Laramide Compression Research Articles

Thinning and Heating of Laramide Continental Lower Crust Recorded by Zircon Petrochronology

AbstractZircon grains from the metasedimentary lower crust of the Rio Grande Rift, New Mexico, preserve a metamorphic record of the transition from Laramide compression to Eocene extension. Zircon U‐Pb isotopes and trace‐element concentrations from five two‐pyroxene metaigneous granulite xenoliths define discrete populations: older zircon cores (∼15–50 Ma) that are depleted in heavy rare‐earth elements (HREE) but Ti‐rich, and younger zircon rims (∼3–15 Ma) with elevated HREE and lower Ti concentrations. Coupled phase equilibria and garnet‐melt‐zircon trace‐element partitioning calculations show that the older zircon cores equilibrated in thick (>40 km), hot (800–900°C), garnet‐bearing lower crust during the cessation of compression at the end of the Laramide orogeny. Zircon rim domains equilibrated at lower pressures, consistent with >9 km of thinning of the lower crust. Thermal‐kinematic calculations show that these pressure‐temperature‐time constraints require thinning of the lithospheric mantle prior to and during regional Cenozoic extension. Convective erosion of the mantle lithosphere over tens of millions of years, possibly facilitated by dynamics of the Farallon slab, provides a mechanism to facilitate lower crustal heating and extension.

Cenozoic sediment bypass versus Laramide exhumation and erosion of the Eagle Ford Group: Perspective from modelling of organic and inorganic proxy data (Maverick Basin, Texas, USA)

Abstract The Cenozoic unconformity above the Late Cretaceous carbonates within the Maverick Basin is a unique feature of Texas (USA). Hypotheses accounting for the unconformity include (1) Cenozoic sediment bypass, and (2) ~6400 m of erosion during the Laramide orogeny. Both hypotheses have different implications for the burial history of the Eagle Ford Group (EFG) and for our understanding of the Laramide orogeny. We generated clumped isotope data and organic maturation proxies from the same location. Carbonate clumped isotope temperatures obtained (113 ± 9 °C) represent recrystallization during burial and a minimum estimate of the maximum burial temperature. This constraint is significantly warmer than apparent organic temperatures derived using an Arrhenius equation (40–55 °C). Organic matter transformation and carbonate recrystallization respond to temperature over different time scales and therefore capture snapshots of the thermal history particular to the chemical reactions that control the respective processes. Using numerical forward modeling on the combination of two different temperature proxies, we derived a new hypothesis: ~2800 m of Cenozoic sediments were accumulated and then eroded during late Laramide compression. This is significantly less erosion than previous estimates, indicating the impact of the Laramide orogeny in the basin may have been less severe than previously thought.

Cenozoic structural evolution of the Catalina metamorphic core complex and reassembly of Laramide reverse faults, southeastern Arizona, USA

AbstractThis study investigates the Late Cretaceous through mid-Cenozoic structural evolution of the Catalina core complex and adjacent areas by integrating new geologic mapping, structural analysis, and geochronologic data. Multiple generations of normal faults associated with mid-Cenozoic extensional deformation cut across older reverse faults that formed during the Laramide orogeny. A proposed stepwise, cross-sectional structural reconstruction of mid-Cenozoic extension satisfies surface geologic and reflection seismologic constraints, balances, and indicates that detachment faults played no role in the formation of the core complex and Laramide reverse faults represent major thick-skinned structures.The orientations of the oldest synextensional strata, pre-shortening normal faults, and pre-Cenozoic strata unaffected by Laramide compression indicate that rocks across most of the study area were steeply tilted east since the mid-Cenozoic. Crosscutting relations between faults and synextensional strata reveal that sequential generations of primarily down-to-the-west, mid-Cenozoic normal faults produced the net eastward tilting of ∼60°. Restorations of the balanced cross section demonstrate that Cenozoic normal faults were originally steeply dipping and resulted in an estimated 59 km or 120% extension across the study area. Representative segments of those gently dipping faults are exposed at shallow, intermediate (∼5–10 km), and deep structural levels (∼10–20 km), as distinguished by the nature of deformation in the exhumed footwall, and these segments all restore to high angles, which indicates that they were not listric. Offset on major normal faults does not exceed 11 km, as opposed to tens of kilometers of offset commonly ascribed to “detachment” faults in most interpretations of this and other Cordilleran metamorphic core complexes. Once mid-Cenozoic extension is restored, reverse faults with moderate to steep original dips bound basement-cored uplifts that exhibit significant involvement of basement rocks. Net vertical uplift from all reverse faults is estimated to be 9.4 km, and estimated total shortening was 12 km or 20%. This magnitude of uplift is consistent with the vast exposure of metamorphosed and foliated cover strata in the northeastern and eastern Santa Catalina and Rincon Mountains and with the distribution of subsequently dismembered mid-Cenozoic erosion surfaces along the San Pedro Valley. New and existing geochronologic data constrain the timing of offset on local reverse faults to ca. 75–54 Ma.The thick-skinned style of Laramide shortening in the area is consistent with the structure of surrounding locales. Because detachment faults do not appear to have resulted in the formation of the Catalina core complex, other extensional systems that have been interpreted within the context of detachments may require further structural analyses including identification of crosscutting relations between generations of normal faults and palinspastic reconstructions.

