Thrust Belt Deformation Research Articles

A new Late Cretaceous paleomagnetic pole from the Adel Mountains, west central Montana

North America's apparent polar wander path has been poorly defined between the mid‐Cretaceous and Paleocene reference pole positions. Existing data allowed 13° of apparent polar motion over about 22 m.y. (87–65 Ma) roughly coinciding with the beginning of Laramide deformation (∼80 Ma). We report on a paleomagnetic study of the Adel Mountain Volcanic rocks to refine the North American apparent polar wander path for this interval. The shonkinite rocks of the Adel Mountain Volcanic field are on the eastern edge of the Cretaceous‐Paleocene fold and thrust belt; some of these structures disturb the western edge of the volcanic pile. We obtained two new K‐Ar dates from the Adel rocks. One date, on biotite ( from a shonkinite dike that crosscuts most of the volcanic rocks, is 71.2±2.7 Ma. The other, a whole rock date from a flow deep in the volcanic pile, is 81.1±3.5 Ma. We collected six to nine paleomagnetic samples from each of 34 sites in roadcuts and natural outcrops of flows, dikes, and laccoliths. Positive fold and conglomerate tests, along with alternating field and thermal demagnetization, indicate that our characteristic remanent directions are primary magnetizations acquired before Late Cretaceous to Paleocene thrust belt deformation. Averaging the virtual geomagnetic poles from 26 reliable sites, all of normal polarity, yields a paleopole at 82.2°N, 209.9°E (α95 = 6.80°, k = 18.38). This pole is concordant with the Paleocene reference pole (82.0°N, 170.2°E, α95 = 3.5°, k = 18.6 (Diehl et al., 1983)) and is 11.6° from the Globerman and Irving (1988) mid‐Cretaceous pole at 71°N, 196°E. The youngest information in the Cretaceous stillstand pole is from the Niobrara Formation (Shive and Frerichs, 1974) at about 85–89 Ma. If we take the average age of the Adel Mountain Volcanics to be 76 Ma, then ∼12° of apparent polar motion occurred between 87 Ma and 76 Ma. Thus, rapid apparent polar motion correlates well with the onset of Laramide deformation.

Structural evolution of the Cape Smith Thrust Belt and the role of out‐of‐sequence faulting in the thickening of mountain belts

Out‐of‐sequence thrusting represents a mechanism capable of heterogeneous internal deformation in thrust belts and was largely responsible for internal thickening in the early Proterozoic Cape Smith Belt. The south verging thrust belt is marked by a complex history of accretion, internal deformation, thickening‐induced metamorphism, uplift, and erosion. An overall chronology for thrust belt evolution is built around the timing of thermal peak metamorphism. Prethermal peak deformation includes (1) initial piggyback stacking of basalt‐dominated thrust sheets above a basal décollement localized at the basement‐cover contact; (2) greenschist to amphibolite grade metamorphism accompanied by development of a ductile shear zone at the base of the thrust belt; and (3) out‐of‐sequence thrusting, precipitating the metamorphic thermal peak both above and below these faults. Synthermal peak out‐of‐sequence thrusting was responsible for accretion of autochthonous crystalline basement slices and eventual abandonment of the external basal décollement. Postthermal peak deformation is characterized by the continued development of out‐of‐sequence thrusts which appear to young toward the hinterland (north). Internal thickening by development of flattening cleavages or folding appears to be regionally insignificant when compared to that accomplished by the out‐of‐sequence faults. This suggests that (1) an eroding and accreting thrust wedge may be able to maintain its critical shape by out‐of‐sequence thrusting; and (2) this style of internal deformation is favored in belts dominated by relatively strong material (e.g., basalt, as in the Cape Smith Belt).

