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Deformation and Brittle Fracture
I. Primary Rock Structure
A. Tectonic Control of Rock Structure
- Lithospheric Plates
a. plate = crust + upper mantle above asthenosphere (1) Layered Crust (a) oceanic crust = i) volcanic/"basalt" ii) oceanic sediments (b) continental crust i) crystalline/"granite" ii) sedimentary cover a) marine sediments b) continental sediments (c) depth of crust: 5-10 km
b. plate motion response to internal heat flux (1) internal heat source: radioactive decay (2) plate motion = rock motion = "geologic work" (a) work = (force)(displacement) (b) plate motion = force applied to rocks
B. Primary Rock Structure/Architecture
- Primary Structure/Architecture:
a. geometries formed at the time of rock origin (1) deposition or crystallization b. Secondary Structure (1) geometries imparted by strain in response to stress applied after the rock forms
- Rock Layering (1) Tendency for layered stratification in volcanic and sedimentary environments (2) Layered Plutonic Rocks (a) magmatic differentiation processes (b) diapiric intrusives (3) Deposition under force of gravity = (a) "blanket deposits" (b) ~parallel to horizontal
- Examples of Primary Rock Structure
a. Stratified bedding (volcanic or sedimentary) (1) beds = layers > 1 cm (2) laminae = layers < 1 cm
b. bedding contacts = breaks or discontinuities in depositional
process (1) Textural and compositional discontinuities (2) stratigraphy = study of time with respect to the rock record
c. Unconformities = significant bedding contacts (1) = break or gap in rock-stratigraphic record (a) erosional truncation or removal (b) periods of non-deposition
d. Internal stratification: internal organization of sediment within bed
(1) Graded Bedding: a layer of sediment in which particle sizes change systematically in a vertical and/or lateral direction (applicaple to both beds and laminae)
e. Cross-bedding: Sets of internal strata, beds and/or laminae are not oriented parallel to the bounding surfaces of the bedset. Applicable to both beds (>1 cm) and laminae (<1 cm).
(1) Tabular Cross-stratification: bounding surfaces are planar (2) Trough Cross-stratification: bounding surfaces are curved and intersecting
f. Common Primary Sedimentary Structures
(1) Current Ripples: found at relatively low flow strengths in sands less than 0.7 mm in diameter, asymmetric in cross- sectional profile.
(2) Flute Casts: current-formed erosion structure, bulbous cast formed by scouring of sediment interface, bulbous end generally points up-current.
(3) Load Casts: irregular knobs found on sandstones overlying shale beds.
(4) Tool Marks: groove casts; infilling of mold formed by dragging object across sediment interface
(5) Imbricate Structure (Pebbles): under high energy flow conditions pebbles may take on inclined imbricate orientation, with inclination pointing upstream.
(6) Brush, Bounce, and Prod Marks: formed by dragging of sticks or pebbles across sediment interface.
(7) Scour and Fill (Gutter Casts): channelized sediments can become back-filled with overlying deposits.
g. Common Deformational Structures in Sedimentary Rocks
(1) Convolute Laminations/Slump Structure: complexly folded
(b) youngest at top
b. Problem: Structurally Overturned Beds
(1) Intense rock folding may result in overturning of beds... (a) stratigraphic up may be oriented down
c. Indicators of up
(1) Fossil shell lags: convex portion of shell commonly up (2) Paleontological relations/stratigraphy (3) Ripple laminations: crest = up (4) Mudcracks: curved up morphology (5) Planar cross-bed morphology (a) commonly asymptotic at base of bed (6) sole markings, scour features on base of beds (7) vertical burrows: commonly from top, down (8) Vesiculated zones in volcanic rocks: commonly at top (9) Pillow morphology in volcanic rocks (10) erosional lag conglomerates; commonly overlie abrupt erosional discontinuities
II. Overview of Secondary Rock Deformation
A. Process: subjecting primary rock sequences to tectonic stresses over time
- Result: rock deformation
B. Critical Factors
- Driving Forces:
a. Heat transfer, plate tectonic motion b. Gravity: downward compression
- Physical Conditions
a. Temperature (1) depth controlled (2) internal heat flux/volcanism b. Pressure (1) depth controlled (2) horizontal plate motion c. Chemical Conditions (1) Interactions of chemically-active fluids with rock under conditions of temperature and pressure
- Resisting Medium
a. Rock Body (1) lithologic composition (a) Lithologic control of mechanical properties of rock
body (b) From Field Trip i) defomation in shale vs. limestone (2) pre-existing primary structure (a) bedding, sed. structures, etc. (3) pre-existing secondary structure (a) faults, joints, folds, etc.
- Stress vs. Strain
a. stress = force applied per unit area of rock material
(1) "vector" = magnitude + direction in space
b. Strain = deformational response of rock to stress applied
C. Deformational Products of Stress Applied to Bodies of Rock
- fractures a. vein fillings
- faults
- folding
- rock cleavage, schistosity, foliation