U-Pb dating of calcite veins reveals complex stress evolution and thrust sequence in the Bighorn Basin, Wyoming, USA: REPLY

We report U-Pb absolute ages of calcite cements from a diffuse vein network documented in the Bighorn Basin (Wyoming, USA), where distinct systematic vein sets developed at the front of the thin-skinned Sevier orogen, during Laramide layer-parallel shortening, and during thick-skinned Laramide thrusting and folding. The U-Pb age distribution illustrates: (1) an outward (eastward) transmission of Sevier orogenic stress (from 89.7 ± 2.9 Ma [2σ] in the west, and from 75.3 ± 2.8 Ma in the east); and (2) an inward (westward) development of Laramide-related fracturing and folding (72 ± 3.0 Ma and 45.4 ± 1.8 Ma, respectively, in the east; 60.5 ± 4.6 Ma and 27.9 ± 1.1 Ma, respectively, in the west), which is in accordance with the known sequence of exhumation of the major Laramide basement arches. Our results also show that the stress related to Laramide compression first overprinted the stress related to Sevier compression in the sedimentary cover around major basement uplifts. This study highlights the utility of U-Pb calcite geochronology as a powerful tool for constraining complex sequences of deformation in orogenic forelands.

U-Pb dating of calcite veins reveals complex stress evolution and thrust sequence in the Bighorn Basin, Wyoming, USA

We report U-Pb absolute ages of calcite cements from a diffuse vein network documented in the Bighorn Basin (Wyoming, USA), where distinct systematic vein sets developed at the front of the thin-skinned Sevier orogen, during Laramide layer-parallel shortening, and during thick-skinned Laramide thrusting and folding. The U-Pb age distribution illustrates: (1) an outward (eastward) transmission of Sevier orogenic stress (from 89.7 ± 2.9 Ma [2σ] in the west, and from 75.3 ± 2.8 Ma in the east); and (2) an inward (westward) development of Laramide-related fracturing and folding (72 ± 3.0 Ma and 45.4 ± 1.8 Ma, respectively, in the east; 60.5 ± 4.6 Ma and 27.9 ± 1.1 Ma, respectively, in the west), which is in accordance with the known sequence of exhumation of the major Laramide basement arches. Our results also show that the stress related to Laramide compression first overprinted the stress related to Sevier compression in the sedimentary cover around major basement uplifts. This study highlights the utility of U-Pb calcite geochronology as a powerful tool for constraining complex sequences of deformation in orogenic forelands.

