Kenneth M. Cruikshank

Analysis of minor fractures associated with joints and faulted joints

Abstract In this paper, we use fracture mechanics to interpret conditions responsible for secondary cracks that adorn joints and faulted joints in the Entrada Sandstone in Arches National Park, U.S.A. Because the joints in most places accommodated shearing offsets of a few mm to perhaps 1 dm, and thus became faulted joints, some of the minor cracks are due to faulting. However, in a few places where the shearing was zero, one can examine minor cracks due solely to interaction of joint segments at the time they formed. We recognize several types of minor cracks associated with subsequent faulting of the joints. One is the kink, a crack that occurs at the termination of a straight joint and whose trend is abruptly different from that of the joint. Kinks are common and should be studied because they contain a great deal of information about conditions during fracturing. The sense of kinking indicates the sense of shear during faulting: a kink that turns clockwise with respect to the direction of the main joint is a result of right-lateral shear, and a kink that turns counterclockwise is a result of left-lateral shear. Furthermore, the kink angle is related to the ratio of the shear stress responsible for the kinking to the normal stress responsible for the opening of the joint. The amount of opening of a joint at the time it faulted or even at the time the joint itself formed can be estimated by measuring the kink angle and the amount of strike-slip at some point along the faulted joint. Other fractures that form near terminations of pre-existing joints in response to shearing along the joint are horsetail fractures. Similar short fractures can occur anywhere along the length of the joints. The primary value in recognizing these fractures is that they indicate the sense of faulting accommodated by the host fracture and the direction of maximum tension. Even where there has been insignificant regional shearing in the Garden Area, the joints can have ornate terminations. Perhaps the simplest is a veer, where the end of one joint segment turns gradually toward a nearby joint segment. The veer is a result of a nearby, shear-stress-free face such as a joint surface. Our greatest difficulty has been explaining long overlap of parallel joint segments, that is, the lack of veer. The only plausible explanation we know is suggested by the research of Cottrell and Rice, that high compression parallel to the joint segments will tend to prevent the joints from turning toward one another. The most interesting and puzzling fractures are stepped joints and associated echelon cracks, in which the slight misalignment of the stepped joints suggests mild left-lateral shear, while the strong misalignment of echelon cracks that continue the traces of the stepped joints suggests strong right-lateral shear. The stepped joints are thought to reflect local left-lateral shearing that acted over an area of several thousand square metres, whereas the stepped echelon cracks reflect local interaction between the tips of nearby joints propagating in different directions.

Duplex structures connecting fault segments in Entrada Sandstone

Abstract All stages in the development of a duplex structure—from isolated, stepped fault segments, to segments joined by a single ramp, to segments joined by tens of ramps—are preserved along strike-slip and normal faults in Entrada Sandstone in Arches National Park, Utah. Bedding is either absent or at a high angle to the duplex-like structures in Entrada Sandstone, thus it had no significant role in constraining their geometry. We can reproduce the essential features of a duplex structure along a normal fault with mechanical and kinematic models previously used to simulate duplex structures along thrust faults. However the models do not account for the amount of observed thickening at the step where the structure forms. This suggests that the geometry of duplex-like structures along these strike-slip faults may be a result of interaction between the fault segments.

Unweaving the joints in Entrada Sandstone, Arches National Park, Utah, U.S.A.

Abstract On the southwest limb of Salt Valley Anticline, Arches National Park, Utah three sets of joints are developed in the Entrada Sandstone covering an area of about 6 km2. Within the 20 m thick Moab Member, a single joint set is is found in three distinct areas, separated by a second set of joints at a 35° angle to the first set. Joint interaction features show that the second set is younger than the first. This illustrates that joints of a single set do not have to fill the entire area across which the stresses that formed the joints were acting. The underlying Slickrock Member contains a third set of joints, which is at an angle of 5°–35° to joints in the Moab Member. The Slickrock set nucleated from the lower edges of joints of all orientations in the overlying Moab Member. Thus, the fracture pattern evolved both horizontally, within the same unit, and vertically between units. The sequence of jointing is determined by establishing the relative ages of each joint set. Each joint orientation is best interpreted as representing a direction of maximum compression, ruling out the possibility that the joints are a conjugate set. The joints, and an earlier set of deformation bands, record a 95° counterclockwise rotation of the direction of maximum compression.

