Richard O. Lease

Signatures of mountain building: Detrital zircon U/Pb ages from northeastern Tibet

Although detrital zircon has proven to be a powerful tool for determining provenance, past work has focused primarily on delimiting regional source terranes. Here we explore the limits of spatial resolution and stratigraphic sensitivity of detrital zircon in ascertaining provenance, and we demonstrate its ability to detect source changes for terranes separated by only a few tens of kilometers. For such an analysis to succeed for a given mountain, discrete intrarange source terranes must have unique U/Pb zircon age signatures and sediments eroded from the range must have well-defi ned depositional ages. Here we use ~1400 single-grain U/Pb zircon ages from northeastern Tibet to identify and analyze an area that satisfi es these conditions. This analysis shows that the edges of intermontane basins are stratigraphically sensitive to discrete, punctuated changes in local source terranes. By tracking eroding rock units chronologically through the stratigraphic record, this sensitivity permits the detection of the differential rock uplift and progressive erosion that began ca. 8 Ma in the Laji Shan, a 10‐25-km-wide range in northeastern Tibet with a unique U/Pb age signature.

Middle Miocene reorganization of deformation along the northeastern Tibetan Plateau

Temporal variations in the orientation of Cenozoic range growth in northeastern Tibet define two modes by which India-Asia convergence was accommodated. Thermochronological age-elevation transects from the hanging walls of two major thrust-fault systems reveal diachronous Miocene exhumation of the Laji-Jishi Shan in northeastern Tibet. Whereas accelerated growth of the WNW-trending eastern Laji Shan began ca. 22 Ma, rapid growth of the adjacent, north-trending Jishi Shan did not commence until ca. 13 Ma. This change in thrust-fault orientation refl ects a Middle Miocene change in the kinematic style of plateau growth, from long-standing NNE-SSW contraction that mimicked the plate convergence direction to the inclusion of new structures accommodating east-west motion. This kinematic shift in northeastern Tibet coincides with expansion of the plateau margin in southeastern Tibet, the onset of normal faulting in central Tibet, and accelerated shortening in northern Tibet. Together these phenomena suggest a plateau-wide reorganization of deformation.

Stable isotope evidence for topographic growth and basin segmentation: Implications for the evolution of the NE Tibetan Plateau

Lithologic, magnetostratigraphic, and stable isotope records from the Neogene Xunhua and Linxia basins along the Tibetan Plateau9s northeastern margin suggest that topography in the intervening Jishi Shan mountain range began to develop between 16 and 11 Ma. Perturbations to local climate patterns resulting from the evolution of local topography are tracked through comparison of stable isotope compositions of calcareous basin-fill materials across the Jishi Shan. Similarity of isotopic compositions is interpreted to reflect the presence of integrated basins, whereas distinct isotopic compositions reflect unique basin hydrologies. Divergent isotope trends develop between ca. 16 and 11 Ma and are indicative of hydrologic separation in the adjacent Xunhua and Linxia basins and increased aridity in the leeward Xunhua basin. The development of aridity in the lee of the growing topography along the plateau9s northeast margin highlights the importance of evaporative enrichment in this extremely continental setting and explains the presence of anomalously positive δ 18 O values in modern rainfall. Our findings add to a growing body of evidence for deformation along the plateau9s north and northeastern margins in the middle to late Miocene.

Pulsed Miocene range growth in northeastern Tibet: Insights from Xunhua Basin magnetostratigraphy and provenance

Author(s): Lease, RO; Burbank, DW; Hough, B; Wang, Z; Yuan, D | Abstract: Sedimentary rocks in Tibetan Plateau basins archive the spatiotemporal patterns of deformation, erosion, and associated climate change that resulted from Cenozoic continental collision. Despite growing understanding of basin development in northeastern Tibet during initial India-Asia collision, as well as in the late Miocene-Holocene, surprisingly little is known about the intervening period: a time when the plateau may have undergone fundamental tectonic changes. To fill this gap, we present new magnetostratigraphy from a g2300-m-thick fluviolacustrine succession that spans ca. 30-9.3 Ma. An integrated analysis of sedimentology, subsidence, and provenance from this section reveals the sequential, pulsed erosion of multiple ranges bordering the Xunhua Basin. Emergence of the WNW-trending Laji Shan is highlighted by a doubling of sediment accumulation rates between 24 and 21 Ma and a transition to coarse allu vial facies at 20.3 Ma. Detrital zircon U/Pb age spectra show that these coarse sediments came from basement terranes within the Laji Shan. Together these observations suggest accelerated growth of the Laji Shan and its coupled foreland basin at ca. 22 Ma. The most rapid accumulation rates in Xunhua Basin occur within the finest-grained strata and suggest an underfilled basin during the fastest interval of Laji Shan deformation. Growth of the Laji Shan occurred northward of the contemporaneous plateau margin, which had been defined since ca. 45-50 Ma by the West Qinling, lying ~60 km farther south. Hence, following ~20-25 m.y. of apparent stability, the deformation front in this region jumped ~60 km to the north at ca. 22 Ma. Subsequently, growth of the north-trending Jishi Shan occurred at ca. 13 Ma and is highlighted by an acceleration in Xunhua Basin accumulation rates between 12 and 9 Ma, as well as by a significant change in detrital zircon provenance of nearby Linxia Basin deposits by 11.5 Ma. Initial growth of the WNW-trending Laji Shan in the early Miocene and subsequent growth of the north-trending Jishi Shan ~10 m.y. later support interpretations of a middle Mio cene kinematic reorganization in northeastern Tibet.

