The authors introduced two kinds of newly found soft-sediment deformation-syn-sedimentary extension structure and syn-sedimentary compression structure,and discuss their origins and constraints on basin tectonic evolution.One representative of the syn-sedimentary extension structure is syn-sedimentary boudinage structure,while the typical example of the syn-sedimentary compression structure is compression sand pillows or compression wrinkles.The former shows NW-SE-trending contemporaneous extension events related to earthquakes in the rift basin near a famous Fe-Nb-REE deposit in northern China during the Early Paleozoic(or Mesoproterozoic as proposed by some researches),while the latter indicates NE-SW-trending contemporaneous compression activities related to earthquakes in the Middle Triassic in the Nanpanjiang remnant basin covering south Guizhou,northwestern Guangxi and eastern Yunnan in southwestern China.The syn-sedimentary boudinage structure was found in an earthquake slump block in the lower part of the Early Paleozoic Sailinhudong Group,20 km to the southeast of Bayan Obo,Inner Mongolia,north of China.The slump block is composed of two kinds of very thin layers—pale-gray micrite(microcrystalline limestone)of 1?2 cm thick interbedded with gray muddy micrite layers with the similar thickness.Almost every thin muddy micrite layer was cut into imbricate blocks or boudins by abundant tiny contemporaneous faults,while the interbedded micrite remain in continuity.Boudins form as a response to layer-parallel extension(and/or layer-perpendicular flattening)of stiff layers enveloped top and bottom by mechanically soft layers.In this case,the imbricate blocks cut by the tiny contemporaneous faults are the result of abrupt horizontal extension of the crust in the SE-NW direction accompanied with earthquakes.Thus,the rock block is,in fact,a kind of seismites.The syn-sedimentary boudins indicate that there was at least a strong earthquake belt on the southeast side of the basin during the early stage of the Sailin