Organic geochemistry and petrology of subsurface Paleocene–Eocene Wilcox and Claiborne Group coal beds, Zavala County, Maverick Basin, Texas, USA

Coal samples from a coalbed methane exploration well in northern Zavala County, Maverick Basin, Texas, were characterized through an integrated analytical program. The well was drilled in February, 2006 and shut in after coal core desorption indicated negligible gas content. Cuttings samples from two levels in the Eocene Claiborne Group were evaluated by way of petrographic techniques and Rock–Eval pyrolysis. Core samples from the Paleocene–Eocene Indio Formation (Wilcox Group) were characterized via proximate–ultimate analysis in addition to petrography and pyrolysis. Two Indio Formation coal samples were selected for detailed evaluation via gas chromatography, and Fourier transform infrared (FTIR) and 13C CPMAS NMR spectroscopy. Samples are subbituminous rank as determined from multiple thermal maturity parameters. Elevated rank (relative to similar age coal beds elsewhere in the Gulf Coast Basin) in the study area is interpreted to be a result of stratigraphic and/or structural thickening related to Laramide compression and construction of the Sierra Madre Oriental to the southwest. Vitrinite reflectance data, along with extant data, suggest the presence of an erosional unconformity or change in regional heat flow between the Cretaceous and Tertiary sections and erosion of up to >5km over the Cretaceous. The presence of liptinite-rich coals in the Claiborne at the well site may indicate moderately persistent or recurring coal-forming paleoenvironments, interpreted as perennially submerged peat in shallow ephemeral lakes with herbaceous and/or flotant vegetation. However, significant continuity of individual Eocene coal beds in the subsurface is not suggested. Indio Formation coal samples contain abundant telovitrinite interpreted to be preserved from arborescent, above-ground woody vegetation that developed during the middle portion of mire development in forested swamps. Other petrographic criteria suggest enhanced biological, chemical and physical degradation at the beginning and end of Indio mire development. Fluorescence spectra of sporinite and resinite are consistent and distinctly different from each other, attributed to the presence of a greater proportion of complex asphaltene and polar molecules in resinite. Gas chromatography of resinite-rich coal shows sesquiterpenoid and diterpenoid peaks in the C14–17 range, which are not present in resinite-poor coal. Quantities of extracts suggest bitumen concentration below the threshold for effective source rocks [30–50mg hydrocarbon/g total organic carbon (HC/g TOC)]. Saturate/aromatic and pristane/phytane (Pr/Ph) ratios are different from values for nearby Tertiary-reservoired crude oil, suggesting that the Indio coals are too immature to source liquid hydrocarbons in the area. However, moderately high HI values (200–400mg HC/g rock) may suggest some potential for naphthenic–paraffinic oil generation where buried more deeply down stratigraphic/structural dip. Extractable phenols and C20+ alkanes are suggested as possible intermediates for acetate fermentation in microbial methanogenesis which may, however, be limited by poor nutrient supply related to low rainfall and meteoric recharge rate or high local sulfate concentration.

Origin of the Colorado Mineral Belt

The Colorado Mineral Belt (CMB) is a northeast-trending, ∼500-km-long, 25–50-km-wide belt of plutons and mining districts (Colorado, United States) that developed within an ∼1200-km-wide Late Cretaceous–Paleogene magma gap overlying subhorizontally subducted segments of the Farallon plate. Of the known volcanic gaps overlying flat slabs in subduction zones around the Pacific Basin, none contains zones of magmatism analogous to the CMB. I suggest that the primary control of the CMB was a northeast-trending segment boundary within the underlying Farallon flat slab. The boundary was dilated during warping of slab segments by the overriding thick (∼200 km) lithospheres of the Wyoming Archean craton and the continental interior craton during acceleration of Farallon–North American convergence beginning in mid-Campanian time (ca. 75 Ma). Because the primary control was not in the North American plate, the CMB cut indiscriminately across the geologic grain of Colorado, seemingly independent of the tectonic elements it crossed. A series of discontinuous shear zones of Proterozoic ancestry provided some local control at the district level but were not the primary control. Geologic contrasts north and south of the CMB reflect its relationship to a segment boundary in the Farallon plate. The dominant trends of Laramide basement-cored uplifts are northwestward north of the CMB but northward south of the CMB. Laramide sedimentary deposits of Late Cretaceous and Paleogene age (exclusive of the Sevier foredeep) are as much as 6 km thick north of the CMB versus only ≤3 km south of the CMB. The Farallon segment south of the CMB rolled back to the southwest and sank into the mantle beginning ca. 37 Ma with resultant major ignimbrite volcanism and generation of the large San Juan and Mogollon-Datil volcanic fields. Volcanism in the Rocky Mountains north of the CMB was sparse. Laramide plutons (ca. 75–43 Ma) are mainly alkaline monzonites and quartz monzonites in the northeastern CMB, but dominantly calc-alkaline granodiorites in the central CMB. Geochemical and isotopic studies indicate that CMB magmas were generated mainly in metasomatized Proterozoic intermediate to felsic lower crustal granulites and mafic rocks (± mantle). Late Eocene–Oligocene rollback magmatism superimposed on the CMB during waning of Laramide compression (ca. 43–37 Ma) resulted in world-class sulfide replacement ores in the Leadville area. Overprinting of the CMB by Rio Grande Rift extension beginning ca. 33 Ma resulted in intrusion of evolved alkali-feldspar granites and generation of major porphyry molybdenum deposits at Climax and Red Mountain.

