Active Spreading Centers Research Articles

Permian hydrothermal deposits in the Mino Terrane, central Japan: Implications for hydrothermal plumes in an ancient ocean basin

Geochemical features of metalliferous deposits, cherts and basalts from the Early Permian Hashikadani Formation in the Mino Terrane, central Japan were studied. The metalliferous deposits occur as thin Fe-rich layers about 5 cm thick and Mn-rich crusts up to 30 cm thick in cherts overlying basalt. The Fe-rich layers contain large amounts of magnetite and are intercalated in the massive chert unit just above the basal basalt. The Mn-rich crusts occur in the bedded chert unit, which always overlies massive cherts, and their dominant Mn-mineral is braunite. These metalliferous deposits show highly fractionated MnO/Fe 2O 3 ∗ (total iron as Fe 2O 3) values and are depleted in such elements as Ni, Co, Cu and Zn; these features indicate their hydrothermal origin. The values of Fe 2O 3/FeO and MnO/Fe 2O 3 ∗ in samples abruptly increase above the lithologic boundary between the massive chert and the bedded chert units. This stratigraphic change of lithofacies and chemical characteristics can be attributed to the precipitation of Fe-oxides prior to Mn-oxides from a hydrothermal solution that was gradually diluted with seawater and became oxic. The formation process of the Permian metalliferous deposits from central Japan can be inferred by analogy with modern hydrothermal deposits recently discovered along active spreading centres.

Attenuation structure beneath the Lau Back Arc Spreading Center from teleseismic S phases

Teleseismic sS and sScS waveforms provide constraints on the attenuation structure above deep seismic rones. We estimate the average Qβ along sS and sScS paths beneath the Lau back arc by comparison with S and ScS waveforms. Q is determined using both time and frequency domain analyses of transverse long period records assuming Q is frequency independent. In the frequency domain technique, spectra from reflected (sS or sScS) and direct (S or sScS) waveforms are analyzed using least squares. The time domain method uses a grid search to determine the attenuation operator that, when convolved with the direct phase, produces the best fit to the reflected phase. In both methods the direct phase is convolved with a crustal response function to simulate the crustal interaction of the reflected phase. Large events subject to source directivity effects and source‐station combinations involving ray propagation down the subducting slab are not used. The resulting attenuation estimates show low Q values which increase with focal depth, suggesting high attenuation in the upper mantle and a rapid decrease in attenuation below 200–300 km. Lateral variations are also indicated with very high attenuation (Q ∼ 20–35) in the upper mantle near the active spreading centers and lower attenuation west of the Lau Ridge. Q‐temperature relationships extrapolated from laboratory experiments suggest that the low Q regions represent zones of asthenospheric partial melting.

Review of past and present geotectonic concepts of eastern indonesia

Review of past and present geotectonic concepts of eastern indonesia

Influence of Shimada Seamount on sediment composition in the eastern tropical North Pacific

Shimada Seamount is a large, young volcanic edifice in the east-central Pacific that is not associated with any active spreading center or known hot spot. The sediments on the abyssal plain surrounding Shimada Seamount consist of pelagic clay with ferromanganese micronodules and zeolites. The pelagic clay is mostly barren of microfossils except for a few occurrences of highly corroded specimens of Radiolaria and diatoms. Eolian terrigenous material is the dominant component of the pelagic clay to a depth of at least 8 m below sea floor, with minor contributions from volcanic debris and hydrothermal and hydrogenous sources. The average amount of basaltic debris is only 0.25%, but concentrations are as high as 10% in some samples. The average hydrothermal component (metalliferous sediment) is 8.8% with a maximum of about 13% at 7.5 m below sea floor in one core. The hydrogenous component, mostly as ferromanganese micronodules, averages 4.1% with a maximum of 5.6%. There is no calcareous biogenic debris and essentially no siliceous biogenic debris. In the past, a decrease in hydrothermal components through time may have been the result of a decrease in relative importance of hydrothermal influences, or an increase in the flux of terrigenous debris transported by the northeast trade winds. Because volcanic activity is still active on Shimada Seamount, or has been in the recent past, the observed increase in relative abundance of terrigenous components probably was the result of increased wind transport and not decreased hydrothermal activity. Shimada Seamount may be an important local source of metalliferous sediment in the eastern equatorial North Pacific, and may have been an even more important source in the past.

