Jean-François Ritz

Active transtension inside central Alborz: A new insight into northern Iran–southern Caspian geodynamics

The tectonic activity in the Alborz mountain range, northern Iran, is due both to the northward convergence of central Iran toward Eurasia, and to the northwestward motion of the South Caspian Basin with respect to Eurasia inducing a left-lateral wrenching along this range. These two mechanisms give rise to a NNE-SSW transpressional regime, which is believed to have affected the entire range for the last 5 6 2 m.y. In this paper, we show that the internal domain of central Alborz is not affected by a transpressional regime but by an active transtension with a WNW-ESE extensional axis. We show that this transten- sion is young (middle Pleistocene). It postdates an earlier N-S compression and may have been initiated when the South Caspian Basin started moving. Consequently, our results suggest that the South Caspian Basin motion may have taken place more recently than previously proposed.

Slip rates along active faults estimated with cosmic-ray–exposure dates: Application to the Bogd fault, Gobi-Altaï, Mongolia

Dating morphological features displaced along active faults presents a major difficulty in evaluation of slip rates. We used in-situ–produced 10 Be to calculate minimum ages for alluvial surfaces misaligned by movement along a major active fault in the Gobi-Altai (western Mongolia). The maximum slip rate of ≈1.2 mm/yr suggested by this method contrasts strongly with rates of ≈20 mm/yr that we estimated by correlation of alluvial deposition with warm humid periods associated with the last glacial termination estimated to have occurred about 12 ka in western Tibet. The 10 Be-based slip rate indicates that strong earthquakes can occur along faults with low slip rates and demonstrates the contribution of cosmic-ray–exposure dating in Quaternary tectonic analyses.

Field examples of strike‐slip fault terminations in Mongolia and their tectonic significance

Deformation at the ends of large intracontinental strike-slip faults that do not simply link other major structures often involves rotations about a vertical axis. We use earthquake slip vectors, surface rupture in earthquakes, and geomorphology to examine the ends of three major strike-slip faults in Mongolia. In these places a simple pattern is seen, consisting of a thrust fault on one side, with a displacement that decreases away from the strike-slip fault, consistent with local rotational deformation. Strike-slip faults that terminate in this way allow the style of faulting to change spatially within a deforming area, for example, from dominantly strike-slip to dominantly dip-slip, while still accommodating the overall deformation required by larger-scale regional motions. Such a change in fault style should also be accompanied by a change in the rotation rate about a vertical axis, which may be detected paleomagnetically. The kind of strike-slip fault termination described here may have consequences for how large strike-slip fxaults evolve and grow and for the variation in displacement along their length.

Late Pleistocene to Holocene slip rates for the Gurvan Bulag thrust fault (Gobi-Altay, Mongolia) estimated with 10Be dates

[1] We surveyed morphotectonic markers along the central part of the Gurvan Bulag thrust, a fault that ruptured with the Bogd fault during the Gobi-Altay earthquake (1957, M 8.3), to document climatic and tectonic processes along the fault for the late PleistoceneHolocene period. The markers were dated using 10 Be produced in situ. Two major periods of alluviation ended at 131 ± 20 and 16 ± 4.8 ka. These appear to be contemporaneous with global climatic changes at the terminations of marine isotope stages (MIS) 6 and 2. The vertical slip rates, determined from offset measurements and surfaces ages, are 0.14 ± 0.03 mm/yr over the late Pleistocene-Holocene and between 0.44 ± 0.11 and 1.05 ± 0.25 mm/yr since the end of the late Pleistocene. The higher of these slip rates for the last � 16 kyr is consistent with paleoseismic investigations along the fault [Prentice et al., 2002], and suggests that, at the end of late Pleistocene, the fault evolved from quiescence to having recurrence intervals of 4.0 ± 1.2 kyr for surface ruptures with � 4 m vertical offset (similar to that of 1957). The inferred recurrence interval is comparable to that of the Bogd fault (3.7 ± 1.3 kyr) suggesting that the two faults may have ruptured together also earlier during the last � 16 kyr. INDEX TERMS: 7221 Seismology: Paleoseismology; 1208 Geodesy and Gravity: Crustal movements—intraplate (8110); 1824 Hydrology: Geomorphology (1625); 7230 Seismology: Seismicity and seismotectonics; 8107 Tectonophysics: Continental neotectonics; KEYWORDS: Late Pleistocene, Holocene, thrust fault, slip rate, 10Be dating, Mongolia

Postseismic stress transfer explains time clustering of large earthquakes in Mongolia

Abstract Three M>8 earthquakes have occurred in Mongolia during a 52-year period (1905–1957). Since these earthquakes were well separated in space (400 km), the coseismic stress change is far too low (0.001 bar) to explain mutual earthquake triggering. By contrast, postseismic relaxation gradually causes a significant stress change (0.1–0.9 bar) over large distances. Using a spring-slider model to simulate earthquake interaction, we find that viscoelastic stress transfer may be responsible for the earthquake time clustering observed in active tectonic areas. Therefore, revealing postseismic strain by satellite geodesy and modeling earthquake clusters could improve our understanding of earthquake occurrence, especially in zones where large earthquakes have already struck in past decades.

