T14C-01 INVITED 16:00h
Relationships Between Incremental and Cumulative Fold Growth With Neotectonic Examples From the Southern Tianshan, China
Some mechanisms of upper crustal fault-related folding display profiles of incremental fold growth that are collocated with the distribution of finite fold uplift, whereas others do not. For example instantaneous uplift in fault-bend folding is not coincident with cumulative uplift because fault bends are velocity discontinuities through which rock moves progressively. This causes uplifted rock to move away from the instantaneous locus of uplift. In contrast, some models of detachment folding assume that instantaneous uplift rate is coincident with and proportional to cumulative uplift. We show how these concepts play out quantitatively in two well imaged actively forming anticlines in the southern Tianshan. The Yakeng detachment fold shows collocated growth in which incremental uplift is linearly proportional to cumulative uplift with a high statistical significance. This has the practical result of allowing one to measure fractional uplift quite accurately. In contrast Quilitak anticline is a complex fault bend fold that shows non-collocated fold growth at several scales, which we illustrate with [1] a detailed analysis of progressive motion of young strata through a fold hinge and [2] large-scale folding of the land surface producing giant fold scarps as high as 800 m.
T14C-02 16:20h
Surface rupture along the Chon Aksu and Aksu (eastern) segments of the 1911 Kebin (Chon-Kemin) earthquake, Tien Shan, Kyrgyzstan
The 1911 Ms 8.2 Kebin (Chon-Kemin) earthquake is one of the largest intraplate reverse-faulting events to occur historically. It ruptured a 200 km E-W trending zone in the northern Tien Shan. Description of the characteristics of major historic earthquakes, such as the Chon-Kemin event, provide data on the primary deformation and its initial geomorphic degradation, thus informing paleoseismological investigations. Trace geometry and offset distribution are key parameters for the interpretation of the seismotectonic setting and mechanical interaction with other regional structures. The Chon-Kemin earthquake's relationship to other large regional earthquakes, notably the 1887 Ms 7.3 Verny and 1889 Ms 8.3 Chilik events, indicates a strong interaction between structures in this portion of the Tien Shan. We reconnoitered most of the Chon-Kemin rupture belt and associated mass movements and conclude that many of the 1911 features are still well preserved. We emphasized mapping and description of the easternmost 50 km of the rupture, along the Aksu (easternmost) and Chon-Aksu segments. Moving from east to west, 1-3 m high fault scarps and warped Holocene terraces discontinuously cut the piedmonts north and northeast of the town of Anan'evo. The rupture closely follows the mountain front and enters the range just below the Anan'evo landslide (formed in the earthquake). Scarp heights are 2-4 m. West of the Tegermenty River, the left-stepping rupture is continuous and consists of sub parallel strands in places. Between the Sutubulak and the Aksu River crossing, some of the tallest scarps are present, with heights of 6 - 10 m. The generally north-dipping fault zone has low south dips in the near surface as the thrust has driven the hanging wall over the south-sloping piedmont. This change in dip is likely responsible for the formation of E-W extensional faults in the hanging wall. Given the shallow fault dips and tall scarps in this area, 1911 displacement is probably $>$ 10 m. The Aksu segment ends in a 5 km wide left step through the Kok Bel Pass into the Chon Asku Valley. Fault scarps are 2-4 m high and a small corral, offset in 1911, shows no evidence for strike-slip displacement. In the Kok Bel Pass, the hanging wall (northwestern) is crossed by numerous clear and straight fault scarps trending mostly NE. The eastern 9 km of the Chon Aksu segment preserves the most spectacular tectonic landforms from the 1911 rupture. Scarps are typically uphill-facing and 6-8 m high in this reach with some domal uplifts deforming Pleistocene fluvial terraces. Two tectonically dammed lakes were formed by the 1911 earthquake. In addition, the Chon Aksu river was offset with the upstream side uplifted about 6 m. In the upper Chon Aksu Valley, the rupture is not well expressed, because of vigourous fluvial, periglacial and glacial activity. Notable fractures on ridgetops on the south side of the valley across from the Kulugan Tash rockfall are well preserved, but their formation as primary ground rupture or shaking induced settling is ambiguous. For 5 km west of the Kulugan Tash, no landforms were unambiguously attributable to ground rupture. Together with 1911 earthquake features in the study area, we also observed scarps and mass movements belonging to paleoseismic catastrophes. They indicate recurrence of major earthquakes along this structure during the Holocene and late Pleistocene.
