Holocene displacement along branch and secondary faults in the San Andreas fault zone, southern California

Detailed geologic mapping of the San Andreas fault zone in Los Angeles County since 1972 has revealed evidence for diverse histories of displacement on branch and secondary faults near Palmdale. The main trace of the San Andreas fault is well defined by a variety of physiographic features. The geologic record supports the concept of many kilometers of lateral displacement on the main trace and on some secondary faults, especially when dealing with pre-Quaternary rocks. However, the distribution of upper Pleistocene rocks along branch and secondary faults suggests a strong vertical component of displacement and, in many locations, Holocene displacement appears to be primarily vertical. The most recent movement on many secondary and some branch faults has been either high-angle (reverse and normal) or thrust. This is in contrast to the abundant evidence for lateral movement seen along the main San Andreas fault. We suggest that this change in the sense of displacement is more common than has been previously recognized.The branch and secondary faults described here have geomorphic features along them that are as fresh as similar features visible along the most recent trace of the San Andreas fault. From this we infer that surface rupture occurred on these faults in 1857, as it did on the main San Andreas fault. Branch faults commonly form “Riedel” and “thrust” shear configurations adjacent to the main San Andreas fault and affect a zone less than a few hundred meters wide. Holocene and upper Pleistocene deposits have been repeatedly offset along faults that also separate contrasting older rocks. Secondary faults are located up to 1500 m on either side of the San Andreas fault and trend subparallel to it. Moreover, our mapping indicates that some portions of these secondary faults appear to have been “inactive” throughout much of Quaternary time, even though Holocene and upper Pleistocene deposits have been repeatedly offset along other parts of these same faults. For example, near 37th Street E. and Barrel Springs Road, a limited stretch of the Nadeau fault has a very fresh normal scarp, in one place as much as 3 m high, which breaks upper Pleistocene or Holocene deposits. This scarp has two bevelled surfaces, the upper surface sloping significantly less than the lower, suggesting at least two periods of recent movement. Other exposures along this fault show undisturbed Quaternary deposits overlying the fault. The Cemetery and Little Rock faults also exhibit selected reactivation of isolated segments separated by “inactive” stretches.Activity on branch and secondary faults, as outlined above, is presumed to be the result of sympathetic movement on limited segments of older faults in response to major movement on the San Andreas fault. The recognition that Holocene activity is possible on faults where much of the evidence suggests prolonged inactivity emphasizes the need for regional, as well as detailed site studies to evaluate adequately the hazard of any fault trace in a major fault zone. Similar problems may be encountered when geodetic or other studies, Which depend on stable sites, are conducted in the vicinity of major faults.     

Classification of Streaky Bay,Mangalo, Ethiudna and Crockers Well: Stony meteorites from South Australia

Abstract— Four meteorites from South Australia are described and classified. Streaky Bay (L4b) and Mangalo (L6e) were received by the South Australian Museum in December, 1989. Ethiudna (L4a) and Crockers Well (LL7) were previously thought to be paired and were previously described only as chondrites.     

Experimental petrology of the basaltic shergottite Yamato‐980459: Implications for the thermal structure of the Martian mantle

Abstract— The Martian meteorite Yamato (Y‐) 980459 is an olivine‐phyric shergottite. It has a very primitive character and may be a primary melt of the Martian mantle. We have conducted crystallization experiments on a synthetic Y‐980459 composition at Martian upper mantle conditions in order to test the primary mantle melt hypothesis. Results of these experiments indicate that the cores of the olivine megacrysts in Y‐980459 are in equilibrium with a melt of bulk rock composition, suggesting that these megacrysts are in fact phenocrysts that grew from a magma of the bulk rock composition. Multiple saturation of the melt with olivine and a low‐calcium pyroxene occurs at approximately 12 ± 0.5 kbar and 1540 ± 10°C, suggesting that the meteorite represents a primary melt that separated from its mantle source at a depth of ?100 km. Several lines of evidence suggest that the Y‐980459 source underwent extensive melting prior to and/or during the magmatic event that produced the Y‐980459 parent magma. When factored into convective models of the Martian interior, the high temperature indicated for the upper Martian mantle and possibly high melt fraction for the Y‐980459 magmatic event suggests a significantly higher temperature at the core‐mantle boundary than previously estimated.     

Hot springs,thermal springs and warm springs. What's the difference?

Springs rising with water temperatures well above ambient have a fascination borne out of curiosity, spectacle and sometimes a need to understand the origins of the groundwater and its composition. While hot springs bring to mind boiling waters, explosive emissions of steam and volcanism, many exit temperatures are far lower than the boiling point of water, yet remain elevated above the mean air temperature or those of other springs in the surrounding country. This has led to some confusion in naming these springs. When does a hot spring cease to be hot?     

The Balderton Sand and Gravel: Pre-Ipswichian cold stage fluvial deposits near Lincoln,England

The Balderton Terrace marks a former course of the River Trent between Newark and the Lincoln Gap. The principal deposit, the Balderton Sand and Gravel, is interpreted as a braided river sediment. Ice wedge casts truncated by intraformational erosion surfaces at many levels indicate syndepositional permafrost. Remnant cover deposits overlying the Balderton Sand and Gravel include the partly aeolian Whisby Sand. Locally, both the upper part of the Balderton Sand and Gravel and the cover deposits exhibit features indicative of temperate climate pedogenesis. All these deposits are affected by subsequent cryoturbation. On the basis of these features and the geomorphological and topographical relationship to other terrace deposits of the area, the Balderton Sand and Gravel and Whisby Sand are regarded as post-Hoxnian and pre-lpswichian, i.e. Wolstonian. Electron spin resonance age determinations for fossil elephant teeth and amino acid analyses on molluscs from the Balderton Sand and Gravel suggest correlation with Oxygen Isotope Stage 6. The Balderton Sand and Gravel has yielded a cold-climate mammalian fauna dominated by woolly mammoth and woolly rhinoceros, though rarer species suggest periods of milder climate. Silts from channels near the base of the deposit have produced pollen, mollusc, ostracod and beetle assemblages also indicating a cold climate.     

The critical inclination problem: A simple treatment

A short analysis is presented in the hope of clarifying the situation.