Les mineralisations a plomb et zinc du nord-ouest de l'Algerie sont localisees, essentiellement, dans les hauts plateaux (Touahri, 1991). Les plus fortes concentrations d'indices et de gisements qui ont un potentiel economique et une grande extension geographique sont recensees, entre autres, dans la region de Saida et sont encaissees dans les dolomies du Jurassique. Dans le socle paleozoique du horst de Tifrit, les zones mineralisees, dont l'extension est modeste, sont generalement de direction NE et sont localisees le long de zones de failles (Touahri, 1991). La zone d'etude, qui appartient aux monts de Saida, est situee a l'est du horst de Tifrit. Elle fait partie des hauts plateaux qui sont limites au nord par l'Atlas Tellien et au sud par l'Atlas Saharien. Pour cartographier les dolomies du Jurassique, qui ont une densite proche de celle du socle mais une susceptibilite magnetique plus faible, nous avons realise cette etude basee sur l'utilisation combinee des donnees gravimetriques et aeromagnetiques. L'interpretation des cartes obtenues a partir 342 mesures gravimetriques a permis de mettre en evidence une anomalie positive de direction generale SW-NE, conforme a la direction du socle dans la region de Saida. Cette anomalie qui est limitee en profondeur, par deux discontinuites, indique une structure caracterisee par une forte densite. Cette structure, qui s'approfondit en allant vers le SW, peut etre associee soit a une remontee du socle ou a une forte epaisseur de dolomies. Pour differencier entre le socle et la couverture dolomitique, nous avons utilise les donnees aeromagnetiques. La carte du champ magnetique reduite au pole montre une forte anomalie negative allongee dans la direction SE-NW. Situee dans la zone ou la structure gravimetrique est la plus profonde, elle indiquerait la zone ou l'epaisseur de la couverture dolomitique est la plus importante. A travers cette etude, on peut conclure que le centre de la region de Ain-Dez est caracterise par un soulevement du socle de direction SW-NE qui s'approfondit rapidement vers le Sud ou il est recouvert par une importante epaisseur dolomitique qui va du SE vers le NW.
Clastic dikes may form by simultaneous fracture propagation in rocks and injection of clastic material into the fractures resulting from strong seismic shaking. We studied the mechanisms of clastic‐dike formation within the seismically active Dead Sea basin, where hundreds of clastic dikes cross‐cut the soft rock of the late Pleistocene lacustrine Lisan Formation. We analyzed the anisotropy of magnetic susceptibility (AMS) of dikes with known formation mechanisms and defined the characteristic AMS signatures, mainly of dikes developed by injection process. Most of the dikes were emplaced due to fluidization of clay‐rich sediment and are characterized by triaxial AMS ellipsoids. The dominant triaxial AMS ellipsoids along the dike widths suggest that the fluidization mechanism of clay‐rich sediment is different from the known liquefaction process of sand. The AMS analysis supported by field evidence indicates that the injection of clay‐rich sediment is characterized by two main regimes: (1) Vertical flow characterized by subvertical V 2 axes and subhorizontal V 1 and V 3 axes. The V 2 axes may indicate the flow directions during fast flow. (2) Horizontal slow flow characterized by subvertical V 3 axes and subhorizontal V 1 and V 2 axes. A streaked AMS pattern mainly composed of V 2 and V 3 axes represents a turbulent flow that generated local eddies simultaneously with the clastic transport. The AMS parameters along the dikes and possible grain imbrications along dike walls support organization of grains under high strain rates. This application of the AMS method provides a petrofabric tool for identifying seismites and inferring their flow kinematics in complex geologic areas.