T14C-03 16:35h
Coastal Uplift and Thrust Faulting Associated With the Mw=6.8 Zemmouri (Algeria) Earthquake of 21 May, 2003
A shoreline uplift marked by a continuous white band visible at rocky headlands occurred during the 21 May 2003 earthquake (Mw 6.8) in northern Algeria. We measured the amount of coastal uplift on a white band (emerged algae) and harbors quays between Boumerdes and Dellys. Most of measured points were collected using tape and differential GPS on rocky headlands with a ,b 0.15 m error bar (tidal prism). Leveling lines running parallel and orthogonal to the coast also provide the precise amount of uplift in the epicentral area. The uplift distribution shows an average 0.55 m along the shoreline with a maximum 0.75 m east of Boumerdes and a minimum close to 0 near Cap Djinet. The active deformation related to a thrust fault is modeled along the 55 km coastline. The dislocation model predicts surface slip on a N 54>XE trending reverse fault, dipping 50>X SE in agreement with CMT solution and coastal uplift. The faulting characteristics imply a fault geometry with possible sea bottom ruptures between 5 to 10 km offshore.
T14C-04 16:50h
Analysis of the Growth of Active Detachment Folds Applying the new Thickness Relief Method, With Examples From the Tien-Shan and Nankai Trough
Active folds develop distinctive stratigraphic geometries resulting from the interactions of sedimentation and deformation, which provides a quantitative record of deformation history. Modern seismic with excellent images of growth deposits, allow us to extract this record of deformation in a detailed and quantitative way applying the new thickness-relief method. We apply the thickness relief method to growth strata of two actives detachment folds, one in Yakeng anticline of the southern Tien-Shan China and another at the front of the Nankai Trough Japan (ODP legs 190, 196). The method involves determination of areas of structural relief as a function of height based on thickness variations of all imaged horizons. This yields a high-resolution profile of shortening, as a function of height since shortening is the derivative of the area-height curve. The thickness-relief method has been successfully applied to pre-growth sequences in a number of structures. Here we extend the method to growth strata. The onset of growth is represented in the area-height plot as an upward decrease rate of growth of fold area, which is most easily analyzed through modeling. A wide variety of distinctive behaviors are expected for various ratios of sedimentation to deformation rate, depending also on the depth to detachment. Amplitude obtaining valuable information about the shortening and how this is accommodated into detachment folds. The slope of best-fit line represents shortening or displacement in the area relief graph. So, as deformation and sedimentation starts to interact the geometry of deposits change and area-relief, height, and shortening relationship is affected. Deviation from the linear trend, negative or a decrease in slope, are signatures of the growth in area relief graph, however data are restricted and theoretical models are necessary to understand the behavior of the graph. We use these models to fit real data on growth strata of Nankai and Yakeng detachment folds, which show substantially different behavior. We found that the very young and rapidly growing Nankai fold at a fast-moving plate boundary displays a sedimentation rate that is no more than 25 to 30 % of shortening rate. In contrast the Yakeng anticline in the more slowly deforming Tien-Shan of central Asia shows a sedimentation rate that is 167 % the shortening rate. We make use of available age control to transform these observations into sedimentation and deformation rates.
T14C-05 17:05h
Along-Strike Variation in Geometry and Kinematics of a Major, Active Intracontinental Thrust System: the Pred-Terskey Fault Zone, Kyrgyz Tien Shan, Central Asia
The Pred-Terskey fault zone defines the southern margin of the Issyk-Kul basin, extending eastward over 250 km from at least the Chu River to the Kazakhstan border, and appears to be one of the most active zones in the Kyrgyz Tien Shan. Despite a diversity of structural styles and changes of vergence at the surface, the lateral continuity and overall geometry of the zone is consistent with a single north vergent thrust at depth, which uplifts the Terskey Range and generally tilts the south margin of the basin to the north. This northward tilting of the margin is probably due to a flattening of the fault as it approaches the surface. In spite of historical quiescence, it is likely capable of producing great earthquakes. We have conducted detailed field mapping coupled with terrace profiling and dating at seven representative, well-exposed areas of the fault zone. Based on these field observations and satellite image and air photo interpretation along the entire zone, we identify three major divisions in structural style expressed at the surface. The western segment is typified by the Tura-Su, Ak-Terek and Ton areas. A series of left-stepping, south-vergent, basement-involved reverse faults and folds are uplifting the southern margin of the Issyk-Kul basin in this area. The resulting uphill-facing scarps have trapped and diverted many of the rivers flowing north from the Terskey Range. Tertiary strata and Quaternary geomorphic surfaces show consistent, progressive northward tilting across the entire zone. The west-central segment is represented by the Kajy-Say area. South-vergent reverse faults and a north-vergent backthrust have uplifted an arcuate granite block. Offshore of this area, the lake floor descends to a sharp break in slope with a low relief area at a depth of about 650 m. Late Quaternary geomorphic features do not show evidence of tilting. In contrast to the areas east and west, the major north-dipping thrust is likely planar over this segment and daylights at the lake floor break in slope. The east-central segment is exemplified by the Barskaun and Jety Oguz areas. A high angle reverse fault juxtaposes Paleozoic rock against Tertiary sediments. To the north, a thrust fault with a sinuous trace places north-dipping Tertiary rock over the nearly horizontal basin floor. Quaternary terraces in the hanging wall of this fault record progressive northward tilting. North of the thrust fault a series of anticlines are growing out of the basin sediments. The eastern segment, which includes the Jergalan River valley, lacks a low angle thrust fault at the basin margin. Along this segment, the basement reverse fault uplifts Paleozoic rock against Quaternary basin sediment. To the north of this range-bounding structure, late Quaternary terraces are offset by south-vergent scarps. We are calculating geologic slip rates for each of the seven sites along the Pred-Terskey zone by dating terraces and constructing structural models consistent with both the rock and terrace records. Based on preliminary radiocarbon dates, a prominent Jety Oguz River terrace is 50 +/- 10 ka. The terrace is tilted $0.5\deg$ relative to the modern river, and with the low angle fault branching off of the basement reverse fault at dips ranging between $45\deg$ and $90\deg$, the slip rate of this fault is 6 +/- 4 mm/yr. This is consistent with the GPS shortening rate across the Pred-Terskey zone at this longitude.