Paleomagnetic and isotopic dating of thrust-belt deformation along the eastern edge of the Helena salient, northern Crazy Mountains Basin, Montana

Thrust-belt deformation along the eastern edge of the Disturbed Belt in the Helena salient of Montana has not been well dated owing to a lack of syn- or postorogenic strata. In situ paleomagnetic data from alkaline intrusions exposed in the easternmost folds of the salient in the northern Crazy Mountains Basin, previously described as pre-, syn-, or posttectonic with respect to deformation, are well grouped (Dec. = 343°, Inc. = 61°, α95 = 5.0°, k = 46, n = 16 sites); a negative fold test is significant at minimally 95% confidence. Intrusion and magnetization acquisition thus postdate fold and thrust deformation. K-Ar age determinations of selected intrusions range from 52 to 48 Ma. Paleomagnetic and isotopic age data, combined with stratigraphic information, indicate that folding in the northern Crazy Mountains Basin is middle to late Paleocene in age. This age is in agreement with suggested dates for deformation in the northern Montana Disturbed Belt but is recognizably older than the youngest episodes of frontal deformation in the Utah-Idaho-Wyoming salient. Data from this study and existing structural and stratigraphic information demonstrate that deformation in the overthrust belt and foreland provinces of southwest Montana overlapped temporally and spatially, ranged from Late Cretaceous to earliest Eocene in age, and progressed from west to east through time.

Interaction of basement uplift and thin-skinned thrusting, Moxa arch and the Western Overthrust Belt, Wyoming: A hypothesis

Immediately north of Big Piney in western Wyoming, the Moxa arch is modeled as emplaced during the Late Cretaceous along an east-dipping, low-angle thrust (Moxa thrust) that has Precambrian basement and Paleozoic and younger cover rocks in the hanging wall. The west-verging Moxa thrust cut up-section from the basement-cover contact and flattened to the west along a detachment in Lower Triassic rocks (Thaynes detachment). During motion on the Moxa thrust, the leading edge of the hanging wall wedged westward along the Thaynes detachment, peeling back Triassic and younger rocks and thrusting them relatively eastward along the ancestral Prospect thrust. The 5.3 km of westward movement along the Moxa thrust was matched by eastward movement along the west-dipping ancestral Prospect thrust. The ancestral Prospect thrust moved an additional 5.1 km during the late Paleocene when thrust-belt deformation progressed eastward to the Moxa arch. It appears that in the Snider Basin area, the Prospect thrust does not share a ramp with the Darby thrust and that the emplacement of the Moxa arch and Prospect thrust determined the position of the later and more westerly Darby thrust.

Thrust-surface geometry: implications for thrust-belt evolution and section-balancing techniques

The application of modern concepts of thrust tectonics to orogenic belts has yielded many detailed restorable sections. Thrusts generally restore with a staircase geometry onto the undeformed stratigraphic template. Since the work of Rich (1934) it has been generally assumed that thrusts have propagated with a staircase trajectory and that hangingwall structures result from the movement of the thrust sheet up footwall ramps. However, thrusts often cut through previously folded strata with a smooth trajectory. Smooth-trajectory thrusts can be identified from footwall geometry and strain state. Once a smooth-trajectory thrust has been identified it is insufficient to produce a balanced deformed section and its restored counterpart. Sequential balanced sections must be constructed reversing the deformation history. The restoration of thrusts to a staircase trajectory has no mechanical significance as the thrust is merely acting as a marker if restored back beyond the time at which it propagated. Thrust ramps are not necessarily actual steps in the thrust surface, they are often just zones where the thrust cuts through bedding in a smoothly curving trajectory. Ductile deformation in thrust belts can reduce bed length at depth thus removing the need for structures which are necessary to preserve constant bed lengths. Ductile deformation in the internal zones of orogenic belts renders the construction of balanced sections in these zones highly problematical.

Fluvial Systems of Upper Cretaceous Mesaverde Group and Paleocene North Horn Formation, Central Utah: Record of Transition from Thin-Skinned Deformation in Foreland Region: ABSTRACT