Research Article| February 01, 2006 Earthquake-induced clastic dikes detected by anisotropy of magnetic susceptibility Tsafrir Levi; Tsafrir Levi 1Department of Geological and Environmental Sciences, Ben Gurion University of the Negev, Beer Sheva, Israel, and Geological Survey of Israel, 30 Malkhe Yisrael Street, Jerusalem 95501, Israel, and Ramon Science Center, Ben-Gurion University of the Negev, P.O. Box 194, Mizpe Ramon 80600, Israel Search for other works by this author on: GSW Google Scholar Ram Weinberger; Ram Weinberger 2Geological Survey of Israel, 30 Malkhe Yisrael Street, Jerusalem 95501, Israel Search for other works by this author on: GSW Google Scholar Tahar Aïfa; Tahar Aïfa 3Géosciences-Rennes, CNRS UMR6118, Université de Rennes l, Campus de Beaulieu, 35042 Rennes cedex, France Search for other works by this author on: GSW Google Scholar Yehuda Eyal; Yehuda Eyal 4Department of Geological and Environmental Sciences, Ben Gurion University of the Negev, Beer Sheva, Israel Search for other works by this author on: GSW Google Scholar Shmuel Marco Shmuel Marco 5Department of Geophysics and Planetary Sciences, Tel Aviv University, Tel Aviv 69978, Israel Search for other works by this author on: GSW Google Scholar Author and Article Information Tsafrir Levi 1Department of Geological and Environmental Sciences, Ben Gurion University of the Negev, Beer Sheva, Israel, and Geological Survey of Israel, 30 Malkhe Yisrael Street, Jerusalem 95501, Israel, and Ramon Science Center, Ben-Gurion University of the Negev, P.O. Box 194, Mizpe Ramon 80600, Israel Ram Weinberger 2Geological Survey of Israel, 30 Malkhe Yisrael Street, Jerusalem 95501, Israel Tahar Aïfa 3Géosciences-Rennes, CNRS UMR6118, Université de Rennes l, Campus de Beaulieu, 35042 Rennes cedex, France Yehuda Eyal 4Department of Geological and Environmental Sciences, Ben Gurion University of the Negev, Beer Sheva, Israel Shmuel Marco 5Department of Geophysics and Planetary Sciences, Tel Aviv University, Tel Aviv 69978, Israel Publisher: Geological Society of America Received: 30 Jun 2005 Revision Received: 26 Sep 2005 Accepted: 27 Sep 2005 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2006) 34 (2): 69–72. https://doi.org/10.1130/G22001.1 Article history Received: 30 Jun 2005 Revision Received: 26 Sep 2005 Accepted: 27 Sep 2005 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Tsafrir Levi, Ram Weinberger, Tahar Aïfa, Yehuda Eyal, Shmuel Marco; Earthquake-induced clastic dikes detected by anisotropy of magnetic susceptibility. Geology 2006;; 34 (2): 69–72. doi: https://doi.org/10.1130/G22001.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Clastic dikes form either by passive deposition of clastic material into preexisting fissures or by fracturing and injection of clastic material during seismic shaking or passive overpressure. Because of their similar final geometry, the origin of clastic dikes is commonly ambiguous. We studied the mechanisms of clastic dike formation within the seismically active Dead Sea basin, where hundreds of clastic dikes crosscut soft rock of the late Pleistocene lacustrine Lisan Formation. We analyzed the anisotropy of magnetic susceptibility (AMS) of clastic dikes of known origin and defined characteristic AMS signatures of depositional or injection filling. We discovered that passively filled dikes, which contain brownish silt resembling local surface sediments, are characterized by an oblate AMS ellipsoid and vertical minimum susceptibility axis V3. Dikes that contain green clayey sediment connected to a mineralogically identical detrital layer of the Lisan Formation are characterized by a triaxial AMS ellipsoid, well grouped subhorizontal and parallel to the dike walls' maximum susceptibility axis V1, and subvertical intermediate susceptibility axis V2. Field evidence and AMS analysis indicate that most of these dikes were emplaced by injection inferred to be due to seismically triggered fluidization. This novel application of the AMS provides a petrofabric tool for distinguishing passively filled dikes from injection dikes and, where appropriate, for identifying the latter as seismites. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
En Tunisie nord-orientale, l'analyse des donnees de surface (microtectoniques, coupes et logs lithostratigraphiques) et de subsurface (forages petroliers, profils sismiques en onshore/offshore), montre: (i) une variation laterale et en profondeur des series lithostratigraphiques, (ii) une zone tres faillee en subsurface et des bassins associes aux decrochements. Les deformations tectoniques reconnues par les donnees sismiques affectent des zones orientees N45°, N100°-120° et N160°-180°, des plis de direction N45°, des failles inverses et des cisaillements associes a des decrochements N90°-110° dextres et N160°-180° senestres. Les donnees microtectoniques devoilent une dominance des fractures NW-SE, NNE-SSW et NE-SW a ENE-WSW respectivement sur les formations du Valanginien- Tortonien, Aptien et Ypresien, et Aptien, Ypresien et Langhien. Elles ont permis de mettre en evidence plusieurs phases tectoniques: (a) une compression de direction NW-SE pendant l'Eocene, (b) une extension de direction NE-SW a l'Oligocene, (c) une compression de direction NW-SE au Tortonien, (d) une distension NNW-SSE au Messinien et enfin (e) une compression au Pliocene de direction NW-SE. Cette tectonique polyphasee a cree des inversions structurales et a amplifie la tectonique tangentielle pendant la phase atlasique. Les calcaires fractures des formations Abiod (Campanien superieur-Maastrichtien inferieur), Metlaoui (Ypresien), Ain Grab (Langhien inferieur) et du groupe Oum Dhouil (Langhien- Messinien) forment les reservoirs petroliers. La couverture est assuree par les argiles de la formation Souar (Lutetien-Priabonien) et Oum Dhouil. Ces reservoirs sont alimentes par des roches meres du Cretace.
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