ABSTRACT This one-day field trip will visit northern Erie County in northwestern Pennsylvania, USA, and is divided into two parts. The first part will highlight brittle and ductile deformation preserved in Upper Devonian bedrock. It is possible that this deformation occurred in relation to the Alleghenian orogeny during the late Paleozoic. However, some of this deformation displays possible evidence of postdating Wisconsin glaciation, but this interpretation is speculative and based largely on possible topographic disruption of Quaternary river terrace deposits. Morphologic characteristics of outcrops in creeks that expose these tectonic structures will be used as a starting point to discuss structure origin, timing, and their implications. The second part of the trip will visit Erie Bluffs State Park (EBSP) and will examine ongoing mass wasting of bluffs along the southern Lake Erie shoreline. The timing (as determined from dendrochronological analyses) and styles of mass wasting that have occurred at EBSP will be discussed and compared to two other locations along Pennsylvania’s Lake Erie shoreline. An appended road log serves as a geologic guide to the features and processes highlighted on this trip.

ABSTRACT With waterfalls and the deepest gorge in Pennsylvania, Ohiopyle State Park provides opportunities to observe a variety of habitats and three-dimensional (3-D) exposures of the Pennsylvanian sandstone most responsible for shaping Laurel Highlands landscapes. Evidence for the relationship between bedrock, ancient climates, and the landscape can be observed at some of the most scenic natural features of the park: Baughman Rock Overlook, Cucumber Falls, Ohiopyle Falls, Meadow Run Waterslide and Cascades, and Youghiogheny River Entrance Rapid. Channel azimuths and lateral variations in thickness of upper Pottsville fluvial/deltaic sandstone suggest that deposition was influenced by deformation of this part of the Allegheny Plateau during the Alleghanian orogeny. Geologic features of Pottsville sandstone outcrops include a 10-m- (~33-ft-) long Lepidodendron fossil and a 3-D exposure of a meter-high Pennsylvanian subaqueous sand dune and scour pit. Cosmogenic age dating has indicated very slow erosion of hard sandstone in an upland location at Turtlehead Rock and informed estimation of Pleistocene/Holocene waterfall retreat rates of Ohiopyle and Cucumber Falls. Bedrock exposures supporting scour habitats along the Youghiogheny River occur only in a limited area of Youghiogheny Gorge where knickpoint migration and bedrock erosion were relatively recent. Geologic factors, including locations of major tributaries, development of bars that constrict river flow, and proximity of Homewood sandstone outcrops as sources of boulder obstacles in the river, contributed to the class, location, and nature of whitewater rapids in the lower Youghiogheny River.

ABSTRACT The Neoproterozoic to Cambrian rifting history of Laurentia resulted in hyperextension along large segments of its Paleozoic margins, which created a complex paleogeography that included isolated continental fragments and exhumed continental lithospheric mantle. This peri-Laurentian paleogeography had a profound effect on the duration and nature of the Paleozoic collisional history and associated magmatism of Laurentia. During the initial collisions, peri-Laurentia was situated in a lower-plate setting, and there was commonly a significant time lag between the entrance of the leading edge of peri-Laurentia crust in the trench and the arrival of the trailing, coherent Laurentian landmass. The final Cambrian assembly of Gondwana was followed by a global plate reorganization that resulted in Cambrian (515–505 Ma) subduction initiation outboard of Laurentia, West Gondwana, and Baltica. Accretion of infant and mature intra-oceanic arc terranes along the Appalachian-Caledonian margin of the Iapetus Ocean started at the end of the Cambrian during the Taconic-Grampian orogenic cycle and continued until the ca. 430–426 Ma onset of the Scandian-Salinic collision between Laurentia and Baltica, Ganderia, and East Avalonia, which created the Laurussian continent and closed nearly all vestiges of the Iapetus Ocean. Closure of the Iapetus Ocean in the Appalachians was followed by the Devonian Acadian and Neoacadian orogenic cycles, which were due to dextral oblique accretion of West Avalonia, Meguma, and the Suwannee terranes following the Pridolian to Lochkovian closure of the Acadian seaway and subsequent outboard subduction of the Rheic Ocean beneath Laurentia. Continued underthrusting of Baltica and Avalonia beneath Laurentia during the Devonian indicates that convergence continued between Laurentia and Baltica and Avalonia, which, at least in part, may have been related to the motions of Laurentia relative to its converging elements. Cambrian to Ordovician subduction zones formed earlier in the oceanic realm between Laurentia and Baltica and started to enter the Arctic realm of Laurentia by the Late Ordovician, which resulted in sinistral oblique interaction of the Franklinian margin with encroaching terranes of peri-Laurentian, intra-oceanic, and Baltican provenance. Any intervening seaways were closed during the Middle to Late Devonian Ellesmerian orogeny. Exotic terranes such as Pearya and Arctic Alaska became stranded in the Arctic realm of Laurentia, while other terranes such as Alexander and Eastern Klamath were translated further into the Panthalassa Ocean. The Middle/Late Devonian to Mississippian Antler orogeny along the Cordilleran margin of Laurentia records the first interaction with an outboard arc terrane built upon a composite block preserved in the Northern Sierra and Eastern Klamath terranes. The Carboniferous–Permian Alleghanian-Ouachita orogenic cycle was due to closure of the vestiges of the Rheic Ocean and assembly of Pangea. The narrow, continental transform margin of the Ouachita embayment of southern Laurentia had escaped accretion by outboard terranes until the Mississippian, when it collided with an outboard arc terrane.

