North American Lithosphere Research Articles

Upper mantle deformation beneath the North American–Pacific plate boundary in California from SKS splitting

In order to constrain the vertical and lateral extent of deformation and the interactions between lithosphere and asthenosphere in a context of a transpressional plate boundary, we performed teleseismic shear wave splitting measurements for 65 permanent and temporary broadband stations in central California. We present evidence for the presence of two anisotropic domains: (1) one with clear E–W trending fast directions and delay times in the range 1.5 to 2.0 s and (2) the other closely associated with the San Andreas Fault system with large azimuthal variations of the splitting parameters that can be modeled by two anisotropic layers. The upper of the two layers provides fast directions close to the strike of the main Californian faults and averaged delay times of 0.7 s; the lower layers show E–W directions and delay times in the range 1.5 to 2.5 s and thus can be compared to what is observed in stations that require a single layer. We propose the E–W trending anisotropic layer to be a 150 to 200 km thick asthenospheric layer explained by the shearing associated with the absolute plate motion of the North American lithosphere. The shallower anisotropic layer ought to be related to the dynamics of the San Andreas Fault system and thus characterized by a vertical foliation with lineation parallel to the strike of the faults localized in the lithosphere. We also propose that the anisotropic layer associated with each fault of the San Andreas Fault system is about 40 km wide at the base of the lithosphere.

Lithospheric mantle duplex beneath the central Mojave Desert revealed by xenoliths from Dish Hill, California

Low‐angle subduction of oceanic lithosphere may be an important process in modifying continental lithosphere. A classic example is the underthrusting of the Farallon plate beneath North America during the Laramide orogeny. To assess the relevance of this process to the evolution and composition of continental lithosphere, the mantle stratigraphy beneath the Mojave Desert was constrained using ultramafic xenoliths hosted in Plio‐Pleistocene cinder cones. Whole‐rock chemistry, clinopyroxene trace element and Nd isotope data, in combination with geothermometry and surface heat flow, indicate kilometer‐scale compositional layering. The shallow parts are depleted in radiogenic Nd (ɛNd = −13 to −6.4) and are interpreted to be ancient continental mantle that escaped tectonic erosion by low‐angle subduction. The deeper samples are enriched in radiogenic Nd (ɛNd = +5.7 to +16.1) and reveal two superposed mantle slices of recent origin. Within each slice, compositions range from fertile lherzolites at the top to harzburgites at the bottom: the latter formed by 25–28% low‐pressure melt depletion and the former formed by refertilization of harzburgites by mid‐ocean‐ridge‐basalt‐like liquids. The superposition and internal compositional zonation of the slices preclude recent fertilization by Cenozoic extension‐related magmas. The above observations imply that the lower Mojavian lithosphere represents tectonically subcreted and imbricated lithosphere having an oceanic protolith. If so, the lherzolitic domains may be related to melting and refertilization beneath mid‐ocean ridges. The present Mojavian lithosphere is thus a composite of a shallow section of the original North American lithosphere underlain by Farallon oceanic lithosphere accreted during low‐angle subduction.

Creation of a continent recorded in zircon zoning

We have discovered a robust microcrystalline record of the early genesis of North American lithosphere preserved in the U-Pb age and oxygen isotope zoning of zircons from a lower crustal paragneiss in the Neoarchean Superior province. Detrital igneous zircon cores with δ 18 O values of 5.1‰–7.1‰ record creation of primitive to increasingly evolved crust from 2.85 ± 0.02 Ga to 2.67 ± 0.02 Ga. Sharp chemical unconformity between cores and higher δ 18 O (8.4‰–10.4‰) metamorphic overgrowths as old as 2.66 ± 0.01 Ga dictates a rapid sequence of arc unroofing, burial of detrital zircons in hydrosphere-altered sediment, and transport to lower crust late in upper plate assembly. The period to 2.58 ± 0.01 Ga included ∼80 m.y. of high-temperature (∼700–650 °C), nearly continuous overgrowth events reflecting stages in maturation of the subjacent mantle root. Huronian continental rifting is recorded by the youngest zircon tip growth at 2512 ± 8 Ma (∼ 600 °C) signaling magma intraplating and the onset of rigid plate behavior. This >150 m.y. microscopic isotope record in single crystals demonstrates the sluggish volume diffusion of U, Pb, and O in zircon throughout protracted regional metamorphism, and the consequent advances now possible in reconstructing planetary dynamics with zircon zoning.

