George E. Hilley

Resolving vertical tectonics in the San Francisco Bay Area from permanent scatterer InSAR and GPS analysis

Using a combination of GPS-measured horizontal velocities of 200 sites and 115,487 range-change rates determined with the permanent scatterer interferometric synthetic aperture radar (InSAR) method in the San Francisco Bay Area, we resolve vertical motions in the region at sub-mm/yr precision. The highest displacement rates are due to nontectonic processes, such as active landslides, subsidence and rebound over aquifers, and rapid settling of unconsolidated sediments along the bay margins. Residual displacement rates are determined by removing the contribution of the GPS-derived horizontal velocity field from the InSAR range-change rates. To isolate vertical tectonic rates, we use only those InSAR measurements made on material that was not Quaternary substrate, which is susceptible to nontectonic and seasonally varying ground motions. The InSAR residuals indicate significant uplift over the southern foothills of the active Mount Diablo anticlinorium, the Mission Hills stepover region of the Hayward and Calaveras faults, and the central and southern Santa Cruz Mountains located along a restraining bend of the San Andreas fault.

Uplift, Erosion, and Phosphorus Limitation in Terrestrial Ecosystems

A BSTRACTPrimary productivity on old, weathered soils often is assumed to be limited by phosphorus (P), especially in the lowland tropics where climatic conditions promote the rapid depletion of rock-derived nutrients. This assumption is based on a static view of soils weathering in place with no renewal of the bedrock source. In reality, advection of material through the soil column introduces a spatially variable supply of rock-derived nutrients. This flux is dependent on the residence time of soil, which can range from a few hundred years in rapidly uplifting collisional mountain belts to tens of millions of years in tectonically quiescent tropical cratons. We modeled the effects of tectonic uplift, erosion, and soil depth on the advection of P through the soil column and P availability, calibrating rate of change in biologically available P over time with data from two basaltic chronosequences in Hawai’i and a series of greywacke terraces in New Zealand. Combining our model with the global distribution of tectonic uplift rates and soil depths, we identified tectonic settings that are likely to support P-depleted ecosystems—assuming that tectonic uplift and erosion are balanced (that is, landscape development has reached steady state). The model captures the occurrence of transient P limitation in rapidly uplifting young ecosystems where mineral weathering is outpaced by physical erosion—a likely occurrence where biological N fixation is important. However, we calculate that P depletion is unlikely in areas of moderate uplift, such as most of Central America and Southeast Asia, due to the continuous advection of P into the rooting zone. Finally, where soil advection is slow, such as the Amazon Basin, we expect widespread P depletion in the absence of exogenous nutrient inputs.

Oligocene range uplift and development of plateau morphology in the southern central Andes

[1] The Puna-Altiplano plateau in South America is a high-elevation, low internal relief landform that is characterized by internal drainage and hyperaridity. Thermochronologic and sedimentologic observations from the Sierra de Calalaste region in the southwestern Puna plateau, Argentina, place new constraints on early plateau evolution by resolving the timing of uplift of mountain ranges that bound present-day basins and the filling pattern of these basins during late Eocene-Miocene time. Paleocurrent indicators, sedimentary provenance analyses, and apatite fission track thermochronology indicate that the original paleodrainage setting was disrupted by exhumation and uplift of the Sierra de Calalaste range between 24 and 29 Ma. This event was responsible for basin reorganization and the disruption of the regional fluvial system that has ultimately led to the formation of internal drainage conditions, which, in the Salar de Antofalla, were established not later than late Miocene. Upper Eocene-Oligocene sedimentary rocks flanking the range contain features that suggest an arid environment existed prior to and during its uplift. Provenance data indicate a common similar source located to the west for both the southern Puna and the Altiplano of Bolivia during the late EoceneOligocene with sporadic local sources. This suggests the existence of an extensive, longitudinally oriented foreland basin along the central Andes during this time. Citation: Carrapa, B., D. Adelmann, G. E. Hilley, E. Mortimer, E. R. Sobel, and M. R. Strecker (2005), Oligocene range uplift and development of plateau morphology in the southern central Andes, Tectonics, 24, TC4011, doi:10.1029/ 2004TC001762.

Processes of oscillatory basin filling and excavation in a tectonically active orogen: Quebrada del Toro Basin, NW Argentina

Intramontane basins may act as important sediment storage areas, serve as recorders of the history of deformation, record syntectonic deposition, and document the evolution of climatic conditions during deposition. We document the timing, cyclicity, and processes that led to the filling and reexcavation of the intramontane Quebrada del Toro basin in NW Argentina. Geomorphic and geologic observations indicate that the basin was filled with sediment that has been subsequently excavated at least two times in the last ∼8 m.y. The last filling and excavation cycle occurred within the last 0.98 m.y. and has led to the deposition and removal of ∼61.4 km 3 of material from the basin, leading to a basin-wide averaged minimum denudation rate of 0.16 mm/yr. Aggradation within the basin took place due to channel steepening of the downstream fluvial system that connects the intramontane basin to the foreland. This portion of the fluvial system is actively incising through an uplifting bedrock zone. We use observations within the Toro to test a quasi-physically based model of channel aggradation behind a rising base level that rises due to downstream channel steepening. Our work shows that the bedrock incision rate constant required to reproduce conditions observed within the Toro basin is consistent with values measured independently in similar rock types. Therefore, in intramontane basins that experience similar processes of filling and evacuation, this model may be used to assess the relative importance of tectonic rock uplift, bedrock resistance to fluvial incision, and climate in determining the geomorphic and sedimentologic history of these basins.

