EN ECHELON PATTERNS OF CALYPTOGENA COLONIES IN THE JAPAN TRENCH

The distribution of Calyptogena phaseoliformis colonies in right-stepp ing en echelon patterns was observed by the Japanese submersible Shink ai 6500 at the foot of the landward escarpment of the northern Japan T rench at around 6437-6274 m depth. The north-south-trending Sanriku Es carpment has a thrust origin and is subparallel to the trench axis alo ng which the Pacific plate is being subducted beneath the North Americ a or Okhotsk plate at about 300 degrees at a rate of about 7.8 to 8.3 cm/yr, The trends of colonies are concentrated at 250 degrees, 300 deg rees, and 330 degrees: each trend matches either an antithetic riedel shear, extension fracture, or synthetic riedel shear, respectively, wi thin a left-lateral shear regime caused by the oblique subduction, Met hane- and hydrogen sulfide-bearing fluid advection from depth occurs e ssentially along the thrust fault, but finally seeps along the fractur es at the sea floor. This supplies energy to the food chain through ba cteria utilizing hydrogen sulfide, then eventually sustains the Calypt ogena colonies, Because the clams select the best places to survive, t he geometric arrangement of the clam colonies provides a kinematic ind icator of relative plate motions.