Sedimentary and diagenetic controls on the multiscale fracturing pattern of a carbonate reservoir: The Madison Formation (Sheep Mountain, Wyoming, USA)

A multidisciplinary approach has been applied to characterize the Madison Formation in the Sheep Mountain anticline at different scales, in order to illustrate the relationship between sedimentology, diagenesis and fracturing pattern. Sedimentary facies are organized into three types of elementary facies sequences showing different vertical evolutions of their petrophysical properties. This is controlled by the double influence of the initial sedimentary facies and the diagenetic evolution. Three main sets of fractures (related to the Sevier and Laramide compressive phases and to the folding of Sheep Mountain) have been described at three orders of scale. At small scale, the Sevier-related set I and folding related set III are the most abundant. Set II, dated from the early stage of the Laramide compression, exhibits the highest fracture intensity. There is also a clear faciologic control on the fracture intensity. Small-scale mechanical units correspond to entire facies sequences when no or little petrophysical property contrasts between beds are observed. Conversely, small-scale mechanical units correspond to each bed of a facies sequence in the case of high contrasts of petrophysical properties between beds. At intermediate scale, set II is not present and sets I and III show the highest fracture intensities. The latter are partly controlled by the mechanical unit thickness. Intermediate and large-scale mechanical unit distribution is mainly controlled by the overall faciologic vertical stacking pattern and by the position across the Sheep Mountain anticline (forelimb or backlimb). Sheep Mountain outcrops provide a good spatial representation of the carbonate reservoir heterogeneities from the micro- to the field scale, illustrating the complexity of fractured-carbonate reservoirs.

The interplay of fold mechanisms and basement weaknesses at the transition between Laramide basement-involved arches, north-central Wyoming, USA

Horizontally-shortened, basement-involved foreland orogens commonly exhibit anastomosing networks of bifurcating basement highs (here called arches) whose structural culminations are linked by complex transition zones of diversely-oriented faults and folds. The 3D geometry and kinematics of the southern Beartooth arch transition zone of north-central Wyoming were studied to understand the fold mechanisms and control on basement-involved arches. Data from 1581 slickensided minor faults are consistent with a single regional shortening direction of 065°. Evidence for oblique-slip, vertical axis rotations and stress refraction at anomalously-oriented folds suggests formation over reactivated pre-existing weaknesses. Restorable cross-sections and 3D surfaces, constrained by surface, well, and seismic data, document blind, ENE-directed basement thrusting and associated thin-skinned backthrusting and folding along the Beartooth and Oregon Basin fault systems. Between these systems, the basement-cored Rattlesnake Mountain backthrust followed basement weaknesses and rotated a basement chip toward the basin before the ENE-directed Line Creek fault system broke through and connected the Beartooth and Oregon Basin fault systems. Slip was transferred at the terminations of the Rattlesnake Mountain fault block by pivoting to the north and tear faulting to the south. In summary, unidirectional Laramide compression and pre-existing basement weaknesses combined with fault-propagation and rotational fault-bend folding to create an irregular yet continuous basement arch transition.

