Active tectonism in the intracontinental Middle Atlas Mountains of Morocco: Synchronous crustal shortening and extension

This material has been published in The Journal of the Geological Society of London, Volume 153, the only definitive repository of the content that has been certified and accepted after peer review. Copyright and all rights therein are retained by The Geological Society of London. Copyright ? 2006 The Geological Society of London See also: http://www.geolsoc.org.uk/template.cfm?name=journals_jgs_home_page; http://atlas.geo.cornell.edu/morocco/publications/gomez1996.htmGeological field observations are integrated with digital topography, LANDSAT imagery, and earthquake focal mechanisms to investigate the Middle and Late Quaternary tectonism in the intracontinental Middle Atlas mountain belt in northern Morocco. The NE-SW-trending Middle Atlas Mountains, approximately 80 km in width and about 200 km long, are part of the Atlas system of northwestern Africa and represent an inverted rift that developed into an intracratonic mountain system in the foreland of the Alpine collisional zone. The Middle Atlas is composed of two provinces, the Folded and Tabular Middle Atlas, representing the palaeo-rift and a flank of the palaeo-rift, respectively. Evidence for Late Quaternary tectonism is provided by the analysis of stream morphology in addition to geological relations. Kinematic analysis of fault-slip data and earthquake focal mechanisms demonstrate the coexistence of both extensional and compressional deformation in different areas of the Middle Atlas with a common sinistral component of slip along NE-SW-striking fault zones. Compressional features dominate the Folded Middle Atlas, whereas extension predominates in the Tabular Middle Atlas. Extension is also manifested by widespread Middle to Late Quaternary alkali volcanism. The observed kinematic variations appear to correlate with the Mesozoic palaeogeography; one possible model may involve tectonic escape. This suggests that differences in the structures inherited from the Mesozoic and Palaeozoic may influence the responses of the different regions to the Cenozoic Alpine collision between Europe and northwest Africa

Structure and evolution of the Neogene Guercif Basin at the junction of the Middle Atlas Mountains and the Rif thrust belt

Copyright 2000, AAPG. See also: http://www.aapg.org/bulletin/index.cfm; http://atlas.geo.cornell.edu/morocco/publications/gomez2000AAPG.htmThe Guercif basin of northern Morocco occupies a 50 x 60 km area where the transpressional Middle Atlas mountains terminate and abut the Rif thrust belt. Analysis of over 800 km of 2-D (two-dimensional) seismic reflection profiles and eight exploratory wells, in combination with existing geological data, suggests a late Miocene episode of extension (4%, or 1.7 km, maximum) and a subsequent episode of contraction since the end of the Miocene. Most of the late Miocene deposition was concentrated in a narrow graben (herein referred to as the Guercif graben), which contrasts with the wider physiographic expression of the basin today. Geohistory analysis indicates that tectonic subsidence persisted until the Messinian, and sediment loading continued to drive subsidence even after extension stopped. Timing constraints demonstrate the contemporaneity of the Guercif graben and west-southwest-vergent thrust tectonics of the Rif thrust belt. Similar timing and proximity with the Rif, as well as the graben geometry, suggest that extension in the Guercif basin, in addition to other smaller extensional basins in the northern Middle Atlas region adjacent to the Rif, may represent the distal effects of a broad lateral shear zone bounding the thrust belt. The Neogene Guercif basin is superimposed on the Mesozoic Middle Atlas rift, which experienced basin inversion during the Cenozoic, and seismic reflection interpretations in the southern Guercif basin depict old Mesozoic rift faults reactivated as reverse faults. Unconformities illustrate that the uplift of the Middle Atlas appears to be primarily a late Cenozoic phenomenon. The Guercif basin offers a special opportunity for petroleum exploration within an aborted rift basin such as the Middle Atlas. Mesozoic source rocks in the Middle Atlas may have been sufficiently buried beneath Neogene basin sediments to reach maturity, and the late Cenozoic timing of contraction can produce suitable structural traps

