Bryan L. Isacks

Andean tectonics related to geometry of subducted Nazca plate

Seismological and geological data show that tectonic segmentation of the Andes coincides with segmentation of the subducted Nazca plate, which has nearly horizontal segments and 30° east-dipping segments. Andean tectonics above a flat-subducting segment between 28°S to 33°S are characterized by (from west to east): (1) a steady topographic rise from the coast to the crest of the Andes; (2) no significant Quaternary, and possibly Neogene, magmatism; (3) a narrow belt of eastward-migrating, apparently thin-skinned, Neogene to Quaternary shortening of the Andes; and (4) Plio-Pleistocene uplift of the crystalline basement on reverse faults in the Pampeanas Ranges. From about 15° to 24°S, over a 30°-dipping subducted plate, a west to east Andes cross section includes: (1) a longitudinal valley east of coastal mountains; (2) an active Neogene and Holocene andesitic volcanic axis; (3) the Altiplano-Puna high plateau; (4) a high Neogene but inactive thrust belt (Eastern Cordillera); and (5) an active eastward-migrating Subandean thin-skinned thrust belt. Tectonics above a steeply subducting segment south of 33°S are similar west of the volcanic axis, but quite different to the east. Early Cenozoic tectonics of western North America were quite similar to the Neogene Andes. However, duration of segmentation was longer and the width of deformation was greater in the western United States. Patterns of crustal seismicity are systematically related to Plio-Quaternary structural provinces, implying that current deformational processes have persisted since at least the Pliocene. Horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench. Above flat-subducting segments, crustal seismicity occurs over a broad region, whereas over steep segments, it is confined to the narrow thrust belt. Strain patterns in the forearc region are complex and perhaps extensional, and a broad region of the Altiplano-Puna and Eastern Cordillera appears to be aseismic.

Spatial distribution of earthquakes and subduction of the Nazca plate beneath South America

A detailed study of the spatial distribution of precisely located hypocenters of South American earthquakes that occurred between lat 0° and 45°S shows that the data can be explained by the simple model of a descending oceanic plate beneath a continental plate and that the following conditions obtain: (1) The hypocenters clearly define five segments of inclined seismic zones, in each of which the zones have relatively uniform dips. The segments beneath northern and central Peru (about lat 2° to 15° S) and beneath central Chile (about lat 27° to 33° S) have very small dips (about 10°), whereas the three segments beneath southern Ecuador (about lat 0° to 2°S), beneath southern Peru and northern Chile (about lat 15° to 27°S), and beneath southern Chile (about lat 33° to 45°S) have steeper dips (25° to 30°). No clear evidence exists for further segmentation of the descending Nazca plate beneath South America. If the two flat segments are in contact with the lower boundary of the continental plate, the thickness of that plate is less than approximately 130 km. This is in marked contrast to the reports of thicknesses exceeding 300 km for the South American continental plate. (2) There is considerable seismic activity within the upper 50 km of the overriding South American plate. This seismic activity is well separated from the inclined seismic zones and probably occurs in the crustal part of the South American plate. Thus, hypocenters in South America are not evenly distributed through about a 300-km-thick zone as previously described. (3) A remarkable correlation exists between the two flat segments of the subducted Nazca plate and the absence of Quaternary volcanism on the South American plate. (4) The transition from the flat Peru segment to the steeper Chile segment is abrupt and is interpreted as a tear in the descending Nazca plate. The tear is located approximately beneath the northern limit of the Altiplano (a high plateau in the Andes), and about 200 km south of the projection of the oceanic Nazca ridge down the subduction zone. (5) A gap in seismic activity exists between depths of 320 and 525 km.

Seismicity and shape of the subducted Nazca Plate

An updated compilation of earthquake locations and focal mechanism solutions from the International Seismological Centre and Preliminary Determination of Earthquakes is the basis of a comprehensive study of the geometry of the Wadati-Benioff zone beneath western South America. The new data support previous mapping of a sharp flexure rather than a tear in the subducted Nazca plate beneath southern Peru and provide evidence for a similar flexure in the southward transition from nearly horizontal subduction to a slab with ∼30° dip at latitude 33°S. In contrast, the transition from 30° slab dip beneath Bolivia to a nearly horizontal dip in the region between 28°S–32°S is more gradual, occurring over several hundred kilometers of along-strike distance between 20°S and 32°S. This southward flattening corresponds to a broadening of a horizontal, benchlike part of the subducted plate formed between 100 and 125 km depth. The transition in continental tectonic style near 27°S–28°S, from a wide, volcanically active plateau to a narrow, nonvolcanic cordillera, appears not to be associated with the main slab flattening, which begins to the north of these latitudes, but with a more abrupt change in curvature of the subducted slab, from convex upward to concave upward, immediately below the plate boundary interface. The concept of Gaussian curvature is applied to slab bending to explain how subduction geometry is affected by the shape of the South American plate. We hypothesize that the polarity of vertical curvature in the subducting slab is governed by the orientation of lateral curvature of the plate margin. Focal mechanism solutions for intermediate and deep earthquakes are grouped by geographic region and inverted for the orientation and relative magnitudes of the principal stresses. Results of the inversion indicate that downdip extension dominates in the slab above 350 km while downdip compression dominates at greater depths.

