| Abstrakt: |
The brittle‐plastic transition (BPT), the strongest part of the crust, is critical to continental geodynamics but is poorly understood relative to simpler crust above and below. It is typically represented as a depth transition from brittle/frictional to plastic/viscous deformation controlled by temperature and pressure. Footwalls of low‐angle normal faults (LANFs) exhumed through the BPT provide rock records that challenge this view. Three well‐studied LANF footwalls are reviewed. All record geochemical, mineralogical and fluid‐related controls on embrittlement, not just monotonic P‐T decrease. Two quartz‐rich examples record embrittlement at unexpectedly high T (≥450–500°C) that was modulated by wetting characteristics of fluids. One had an inverted BPT: brittle fracture beneath contemporaneous mylonites. In another study, a brittle LANF grew from plastic mylonites due to mineralogic changes that strengthened parts, causing initial frictional slip and cataclasis on weak planes that ultimately linked. In all, geologically abrupt small‐scale processes controlled behavior at kilometer scales. Similar processes likely affect other tectonic settings and seismic cycles. Such processes offer fertile research opportunities in continental geodynamics; they will be increasingly tractable as computational abilities improve. Adaptive, multi‐scale approaches including the effects of fluid‐rock geochemistry and mineralogical changes on rock strength and deformation are needed. Thoughtful modeling approaches may yield key insights into the positive and negative feedbacks that are likely. Discontinuous deformation is probably needed explicitly along with exploration of initial and boundary conditions. Plain Language Summary: The strongest part of continental crust is the brittle‐plastic transition (BPT) in the midcrust at ∼10–20 km depth. There, brittle behavior (fault slip, fracturing, fragmentation), which increases in strength downward, gives way to "ductile" behavior dominated by crystal plasticity of quartz, which decreases in strength downward. The BPT is less well understood than either brittle crust above or plastic crust below. The transition is generally modeled and thought of as being dominated by a downward increase in temperature and pressure, but studies of faults that evolved from being plastically deforming shear zones to brittle slip surfaces suggest that abrupt and localized geochemical, mineralogical and fluid processes are also very important. Such processes are currently little represented in numerical geodynamic models of the crust. Their inclusion in future models will probably lead to important insights into crustal geodynamics. Key Points: A detailed study of the evolution of three large‐slip low‐angle normal faults in the continental brittle‐plastic transition shows that, in addition to monotonic decreases in pressure and temperature, diverse, localized, and/or geologically abrupt geochemical, fluid‐related, and mineralogical processes were critical to fault evolutionThese processes commonly occurred over small spatial (sub‐millimeter to several meter) and temporal (seconds to years?) scales but affected geodynamic factors such as crustal strength and deformation style on larger and longer (kilometer and Myr) scalesSuch processes are presently only rarely incorporated into geodynamic models. Anticipated improvements in modeling methods and computational ability should allow their future incorporation, which will likely provide fresh insights into crustal geodynamics [ABSTRACT FROM AUTHOR] |
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