William R. Seager

Accretionary tectonics of Burma and the three-dimensional geometry of the Burma subduction zone

The geometry of the Burma Wadati-Benioff zone (WBZ) has been determined by fitting a trend surface parameterized with eight effective degrees of freedom to 184 well-located hypocenters. The dip of this surface, which passes through the middle of the WBZ, varies from about 50° in the north near the eastern Himalayan syntaxis to about 30° in the Bay of Bengal area. The eastern edge of the Indo-Burman ranges closely follows the map projection of the 60 km depth contour of the WBZ. The curvature of the Indo-Burman ranges is controlled by the geometry of the interface between the more steeply dipping part of the Indian plate and the leading edge of the overriding Burma platelet. Shallow earthquakes beneath the Indo-Burman ranges are primarily confined to the underthrusting Indian plate. Their focal mechanisms indicate strike-slip faulting and north-south shortening parallel to the eastern margin of the Indian plate.

New K-Ar dates from basalts and the evolution of the southern Rio Grande rift

In the southern Rio Grande rift, two extensional regimes of different origin (but transitional with each other through the Miocene) can be interpreted from structures and rocks formed within the past 28 to 29 m.y. The earlier regime, which began about 28 to 29 m.y. B.P., is characterized by emplacement of “basaltic andesite” flows with relatively high strontium isotope ratios; formation of broad, relatively deep, northwest-trending basins; and incipient uplift of some of the region9s fault-block mountains. This regime appears to have developed in a back-arc setting, perhaps behind a rapidly steepening slab and a westward-sweeping arc system. The younger episode seemingly represents a renewal or acceleration of block faulting and volcanism during the latest Miocene and Pliocene, 9 to 3 m.y. B.P., after a long transitional period during the early and mid-Miocene when volcanism was absent and tectonism was less vigorous. The latest Miocene-Pliocene episode produced the modern northerly-trending rift basins and uplifts, regional uplift of the rift 1 to 2 km above sea level, and renewal of volcanism, this time dominated by relatively primitive alkali-olivine basalt. New basalt dates reveal that in the southern rift, modern ranges and basins were almost fully developed and that near-modern drainage ways were established across uplifts into bolsons by about 5.0 m.y. B.P. An ancestral Rio Grande had extended itself southward into the southern rift by 3 to 4 m.y. B.P., and the river entrenched itself into its modern valley between 0.7 and 0.5 m.y. B.P. Horst-graben development of the southern Basin and Range province, as well as associated basaltic volcanism, swept progressively eastward from southeastern California in the past 20 m.y., culminating in formation of the Rio Grande rift and other fault-block terrane in west Texas, New Mexico, and northern Chihuahua in the latest Miocene and Pliocene. Late Quaternary Basin and Range fault scarps increase in density eastward, which also suggests that more easterly parts of the province are youngest. These relationships support a previous model of an eastward-expanding, slab-free triangle (related to growth of the San Andreas transform), through which mantle upwelling triggers eastward-younging patterns of tectonism, volcanism, and uplift and promotes lithospheric thinning and increased heat flow. Across most of the southern Basin and Range and Rio Grande rift, the horst-graben structures related to growth of this triangle are superimposed on somewhat older (late Oligocene-middle Miocene) extensional terrane that appears to have formed in a back-arc or arc setting.

Tectonic control on facies distribution of the Camp Rice and Palomas Formations (Pliocene-Pleistocene) in the southern Rio Grande rift

The Pliocene-Pleistocene Camp Rice and Palomas Formations in the Rio Grande rift of southern New Mexico provide an excellent test of the role of basin symmetry in the distribution of piedmont and axial-fluvial facies. In the asymmetrical Palomas and northern Mesilla basins, the axial-fluvial facies is characterized by multistory channel sands and sandstones and is concentrated near the locus of maximum subsidence within a few kilometers of the footwall scarp. Fanglomerate derived from the footwall uplift extends only a few kilometers or less from the footwall scarp, whereas alluvial-fan and alluvial-flat conglomerate, sand, and mudstone deposited on the hanging-wall dip slope occupy a much wider outcrop belt. In the symmetrical Hatch-Rincon basin, the axial-fluvial facies extends to within a few kilometers of both the northern and southern basin margins and has a much higher percentage of fine-grained overbank deposits, some of which contain calcareous paleosols. In all three basins, a tongue of fanglomerate as much as 30 m thick prograded over the axial-fluvial facies near the end of Camp Rice and Palomas deposition. The distribution of facies of the Camp Rice and Palomas Formations supports previously published models that predict a two-stage history of asymmetrically subsiding basins. During tectonically active periods, axial-river channels preferentially avulse into the topographically lowest area of the alluvial plain, which is directly above the axis of maximum subsidence. During the postorogenic stage, when erosion rate exceeds subsidence rate, coarse, transverse-dispersed sediment progrades toward the center of the basin.

