Hervé Philip

Geodynamics of the northern Andes: Subductions and intracontinental deformation (Colombia)

New regional seismological data acquired in Colombia during 1993 to 1996 and tectonic field data from the Eastern Cordillera (EC) permit a reexamination of the complex geodynamics of northwestern South America. The effect of the accretion of the Baudo-Panama oceanic arc, which began 12 Myr ago, is highlighted in connection with mountain building in the EC. The Istmina and Ibague faults in the south and the Santa Marta-Bucaramanga fault to the northeast limit an E-SE moving continental wedge. Progressive indentation of the wedge is absorbed along reverse faults located in the foothills of the Cordilleras (northward of 5°N) and transpressive deformation in the Santander Massif. Crustal seismicity in Colombia is accurately correlated with active faults showing neotectonic morphological evidences. Intermediate seismicity allows to identify a N-NE trending subduction segment beneath the EC, which plunges toward the E-SE. This subduction is interpreted as a remnant of the paleo-Caribbean plateau (PCP) as suggested by geological and tomographic profiles. The PCP shows a low-angle subduction northward of 5.2°N and is limited southward by a major E-W transpressive shear zone. Normal oceanic subduction of the Nazca plate (NP) ends abruptly at the southern limit of the Baudo Range. Northward, the NP subducts beneath the Choco block, overlapping the southern part of the PCP. Cenozoic shortening in the EC estimated from a balanced section is ∼120 km. Stress analysis of fault slip data in the EC (northward of 4°N), indicates an ∼E-SE orientation of σ1 in agreement with the PCP subduction direction. Northward, near Bucaramanga, two stress solutions were observed: (1) a late Andean N80°E compression and (2) an early Andean NW-SE compression.

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.

Gigantic paleolandslide associated with active faulting along the Bogd fault (Gobi-Altay, Mongolia)

On the basis of analyses of satellite imagery, aerial photographs, and field observations, we describe the occurrence of one of the largest paleolandslides (50 km3) ever recognized in an intracontinental domain. The slide occurred along the active Bogd fault in the Gobi-Altay mountain range in Mongolia. Morphological and structural analyses of the relationships between the landslide and the area affected by active tectonics suggest that this gigantic mass movement was associated with surface faulting during a strong earthquake.

Active tectonics in the Lesser Caucasus: Coexistence of compressive and extensional structures

Detailed SPOT image analysis which completed field data and a microseismicity study was done on the central Caucasian mountain belt (Georgia, Armenia, and eastern Turkey), north of the front of the Arabian collision in order to clarify the relationships between compression, extension, and volcanism. In fact, this region is characterized by relatively complex active tectonics, associating both N-S compressive (E-W thrusts and folds) and E-W extensional (N-S normal faults and dikes) structures accompanied by considerable Neogene to Quaternary volcanism, and NE-SW left-lateral and NW-SE right-lateral strike-slip faults. These different structures are all intricately imbricated and can be observed at different scales. The general layout implies an important variation in the stress state; for instance, this passes from a N-S compression in the Spitak earthquake fault zone to an E-W extension at the Aboul-Samsar volcanic axis. These results agree well with the N-S convergence between Arabia and Eurasia, and the E-W lateral expulsion of the Anatolian and Iranian blocks. A detailed cartographic network of active structures was drawn and their kinematic relationships were specified. In places, recent offsets on strike-slip faults were estimated at about 500 to 1000 m. Suitable sites for future determination of fault velocity displacements were also selected. This study shows that some strike-slip faults behave partly like faults which transform the E-W extension of the N-S striking normal faults into a N-S compression along the E-W thrusts. One of the characteristics of this region is the continuous important volcanic activity at least from the Jurassic until now. This persistence evidences a lithospheric thinning, which remained in spite of the recent Arabian-Eurasian collision because of the E-W extension linked to the opposite lateral expulsion of the Anatolian and Iranian blocks. This geodynamic evolution can explain the juxtaposition and superimposition of volcanic structures and thrusts in a general compressive context.

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.