Post-Laramide and pre-Basin and Range deformation and implications for Paleogene (55–25 Ma) volcanism in central Mexico: A geological basis for a volcano-tectonic stress model

At central-eastern Mexico, in the Mesa Central province, there are several ranges that were formed after the K/T Laramide compression but before the Basin and Range peak extensional episodes at middle–late Oligocene. Two important volcano-tectonic events happened during this time interval, 1) uplift of crustal blocks exhuming the Triassic–Jurassic metamorphic sequence and formation of basins that were filled with red beds and volcanic sequences, and 2) normal faulting and tilting to the NE of these blocks and fanglomerate filling of graben and half-graben structures. The first event, from late Paleocene to early Eocene, was related to NNE and NNW oriented dextral strike-slip faults. These faults were combined with NW–SE en echelon faulting in these blocks through which plutonism and volcanism occurred. The second event lasted from early Oligocene to early Miocene and coincided with Basin and Range extension. Intense volcanic activity occurred synchronously with the newly-formed or reactivated old fault systems, producing thick sequences of silicic pyroclastic rocks and large domes. Volcano-tectonic peaks occurred in three main episodes during the middle–late Oligocene in this part of Mexico, at about 32–30 Ma, 30–28 Ma, and 26–25 Ma. The objectives of this work is to summarize the volcano-tectonic events that occurred after the end of the Laramide orogeny and before the peak episodes of Basin and Range faulting and Sierra Madre Occidental Oligocene volcanism, and to discuss the influence of these events on the following Oligocene–Miocene volcano-tectonic peak episodes that formed the voluminous silicic volcanism in the Mesa Central, and hence, in the Sierra Madre Occidental. A model based upon geological observations summarizes the volcanic-tectonic evolution of this part of Mexico from the late Paleocene to the Early Miocene.

Basin architecture and syndepositional fault activity during deposition of the Neoproterozoic Mackenzie Mountains supergroup, Northwest Territories, CanadaNorthwest Territories Geoscience Office Contribution 0040.

The early Neoproterozoic Mackenzie Mountains supergroup (MMSG; ∼4 km thick, <1083 Ma, >779 Ma) was deposited in a large but poorly understood basin. Analysis of new and existing stratigraphic data reveals unexpected, abrupt changes in thickness and lithofacies patterns in formation scale and smaller units over short distances parallel and perpendicular to depositional strike. These isopach and lithofacies patterns depict irregularly subsiding syndepositional sub-basins. These basins may have developed during basin extension in a lower-plate segment of an evolving passive margin, as the result of episodic, excessive subsidence along growth faults parallel to the basin margin, and crustal-scale (transfer) faults at a high angle to the margin. The extent of the lower-plate area is inferred to correspond to the distribution of the MMSG (∼500 km strike length); upper-plate segments are inferred to flank the MMSG embayment to the northwest and southeast. A lower-plate model for the arcuate MMSG embayment has important implications for the Mackenzie Mountains zinc district. The present-day regional distribution of known carbonate-hosted base-metal showings suggests that the configuration of the thinned, faulted Neoproterozoic lower-plate segment, together with later reactivation of basement-rooted transfer faults during Neoproterozoic–Paleozoic rifting or Laramide compression, may have been important in focussing metalliferous fluids into overlying units.

The role of fractures in the structural interpretation of Sheep Mountain Anticline, Wyoming

The development of fractures in the sedimentary layers of Sheep Mountain Anticline, a Laramide asymmetric fault-cored fold of the Bighorn Basin, is documented and interpreted to constrain the kinematic evolution of the fold. The fracture pattern is interpreted to identify five main fracture sets. A fracture set striking 110° (Set I), oblique to the future fold trend, is interpreted as a regional fracture set that predates Laramide compression. A joint set (Set II), striking 045° and present in the hinge and backlimb, is related to the NE-oriented Laramide compression just prior to and during initial anticline growth. Joints striking 135° (Set III), parallel to the fold trend, are found within the hinge and are interpreted to have developed in response to the bending of layers. The two youngest fracture sets are attributed to a late stage of fold growth: a joint set (Set IV) in the backlimb striking parallel to the Set I fractures, but vertical; and a fracture set in the forelimb consisting of Set I fractures reactivated as reverse faults. The relative chronology, mode of formation (opening vs. shearing), and structural locations of these fractures suggest the following kinematics of folding at Sheep Mountain Anticline: little or no lateral fold propagation; no hinge migration; and limb rotation with only minor limb flexure and stretching.

