STATE OF INTRAPLATE STRESS AND TECTONISM OF NORTHEASTERN AMERICA SINCE CRETACEOUS TIMES, WITH PARTICULAR EMPHASIS ON THE NEW-ENGLAND QUEBECIGNEOUS PROVINCE

A paleostress analysis based on inversion of fault-slip data has been conducted in the Quebec Appalachians and the St, Lawrence Lowlands in order to characterize the direction and state of stress during and aft er the emplacement of Cretaceous Monteregian plutons, Two major events with contrasting directions of extension are recognized: (1) an early , widely distributed NE-SW-directed extension (similar to 140 Ma), and (2) a N-S-oriented extension associated with the main phase of magmat ism (similar to 125 Ma). Both these extensional paleostress regimes ar e consistent in direction with the result of statistical analysis of C retaceous pluton and dyke trends in southern Quebec and New England. T he NE-SW-directed far-field tensional stress of the first event favour s the reactivation of the earlier E-W-trending Proterozoic Ottawa-Bonn echere Graben and the reorientation of the local stress field. As a re sult, a restricted zone of N-S-directed extension was developed synchr oneously with the emplacement of the Monteregians plutons, Cretaceous intrusions of the studied area are thus interpreted as the result of m agmatism along reactivated Proterozoic basement faults, rather than in terms of the North American plate moving over a hot spot,In Late Cret aceous-early Tertiary times, the extensional stress regime changed to an ENE-WSW-directed compressional stress field, This compression is ch aracterized by strike-slip faults and represents the youngest tectonic event in the area, Directions of compressional paleostress axes compa re well to the present-day maximum compressive stress in northeastern America, The stress regimes inferred in the Quebec-New England igneous province call be attributed to the Early Cretaceous rifting between L abrador and Greenland. Variations of spreading rate and plate boundary conditions of North America in the Late Cretaceous-early Tertiary led to stress inversion in eastern North America and to the establishment of a durable compressional stress field that is still present today.