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SCIENCE SUMMARIES 2006 IRIS 5-YEAR PROPOSAL The Continental Lithosphere and Seismology Alan Levander, Adrian Lenardic • Rice University Karl Karlstrom • University of New Mexico The success of plate tectonics in describing the evolution of the ocean basins lies in the regular progression of oceanic lithosphere from its generation at spreading ridges to its consumption at subduction zones, and the simple description of the tectonics of oceanic lithosphere in terms of translation, rotation, and consumption of large “rigid,” nearly uniform plates obeying fairly simple geometric laws at their boundaries. This process dominates global tectonics at scales of hundreds to thousands of kilometers and durations of ~0.2 Ga and is often taken as the surface expression of mantle convection, with the crust and mantle forming the upper thermal boundary layer. Differentiation processes at ocean ridges are also relatively simple, creating plate-wide layered structures that are largely recycled by subduction, and the relationships between the age of the lithosphere, its thickness, thermal state, and density are well known. The Earth, unique among the planets, has a bimodal distribution of topography, with ~40% of its surface made up of continents, whose surfaces reside above to slightly below sea level, and ~60% oceans with average elevation of ~ - 4km. This is the most obvious difference between the continents and oceans, but there are equally profound differences extending to depths of 200-400 km first recognized seismologically almost 30 years ago. In contrast to the 60% of the Earth forming the oceanic plates, the 40% comprising the continents has grown over the last ~4.5 Ga through a complex sequence of differentiation and deformation processes that operate on scales of hundreds of kilometers and smaller, including highly localized, phenomena such as faulting and pluton emplacement. The resulting continental lithosphere consists of a complex collage of fragments of various tectonic affinities. While these fragments can be collected to form supercontinents or dispersed by the same convection system that creates and recycles oceanic plates, the development of continental lithosphere itself, unlike the oceanic lithosphere, cannot be viewed in terms of the creation of a thermal boundary layer in the convecting mantle system, and continental tectonics cannot be simply described by translation, rotation, and consumption of large, “rigid,” nearly-uniform plates obeying fairly simple geometric laws at their boundaries. In general, continental crust does not participate in global mantle overturn and this also appears to be the case for the subcontinental mantle beneath many cratons. Thus, the continental lithosphere is not a simple active upper thermal boundary layer within the convecting mantle system, and understanding the structure of continents requires understanding the cumulative products of magmatic and deformational processes that have acted over billions of years of Earthʼs history. The thickness and chemical buoyancy of continental crust makes continents resistant to subduction. That this could effect large-scale mantle dynamics was proposed almost 70 years ago by Pekeris (1935). Continents represent the products of repeated differentiation cycles whereby less-dense, intermediateto-felsic, crust is upwardly stratified by combinations of tectonomagmatic processes. Accretion of uniquely continental materials takes place above subduction zones in magmatic arcs and orogenic plateaus, but formation of High-resolution, teleseismic imaging of the lithosphere beneath the Abitibi 1996 broadband array (IRIS-PASSCAL, Lithoprobe). a)Map of study area. b) CCP receiver function profile. Red and blue pulses denote discontinuities with positive and negative downward velocity gradients, respectively. Black lines show interpretation. Abbreviations: GF = Grenville Front, M = Moho. c) S-velocity perturbation profile obtained by 2D Generalized Radon Transform inversion (Rondenay et al., 2005), with red to blue colour scale representing negative (slower) to positive (faster) velocity perturbations. 6
2006 IRIS 5-YEAR PROPOSAL SCIENCE SUMMARIES true continental lithosphere requires multiple differentiation events to reach the average composition of continents. These events take place during plate convergent events such as collisions of arcs with arcs, arcs with continents, and continents with continents. They can also take place during incipient rifting of continents via interactions between lithosphere and asthenosphere. In terms of the differentiation processes that make continental crust over time, the important physical quantities are heat sources, both mantle and crustal, magma sources within the mantle (i.e. fertile mantle), other fluid sources in the crust and mantle (e.g., H 2 O and CO 2 ), and migration pathways provided by existing fractures and shear zones, or in the case of highly-pressured magmas, the regional stress field that controls the orientations and locations of hydrofracturing. It is generally believed that the radiogenic heat production in the crust is inadequate to produce crustal magmas, except in the greatly thickened, probably hydrated, crust occurring near or within orogenic plateaus. Most crustal melting results from heat originating in the mantle that is advectively transferred to the crust in the form of Interpreted cross section showing compressional velocities (contoured at 0.5-km/s intervals), relocated seismicity (black circles: continental crustal events; colored symbols: intraslab events), and Moho reflector (blue line). Interpreted top of subducting plate (red line) is drawn 7 km above reflector. The region between these lines is interpreted to be the subducting oceanic crust, composed of basalt above 40-km depth (horizontal green line) and beginning to transform to eclogite below. Subducting mantle is below the blue line. Low velocities in the mantle wedge imply the presence of serpentinite. There is no vertical exaggeration. mantle-derived mafic melts, with the melting process facilitated by other fluids residing in the crust, or transferred to the crust from the mantle: for example, hydration melting above subduction zones. Seismology has played a transformational role in our understanding of the continents and the ocean basins, particularly so in the search for hydrocarbon reserves and (more recently) groundwater, in the recognition and mitigation of earthquake hazards, and in our understanding of Earthʼs structure, processes, and history. Modern detailed seismic investigations image details of structural and sedimentary geology, identify magmatic and other fluids in the crust, map details of structure within fault zones, and image migration of the seismicity associated with volcanic eruptions and volcanic systems. Seismic investig to make the links between brittle and ductile deformation in the crust. Discussion of the deformation of the continental crust is virtually impossible without consideration of the mantle underneath it, part of the remote Earth that is only accessible to seismic and electromagnetic probing. In both ancient cratonic and active modern provinces, surface structures appear to be highly correlated with various mantle structures, giving structural geologists constraints on large-scale lithospheric processes that formed and stabilized the continents. In particular, recent seismic results around the peripheries of the North American craton have led to a thrust-stacking model for the growth and stabilization of the cratonic lithosphere in the late Archean and early Proterozoic. The density of recording in modern seismic experiments, both active and passive, permits wavefield imaging to provide unprecedented resolution of the crust and mantle throughout the transition zone. (Such direct imaging is even being applied to D” and the Core-Mantle Boundary). Seismic heterogeneities observed in the mantle resulting from subduction, delamination or rifting can be used to infer some of the forces driving modern crustal deformations, providing a vital link to surface deformation measurements provided by modern geodesy. Seismology, and by inference IRIS, is one of the drivers in modern Earth science, providing images interpreted jointly by a wide variety of researchers investigating virtually every aspect of the continental system. The expanding PASSCAL instrument base permits use of seismic imaging techniques drawing on the most advanced aspects of practices in the petroleum industry, medical imaging fields, and astronomy. This pool of seismometers will continue to provide the primary means by which we will discover the true nature of the structure and evolution of the continents. 7
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