Incision into the Eastern Andean Plateau During Pliocene Cooling

Bringing Down the Andes Mountain ranges, like the Andes in South America, have a number of forces acting on them that control their elevation. High rates of precipitation can induce rapid incision of canyons, but tectonic forces from deep within the mountain range may balance or even exceed the rate of erosion. Lease and Ehlers (p. 774) examined the exhumation histories of the northeastern Andean Plateau. The erosion of sediments older than ∼10 million years was controlled largely by tectonic processes. However, more recent sediments suggest that a shift to cooler temperatures increased precipitation 3 to 4 million years ago. Climate had stronger control of canyon incision than tectonics in the eastern Andes 4 million years ago. Canyon incision into mountain topography is commonly used as a proxy for surface uplift driven by tectonic or geodynamic processes, but climatic changes can also instigate incision. The ~1250-kilometer (km)–long eastern margin of the Andean Plateau hosts a series of 1.5- to 2.5-km-deep canyons that cross major deformation zones. Using (U-Th)/He thermochronology, we document a transition from Miocene faulting to Pliocene canyon incision across the northeastern plateau margin. Regionally, widespread Pliocene incision into the eastern plateau margin is concurrent with a shift in global climate from early Pliocene warmth to late Pliocene cooling. Enhanced moisture transport onto the Andean Plateau driven by sea surface temperature changes during cooling is the likely pacemaker for canyon incision.

Changing exhumation patterns during Cenozoic growth and glaciation of the Alaska Range: Insights from detrital thermochronology and geochronology

Cenozoic growth of the Alaska Range created the highest topography in North America, but the space-time pattern and drivers of exhumation are poorly constrained. We analyzed U/Pb and fission-track double dates of detrital zircon and apatite grains from 12 catchments that span a 450 km length of the Alaska Range to illuminate the timing and extent of exhumation during different periods. U/Pb ages indicate a dominant Late Cretaceous to Oligocene plutonic provenance for the detrital grains, with only a small percentage of grains recycled from the Mesozoic and Paleozoic sedimentary cover. Fission-track ages record exhumation during Alaska Range growth and incision and reveal three distinctive patterns. First, initial Oligocene exhumation was focused in the central Alaska Range at ~30 Ma and expanded outward along the entire length of the range until 18 Ma. Oligocene exhumation, coeval with initial Yakutat microplate collision >600 km to the southeast, suggests a far-field response to collision that was localized by the Denali Fault within a weak Mesozoic suture zone. Second, the variable timing of middle to late Miocene exhumation suggests independently evolving histories influenced by local structures. Time-transgressive cooling ages suggest successive rock uplift and erosion of Mounts Foraker (12 Ma) through Denali (6 Ma) as crust was advected through a restraining bend in the Denali Fault and indicate a long-term slip rate ~4 mm/yr. Third, Pliocene exhumation is synchronous (3.7–2.7 Ma) along the length of the Alaska Range but only occurs in high-relief, glacier-covered catchments. Pliocene exhumation may record an acceleration in glacial incision that was coincident with the onset of Northern Hemisphere glaciation.

Quantifying Dextral Shear on the Bristol-Granite Mountains Fault Zone: Successful Geologic Prediction from Kinematic Compatibility of the Eastern California Shear Zone

For regional kinematic compatibility to be a valid boundary condition for continental tectonic reconstructions, there must be tests that validate or invalidate kinematic model predictions. In several reconstructions of western North America, the displacement history of the Mojave block continues to be unresolved. The magnitude of displacement along the Bristol-Granite mountains fault zone (BGMFZ), which is the eastern margin of the Eastern California Shear Zone (ECSZ) in the Mojave block, is a key example of a long-standing kinematic prediction that has defied a positive field test until now. The ECSZ is a network of late Neogene and Quaternary right-lateral strike-slip faults that extend from the Gulf of California north through the Mojave Desert, linking Pacific–North America plate motion with Basin and Range extension. This network of faults accounts for ∼15% of post-16-Ma plate transform motion. Geologic estimates of net dextral offset along the Mojave portion of the ECSZ (53 ± 6 km) are approximately half that measured to the north in the Owens Valley–Death Valley region (∼100 ± 10 km). Previous geological estimates of BGMFZ slip range from 0 to 15 km. Models of right-lateral displacement that are based on kinematic compatibility suggest 21–27 km of BGMFZ displacement. We map and describe a tuff- and gravel-filled paleovalley offset by the BGMFZ. The orientation of the Lost Marble paleovalley is constrained by the position of gravel outcrops, provenance, and tuff anisotropy of magnetic susceptibility. Reconstruction of the paleovalley indicates at least 24 km of post-18.5-Ma dextral offset, confirming a significant, previously undocumented component of dextral slip in the Mojave portion of the ECSZ.

THE TECTONIC EVOLUTION OF THE CENTRAL ANDEAN PLATEAU AND GEODYNAMIC IMPLICATIONS FOR THE GROWTH OF PLATEAUS

Current end-member models for the geodynamic evolution of orogenic plateaus predict (1) slow-and-steady rise during crustal shortening and ablative subduction (i.e., continuous removal) of the lower lithosphere, or (2) rapid surface uplift following shortening, associated with punctuated removal of dense lower lithosphere and/or lower crustal flow. We will review results from a recent multidisciplinary study of the modern lithospheric structure, geologic evolution, and surface uplift history of the Central Andean Plateau to evaluate the geodynamic processes that have formed the Plateau. Comparison of the timing, magnitude, and distribution of shortening and surface uplift, in combination with other geologic evidence, highlights the pulsed nature of plateau growth. We will discuss specific regions and time periods that show evidence for end-member geodynamic processes, including middle-late Miocene surface uplift of the southern Eastern Cordillera and Altiplano associated with shortening and ablative subduction, latest Oligocene-early Miocene and late Miocene-Pliocene punctuated removal of dense lower lithosphere in the Eastern Cordillera and Altiplano, and late Miocene-Pliocene crustal flow in the central and northern Altiplano.