Eric Kirby

Quantifying differential rock-uplift rates via stream profile analysis

Despite intensive research into the coupling between tectonics and surface processes, our ability to obtain quantitative information on the rates of tectonic processes from topography remains limited due primarily to a dearth of data with which to test and calibrate process rate laws. Here we develop a simple theory for the impact of spatially variable rock-uplift rate on the concavity of bedrock river profiles. Application of the analysis to the Siwalik Hills of central Nepal demonstrates that systematic differences in the concavity of channels in this region match the predictions of a stream power incision model and depend on the position and direction of the channel relative to gradients in the vertical component of deformation rate across an active fault-bend fold. Furthermore, calibration of model parameters from channel profiles argued to be in steady state with the current climatic and tectonic regime indicates that (1) the ratio of exponents on channel drainage area and slope ( m / n ) is ∼0.46, consistent with theoretical predictions; (2) the slope exponent is consistent with incision either linearly proportional to shear stress or unit stream power ( n = 0.66 or n = 1, respectively); and (3) the coefficient of erosion is within the range of previously published estimates (mean K = 4.3 × 10 −4 m 0.2 /yr). Application of these model parameters to other channels in the Siwalik Hills yields estimates of spatially variable erosion rates that mimic expected variations in rock-uplift rate across a fault-bend fold. Thus, the sensitivity of channel gradient to rock- uplift rate in this landscape allows us to derive quantitative estimates of spatial variations in erosion rate directly from topographic data.

Late Cenozoic evolution of the eastern margin of the Tibetan Plateau: Inferences from 40Ar/39Ar and (U‐Th)/He thermochronology

High topography in central Asia is perhaps the most fundamental expression of the Cenozoic Indo-Asian collision, yet an understanding of the timing and rates of development of the Tibetan Plateau remains elusive. Here we investigate the Cenozoic thermal histories of rocks along the eastern margin of the plateau adjacent to the Sichuan Basin in an effort to determine when the steep topographic escarpment that characterizes this margin developed. Temperature-time paths inferred from ^(40)Ar/^(39)Ar thermochronology of biotite, multiple diffusion domain modeling of alkali feldspar ^(40)Ar release spectra, and (U-Th)/He thermochronology of zircon and apatite imply that rocks at the present-day topographic front of the plateau underwent slow cooling ( 30°–50°C/m.y.) coincident with exhumation from inferred depths of ∼8–10 km, at denudation rates of 1–2 mm/yr. Samples from the interior of the plateau continued to cool relatively slowly during the same time period (∼3°C/m.y.), suggesting limited exhumation (1–2 km). However, these samples record a slight increase in cooling rate (from <1 to ∼3°C/m.y.) at some time during the middle Tertiary; the tectonic significance of this change remains uncertain. Regardless, late Cenozoic denudation in this region appears to have been markedly heterogeneous, with the highest rates of exhumation focused at the topographic front of the plateau margin. We infer that the onset of rapid cooling at the plateau margin reflects the erosional response to the development of regionally significant topographic gradients between the plateau and the stable Sichuan Basin and thus marks the onset of deformation related to the development of the Tibetan Plateau in this region. The present margin of the plateau adjacent to and north of the Sichuan Basin is apparently no older than the late Miocene or early Pliocene (∼5–12 Ma).

Geomorphic limits to climate-induced increases in topographic relief

Recognition of the potential for strong dynamic coupling between atmospheric and tectonic processes has sparked intense cross-disciplinary investigation and debate on the question of whether tectonics have driven long-term climate change or vice versa. It has been proposed that climate change might have driven the uplift of mountain summits through an isostatic response to valley incision. Because isostasy acts to compensate mean elevations, the debate hinges on the question of whether climate change can significantly increase topographic relief or, more precisely, increase the volume of ‘missing mass’ between summits and ridges. Here we show that, in tectonically active mountain ranges, geomorphic constraints allow only a relatively small increase in topographic relief in response to climate change. Thus, although climate change may cause significant increases in denudation rates, potentially establishing an important feedback between surficial and crustal processes, neither fluvial nor glacial erosion is likely to induce significant isostatic peak uplift.

Stress changes from the 2008 Wenchuan earthquake and increased hazard in the Sichuan basin

On 12 May 2008, the devastating magnitude 7.9 (Wenchuan) earthquake struck the eastern edge of the Tibetan plateau, collapsing buildings and killing thousands in major cities aligned along the western Sichuan basin in China. After such a large-magnitude earthquake, rearrangement of stresses in the crust commonly leads to subsequent damaging earthquakes. The mainshock of the 12 May earthquake ruptured with as much as 9 m of slip along the boundary between the Longmen Shan and Sichuan basin, and demonstrated the complex strike–slip and thrust motion that characterizes the region. The Sichuan basin and surroundings are also crossed by other active strike–slip and thrust faults. Here we present calculations of the coseismic stress changes that resulted from the 12 May event using models of those faults, and show that many indicate significant stress increases. Rapid mapping of such stress changes can help to locate fault sections with relatively higher odds of producing large aftershocks.

