Search

New geologic mapping and tephrochronologic assessment of strata in extensional basins surrounding Knoll Mountain (Nevada, USA) reveal a geologic history linked to tectonic development of the Yellowstone hotspot and Snake River Plain to the north, and to the Ruby--East Humboldt--Wood Hills metamorphic core complex to the south. Data from these areas are utilized to present a paleogeographic reconstruction of northeastern Nevada--southcentral Idaho depicting the architecture of extensional faulting and basin development during collapse of the Nevadaplano over the past 17 m.y. Knoll Mountain is a northeast-trending horst along the southern margin of the Snake River Plain and track of the Yellowstone hotspot. The horst is bounded on the east by the Thousand Springs fault system and basin, and on the west by the Knoll Mountain fault and basin, where streams currently drain north into the Snake River Plain. The Knoll and Thousand Springs basins form half-grabens that are filled with the ca. 16 Ma to ca. 8-5 Ma Humboldt Formation, which was deposited in alluvial, eolian, and lacustrine environments during slip along range-bounding faults and a series of late-stage synthetic intrabasin faults. Structural, chronologic, and sedimentologic assessment of the Humboldt Formation in the Knoll basin indicates that it records overall southward fluvial drainage with slip along the Knoll Mountain fault beginning ca. 16 Ma and continuing to at least 8 Ma, and that between 8 and ca. 5 Ma, a west-dipping intrabasin fault system had developed. Between ca. 8-5 Ma to ca. 3 Ma, several fundamental changes took place, beginning with the cessation of faulting followed by widespread erosion that in turn was followed by deposition of older alluvium. The reversal of drainage direction from south to north flowing in the Knoll basin also took place during this time period, but its age relative to the widespread erosion or older alluvium is unknown. An integration of our work with previous studies north of Knoll Mountain reveal that the Knoll Mountain and intrabasin faults terminate to the north in the vicinity of the Jurassic Contact pluton, and that this area forms an accommodation zone separating broadly coeval and colinear faults bounding the ca. 10-8 Ma north-trending Rogerson graben, the northern end of which merges with the Snake River Plain. Furthermore, an integration of our work with previous work south of Knoll Mountain reveals that the Knoll Mountain fault formed part of a >190-km long, west-dipping fault zone that included the Ruby--East Humboldt detachment. This fault zone, which we refer to as the Knoll-Ruby fault system, had an extensive hanging-wall basin, the Knoll- Ruby basin. The Knoll-Ruby fault system was a prominent structure facilitating collapse of the Nevadaplano in northeastern Nevada between ca. 16 and ca. 8-5 Ma, and its central part produced partial exhumation of high-grade, mid-crustal metamorphic rocks in the Ruby--East Humboldt--Wood Hills metamorphic core complex. By 8-5 Ma, during the waning stages of extension along the Knoll-Ruby fault system, a series of intrabasin faults developed at about the same time as the integration of streams to form the incipient eastern reaches of the Humboldt River system. Profound changes in tectonics and paleogeography took place between ca. 8-5 Ma and ca. 3 Ma, that included the extinction of the Knoll-Ruby and intrabasin basin fault systems followed by southward migration of significant tectonism away from the Snake River Plain, resulting in development of a set of modern normal faults responsible for uplift of the southern Snake Mountains, Ruby Mountains, East Humboldt Range, and Pequop Mountains. These new faults cut and dismembered the central and southern part of the Knoll-Ruby fault system and basin, effectively ending any fluvial connection between the northern and southern parts of the Knoll-Ruby basin. Since ca. 8-5 Ma to the present, the Knoll Mountain region has remained relatively tectonically quiescent, and continued subsidence in the Snake River Plain to the north induced capture of the drainage system in the Knoll basin and reversed the drainage direction from south to north flowing. Our new findings indicate that (1) the Knoll-Ruby fault system and associated intrabasin faults were active until ca. 8-5 Ma, which is younger than the 12-10 Ma age generally recognized for cessation of major extension elsewhere in the northern Nevada region; (2) although this fault system was responsible for partial exhumation of core-complex metamorphic rocks, it extended well beyond the confines of the core complex proper; and (3) slip along faults in the Knoll Mountain region occurred before, during, and after passage of the hotspot at the longitude of Knoll Mountain. With the exception of significant faulting postdating passage of the hotspot, the timing of faulting in the Knoll Mountain area is consistent in a general way with the space-time pattern of extension recognized elsewhere along the southern margin of the Snake River Plain. However, it is unknown if the rate of fault slip increased during passage of the hotspot as it did in other areas. [ABSTRACT FROM AUTHOR]

