Hagai Ron

Long-term earthquake clustering: A 50,000-year paleoseismic record in the Dead Sea Graben

The temporal distribution of earthquakes in the Dead Sea Graben is studied through a 50,000-year paleoseismic record recovered in laminated sediments of the Late Pleistocene Lake Lisan (paleo-Dead Sea). The Lisan represents more than 10 times the 4000 years of historical earthquake records. It is the longest and most complete paleoseismic record along the Dead Sea Transform and possibly the longest continuous record on Earth. It includes unique exposures of seismite beds (earthquake-induced structures) associated with slip events on syndepositional faults. The seismites are layers consisting of mixtures of fragmented and pulverized laminae. The places where the seismites abut syndepositional faults are interpreted as evidence for their formation at the sediment-water interface during slip events on these faults. Thicker sediment accumulation above the seismites in the downthrown blocks indicates that a seismite formed at the water-sediment interface on both sides of the fault scarps. Modern analogs and the association with surface ruptures suggest that each seismite formed during a M L ≥5.5 earthquake. The 230 Th- 234 U ages of a columnar section, obtained by thermal ionization mass spectrometry, give a mean recurrence time of ∼1600 years of M L ≥5.5 earthquakes in the Dead Sea Graben. The earthquakes cluster in ∼10,000-year periods separated by quiet periods of similar length. This distribution implies that a long-term behavior of the Dead Sea Transform should be represented by a mean recurrence of at least 20,000 year record. This observation has ramifications for seismic hazard assessment based on shorter records.

Fault mechanics and the kinematics of block rotations

In many strike-slip tectonic settings, large rotations (up to 100°) of crustal blocks have been inferred from paleomagnetic data. These blocks are bounded by sets of parallel faults, which accommodate the relative motion between the blocks as regional deformation progresses. Simple geometrical considerations require that the faults must also rotate. In this paper we show that on the basis of mechanical considerations, the amount of fault rotation permissible under a stationary stress field is limited to 20° to 45°. Consequently, block rotations that are larger than 40° or 45° require more than one set of accompanying faults to accommodate the block rotation. Examples of such multiple sets with 40° to 45° between them, as predicted by the model, were recognized in Sistan, Iran; in Yerington, the Lake Mead area, Nevada; and in southern California.

SEISMICITY OF THE EASTERN MEDITERRANEAN REGION : PERSPECTIVE FROM THE SINAI SUBPLATE

We conducted a comprehensive study of the seismicity of the eastern Mediterranean and examined its relation to the regional plate tectonics. Based on various published catalogs, we have constructed a new list of ML ≥ 4 earthquakes which were recorded during the years 1900–1991 and represents most of the seismic moment released in the area over this period. b-values are 1.02, 1.0 and 1.07 for the whole Sinai subplate, the Dead Sea transform and the Cypriot arc, respectively. Seismic efficiency of the Dead Sea transform is very low (about 7%!), stressing the role of aseismic deformation in that plate border. Most of the major and moderate events, ML ≥ 5, occur in belts associated with the geologically documented borders of the Sinai subplate: the Cypriot convergent arc in the north, the Dead Sea transform in the east and the rift of Suez in the southwest (the latter, like the Gulf of Eilat (Aqaba), was considered aseismic during the first half of the century). The northwestern border, however, could not be delineated. Low-level activity appears within the subplate, especially north of latitude 32°N, reflecting the breakdown of this part of Sinai as it approaches the Cypriot convergence zone. Though most of the seismic moments tend to concentrate along the subplate borders, some moderate activity spreads out in wide belts which reflect the complexity of deformation accompanying motion along the subplate borders.

Radiometric dating of the Earlier Stone Age sequence in Excavation I at Wonderwerk Cave, South Africa: preliminary results

We present here the results of 44 paleomagnetic measurements, and single cosmogenic burial and optically stimulated luminescence ages for the Earlier Stone Age deposits from Wonderwerk Cave, Northern Cape, South Africa. The resulting paleomagnetic sequence: N>R>N>R>N constrains the Earlier Stone Age strata in this part of the site to between approximately 0.78-1.96 Ma. A single cosmogenic date of approximately 2.0 Ma from the base of the section offers some corroboration for the paleomagnetic sequence. Preliminary results indicate that the small lithic assemblage from the basal stratum may contain an Oldowan facies. This is overlain by several strata containing Acheulean industries. The preliminary radiometric dates reported here place the onset of the Acheulean at this site to approximately 1.6 Ma, which is roughly contemporaneous with that of East Africa.

