Fault Trench Research Articles

Paleoseismicity in France: Fault trench studies in a region of moderate seismicity

France is a country of moderate to low seismicity with a ten-century record of historical seismicity. Paleoseismicity appears to be the only available tool which is able to extend the record of seismic activity beyond this short time-window. However, moderate to low seismicity should be associated with low slip rates and, thus, geomorphic evidence for detecting active fault traces should be weak, impeding the development of paleoseismic fault studies. To test the paleoseismic method in France, we have developed a programme on paleoseismology during the past few years. First, we performed a critical reappraisal of existing data on French active tectonics, then we selected several suitable sites, and finally we trenched on some sites. Much of the revised evidence from recent and active tectonics was not selected because their origins were either dubious or non-tectonic. In particular, many reported sites from the Alps, seen in moraines, actually correspond to glacial processes (‘glacitectonics’). In addition, some clearly observed scarps could result from gravitational processes such as post-glacial rebound. Among the selected sites, three were studied in detail; they correspond to the Nimes, Moyenne Durance, and Argentera fault zones. The most reliable information was provided by trenching observations on the Moyenne Durance Fault. The trench site exhibited a N35 °E-striking knee fold, parallel to the Moyenne Durance Fault, covered unconformably by colluvial deposits. Radiocarbon dates indicate that folding occurred during the latest Pleistocene or earliest Holocene. The observed stratigraphical relations strongly suggest that folding resulted from a single coseismic slip event associated with a strong earthquake of magnitude Mw = 6.4–6.9. Our data also indicate long recurrence intervals for such earthquakes, at least of the order of 25 ka. Paleoseismic data collected on the other studied fault zones confirm that nearly Mw = 7 events may be generated by French active faults and that the recurrence intervals should be over 10 ka. Thus, the paleoseismic study of active faults is a robust tool to reconstruct seismic history where slip rates are low to moderate; however, specific exploration methods should be developed to localize fault traces more accurately and study hidden faults.

Preliminary Observations on Quaternary Reverse Faulting Along the Southern Front of the Northern Range of Trinidad

Several geomorphological evidences of Quaternary reverse faulting are observed along the southern front of the Northern Range in Trinidad between Port-of-Spain and Matura Point. Such a mountain front is associated to a reverse fault system showing an imbricated pattern southward. In the north, the system is limited by a structural feature showing an important vertical component. Southward this system progressively changes to low angle faults. This geometry is corroborated by seismic profiling in the continent shelf. The active faulting evidences consist in lateral drainage offsets, fault trenches, sag-ponds, triangular facets, and saddles. Some quaternary terraces show fault scarps and tilting. We postulate that these reverse fault systems as Arima Fault instead of El Pilar fault as it is not actually connected to the San Sebestian-El Pilar right-lateral slip system, due to the southward prolongation of the southern limit of the Caribbean Plate through the fault system of Los Bajos-El Soldado.

Neotectonics of the Caribbean

A compilation of earthquake focal mechanism solutions together with the interpretation of inclined, intermediate‐depth seismic zones, Neogene strike‐slip faults, fault‐controlled basins, and volcanism is used to test three existing models for the neotectonic (i.e., Neogene or post‐Oligocene) development of the Caribbean. The current models, although based on different hypotheses using different data sets, are similar and suggest varying degrees of convergence between the Caribbean and either North or South American plates or convergence between the Caribbean plate and continental fragments of the northwestern corner of South America. Using model 1, previous workers have interpreted the disposition of fracture zones and magnetic anomalies in the central and South Atlantic as indicating that the Caribbean plate is being compressed by north‐south convergence between the Americas with the result that it is being driven eastward relative to the Americas along strike‐slip faults while producing east‐west trending subductionlike convergent zones along northern and southern plate margins. Using model 2, previous workers have interpreted the azimuths of active strike‐slip faults and focal mechanism solutions and closure of three‐plate velocity circuits as indicating that the South American plate converges in a west‐northwesterly direction on the southern margin of the Caribbean plate and has produced compressional features, while strike‐slip motion dominates along the northern boundary. Using model 3, previous workers have interpreted focal mechanism solutions and the pattern of strike faulting and basin development in northwestern South America to suggest right‐lateral displacement of the northwestern corner of South America relative to the craton with convergence and compression in the south central Caribbean. Our integration of geologic information on faults, basins, and volcanism considerably extends the limited record of plate interactions known from historical reports of the past few hundred years and recorded earthquakes and suggests that (1) relative motion between the North American and Caribbean plates is dominantly east‐west strike‐slip motion, although areas of complex internal deformation associated with restraining bend or compressional strike‐slip fault segments have been identified in Jamaica, Hispaniola and Central America (2) relative motion between the South American and Caribbean plates is dominantly east‐west strike‐slip motion, although an area of restraining bend deformation has been identified in the Araya Peninsula of Venezuela; (3) relative motion between a triangular continental fragment of northwestern South America (the “Maracaibo Block”) ranges from northwest to northeast along the south central margin of the Caribbean and appears to be driven mostly by the late Miocene‐Pliocene collision of the Panama arc, although additional driving forces may include the collision of the Carnegie Ridge and/or oblique subduction (to the northeast) in Ecuador; and (4) relative motion (internal deformation) may be occurring at a very slow rate between fault‐bounded blocks within the Caribbean plate as a result of the convergence of the Maracaibo Block and/or eastward motion of the Caribbean plate relative to a continental promontory of North America in northern Central America. Because the Caribbean margins are composed mainly of anisotropic, easily deformed continental and arc lithosphere, plate boundaries are zones of diffuse deformation, and it is difficult to assess the degree of convergence between the Americas, although data from the Caribbean itself suggest that it may be smaller than the amount inferred by some workers using model 1 (fracture zone and magnetic anomalies in the Atlantic). Some of the compressional features cited as evidence for the relative convergence of the Americas may be instead related to the convergence between the Maracaibo Block relative to the southern margin of the Caribbean. Many aspects of Caribbean neotectonics remain less than completely understood, but more integration of seismic and geologic studies (particularly exploratory fault trenching of onshore faults and seismic profiling of offshore faults) should lead to rapid advances.