Inversion of synrift normal faults in the High Atlas Mountains, Morocco

Copyright 1997, Society for Exploration Geophysics. See also: http://segdl.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=LEEDFF000016000008001171000001&idtype=cvips&gifs=Yes; http://atlas.geo.cornell.edu/morocco/publications/beauchamp1997.htmStructural inversion related to intracontinental rifting occurs when extensional rift faults reverse their sense of motion during subsequent episodes of compressional tectonics. Features generated by extension, such as half grabens, are uplifted to form positive anticlinal structures

Mesozoic and Cenozoic deformation inferred from seismic stratigraphy in the southwestern intracontinental Palmyride fold-thrust belt, Syria

This paper was published in the Geological Society of America Bulletin. The Geological Society of America retains the copyright to this paper. Geological Society of America, P.O. Box 9140 , Boulder, CO 80301-9140 USA See also: http://www.geosociety.org/; http://atlas.geo.cornell.edu/syria/chaimov_gsa_bull_1992.htmlThe Palmyride fold belt in central Syria is an intracontinental northeast-trending, 400 by 100 km transpressive belt embedded in the northern Arabian platform. During the Late Paleozoic and most of the Mesozoic the region of the present-day mountains was a rift-like trough that collected over 5 km of sediments, for a total Phanerozoic thickness of over 10 km. The southwestern sector of the fold belt is bounded in the north by the Jhar fault and in the south by the south-vergent frontal thrust faults of the Palmyrides, with the broad Al-Daww depression in between. Structural features that characterize the southern and southwestern region of the Palmyrides include a short wavelength, typically 5-10 km, fold style controlled by a regional low-angle decollement within Triassic beds, and small inverted Jurassic and Early Cretaceous normal faults. Small intermontane basins (about 10 X 30 km) whose strata can be used to document the history of Palmyride deformation flank growth fault-bend folds and are mainly a product of Cenozoic shortening in the belt. These structures are elucidated by about 2000 km of newly available seismic reflection data in the Palmyrides. Synthetic seismic traces generated solely from forward modeling of outcrop information constrain seismic stratigraphic picks in two small basins about 100 km northeast of Damascus. There, minor Late Cretaceous uplift caused local onlap, marking the first inversion phase of the Palmyride trough. Tectonic quiescence throughout the Paleogene, interrupted only in the Middle Eocene by minor tectonism, resulted in monotonous deposition of about 2500 m of mostly limestone. Marked onlap and probable downlap of Lower Miocene strata onto an Oligocene angular unconformity indicate accelerated tectonism by Late Oligocene to Early Miocene time. This marks the beginning of the major phase of inversion and uplift of the Palmyrides. Recent seismicity indicates that transpression continues today. Despite its relative remoteness from convergent plate boundaries (the nearest, the Bitlis suture in southern Turkey, is about 300 km distant), the Late Cretaceous, Middle Eocene, and Neogene phases of deformation in the intraplate setting of the Palmyrides have a direct temporal relationship with major regional tectonism that occurred along the surrounding Arabian plate boundaries. The Palmyride trough was inverted in Late Cretaceous time and, subsequently, developed into a transpressive zone throughout Neogene and Quaternary times. Thus, the initiation of inversion in the Palmyrides, an integral part of the Syrian Arc, which extends from central Syria southward to central Sinai, apparently predates development of the Red Sea/Dead Sea plate boundary. In contrast, the intense Neogene through Quaternary deformational episode is clearly related to development of the Red Sea/Dead Sea fault system and to convergence along the northern boundary of the Arabian plate in southern Turkey

Upper crustal velocity structure and basement morphology beneath the intracontinental Palmyride fold-thrust belt and north Arabian platform in Syria