Pangaean megalineaments: geophysical indications for the existence of Laurentian counterparts of the 6000 km Trans-Saharan Tibesti Lineament, and implications for lithospheric tectonics and mineral deposits

A major 6000-km long west-northwest to east-southeast-trending Trans-Saharan lineament, extending from Kenya to Morocco, referred to as the Tibesti Lineament, has been described by Guiraud et al. (2000). This lineament shows up on satellite images, air-photo mosaics and terrain elevation models, as swarms of parallel faults. It corresponds to a major discontinuity of surface wave velocities in the mantle, suggesting that it deeply affects the lithosphere. The highest heat flow measurements in the West African shield are found adjacent to the Tibesti Lineament in the northern Hoggar uplift. Geophysical, geological, and geomorphological evidence suggests that the Tibesti lineament may have continued into North America (Laurentia), which collided with northwest Africa during the Appalachian-Mauritanide orogeny, coinciding with the late Palaeozoic assembly of the supercontinent Pangaea. The contiguous Laurentian lineaments which may be extensions of the Tibesti Lineament, are the here newly recognised Chesterfield Inlet Lineament and the Cabot Strait Lineament, both of which are situated in Canada. These lineaments are defined as linear pattern breaks in processed aeromagnetic anomaly maps of Canada. Some portions of them also can be detected in refraction and reflection seismic survey data ( e.g. , under Hudon Bay), and to a lesser extent, in the Bouguer gravity anomaly map of Canada. The lineaments have present-day (neotectonic) surface expressions in the form of inlets, submarine channels, straight river segments, and strings of linear lakes, extending over 3700 km, from Nunavut Province northwest of Hudson Bay, across Quebec and Newfoundland, and to the Cabot Strait separating Newfoundland from Nova Scotia. The highest heat flow in the Canadian Maritimes is found in the Cabot Strait. The Tibesti-Cabot-Strait-Chesterfield Inlet lineaments reflect deep-seated fracturing of the North African and North American lithosphere, involving both the crust and rigid upper mantle. These lineaments cut right across the grain of numerous orogenic belts dating from the Palaeoproterozoic, Late Mesoproterozoic, and Palaeozoic. By analogy with other large transcontinental lineaments which contain ore deposits, these newly identified megalineaments may be prospective for economic mineral deposits, such as hydrothermal gold and base metal mineralization, and diamondiferous kimberlites.

Trace Element Evidence for Hydrous Metasomatism at the Base of the North American Lithosphere and Possible Association with Laramide Low‐Angle Subduction

Abstract Trace element signatures of mantle xenoliths along an east‐west transect extending from the Sierra Nevada to the Colorado Plateau were investigated in this study. Those beneath the Sierra Nevada are highly enriched in fluid‐mobile elements (Cs, Pb, U, and Sr) and, in particular, are characterized by low U/Pb and high Sr/Nd. These signatures imply that aqueous fluids derived by dehydration of seawater‐altered lithologies have been introduced into the Sierran xenoliths. Peridotite xenoliths from beneath the Colorado Plateau, lying ∼1000 km inboard, are also enriched in fluid‐mobile elements, but the extent is lower, the U/Pb ratios are high, the Sr/Nd ratios are low, and there is a strong enrichment in the light rare earth elements but negligible enrichment in the heavy rare earths. These features require the introduction of an aqueous component and a silicate melt component into the plateau xenoliths, with the latter, in particular, derived from a garnet‐rich source such as an eclogite that has al...