A strike-slip fault is present outboard and subparallel to the Wassuk Range front within the central Walker Lane (Nevada, USA). Recessional shorelines of pluvial Lake Lahontan that reached its highstand ca. 15,475 ± 720 cal. yr B.P. are displaced ~14 m and yield a right-lateral slip-rate estimate approaching 1 mm/yr. The strike-slip fault trace projects southeastward toward the eastern margin of Walker Lake, which is ~15 km to the southeast. The trace is obscured in this region by recessional shorelines features that record the historical dessication of the lake caused by upstream water diversion and consumption. High-resolution seismic CHIRP (compressed high intensity radar pulse) profi les acquired in Walker Lake reveal ~20 k.y. of stratigraphy that is tilted westward ~20- 30 m to the Wassuk Range front, consistent with ~1.0-1.5 mm/yr (20-30 m/20 k.y.) of vertical displacement on the main rangebounding normal fault. Direct evidence of the northwest-trending right-lateral strikeslip fault is not observed, although a set of folds and faults trending N35°E, conjugate to the trend of the strike-slip fault observed to the north, is superimposed on the west-dipping strata. The pattern and trend of folding and faulting beneath the lake are not simply explained; they may record development of Riedel shears in a zone of northwest-directed strike slip. Regardless of their genesis, the faults and folds appear to have been inactive during the past ~10.5 k.y. These observations begin to reconcile what was a mismatch between geodetically predicted deformation rates and geological fault slip rate studies along the Wassuk Range front, and provide another example of strain partitioning between predominantly normal and strikeslip faults that occurs in regions of oblique extension such as the Walker Lane. [ABSTRACT FROM AUTHOR]

Alluvial fan evolution and morphology are often considered to respond primarily to external forcing (e.g., tectonics, climate, and base-level change). Here we present a numerical model of alluvial fan evolution that shows that dramatic and persistent fan entrenchment may occur in the absence of such forcing. This process is driven by positive autogenic feed-backs between flow width, sediment transport, and rate of fan aggradation. Entrenchment is initiated where sediment accommodation space limits continued fan growth. Our results highlight a need to rethink both the representation of fluvial width adjustment in landscape evolution models and the established framework for the interpretation of fluvial landforms as archives of environmental change. [ABSTRACT FROM AUTHOR]

Rock uplift and erosional denudation of orogenic belts have long been the most important geologic processes that serve to shape continental surfaces, but the rate of geomorphic change resulting from these natural phenomena has now been outstripped by human activities associated with agriculture, construction, and mining. Although humans are now the most important geomorphic agent on the planet's surface, natural and anthropogenic processes serve to modify quite different parts of Earth's landscape. In order to better understand the impact of humans on continental erosion, we have examined both long-term and short-term data on rates of sediment transfer in response to glacio-fluvial and anthropogenic processes. Phanerozoic rates of subaerial denudation inferred from preserved volumes of sedimentary rock require a mean continental erosion rate on the order of 16 m per million years (m/m.y.), resulting in the accumulation of ∼5 gigatons of sediment per year (Gt/yr). Erosion irregularly increased over the ∼542 m.y. span of Phanerozoic time to a Pliocene value of 53 m/m.y. (16 Gt/yr). Current estimates of large river sediment loads are similar to this late Neogene value, and require net denudation of ice-free land surfaces at a rate of ∼62 m/m.y. (∼21 Gt/yr). Consideration of the variation in large river sediment loads and the geomorphology of respective river basin catchments suggests that natural erosion is primarily confined to drainage headwaters; ∼83% of the global river sediment flux is derived from the highest 10% of Earth's surface. Subaerial erosion as a result of human activity, primarily through agricultural practices, has resulted in a sharp increase in net rates of continental denudation; although less well constrained than estimates based on surviving rock volumes or current river loads, available data suggest that present farmland denudation is proceeding at a rate of ∼600 m/ m.y. (∼75 Gt/yr), and is largely confined to the lower elevations of Earth's land surface, primarily along passive continental margins; ∼83% of cropland erosion occurs over the lower 65% of Earth∼s surface. The conspicuous disparity between natural sediment fluxes suggested by data on rock volumes and river loads (∼21 Gt/yr) and anthropogenic fluxes inferred from measured and modeled cropland soil losses (75 Gt/yr) is readily resolved by data on thicknesses and ages of alluvial sediment that has been deposited immediately downslope from eroding croplands over the history of human agriculture. Accumulation of postsettlement alluvium on higher-order tributary channels and floodplains (mean rate ∼12,600 m/m.y.) is the most important geomorphic process in terms of the erosion and deposition of sediment that is currently shaping the landscape of Earth. It far exceeds even the impact of Pleistocene continental glaciers or the current impact of alpine erosion by glacial and/ or fluvial processes. Conversely, available data suggest that since 1961, global cropland area has increased by ∼11%, while the global population has approximately doubled. The net effect of both changes is that per capita cropland area has decreased by ∼44% over this same time interval; ∼1% per year. This is ∼25 times the rate of soil area loss anticipated from human denudation of cropland surfaces. In a context of per capita food production, soil loss through cropland erosion is largely insignificant when compared to the impact of population growth. [ABSTRACT FROM AUTHOR]

