Abstract Integration of new geological mapping, detrital zircon geochronology, and sedimentary and metamorphic petrography south of the Muskol metamorphic dome in the Central Pamir terrane provides new constraints on the evolution of the Pamir orogen from Triassic to Late Oligocene time. Zircon U–Pb data show that the eastern Central Pamir includes Triassic strata and mélange that are of Karakul–Mazar/Songpan–Ganzi affinity and comprise the hanging wall of a thrust sheet that may root into the Tanymas Fault c. 35 km to the north. The Triassic rocks are unconformably overlain by Cretaceous strata that bear similarities to coeval units in the southern Qiangtang terrane and the Bangong Suture Zone of central Tibet. Finally, Oligocene or younger conglomerate and interbedded siltstone, the youngest documented strata in the Pamir Plateau proper, record an episode of juvenile magmatism at c. 32 Ma, which is absent in the extant rock record and other detrital compilations from the Pamir but overlaps in age with ultrapotassic volcanic rocks in central Tibet. Zircon Hf isotopic data from the Oligocene grains ( ε Hf (t) ≈ +9.6) suggest a primary mantle contribution, consistent with the hypothesis of Late Eocene lithospheric removal beneath the Pamir Plateau.

Abstract: Fault growth could be achieved by (1) synchronous increases in displacement and length or (2) rapid fault propagation succeeded by displacement-dominated growth. The second of these growth models (here referred to as the constant length model) is rarely applied to small outcrop-scale faults, yet it can account for many of the geometric and kinematic attributes of these faults. The constant length growth model is supported here using displacement profiles, displacement–length relationships and tip geometries for a system of small strike-slip faults (lengths of 1–200 m and maximum displacements of 0.001–3 m) exposed in a coastal platform in New Zealand. Displacement profiles have variable shapes that mainly reflect varying degrees of fault interaction. Increasing average displacement gradients with increasing fault size (maximum displacement and length) may indicate that the degree of interaction increases with fault size. Horsetail and synthetic splays confined to fault-tip regions are compatible with little fault propagation during much of the growth history. Fault displacements and tip geometries are consistent with a two-stage growth process initially dominated by propagation followed by displacement accumulation on faults with near-constant lengths. Retardation of propagation may arise due to fault interactions and associated reduction of tip stresses, with the early transition from propagation-to displacement-dominated growth stages produced by fault-system saturation (i.e. the onset of interactions between all faults). The constant length growth model accounts for different fault types over a range of scales and may have wide application.

Abstract The Scottsville Basin in the central Virginia Piedmont forms one of the westernmost Mesozoic sedimentary basins in eastern North America. This small basin has received limited scientific attention during the past 50 years; this field trip focuses on recent stratigraphic and structural research concerning the Scottsville Basin and surrounding region. The ∼110 km 2 Scottsville Basin and adjacent ∼5 km 2 Midway Mills Sub-basin formed astride the boundary between the eastern Blue Ridge and western Piedmont. The Scottsville Basin is a half-graben, bound on its northwest margin by a segmented normal fault that places Neoproterozoic to early Paleozoic metamorphic rocks in the footwall against Triassic strata in the hanging wall. Basin strata dip to the northwest toward the boundary fault, and dip angles increase from west to east. The southeastern basin boundary, previously interpreted as a small displacement normal fault, is an unconformity with phyllitic rocks of the western Piedmont. Strata within the basin include 2–3 km of boulder to pebble conglomerate, breccia, arkosic sandstone, and siltstone. Sedimentary rocks in the Scottsville Basin were sourced primarily from Proterozoic rocks in the Blue Ridge province to the west of the basin. The age of Triassic strata in the Scottsville Basin is poorly constrained. The Midway Mills Sub-basin was originally contiguous with the Scottsville Basin, but now forms an erosional outlier. A suite of north-northwest–striking Jurassic diabase dikes crosscuts Triassic sedimentary rocks and is subparallel to the dominant extensional fracture set in basin sedimentary rocks.

