Class Outline Deformation of the lithosphere, an overview Basic Techniques in Structural Geology and Tectonics Basic of Continuum mechanics- strain Basic of Continuum mechanics- stress Deformation Mechanism…

II. Basic Techniques in Structural Geology Fundamental principles Field measurements and mapping Terminology on faults and folds Stereographic projections Reading and preparing maps and constructing cross-sections Seismic Imaging

Fundamental Principles (Steno, 1669) Superposition : Younger on top of older. Initial Horizontality : Original stratigraphy is approximately flat and horizontal. Strata continuity : Beds are continuous laterally. Cross-cutting relationships : ‘If a body or discontinuity cuts across a stratum, it must have formed after that stratum.’ Pinciples 1-3 apply to sedimentary strata Principle is very general

‘Primary’ and ‘Secondary’ structures. Definition: Primary structures are those formed at the time of formation of the rock unit (sedimentary or igneous fabric). Secondary structures are those related to the deformation of the rock unit posterior to its formation. In structural geology we care about secondary structures only but you must be able to recognize both types of structures because -1- Primary structures can sometime be misinterpreted as secondary structures -2- The identification of primary structures help identify deformation.

I. Basic Techniques in Structural Geology Fundamental principles Field measurements and mapping Terminology on faults and folds Stereographic projections Preparing maps and constructing cross-sections Seismic Imaging

Field measurements Preparation of field work Field observations prior to any structural measurements Measurements at the outcrop scale Measurements on maps

Preliminary to field work Before any field survey the geologist needs to get acquainted with the study area through reviewing the existing literature (maps and articles); inspection of the topography and geography (Google Earth is great for this) Places of interest for field work can be spotted ahead of actual field survey.

Field measurements Preparation of field work Field observations prior to any structural measurements Measurements at the outcrop scale Measurements on maps

Once in the field Before detailed observations are collected it is important that the geologist be familiar with the regional stratigraphy (‘calibration’). At the outcrop scale identify: Rock units; Distinguish ‘Primary’ and ‘Secondary’ structures. Bed polarities; Unconformities and contacts (stratigraphic, intrusive or tectonic); Deformation markers; Establish the relative chronology of ‘events’ (sedimentation and deformation) based on simple logic (Steno’s principles’).

We care about bed polarities:

We care about bed polarities: We want to know if the sequence if still upward or is upside down as often happen in thrust tectonics (‘overturned beds’)

Some common indicators of bed polarities in sedimentary rocks Grading Cross-bedding stratification Bottom bed markings Flutecasts Loadcasts Ripple marks Bioturbation markers

(finer over coarser sed.) Cr Grading (finer over coarser sed.)

Graded bedding. Graded bedding results from a rapid decrease in flow velocity that causes sediment to drop out of suspension. Larger particles settle fastest, therefore they accumulate at the bottom of the bed. Houcheng Formation, Jurassic, Hebei Province, China. http://www.depauw.edu/acad/geosciences/tcope/sedstruct.html

Graded beds, load casts and flame structures Graded beds, load casts and flame structures. Rapid sediment fallout from suspension often loads underlying fine-grained sediment to the point of failure, causing foundering of the overlying sediment and formation of structures termed flames. Permian, Inyo County, California http://www.depauw.edu/acad/geosciences/tcope/sedstruct.html

Cross-bedding stratification (NB: cross beds are concave upward) Ripple morphology and terminology (Jobe, Lowe and Morris, 2012, modified from Jopling & Walker, 1968; Allen, 1971). Under normal bedload transport conditions, the lee and stoss sides of current ripples are depositional and erosional, respectively.

Cross-bedding structures

JP Avouac Cross-bedding structures in the Canyon de Chelly Sandstone (Canyon de Chelly National Park, Az). The large-scale cross bedding developed in the Permian as successive layers of fine dune sand, blowing up gentle windward dune slopes, were deposited on steeper leeward slopes where wind velocity lessened. The sandstone was deposited in an ancient desert similar to today's Sahara at the time of the Pangea supercontinent.

JP Avouac Cross-bedding structures in the Canyon de Chelly Sandstone (Canyon de Chelly National Park, Az). The large-scale cross bedding developed in the Permian as successive layers of fine dune sand, blowing up gentle windward dune slopes, were deposited on steeper leeward slopes where wind velocity lessened. The Sandstone was deposited in an ancient desert similar to today's Sahara at the time of the Pangea supercontinent.

These steep foresets are typical of eolian deposition These steep foresets are typical of eolian deposition. Late Jurassic, Liaoning Province, China. Primary structure! http://www.depauw.edu/acad/geosciences/tcope/sedstruct.html

Gilbert Delta foresets Gilbert Delta foresets. These are composite, large-scale foresets that indicate deposition into still water. The height of the foresets indicates the still water depth. Mecca Formation, California. Primary structure! http://www.depauw.edu/acad/geosciences/tcope/sedstruct.html

Herrigbone cross-stratification Herrigbone cross-stratification. Bi-directional cross beds such as these are indicative of a tidal origin. Curtis Formation, Jurassic, Utah. Which way is up? Do we really care in that case? http://www.depauw.edu/acad/geosciences/tcope/sedstruct.html

Ripple Marks. Casts of wave ripples on the base of a sandstone bed Ripple Marks. Casts of wave ripples on the base of a sandstone bed. Cretaceous, Hebei Province, China. http://www.depauw.edu/acad/geosciences/tcope/sedstruct.html

Note sharp peaks and rounded troughs

Flute casts. Flutes are caused by erosional eddies at the base of a turbulent flow. The deepest scour is on the upstream end of the flute, and the scours widen and become shallower downflow. Shiguai Formation, Inner Mongolia, China. http://www.depauw.edu/acad/geosciences/tcope/sedstruct.html

Pillow Lavas http://soer.justice.tas.gov.au/2003/image/546/index.php http://geology.about.com/od/structureslandforms/ig/pillowlava/Fresh-Pillow-Lavas.htm http://soer.justice.tas.gov.au/2003/image/546/index.php