in the Karoo Basin in South Africa occurs around water- filled fractures, originally produced by thermal contraction or by externally imposed stresses. This chemical weathering causes local expansion of the rock matrix and generates elastic stresses. On a mm to cm scale, these stresses lead to mechanical layer-by-layer spalling, producing the characteristic spheroidal weathering patterns. However, our field observations and computer simulations demonstrate that in confined environments, the spalling process alone is unable to relieve the elastic stresses. In such cases, chemical weathering drives a much larger scale hierarchical fracturing process in which fresh dolerite undergoes a continuous domain division that effectively regenerates fresh surfaces. This process produces the characteristic weathering patterns seen both in Karoo and a wide-range of other geological environments. In summary, hierarchical fracturing leads to a continuous production of fresh reactive surface area during hydration processes such as serpentinization and weathering, and provides first-order rate control during both serptinization and weathering. It thus has wide ranging implications for global geochemical budgets, landscape evolution, and a number of other important geological features.
DE: 1039 Alteration and weathering processes (3617)
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3653 Fluid flow
DE: 4460 Pattern formation
DE: 8010 Fractures and faults
SC: Tectonophysics [T]
MN: 2007 Fall Meeting