How flat is Tibet

High-resolution digital topography (three arc-second grid) for most of Tibet provides new information to characterize the relief of the highest and largest plateau on Earth. The arid to semiarid central and northern part of the plateau interior has low relief (average slopes of ∼; over 250 m windows) and a mean elevation of 5023 m above sea level. At moderate wavelengths of ∼m, relief is ∼or less for most of Tibet, as opposed to the much higher relief of up to 6 km on the plateau edges, where glacial and fluvial dissection is greater because of higher levels of precipitation. The only faults manifesting significant topographic relief are the relatively small scale, generally north-trending graben systems, primarily in southern Tibet, and several large-scale fault systems near the edges of Tibet. The flatness of Tibet implies that (1) there has been little deformation (especially shortening) of the uppermost crust north of the graben systems during the late Cenozoic, and (2) shallow crustal isostatic compensation has been acting to level the surface of the plateau.

Stream flow characterization and feature detection using a discrete wavelet transform

An exploration of the wavelet transform as applied to daily river discharge records demonstrates its strong potential for quantifying stream flow variability. Both periodic and non-periodic features are detected equally, and their locations in time preserved. Wavelet scalograms often reveal structures that are obscure in raw discharge data. Integration of transform magnitude vectors over time yields wavelet spectra that reflect the characteristic time-scales of a rivers flow, which in turn are controlled by the hydroclimatic regime. For example, snowmelt rivers in Colorado possess maximum wavelet spectral energy at time-scales on the order of 4 months owing to sustained high summer flows; Hawaiian streams display high energies at time-scales of a few days, reflecting the domination of brief rainstorm events. Wavelet spectral analyses of daily discharge records for 91 rivers in the US and on tropical islands indicate that this is a simple and robust way to characterize stream flow variability. Wavelet spectral shape is controlled by the distribution of event time-scales, which in turn reflects the timing, variability and often the mechanism of water delivery to the river. Five hydroclimatic regions, listed here in order of decreasing seasonality and increasing pulsatory nature, are described from the wavelet spectral analysis: (a) western snowmelt, (b) north-eastern snowmelt, (c) mid-central humid, (d) south-western arid and (e) ‘rainstorm island’. Spectral shape is qualitatively diagnostic for three of these regions. While more work is needed to establish the use of wavelets for hydrograph analysis, our results suggest that river flows may be effectively classified into distinct hydroclimatic categories using this approach.

Erosion and tectonics at the margins of continental plateaus

The topography across the eastern margin of the central Andean plateau north of 18°S (Beni region) bears a strong resemblance to the topography of the southern margin of the Tibetan plateau (Nepal Himalaya), with both regions featuring a steep frontal slope and high peaks at the plateau edge. In contrast, the topography of the eastern margin of the central Andean plateau south of 18°S (Pilcomayo region) tapers toward the foreland more gently and has no line of high peaks at the margin. Both the Himalayan and the Beni regions have been the sites for large amounts of denudation, and in both regions, geologic evidence suggests that erosion has been sufficiently vigorous for the physiographic plateau margin to have retreated toward the plateau interior during the Neogene. We hypothesize that the steep frontal slope and high peaks of the Beni region and Himalayan front largely reflect the high orographic precipitation and high erosion rates occurring in these regions and that the more gentle topography of the semiarid Pilcomayo region reflects a tectonic landform only slightly modified by erosion. We propose that orographic precipitation impinging on a plateau margin will generally tend to drop moisture low on the slope, eroding back the plateau while enhancing or maintaining the steep long-wavelength slope. A numerical model coupling orographic precipitation, erosion, and tectonic uplift demonstrates the plausibility of this hypothesis. The erosional efflux in both the Beni and Nepal Himalaya have been considerable, and simple mass balance calculations for the Himalaya suggest that during the Neogene, the erosional mass efflux has generally outpaced the tectonic mass influx. This contrasts with the apparent prior domination of tectonic influx and may reflect a decrease in the rate of tectonic addition during the same period, and/or increased late Cenozoic erosion rates.

