AGE AND TECTONICS OF MIDDLE TERTIARY BASALTIC VOLCANISM AND EFFECTS ON THE LANDSCAPE OF WEST-CENTRAL MONTANA

The distribution and age of basalt flows in west-central Montana, from Helena east for 125 km, define a distinct episode of eruptive igneous activity, mark the redirection of drainages of major rivers, and serve as datums by which to estimate amounts of vertical uplift of mountain blocks relative to adjacent valleys during late Tertiary and Quaternary time. New Ar40/Ar39 dates define a short time interval from 32.8 to 30.4 million years before present, during which basaltic magma erupted at the east end of Lewis and Clark tectonic zone and along recurrently active, generally west-trending faults in the Little Belt Mountains. The latter faults intersect a major fault zone beneath the northern part of the Big Belt Mountains where basalt flows are widespread. The flows are concentrated along the south side of the tectonic zone that has served as a transform fault during crustal extension. Accumulations of flows range from a few meters to 175 m thick and rest on gently undulating ancestral surfaces or broad stream valleys eroded across Middle Proterozoic and Phanerozoic strata and Mesozoic and early Tertiary intrusive igneous rocks. Distribution of the flows suggests local vertical displacement between the mountain ranges and adjacent valleys since late Oligocene time of as much as 680 m. At the north end of the Smith River Valley, ejecta from the eruptions are locally tilted 26 degrees south, and the ancestral valley, filled with remnants of basalt flows, is tilted south. Flows forced the courses of the Smith and Musselshell Rivers to migrate west and south, respectively, ultimately to incise adjacent older, harder rocks. Similarly, west-flowing Sheep Creek in the Little Belt Mountains was directed south in part by the volcanic eruptions, in part by tilting, to incise across resistant Early Proterozoic granite gneiss. West-flowing Tenderfoot Creek is incised along much of its length into the gneisses, Eocene intrusive rocks, and resistant Phanerozoic rocks directly along a fault zone that displays recurrent vertical and lateral movement. Thus, the central Montana landscape is defined in part by mid-Tertiary volcanism and younger structural activity manifested along long-lived structures.