Eric Barrier

The tectonic development of the hellenic arc and the sea of crete: A synthesis

Abstract We use field analyses together with interpretation of aerial photographs, orbital imagery and Sea-Beam data to define more accurately the geometry of fault systems in the Hellenic arc. The tectonics of the area are dominated by extensional processes related to normal faulting. We then use Sea-Beam data, previous oceanographic studies and extrapolation of field analyses on surrounding islands to describe the fault pattern of the Cretan Sea basin, which is found consistent with the land fault pattern. The mechanisms of the late Neogene and Quaternary deformation of the central and southern Aegean region are analysed in terms of stress trajectories, as revealed by field studies of fault populations. We then examine the subsidence of the Sea of Crete in terms of a model of uniform stretching of the lithosphere. The opening of the Sea of Crete was cylindrical and did not result in bending of Crete. This leads us to modify the southern portion of an earlier kinematic reconstruction of Aegea, in agreement with recent paleomagnetic results. The implications of the opening of the Sea of Crete on the tectonic evolution of the straits to the west and east of Crete are discussed. Finally, the uplift of the outer arc is explained by underplating due to subduction.

Tectonic evolution of the northern African margin in Tunisia from paleostress data and sedimentary record

Abstract A reconstruction of the tectonic evolution of the northern African margin in Tunisia since the Late Permian combining paleostress, tectonic stratigraphic and sedimentary approaches allows the characterization of several major periods corresponding to consistent stress patterns. The extension lasting from the Late Permian to the Middle Triassic is contemporaneous of the rifting related to the break up of Pangea. During Liassic times, regional extensional tectonics originated the dislocation of the initial continental platform. In northern Tunisia, the evolution of the Liassic NE–SW rifting led during Dogger times to the North African passive continental margin, whereas in southern Tunisia, a N–S extension, associated with E–W trending subsiding basins, lasted from the Jurassic until the Early Cretaceous. After an Upper Aptian–Early Albian transpressional event, NE–SW to ENE–WSW trending extensions prevailed during Late Cretaceous in relationship with the general tectonic evolution of the northeastern African plate. The inversions started in the Late Maastrichtian–Paleocene in northern Tunisia, probably as a consequence of the Africa–Eurasia convergence. Two major NW–SE trending compressions occurred in the Late Eocene and in the Middle–Late Miocene alternating with extensional periods in the Eocene, Oligocene, Early–Middle Miocene and Pliocene. The latter compressional event led to the complete inversion of the basins of the northwestern African plate, originating the Maghrebide chain. Such a study, supported by a high density of paleostress data and including complementary structural and stratigraphic approaches, provides a reliable way of determining the regional tectonic evolution.

Active collision in eastern Taiwan: the coastal range

Abstract A detailed analysis of Late Cenozoic faulting has been carried out in the Coastal Range of eastern Taiwan. 4000 measurements have been collected in 60 sites where faults affect formations that range from upper Miocene to Pleistocene and scattered Quaternary deposits. This large data set enables us to accurately determine paleostress axes in the area. During the major tectonic event (the Pleistocene Coastal Range orogeny), the direction of the maximum compressional stress σ 1 has remained constant. 25% of reconstructed σ 1 axes trend approximately 120; 90% trend between 090 and 140. Taking into account the internal deformation in the Coastal Range during folding and thrusting, these reconstructed directions of compression are remarkably homogeneous. Present faulting in the Longitudinal Valley is consistent with the paleostress pattern recorded in the adjacent Coastal Range formations. The Longitudinal Valley fault is a thrust, with a minor left-lateral strike-slip component (20% of the total movement). This quantitative estimate of motion is compatible with reconstructed Pleistocene paleostress and more general reconstructions of recent plate motion in the vicinity of Taiwan. The analysis of the focal mechanisms of earthquakes provides similar results. Finally, the analysis of the Coastal Range faults systems related to the collision of the volcanic Luzon arc (including the Luzon trough) with the Central Range of Taiwan provides a consistent picture of the distribution of stress patterns and deformations. The strikes of folds and thrusts in the Coastal Range are mainly controlled by the direction of convergent plate motion (125), whereas the overall trend of the Coastal Range is similar to that of the Central Range (020). The obliquity of convergent motion has resulted in the complex en echelon structure of the Coastal Range. The relative plate motion is not the sole factor controlling the development of a collision chain such as the Coastal Range; the existence of major pre-existing structures also plays an important role.