The Nature of the Landers-Mojave Earthquake Line

The Landers, California, earthquake of 28 June 1992 (magnitude = 7.3) is the latest of six significant earthquakes in the past 60 years whose epicenters and slip directions define a 100-kilometer alignment running approximately N15�W across the central Mojave region. This pattern may indicate a geologically young throughgoing fault that replaces numerous older strike-slip faults by obliquely cutting across them. These older faults, and perhaps also the bend in the San Andreas fault, may be losing their ability to accommodate upper crustal deformation because they have become unfavorably oriented with respect to the regional stress field.

Geometric changes of plate boundaries along part of the northern Dead Sea Transform: Geochronologic and paleomagnetic evidence

The Korazim block, an elevated pressure-ridge, along with the faults bounding the Sea of Galilee and Hula pull-apart basins are internal structures of the Dead Sea Transform plate boundary. K-Ar dates and paleomagnetic data from basalt flows and sedimentary strata are utilized to determine the stratigraphy and the deformation of the region. The integration of the two methods, together with structural data, yield high-resolution age determination of the stratigraphic sequence and provides insight into the timing and style of deformation. The Korazim block is covered in its southern and central areas by Pliocene basalt flows (the Cover Basalt, 5.1–3.6 Ma, and Ruman Basalt, 2.9–2.2 Ma, formations) and in its northern area by basalts (Yarda Basalt, 0.9–0.8 Ma) and sediments (3.0–2.3 and 1.6–0.9 Ma) of Plio-Pleistocene age. The magnetic polarity of a total of 29 lava flows and intercalated sedimentary strata is mixed; the mean magnetic declination of the 29 sites shows an anomalous direction which yields 11.4° ± 4.0° counterclockwise rotation with respect to the expected field. The mean inclination of 46.0° ± 4.1° does not significantly deviate from the expected field. The anomalous declination is interpreted as the consequence of faulting and block rotation accommodating internal deformation. This deformation did not begin before late Pliocene to early Pleistocene, probably not before 0.9 Ma, and may still be active. Simultaneous right-lateral slip (of 50–175 m) with a small reverse component (of 30–90 m) along NW trending faults and 11° counterclockwise rotation created internal deformation of the Korazim pressure ridge. This deformation is the consequence of left-lateral slip along the Almagor strand of the Dead Sea transform, subparallel to the plate slip vector. Thus a compression component was formed which presumably led to the internal deformation of the Korazim block. The recent deformation of the Korazim block in relation to the age of adjacent structures and to the age of the Dead Sea Transform permits temporal resolution of geometric changes which must typify transform boundaries such as the Dead Sea Transform. Internal structures within transforms may change, along with the transform internal geometry, and become inactive, while new structural features may develop.

Strike-slip faulting and block rotation in the Lake Mead fault system

Strike-slip faults in the Basin and Range province have often been considered passive boundaries between differentially extended domains of tilted normal faults and are thus considered secondary in accommodating regional horizontal deformation. Paleomagnotic investigation of late Miocene age volcanic rocks, displaced by the left-lateral fault system of Lake Mead, Nevada, shows: (1) that these rocks have not been affected by significant structural tilt, the difference between observed and expected inclinations being only −0.6° ± 14.9° and (2) a significant horizontal counterclockwise rotation of −29.4° ± 8.5° about a vertical axis. This rotation was accommodated by slip on northwest-trending, right-lateral strike-slip faults; this implies significant west-northwest elongation. Results of the investigation indicate that strike-slip faulting is the primary process accommodating crustal deformation along the Lake Mead fault system and that tilting in response to normal faulting is secondary.

