Rock Structure and Fault Activity

Rock Structure and Fault Activity chapter 9

What is structural geology The study of the forms of the Earth’s crust and the processes which have shaped it • analysis of displacement and changes in shape of rock bodies (strain) • reconstruct stress that produced strain

Structural Deformation Rocks deform when stresses placed upon them exceed the rock strength • Brittle deformation (e.g. fractures) • ductile deformation (e.g. folds)

Driving Forces • Plate tectonics – plate convergence and ridge spreading • Deep burial of sediments • Forceful intrusion of magma into the crust • Meteorite impacts

Evidence of Crustal Deformation • Folding of strata • Faulting of strata • Tilting of strata • Joints and fractures

Applications of structural geology • subsurface exploration for oil and gas • mining exploration • geotechnical investigations • groundwater and environmental site assessment

Geological structures • Geologic bed contacts • Primary sedimentary structures • Primary igneous structures • Secondary structures

Fundamental Structures Three fundamental types of geologic structures: • bed contacts • primary structures - produced during deposition or emplacement of rock body • secondary structures - produced by deformation and other process after rock is emplaced

Bed Contacts Boundaries which separate one rock unit from another • two types: 1. Normal conformable contacts 2. Unconformable contacts (‘unconformities’)

Conformable Bed Contacts Horizontal contact between rock units with no break in deposition or erosional gaps • no significant gaps in geologic time Book Cliffs, central Utah

Unconformable Contacts Erosion surfaces representing a significant break in deposition (and geologic time) • angular unconformity • disconformity • non-conformity

Angular Unconformity Bedding contact which discordantly cuts across older strata • discordance means strata are at an angle to each other • commonly contact is erosion surface

Formation of an angular unconformity

Disconformity Erosional gap between rock units without angular discordance • example: fluvial channel cutting into underlying sequence of horizontally bedded deposits

Nonconformity Sedimentary strata overlying igneous or metamorphic rocks across a sharp contact • example: Precambrian-Paleozoic contact in Ontario represents a erosional hiatus of about 500 ma Grand Canyon, USA

Structural Relations The structural relations between bed contacts are important in determining: 1. presence of tectonic deformation/uplift and; 2. relative ages of rock units • principle of original horizontality • principle of cross-cutting • principle of inclusion

Principle of Original Horizontality Sedimentary rocks are deposited as essentially horizontal layers • exception is cross-bedding (e.g. delta foresets) • dipping sedimentary strata implies tectonic uplift and tilting or folding of strata

Principle of Cross-cutting Igneous intrusions and faults are younger than the rocks that they cross-cut Mafic dike cutting across older sandstones

Cross-cutting Relations Often several cross-cutting relationships are present • how many events in this outcrop?

Principle of Inclusion Fragments of a rock included within a host rock are always older than the host

Fundamental Structures Three fundamental types of structures: • bed contacts • primary structures • secondary structures

Primary Sedimentary Structures Structures acquired during deposition of sedimentary rock unit Stratification- horizontal bedding is most common structure in sedimentary rocks

Primary Sedimentary Structures Cross-bedding - inclined stratification recording migration of sand ripples or dunes

Primary Sedimentary Structures Ripples - undulating bedforms produced by unidirectional or oscillating (wave) currents

Ripple marks

Primary Sedimentary Structures Graded bedding- progressive decrease in grain size upward in bed • indicator of upwards direction in deposit • common feature of turbidites

Primary Sedimentary Structures Mud cracks - cracks produced by dessication of clays/silts during subaerial exposure

Primary Sedimentary Structures Sole marks- erosional grooves and marks formed by scouring of bed by unidirectional flows • good indicators of current flow direction

Primary Sedimentary Structures Fossils – preserved remains of organisms, casts or moulds • good strain indicators • determine strain from change in shape of fossil • relative change in length of lines/angle between lines

Primary Igneous Structures Flow stratification • layering in volcanic rocks produced by emplacement of successive lava sheets • stratification of ash (tephra) layers

Primary Igneous Structures Pillow lavas - record extrusion and quenching of lava on sea floor

Importance of Primary Structures 1. Paleocurrents - determine paleoflow directions 2. Origin – mode of deposition, environments 3. Way-up - useful indicators of the direction of younger beds in stratigraphic sequence 4. Dating - allow relative ages of rocks to be determined based on position, cross-cutting relations and inclusions 5. Strain indicators - deformation of primary structures allows estimates of rock strain

Secondary Structures Secondary structures - deformation structures produced by tectonic forces and other stresses in crust Principle types: • fractures/joints • faults/shear zones • folds • cleavage/foliation/lineation

Fractures and Joints Fractures – surfaces along which rocks have broken and lost cohesion Joints - fractures with little or no displacement parallel to failure surface • indicate brittle deformation of rock

Fractures and Joints

Faults • Faults - fracture surfaces with appreciable displacementof strata • • single fault plane • • fault zone - set of associated shear fractures • • shear zone - zone of ductile shearing

Shear Zones • Shear zone - zone of deformed rocks that are more highlystrained than surrounding rocks • • common in mid- to lower levels of crust • • shear deformation can be brittle or ductile

Fault Terminology Hanging wall block- fault block toward which the faultdips Footwall block - fault block on underside of fault Fault plane – fault surface

Fault Slip Slip is the fault displacement described by: • direction of slip • sense of slip • magnitude of slip

Fault Types Dip-slip faults - slip is parallel to the fault dip direction normal reverse thrust

Fault Types Normal fault - footwall block dispaced up

Fault Types Reverse (thrust) fault - footwall block displaced down

Fault Types Strike-slip – fault slip is horizontal, parallel with strike ofthe fault plane • right-handed (dextral) • left-handed (sinistral)

Fault Types Oblique slip – Combination of dip- and strike-slip motion • dextral-normal • dextral-reverse • sinistral-normal • sinistral-reverse

Faults What type of faults are shown here?

Rock Structure and Fault Activity

Rock Structure and Fault Activity

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