Neogene tectonic, stratigraphic, and play framework of the southern Laguna Madre–Tuxpan continental shelf, Gulf of Mexico

Neogene shelf, slope, canyon, and slope-to-basin-floor transition plays in the southern Laguna Madre–Tuxpan (LM-T) continental shelf reflect a variety of structural and stratigraphic controls, including gravity sliding and extension, compression, salt evacuation, and lowstand canyon and fan systems. The Neogene in the LM-T area was deposited along narrow shelves associated with a tectonically active coast affected by significant uplift and erosion of carbonate and volcanic terrains. This study characterizes 4 structurally defined trends and 32 Neogene plays in a more than 50,000-km2 (19,300-mi2) area linking the Veracruz and Burgos basins. The Caonero trend in the southern part of the LM-T area contains deep-seated basement faults caused by Laramide compression. Many of these faults are directly linked to the interpreted Mesozoic source rocks, providing potential pathways for vertically migrating hydrocarbons. In contrast, the Lankahuasa trend, north of the Caonero trend, contains listric faults, which detach into a shallow horizon. This trend is associated with thick Pliocene shelf depocenters. The dominant plays in the Faja de Oro–Nyade trend in the central part of the LM-T area contain thick lower and middle Miocene successions of steeply dipping slope deposits, reflecting significant uplift and erosion of the carbonate Tuxpan platform. These slope plays consist of narrow channel-fill and levee sandstones encased in siltstones and mudstones. Plays in the north end of the LM-T area, in the southern part of the Burgos basin, contain intensely deformed strata linked to salt and shale diapirism. Outer-shelf, slope, and proximal basin-floor plays in the Lamprea trend are internally complex and contain muddy debris-flow and slump deposits. Risk factors and the relative importance of play elements vary greatly among LM-T plays. Reservoir quality is a critical limiting play element in many plays, especially those in the Caonero trend directly downdip from the trans-Mexican volcanic belt, as well as carbonate-rich slope plays adjacent to the Tuxpan platform. In contrast, trap and source are low-risk play elements in the LM-T area because of the abundance of large three-way and four-way closures and the widespread distribution of organic-rich Upper Jurassic Tithonian-age source rock. The potential for hydrocarbon migration in LM-T plays is a function of the distribution of deep-seated faults inferred to intersect the primary Mesozoic source. Their distribution is problematic for the Lankahuasa trend, where listric faults sole out into the Paleocene. Seal is poorly documented for LM-T plays, although the presence of overpressured zones and thick bathyal shales is favorable for seal development in middle and lower Miocene basin and slope plays.

Stratigraphy, structure and tectonic history of the Pink Mountain Anticline, Trutch (94G) and Halfway River (94B) map areas, northeastern British Columbia

Abstract Pink Mountain Anticline stands out in front of the foothills of northeastern British Columbia (57°N, 123°W). Geologic mapping and prestack depth-migrated seismic sections show that it is localized above and west of a northwest-trending subsurface normal fault. Along with isopach maps these data demonstrate episodic normal movement on the fault during deposition of the Carboniferous Stoddart Group, Triassic Montney Formation and possibly the Jurassic–Cretaceous Monteith–Gething formations. West of this fault, during Laramide compression, a pair of backthrusts nucleated on either side of a minor east–west trending Carboniferous fault and propagated across it in an en echelon pattern. One backthrust ramped laterally across the area and separated the Pink Mountain and Spruce Mountain structures, both of which are contained within a 30+ km long pop-up structure above the Besa River Formation decollement. Glomerspirella fossils confirm the existence of the Upper Jurassic Upper Fernie Formation and Upper Jurassic to Lower Cretaceous Monteith Formation at Pink Mountain.

Paleomagnetic and paleointensity study of Oligocene volcanic rocks from Chihuahua (northern Mexico)