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 Paleozoic plate boundary zone between Laurussia and Gondwana in western Pangea hosts major magmatic and hydrothermal Sn-W-Ta, Au, and U mineralization. Individual mineral deposits represent the results of the superposition of a series of exogenic and endogenic processes. Exogenic processes controlled (1) the enrichment of the ore elements in sedimentary protoliths via residual enrichment during intense chemical weathering and via climatically or tectonically controlled redox traps, (2) the spatial distribution of fertile protoliths, and, thus, eventually (3) the spatial distribution of mineralization. Endogenic processes resulting in metamorphism and crustal melting controlled the mobilization of Sn-W, Au, and U from these enriched protoliths and, thus, account for the age distribution of Sn-W and Au mineralization and U-fertile granites. It is the sequence of exogenic and endogenic processes that eventually results in the formation of mineralization in particular tectonic zones. Whereas the endogenic processes were controlled by orogenic processes during the assembly of western Pangea itself, the exogenic processes were linked to the formation of suitable source rocks for later mineralization. The contrasting distribution of magmatic and hydrothermal Sn-W-Ta, Au, and U mineralization on the Laurussia and Gondwana sides of the plate boundary zone reflects the contrasting distribution of fertile protoliths and the contrasting tectonic situation on these margins. The Laurussian margin was an active margin during most of the Paleozoic, and the distribution of different mineralization types reflects the distribution of terranes of contrasting provenance. The Gondwanan margin was a passive margin during most of the Paleozoic, and the similar distribution of a wide range of different metals (Sn, W, Ta, Au, and U) reflects the fact that the protoliths for the various metals were diachronously accumulated on the same shelf, before the metals were mobilized during Acadian, Variscan, and Alleghanian orogenic processes.

ABSTRACT The Baie Verte Line in western Newfoundland marks a suture zone between (1) an upper plate represented by suprasubduction zone oceanic crust (Baie Verte oceanic tract) and the trailing continental Notre Dame arc, with related upper-plate rocks built upon the Dashwoods terrane; and (2) a lower plate of Laurentian margin metasedimentary rocks with an adjoining ocean-continent transition zone (Birchy Complex). The Baie Verte oceanic tract formed during closure of the Taconic seaway in a forearc position and started to be obducted onto the Laurentian margin between ca. 485 and 476 Ma (early Taconic event), whereas the Birchy Complex, at the leading edge of the Laurentian margin, was subducted to maximum depths as calculated by pseudosection techniques (6.7–11.2 kbar, 315–560 °C) by ca. 467–460 Ma, during the culmination of the Taconic collision between the trailing Notre Dame arc and Laurentia, and it cooled isobarically to 9.2–10.0 kbar and 360–450 °C by 454–449 Ma (M 1 ). This collisional wedge progressively incorporated upper-plate Baie Verte oceanic tract rocks, with remnants preserved in M 1 high-pressure, low-temperature greenschist-facies rocks (4.8–8.0 kbar, 270–340 °C) recording typical low metamorphic gradients (10–14 °C/km). Subsequently, the early Taconic collisional wedge was redeformed and metamorphosed during the final stages of the Taconic cycle. We relate existing and new 40 Ar/ 39 Ar ages between 454 and 439 Ma to a late Taconic reactivation of the structurally weak suture zone. The Taconic wedge on both sides of the Baie Verte suture zone was subsequently strongly shortened (D 2 ), metamorphosed (M 2 ), and intruded by a voluminous suite of plutons during the Salinic orogenic cycle. Calculated low- to medium-pressure, low-temperature M 2 conditions in the Baie Verte oceanic tract varied at 3.0–5.0 kbar and 275–340 °C, with increased metamorphic gradients of ~17–25 °C/km during activity of the Notre Dame arc, and correlate with M 2 assemblages in the Birchy Complex. These conditions are associated with existing Salinic S 2 white mica 40 Ar/ 39 Ar ages of ca. 432 Ma in a D 2 transpressional shear zone and synkinematic intrusions of comparable age. A third metamorphic event (M 3 ) was recorded during the Devonian with calculated low-pressure, low-temperature conditions of 3.2–3.8 kbar and 315–330 °C under the highest metamorphic gradients (23–30 °C/km) and associated with Devonian–early Carboniferous isotopic ages as young as 356 ± 5 Ma. The youngest ages are related to localized extension associated with a large-scale transtensional zone, which reused parts of the Baie Verte Line suture zone. Extension culminated in the formation of a Middle to Late Devonian Neoacadian metamorphic core complex in upper- and lower-plate rocks by reactivation of Baie Verte Line tectonites formed during the Taconic and Salinic cycles. The Baie Verte Line suture zone is a collisional complex subjected to repeated, episodic structural reactivation during the Late Ordovician Taconic 3, Silurian Salinic, and Early–Late Devonian Acadian/Neoacadian orogenic cycles. Deformation appears to have been progressively localized in major fault zones associated with earlier suturing. This emphasizes the importance of existing zones of structural weakness, where reactivation took place in the hinterland during successive collision events.