Regional Tectonics

1. Regional Tectonics Mihai Ducea Geos 425/525 Fall 2014 LECTURE 1, 37 slides, 75 min

2. Who takes this class and why • Target: Geosciences majors and graduate students working on geologic problems from a variety of approaches, geophysics, geochemistry, structure, petrology; • Goals: to provide a foundation for understanding tectonic processes at the kilometer to hundreds of kilometer scale, I.e. the scale of orogenic belts

3. What is tectonics Tectonics is the integrated study of large scale processes on Earth, formation of mountain belts, basins, the relationships between continents and ocean basins, and between different Earth layers.

4. Applied tectonics • Extremely important in understanding the distribution of natural resources; • Has impact on human infrastructure, from providing predicting/ forecasting/ mitigating info on hazards;

5. Length scales • Global - plate tectonics, the basic framework of all modern tectonic research; • “Regional”, km - 100’s km (focus of this course); • Local to micro - structural geology and other fields (stratigraphy, petrology, etc.)

6. Tectonic time scales • Seconds - earthquakes, faults moving; • Years - rates of motion at the surface meaured by geodesy; • Thousands of years - specialized geochronology. E.g. shoreline changes at plate boundaries; • Millions to tens of millions of years - the life cycle of an orogenic belt; • Hundreds of millions of years - continental assembly and fagmentation. Main focus of this course

7. Introductory concepts ( a refresher of general info) - puts our “regional” focus in perspective • The composition and planetary evolution of the Earth’s interior (today) • Basic tenets of the theory of plate tectonics, with special emphasis on what will be used in this course (the following two lectures, lecture 2 and 3).

8. The Earth’s Interior • Fields that constrain the composition of Earhs’ interior: - Astronomy and physics - Seismology - Geochemistry and planetary geochemistry

9. assumed density increased smoothly with depth due to increase of pressure with depth estimate for center of Earth was 10,000-12,000 kg/m3 differentiation of Earth early in its history breakthrough came with idea that seismic waves generated by earthquakes could travel through the entire Earth and be recorded elsewhere on the surface: seismology

11. earthquake seismic station different behavior of P and S waves led to idea of liquid layer in interior seismic station travel paths of seismic waves generated by earthquakes; directly through the earth or reflected by discontinuities S waves cannot travel through liquid creates S shadow zone from: http://www.seismo.unr.edu/ftp/pub/louie/class/plate

12. arrival times and paths of P waves …paths are curved due to refraction through material whose ability to transmit seismic energy increases with depth… …Snell’s Law… …use to generate velocity models (how seismic velocities vary)… e.g. PREM: Prelimary Reference Earth Model from http://earth.leeds.ac.uk/dynamicearth/internal/

13. P, a P-wave in the mantle S, an S-wave in the mantle p, a P-wave reflected from the surface of the earth close to the earthquake hypocenter s, an S-wave reflected from the surface of the earth close to the earthquake hypocenter K, a P-wave through the outer core I, a P-wave through the inner core J, an S-wave through the inner core c, a reflection from the mantle-outer core boundary i, a reflection from the outer core-inner core boundary PKIKP, started as P-wave, passed through mantle and outer core, then through inner core and up through outer core and mantle sSP, travelled as S-wave to earth's surface close to earthquake focus, reflected, then travelled through mantle as S-wave, was reflected again at surface, and converted to P-wave and travelled through mantle

14. now have a good idea of seismic velocity with depth seismic discontinuities define: crust; upper mantle; transition zone; lower mantle outer core; inner core indicate changes in physical properties with depth (mostly density and elastic modulii) still only a model to fit existing measurements…. debate continues on exact position of discontinuities from: http://www.personal.umich.edu/~vdpluijm/gs205.html

15. The core is not involved in mass exchange with the outer layers.

16. Structure of mantle revealed by S-wave velocity • Moho • Strong lithosphere • Weak asthenosphere with zero to small fraction partial melt • Phase changes—olivine / spinel / perovskite

17. why density variation in Earth? • changes in chemical composition (compositional changes) • changes in mineral structures (phase changes) what changes occur where is a large area of research… ……cannot make direct observation! draw from geochemistry, mineral physics, meteoritics, igneous petrology, seismology crust: felsic (shallow) to mafic mantle: ultramafic (peridotite) outer core: liquid iron alloy inner core: solid iron alloy crust/mantle: Mohorovicic discontinuity (Moho)--compositional mantle/core: Gutenberg discontinuity--compositional inner/outer core: phase (liquid to solid) 400 km discontinuity: phase (olivine to spinel structure) 670 km discontinuity: phase (spinel structure to perovskite)

