Spatial analysis of extension fracture systems: A process modeling approach

Little consensus exists on how best to analyze natural fracture spacings and their sequences. Field measurements and analyses published in geotechnical literature imply fracture processes radically different from those assumed by theoretical structural geologists. The approach adopted in this paper recognizes that disruption of rock layers by layer-parallel extension results in two spacing distributions, one representing layer-fragment lengths and another separation distances between fragments. These two distributions and their sequences reflect mechanics and history of fracture and separation. Such distributions and sequences, represented by a 2 × nmatrix of lengths L,can be analyzed using a method that is history sensitive and which yields also a scalar estimate of bulk extension, e(L).The method is illustrated by a series of Monte Carlo experiments representing a variety of fracture-and-separation processes, each with distinct implications for extension history. Resulting distributions of e(L) are process-specific, suggesting that the inverse problem of deducing fracture-and-separation history from final structure may be tractable.     

Research Note: Overpressure prediction from C‐wave seismic data

Recently, mode converted shear waves (C‐waves) have been shown to enable overpressure prediction in media where primary wave acquisition is inhibited by gas and fluid effects – C‐wave moveout is analysed and a long standing relationship between differential stress and primary‐wave (P‐wave) velocity is modified and employed. Though pore‐pressure prediction based on C‐waves is supported by empirical evidence from laboratory and field experiments, a theoretical justification has yet to be developed. In this research note, we provide a supporting algebra for the original relationship between pore pressure and C‐wave velocity.     

Cleavage strain in the Variscan fold belt,County Cork,Ireland, estimated from stretched arsenopyrite rosettes

In south-west Ireland, hydrothermally formed arsenopyrite crystals in a Devonian mudstone have responded to Variscan deformation by brittle extension fracture and fragment separation. The interfragment gaps and terminal extension zones of each crystal are infilled with fibrous quartz. Stretches within the cleavage plane have been calculated by the various methods available, most of which can be modified to incorporate terminal extension zones. The Strain Reversal Method is the most accurate currently available but still gives a minimum estimate of the overall strain. The more direct Hossain method, which gives only slightly lower estimates with this data, is more practical for field use. A strain ellipse can be estimated from each crystal rosette composed of three laths (assuming the original interlimb angles were all 60°) and, because actual rather than relative stretches are estimated, this provides a lower bound to the area increase in the plane of cleavage. Based on the average of our calculated strain ellipses this area increase is at least 114% and implies an average shortening across the cleavage of at least 53%. However, several lines of evidence suggest that the cleavage deformation was more intense and more oblate than that calculated, and we argue that a 300% area increase in the cleavage plane and 75% shortening across the cleavage are more realistic estimates of the true strain. Furthermore, the along-strike elongation indicated is at least 80%, which may be regionally significant. Estimates of orogenic contraction derived from balanced section construction should therefore take into account the possibility of a substantial strike elongation, and tectonic models that can accommodate such elongations need to be developed.     

Ground‐penetrating radar and sedimentological analysis of Holocene floodplains: Insight from the Tuross valley,New South Wales

Ground‐penetrating radar (GPR) has been used on an array of floodplain types on the lower Tuross River, in southeastern New South Wales, as part of an investigation into controls on channel‐floodplain relationships. Ground‐penetrating radar transects from two floodplains are presented, along with sedimentological detail from trenches dug along the profiles at key locations. Sedimentological investigations showed that 100 MHz antenna gave an approximation of overall bedding trends in the upper 3 m when automatic gain control processing was used. Spreading and exponential compensation processing provided insight into textural changes associated with increased silt content distal of the levee crest. One trench showed that thinning beds were responsible for onlapping reflectors. Signal attenuation at ～4 m depth below the raised floodplain surface resulted from a >50 cm‐thick bed of sandy clay. The close integration of GPR and sedimentological data produced an excellent dataset, that enabled form‐process associations and floodplain evolution to be established for these sandy floodplains. However, accurate subsurface assessment and interpretation must stem from carefully combined GPR and sedimentological datasets.     

Appraisal of the new occurrence of götzenitess, khibinskite and apophyllite in kalsilite-bearing lavas from San Venanzo and Cupaello (Umbria), Italy

Minerals corresponding in chemical composition to probable götzenite solid solutions, götzenite_ss, ideally (Ca, Na)₃ (Ti, Zr) (Si₂O₇) (F, O, OH)₂, khibinskite (K₄Zr₂Si₄O₁₄) and apophyllite (KCa₄Si₈O₂₀(OH, F)) were first reported in an aphyric, glass-bearing kalsilitite from Cupaello (Umbria), Italy, by Cundari and Ferguson (1991). To the authors knowledge, this occurrence of khibinskite is only the second recorded. Götzenite_ss was also found in a leucite-, melilite-rich pegmatoid in an olivine-melilitite from San Venanzo, S. Venanzo, in the same region.

