PRECISE P-WAVE AND S-WAVE VELOCITY STRUCTURES IN THE KITAKAMI MASSIF,NORTHERN HONSHU, JAPAN, FROM A SEISMIC-REFRACTION EXPERIMENT

The Kitakami massif, which is located in the eastern part of Northern Honshu, Japan, is composed of two geological units. The northern Kitak ami terrane is characterized as a Jurassic accretionary complex, while the southern Kitakami terrane consists of pre-Silurian basement and S ilurian-lower Cretaceous marine sediments. The boundary region of thes e two units, called the Hayachine tectonic belt (HTB), is composed of mafic to ultramafic rocks. The Kitakami massif experienced intense gra nitic intrusions in the Cretaceous. We present a detailed crustal stru cture model for the eastern part of the massif derived from an extensi ve seismic refraction experiment conducted on a 194-km N-S line. The u ppermost crust is covered with a very thin (0.5-1 km) surface layer wi th a velocity of 3.1-5.4 km/s. The velocity structure below this layer shows remarkable lateral variation. In the northern Kitakami terrane the P wave velocity and V-p/V-s at the top of the basement are 5.85-5. 95 km/s and 1.68-1.70, respectively. The seismic attenuation in this r egion is high (Q(p) = 150-200 and Q(s) = 70-100). In contrast, the upp ermost crust in the southern Kitakami terrane is characterized by a hi gh P wave velocity (6.05-6.15 km/s) and V-p/V-s (1.74-1.77). The Q(p) and Q(s) also show high values of 300-400 and 150-200, respectively. S uch a structural difference persists to 14- to 16-km depth, at which t he P wave velocity increases to 6.45 km/s. The low velocity and high a ttenuation in the northern Kitakami terrane represent a highly deforme d structure of the accretionary complex. The high P wave velocity and V,IV, in the southern Kitakami terrane indicate the relatively mafic c rustal composition, which may result from the fragment of the oceanic crust incorporated by the accretion process or the uplifting in the la test Jurassic-early Cretaceous. A midcrustal interface determined from wide-angle reflections shows an abrupt southward depth decrease from 25 to 20 km under the HTB. The P wave velocity and V-p/V-s between 14- and 16-km depth and the midcrustal interface are p 6.45-6.55 km/s and 1.74-1.78, respectively. The Moho depth under the northern Kitakami t errane decreases southward from 34 to 32 km. In the southern Kitakami terrane the Moho dips slightly southward. The P wave velocity and the V-p/V-s ratio in the lower crust are 6.9-7.0 km/s and 1.75-1.76, respe ctively. The P wave velocity in the uppermost mantle is not well resol ved but is probably less than 7.7 km/s. The S wave velocity derived fr om relatively clear S-n is 4.35-4.40 km/s. Our results show that the H TB is a prominent structural boundary extending to the Moho. The crust of Kitakami massif was not homogenized by the Cretaceous granitic int rusions, and the original structural difference remains in the upper c rust.