T14C-06 17:20h
Anatomy of a Thrust System From Geological and Seismological Evidence: the Case of the Northern Apennines (Italy)
The northern Apennines thrust belt has an arcuate trend made up of several fronts progressively younger to the NE. The youngest are buried in the southern Po Plain, and deform the Marche and the northern Abruzzi coastal regions. Instrumental and historical seismicity, fault plane solutions, and a large dataset of seismic exploration lines testify that blind thrust faults produce significant deformation and moderate-size earthquakes up to M=6.0. In spite of their size, earthquakes can still be damaging and pose a substantial hazard to the population and to the civil and industrial facilities. Slip rates lower than depositional rates, blindness of thrust faults and subtle topographic expression of the active anticlines hinder the identification of individual seismic sources. The detection of the active structures is also made difficult by the low seismic release and by the small number of well constrained instrumental earthquakes. However the drainage network and river and coastal terraces faithfully register the surface deformation. In the subsurface, seismic lines and structural horizons document the recent activity of the thrusts and the geometry of individual faults and of the system. The identification and characterization of the most active thrust fronts is critical for a correct evaluation of the seismic hazard. We tackle this problem by combining seismological data with subsurface geology and geomorphology. We present the result of several transects oriented perpendicular to the chain trend highlighting the longitudinal continuity of activity of the thrust fronts and stressing the existence of gaps and shifts in seismic release. Our study shows that seismogenesis in the Northern Apennines follows well defined structural trends. Moving from W to E, in the southern Po Plain seismicity is mainly concentrated along the emergent Pedeapenninic Thrust Front (PTF), as well as along buried outer fronts, although with a lower moment release. However, activity of the PTF is not continuous along strike. In the Marche coastal region deformed marine terraces and river anomalies highlight the position of active coastal anticlines driven by blind thrusts, associated with historical and instrumental earthquakes. Southward, seismicity becomes sparser, less frequent, of lower magnitude, and distributed over a wider area.
T14C-07 17:35h
Identifying Blind Seismogenic Faults at the Apennine Thrust Front: Implications for the Seismic Hazard of the Northern Marche (Central Italy) Coastal Belt
The structural architecture of the northern Apennines is dominated by NE-verging arc-shaped folds and thrusts that developed through progressive migration of the contractional process combined with regional uplift. Migration and shortening of the Apennine fold-and-thrust system appear to be constant over space (from SW to NE) and time (since the Burdigalian; ca. 19 My), indicating that the two major frontal thrusts are active. The coastal area of the northern Marche - one of the most densely populated regions in central Italy - extends between the axes of these two structures. We reconstructed the deep geometry, size, and style of deformation of these youngest folds and blind thrusts by using seismic reflection lines. Anomalously behaving river courses and warped coastal and fluvial terraces provided insights on the rates of their recent activity. The occurrence of several historical and instrumental earthquakes suggests that these structures can also be seismogenic. The results of a detailed analysis on one of these structures allowed us to constrain the source of the Senigallia, 1930, earthquake (Me 5.9) as a blind thrust fault accompanied by anticlinal growth. Prospectively, our findings can be extended at the entire fault system to (i) outline its possible segmentation, (ii) identify the areas that have been historically silent and that could be regarded as potential seismic gaps, and (iii) evaluate the seismic potential of the region. Successfully addressing the above three points is a necessary step for individual-source, time-dependent seismic hazard assessment.