A principal aim of future work is to provide crystal structure profiles at margins that differ in 

 their physiography, sediment history, and tectonic style. Multichannel seismic experiments and 

 ocean drilling will be part of studies of margin evolution that include lithologic and acoustic 

 stratigraphy, structural analysis, subsidence analysis, and heat flow, gravity.and magnetic 

 measurements. Correlation and integration with onshore geological studies will be required. 

 This program is of fundamental geologic interest, but it is facilities-limited. Therefore, a 

 major increase in support for this program area is requested as part of the Tectonics and 

 Structures of Submerged Continental Margins component of the Global Program. 



2. Tectonics and Structure of Midocean Ridges and Basins. Studies using multibeam 

 bathymetric systems, side-scan sonars, navigated cameras, and research submersibles have 

 provided new perspectives on the complex and poorly understood spatial and temporal scales of 

 tectonic and volcanic processes along ridge crests and transform faults. Ridge crest 

 reorganization and migration, changes in spreading rate, and fracture zone orientation can 

 imprint the ocean crust with a complex structural and geochemical signature. Episodic 

 volcanism may control both distribution of ridge crest morphologies and hydrothermal vents. 

 Major limitations are a lack of knowledge of the internal structure of ridge systems, including 

 the shape and distribution of magma chambers, and the spatial and temporal scales of volcanic 

 activity and faulting which are needed to constrain dynamic models. 



A major goal of future work is to provide a three-dimensional view of the structure and 

 evolution of spreading ridges and fracture zones including closely-spaced surveys at critical 

 locations using detailed bathymetric, magnetic, seismic, and heat-flow measurements. 

 Complementary studies in off-axis, old ocean basins will examine effects of plate motions and 

 geologic age on lithosphere chemistry and rheology. This area is also of basic scientific 

 importance and is also facilities-limited. Therefore, the Oceanic Lithosphere component of the 

 Global Program on Ridge Crest Processes provides for significant new efforts in this program 

 element. 



3. Geochemical Evolution of the Oceanic Crust. One square kilometer of new seafloor 

 is created each year along midocean ridges. Research on this process and the resulting volcanic 

 products has documented the large-scale geochemical heterogeneity of the crust and upper 

 mantle; the rate of chemical and heat exchange between the ocean, oceanic crust, and mantle; and 

 secondary mineralization of the crust as it ages. Application of modern analytic techniques for 

 isotope and trace element variations provides insight into magma evolution, mixing processes, 

 and thermodynamics of the volcanic system at ridge crests, back-arc basins, and seamounts. 

 Detailed studies of the geochemistry of hydrothermal vents are relating the composition and 

 structure of oceanic crust to the nature and origin of associated mineral deposits, though studies 

 are limited by the lack of adequate control on chemical and thermal fluxes. 



A major emphasis of future work is to couple detailed geochemical sampling and analysis with 

 studies of the spatial and temporal scales of seafloor creation and evolution. Such studies will 

 require coordinated and comprehensive geochemical analysis of samples recovered by dredging, 

 submersibles, and crustal drilling. These studies will examine chemical heterogeneity produced 

 by overlapping and propagating spreading centers, the location of transform faults and mantle 

 plumes, and magma chamber evolution. This significant new effort is proposed as part of the 

 Ridge Crest Processes component of the Global Program. 



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