A detailed rock-magnetic, paleomagnetic and paleointensity study was carried out on Oligocene volcanic formations from Chihuahua (northern Mexico) in order to obtain some decisive constraints for the tectonic evolution of the southern Cordilleran Orogenic Belt of North America and for the geomagnetic field strength during Oligocene time. Age of the volcanic units lies between 33 and 25 Ma according to available radiometric data. Rock-magnetic experiments reveal that remanence is carried in most cases by Ti-poor titanomagnetite, resulting of oxy-exsolution of original titanomagnetite during the initial flow cooling. Unblocking temperature spectra and relatively high coercivities point to ‘small’ pseudo-single domain magnetic grains for these (titano)magnetites. Single-component, linear demagnetization plots were observed in most cases. Nine sites yield reverse polarity magnetization, three are normally magnetized and one unit shows intermediate polarity magnetization. An evidence of strong lightning-produced magnetization overprint was detected for one site. Combining all paleomagnetic data currently available for northern Mexico, we obtained a well-defined Eocene–Oligocene mean paleomagnetic direction with I=48.5°, D=337.1°, k=20, α 95=6.8°, N=24, which deviates counterclockwise from the expected direction estimated from the North American apparent polar wander path. This suggests a vertical-axis tectonic counterclockwise rotation of about 16° relative to stable North America. Transition from Laramide compression to Basin and Range extension occurred during Oligocene, around 32–30 Ma. Tectonic rotation may then reflect east–northeast extension in the mid- to late-Cenozoic. Twenty-three samples were pre-selected for Thellier palaeointensity experiments because of their low viscosity index, stable remanent magnetization and reasonably reversible continuous thermomagnetic curves. Only 12 samples, coming from three individual basaltic lava flows, yielded reliable paleointensity estimates with the flow-mean virtual dipole moments (VDM) ranging from 3.96 to 4.65×10 22 Am 2. Combination of Mexican data with the available comparable quality Oligocene paleointensity results yield a mean VDM of 4×10 22 Am 2, which is comparable or slightly higher than the mean VDMs calculated for the Mesozoic low field period, but significantly lower than present day value. This low intensity may correlate with the relatively high paleosecular variation rate invoked around 30 Ma by several authors.

ABSTRACT: Effects of Laramide Compression and Tertiary Extension on Hydrocarbon Exploration in The Rio Grande Rift

ABSTRACT: Effects of Laramide Compression and Tertiary Extension on Hydrocarbon Exploration in The Rio Grande Rift

Monocline development by oblique-slip fault-propagation folding: the East Kaibab monocline, Colorado Plateau, Utah

Fault relationships along a 50-km stretch of the East Kaibab monocline in southern Utah suggest that Late Cretaceous/early Tertiary development of the structure involved a significant component of right-lateral strike-slip displacement, accommodated by basement-rooted faulting and fault-propagation folding. Evidence of oblique slip is provided mainly by pervasive map-scale and outcrop-scale faults that define a shear zone occupying the steep east-dipping limb of the monocline for at least its northernmost 50 km. Dominant fault orientations are synthetic and antithetic to the shear zone, and accommodate reverse-right-lateral and reverse-left-lateral slip, respectively. Structural style within the shear zone changes character and increases in intensity with progressively lower structural and stratigraphic levels in the fold, suggesting that the shear zone propagated upward from a basement-rooted fault during monocline formation. We conclude that horizontal, ENE-directed, Laramide compression drove reverse-right-lateral slip on the basement fault zone beneath the developing East Kaibab monocline. The resulting transpressional fault-propagation fold is marked in southern Utah by 1600 m of reverse displacement and possibly 8000 m of right-lateral displacement across the shear zone and associated monoclinal flexure.

Crust and mantle structure across the Basin and Range‐Colorado Plateau boundary at 37°N latitude and implications for Cenozoic extensional mechanism

We present new evidence on the seismic velocity and density of the crust and upper mantle along a 200‐km‐long transect across the eastern Basin and Range and western Colorado Plateau at 37°N latitude. Receiver functions computed from the P waveforms recorded with 10 portable broadband stations deployed along the transect were used to estimate crustal thickness variations. The crust is 30–35 km thick within the eastern Basin and Range and increases over a distance of ∼100 km at the western edge of the Colorado Plateau, reaching a maximum of approximately 45 km east of the Hurricane fault. The timing of crustal multiples within the reciever functions were used to estimate the Vp/Vs of the crust along the profile, and we found that the western Colorado Plateau crust is characterized by a high Poisson's ratio (0.28–0.29) indicative of a crust with an average mafic composition. We estimated the upper mantle lid thickness along our profile based on teleseismic P wave travel times and constraints provided by gravity data. Our data and available geophysical constraints are most consistent with a lithosphere that thickens from an average thickness of 60 km beneath the Basin and Range to 100 km beneath the western Colorado Plateau, although the Basin and Range lithosphere may have significant thickness variations. The thick, strong mafic crust and thicker mantle lid under the Colorado Plateau can account for the relative geologic stability and subdued magmatism of the plateau during Laramide compression and Cenozoic extension compared to surrounding regions. The crustal and lithospheric thinning across the tectonic boundary occurs over a short distance (∼100 km), suggesting it is a geologically young feature produced by a predominantly mechanical response to late Cenozoic extension. Our new lithosphere model at 37°N latitude is consistent with the existence, in early Cenozoic time, of a flat subducted slab at 100 km depth and a relict Sevier‐Laramide 50–60 km thick crustal welt, and 60–100% pure shear extension (β values of 1.6–2.0) during the late Cenozoic.