Philippine fault: A key for Philippine kinematics

On the basis of new geologic data and a kinematic analysis, we establish a simple kinematic model in which the motion between the Philippine Sea plate and Eurasia is distributed on two boundaries: the Philippine Trench and the Philippine fault. This model predicts a velocity of 2 to 2.5 cm/yr along the fault. Geologic data from the Visayas provide an age of 2 to 4 Ma for the fault, an age in good agreement with the date of the beginning of subduction in the Philippine Trench. The origin of the Philippine fault would thus be the flip of subduction from west to east after the locking of convergence to the west by the collision of the Philippine mobile belt with the Eurasian margin.

Plate collision and paleostress trajectories in a fold-thrust belt: The foothills of Taiwan

Abstract Field analyses of compressional faulting and folding in the Foothills of western Taiwan enable us to reconstruct paleostress trajectories over a large area and to establish the relative chronology of tectonic events. Two main compressional events have contributed to the present structure of the fold-thrust belt. Stratigraphic data show that these events are Plio-Pliocene in age. Older normal faulting indicates NNW-SSE extension across the Chinesse passive continental margin during the Neogene. The two main compressional events of the Taiwan collision correspond to similar fan-shaped distributions of maximum compressive stress trajectories, with a counterclockwise shift of 30°–50° between the two events. Using the relationship between recent stress trajectories and the direction of recent plate motion as a guide, we reconstruct the direction of plate convergence for the older event. We suspect that the relative motion Philippine Sea plate-Eurasia has rotated counterclockwise of at least 35°–45° in Taiwan during collision. This conclusion is in agreement with independent plate tectonic reconstructions. Several problems provide objectives to further tectonic and paleomagnetic studies, including the duration and diachronism of compressional events as well as possible clockwise rotation of northernmost Taiwan.

Miocene to recent calc-alkalic volcanism in eastern Taiwan: K-Ar ages and petrography

Abstract K-Ar radiometric datings are presented for volcanic rocks from the Coastal Range of Taiwan and from Lanhsu and Lutao islands. The samples involved are basalts, andesites and dacites which show the main petrological and geochemical characteristics of island arc magmas. The K-Ar data show that volcanic activity occurred from Early Miocene to Early Pliocene times in the Coastal Range and in Lanhsu Island, and during Pliocene times (ca. 1.3–4.3 Ma) in Lutao Island. The geological significance of Early and Middle Miocene ages is discussed with respect to hydrothermal/metasomatic alterations which have affected most of the samples. A significant increase in incompatible elements (e.g., K and Sr) is shown to have occurred during Pliocene to Recent times, and is exemplified by the compositions of the Lanhsu, and Lutao volcanic rocks. The origin of these chemical variations is related to the magmatic effects of crustal thickening linked to the transition from subduction to collision regimes.

The Philippine Fault in the Late Cenozoic tectonic evolution of the Bondoc-Masbate-N. Leyte area, Central Philippines

Abstract This study of the Bondoc-Masbate-N. Leyte region contributes to an understanding of its Late Cenozoic tectonic evolution. In this region, the Philippine Fault, an active left-lateral slip fault, dissects a region of complex structural history. This structural complexity is attributed to a superposition of three compressive and two tensional tectonic events. The first compressional event, of Middle Oligocene age, appears to be a significant tectonic phase, previously unrecognized. The second compressional event is related to the well-documented end of Miocene collision between the North Palawan Block and the western edge of the Philippine Mobile Belt, whereas the last one represents a compressive tectonic phase associated with the Philippine Fault. The two tensional events occurred during Late Oligocene to Middle Miocene and Pliocene to Pleistocene periods. The present configuration of the Philippine Fault is interpreted to be a young feature (not older than upper Early Pliocene), postdating the end-of-Miocene collision. This observation does not, however, preclude the existence of older strike-slip features believed to have traversed the Philippine Archipelago since mid-Tertiary times.