Abstract We compiled paleostress analyses from previous research works collected at 591 localities of striated fault planes in rocks ranging in age from Late Cretaceous to Quaternary in the circum-Caribbean and Mexico. The purpose of the study is to quantify a progressive clockwise rotation of the Caribbean plate during its Late Cretaceous to recent subduction of the proto-Caribbean seaway. Paleostress analysis is based on the assumption that slickenside lineations indicate both the direction and sense of maximum resolved shear stress on that fault plane. We have plotted directions of maximum horizontal stress onto plate tectonic reconstructions of the circum-Caribbean plate boundaries and infer that these directions are proxies for paleo-plate motion directions of the Caribbean plate. Plotting these stress directions onto reconstructions provides a better visualization of the relation of stress directions to blocks at their time of Late Cretaceous to recent deformation. Older, more deformed rocks of Late Cretaceous to Eocene ages yield a greater scatter in derived paleostress directions as these rocks have steeper dips, more pervasive faulting, and likely are affected by large rotations as known from previous paleomagnetic studies of Caribbean plate margins. Despite more scatter in measurements from older rock units, four major events that affected the Caribbean plate and the Great Arc of the Caribbean (GAC) are recognizable from changing orientations of stress directions: (1) Late Cretaceous collision of the GAC with southern Mexico and Colombia is consistent with a northeast direction of maximum compression in rocks of this age range in southern Mexico and east-west direction in Colombia, as the GAC approached the proto-Caribbean seaway; (2) Paleocene-Eocene collision of the GAC with the Bahamas platform in Cuba and Hispaniola and with the South American plate in Venezuela is consistent with clockwise rotation of stress directions in rocks of these ages in the northern Caribbean and counter-clockwise rotation of these rocks in the southern Caribbean; (3) Late Miocene collision and indentation of the Panama arc with northwestern South America is consistent with east-west directions in rocks of these ages; and (4) Oligocene to recent strike-slip faulting along the northern and southern boundaries of the Caribbean shows consistent directions for the northern (northeast) and southern (northwest) Caribbean. Stress directions document the progressive clockwise rotation of the Caribbean plate and the GAC motion from northeast in the Late Cretaceous, to east-northeast in the Paleogene, to east-west in the Neogene.

Abstract The Virgin Islands basin is a 4.5-km-deep passage that connects the Atlantic and Caribbean seas. A variety of models have been proposed to explain its tectonic origin, which range from right- and left-lateral pull-apart basins to a rotational-type basin or even a simple, orthogonal rift basin. This study integrates three data types to better understand the Miocene to recent kinematics of basin opening and its present-day tectonics known from a parallel zone of earthquakes and GPS results that span the basin from Puerto Rico to St. Croix (U.S. Virgin Islands). A grid of 400 km of 2D seismic lines provided courtesy of the Danish Galathea 3 expedition reveals the geometry of faults underlying the offshore basin to a depth of 7.5 seconds two-way time. The basin is asymmetrical having more throw along the southeastern normal fault than the normal fault along its northwestern edge. The island of St. Croix, is the uplifted footwall of the southeastern normal fault while the island of Vieques, Puerto Rico, is the uplifted footwall of the northwestern fault. Seismic data show that both bounding normal faults are listric and have associated rollover anticlines in the basin center. A linear, possibly strike-slip fault system can be traced for a distance of 4.7 km in the center of the basin. Fourteen normal fault planes have been measured in Miocene and younger rocks on the footwall blocks of Vieques and St. Croix and revealed dip-slip normal faults having fault planes oriented northeast to east-northeast and parallel to the long axis of the offshore basin. A GPS baseline between continuously recording sites in southwest Puerto Rico and St. Croix reveals that that the basin is presently opening in a direction of 100° - or roughly at right angles to its long axis - at a rate of 2.5 mm/yr. We conclude based on seismic data, striated fault planes, and GPS results that the present-day opening of the basin and perhaps its early evolution is the result of simple, orthogonal rifting in a northwest-southeast or west-northwest/east-southeast direction.

ABSTRACT The Ice Age National Scenic Trail leads hikers on a 1200-mi (1900-km) tour of glacial and other geologic features across the State of Wisconsin. This one-day field trip highlights glacial landforms of the Superior Lobe of the southern Laurentide Ice Sheet in northwestern Wisconsin. Here the Ice Age Trail features spectacular end moraines, low-relief and high-relief hummocky topography, ice-walled-lake plains, eskers, tunnel channels, striations, and water-scoured features on basalt. The field trip involves several short hikes on parts of the trail, including one on a classic esker located in a tunnel channel. We argue that there is paleoglaciological significance to differing landform assemblages on the older, low-relief Emerald Phase land surface and the younger St. Croix Phase moraine, which has numerous high-relief hummocks, ice-walled-lake plains, and tunnel channels. Large potholes from the drainage of glacial Lake Superior are present at the Interstate State Park Unit of the Ice Age National Scientific Reserve.