Relict subduction zones in Canada

Relict subduction zones that are interpreted on 10 Lithoprobe deep seismic reflection profiles in Canada provide key observational evidence that lithosphere plate interactions have operated with essentially the same resultant geometry for at least 2.7 Gyr. Subduction zone reflections are generally found beneath known collisional belts, and their orientations are often consistent with the locations of exposed arcs, sutures, and subduction complexes. They project from the lower crust into the upper mantle and provide key geometric relationships between the crust, Moho, and upper mantle. The interpretation of subduction zone reflections as being syntectonic establishes a minimum age for the overlying lower crust and mantle wedge and precludes inferred formation of the Moho and upper mantle by postorogenic delamination in these areas. The orientation of subduction zone reflections may be synthetic or antithetic to exposed sutures, indicating juxtaposition by subcretion or by delamination, respectively. The mantle wedge above the subduction zone reflections may exhibit sparse reflectivity. Subduction zone reflections commonly dip toward the craton, indicating a preferred dip of subduction during the last stages of convergence within an orogen, or else a higher likelihood of preservation beneath older domains. Interpreted ages of the Canadian examples range from Recent to Neoarchean (∼2.7 Ga), providing observational evidence that plate tectonics or similar processes have been active since at least the Neoarchean. The locations and geometries of these relict subduction zones provide a snapshot of accretion boundaries in the mantle and establish a framework for the Neoarchean‐Recent accretionary architecture of the North American lithosphere.

Missing Idaho arc: Transpressional modification of the 87Sr/86Sr transition on the western edge of the Idaho batholith

The North American Cordillera is characterized by subduction-related Mesozoic batholiths with voluminous middle Cretaceous plutonism. In the Cordillera, the Idaho batholith is anomalous, containing only a narrow (<10 km) band of middle Cretaceous plutons on its extreme western margin. These plutons are spatially coincident with the Late Cretaceous, crustal-scale, dextral transpressional western Idaho shear zone. Finite strain analysis indicates that there is significant east-west shortening in the western Idaho shear zone. A reconstruction using isotopic variations as strain markers, and comparing them to the undeformed middle Cretaceous Sierra Nevada batholith, results in a middle Cretaceous magmatic arc in western Idaho similar in width (∼85– 100 km) to other sections of the Cordilleran magmatic arc. The tectonic implications include: (1) a full magmatic arc existed in Idaho in the middle Cretaceous prior to Late Cretaceous deformation; (2) the currently sharp 87Sr/86Sr transition in western Idaho represents structural modification of the batholith after emplacement (i.e., the steep isotopic gradient results from deformation); and (3) as a result of Late Cretaceous shortening in the western Idaho shear zone, the main part of the Idaho batholith (i.e., post–ca. 90 Ma) intruded farther eastward into thick North American lithosphere. Consequently, the main part of the Idaho batholith has a stronger continental signature than the other coastal batholiths.

How Laramide-Age Hydration of North American Lithosphere by the Farallon Slab Controlled Subsequent Activity in the Western United States

Starting with the Laramide orogeny and continuing through the Cenozoic, the U.S. Cordilleran orogen is unusual for its width, nature of uplift, and style of tectonic and magmatic activity. We present teleseismic tomography evidence for a thickness of modified North America lithosphere <200 km beneath Colorado and >100 km beneath New Mexico. Existing explanations for uplift or magmatism cannot accommodate lithosphere this thick. Imaged mantle structure is low in seismic velocity roughly beneath the Rocky Mountains of Colorado and New Mexico, and high in velocity to the east and west, beneath the tectonically intact Great Plains and Colorado Plateau. Structure internal to the low-velocity volume has a NE grain suggestive of influence by inherited Precambrian sutures. We conclude that the high-velocity upper mantle is Precambrian lithosphere, and the lowvelocity volume is partially molten Precambrian North America mantle. We suggest, as others have, that the Farallon slab was in contact with the lithosphere beneath most of the western U.S. during the Laramide orogeny. We further suggest that slab de-watering under the increasingly cool conditions of slab contact with North America hydrated the base of the continental lithosphere, causing a steady regional uplift of the western U.S. during the Laramide orogeny. Imaged low-velocity upper mantle is attributed to hydration-induced lithospheric melting beneath much of the southern Rocky Mountains. Laramide-age magmatic ascent heated and weakened the lithosphere, which in turn allowed horizontal shortening to occur in the mantle beneath the region of Laramide thrusting in the southern Rocky Mountains. Subsequent Farallon slab removal resulted in additional uplift through unloading. It also triggered vigorous magmatism, especially where asthenosphere made contact with the hydrated and relatively thin and fertile lithosphere of what now is the Basin and Range. This mantle now is dry, depleted of basaltic components, hot, buoyant, and weak.

