GLOBAL GEOMORPHOLOGY: OROGENESIS; TECTONIC STRUCTURES OF NORTH AMERICA.

1. GLOBAL GEOMORPHOLOGY: OROGENESIS; TECTONIC STRUCTURES OF NORTH AMERICA. Harry Williams, Geomorphology

2. ISOSTACY RELIEF OF THE EARTH’S SURFACE 29% of the earth's surface is above sea-level - a volume of about 1.3 x 108 km3. About 13.6 km3 is eroded from continents each year; therefore it would take only about 10 million years to completely flatten the land surfaces - but the earth is about 4.6 billion years old. Clearly processes are operating to maintain RELIEF (i.e. uplift to compensate for erosional lowering). Isostatic uplift offsets some of the erosional lowering (lower density continental crust rises up as weight is removed), but isostasy can’t explain the creation of new mountain belts. This process is OROGENESIS - part of the theory of plate tectonics. Harry Williams, Geomorphology

3. Shallow structure of the earth - review. Continental crust: averages about 35 km thick. Rich in feldspars and "granitic" rocks. Oceanic crust: 5-12 km thick. Rich in iron and magnesium - basaltic rock. Upper mantle: the uppermost part of the mantle (approx. 100 km) is cooler and rigid, compared to the deeper mantle. Rich in iron and magnesium - peridotite rock (olivine and pyroxene minerals). This part of the mantle together with the overlying crust is the LITHOSPHERE (approx. the upper 100 km of the earth). The Asthenosphere: this part of the mantle, between about 100 and 700 km deep, is partially molten and capable of flowing slowly (especially between 100-200 km). Harry Williams, Geomorphology

4. The lithosphere is not continuous, but is fractured into a number of LITHOSPHERIC PLATES (about 7 major plates). The plates are moving, as shown by the arrows, at typical velocities of 1 -10 cm/year (fingernail growth rate). Harry Williams, Geomorphology

5. Orogenesis occurs mainly at convergent boundaries. DIVERGENT TRANSFORM CONVERGENT Harry Williams, Geomorphology

6. OROGENESIS AT CONVERGENT BOUNDARIES 1. Passive Margins: prior to orogenesis, the continental boundary is a PASSIVE margin. Sedimentation at passive margins reflects the progressive increase in water depth. Nearshore deposits are coarser - sand grading to silt and clay; further out on the continental shelf in clean shallow water, carbonate reefs form in tropical regions. Together these sediments are known as MIOGEOCLINAL DEPOSITS. On the deeper continental slope and rise, mostly fine-grained clastic sediments accumulate, forming SHALES and GREYWACKES. These are EUGEOCLINAL DEPOSITS. A common feature of continental margins undergoing extensive sedimentation is SUBSIDENCE, due to the weight of sediment ISOSTATICALLY DEPRESSING the crust; in this way shallow water deposits (e.g. 100’ depth) can build up thicknesses of 1000's of feet. Harry Williams, Geomorphology

7. Cordilleran-Type Orogenesis 1. Passive stage (pre-convergence) -> marginal deposits form. hills mountains cliffs ridges Harry Williams, Geomorphology

8. valleys 2. early subduction -> marginal deposits are deformed by compression. Folds and thrust faults are formed. 3. Volcanic arc forms. 4. Lateral growth by accretion; emplacement of igneous masses; metamorphism; further deformation of marginal deposits. Mountain chain begins to form. canyons deltas beaches Harry Williams, Geomorphology

9. 5. Continued uplift and deformation results from continuing plate convergence. 6. Erosion forms a sediment wedge in the backarc basin. Examples = Andes of western south America. Sedimentation in back arc basin Harry Williams, Geomorphology

10. Continental Collision-Type Orogenesis Initial growth is similar to Cordilleran-type orogenesis; however, when the continents collide one of them can not be subducted (too thick and buoyant), therefore the plates are welded together forming a SUTURE ZONE and producing a large mountain chain, containing sedimentary, igneous and metamorphic rock. 17.23a Harry Williams, Geomorphology

11. The collision that created the Himalayas is the classic example of continent-continent orogenesis. The Himalayas are very high because they are very young (geologically). Uplift continues and erosion hasn’t had long to wear the mountains down. Harry Williams, Geomorphology

12. The suturing of land masses is an example of continental growth by accretion - if a relatively small crustal fragment is accreted to a larger continent, the small addition is known as an EXOTIC TERRANE (meaning it originated elsewhere). Although tectonics can explain a wide range of continental and oceanic features, we will deal only with large-scale continental features which provide a structural framework upon which landforms develop. cordillera Exotic terranes Harry Williams, Geomorphology

13. Orogenic belt Passive margin coastal plain The “Big Picture” created by tectonics provides a large-scale structural framework upon which smaller-scale landforms develop. Harry Williams, Geomorphology

14. TECTONIC REGIONS OF NORTH AMERICA The distribution of tectonic activity around continents (including North America) usually provides a clear contrast between continental interiors and margins; and between leading margins (converging or active) and passive margins (diverging or trailing). This division provides us with distinct large-scale structural regions: Activemargin Passive margin Harry Williams, Geomorphology Harry Williams, Geomorphology 14