Crustal structure, gravity anomalies and heat flow in the southern Rio Grande rift and their relationship to extensional tectonics

Abstract The Rio Grande rift is a Neogene structure which has undergone a two-phase history of extension and volcanism. Significant amounts of new data are available in the southern Rio Grande rift region which is the area of this study. The first phase of extension (15–30 Ma) was of the order of 30–50% and was directed NE-SW. The second phase (3–10 Ma) involved extension of about 10% which was directed E-W. Crustal structure in the region of the rift can be mapped using recent seismic and gravity data and the observed thinning associated with the rift is consistent with extension of about 10%. However, heat flow data in the southern rift cannot be explained simply in terms of extension. Both the heat flow and uplift history of the area require crustal thickening possibly by intrusion in the lower crust, a process not directly related to extension.

Late oligocene and miocene faulting and sedimentation, and evolution of the southern Rio Grande rift, New Mexico, USA

Abstract The distribution of nonmarine lithofacies, paleocurrents, and provenance data are used to define the evolution of late Oligocene and Miocene basins and complementary uplifts in the southern Rio Grande rift in the vicinity of Hatch, New Mexico, USA. The late Oligocene-middle Miocene Hayner Ranch Formation, which consists of a maximum of 1000 m of alluvial-fan, alluvial-flat, and lacustrine-carbonate lithofacies, was deposited in a narrow (12 km), northwest-trending, northeast-tilted half graben, whose footwall was the Caballo Mountains block. Stratigraphic separation on the border faults of the Caballo Mountains block was approximately 1615 m. An additional 854 m of stratigraphic separation along the Caballo Mountains border faults occurred during deposition of the middle-late Miocene Rincon Valley Formation, which is composed of up to 610 m of alluvial-fan, alluvial-flat, braided-fluvial, and gypsiferous playa lithofacies. Two new, north-trending fault blocks (Sierra de las Uvas and Dona Ana Mountains) and complementary west-northwest-tilted half graben also developed during Rincon Valley time, with approximately 549 m of stratigraphic separation along the border fault of the Sierra de las Uvas block. In latest Miocene and early Pliocene time, following deposition of the Rincon Valley Formation, movement continued along the border faults of the Caballo Mountains, Dona Ana Mountains, and Sierra de las Uvas blocks, and large parts of the Hayner Ranch and Rincon Valley basins were segmented into smaller fault blocks and basins by movement along new, largely north-trending faults. Analysis of the Hayner Ranch and Rincon Valley Formations, along with previous studies of the early Oligocene Bell Top Formation and late Pliocene-early Pleistocene Camp Rice Formation, indicate that the traditional two-stage model for development of the southern Rio Grande rift should be abandoned in favor of at least four episodes of block faulting beginning 35 Ma ago. With the exception of two northwest-trending border faults of the Caballo Mountains block that may be reactivated along Eocene compressional structures, the majority of border faults and complementary basins throughout the history of the southern Rio Grande rift were north-trending, which challenges the conventional idea of a clockwise change in stress through time.

Structural kinematics and depositional history of a Laramide uplift-basin pair in southern New Mexico: Implications for development of intraforeland basins