Shear wave splitting and mantle flow beneath LA RISTRA

Shear‐wave splitting parameters (fast polarization direction and delay time) are determined using data from LA RISTRA (Colorado pLAteau RIo Grande Rift/Great Plains Seismic TRAnsect), a deployment of broadband seismometers extending from the Great Plains, across the Rio Grande Rift and the Jemez Lineament, to the Colorado Plateau. Results show that the fast polarization directions are sub‐parallel to North American absolute plate motion. The largest deviations from the plate motion are observed within the western edge of the Great Plains and in the interior of the Colorado Plateau where lithospheric anisotropy may be significant. Delay times range from 0.8 to 1.8 seconds with an average value of 1.4 seconds; the largest values are along the Jemez Lineament and the Rio Grande Rift which are underlain by an uppermost mantle low velocity zone extending to depths of ∼200 km. The anisotropy beneath the central part of LA RISTRA shows a remarkably consistent pattern with a mean fast direction of 40° ± 6°. Seismic anisotropy can be explained by differential horizontal motion between the North American lithosphere and westerly to southwesterly flow of the asthenospheric mantle. The approximately N‐S fast direction found beneath western Texas is similar to that observed beneath the southern rift and may reflect a different dynamic regime.

Subduction‐induced strain in the upper mantle east of the Mendocino triple junction, California

We observe splitting of teleseismic shear waves at five stations of the Berkeley Digital Seismic Network located east of the Mendocino triple junction in northeastern California that is dependent on the arrival direction of the seismic phases. The observed variations with back azimuth cannot be explained with laterally varying anisotropy with a horizontal symmetry axis and are attributed to the presence of fabric with an inclined symmetry axis. We assume that the anisotropy is caused by the preferred alignment of olivine crystals. A grid search over possible orientations of the olivine a axes reveals that south of the Mendocino triple junction they dip to the east, whereas north of the triple junction they dip to the northeast. On the basis of a comparison of the ray paths of our data to the spatial distribution of fast and slow P wave velocity anomalies in the upper mantle, we conclude that the anisotropy is located within seismically slow regions and that the directions are controlled by the geometry of a steeply dipping fast P wave velocity anomaly. Assuming that the fast P velocity anomaly represents subducted slab material, we conclude that the fabric beneath the stations north of the triple junction is most likely caused by the differential motion between this rigid, strong down going plate and the surrounding mantle. South of the triple junction the fabric may have developed while subduction of the Farallon plate was still ongoing in this region (prior to 6 Ma). However, we prefer to attribute the observations to more recent asthenospheric flow associated with the opening of a slabless window beneath the North American lithosphere. The flow is modulated by the presence of rigid lithosphere to the north and east.

Wave‐field Tau‐p analysis for 2‐D velocity models: Application to western North American lithosphere

The international Deep Probe refraction experiment is a continental‐scale study of the crust and upper mantle in western North America to determine well‐constrained mantle velocity‐depth structures for the stable Archean Hearne and Wyoming provinces and the tectonically modified Proterozoic lithosphere of the southern Rocky Mountains and Colorado Plateau. An initial 2‐D velocity interpretation for Deep Probe has been accomplished by the application of the downward‐continued wave‐field τ‐p transform method. This approach involves two linear transforms for each shot gather from the experiment: a τ‐p transform (or slant stack) of the input data followed by a downward continuation to z‐p space using an appropriate velocity function. The determination of this velocity function is an iterative process which converges when the downward‐continued wave field images a velocity function which is the same as that used to create it. The resulting 1‐D velocity function, υ(z), from each gather is converted to a 2‐D function, υ(x, z) using ray theory. Eleven profiles along the Deep Probe corridor have been analysed. The resulting functions have been combined, gridded and contoured to produce a velocity structure section for the project, which provides an initial model for the application of more advanced interpretation procedures. As a result, the lithospheric structures of three major tectonic provinces have been identified in an efficient and effective manner that did not require ‘picking’ of travel times.