Estimating slip rates and recurrence intervals for strong earthquakes along an intracontinental fault: example of the Pambak–Sevan–Sunik fault (Armenia)

North of the Arabian plate, active tectonics is characterised by both N–S compression and E–W extension associated with strike-slip faults. The Pambak–Sevan–Sunik fault (PSSF) zone in Armenia is one of the major active structures of the region. The fault is comprised of four main segments and it displays morphological evidence for dextral movement during the Holocene. However, no large earthquake (M >7) has occurred in the northern or central parts of the fault during the last 2000 years. We undertook a geomorphological and paleoseismological investigation along the Pambak–Sevan–Sunik fault with the aim of estimating the long-term slip rate and recurrence interval of strong earthquakes. Trenches were excavated at three sites. Detailed studies of trench cross sections and dating (radiocarbon and ceramics) show three faulting events that occurred in the Vanadzor–Artanih segment (Fioletovo and Semionovka areas), whereas a single event took place in the Artanish–Sunik segment (Khonarhasar area). In both areas, we estimated the average slip rate using (i) the offset of rivers along the Vanadzor– Artanish segment (2.24±0.96 mm/year over an interval of 120–300 ka), and (ii) the offset of volcanic cones along the Artanish–Sunik segment (0.53±0.04 mm/year over an interval of 1.4 Ma). These results suggest that a greater slip rate characterises the Vanadzor–Artanish segment (Fioletovo site) from the Artanish–Sunik (Khonarhasar site) segment. Division of the Pambak–Sevan–Sunik fault zone into two main branches, east of the Artanish peninsula (Sevan Lake) could explain the difference in slip rate. In addition to its segmented seismic behaviour, the Pambak–Sevan–Sunik fault is a well-documented example of a fault that generates strong earthquakes with long recurrence time intervals (about 3000–4000 years). D 2001 Elsevier Science B.V. All rights reserved.

Transpressional tectonics and stream terraces of the Gobi‐Altay, Mongolia

We studied the patterns, rates and evolution of fluvial terraces and fault system during the building process of an intracontinental transpressional mountain in the Gobi-Altay (Mongolia). By analyzing incisions and offsets of fluvial terraces and alluvial fans, we show that the massif has grown by outward migration of thrust faults through time. On the northern flank, the present bounding thrust fault began its activity ~600 ka ago, while a more internal sub-parallel fault was still active until ~200-100 ka. Vertical offset of an alluvial fan abandoned ~100 ka ago allows an estimate of 0.1 mm/yr Upper Pleistocene - Holocene uplift rate. The morphology of the catchment-piedmont system strongly suggests a periodical formation of the alluvial surfaces, controlled by the climatic pulses, at the beginning of the wet interglacial periods. The abandonment of the alluvial terraces lags by several thousand years the abandonment of the alluvial fans, showing a diachronous incision propagating upstream. The incision rate deduced from the different elevations of straths exceeds of one order of magnitude the rock uplift rate. This excess is mostly due to ongoing drainage network growth at the core of the massif, and incision due to alluvial apron entrenchment near the outlet. This implies that fluvial response is mainly controlled by drainage growth, interaction with piedmont and cyclic climatic variations, rather than by rock uplift.

Active tectonics of the east Alborz mountains, NE Iran: Rupture of the left-lateral Astaneh fault system during the great 856 A.D. Qumis earthquake

