Abstract A field trip to the Baraboo District provides an amazing opportunity to teach (or learn) many important aspects of structural geology. For example, students can define the regional-scale shape of the Proterozoic-age south-verging Baraboo Syncline from data on bedding attitudes and facing indicators and then can compare this shape to a digital elevation model of the district to see relationships between the dip of a stratigraphic unit and the width of its outcrop belt. Key outcrops of the Baraboo District, which we describe in detail, allow students to identify and sketch mesoscopic tectonic structures (joints, spaced and phyllitic cleavage, veins, faults, slip lineations, parasitic folds, boudinage, crenulation cleavage, and kink folds) and to interpret the kinematic significance of these structures. Students will leave Baraboo with a clear image of how progressive crustal shortening can be accommodated under lower-greenschist conditions.

ABSTRACT The Baraboo District includes an exceptional array of outcrops that have provided geological enlightenment to students and professionals, alike, for 150 years. In the late nineteenth century, several fundamental structural principles were developed here, such as criteria for determining stratigraphic facing and the significance of cleavage-bedding relations. More recent studies of deformational features in the folded Baraboo Quartzite, such as crenulation cleavage and quartz fabrics, have yielded insights into the kinematics of folding in the District and the significance of regional tectonics in the context of the Proterozoic assembly of North America. Additional petrologic, geochemical, and isotopic studies have established the age of the Baraboo Quartzite (≤1700 Ma), identified a Paleoproterozoic weathering profile, confirmed the supermature composition of the Baraboo Quartzite, established the presence of geon 14 hydrothermal alteration, and elucidated the Proterozoic tectonothermal evolution of the District, all of which bear importantly on Proterozoic tectonic, atmospheric, and climatic conditions in the southern Lake Superior region. By Late Cambrian time, the Baraboo Quartzite was a ring of islands, which was abutted by spectacular conglomerates deposited by tropical storms. These were surrounded by more distal sandstones and were eventually buried by Ordovician dolomite and sandstone. During the field trip, we will visit eleven localities, which have been selected to illustrate the key geological features of this North American classic.

Abstract The Hudson Valley fold-thrust belt of eastern New York State involves a relatively thin sequence of shallow-marine Silurian and Lower Devonian strata. Because of the thinness of this sequence, structures of the belt are relatively small. Thus, first-order ramps and flats, and fault-related folds can be seen in their entirety at a single roadcut. A field trip in the southern half of the Hudson Valley fold-thrust belt, from the latitude of Catskill to the latitude of Rosendale, provides an opportunity to see many examples of these structures, and to discuss the three-dimensional architecture of fold-thrust belts in an orocline. In particular, we will see how along-strike changes in stratigraphy affect fold wavelength and the depth of detachment horizons. The trip also provides the opportunity to examine the relationship between mesoscopic structures (e.g., solution cleavage and veining) and first-order structures.

ABSTRACT The problem of how map-view curves (variously named salients, recesses, arcs, oroclines, virgations, festoons, bends, oroflexes, and syntaxes) in fold-thrust belts originate has caught the attention of geologists for more than 200 years. This chapter reviews the advances in understanding curves. Early geologists recognized that by understanding curve formation, one might gain insight into the process of orogeny. In recent decades, researchers have proposed several geologically reasonable models to explain curve formation; no single explanation can work for all curves. The majority of curving fold-thrust belts can be called “basin controlled,” in that their presence reflects the architecture of the predeformational sedimentary basin from which the curve formed. Factors such as depth to detachment, rock strength, detachment strength, and detachment slope all affect the width of a fold-thrust belt for a given amount of hinterland displacement, as predicted by critical-taper theory. Therefore, along-strike variation in these factors leads to the inception of thrust belts that vary in width along strike, and thus have curved traces. However, not all curved thrust belts are basin controlled. Other causes for curve formation include interaction of a thrust belt with foreland obstacles or promontories, hinterland collision of an indenter, interaction with subsequent strike-slip faults, and warping of the downgoing (underthrust) plate. Not all curve-forming processes lead to “oroclinal” bending of a fold-thrust belt, in that not all curves involve rotation of segments of the thrust belt around a vertical axis. Thus, not all curves are oroclines, where the term “orocline” specifically refers to a mountain belt bent in plan. Basin-controlled curves and curves formed in front of indenters generally initiate with a curved trace, whereas curves formed in response to interactions with foreland obstacles or with strike-slip faults involve oroclinal bending.

The Penokean orogen of Michigan's Upper Peninsula includes a belt of dome-and-keel structure presently defined by deep troughs, or “keels,” of Paleoproterozoic Marquette Range Supergroup strata between gneiss domes composed of Archean basement rock. Structural, metamorphic, and geochronological data from the Southern Complex indicates that dome-and-keel structure developed in two stages. The first stage involved rise (intrusion, possibly diapirically) of the 2.6 Ga Bell Creek Assemblage (a gneissic megacrystic granite) into the Twin Lake Assemblage (migmatitic mafic to felsic gneiss). Flow folding in gneisses and migmatites indicate that this Archean event involved plastic flow of basement. The second stage occurred after the ca. 1.8 Ga Penokean orogeny, subsequent to the formation of a fold-thrust belt involving Paleoproterozoic Marquette Range Supergroup strata. During this stage, deep, narrow troughs developed in the region that had been the fold-thrust belt. Analysis of structures bordering the Republic Trough indicates that Paleoproterozoic keel borders are shear zones; keel rocks moved down relative to dome rocks. In effect, the Paleoproterozoic keels are steep- to vertical-sided grabens, suggesting that the dome-and-keel architecture is a consequence of extensional faulting. Amphibolite facies metamorphism occurred in Paleoproterozoic keel strata along dome-keel borders. Peak-metamorphism developed adjacent to dome borders at the time keel-bounding shear zones were active. The relative timing of Paleoproterozoic keel formation supports the model that this stage reflects collapse of the Penokean orogen. Our results show that the present dome-and-keel structure of the Southern Complex region represents superposition of Paleoproterozoic collapse structures on preexisting Archean gneiss domes.