Origin of the Columbia River basalts: melting model of a heterogeneous plume head

In order to study the origin of the Grande Ronde basalts (GRs) erupted in the climax stage of the Columbia River basalts (CRBs), we carried out high pressure melting experiments on four of the most primitive rock compositions representing the Yakima group of the CRBs. The voluminous GRs (constituting >80 vol% of CRBs) are totally aphyric basaltic andesites. GRs show very narrow and coherent chemical trends both in major and trace elements as well as isotopes. The silica-rich GRs (SiO 2 = 52–56 wt%) can be produced by direct partial melting of a MORB like source material (CRB72-31) at ∼2 GPa or ∼70 km depth. By 30–50% partial melting of the CRB72-31, the entire compositional range of the GRs can be produced in a narrow temperature interval (1300–1350°C) at ∼2 GPa. The aluminous clinopyroxene that appears in the above melting range is consistent as the major controlling phase of the GR trends. The partial melts are very similar to the GRs except for Al 2O 3 and FeO which could be due to the mismatch in the source rock composition. Judging from the variation in REE, involvement of garnet in GR magma genesis can be ruled out. Small amounts of plagioclase (10–30 wt%) may be present in the partial melting residue. Judging from REE patterns and Nd isotopes of the GRs, the source rock should be unfractionated in REE. Based on the melting experiments, a heterogeneous plume model is proposed for the initial stage of the Yellowstone hot spot. Large lithologically distinct blobs of old oceanic crust components were included in the plume head. The GR magmas were produced by partial melting of the oceanic crust components at the bottom of the North American lithosphere. Similar melting processes of basalt/peridotite composite source may be operating in other LIPs (large igneous provinces). The GR type genuine oceanic crust derived melts may be seen where the ambient peridotite remains under subsolidus conditions. Volume and temperature of mantle plumes may have been overestimated, because contributions from the recycled oceanic crust is so large and the current mantle melting models concern only peridotite source.

Basin development and inversion at the Appalachian structural front, Port au Port Peninsula, western Newfoundland Appalachians

In the Humber Zone of the Newfoundland Appalachians, Cambro-Ordovician shelf and foreland basin successions are affected by Middle Ordovician (Taconian orogeny) and Devonian (Acadian orogeny) deformation. On Port au Port Peninsula the presence of the Late Ordovician to Late Silurian Long Point – Clam Bank succession allows these episodes to be separated. The Taconian foreland basin stratigraphy on Port au Port Peninsula is highly variable. On the west coast, platform carbonates are overlain by megaconglomerates of the Cape Cormorant Formation, which record progressive exposure of 1 km of the platform succession. The conglomerates are restricted to a narrow zone, consistent with derivation from a fault scarp originally immediately west of the outcrops (in palinspastic restoration). Farther east, at Victors Brook, the Cape Cormorant Formation is absent, but the overlying, almost undeformed Goose Tickle Group contains conglomerate derived both from the upper part of the platform succession and from the Taconian Humber Arm Allochthon. Southeast of Victors Brook, the top of the platform is overlain directly by scaly shales and mélange of the Humber Arm Allochthon, which includes deformed equivalents of the foreland basin succession. The distribution of conglomeratic units, the presence and configuration of faults, and the preservation of the Goose Tickle Group in the Victors Brook area imply that a fault-bounded basin developed in advance of the Humber Arm Allochthon during the Taconian orogeny. This basin is interpreted to have resulted from flexural extension of North American lithosphere. The close spatial coincidence between later Acadian structures and the Taconian basin boundaries implies that the basin-bounding faults were reactivated as thrusts and reverse faults, and that the basin underwent inversion during Acadian thrusting. The western basin-bounding fault, modified by the development of a "short cut" thrust, developed into the present-day Round Head thrust.

Curving cross joints and the lithospheric stress field in eastern North America

Cross joints are late-formed nonsystematic fractures that extend across intervals between systematic joints. Traces of such cross joints are seen on bedding-plane surfaces of Devonian Catskill clastic sedimentary rocks of the Appalachian Plateau of western New York State, where the maximum horizontal principal stress (S H ) is oriented ∼N65°E, as indicated by in situ stress measurements. Between pairs of closely spaced systematic joints, traces of cross joints are commonly planar and orthogonal to the preexisting joints. However, in the mid-region between some widely spaced systematic joints in western New York, cross joints strike parallel to the S H of the present lithospheric stress field, but then curve to abut the preexisting joints at right angles. A curving trace reflects a local perturbation of the regional stress field in the vicinity of preexisting joints, and the perpendicular termination indicates that the preexisting joints were open. Depending on their age, the strike of the mid-region of curving cross joints denotes the orientation of either the neotectonic stress field or its Tertiary predecessor in the North American lithosphere.