Neotectonics of the Min Shan, China: Implications for mechanisms driving Quaternary deformation along the eastern margin of the Tibetan Plateau

The Min Shan region, located along the eastern margin of the Tibetan Plateau north of the Sichuan Basin, provides an important natural laboratory in which to study the rates and patterns of deformation and their relationship to mountain building at the margin of the plateau. The topographic margin of the plateau is coincident with a north-trending mountain range, the Min Shan, that stands nearly 2 km above the mean elevation of the plateau (~3500 m in this region). We exploit the preservation of a series of variably deformed Quaternary sediments along the western flank of the range to investigate the Pleistocene-Holocene deformation field within the Min Shan region. Mapping and field observations of remnant alluvial fans of late Pleistocene age indicate that deformation within the Min Shan involved substantial (~10°), rapid, down-to-the-northwest tilting. The geometry of the deposits and the partial preservation of an erosion surface beneath the basin suggest that much of the modern relief of the Min Shan relative to the Tibetan Plateau is a consequence of this late Pleistocene tilting. Rates of tilting inferred from luminescence dating of interbedded loess have been remarkably rapid (~10 ‐8 rad/yr). Similarly rapid rates of Holocene differential rock uplift are inferred from tilted lacustrine sediments in the southwestern part of the range. The range is bounded on the west by the Min Jiang fault zone, an east-vergent reverse fault. However, Holocene alluvial terraces in headwaters of the Min River are preserved across the fault in several places, indicating that displacement rates on the Min Jiang fault are <1 mm/yr. Active faulting only occurs along the eastern foot of the range (Huya fault) for a short distance (~60 km), despite 3 km of relief on the eastern range front. The relationship between these structures and the tilting observed in the Min Jiang basin is enigmatic; the faults do not appear to exert a strong control on the rates and pattern of deformation within the basin. A simple flexural model demonstrates that rates of tilting on the western flank of the Min Shan are too high to be simply attributed to an isostatic response to surficial loading and unloading of the lithosphere. Present-day horizontal shortening across the Min Shan is geodetically determined to be less than 2‐3 mm/yr, suggesting that only a small part of the observed tilting can be attributed to horizontal shortening. Thus, tilting and concomitant differential rock uplift in the Min Shan appear to require an additional driving component. We suggest that Quaternary deformation along the western Min Shan may reflect the surface response to thickening of a weak lower crust at the margin of the Tibetan Plateau.

Mantle-driven dynamic uplift of the Rocky Mountains and Colorado Plateau and its surface response: Toward a unified hypothesis

The correspondence between seismic velocity anomalies in the crust and mantle and the differential incision of the continental-scale Colorado River system suggests that significant mantle-to-surface interactions can take place deep within continental interiors. The Colorado Rocky Mountain region exhibits low-seismic-velocity crust and mantle associated with atypically high (and rough) topography, steep normalized river segments, and areas of greatest differential river incision. Thermochronologic and geologic data show that regional exhumation accelerated starting ca. 6–10 Ma, especially in regions underlain by low-velocity mantle. Integration and synthesis of diverse geologic and geophysical data sets support the provocative hypothesis that Neogene mantle convection has driven long-wavelength surface deformation and tilting over the past 10 Ma. Attendant surface uplift on the order of 500–1000 m may account for ∼25%–50% of the current elevation of the region, with the rest achieved during Laramide and mid-Tertiary uplift episodes. This hypothesis highlights the importance of continued multidisciplinary tests of the nature and magnitude of surface responses to mantle dynamics in intraplate settings.

Tectonic geomorphology along the eastern margin of Tibet: insights into the pattern and processes of active deformation adjacent to the Sichuan Basin

Abstract We present a review and synthesis of the tectonic geomorphology along the eastern margin of the Tibetan Plateau adjacent to and north of the Sichuan Basin. Re-evaluation of spatial variations in the form of fluvial longitudinal profiles provides a refined image of the distribution of anomalously steep channels. Three new analyses demonstrate that these variations in channel steepness reflect variations in the locus and rate of differential rock uplift. First, measurements of channel width along trunk streams reveal systematic co-variations in channel hydraulic geometry and slope that suggests channels are dynamically adjusted to spatial variations in erosion rate. Second, recent determinations of the functional relationship between channel steepness indices and erosion rate allow a quantitative estimation of erosion rate from channel profile form. Third, comparison of rock uplift patterns to variations in the distribution of slip associated with the 2008 Wenchuan earthquake confirms that channel gradients reflect differential rock uplift. Our analysis suggests that reactivated fault systems adjacent to the Sichuan Basin are primarily responsible for accommodating differential rock uplift, but that rock uplift northward along the margin is not associated with active faults and is likely sustained by flow and thickening in the deep crust.