Role of fracture localization in arch formation, Arches National Park, Utah

Spectacular rock fins on the flanks of Salt Valley anticline in southeast Utah are formed by erosion along zones of joints. Within a rock fin, arches form where intense fracturing is localized. Fracture localization is controlled by shear displacement along existing horizontal or vertical discontinuities. Horizontal discontinuities may be shale layers, shale lenses, or bedding planes, whereas vertical discontinuities are usually preexisting joint segments. The roof and overall shape of an arch is controlled by existing shale layers, interfaces between sandstones of different properties, or secondary fractures due to shear on vertical joints. Joints that bound rock fins are related to the formation of the diapir-cored Salt Valley anticline. Shear displacement along existing discontinuities, which localizes intense fracturing, is probably related to the growth of Salt Valley anticline and its subsequent collapse due to dissolution of the anticlines salt core.

Propagation of long fractures in the Ronne Ice Shelf, Antarctica, investigated using a numerical model of fracture propagation

Long rifts near the front of the Ronne Ice Shelf, Antarctica, are observed to begin as fractures along the lateral boundaries of outlet streams feeding the shelf. These flaws eventually become the planes along which tabular icebergs calve. The fractures propagate laterally as they advect through the shelf, with orientations that can be explained by the glaciological stress field. Fracture length remains constrained over much of the advective path, and locations of crack tip arrest are observed to coincide with structural boundaries, such as suture zones between ice from adjacent outlet glaciers. Geomechanical principles and numerical models demonstrate that in the absence of these suture zones crack tips are unlikely to arrest in these locations. We conclude that lateral inhomogeneity in the ice plays an important role in fracture mechanics through most of the ice shelf. Only near the shelf front are these local structural effects overcome such that the large rifts required for tabular iceberg production develop.

High-amplitude folding of linear-viscous multilayers

Abstract High-amplitude folding of viscous multilayers during shortening can be analyzed with a theoretical solution motivated by first-order theoretical analysis of folding by Raymond Fletcher and Ronald Smith. The solution method can better match boundary conditions along irregular interfaces than the first-order method, so it increases the range of slopes over which linear-viscous folding theory can be applied. In our method, rather than solving algebraically for a small number of constants in the flow equations, we numerically solve for a large number of constants, the values of which are chosen so that they minimize errors in matching conditions at the interfaces in a least-squares sense. A similar method has been applied to problems of density instability involving a single deformable interface with bonded contacts; however, we extend the method to include shortening parallel to interfaces and many deformable interfaces so that we can deal with problems of multilayer folding. Contacts between the layers can be firmly bonded, slip freely, or slip with viscous resistance. We use the solution to produce high-amplitude folds in single layers embedded in soft media, and in simple repetitive multilayers confined above and below by stiff or soft media. We show that the folding of linear-viscous multilayers can largely reproduce the gross forms of some small folds in the Huasna syncline in the central California Coast Range as well as the Berry-Buffalo syncline in the central Pennsylvania Appalachians. However, the sharp, chevron-like forms in these natural examples are notably missing in the simulations based on linear-viscous theory.

Coseismic Subsidence and Paleotsunami Run-Up Records from Latest Holocene Deposits in the Waatch Valley, Neah Bay, Northwest Washington, U.S.A.: Links to Great Earthquakes in the Northern Cascadia Margin