The Nootka Fault Zone -- a new plate boundary off western Canada

Summary. Relative motion across a boundary between the main Juan de Fuca plate and its northern extension, the Explorer plate, had earlier been suggested from sea-floor magnetic anomaly analysis and from earthquakes recorded on the western Canada land seismic network. The location of the boundary, called the Nootka fault zone, and the motion across it have been examined through seismic reflection profiles, accurate location of earthquakes with an array of ocean bottom seismometers and through analysis of magnetic, gravity and bathymetric data. The fault zone extends from a ridge-fault—fault triple point at the northern end of the Juan de Fuca ridge to a fault—trench—trench triple junction at the margin off north-central Vancouver Island. The active portion of the fault zone is about 20 km wide, and has produced extensive disturbance in the 0.5 to 1 km of overlying sediments. Magnetic anomaly analysis suggests present left-lateral strike slip motion of about 3 cm/yr, with convergence at the margin being more rapid to the south than to the north of the fault zone. Because of rapidly changing spreading parameters on the Explorer and Juan de Fuca ridges over the past 5 Myr the Nootka fault zone has had a very complex history.

TECTONIC MORPHOGENESIS IN THE NOSAKA MOUNTAINS, NORTH OF LAKE BIWA, CENTRAL JAPAN

The Nosaka Mountains are situated on the western side of the Tsuruga Bay-Ise Bay Tectonic Line which is one of the main tectonic lines in Southwest Japan, and occupy the northeasternmost part of the Setouchi tectonic depression. In this mountain area, tectonic landforms of various kinds are remarkably developed. This fact has been recognized by many investigators, but the process of tectonic morphogenesis in this area has not inten-sively been analysed since the Yamazaki & Tada's study (1927). The author investigated the tectonic m_orphogenesis in this mountain area, chiefly based on interpretations of summit level maps and aerial photographs, together with field surveys. The followings are the major results obtained ; (1) On summits of the Nosaka Mountains, erosion surfaces with low relief are recognized. After these erosion surfaces with low relief had been formed, the Nosaka Mountains were undulatingly upwarped trending WNW-ESE, and then disintegrated into many small mountain blocks by faulting trending N-S, NE-SW and NW-SE. However, the WNW-ESE alignment of the undulating tectonic relief, has not been dislocated horizontally and is clearly recognized even after the disintegration. This fact suggests that these faults are dip-slip faults. (2) Many active faults are ascertained by photo-interpretations and field surveys in the Nosaka Mountains. It is relatively easy to find active faults in this area because each of them is characterized by a remarkable photogeologic linca.ment and tectonic landforms such as offset stream, shutter ridge, fault trench, scarplet, wind gap and so on. These active faults are trending NE-SW and NW-SE, and most of them are strike-slip faults, among which the NE-trending ones are dextral and the NW-trending ones sinistra.l. (3) Except several parts, these active faults are inconsistent in distribution from the above-mentioned faults by which the Nosaka Mountains were disintegrated into many blocks. And, as already mentioned, most of the former are strike-slip faults, while the latter are dip-slip faults. Besides, one of the active faults is confirmed by field survey to have displaced one of the latter. From these facts, it can be concluded that these active faults have been active since the Nosaka Mountains were disintegrated into many blocks.