The kinematic and erosional history of the Rio Grande uplift, a large northwest-trending, basement-involved, thrust-bounded, block uplift of Laramide age (latest Cretaceous–early Tertiary) in south-central New Mexico, is documented by clastic rocks that accumulated in the complementary Love Ranch basin. The synorogenic to postorogenic McRae and Love Ranch Formations are as much as 1460 m thick; they filled the Love Ranch basin and onlapped the Rio Grande uplift. Present outcrops of the two formations cover an area of 100 km2 and reveal the stratigraphic architecture of the basin fill in three dimensions. Lithofacies distribution, clast size and composition, paleoflow data, syndepositional structures, nature of the basal unconformities, and ages of basin fill provide essential data for constraining uplift history. Laramide shortening began between Campanian and latest Maastrichtian time, and initially created open, symmetrical, northwest-trending folds, as well as a broad, approximately symmetrical uplift. This incipient Rio Grande uplift was capped by Upper Cretaceous volcanic rocks of intermediate and silicic composition, which were the primary source of volcanic detritus in the latest Cretaceous–Paleocene(?) McRae Formation. At this stage of uplift, the northeastern flank dipped gently northeastward away from the crest and served largely as a sediment transport surface; McRae strata accumulated only on distal parts of the surface in an embryonic Love Ranch basin to the northeast. Following an interruption in tectonism lasting as much as 10 m.y., renewed shortening in Paleocene(?) time elevated the Rio Grande uplift and formed the Love Ranch basin. At least 900 m of upward-fining, clastic Love Ranch strata of Paleocene-Eocene age accumulated in the basin. The Love Ranch lithofacies record a gradual southwestward shift of alluvial-fan depocenters, which resulted from growth of basin-margin structures and increasing basin asymmetry. Syndepositional synclines and angular unconformities record the growth of both intrabasinal folds and basin-margin thrusts. Clast compositions document progressive erosional unroofing of the Rio Grande uplift from Upper Cretaceous volcanic rocks into Precambrian granite and metamorphic rocks. Canyons 0.4 km deep locally drained the uplift, and maximum topographic relief may have approached 2 km. By late Eocene time, Love Ranch piedmont-slope deposits onlapped the uplift, burying all but the higher granite peaks. At this stage, the Love Ranch basin broadened and was the site of broad alluvial plains crossed by braided rivers draining to saline lakes. Our analysis of syntectonic sedimentary rocks in the Love Ranch basin supports recent models of evolution of Laramide basement-involved block uplifts in which early stages produce approximately symmetrical structures, and sediment derived from the uplift is transported across most of the uplift flank to be deposited in a distal setting. At this stage the future footwall of uplift-boundary faults dips basinward in a ramp-like fashion, providing a sediment transport surface. As boundary thrust faults and fault-propagation folds evolve and grow, basin asymmetry rapidly develops, causing depocenters to shift toward footwall positions near the overthrust margins. This evolution from symmetrical to asymmetrical structures is reflected in an overall upward-fining sequence in the basin fill.

Resurgent Volcano-Tectonic Depression of Oligocene Age, South-Central New Mexico

The Goodsight-Cedar Hills volcano-tectonic depression in south-central New Mexico formed concurrently with and following deposition of about 550 km 3 of Oligocene volcanic and volcaniclastic rocks. The depression is an asymmetric basin about 80 km long and 38 km wide. It is filled to a maximum depth of about 550 m by early rhyolitic ash-flow tuff, medial epiclastic strata derived partly from marginal raised rims (Bell Top Formation), and late basaltic andesite (Uvas Basalt). Much of the 295 km 3 of effusive rock was erupted from vents located near the center of the depression and from a major subsidence fracture zone along the eastern margin. Subsidence of the depression floor was noncatastrophic and approximately kept pace with basin filling, except along the eastern margin. Following eruption and broad, regional subsidence of the Uvas Basalt, the central floor of the depression was arched upward to form the Sierra de las Uvas dome and adjacent synclinal moat. The fault pattern of the dome and its association with known vents and the thickest part of the volcanic pile suggest that it formed by vertical movement of magma from an underlying chamber. Its development, following subsidence of the Uvas Basalt, suggests that it is essentially resurgent in origin. Although no postdoming volcanism is known, indirect evidence indicates that intrusion of silicic magma probably caused resurgence. It is clear that the Goodsight-Cedar Hills depression is not a resurgent cauldron of the classic Valles or Toba types. Rather, it appears to be a structure transitional in character between a cauldron and a fault trough. Subsidence apparently followed volcanism. Resurgence followed eruption of basaltic andesite rather than the usual ash-flow sequence, and the 10- to 12-m.y. history of the Goodsight-Cedar Hills depression is 5 to 6 times as long as other well-known resurgent cauldron cycles. Locally, deposition in the depression continued without interruption into early late Tertiary time, when extensional faulting occurred. The depression is elongated parallel to the late Tertiary Rio Grande Rift and may be a precursor of Basin and Range structure in the area. The late Tertiary fault pattern in the area indicates that earlier volcano-tectonic structures had important effects on the pattern of late Tertiary Basin and Range structures. Locally, late Tertiary faults were inherited from and duplicate the position of earlier subsidence faults or the synclinal moat. Elsewhere, the north-trending regional fault pattern was considerably modified by structural and litho-logic inhomogeneities of volcano-tectonic origin, particularly the Sierra de las Uvas dome and its buried intrusive masses.