Structural analysis of the superficial deformation of the 1969 Pariahuanca earthquakes (Central Peru)

Abstract An analysis of the superficial structures related to the Huaytapallana fault is presented. This fault was formed during the Pariahuanca earthquakes of July 24 and October 1, 1969. The fault trends N120E and is 3.5 km long, the displacement is 0.70 m left-lateral slip and 1.6 vertical upthrust. At most places the fault appears as a steep flexure associated with open fractures and mostly gravity folds, all of them almost parallel to it. Oblique, “en echelon” shears, often associated with small folds, are developed in the turf which is partly detached from its substratum. The analysis of these fractures is consistent with a mean N65E shortening direction. The compressional and dilatational quadrants obtained by means of this analysis fit rather well with the focal-mechanism data. On the other hand, some of the en echelon fold and fault systems provide a model of similar features with a magnitude of several kilometers.

Recent tectonic stress evolution in the Lesser Caucasus and adjacent regions

Abstract The stress indicators describing the recent (provided by active tectonics framework) and palaeo-stress (provided by micro-fault kinematics and volcanic cluster) patterns show the scale and temporal changes in stress states since the beginning of Arabian–Eurasian collision. The recent stress derived from the active fault kinematics in the Lesser Caucasus and adjacent area corresponds to a strike–slip regime with both transtension and transpression characteristics. The kinematics of active structures of various scale are conditioned by tectonic stress field with general north–south compression and east–west extension. The distribution of Neogene to Quaternary volcanic cluster geometries and micro-fault kinematic data evidence the time and orientation variability of the stress field since the beginning of the Arabian–Eurasian collision. In addition to the general north–south compression orientation, two other – NW–SE and NE–SW – secondary orientations are observed. The first one was dominant between the Palaeogene and the late Early Miocene and the second one has prevailed between the Late Miocene and the Quaternary. Since the continental collision of Arabia with Eurasia the tectonic stress regime in the Lesser Caucasus and adjacent area changed from compression (thrusting and reverse faulting) to transtension-transpression (strike–slip faulting with various vertical components).

Coseismic elastic models of folds above blind thrusts in the Betic Cordilleras (Spain) and evaluation of seismic hazard

Abstract Although it is generally considered that near-surface earthquakes result from movements along faults that cut through the surface, several recent large earthquakes have been partly attributed to blind thrusts. Movements along blind thrusts lead to the formation of surface folds, which are highly dependent upon fault geometry at depth and often not considered in seismic hazard evaluation. Several authors have studied the relationship between surface folding and thrusting for geological situations in which fault geometries are quite simple. However, active fault geometries can be quite complex e.g., segmented thrust faults associated with strike-slip faults. The aim of this contribution is to reconstruct the fault kinematics at depth for a relatively complex geological structure located in the Eastern Betic Cordilleras (Orihuela-Guardamar-Torrevieja region) using the patterns of kilometre-scale folds observed in the field. In order to model surface deformation, the assumption is made that surface km-scale folds have been created by coseismic deformation associated with movement along blind thrusts. By means of a coseismic deformation model, movements at depth have been calculated for three possible hypotheses. Hypothesis 1 assumes that each superficial fold is created by an independent fault. Hypotheses 2 and 3 assume that a sequence of two superficial folds can be created by movement along a single fault displaying a flat and ramp geometry. In Hypothesis 2, the flat is a superficial decollement level between the sedimentary cover and the Betic basement; in Hypothesis 3, it is a deeper decollement level within the Betic basement. Knowing the approximate age of surface deformation, rough estimates of fault slip-rates and recurrence periods for two possible earthquake magnitudes (7 Ms and 6.7 Ms) have been made, from calculated dislocations at depth. Slip-rates and recurrence periods for flat and ramp fault geometries are in the range of 0.75–1 mm/yr and 1000–2000 yr, respectively. These values are close to those calculated by direct methods in similar seismotectonic contexts.

Neogene to Quaternary stress field evolution in Lesser Caucasus and adjacent regions using fault kinematics analysis and volcanic cluster data

In the Great Caucasus, the Lesser Caucasus and Eastern Turkey, the distribution of Neogene to Quaternary volcanic cluster geometries, paleo-stress field data of the Lesser Caucasus area (Republic of Armenia) and the P axes of earthquakes focal mechanisms show the scale and time variability of the stress field since the beginning of the Arabia-Eurasian collision. In addition to the general N-S compression orientation, two other NW-SE and NE-SW secondary orientations are observed. Both orientations were successively significant for some period of tectonic activity. The first one was dominant between the Paleogene and the end of the Lower Miocene and the second one has prevailed between the Upper Miocene and the Quaternary. On a regional scale the principal stress axes orientations are mainly controlled by the Arabian-Eurasian plate convergence and have changed with time. Local stress orientations have been significantly influenced by secondary blocks motions and their geometries.