The S. Venanzo phase, with significant Zr (0.18 atoms per formula unit) is transitional to rosenbuschite, which is the Zr-rich end-member of the isostructural series götzenite-rosenbuschite, while that from Cupaello, virtually Zr free, is close to the ideal götzenite end-member. The S. Venanzo phase crystallized as the preferred Zr phase from the relatively low-Zr (Zr=335 ppm) S. Venanzo lava. This probably inhibited khibinskite crystallization in the S. Venanzo lava, whereas khibinskite is the preferred Zr phase in the Cupaello lava (Zr=760 ppm), coexisting with low-Zr götzenite_ss.

The structural formula of the Cupaello khibinskite, based on 10 cations, yielded: K_3.89[Zr_1.99Mg_0.01Fe_0.04²⁺Ti_0.02]_2.06Si_4.05O_14.1 which is very close to the composition first reported by Khomiakov et al. (1974).

Apophyllite, also occurring in the Cupaello assemblages and in cavities in the lava, yielded a composition close to the ideal formula and supports the important role of alkali-rich fluids in the crystallization history of this lava.

The rare records of these minerals reflect, at least in part, the difficulty in identifying them. Their role as potential acceptors of both large ion lithophile and high field strength elements in peralkaline rock compositions is important in understanding the geochemistry of lamproitic rocks and in detecting possible genetic links with related rocks.     

A laboratory and field comparison of sediment polycyclic aromatic hydrocarbon bioaccumulation by the cosmopolitan estuarine polychaete Streblospio benedicti (Webster)

Polycyclic aromatic hydrocarbon (PAH) bioaccumulation by the polychaete worm Streblospio benedicti (Webster) was measured under exposure to PAH-contaminated sediments in the field and for 28 d in the laboratory. Streblospio benedicti collected from field sediments contaminated at 2.94, 1.07, and l.52 μg g⁻¹ fluoranthene (FLU), benz[a]anthracene (BAA), and benzo[a]pyrene (BAP), bioaccumulated those PAHs at 1.53, 0.215, and 0.332 μg g⁻¹, while worms isolated from less contaminated field sediments (0.399, 0.228, 0.288 μg g⁻¹ FLU, BAA, and BAP) had FLU, BAA, and BAP body burdens of 0.543, 0.236, and 0.083 μg g⁻¹. Worms incubated for 28 d in PAH-spiked sediments (1.52, 0.991, 0.504 μg g⁻¹ FLU, BAA, and BAP) bioaccumulated those PAHs at 0.382, 0.966, and 0.602 μg g⁻¹, respectively. Data normalization to organism lipid and sediment organic carbon (biota-sediment accumulation factors [BSAFs] strongly suggest that Streblospio PAH bioaccumulation was directly related to percent sediment organic carbon, but BSAFs were substantially lower than predicted by equilibrium partitioning theory. BSAFs decreased with increasing PAH log K_ow, in worms collected from field sediments, but in spiked sediments BSAFs increased with increasing PAH hydrophobicity. This disparity may have been caused by insufficient spiked-sediment equilibration time (1.5h) in the case of the laboratory test sediments.     

An Early Pliocene lake and its surrounding vegetation in Zhejiang,East China

The palynomorph composition of an Early Pliocene assemblage from Du’ao Lake, Zhejiang Province, East China, including sporomorphs and algae, was analyzed to reconstruct the vegetation and climate around the lake, as well as the environmental conditions in the lake. A subtropical evergreen and deciduous broad-leaved mixed forest surrounding the lake is inferred from the pollen data. The composition of the green algae community indicates a clear, shallow (about 5–6 m deep), mesotrophic freshwater lake. The inferred pH was about 7.0–8.0 during the algae growing season. Applying the Coexistence Approach, the climatic conditions in Early Pliocene Du’ao were: (1) mean annual temperature ranged from 18.1 to 22.0°C, (2) difference in temperature between the coldest and warmest months ranged from 14.2 to 15.1°C, (3) mean temperature of the coldest month varied from 10.7 to 12.1°C, (4) mean temperature of the warmest month ranged from 23.5 to 25.4°C, (5) mean annual precipitation varied from about 994 to 1,255 mm, (6) minimum monthly precipitation ranged from about 9 to 11 mm, and (7) maximum monthly precipitation varied from approximately 219 to 245 mm. These values indicate that the Early Pliocene climate was subtropical.