An edited version of this paper was published in Geophysical Journal International by Blackwell Publishing. Copyright 1993, Blackwell Publishing. See also: http://www.blackwellpublishing.com/journal.asp?ref=0956-540X&site=1; http://atlas.geo.cornell.edu/syria/seber_gji_1993.htmThe intracontinental Palmyride fold-thrust belt, which is the site of an inverted Mesozoic rift, is sandwiched between two crustal blocks, the Aleppo plateau in the north and the Rutbah uplift in the south. The 400 x 100 km belt merges with the Dead Sea fault system in the southwest and gradually ends near the Euphrates depression in the northeast. Very dense (i.e., 100 m geophone spacing), reversed and multifold seismic refraction profiling was carried out to map approximately the upper 15 km of the crust in the early 1970s. These refraction data are utilized to model sedimentary rock thickness, seismic velocity, and basement morphology. Extensive data coverage also enables identification of the major faults of the region. A 2-D ray tracing technique is used in the modeling. Interpretation of these data indicates that five distinct velocity layers characterize the upper crust of the northern Arabian platform in Syria. The P-wave velocities within these layers are (in km s-1): 2.0-2.8, 4.0-4.4, 5.2-5.3 , 5.5-5.7, corresponding to sedimentary rocks from Quaternary to late Precambrian in age, and 5.9-6.0, corresponding to metamorphic basement. A comparison of the velocity models with the available drill hole information and seismic reflection profiles shows strong velocity variations in a given geologic formation, depending on the depth and location of the formation. The depth to metamorphic basement beneath the Palmyride fold belt clearly shows a deep trough, filled with Phanerozoic sedimentary rocks. These rocks decrease in thickness from about 11 km in the southwest to about 9 km in the central segment of the belt. The basement depth is about 6 km in the Aleppo plateau and not less than 8 km in the Rutbah uplift. Deeper basement in the Rutbah uplift is probably the result of a Precambrian rifting episode, clearly identified to the south in Jordan and Saudi Arabia. Cenozoic crustal shortening of about 20-25% across the southwestern segment of the Palmyride belt has not been sufficient to substantially reduce the size of the basement trough beneath this mountain belt. Finally, northeast decreasing basement depth in the Palmyrides supports the idea that the Palmyride Mesozoic rifting was developed as an aulacogen of the rifted Levantine margin along the eastern Mediterranean

Tomographic Pn velocity and anisotropy structure beneath the Anatolian plateau (eastern Turkey) and the surrounding regions

An edited version of this paper was published by the American Geophysical Union (AGU). Copyright 2003, AGU. See also: http://www.agu.org/pubs/crossref/2003.../2003GL017391.shtml; http://atlas.geo.cornell.edu/turkey/publications/Al-Lazki-et-al_2003.htmWe use Pn phase travel time residuals to invert for mantle lid velocity and anisotropy beneath northern Arabia eastern Anatolia continent-continent collision zone. The primary phase data were obtained from the temporary 29- station broadband PASSCAL array of the Eastern Turkey Seismic Experiment. These data were supplemented by phase data from available stations of the Turkish National Seismic Network, the Syrian National Seismic Network, the Iranian Long Period Array, and other stations around the southern Caspian Sea. In addition, we used carefully selected catalog data from the International Seismological Centre and the National Earthquake Information Center bulletins. Our results show that low (<8 km/s) to very low (<7.8 km/s) Pn velocity zones underlie the Anatolian plateau, the Caucasus, and northwestern Iran. Such low velocities are used to infer the presence of partially molten to absent mantle lid beneath these regions. In contrast, we observed a high Pn velocity zone beneath northern Arabia directly south of the Bitlis-Zagros suture indicating the presence of a stable Arabian mantle lid. This sharp velocity contrast across the suture zone suggests that Arabia is not underthrusting beneath the Anatolian plateau and that the surface suture extends down to the uppermost mantle. Pn anisotropy orientations within a single plate (e.g. Anatolia plate) show a higher degree of lateral variation compared to Pn velocity. Areas of coherent Pn anisotropy orientations are observed to continue across major fault zones such as the EAF zone