ABSTRACT Peterson, C.D.; Cruikshank, K.M.; Darienzo, M.E.; Wessen, G.C.; Butler, V.L., and Sterling, S.L., 2013. Coseismic subsidence and paleotsunami run-up records from latest Holocene deposits in the Waatch Valley, Neah Bay, northwest Washington, U.S.A.: links to great earthquakes in the northern Cascadia Margin. Representative shallow cores (1–2-m depth) from the Waatch Valley (n = 10) and from Neah Bay back-barrier wetlands (n = 7) record four coseismic subsidence events and associated paleotsunami inundations during the last 1300 years in the North Central Cascadia Margin. Three of the subsidence events (SUB1, SUB2b, and SUB3) correlate to reported great earthquakes dated at AD 1700, about 1.1 ka, and about 1.3 ka. An additional subsidence horizon (SUB2a), which is newly discovered in the study area, might correlate to a widely reported paleotsunami inundation, dated between 0.7 and 0.9 ka in the study region. The magnitudes of paleosubsidence in the Waatch Valley are modest (about 0.5−1.0 m), as based on macofossil evidence of abrupt wetland burial. Paleotsunami origins of the four landward thinning sand sheets are confirmed by the presence of ocean diatom taxa and beach sand grains. Long wave run-up in the low-gradient Waatch floodplain ranged from 2.5 to 4.5 km up-valley distance from the present tidal inlet shoreline. Paleotsunami overtopping of the Neah Bay barrier ridge (6–8-m elevation North American Vertical Datum of 1988 [NAVD88]) provides the first estimates of paleotsunami minimum run-up height at the entrance to the Juan de Fuca Strait.

Computer simulation of growth of duplex structures

Abstract The geometry of hinterland-dipping duplex structures, produced by deformation over several successive ramp faults, can be explored using composite kinematic and mechanical models which were developed to describe the deformation in the vicinity of an isolated ramp. The composite kinematic model permits the relationships between ramp height, angle, spacing, and displacement to be calculated. A special case of hinterland-dipping duplex structures, approximately flat-topped structures, require a specific amount of displacement that depends on ramp angle, height, and spacing. The requirement is very sensitive to final ramp spacing, and relatively insensitive to displacement.

Distal Run-up Records of Latest Holocene Paleotsunami Inundation in Alluvial Flood Plains: Neskowin and Beaver Creek, Oregon, Central Cascadia Margin, West Coast U.S.A.

Abstract Paleotsunami records in two localities of the central Cascadia margin, Neskowin and Beaver (West Coast, U.S.A., Northeast Pacific Ocean coast), are extended landward to distal flood plain settings. Three paleotsunami sand sheets are correlated to Cascadia subduction zone earthquakes, between 0.3 and ∼1.3 ka in age. One older paleotsunami layer (2960–3220 cal YBP) is apparent in some deeper core sites from the Beaver Creek locality. Marine sand (22%–100%) and marine diatoms (40%–100%) from the distal sand sheets distinguish the catastrophic marine inundations from creek floods. The greatest inundations are correlated to two Cascadia paleotsunami events, #3 at ∼1.3 ka and an older event between ∼2.6 and ∼3.2 ka, based on radiocarbon dating and great earthquake sequence. The best-preserved records are from paleotsunami #3, which reached 4.1 km in overland inundation up the North Beaver flood plain (3 m elevation North American Vertical Datum). At the Neskowin locality, a sand sheet from the #3 paleotsunami was traced to 8.3 m elevation in the Hawk flood plain. Adjusting for paleosea level at 1.3 ka, we estimate that the #3 paleotsunami run-up height reached 9 m at a landward distance of 1.0 km in Neskowin. The paleotsunami sand sheets in Neskowin and Beaver represent the maximum recorded distal run-up for Cascadia paleotsunami reported to date. The potential for preservation of marine surge deposits in alluvial flood plains should greatly extend the geologic record of prehistoric inundations in other susceptible coastlines.

Paleotsunami Inundation of a Beach Ridge Plain: Cobble Ridge Overtopping and Interridge Valley Flooding in Seaside, Oregon, USA

The Seaside beach ridge plain was inundated by six paleotsunamis during the last ~2500 years. Large runups (adjusted >10 m in height) overtopped seawardmost cobble beach ridges (7 m elevation) at ~1.3 and ~2.6 ka before present. Smaller paleotsunami (6–8 m in height) likely entered the beach plain interior (4-5 m elevation) through the paleo-Necanicum bay mouth. The AD 1700 Cascadia paleotsunami had a modest runup (6-7 m height), yet it locally inundated to 1.5 km landward distance. Bed shear stresses (100–3,300 dyne ) are estimated for paleotsunami surges (0.5–2 m depths) that flowed down slopes (0.002–0.017 gradient) on the landward side of the cobble beach ridges. Critical entrainment shear stresses of 1,130–1,260 dyne were needed to dislodge the largest clasts (26–32 cm diameter) in paleotsunami coulees that were cut (100–200 m width) into the landward side of the cobble ridges.