Oblique Convergence During Northeast-Southwest Laramide Compression Along the East-West Owl Creek and Casper Mountain Arches, Central Wyoming

Understanding the diversity of structural trends in the Laramide foreland of the conterminous United States is important to understanding the location, geometry, and fracturing of hydrocarbon reservoirs. East-west basement-cored arches in central Wyoming are oblique to the average northwesterly trend of foreland faults and folds. Tectonic models predict that these arches formed by one of the following mechanisms: north-south-directed thrust faulting; sinistral strike-slip faulting; or northeast-southwest-directed, oblique-slip thrust faulting. In the eastern Owl Creek Mountains, average slip directions given by slickenline directions trend from N37°E to N57°E. Geometric analysis of conjugate faults and stress inversion of minor fault data indicate nearly horizontal compression trending between N48°E and N65°E. In the east-west Casper Mountain structure, more limited minor fault data are consistent with the northeast-southwest compression seen in the eastern Owl Creek arch and indicate an additional stage of extension by normal faulting. The northeast-southwest compression documented by minor faults suggests oblique thrusting with a component of sinistral strike-slip on the underlying, east-west-striking Owl Creek and Casper Mountain thrusts. In this area of the Laramide foreland, east-west arches probably formed during a single stage of oblique slip on thrust ramps connecting northwest-trending arch culminations. This conclusion indicates that trap geometries and reservoir characteristics of foreland hydrocarbon accumulations are dependent on their obliquity to the regional stress field.

Late Laramide Thrust-Related and Evaporite-Domed Anticlines in the Southern Piceance Basin, Northeastern Colorado Plateau (1)

New seismic and gravity data across the hydrocarbon-producing Divide Creek and Wolf Creek anticlines in the southern Piceance basin reveal contrasting styles of deformation within two widely separated time frames. Seismic data indicate that prebasin Paleozoic deformation resulted in block faulting of the Precambrian crystalline basement rocks and overlying Cambrian through Middle Pennsylvanian strata. Movement along these block faults throughout much of Pennsylvanian time, during northeast-southwest crustal extension, likely influenced distribution of the Middle Pennsylvanian (Desmoinesian) evaporite-rich facies. Younger rocks, including the thick succession of Cenozoic basin strata, then buried the Paleozoic structures. Tectonic reconfiguration of the basin's eastern margin occurred during late Laramide northeast-southwest compression, when a basement-involved thrust block, whose surface expression is the Grand Hogback monocline, moved into the Piceance basin. A decollement developed in front of the thrust block within the mechanically weak Desmoinesian evaporites and splayed out basinward as small-scale imbricate thrusts in the Upper Cretaceous Mancos Shale. The Divide Creek anticline formed above these splays as thrusting locally overthickened the shale and repeated the sandstone units between it and the evaporites. The Wolf Creek anticline to the east, however, is due to both depositional and tectonic thickening of the evaporite section along the decollement. Gravity data confirm that excess mater al of relatively low density exists beneath the Wolf Creek structure, whereas material of relatively higher density (overthickened shale) is found beneath the Divide Creek anticline. Thrust-related basin margins and intrabasin folds structurally analogous to the Divide Creek and Wolf Creek anticlines may be more common than presently recognized in the Rocky Mountain foreland. One well-documented example is the Pinedale anticline in the northern Green River basin, Wyoming, which, like the Divide Creek anticline, developed above a zone of splay faults from a decollement in front of a large thrust block.