Constraints on India-Eurasia collision in the Arabian Sea region taken from the Indus Group, Ladakh Himalaya, India

Abstract The Indus Group is a Paleogene, syntectonic sequence from the Indus Suture Zone of the Ladakh Himalaya, India. Overlying several pre-collisional tectonic units, it constrains the timing and nature of India’s collision with Eurasia in the western Himalaya. Field and petrographic data now allow Mesozoic-Paleocene deep-water sediments underlying the Indus Group to be assigned to three pre-collisional units: the Jurutze Formation (the forearc basin to the Cretaceous-Paleocene Eurasian active margin), the Khalsi Flysch (a Eurasian forearc sequence recording collapse of the Indian continental margin and ophiolite obduction), and the Lamayuru Group (the Mesozoic passive margin of India). Cobbles of neritic limestone, deep-water radiolarian chert and mafic igneous rocks, derived from the south (i.e. from India), are recognized within the upper Khalsi Flysch and the unconformably overlying fluvial sandstones of the Chogdo Formation, the base of the Indus Group. The Chogdo Formation is the first unit to overlie all three pre-collisional units and constrains the age of India-Eurasia collision to being no younger than latest Ypresian time (>49 Ma), consistent with marine magnetic data suggesting initial collision in the Arabian Sea region at c. 55 Ma. The cutting of equatorial Tethyan circulation north of India at that time may have been a trigger to the major changes in global palaeoceanography seen at the Paleocene-Eocene boundary. New 40Ar/39Ar, apatite fission-track and illite crystallinity data from the Ladakh Batholith and Indus Group show that the batholith, representing the old active margin of Eurasia, experienced rapid Eocene cooling after collision, but was not significantly reheated when the Indus Group basin was inverted during north-directed Miocene thrusting (23–20 Ma). Subsequent erosion has preferentially removed 5–6 km (c. 200°C) over much of the exposed Indus Group, but only c. 2 km from the Ladakh Batholith. Reworking of this material into the Indus fan may complicate efforts to interpret palaeo-erosion patterns from the deep-sea sedimentary record.

Late Miocene–Pliocene range growth in the interior of the northeastern Tibetan Plateau

The time-space patterns of deformation throughout the Indo-Asian collision zone can place constraints on the processes responsible for the development of high topography. Although most agree that high topography associated with the Tibetan Plateau expanded throughout the Cenozoic, it is increasingly being recognized that portions of the present-day plateau experienced a protracted history of deformation starting before or shortly after collision. Deciphering the history of deformation in these regions is central to understanding the dynamics of plateau formation. Here, we report new constraints on the timing of shortening along the southern margin of the Gonghe Basin complex, a broad Tertiary–Quaternary depocenter within the interior region of the northeastern Tibetan Plateau. Deformation of basin strata, lithostratigraphic patterns, and changes in paleocurrents record the growth of structures along the southern margin of the basin. A novel combination of magnetostratigraphy and cosmogenic burial ages from fluvial deposits provides a chronology that suggests that sediment accumulation initiated at ca. 20 Ma and that indicates the basin-bounding structures became active during the late Miocene, between ca. 10 and 7 Ma. The probable onset of basin development in the early Miocene is similar to other regions of the northeastern Tibetan Plateau, and it appears to herald the onset of widespread contractional deformation in the region. Moreover, late Miocene activity on thrusts bounding the southern margin of Gonghe Basin was broadly synchronous with the rise of mountain ranges elsewhere along the periphery of the plateau, suggesting a coordinated pulse of growth of high topography during this time.

Millennial slip rates along the eastern Kunlun fault: Implications for the dynamics of intracontinental deformation in Asia

The role of major strike-slip faults in the Indo-Asian collision zone is central to our understanding of the ways in which continental crust and lithosphere deform in response to continental collision. We investigated how slip varies along the eastern segments of the Kunlun fault in northeastern Tibet. Millennial slip rates were determined based on landforms that are offset by the fault and that were dated using a combination of 14 C and cosmogenic radionuclide exposure dating techniques. We developed estimates for slip rates at four new locations along the fault in addition to four previously published sites. All of these sites are located along the eastern 300 km of the fault system, and our results reveal a systematic eastward decrease in slip rate along this portion of the fault since the late Pleistocene. This displacement gradient is consistent with the termination of the Kunlun fault near ∼102°E. Coincident variations in fault slip rates and geometry reflect complex kinematics along the fault zone. Although other faults exist in the region, our observations suggest that none of these accomplishes transfer of slip from the primary Kunlun fault system. Instead, we interpret that either the eastern Kunlun fault is relatively young and propagating eastward, or that left-lateral slip is absorbed by interaction of the fault zone with regional rotation of the eastern fault tip. Both of these scenarios contrast with previous interpretations and indicate that the Kunlun fault does not accommodate the eastward extrusion of the central Tibetan Plateau lithosphere.