Kinematic evidence for extensional unroofing of the Franciscan Complex along the Coast Range Fault, Northern Diablo Range, California

Franciscan metagraywacke immediately below the Del Puerto ophiolite, an outlier of the Coast Range ophiolite in the northern Diablo Range, was sheared during top‐to‐the‐east displacement on the Coast Range fault. This represents normal faulting and extensional offset. It was accompanied by attenuation of the Coast Range ophiolite and Great Valley sequence in the hanging wall along layer‐parallel normal faults that sole into the Coast Range fault. Extension occurred as the Franciscan Complex moved relatively west, out from under North American lithosphere and across the subducting ocean plate below. This effected a lengthening and thinning in the wedge of material above the down‐going plate, presumably in response to instability brought about by subduction shallowing (Krueger and Jones, 1989) and accretion of the Franciscan Central belt in the latest Cretaceous to early Paleocene. As a result, blueschist facies terranes of the uppermost part of the Franciscan Complex are now juxtaposed directly against hanging wall units that bear only low‐grade metamorphism.

Late Cenozoic volcanism, subduction, and extension in the Lassen Region of California, southern Cascade Range

Hundreds of short‐lived, small‐ to moderate‐volume, mostly mafic volcanoes occur throughout the Lassen region of NE California and surround five longer‐lived, large‐volume, intermediate to silicic volcanic centers younger than 3 Ma. Volcanic rocks older than 7 Ma are scarce in the Lassen region. We identify 537 volcanic vents younger than 7 Ma, and we classify these into five age intervals and five compositional categories based on SiO2 content. Maps of vents by age and composition illustrate regionally representative volcanic trends. By 2 Ma, the eastern limit of voicanism had contracted westward toward the late Quaternary arc. Late Quaternary volcanism is concentrated around and north of the silicic Lassen volcanic center. The belt of most recent volcanism (25–0 ka) has been active since at least 2 Ma. Most mafic volcanism is cakalkaline basalt and basaltic andésite. However, lesser volume of low‐potassium olivine tholeiite (LKOT), a geochemically distinctive basalt type found in the northern Basin and Range province, also has erupted throughout the Lassen segment of the Cascade arc since the Pliocene. Thus models of the mantle source and tectonic control of LKOT magmatism should be applicable both within and behind the subduction‐related arc. Normal faults and linear groups of vents are evidence of widespread crustal extension throughout most of the Lassen region. NNW alignments of these features indicate NNW orientation of maximum horizontal stress (ENE extension), which is similar to the stress regime in the adjacent northwestern Basin and Range and northern Sierra Nevada provinces. The large, long‐lived volcanic centers developed just west of a zone of closely spaced NNW trending normal faults. Within that zone of faulting, pervasive ENE extension has precluded growth of large, long‐lived crustal magma systems. We interpret the western limit of the zone of NNW trending normal faults as the western boundary of the Basin and Range province where it overlaps the Lassen segment of the Cascade arc. In our view, the Lassen volcanic region occurs above the subducting Gorda North plate but also lies within a broad zone of distributed extension that occurs in the North American lithosphere east and southeast of the present Cascadia subduction zone. An episode of ENE extension that began in the late Miocene in the northwestern Basin and Range province appears to have triggered widespread late Miocene to Quaternary mafic volcanism in the Lassen region. The scarcity of volcanic rocks older than 7 Ma suggests that a more compressive lithospheric stress regime prior to the late Miocene extensional episode may have suppressed volcanism, even though subduction probably was occurring beneath the Lassen region.

Variations in effective elastic thickness of the North American lithosphere

THE isostatic response of the lithosphere to loading can often be successfully represented by a model in which the weight of topographic features on the Earth's surface is supported in part by stresses within a flexed elastic plate overlying an inviscid asthenos-phere, and in part by the buoyancy forces associated with the deflection of density interfaces accompanying flexure1,2. Previous estimates3–10 of the apparent thickness of the elastic plate in North America, based on the shapes of sedimentary basins or the deformation accompanying slip on faults, range from a few to more than a hundred kilometres—or equivalently, flexural rigidities ranging from about 1021 to 1025 N m−1. Here we use a technique for estimating flexural rigidity that is not limited to sedimentary basins to map variations in the effective elastic thickness of the North American lithosphere. The effective elastic thickness ranges from a minimum of ∼4 km in the Basin and Range Province to >100 km in the Precambrian core of the continent, supporting the idea that flexural rigidity increases with time since the last thermal event11.