Mesozoic paleostress evolution in the Saharian platform (southern Tunisia)

AbstractA detailed analysis of brittle deformations in the Saharian platform of southern Tunisia is based on studies of fault-slip data sets and joint sets. It allows reconstruction of the Mesozoic paleostress evolution. During the Permo-Triassic, N-S extensions occurred with high late Permian subsidence rates. During the Norian, strike-slip movements reactivated former normal faults. During the Jurassic and the Cretaceous a succession of extensional events was characterized by : (1) a N-S extension which dominated from late Triassic to early Aptian. We relate this extension to the Africa-Eurasia divergence; (2) a ENE-WSW extension during the Cenomanian. We relate this extension to the opening of «he African basins ; (3) a NE-SW Senonian extension that continued during the Cenozoic in the Jeffara and in the Gabes Gulf, during the further evolution of the northern African margin. The various compressional trends recorded in the platform are attributed to Cenozoic events.

Tectonic analysis of a flexed foreland: the Ragusa Platform

Abstract Detailed field studies of fracture patterns and paleostress reconstructions based on analyses of fault-slip data sets allow the identification of the Late Cenozoic stress pattern evolution of the Ragusa Platform. During the Late Miocene to Early Pleistocene period, a complex extensional pattern developed. It was dominated in almost the entire Hyblean Plateau by NW-SE to NNW-SSE extensions related to the flexure of the foreland under the load of the allochthonous units of the Sicilian Fold and Thrust Belt. These extensional tectonics were characterized by NE-SW to ENE-WSW trending normal faults and grabens. In eastern Hyblea, NE-SW to ENE-WSW extension have prevailed at least since the Late Cretaceous. They are linked to the effects of the Malta Escarpment. In the western Ragusa Platform, a third, minor, WNW-ESE extension associated with sets of NNE-SSW trending normal faults, may be correlated with the development of the Gela basin. In addition, these major extensions induced complex patterns of perpendicular normal fault systems through permutations between the principal stress axes σ2 and σ3. The Pliocene-Pleistocene extensional paleostress field evolution is marked in almost the entire platform by the alternation of two or three main extensional stress patterns (NE-SW to NNW-SSE, WNW-ESE and NE-SW to ENE-WSW extensions), each of them controlled by particular boundary conditions. Three minor compressional paleostress orientations have been identified. They correspond to three minor compressional events dominated by 020°–030°, 070°–080° and 100°–110° compressional directions. These events occurred in within the general Pliocene-Pleistocene extensive context. The 100°–110° event is of Early-Middle Pliocene age. Its large regional distribution suggests that this event may have resulted from interactions between the major blocks constituting the Africa-Eurasian boundary. The relative chronology shows that the 020°–030° compression predates the 070°–080° one. The origins of these compressional events are still uncertain due to poor regional control.

Mesozoic extensional brittle tectonics of the Arabian passive margin, inverted in the Zagros collision (Iran, interior Fars)

Abstract The present Zagros mountain belt of SW Iran is known to be the former NE Arabian passive continental margin of the southern Neo-Tethyan basin, which originated by Permian–Triassic rifting, and has a late Cenozoic collisional imbricate structure. We carried out brittle tectonic analyses of syndepositional normal fault slip data in the High Zagros Belt of the Fars Province to reconstruct the extensional deformation of the passive margin during the Mesozoic era in terms of stress tensor inversion. This reconstruction revealed two main directions of extension, developing from a north–south margin-oblique trend to a NE–SW margin-perpendicular one. Considering the basement structures and the existence of the basal Infracambrian salt detachment, we infer that a transtensional extension could have initiated two major periods of crustal stretching: a Permian–Triassic thick-skinned phase with the basement faults developing in an oblique rifting, and a Mesozoic thin-skinned phase with the sedimentary cover being affected by successive extensional structures and block tilting. This extensional tectonic history probably continued during the early Tertiary period, prior to the continental collision. Fault slip geometries and structural patterns of both the Mesozoic extension and the late Cenozoic compression indicate inversion of the inherited structures in the Zagros collision during the subsequent thin- and thick-skinned stages of crustal shortening.