Chapter 1 - Plate Tectonics
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is bounded by two major seismic discontinuities at 410 km and 660 km depth. At each discontinuity new, high-pressure phases are formed. 5 The Earth has two boundary layers with steep tem- perature gradients: the LVZ beneath the lithosphere, and the D" layer at the base of the mantle. Plates move about on the LVZ and mantle plumes may be generated in the D" layer. 6 Plate boundaries are of four types: ocean ridges where new lithosphere is produced; subduction zones where lithosphere descends into the mantle; transform faults where plates slide past each other; and collisional zones, where continents or arcs have collided. 7 Divergent plate boundaries (ocean ridges) are char- acterized by small-magnitude, shallow earthquakes with vertical motions reflecting formation of new lithosphere. Topography in axial rifts varies from high relief to little if any relief in going from slow- to fast-spreading ridges. Ocean ridges grow by lat- eral propagation. 8 Transform faults are characterized by shallow, variable-magnitude earthquakes exhibiting lateral motion. Transform faults may lengthen or shorten with time. 9 Convergent boundaries (subduction zones) are char- acterized by a dipping seismic zone with variable- magnitude earthquakes and, in some instances, seismic gaps suggestive of plate fragmentation. Fault motions vary with depth in the seismic zone and seismicity is strongly correlated with the degree of coupling of the descending slab and mantle wedge. Plates < 50 My in age may be buoyantly subducted and slide beneath the overriding plate. 10 Buoyant subduction is the only recognized tectonic force sufficient to trigger nucleation of a new sub- duction zone. New subduction zones commonly form at zones of weakness such as transform faults or fracture zones. 11 Collisional boundaries are characterized by wide (up to 3000 km) zones of lateral deformation with compressive fault motions dominating near sutures and lateral or vertical motions in areas overlying partially subducted plates. Plate tectonics 35 12 A Wilson Cycle, the opening and closing of an ocean basin, has occurred many times during geo- logic history. 13 The motion of one plate relative to another is that of a spherical cycloid and is a function of the po- sition of the pole of rotation, the direction of rela- tive motion, and the angular velocity of the plate. 14 Plate velocities can be estimated from magnetic anomalies on the sea floor, the azimuths and mo- tions on transform faults, first-motion studies of earthquakes, apparent polar wander paths, and by space geodetic measurements. Average relative velocities of plates range from 1-20 cm/y and the oldest surviving sea floor is about 160 Ma. 15 Computer models indicate that plates move in re- sponse chiefly to slab-pull forces, and that ocean- ridge push forces transmitted from the asthenosphere to the lithosphere are very small. 16 Rocks acquire remanent magnetization in the Earth's magnetic field by cooling through the Curie point of magnetic minerals (TRM), during deposition or diagenesis of a clastic sediments (DRM), and dur- ing secondary processes if new magnetic minerals are formed (CRM). 17 The Earth's magnetic field has reversed its polarity many times in the geologic past. Normal and re- verse polarity intervals in the stratigraphic record allow construction of the Geomagnetic Time Scale. Magnetic reversals show periodicity on several scales and evidence of reversals exists in rocks as old as 3.5 Ga. 18 Polarity intervals correlate with magnetic anomaly distributions on the sea floor allowing seafloor spreading rates to be estimated. The magnetic anomalies are caused by magnetized basalt injected into axial zones of ocean ridges during normal and reversed polarity intervals. 19 During a reversal, which occurs over about 4000 years, the Earth's dipole field decreases in inten- sity and rapid changes occur in declination and inclination. 20 The two most important problems in using paleo- magnetism to reconstruct ancient plate motions are, (1) separation of multiple magnetizations in the same rock, and (2) isotopic dating of the magnetization(s). 21 Apparent polar wander paths show distinct charac- teristics for various plate tectonic scenarios. 22 Chains of volcanic islands and aseismic ridges on the sea floor appear to have formed as oceanic plates more over hotspots, which are the shallow manifes- tations of mantle plumes. Similar, although not as well-defined, trajectories are formed when contin- ents move over hotspots. Lifespans of hotspots are < 100 My. 23 Although hotspots appear to have remained fixed beneath a given plate or beneath adjacent plates, distant hotspots have not remained fixed, but move at velocities approximately an order of magnitude less than plate velocities. 24 A supercontinent is a large continent composed of several or all of the existing continents. A super- continent cycle consists of rifting and break up of one supercontinent, followed by reassembly, in which dispersed cratons collide to form a new super- continent, with most or all fragments in different configurations from the older supercontinent. 25 The youngest supercontinent is Pangea, which formed between 450 and 320 Ma and includes most of the existing continents. Pangea began to frag- ment about 160 Ma and is still dispersing today. Gondwana, comprising Southern Hemisphere con- tinents, formed at 750-550 Ma. The earliest well- documented supercontinent is Rodinia, which formed about 1.3-1.0 Ga, fragmented at 750-600 Ma, and appears to have included most of the continents in a configuration quite different from Pangea. 26 To prepare for the survival of living systems on planet Earth, it is important to understand the na- ture and causes of interactions between Earth sys- tems and between Earth and extraterrestrial systems. Suggestions for further reading Kearey, P. and Vine, F. J. (1996). Global Tectonics (sec- ond edition). Cambridge, MA, Blackwell Scient., 348 pp. Klein, G. D. (1994). Pangea: Paleoclimate, Tectonics, and Sedimentation during Accretion, Zenith, and Breakup of a Supercontinent. Geol. Soc. America, Spec. Paper 288. Moores, E. M. and Twiss, R. J. (1995). Tectonics. New York, W. H. Freeman, 415 pp. Storey, B. C. (1995). The role of mantle plumes in con- tinental breakup: Case histories from Gondwanaland. Nature, 377, 301-308. Windley, B. F. (1995). The Evolving Continents (third Edition). New York, J. Wiley, 526 pp.