1. METAMORPHISM

• response of rocks to increasing heat and pressure, involves changes in mineralogy, texture, and overall bulk chemistry in the solid state (200-800°C). Diagenesis grades into.
• New minerals form at specific P-T conditions by recrystallization, geothermobarometry; increased grain size, mineral alignments and layering.
• Concepts of low- and high-grade metamorphism.

• internal heat - ranges from 20-60°C/km depending on tectonic setting
• confining and directed pressures - ~300 bars/km, denser minerals form, platy (micas) and elongate (hornblende) minerals are aligned to form metamorphic foliation.
• fluids - mostly H2O- and CO2-rich; contain dissolved ions (Na, K, Si), and other soluble metals (Zn, Cu, Au, Ag) and trace ions (Rb, Ba, Sr). Derived from the breakdown of clay minerals (clays) and carbonates in sed rocks. Percolate through grain boundaries or open cracks. Helps metamorphic reactions proceed; explains abundance of qtz and calcite veins in low-grade rocks; causes chemical changes (ie. metasomatism)

• regional - usually moderate to high P-T over large regions, subduction zones, volcanic arcs, associated with orogenic events (ie. mountain-building).
• contact - thin zone around igneous intrusions or below lava flows; usually high T-low P.
• cataclastic - metamorphism due to motion on faults or in shear zones.
• hydrothermal - hot (up to 400°C) fluids expelled near MOR, also associated with fluids expelled from igneous intrusions and high-grade regional metamorphism.
• burial - low-med grade; continuation of diagenesis; deeply buried sediments in sed basins.

• foliation and cleavage - produced by directed pressures that align platy or elongate minerals (micas, chlorite, amphiboles). Preferred orientation relative to compressive or shear stresses. Occurs through recrystallization and/or physical rotation of grains. Slaty cleavage; schistocity, layer banding (gneissocity).
• non-foliated - no or weak preferred orientation; equant mineralogy; defined by composition, not texture (ie. marble, quartzite, amphibolite, granulite).
• large crystal - very large crystals (porphyroblasts) set in finer (usually mica-rich) matrix; garnet- or staurolite-schists. Due to differential nucleation and growth rates.
• deformation textures - produces crushed and pulverized, fine-grained textures.

• Characteristic mineral assemblages are used as gauges of P-T conditions (ie. metamorphic grade). Index minerals define zones that experienced similar P-T conditions (ie. chlorite; biotite; garnet; sillimanite). Zone boundaries on a geologic map are called isograds and mark the first appearance of a particular index mineral.

• Mineral assemblages depend partly on original parent rock; thus different minerals may form in a shale and basalt at the same P-T conditions. Concept of metamorphic facies; simply the P-T conditions regardless of original parent rock.
• Concept of geothermal gradients. Low T-high P (blueschist) at forearc of subduction zones; high T-low P facies in the interior of volcanic arcs in subduction zones. This gives "paired" metamorphic belts along subduction zones. Low to high T- mod to high P (greenschist to granulite; regional metamorphism) = continental collisions (ie. Alps, Tibet, Appalachians).

Prof. Gorring
GEOS 112 PHYSICAL GEOLOGY
Oct. 1, 1998

1. ROCK DEFORMATION
-types of forces; relation to tectonics; brittle and ductile deformation; factors that dictate type of deformation (T, P, and rock strength)
 

2. FOLDS

• fold anatomy and types - anticline/syncline, asymmetrical, overturned, plunging.
• domes and basins

• joints - have no relative motion; formed tectonic forces or pressure release.
• Fault types - normal, reverse, strike-slip, oblique-slip; thrust faults

• Fold-Thrust belts- Appalachians, convergent settings, Mtn building.
• Extensional terranes - western US (Basin & Range); E. African Rift Valley .