Formation of the Rocky Mountains, Western United States: A Continuum Computer Model

One hypothesis for the information of the Rocky Mountain structures in late Cretaceous through Eocene time is that plate of oceanic lithosphere was underthrust horizontally along the base of the North American lithosphere. The horizontal components of the motion of this plate are known from paleomagnetism, and the edge of the region of flat slab can estimated from reconstructed patterns of volcanism. New techniques of finite-element modeling allow prediction of the thermal and mechanical effects of horizontal subduction on the North American plate. A model that has a realistic temperature-dependent rheology and a simple plane-layered initial condition is used to compute the consequences of horizontal underthrusting in the time interval 75 million to 30 million years before present. Successful prediction of this model include (i) the location, amount, and direction of horizontal shortening that has been inferred from Laramide structures; (ii) massive transport of lower crust from southwest to northeast; (iii) the location and timing of the subsequent extension in metamorphic core complexes and the Rio Grande rift; and (iv) the total area eventually involved in Basin-and-Range style extension. In a broad sense, this model has predicted the belt of Laramide structures, the transport of crust from the coastal region to the continental interior, the subsequent extension in metamorphic core complexes and the Rio Grande rift, and the geographic region of late Tertiary Basin-and-Range extension. Its principal defects are that (i) many events are predicted about 5 million to 10 million years too late and (ii) the wave of crustal thickening does not travel far enough to the east. Reasonable modifications to the oceanic plate kinematics and rheologies that were assumed may correct these defects. The correspondence of model predictions to actual geology is already sufficiently close to show that the hypothesis that horizontal subduction caused the Laramide orogeny is probably correct. The Rocky Mountain thrust and reverse faults formed in an environment of east-west to northeast-southwest compressive stress that was caused by the viscous coupling between the oceanic plate and the base of the North American crust. Nonuniform crustal thickening by simple-shear transport also caused relative uplifts; therefore, this model is consistent with both of the range-forming mechanisms that have been inferred (1). A new proposal that arises from this simulation is that horizontal subduction also caused the subsequent extensional Basin-and-Range taphrogeny by stripping away the mantle lithosphere so that the crust was exposed to hot asthenosphere after the oceanic slab dropped away.

Best Student Tectonophysics Paper selected

A paper entitled “Variations in the Effective Elastic Thickness of the North American Lithosphere,” written by Brown University student Timothy Bechtel, has been selected as the best student paper presented at the 1986 AGU Spring Meeting by the Tectonophysics Section. Bechtel, who was assisted on the paper by colleagues from Brown and from Energy, Mines, and Resources Canada, is currently working on his Ph.D. in geophysics. He received his B.S. in geology in 1982 through a cooperative program between Bryn Mawr and Haverford colleges; in 1984, he received an M.S. from Brown as a result of his studies in theoretical structural geology. Bechtel's primary research interests involve continental gravity and tectonics.

Flexure of the North American lithosphere above the subducted Mendocino Fracture Zone and the formation of east‐west faults in the transverse ranges

During the middle and late Tertiary, several prominent topographic scarps, the Farallon plate equivalents of present‐day fracture zones in the Pacific plate, were subducted under the western margin of North America. If subduction occurred at a shallow angle so that the underthrust Farallon plate was in isostatic balance with the overlying North American plate, then differential uplift of the North American plate would have occurred. The Mendocino fracture zone was the tallest of the subducted scarps, and isostasy calculations indicate that the surface of the North American plate south of the trace of the subducted fracture zone should have been 500 to 1000 m higher than the surface north of the trace. Flexure calculations in which the North American plate is treated as a thin slab predict that this uplift should have been spread over a north‐south distance of 70 to 120 km, yielding an average slope of around 0.5°. A regional slope of this magnitude would have been great enough to account for observed late Oligocene and Miocene disruptions of drainage and sedimentation in southern California. Calculated stresses in the upper part of the flexed plate are 50 to 80 MPa. These stresses exceed the tensile failure strength of crustal rocks and suggest that the plate should have failed by tensile cracking. Tensile fracture would decrease the elastic thickness of the plate, enhance bending, and perhaps lead to failure by normal faulting. This mechanism may explain the formation of east‐west faults in southern California.