18. crust and mantle (remember that they are distinguished on the basis of their physical properties) how do we know what is at depth? electrical conductivity: identifies partial melts exposed deep crust: occurs in mountain belts; 50 km originally geochemistry and elemental abundances: tell range of composition gravity anomalies: identifies density differences lithospheric flexure: constrains rheology magnetic anomalies: shows distribution of subsurface rocks mineral physics: measures seismic velocities in rock samples ophiolites: represents oceanic lithosphere xenoliths in volcanic rocks: represents upper mantle seismic reflection: identifies changes in lithology seismic refraction: defines velocities of seismic waves at depth seismic tomography: permits 3D visualization

19. crust obvious from space that Earth has two fundamentally different physiographic features: oceans (71%) and continents (29%) from: http://www.personal.umich.edu/~vdpluijm/gs205.html global topography

20. bimodal distribution of topography is best illustrated with a hypsometric curve (cumulative frequency curve) from: http://www.personal.umich.edu/~vdpluijm/gs205.html high mountains and deep trenches are only a small portion two modes (left) or two plateaus (right) on curve with little transition continental crust: 1000 m oceanic crust: -4000 m

21. why bimodal distribution? differences in composition and thickness of oceanic and continental crust oceanic crust: mafic; denser continental crust: felsic; less dense isostasy: columns of mass must be the same at a certain depth (compensation depth) ~ 50 km continents have roots and stick-up from: http://www.personal.umich.edu/~vdpluijm/gs205.html

22. Geochemistry and planetary geochemistry • The Earth and Solar System are some 4.5 Ga old; • The interior of the planet is primarily a silicate, peridotite; • Makes up the bulk of the Earth’s mantle; • The core is made of Fe (and Ni, maybe others), which differentiated early in the history of the planet; • The crust is differentiated via magmatism; oceanic crust is easier to understand geochemically; • All but the continental crust are found elsewhere in the Solar system and in meteorites;

23. What is a peridotite? • Mg, Si, Fe - rich silicate rock composed predominantly of olivine, followed by orthopyroxene and clinopyroxene, plus an aluminus phase (plagioclase, spinel, or garnet); • Has typically annealed textures, but could be defomed, banded, or even mylonitic; • Think of them as being metamorphic (solid) rocks of the Earth’s interior; occasionally they partially melt and when they do, they lose a fraction of the “lithophile” elements, those that want to go in the crust; • The lower the amount of clinopyroxene, the more “depleted” they are - depleted of melt that is; that is because clinopyroxene, which is resembling a basalt in its chemistry, is what get consumed first during melting; • At greater depths in the Earth, this bulk chemistry remains, but the phases change, adjusting to the higher lithostathic pressures and higher temperatures.

24. All the rocks above the peridotie-websterite line are defined as “peridotites”, but on average, real peridotites found at the Earth’s surface. occupy the yellow area. Classification of ultramafic rocks

25. Meteorites

26. Meteorite summary • A variety of sources extraneous to Earth in the Solar System; • Show the predominantly peridotitic composition of the silicate matter; • Some specimens are iron-rich or entirely Fe and they are taken to represent the equivalent of the Earth’s core; • Chondrites and carbonaceous chondrites contain minerals that resemble the Earth’s crust, among others. They are still rich in Mg, Si, and Fe, overall mantle-like; • Whatever their source, they likely came from relatively shallow parts of solid bodies in the Solar system; they do not reflect equilibrium at many hundreds or thousand of km in a planetary interior.

27. Xenoliths- direct samples of the Earth’s (shallow) mantle Found in some basaltic lavas, very few percentage-wise; Represent “accidental” solid wall-rock fragments; Were recorded to be from as deep as 250 km, no more; Predominantly peridotites, but also pyroxenites, and other mixed ultramafic rocks; Contain plagioclase, spinel, or garnet, depending on the depths of origin; The most direct samples of the Earth’s mantle. When in very young basalts, can be used in conjunction with seismologic studies to study physical properties of regional mantle; Their composition is fully consistent with seismologic and chemical (meteorite) lines of evidence regarding the composition of the Earth’s interior.

28. The Interior of the Earth must be bright green… San Carlos, AZ/

29. Observations from volcanic rocks • Volcanic rocks, especially basaltic rocks, provide indirect information regarding the composition of the mantle, when used in conjunction with experimental data; • Mantle temperatures, and various other parameters can be predicted through studies of basalts;

30. Heat engine- very efficient Earth differentiation- primarily by magmatism Mantle convection- Mostly solid state Melting shallow by adiabatic decompression Lithosphere- the cold lid at the top

32. There are strong hints for the existence of a relationship between surface tectonics and the deep interior beneath North America.

33. Mantle convection • Time scales • Length scales • Plumes

34. T - scale ~ plate motions Length scales - appear much more complicated than the ridge-trench systems

35. Surface interaction of convective cells • Plate boundary -related (mid-ocean ridges, subducting plates) - will discuss next lecture • Non-plate boundary-related; plumes hitting the surface in random “hot spots”; can be very vigorous and break other plates, or not; can be short-lived (<1 My) or persist for many tens of million of years (e.g. Hawaii); can generate flood basalt provinces (see next slide).

37. Next 2 lectures • Plate tectonics