Geomorphic response to uplift along the Dragon's Back pressure ridge, Carrizo Plain, California

We used high-resolution topography, geomorphic mapping of active surface processes, and geologic mapping to study the topographic and erosional response of small drainage basins to rock uplift along the Dragons Back pressure ridge along the San Andreas fault in the Carrizo Plain, California. We infer the history of deformation experienced by ~40 small drainage basins formed in poorly consolidated sedimentary rocks. A space-for-time substitution directly images the erosional and topographic responses to deformation. Progressive deformation and rock uplift are accompanied by increases in channel steepness and basin relief. As uplift ceases, channel concavity rapidly increases, causing channels to undercut hillslopes—this undercutting promotes the consumption of hillslopes by landsliding. This undercutting also causes basin relief to be greatest after uplift has stopped. This analysis indicates that channels of the Dragons Back pressure ridge respond to changes in rock uplift rates over thousands of years, whereas hillslope processes may take more than an order of magnitude longer to adjust to changes in rock uplift rates. Our study directly measures changes in erosional processes due to the initiation and cessation of rock uplift, which can typically only be inferred using numerical models, by direct field observations.

Does the topographic distribution of the central Andean Puna Plateau result from climatic or geodynamic processes

Orogenic plateaus are extensive, high-elevation areas with low internal relief that have been attributed to deep-seated and/or climate-driven surface processes. In the latter case, models predict that lateral plateau growth results from increasing aridity along the margins as range uplift shields the orogen interior from precipitation. We analyze the spatiotemporal progression of basin isolation and fi lling at the eastern margin of the Puna Plateau of the Argentine Andes to determine if the topography predicted by such models is observed. We fithat the timing of basin fi lling and reexcavation is variable, suggesting nonsystematic plateau growth. Instead, the Airy isostatically compensated component of topography constitutes the majority of the mean elevation gain between the foreland and the plateau. This indicates that deep-seated phenomena, such as changes in crustal thickness and/or lateral density, are required to produce high plateau elevations. In contrast, the frequency of the uncompensated topography within the plateau and in the adjacent foreland that is interrupted by ranges appears similar, although the amplitude of this topographic component increases east of the plateau. Combined with sedimentologic observations, we infer that the low internal relief of the plateau likely results from increased aridity and sediment storage within the plateau and along its eastern margin.

Influence of lithosphere viscosity structure on estimates of fault slip rate in the Mojave region of the San Andreas fault system

[1] It is well known that slip rate estimates from geodetic data are nonunique because they depend on model assumptions and parameters that are often not known a priori. Estimates of fault slip rate on the Mojave segment of the San Andreas fault system derived from elastic block models and GPS data are significantly lower than estimates from geologic data. To determine the extent to which the slip rate discrepancy might be due to the oversimplified models of the rheology of the lithosphere, we develop a two-dimensional linear Maxwell viscoelastic earthquake cycle model and simultaneously estimate fault slip rates and lithosphere viscosity structure in the Mojave region. The model consists of episodic earthquakes in an elastic crust overlying layers with different viscosities that represent the lower crust, uppermost mantle, and upper mantle. We use GPS measurements of postseismic relaxation following the 1992 Landers earthquake, triangulation measurements spanning 1932-1977, GPS measurements of the contemporary velocity field, and paleoseismic data along the San Andreas fault. We infer lower crustal (15-30 km depth) viscosity of ∼1019-10 20 Pa s, uppermost mantle (30-60 km) viscosity of ∼10 20-22 Pa s, and underlying upper mantle viscosity of ∼10 18 -10 19 Pa S, consistent with inferences from laboratory experiments of relatively high-viscosity lithospheric mantle and lower-viscosity lower crust and underlying asthenospheric mantle. We infer a 20-30 mm/yr slip rate on the San Andreas fault, in agreement with the lower end of geologic estimates. Inversions of geodetic data with models that do not incorporate layered viscosity structure may significantly misestimate slip rates.

