ABSTRACT The record of Permian–Triassic evolution in eastern North America indicates an important change in the tectonic regime from compression to extension as eastern Laurentia transitioned from the Alleghanian orogeny to continental rifting associated with the breakup of Pangea. The temporal pace (e.g., gradual vs. episodic, diachronous vs. synchronous), the accommodating structures, and the influential processes that characterized this transition provide critical insights into the late Paleozoic evolution of Laurentia and rifted continental margins in general. Connections between the formation of the South Georgia basin and regional cooling of the southernmost Appalachian crystalline rocks, along with the distribution of normal faults and discontinuities in metamorphic grade, indicate extensional collapse of the Alleghanian orogen along an extensive detachment system that was active from ca. 295 to 240 Ma. The 40 Ar/ 39 Ar cooling ages of biotites from low-angle normal shear zones cutting migmatitic gneisses of the southernmost Appalachians are interpreted to document extensional faulting ca. 280 Ma and to provide a snapshot of the prolonged orogenic collapse. The timing, orientation of structures, extent of reactivation, and character of late Alleghanian extension in the central and northern Appalachians provide an orogen-scale framework for this tectonic transition. This contribution focuses on correlations between the beginning of orogenic collapse and the initiation of continental rifting along with the tectonic processes that transformed eastern North America from a convergent to divergent plate boundary following the Alleghanian orogeny.

Abstract The Anaconda and Bitterroot metamorphic core complexes are located in western Montana, along the eastern edge of the Cordilleran hinterland. This multi-tiered extensional terrain contains exceptional exposures that collectively exhibit a crustal cross section through orogenic continental crust (i.e., middle through upper crust). The core complex footwall rocks consist of Late Cretaceous arc-related plutons and Eocene granitic plutons intruded into deformed and metamorphosed Midproterozoic Belt Supergroup and Paleozoic to Cretaceous shelf-platform strata. Late Cretaceous shear zones and folds dominate footwall structure, representing significant thinning of the stratigraphic section. Eocene detachments, mylonites, and plutonic suites distinctly overprint the Late Cretaceous structures. A stark example of this Eocene overprint is the Anaconda detachment, which resulted in eastward translation of the Late Cretaceous, arc-related Boulder batholith. This field trip will cover a transect through the Anaconda core complex from the Philipsburg valley to Butte, Montana. Field trip participants will examine key locations that clarify the distinction between the timing and structural style of Late Cretaceous crustal thickening and/or collapse features versus those related to Eocene core complex development.

Abstract The Lachlan orogen developed as a classic accretionary orogen in an oceanic setting between the palaeo-Pacific subduction zone and the Australian craton. Direct evidence for the composition and age of the lower crust and the basement to the thick Palaeozoic turbidite fan of the Lachlan orogen is limited. Exposures of Cambrian metavolcanic rocks and geophysical data suggest that most of the basement is the mafic oceanic crust along with possible small fragments of older continental crust. The trace element compositions of Cambrian metavolcanic rocks in the western and central Lachlan orogen are similar to those of volcanic rocks formed in modern back-arc and forearc settings. Pb, Nd and Sr isotopic data from these Cambrian rocks suggest a supra-subduction zone setting with little or no influence of continental crust other than subducted sediment.