Nonmarine strata of the upper part of the Mesaverde Group and North Horn Formation exposed between the Wasatch Plateau and the Green River in central Utah record a late Campanian tectonic transition from thrust-belt deformation to basement-cored uplift. Mesaverde Group sediments were deposited by synorogenic braided and meandering rivers. During most of Campanian time, sediment transport was east and northeast away from the thrust belt across a fluvial coastal plain. Subsequent development of the San Rafael swell, a basement uplift, between western and eastern localities caused erosional thinning of the section. Sandstones within the upper part of the Mesaverde Group form two distinct compositional suites, a lower quartzose petrofacies and an upper lithic petrofacies. Lithic grain populations of the upper petrofacies are dominated by sedimentary rock fragments on the west and volcanic rock fragments on the east. Sedimentary lithic grains were derived from the thrust belt, whereas volcanic lithic grains were derived from a volcanic terrane to the southwest. Tributary streams carrying quartzose detritus from the thrust belt entered a northeast-flowing trunk system and caused a basinward dilution of volcanic detritus. Disappearance of volcanic grains and local changes in paleocurrent directions in latest Campanian time reflect initial growth of the San Rafael swell and development of an intermon ane trunk-tributary fluvial system. Depositional onlap across the Mesaverde Group by the post-tectonic North Horn Formation indicates a minimum late Paleocene age for uplift of the San Rafael swell. End_of_Article - Last_Page 854------------

Dedolomitization in Tectonic Veins and Stylolites: Evidence for Rapid Fluid Migration During Deformation: ABSTRACT

Jurassic through Tertiary thrust-belt deformation of the Mississippian Madison Group has introduced complex fracturing, stylolitization, and carbonate vein mineralization. Host rocks are dominantly dolostone and dolomitic limestone. Tectonic veins are mineralized first by dolomite and then by multiple calcite phases. Dolomite and some generations of calcite which line veins are highly luminescent, while host-rock dolomite is non-luminescent. Both vein-lining dolomite and host-rock dolomite have been corroded and replaced by subsequent generations of calcite mineralization. These textural relationships suggest that fluids associated with thrust-belt deformation were in part extraformational and had not equilibrated with host-rock dolomite. Because thrust-belt deformation moved from west to east with time, the isotopic composition (18O, 13C) of vein and stylolite mineralization can be used to evaluate fluid migration during deformation. In three sections located along an east-west transect in the southern overthrust belt, calcite vein mineralization displays a wide range of isotopic compositions that are distinctly depleted relative to the host-rock composition. These End_Page 458------------------------------ vein-lining calcites exhibit systematic compositional changes with both time of deformation and with geographic position relative to major thrust faults. These isotopic changes in vein mineralization and pressure-solution products, together with the textural evidence for calcitization of host-rock and vein dolomite, suggest that these rocks were open to allochthonous fluid migration during deformation. End_of_Article - Last_Page 459------------

Foreland Fold and Thrust Belt Deformation Chronology, Trenton Group Limestones and Overlying Shales, Northwestern Vermont: ABSTRACT

Outcrops of Trenton and younger limestones and shales between Plattsburgh, New York, and Malletts Bay, Vermont, show eastwardly increasing progressive deformation fabric in the foreland fold and thrust belt of northwestern Vermont. The deformation sequence within the calcareous Stony Point and Ibervillle shales is: (1) bed-parallel slip, marked by grooved calcite-covered surfaces; (2) folding accompanied by pressure solution cleavage; (3) overturned folds with frequent faulting along overturned limbs; fault surfaces, marked by calcite slickensides, at high angles to calcite-filled extension fractures; cleavage frequency increases and is rotated into a lower angle with bedding; (4) late-stage features 2-3 km (1.2-1.9 mi) from the Champlain thrust include high-angle faults and pervasive shearing of early fabric. Less than 1 km (0.6 mi) from the Champlain thrust (Malletts Bay area), fold hinges are sheared out and closely spaced cleavage is folded. The Cumberland Head Argillite (transitional between the Trenton Glens Falls Limestones and the younger Stony Point Shale) contains medium to thick micrite beams in a calcareous shale matrix. Deformation in the beams occurs by brittle failure and minor thrusting up ramps in the beams. The surrounding shale deforms by bed-plane slip and pressure-solution cleavage. Glens Falls Limestone in the study area consists of medium to thick beds of fossiliferous micrite. A widely spaced pressure solution cleavage is intensified and rotated in ramp-and-fold zones. The cleavage is crosscut by late high-angle faults. A progressive increase in deformation fabric from west to east across the Ordovician limestones and shales of the northern Champlain Valley allows the fold and thrust chronology to be determined for the different lithologies. End_of_Article - Last_Page 1923------------