15. Craton: old, Precambrian (540 million+) metamorphic and igneous rocks at the "core" of continents. The metamorphic rocks are the "roots" of ancient orogenic belts, long since eroded away. Shield: the exposed part of the craton. The North American craton is exposed mainly in the Canadian Shield. Platform: adjacent to the shield, this is where the craton is covered by newer sedimentary rocks. Cratonic rocks are also called "basement rocks" because they underlie more recent rocks in Platform regions. Orogenic belts - the currently or formerly active edge of continents, severely deformed due to tectonic activity(orogenic = "mountain-building"). Passive margins - the trailing edge of continents; little or no tectonic activity. Harry Williams, Geomorphology Harry Williams, Geomorphology 15

16. The craton tends to be very stable, but has undergone broad warping in the past (because of tectonic movements around the margins) - especially in platform areas, creating basins, domes and tilted strata. View of the Canadian Shield Harry Williams, Geomorphology Harry Williams, Geomorphology 16

17. Platform Regions. Harry Williams, Geomorphology Harry Williams, Geomorphology 17

18. Deformed accretionary wedge sediments in california – a real contrast to mostly flat sedimentary strata in Platform areas such as Texas. Harry Williams, Geomorphology Harry Williams, Geomorphology 18

19. Platforms Platforms occur where the craton is covered by relatively undeformed layers of sedimentary rock, formed when seas occupied these areas (due to tilting/sinking of the craton or rise in sea level – all driven mainly by tectonics). A large part of the interior of North America consists of platform areas. Structure within the platform rocks is usually quite subdued: however, many parts of platforms are not completely flat; instead broad warping of the underlying craton has produced gently tilted beds (< 5o), domes (e.g. the Black Hills of Dakota; Ozark Mountains of Missouri) and basins (e.g. Michigan Basin, Permian Basin). Harry Williams, Geomorphology Harry Williams, Geomorphology 19

20. An exception to the generally subdued structure of platforms, resulting from plate movements, is intracratonic rifting which forms rift valleys, seas and aulacogens in platform regions. Red Sea rift(view to the south) Harry Williams, Geomorphology Harry Williams, Geomorphology 20

21. A relatively new idea in plate tectonics suggests that rifting may actually be initiated by hot spots which cause the continental crust to dome up and split into a triple rift. Commonly, one of the rift arms fails, while the other two become active and contribute to an opening linear ocean basin. So the convection cell that drives sea-floor spreading may actually be a line of "hot spots". The rift valley forms a large linear trough, bounded by fault scarps and floored by volcanic rocks. If the valley extends below sea level, it becomes the site of sedimentation. Harry Williams, Geomorphology Harry Williams, Geomorphology 21

22. The failed arm, represented by a deep linear trough into the adjacent continent, may become a natural location for a major river valley within platform areas - probable examples include the Mississippi and the Amazon. Harry Williams, Geomorphology Harry Williams, Geomorphology 22

23. It is also possible for a failed arm of a rift to become "squeezed shut" by later plate movements - forming an aulacogen. The Arbuckle Mountains lie on the northeastern margin of the southern Oklahoma aulacogen. Closure of the aulacogen resulted in severe folding and faulting of marine sedimentary rocks which had accumulated in the trough - one of the few places that platform strata are so deformed. Harry Williams, Geomorphology Harry Williams, Geomorphology 23

24. Recap: although platform rocks can be severely deformed like those in the Arbuckles, they are more usually flat or gently tilted, like these shown below in Dallas. Harry Williams, Geomorphology Harry Williams, Geomorphology 24

25. Orogenic Belts In the case of North America there are two major orogenic belts - the older Appalachian chain in the east, and the much younger Cordilleran belt in the west. Both were formed by plate convergence and have similar structural features. Both areas underwent uplift, metamorphism, volcanism, folding, thrust faulting and later erosion; some crustal extension was also experienced in the west. SHIELD OROGENIC BELT OROGENIC BELT PLATFORM Appalachians ACTIVE MARGIN COASTAL PLAIN Cordillera PASSIVE MARGIN Ouachitas Harry Williams, Geomorphology Harry Williams, Geomorphology 25

26. The present-day relief of these mountains is a result of erosion - the Appalachians being much older are more subdued and lower and consist mainly of eroded folds (more on this later). Appalachians Cordillera Harry Williams, Geomorphology Harry Williams, Geomorphology 26

27. The western Cordillera has a number of other features related to orogenesis: Erosion has exposed the granitic batholiths emplaced during orogenesis, forming the Sierra Nevada Mountains. Half Dome Mountain Harry Williams, Geomorphology Harry Williams, Geomorphology 27

28. Another part of the crust in the west underwent stretching during the orogeny - forming the Basin and Range Province. These ridges and valleys are formed by normal faulting caused by crustal stretching (more later). Death Valley Harry Williams, Geomorphology Harry Williams, Geomorphology 28

29. Another large region in the west experienced uplift, but with little deformation, forming the large Colorado plateau - rivers cut down through the uplifted strata forming many canyons, including the Grand Canyon. Coloradoplateau Harry Williams, Geomorphology Harry Williams, Geomorphology 29

30. The Atlantic and Gulf Coast margins of the U.S. are PASSIVE MARGINS, characterized by low-lying flat coastal plains extending below sea-level into a wide continental shelf (the submarine part of the continent, overlain by sediments and subject to subsidence - recall the role of these sediments in orogenesis). Harry Williams, Geomorphology Harry Williams, Geomorphology 30