Low-Angle Gravity Glide Structures in the Northern Virgin Mountains, Nevada and Arizona

The Northern Virgin Mountains of northwestern Arizona and adjacent Nevada are an arc-shaped anticlinal uplift that contains marine Paleozoic shelf sedimentary rocks flanking a central core of Precambrian metamorphic rocks. Mesozoic and Tertiary rocks, including Upper Cretaceous beds previously unknown in northwestern Arizona, are present in downfaulted blocks around the edges of the range. Except for marine Lower Triassic beds and Tertiary lacustrine limestones, they consist largely of clastics of continental origin. Tertiary and Laramide uplift in the Virgin Range resulted in gravitative adjustments that took the form of major glide plates. Gravity structures of different styles originated from each of three major uplifts. The oldest uplift, the asymmetric Virgin Mountain Anticline, formed during the Laramide orogenic interval by draping of the sedimentary cover over a basement horst that shows evidence of recurrent activity from Precambrian through Recent time. The northeast-trending anticline produced a structural and topographical high from which the Paleozoic cover slid, by gravity, south-eastward down a slope formed by bedding. By moving across successively younger beds of the southeastern limb, older rocks in the upper plate came to rest on younger. Compression generated during movement created tight folds in the overridden strata and within the glide plate itself. Most important of these is an overturned syncline beneath the glide plate that extends for a distance of 12 mi parallel to the axis of the Virgin Mountain Anticline. Two younger uplifts were formed by the Virgin Mountain fault system on the west border of the range and by the State Line fault on the east border. The strikes of both faults apparently were partly controlled by the structural grain of the basement; the faults collectively produced the late Tertiary uplift of the Virgin Mountain Anticline as a wedge-shaped horst. Along the State Line fault, uplift and westward tilting created a structural high from which a 25-sq-mi plate of sedimentary cover became detached and moved westward down a slope, subparallel to bedding, thereby truncating all older structures in both upper and lower plates. A horst uplifted on two large faults belonging to the Virgin Mountain fault system formed the third structural high in the Virgin Range. Six glide plates, apparently originating from this high, moved westward on bowl- and scoop-shaped faults across steeply dipping strata of the northwestern limb of the Virgin Mountain Anticline. The glide plates both truncate and are cut by faults associated with the frontal Virgin Mountain fault, which suggests that sliding was concurrent with and in response to movements on them. The main distinction between the Laramide and late Tertiary gliding is the nature of the causal uplift and degree to which older rocks were emplaced above younger ones. Gliding from the crest of the Virgin Mountain Anticline under considerable cover resulted in plastic flow, which allowed tight drag folding, the formation of a sharp glide surface, and the emplacement of older rocks above younger ones. Shallower gliding from late Tertiary fault blocks permitted brecciation, minor folding, and resulted mainly in the movement of younger rocks to positions above older ones.

Onset of the Laramide orogeny and associated magmatism in southern New Mexico based on U-Pb geochronology

The Laramide orogeny is a classic yet controversial mountain-building event that resulted, in the southwest United States, in uplifts, sedimentation, and magmatism that can be used to constrain the onset of this event in the region and expand our knowledge of Late Cretaceous to Paleogene tectonism. The McRae Formation marks the onset of deposition in the Laramide Love Ranch Basin, which was located to the northeast of the west-northwest-trending coeval Rio Grande uplift in south-central New Mexico, but its age is not well constrained. A previously published late Maastrichtian age for the McRae Formation was based on the presence of dinosaur bones in the upper of two members of the formation. We obtained new U-Pb dates from one dacite clast and three ash-fall tuffs from the lower Jose Creek Member and from one ash-fall tuff from the lower part of the overlying Hall Lake Member of the McRae Formation. The clast yielded a date of 75.0 ± 1.1 Ma, whereas the ages of the tuffs, in ascending stratigraphic order, are 74.9 ± 0.7 Ma, 74.7 ± 0.6 Ma, 75.2 ± 1.3 Ma, and 73.2 ± 0.7 Ma. These dates indicate that the onset of Laramide deposition in the Love Ranch Basin must have occurred earlier, in late Campanian time, similar to deposition in the Laramide Ringbone Basin in southwestern New Mexico. In addition, U-Pb zircon dates of 75.7 ± 1.3 Ma and 75.0 ± 2.8 Ma were obtained on the Twin Peaks stock and on a dacite sill, respectively, in the Burro Mountains of southwestern New Mexico. These dates are similar to those of other Laramide arc magmatic centers in southern New Mexico, which have a limited range of ages from 75 to 70 Ma, including the Hidalgo Formation in the Little Hatchet Mountains, the Silver City-Pinos Altos region, and the Copper Flat porphyry system. These new and previously published dates indicate that during the onset of Laramide deformation in southwestern and south-central New Mexico, the angle of subduction of the Farallon plate may have been steep enough to allow partial melting of an asthenospheric wedge, resulting in arc magmatism far inboard of the trench.