MICROEARTHQUAKE CHARACTERISTICS AND CRUSTAL VELOCITY STRUCTURE AT 29-DEGREES-N ON THE MID-ATLANTIC RIDGE - THE ARCHITECTURE OF A SLOW-SPREADING SEGMENT

We report the results of a microearthquake and seismic tomography expe riment conducted along the southern half of the Mid-Atlantic Ridge seg ment at 29 degrees N and aimed at investigating the relationship of ea rthquake and seismic structural characteristics to spreading processes . The seismic velocity structure is obtained from two-dimensional (2-D ) and three-dimensional (3-D) tomographic inversions of travel times f rom shots along an axial refraction line. Inversion solutions indicate that the velocity structure in the lower crust is heterogeneous, with higher velocities and relatively thin crust near the segment end and lower velocities and a thicker layer 3 near the central bathymetric hi gh. The thickness of the lower crust at the segment end is asymmetric across axis, with thinner crust beneath the inside corner. The indicat ed variations in crustal thickness are consistent with those inferred from mantle Bouguer gravity anomalies. The microearthquakes located al ong axis during the 41-day recording period cluster in three separate along-axis regions: (1) the southern segment end near 28 degrees 55'N, (2) the central along-axis topographic high at 29 degrees 11'N, near and north of the Broken Spur hydrothermal vent field, and (3) a region midway between, beneath a volcano near 29 degrees 02'N. The greatest level of microearthquake activity was in a diffuse zone off axis benea th the inside corner of a nontransform offset. This pattern of off-axi s microearthquake activity, and the cross-axis asymmetry in crustal th ickness at the segment end, support tectonic models in which normal fa ulting and consequent crustal thinning occur preferentially at inside corner regions. Anomalous focal mechanisms for microearthquakes beneat h the along-axis volcano and the significant seismicity beneath the ax ial volcanic ridge at the segment center, in contrast, may be the resu lt of volcanic and hydrothermal processes, such as magma movement or t hermal stresses generated near cooling plutons. A comparison of microe arthquake characteristics with residual gravity data and velocity stru cture leads to the hypothesis that microearthquakes associated with ar eas of thin crust near the segment end and inside corner are dominantl y tectonic in nature, whereas microearthquakes associated with volcani c and hydrothermal processes are more likely to occur toward the segme nt center in areas of greater rates of magma supply and thicker crust. Along axis, well-resolved focal depths determined with a 3-D velocity model range from 3 to 6 km beneath the seafloor and do not shoal towa rd the segment center. These observations indicate that the thermal st ructure of the crust along this slow spreading ridge segment is not in steady state.