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.     

A normalizing procedure for comparing chemical variations in pyroxenes in mafic granulites

Data from analyses of three coexisting pairs of pyroxenes with a wide range of Fe content from each of two localities are used to show the large systematic variation and predictable correlation of Fe (or Mg) of a pyroxene with its content of Al, Mn and Na in mafic granulites. Comparisons of pyroxenes can then be made more meaningful by normalizing Al, or other elements, to an appropriate Mg value. As both P and T may affect the element distribution of the two pyroxenes differently (especially Al and Na) the factor used in normalizing is found to vary from region to region.     

Aluminium in coexisting pyroxenes as a sensitive indicator of changes in metamorphic grade within the Mafic Granulite Terrane of the Fraser Range,Western Australia

Three pairs of coexisting pyroxenes of mafic granulites from each of two locations 100 km apart show large chemical differences, especially in Al, Fe, Mn, Ti and Na. Al content of the pyroxenes at the higher pressure locality is more nearly independent of Al of the host rock than are the pyroxenes from the lower pressure locality. All the data confirm that although no significant difference in temperature has emerged, there was a large difference in pressure between the two localities. Al is found to be a more effective discriminant of metamorphic conditions than . As the three pairs of pyroxenes cover a wide range of Fe at each locality, the close relationship of Al and Na (and of Ca-tschermak and jadeite) to Fe becomes evident. This shows that a normalizing procedure should be adopted before comparing localities with different Al, Mn, Ti, Na and other elements or derived components such as jadeite and Ca-tschermak.     

Trace-element distribution and ore formation in vein-metasomatised peridotite at Kalskaret,near Tafjord,South Norway

The concentration profiles of the trace elements, S, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Sr, and Y have been determined across a metasomatic vein in peridotite. The introduced elements Ti, V, Sr, and Y show specific enrichment in particular silicate phases in accordance with the availability of suitable lattice sites. In contrast, the other introduced trace elements (Cu and S) behave more like the redistributed elements, Cr, Ni, Mn, and Co which do not show concentration ‘fronts’ that can be simply related to the silicate minerals. Concentration of pentlandite, chalcopyrite, and Cr-magnetite near the boundary between the enstatite and anthophyllite zones gives rise to maxima in the Ni, Cu, S, and Cr distributions, while in the chlorite zone significant concentrations of Cr and Ni occur in the chlorite itself. Control of the distribution of Ni, Cu, and Cr is ascribed to the oxidation/reduction reactions involved in the formation of pentlandite, chalcopyrite and Cr-magnetite, together with the critical role of Al in limiting chlorite formation during metasomatism.     

Aspects of the population dynamics of the polychaeteSabellaria vulgaris Verrill,in the Delaware Bay

Aspects of the population dynamics of the polychaeteSabellaria vulgaris Verrill 1874 were studied by observing the temporal occurrence of larvae in the plankton of Delaware Bay. Vertical plankton samples were collected monthly from July 1970 to October 1971. Four 25-hour plankton studies were conducted within this time period, and on one occasion samples were collected on a transect across the mouth of the bay.Sabellaria vulgaris larvae were present in the bay only from mid-April through October. July (1970) and August (1971) 25-hour plankton studies showed larvae present in the water column at virtually all times of the day and night. Horizontal dispersion of larvae in the plankton was clearly aggregated. However, correlation of larval presence, absence or abundance with the measured physical factors in the estuary was not apparent except on a seasonal scale. Six developmental stages were defined based upon observation of laboratory-reared larvae. Young larvae appeared in the plankton on numerous occasions, indicating that spawning occurred repeatedly during the April–October time period in Delaware Bay. Relative to other habitats within the geographic range ofS. vulgaris, Delaware Bay is a particularly well-suited environment for the construction of massive colonies by the species. Adults living in large aggregates would exhibit greater fitness because of the higher probability of eggs being fertilized. Indications are that a portion of the larvae produced in the Bay are retained in the estuary and undergo settlement there. Delaware Bay may be a population center for the species. A comparison of reproductive phenomena among sabellariid species is presented. It is apparent that the species,S. vulgaris, consists of several physiologically distinct populations, and that this is true of certain other sabellariid species as well.     

Variations in the content and distribution of n-alkanes in a series of carboniferous vitrinites and sporinites of bituminous rank

n-Alkanes in the soluble organic matter extracted from a series of vitrinite and sporinite concentrates have been analysed by gas chromatography. The macerals were isolated from coals ranging in rank from 77.1% to 86.6% carbon (vitrinite: dry, ash-free), and yields of n-alkanes ranged from 10 to 580 ppm for vitrinites and from 20 to 970 ppm for sporinites. The maximum yields were found at a rank of 85.4% C from vitrinites and 86.6% C from sporinites.Distribution maxima of the n-alkanes, as shown by gas chromatography, range from C₂₇ and C₂₉ at lower ranks to as low as C₁₆ at higher ranks. The distributions also show a progressive decrease in the preference of odd-carbon-number homologues with increasing rank. Virtually smooth distributions were attained in high-volatile bituminous A coals. Quantitative data show that the loss of the odd-carbon-number preference occurred, for the most part, while individual long-chain homologues increased in concentration.There is a progressive increase in the amounts of shorter-chain n-alkanes with increasing rank. It is suggested that sequential processes may have occurred whereby the rate of formation of long-chain n-alkanes in high-volatile bituminous A rank macerals becomes slower than their rate of subsequent fragmentation to shorter chain lengths. Consequently, assuming derivation from the insoluble maceral matrices, the chain-length distributions of parent n-alkyl structures within the insoluble material may retain characteristics pertaining more to the nature of the source organic matter at the time of deposition than do the extractable n-alkane patterns, especially at higher ranks.     

Geochronology of Precambrian rocks in the Laramie Range and implications for the tectonic framework of Precambrian southern Wyoming

From Casper Mountain; at its northern end, to the northwestern margin of the Laramie anorthosite—syenite complex, in its central parts, the Laramie Range is underlain by granite and granitic gneiss that has a minimum age of 2.54 ± 0.04 Ga (Rb/Sr whole-rock isochron) and by metasedimentary rocks, including marble and quartzite, that appear to overlie the granitic gneiss nonconformably (minimum age: 1.7 Ga based on several horn-blende K/Ar dates). Southward from the anorthosite—syenite complex into Colorado, the Range is underlain chiefly by the Sherman Granite (1.41 Ga; Peterman and Hedge, 1968) and scattered patches of gneiss that are not dated, but are tentatively correlated wit similar gneiss in the southern Medicine Bow Mountains and in the Colorado Front Range, where they are dated as ? 1.7 Ga (Peterman and Hedge, 1968).The Laramie anorthosite—syenite complex (minimum age: ? 1.42 Ga or ? 1.51 Ga if a hornblende K/Ar date is accepted) apparently intruded the suture separating the old (? 2.5 Ga) continental edge from younger (? 1.7 Ga) geosynclinal rocks. The suture, which manifests itself as the Mullen Creek—Nash Fork shear zone in the Medicine Bow Mountains, also is the boundary between ensialic and ensimatic geosynclinal deposition that occurred during the interval 1.7–2.5 Ga ago.K/Ar dates on biotite and muscovite from rocks north of the anorthosite—syenite complex grade from 2.5 Ga on Casper Mountain down to 1.38 Ga near the complex. Near its northern tip, the Laramie Range is crossed by a geochronologic front, separating 2.5 Ga old gneiss whose K/Ar dates were not lowered by subsequent metamorphism from 2.5 Ga old gneiss whose mica dates were reset between 1.4 and 1.6 Ga ago.