Workshop Report - Ridge 2000 Program

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electromagnetic fields must be measured directly on the seafloor or in boreholes. The second class of data that must be collected is related to defining dynamic processes that can constrain modes of crustal deformation. These studies are more difficult to do because they require measurements over relatively long periods of time (years) mostly with devices that have not been previously used. These include measurements of strain and elevation changes in actively deforming areas with geodetic surveys and possibly strain meters and tilt meters, as well as long-term electromagnetic field measurements. Detailed locations and focal mechanisms for microearthquakes are needed to define fault geometry and kinematics in the brittle crust. These would be especially useful in conjunction with surface mapping and borehole monitoring. Laboratory, Theoretical, and Numerical Developments Important laboratory measurements that influence our understanding of mid-ocean ridge processes include both mechanical and electrical properties of rocks. We need a better understanding of the ductile strength or viscosity of crust and mantle rock particularly as a function of temperature, composition and degree of partial melting. Knowing the frictional resistance to sliding on existing faults and the critical stress intensity factor required to create new faults is essential for determining the brittle strength of the lithosphere. Elastic wave velocities, especially their dependence on temperature, degree of partial melting, and composition are important to interpreting the seismic velocity structure of ridge segments. Electrical properties, particularly the conductivity, will be needed to interpret electromagnetic observations. Laboratory studies will also be required for chemical and isotopic analysis and to develop new dating techniques, particularly ones which allow us to establish ages in the range of 0 to 10 years. Theoretical modelling studies and physical experiments allow us to examine simple physical systems or processes that may be important at ridge segments. Physical experiments include wax models, convection experiments, and porous flow/compaction experiments. Theoretical studies include convective and necking instability, crack propagation and fault growth. These ongoing types of studies will continue to provide new physical insight and understanding. Large-scale numerical experiments made possible by new developments in computer technology will allow us to theoretically model the interaction of individual processes. Seismic tomographic methodologies should be advanced to include amplitude effects. Multichannel seismic imaging of the subsurface requires major developments to account for scattering 35
and rough topography. We further require the development of forward numerical modelling and inverse methods appropriate when the heterogeneities are on the order of seismic wavelengths of 200 to 500 m, and that also account for the effects of surface and volume scattering, attenuation, and anisotropy at these scales. The integrated analysis of swath mapping, acoustic imaging, multichannel profiling, and aero-gravity and magnetics data will require the development of an interactive computer imaging and data analysis capacity. Marine Data Acquisition Our ability to initiate the measurement and sampling programs discussed above rests on the fact that new techniques, some brought into play only recently, are available to gather many of the types of data we need. These include multibeam echo sounding systems, acoustic and optical imaging techniques, ocean bottom seismographs, magnetometers, shipboard and airborne gravimeters and methods for controlled sampling of loose rock. The rates of coverage and the resolution of existing versions of these systems, while adequate for some purposes, leave much room for improvements and specific ideas exist as to how such improvements could be made. Beyond these existing instrument types there are new concepts emerging that could contribute substantially to our understanding of the generation and evolution of the lithosphere, and that could be available within the next few years. These include devices for obtaining oriented, fresh rock samples from selected outcrops; systems for measuring strain, tilt and elevation changes; and high resolution on-bottom gravimeters. Two challenging areas for the future lie in implementing an ability to locate and react rapidly to the occurrence of transient events {volcanic eruptions, earthquakes}; and in developing means for using deep drill holes after departure of the drilling ship. Under-way monitoring of hydrothermal venting would allow us to detect hydrothermal activity while conducting large-scale surveys. In addition to the measurement instruments required for particular types of data acquisition, there are several recent supporting technological advances that will enhance our ability to carry out sea foor tasks effectively. Fiber optic cables, remote operated cable connected and autonomous vehicles, improved computing and recording capacity for shipboard and sea floor data handling, and continuous high-accuracy satellite navigation will all help open new opportunities. Finally, it is clear that this program embodies significant ship requirements supporting a wide range of capability. For global and regional surveys we will need the use of a ship with well integrated survey capabilities for extensive periods. This 36
Page 2 and 3: The Mid-Oceanic Ridge A Dynamic G
Page 4 and 5: OCEAN STUDIES BOARD WALTER H. MUNK,
Page 6 and 7: PREFACE Recent discoveries of the w
Page 8 and 9: o Regional Scale: selected subsets
Page 10 and 11: 1 INTRODUCTION The global mid-ocean
Page 12 and 13: papers were compiled, printed, and
Page 14 and 15: 2 STATEMENT OF GOALS AND OBJECTIVES
Page 16 and 17: experience from land volcanic and m
Page 18 and 19: solid understanding of both the bio
Page 20 and 21: NEXT STEPS TOWARD A RIDGE INITIATIV
Page 22 and 23: 4 REPORTS OF THE WORKING GROUPS INT
Page 24 and 25: Critical Investigations The specifi
Page 26 and 27: Seismic refraction methods provide
Page 28 and 29: New Instrument and Laboratory Capab
Page 30 and 31: GROUP 2: GEOMETRY AND DYNAMICS OF M
Page 32 and 33: FORMATION OF OCEAN CRUST AT SPREADI
Page 34 and 35: 3. Regional Surveys. The third comp
Page 36 and 37: provide efficient and economically
Page 38 and 39: 3. Adaptation of tomographic invers
Page 40 and 41: Consequently, the sin;J1e most impo
Page 42 and 43: Thermal stresses have been suggeste
Page 46 and 47: capability should include at least
Page 48 and 49: In order to understand subseafloor
Page 50 and 51: fossil hydrothermal systems in ophi
Page 52 and 53: instrumented to record the environm
Page 54 and 55: GROUP 5: BIOLOGY OF HYDROTHERMAL SY
Page 56 and 57: There are several key questions abo
Page 58 and 59: Dispersal of Organisms What are the
Page 60 and 61: Strategy Biological studies will em
Page 62 and 63: GROUP 6: WATER COLUMN DYNAMICS AND
Page 64 and 65: On the ridge segment scale, detaile
Page 66 and 67: standard transect sampling and Lagr
Page 68 and 69: APPENDIX I The Mid-Oceanic Ridge: A
Page 70 and 71: Forsyth, Donald Department of Geolo
Page 72 and 73: Lonsdale. Peter Scripps Institution
Page 74 and 75: Reeve, Michael National Science Fou
Page 76: Tunnicliffe, Verena University of V

Workshop Report - Ridge 2000 Program

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