I. Introduction to Structural Geology -Ge106-

Instructor:Jean-Philippe Avouac (301NM; avouac@gps, caltech.edu) Teaching Assistant:Erika Swanson (eswanson@caltech.edu) Administrative Assistant:Heather Steele (302NM, heatherg@gps.caltech.edu) Meetings:Location: MN215Lectures: Tuesdays, Wenesdays and Thursdays, 3:00-4:00 p.m.

Synopsis:- This course is an introduction to the fundamentals of structural geology. It is designed mainly for students with an elementary background in mathematics and physics at the sophomore level. - The components of the course include lectures, problem sets, a lab and a final exam. -Each student will have to make a presentation during the last week of instruction.Grading: •Problem Sets 40%•Participation to class and labs 40%•Final Exam 30%

I. Introduction to Structural Geology -Ge106-

Textbooks

Sources used in Class Material

The material in the presentions was

• kindly provided by coleagues;– John Suppe (for a great deal)

• downloaded from the ‘web’;

• or extracted from publications.

Hopefully sources from the web and literature ar appropriately credited (my apologies to the authors if not)

What’ the purpose of Structural Geology?

Structural Geology provides techniques to help interpret geological and geophysical data in 3D and 4D (‘what is the current geometry and how did we get there?)

Data: – Surface geology (intersection of 3-D bodies with

the 2-D topographic surface)– Subsurface data: seismic, gravity, well logs…

(hardly scratching the uppermost layers of the Earth..)

What is Structural Geology?

• A descriptive, but quantitative, discipline.

• Output:– Geometric model– Kinematic model

• Structural geology concerns all scales.

• Some amount of structural geology is necessary in nearly all areas of geology.

• ‘Structural Geology’ and ‘Tectonics’ are intimately linked

• In some instances, determining the geometry of rock structures is an end in itself. For example for geological resources exploration (oil and gas, ore deposits, ground water ressources …)

• In Tectonic studies the objective is generally a kinematic mode, that is intended to provide insight on the mechanisms governing deformation.

• In some other instances it provide a geometric and kinematic framework to help interpret non-structural data (Seismic Hazard studies, most academic studies).

Class Outline

I. Deformation of the lithosphere, an overview

II. Basic Techniques in Structural Geology and Tectonics

III. Basic of Continuum mechanics

IV. Geological strain

V. Deformation Mechanism

VI. Joints and Fault mechanics

VII. Architecture and structural evolution of orogenic belts

Deformation of the Lithosphere, an overview

• Basics of Plate Tectonics. A convenient framework.

World seismicity (data source: USGS) and velocities relative to ITRF1997 at Geodetic sites (Sella et al, 2000)

The distribution of seismicity, quaternary faulting and geodetic displacement shows, to fisrt order a ‘plate-like’ behavior.

• Why do we have tectonics on Earth and how does it relate to Earth internal structure and dynamics?

(Mackwell et al., 1998)

‘Strength envelopes’ as inferred from laboratory laws.

- The ‘plate like behavior’ of the lithosphere results from its strength at shallow depth- The Lithosphere deforms as a result of both brittle and ductile processes.

(Jackson, 2001)

NB: Plate tectonics could develop on Earth due to 1-the strong viscosity contrast between the Lithosphere and the underlying mantle. This contrast is due to low surface temperature and presence of water in mantle2- strain weakening behvior of the lithosphere itself (again water is key)

The Earth Structure as seen from seismic tomography

Due to chemical and temperature induced variations of density, the Earth is not in an hydrostatic equilibrium. This results in viscous convective flow in the mantle (where temperature is high enough to promote advection). -The Lithosphere is a thermal boundary layer across which heat is transferred by conduction. - Due its low temperature it exhibits a plastic like behavior with localized zone of deformation separating ‘rigid’ plates. Plates move due to the tectonic forces induced by internal load (gravity), and forces applied along plate edges and bottom.

World seismicity (data source: USGS) and velocities relative to ITRF1997 at Geodetic sites (Sella et al, 2000)

A system of connected plate boundaries:• Subduction Zones (converging)• Mid-Oceanic Ridges (diverging)• Transform Faults (horizontal shear)

Transform fault

Mantle convection and plate tectonics are coupled processes but probably not as as this cartoon suggests (there is no evidence for mantle upwelling beneath mid-oceanic ridges).….

Transform fault

I. Deformation of the Lithosphere, an overview

• Basics of Plate Tectonics. A convenient kinematic framework.– Current Kinematics

12 major lithospheric plates

USGS

I. Deformation of the Lithosphere, an overview

• Basics of Plate Tectonics. A convenient kinematic framework.– Current Kinematics– Structures related to plate boundary

processes and longer term kinematics

Ocean-ocean convergence

• Subduction – one oceanic plate subducts (dives) beneath the other

• Volcanic island arc is formed

Ocean-continent convergenceDuring ocean-continent convergence, oceanic

lithosphere subducts beneath continental lithosphere

Continental volcanic arc formedThe continental margin often deforms

‘Active continental margin’

‘Mantle wedge’

‘Active continental margin’

From continental rifting to the development of a an oceanic basin

Passive continentalmargin

Continental shelf

Structure of a passive margin (rifted continental margin)

Continental slope

Continental rise

12 major lithospheric plates

USGS

At this point we have described Solid Earth surface dynamics in terms of rigid bodies, ‘Plates’, undergoing tangential displacements on the sphere and separated by narrow plate boundaries.

• Are plate interiors really rigid bodies?

• What about radial displacements?

ContinentsOceans

The Earth topography is highly bimodal

NB: the computation in b ignores isostasy and crustal flow.

-The fact that continental topography is not even despite erosion, is clear evidence for radial displacements within continents. - Mountain range are a clear demonstration that the continental crust can experience distributed ‘regional’ deformation.

(courtesy of James Jackson)

(courtesy of James Jackson)

World topography and bathymetry