The 856 A.D. Qumis earthquake (M7.9) is the most destructive earthquake to have occurred in Iran, killing more than 200,000 people and destroying the cities of Damghan and the old Parthian capital of Shahr-i Qumis (Hecatompylos). This study combines evidence of historical seismicity with observations of the geomorphology and paleoseismology to provide the first detailed description of active faulting in the Damghan region of the east Alborz mountains, Iran. Regional left-lateral shear is accommodated on the Astaneh, Damghan, and North Damghan faults. Quaternary alluvial fans have been displaced along the Astaneh fault, with 15–20 m stream offsets recording the cumulative displacement over the last two to five earthquakes. A paleoseismology study from a single trench along a 5–10 km segment of the Astaneh fault reveals a rupture prior to 1300 A.D. and significantly later than 600 B.C. Despite the limitations of a single trench in documenting the spatial and temporal evolution of the fault over the late Quaternary, we are nevertheless able to bracket the last event to a time period consistent with the 856 A.D. earthquake. Two older earthquakes were also identified during the Holocene occurring between 600 B.C. and 4600 B.C. and between 4600 B.C. and 9600 B.C. The location of our trench within a bend on the Astaneh fault, which could act as a barrier to rupture propagation, means the three earthquakes recovered from our trench over the Holocene may represent a minimum. Further trenching will reveal how the Astaneh fault ruptures over repeated earthquakes and, consequently, the magnitude and extent of slip during the 856 A.D. earthquake.

GEODETIC MEASUREMENT OF TECTONIC DEFORMATION IN THE SOUTHERN ALPS AND PROVENCE, FRANCE, 1947-1994

Abstract Active deformation at the boundary between the Eurasia and Africa plates varies in style. The belt between the Alpine mountain range and the Mediterranean Sea, for example, differs markedly in its western and eastern parts. In the western part, around southeast France, the mountains are higher, but the seismicity lower, than in the eastern part, around northern Italy and Greece. Yet the inter-plate convergence rate of 6 mm/yr varies by less than 15% between these two areas. To better understand the behaviour of this complex plate boundary, we use geodesy to map the spatial distribution of the deformation. In this paper, we focus on southeast France, a tectonic crossroads between three different domains (Alps, Ligurian Sea, and Massif Central) which exhibits a moderate level of seismicity. Here, the geodetic measurements imply low rates of horizontal deformation. By combining historical triangulation measurements mostly from 1947 to 1983 with Global Positioning System (GPS) surveys in 1993 and 1994, we estimate the rate of angular shear in triangular subnetworks covering the study area. The estimated strain rates in thirteen of nineteen triangles are smaller than their (1 standard deviation) uncertainties of about 0.1 microradian/yr. This value bounds the rate of deformation for a 100-km wide zone in Provence, between Marseilles to the south and the Ventoux massif to the north. The geodetic estimates place an upper bound of 1 to 2 mm/yr on the slip rates of two seismically active structures, the Durance fault and the Nimes fault, assuming a fault zone ∼20 km wide in each case. We also find strain rates as high as 0.20±0.07 microradian/yr in three subnetworks near the epicentre of the magnitude 5.3 Haute-Ubaye earthquake in 1959, in a region which includes the higher summits. This may be interpreted either as pure shear with compression oriented NE–SW across this region or right-lateral simple shear along NNW–SSE-trending faults. Given that this earthquake is the only one of its magnitude recorded in the study area during the time interval spanned by the geodetic measurements, we infer that most of the geodetically observed deformation occurs aseismically. On the whole, the geodetic results suggest that the rate of north–south shortening across the 100-km wide study area is not more than 1 or 2 mm/yr, in agreement with kinematic models based on other types of geophysical data. Since this deformation represents only a small part of the convergence rate of 6 mm/yr predicted by the NUVEL-1 model for the entire boundary between the African and Eurasian plates, the remaining deformation must be accommodated elsewhere. This study illustrates how a careful analysis of historical geodetic data can measure the rate of tectonic deformation, even where it is low, and thus meaningfully bound geophysical models.

Active tectonics of the eastern Himalaya: New constraints from the first tectonic geomorphology study in southern Bhutan

How convergent systems distribute strain among frontal thrusts is a major concern regarding seismic hazard assessment. Along the 2500 km Himalayan arc, the seismic behavior of the Bhutan region is unknown, because it corresponds to the only portion of the arc where no evidence of major earthquakes has been reported. This can be due either to the fact that no active tectonic studies have been conducted or to continental shortening being absorbed by the Shillong plateau 150 km farther south. Analyzing offset fluvial terraces in south-central Bhutan shows that two major earthquakes ruptured the Himalayan frontal thrust during the last millennium, and that a comparable rate of Holocene deformation (∼20 mm/yr) is accommodated across the Himalaya in Bhutan as in central Nepal. Thus, the propensity for great earthquakes in Bhutan is similar to what is observed in neighboring portions of the Himalaya arc. This in turn suggests that the shortening process beneath the Shillong plateau has little effect on how strain accumulates within the Bhutanese Himalaya.