Reconstructing the history of sediment deposition in caves: A case study from Wonderwerk Cave, South Africa

We applied cosmogenic isotope burial dating, magnetostratigraphy, and grain-size distribution analysis to elucidate the history of the sedimentary sequence, composed of fine quartz sands and silts, of Wonderwerk Cave, located on the southern edge of the Kalahari Desert, South Africa. The source for the quartz sand is the Kalahari sand dunes, presently located ∼100 km to the north of the cave. Field observations and grain-size analysis suggest a sediment transport scenario that includes eolian transport of Kalahari sand, abraded to a size of 70–100 µm, to the Kuruman Hills, temporary storage on the hill slopes and valleys surrounding Wonderwerk Cave, and later transport and deposition inside the cave. Our results suggest simple burial ages for sediments from both the front and back of the cave that range between 2.63 ± 0.17 Ma and 1.56 ± 0.10 Ma following initial exposure of 310–620 k.y. However, 26 Al/ 10 Be ratios of 3.98 ± 0.24 and 4.08 ± 0.22 measured in a sand sample collected from the surface outside the cave may imply an initial burial signal equivalent to 0.78 ± 0.15 Ma, thus reducing the possible age range of the buried samples to between 1.85 ± 0.23 and 0.78 ± 0.18 Ma. The paleomagnetic results for the front of the cave gave a polarity sequence of N > R > N||N, where N indicates normal polarity, and R indicates reverse polarity. This sequence can be correlated with both the older and younger cosmogenic burial age ranges. The correlation suggests that in the cave front, cosmogenic burial ages and the acquisition of stable remanent magnetization were not significantly affected by chemical and physical processes and that postburial production of cosmogenic isotopes was insignificant. In contrast, at the back of the cave, the paleomagnetic polarity sequence of R > N cannot be correlated with the cosmogenic burial ages, since the temporal gap between the initial penetration of the sediment into the cave and the final acquisition of a stable remanent magnetization may have been long (∼10 5 yr), and the single polarity transition can be correlated to any reverse-normal transition that occurred during the Quaternary. This highlights the need for caution when cosmogenic burial ages and paleomagnetic sequences are compared. The buried sediments in Wonderwerk Cave show similar grain-size distributions to the fine sand sediment presently exposed at the surface in the vicinity of the cave. Furthermore, calculated preburial 10 Be concentrations for the buried sediment are similar to those measured in sediment outside the cave. These similarities suggest that the environmental conditions and rates of geomorphic processes that persisted during sand deposition in Wonderwerk Cave during the late Pliocene and early Pleistocene may have been similar to those currently experienced in the southern Kalahari, the Kuruman Hills, and the western Ghaap Plain. These conditions favor the transport of fine-grained quartz sand to the vicinity of the cave.

Magnetostratigraphy of the Evron Member--implications for the age of the Middle Acheulian site of Evron Quarry.

The Institute of Earth Sciences, The Hebrew University, Jerusalem 91904, Israel The Geophysical Institute of Israel, P.O.Box 182, Lod 71100, Israel The Geological Survey of Israel, 30 Malkhe Israel St., Jerusalem 95501, Israel The Zinman Institute of Archaeology, The University of Haifa, Haifa 31095, Israel Department of Evolution, Systematics and Ecology, The Hebrew University, Jerusalem 91904, Israel

HIGH-RESOLUTION RECORD OF GEOMAGNETIC SECULAR VARIATION FROM LATE PLEISTOCENE LAKE LISAN SEDIMENTS (PALEO DEAD SEA)

We measured geomagnetic secular variation in Lake Lisan sediments (paleo Dead Sea). More than 1500 oriented samples were collected from a 27.3-m section of alternating aragonite and detritus laminae in the Dead Sea basin ranging in age from 67 to 32 ka. The natural remanent magnetization (NRM) is carried by titanomagnetite in the detrital laminae whereas the aragonite is diamagnetic. The NRM is very stable and was acquired several hundred years after deposition. The mean direction of 878 horizons is DD 005o, ID 45o (95D 1o;D 22). We observed three modes of directional geomagnetic variation as a function of (and by inference, time): very rapid inter-sample changes, slow variation in mean direction, and inclination shallowing of about 1o=m. The overall rate of change in direction is 0:57 0:57o=year, not significantly different from zero. For about 83% of the record the rate of change is less than 1o=year and comparable to historical values. High rates of change are observed more frequently in the Lisan than in historical records, and peak rates are up to ten times faster. A smoothed curve resulting in a maximum rate of change of 0.66o=year and a mean 0:10 0:10o=year may be a more realistic representation of the field behavior. No reverse NRMs were observed, but geomagnetic field excursions may be present where the VGPs deviate by more than 40o from the geographic north at about 52 and 41 ka; the latter may represent the Laschamp event.