Terrestrial source to deep-sea sink sediment budgets at high and low sea levels: Insights from tectonically active Southern California

Sediment routing from terrestrial source areas to the deep sea influences landscapes and seascapes and supply and filling of sedimentary basins. However, a comprehensive assessment of land-to-deep-sea sediment budgets over millennia with significant climate change is lacking. We provide source to sink sediment budgets using cosmogenic radionuclide–derived terrestrial denudation rates and submarine-fan deposition rates through sea-level fluctuations since oxygen isotope stage 3 (younger than 40 ka) in tectonically active, spatially restricted sediment-routing systems of Southern California. We show that source-area denudation and deep-sea deposition are balanced during a period of generally falling and low sea level (40–13 ka), but that deep-sea deposition exceeds terrestrial denudation during the subsequent period of rising and high sea level (younger than 13 ka). This additional supply of sediment is likely owed to enhanced dispersal of sediment across the shelf caused by seacliff erosion during postglacial shoreline transgression and initiation of submarine mass wasting. During periods of both low and high sea level, land and deep-sea sediment fluxes do not show orders of magnitude imbalances that might be expected in the wake of major sea-level changes. Thus, sediment-routing processes in a globally significant class of small, tectonically active systems might be fundamentally different from those of larger systems that drain entire orogens, in which sediment storage in coastal plains and wide continental shelves can exceed millions of years. Furthermore, in such small systems, depositional changes offshore can reflect onshore changes when viewed over time scales of several thousand years to more than 10 k.y.

A framework for predicting global silicate weathering and CO2 drawdown rates over geologic time-scales

Global silicate weathering drives long-time-scale fluctuations in atmospheric CO2. While tectonics, climate, and rock-type influence silicate weathering, it is unclear how these factors combine to drive global rates. Here, we explore whether local erosion rates, GCM-derived dust fluxes, temperature, and water balance can capture global variation in silicate weathering. Our spatially explicit approach predicts 1.9–4.6 × 1013 mols of Si weathered globally per year, within a factor of 4–10 of estimates of global silicate fluxes derived from riverine measurements. Similarly, our watershed-based estimates are within a factor of 4–18 (mean of 5.3) of the silica fluxes measured in the worlds ten largest rivers. Eighty percent of total global silicate weathering product traveling as dissolved load occurs within a narrow range (0.01–0.5 mm/year) of erosion rates. Assuming each mol of Mg or Ca reacts with 1 mol of CO2, 1.5–3.3 × 108 tons/year of CO2 is consumed by silicate weathering, consistent with previously published estimates. Approximately 50% of this drawdown occurs in the worlds active mountain belts, emphasizing the importance of tectonic regulation of global climate over geologic timescales.

Differential structural and geomorphic mountain-front evolution in an active continental collision zone: The northwest Pamir, southern Kyrgyzstan

Western, central, and eastern segments of the Trans Alai mountain front in the northern Pamir of Kyrgyzstan have accommodated varying degrees of approachment of the Pamir orogen with respect to the Tien Shan (Shan = Mountains) to the north. Ongoing collision between the north-western corner of the Indian indenter and Eurasia has resulted in closure of the intramontane Alai Valley, which separates the Tien Shan and Trans Alai (Pamir) ranges. The different segments highlight the processes of shaping tectonically active mountain fronts in a semiarid environment. In this study, we have characterized this variation in processes with compilations of regional tectonic information, detailed geologic and geomorphic maps, topographic analyses, and interpretation of seismic reflection data. Along the sinuous western segment of the mountain front, dextrally oblique thrusting has created a wide (>500 m) zone of highly erodible fault gouge. This fault zone impinges on the southern Tien Shan, but complete basin closure is prevented by erosion due to the westward-flowing Kyzilsu River; the Kyzilsu valley forms the only outlet and is the vestige of a formerly contiguous sedimentary basin linking the Tarim Basin of China with the Tadjik Depression in the west. Numerous large landslides rooted in the fault zone have covered the active fault, which is partially undercut by the Kyzilsu River. Older, large landslides in this setting are associated with different levels of fluvial terraces of the former or present course of the Kyzilsu River, suggesting a causative relationship between lateral fluvial scouring, failure of mechanically weak mountain fronts, ongoing faulting, and mass transfer. Along the linear central segment, deformation is confined to a narrow single south-dipping thrust fault that juxtaposes Pliocene-Pleistocene and Holocene conglomerates. In this sector, the mountain front has numerous Holocene offsets. This prevailing structural style and the long-term deformation are underscored by multiple flights of gently sloping pediments and glaciogenic terrace surfaces that abruptly terminate at the steep mountain front, which also forms the boundary with the wide regraded piedmont. In contrast, closure between the Pamir and Tien Shan is complete along the eastern segment. The eroded and sinuous mountain front has been tectonically inactive during late Quaternary time. Small drainage-basin areas and low stream power apparently were not conducive to maintaining an eastern outlet to the Tarim Basin. Active deformation has stepped back into the orogen and now is concentrated along the Markansu Fault and within the Tien Shan to the north. The large drainage-basin area of the Kyzilsu River and the constant, glacially fed runoff guarantee that an effective interplay between tectonic uplift and erosion is maintained. Therefore, the geomorphically different mountain-front segments highlight the relationships between tectonic uplift and geomorphic processes, which in turn are controlled by lithology, topography, and the history of sediment routing throughout the landscape.