Aluminum-26 and beryllium-10 surface exposure dating on cut-and-fill river-terrace surfaces from the lower Sutlej Valley (northwest Himalaya) documents the close link between Indian Summer Monsoon (ISM) oscillations and intervals of enhanced fluvial incision. During the early Holocene ISM optimum, precipitation was enhanced and reached far into the internal parts of the orogen. The amplified sediment flux from these usually dry but glaciated areas caused alluviation of downstream valleys up to 120 m above present grade at ca. 9.9 k.y. B.P. Terrace formation (i.e., incision) in the coarse deposits occurred during century-long weak ISM phases that resulted in reduced moisture availability and most likely in lower sediment flux. Here, we suggest that the lower sediment flux during weak ISM phases allowed rivers to incise episodically into the alluvial fill. [ABSTRACT FROM AUTHOR]

A major earthquake on the Seattle fault, Washington, Ca. A.D. 900-930 was first inferred from uplifted shorelines and tsunami deposits. Despite follow-up geophysical and geological investigations, the rupture parameters of the earthquake and the geometry of the fault are uncertain. Here we estimate the fault geometry, slip direction, and magnitude of the earthquake by modeling shoreline elevation change. The best fitting model geometry is a reverse fault with a shallow roof ramp consisting of at least two back thrusts. The best fitting rupture is a SW-NE oblique reverse slip with horizontal shortening of 15 m, rupture depth of 12.5 km, and magnitude Mw = 7.5. [ABSTRACT FROM AUTHOR]

We investigated active deformation within the Akato Tagh restraining double bend to determine the age of the active Altyn Tagh fault relative to the Altyn Tagh system and thereby evaluate the extent to which this system evolved by net strain-hardening or softening. Active structures were mapped based on their geomorphology and disruption of Quaternary(?) deposits/surfaces. The style of active faulting is strongly correlated with fault strike: 065°–070° striking segments show pure left-slip whereas faults with more northward or eastward strikes are transtensional or transpressional, respectively. Our mapping further suggests that the 065° to 070°-striking western and eastern segments of the Akato Tagh bend are characterized by pure strike-slip motion, with partitioned transpression along the ∼090°-striking central segment of the double bend. It remains unclear how this active, bend-perpendicular shortening is absorbed. In conjunction with previous work, the present study fails to support the commonly held idea that Tarim-Tibet motion is strongly oblique to the Altyn Tagh system. Estimates for the age of the Akato Tagh bend derived in a companion study suggest the bend is only a few million years old. The current principal trace is probably similarly young because formation of the bend by recent deformation of an old trace should result in transpression along the western and eastern segments, contrary to the pure left-slip shown here. The Altyn Tagh system comprises multiple fault strands in a zone ∼100 km wide across strike. Because the main trace appears to be much younger than the system in which it is embedded, we speculate that this system evolved by the sequential formation and death of short-lived fault strands. In particular, we suggest that geometrically complex strike-slip fault systems such as the Altyn Tagh may form via system strain hardening, where this net response reflects a dynamic competition between hardening and softening processes that are active simultaneously within the fault zone. Hardening mechanisms may include growth of restraining bend topography or material hardening of phyllosilicate-rich gouge, whereas softening processes might include R-P shear linkage or reduction in bend angle by vertical axis rotation. Our analysis suggests that net strain hardening of a fault system can produce continental deformation that is spatially localized over the 1–5 m.y. during which an individual strand is active, but distributed over the 10–100 m.y. corresponding to the life-span of the whole fault system and the collision zone in which it is contained. Thus, time scale is critically important in determining whether or not continental deformation is spatially distributed or localized. [ABSTRACT FROM AUTHOR]