Estimation of Discharge From Three Braided Rivers Using Synthetic Aperture Radar Satellite Imagery: Potential Application to Ungaged Basins

Analysis of 41 ERS 1 synthetic aperture radar images and simultaneous ground measurements of discharge for three large braided rivers indicates that the area of active flow on braided river floodplains is primarily a function of discharge. A power law correlation is found between satellite-derived effective width We and discharge Q, where We is the water surface area within a braided reach divided by the reach length. Synthetic values of We and Q generated from a cellular automata model of stream braiding display a similar power law correlation. Power functions that are fit through plots of We and Q represent satellite-derived rating curves that can subsequently be used to estimate instantaneous river discharge from space, with errors ranging from tens to hundreds of cubic meters per second. For ungaged rivers, changes in relative discharge can be determined from satellite data alone to determine the shape and timing of annual flows in glacierized basins. Absolute discharge can probably be estimated within a factor of 2. More accurate estimates will require either (1) one or more ground measurements of discharge acquired simultaneously with a satellite image acquisition, or (2) successful parameterization of known morphologic controls such as total sinuosity 5;P, valley slope, bank material and stability, and braid channel hydraulic geometry. Values of total sinuosity 5;P derived from satellite imagery and field measurements from two rivers of braid channel width, depth, velocity, water surface slope, and bed material grain size indicate that while the shape of satellite-derived We-Q rating curves may be influenced by all of these variables, the sensitivity of flow area to changing discharge is most dependent upon the degree of braiding. Efforts to monitor river discharge from space will be most successful for intensely braided rivers with high values of total sinuosity. Subsampling of existing daily discharge records from the Iskut River suggests that satellite return times of about 1 week are sufficient for approximating the shape and timing of the seasonal hydrograph in large, glacierized basins. Although errors are large, the presented technique represents the only currently available way to estimate discharge in ungauged braided rivers.

MODERN AND LAST LOCAL GLACIAL MAXIMUM SNOWLINES IN THE CENTRAL ANDES OF PERU, BOLIVIA, AND NORTHERN CHILE

Late Pleistocene snowlines in the central Andes were 500–1200 m lower than at present. Radiocarbon dates imply that the late-Pleistocene glacial maximum in the region occurred prior to 20 14C ka, but lack of maximum limiting ages adds considerable uncertainty to the exact timing. Snowline modeling demonstrates that snowlines in the eastern and western cordilleras of the central Andes respond differently to temperature and precipitation changes. In the eastern cordillera, the snowline is near the level of the annual 0°C isotherm and melting occurs throughout the year. Here snowlines are sensitive to temperature changes, but relatively insensitive to accumulation changes. In the western cordillera, the snowline rises 1000 higher owing to increasing aridity, and the snowline exhibits a much stronger sensitivity to accumulation changes. The consistent 1200 m snowline depression along the eastern cordilleras of the central Andes can be modeled by a mean annual cooling of 5–9°C. This is inconsistent with the <2°C cooling in tropical sea-surface temperatures suggested by CLIMAP reconstructions. The 800–1000 m snowline depression in the western cordillera cannot be accounted for solely by cooling, but also requires an increase in precipitation.

The mechanical state of the lithosphere in the Altiplano-Puna segment of the Andes

Abstract Information about topography, the shape of the geoid, seismicity, Neogene deformation and volcanism in the region of Altiplano-Puna of western South America is used to analyse the state of stress across the convergent plate margin in terms of the effects of topography and simple models of its compensation. An average elevation near 4 km is consistent with compensation by a yet unresolved combination of crustal root and hot uppermost mantle producing a geoid high of 22–27 meters, average horizontal compressive stress (in excess of a reference sea level lithostatic value) of 390 bars in a 150 km thick lithosphere, and an average shear stress of 170 bars along a 30° dipping interplate boundary. The basis for these estimates is evidence for a neutral to extensional stress regime within the high plateau contrasted with a compressional regime on the eastern slopes and along the interplate boundary itself. Comparison with other plateaus in a convergent plate tectonic setting suggests an evolutionary sequence from compressional to extensional tectonics as elevation of the plateau increases.

Lithospheric structure of Tibet and western North America: Mechanisms of uplift and a comparative study

We seek to determine the mantle lid thickness of the lithosphere and to constrain the mechanisms that are responsible for the Cenozoic uplift of the Tibetan plateau located behind the Himalaya collision zone and the western United States located behind the San Andreas transform/convergent plate boundary. Selected first P arrivals of International Seismological Centre (ISC) data at distances less than 22° are analyzed by the two-station method to constrain the upper mantle structure of the Basin and Range (BR), Colorado Plateau (CP), Great Plains (GP) and Tibetan Plateau (TP). The data are presented as plots of apparent velocity versus distance to the second station. We show that these observations of apparent velocities versus distance as well as those that concern the attenuation of Sn provide estimates of the mantle lid thicknesses. We obtain the following mantle lid thicknesses: BR 20–40 km; CP 35–50 km; GP 150–195 km; and TP 135–180 km. Sn is not observed for paths beneath the BR and CP, while Sn efficiently propagates beneath the GP; Sn is also observed beneath most of Tibet except in the north central part of the TP. A possible interpretation of our observations is that Indian continental lithosphere underthrusts only about the southern 2/3 of Tibet.