Low level determination of the rare earth elements in rocks and minerals by inductively coupled plasma mass spectrometry

Seven well-documented and fresh glassy selvages from ocean floor basalt pillows were analyzed by radiochemical neutron activation analysis for Ag, Au, Bi, Br, Cd, Cs, Ge, In, Ir, Ni, Os, Pd, Rb, Re, Sb, Se, Te, Tl, U and Zn. The samples came from active spreading centers in the Indian and Atlantic Ocean. Glasses from DSDP Leg 24, site 238 (Indian Ocean) have a somewhat peculiar trace element pattern, but this is thought to reflect secondary processes operating at shallow depth, not an anomalous source region in the mantle. Our data rather indicate that heterogeneities in the mantle are confined to the highly incompatible lithophile elements.Chemical fractionations during petrogenesis of tholeiitic basalts are discussed in the light of literature data for primitive peridotitic upper mantle nodules. (Ir, Os), Au, Pd, Ni and Re are strongly fractionated from each other in igneous processes; the unfractionated chondritic mantle pattern thus imposes firm constraints on mantle evolution models. The potentially chalcophile elements Ag, Cd, In and Zn do not behave differently from lithophile elements of the same valency and comparable ionic radius. Residual sulfides are not abundant enough to efficiently control the partitioning of these elements during basalt petrogenesis. However, the poor coherence of Tl to Rb and U in ocean floor basalts could point to retention of Tl by residual sulfides during depletion of the MORB source regions. Sb is strongly depleted in the source regions of ocean ridge basalts; most likely, it was present as a highly incompatible Sb5+ cation. The limited Rb/Cs fractionation in oceanic tholeiites, as opposed to continental tholeiites and acidic rocks, appears to reflect the low abundance of volatile constituents and hydrous silicates in normal ocean ridge basalts.

Composition of back-arc basin volcanics, Valu Fa Ridge, Lau Basin: Evidence for a slab-derived component in their mantle source

Samples dredged from 2 localities near the crest of the Valu Fa ridge, an active back-arc basin spreading centre in the Lau Basin, consist of highly vesicular lava fragments of andesitic composition. The samples are characterized by rare, euhedral An 85 plagioclase phenocrysts in a hypocrystalline groundmass of An 60 plagioclase laths, brown glass and rare subhedral clinopyroxene. Samples from within and, to a lesser extent, between the dredge hauls show remarkable isotopic and chemical homogeneity, with: 87Sr/ 86Sr − 0.70330 ± 2; 143Nd/ 144Nd − 0.51303 ± 2; 206Pb/ 204Pb − 18.65 ± 2; 207Pb/ 204Pb − 15.55 ± 1; 208Pb/ 204Pb − 38.34 ± 4; Sr − 165 ppm; Rb − 7 ppm; Cs − 0.17 ppm; K − 3300 to 4200 ppm; Ba − 96 ppm; and REE — LREE depleted with 12–18 × chondritic abundances. On Sr-Nd, Pb-Pb and Sr-Pb plots the volcanics lie just within or on the edge of the MORB fields, overlapping with island-arc volcanics from the Marianas and Tonga. Compared with MORB and ocean-island basalts, the samples show alkali-element enrichment relative to REE and higher Cs relative to Rb. The isotopic and geochemical characteristics of the Valu Fa Ridge volcanics clearly indicate a minor, but significant, slab-derived component in the back-arc basin mantle source.

Structure and evolution of the Kerguelen-Heard Plateau (Indian Ocean) deduced from seismic stratigraphy studies

The structure and evolution of the Kerguelen-Heard Plateau, the northern domain of the Kerguelen-Gaussberg Ridge, is derived from stratigraphic interpretation of multichannel seismic profiles. The Kerguelen-Heard Plateau was created between 130 and 100 Ma at or near an active spreading center in the gap between the Antarctica-Australia plate and the India plate. From the early Late Cretaceous (about 100 Ma) to the Eocene (42–45 Ma) the Kerguelen-Heard Plateau was a shallow marine structure continuously subsiding at a rate of about 20 m Ma −1 and covered by pelagic and shelf sediments. At 42–45 Ma the Kerguelen-Heard Plateau and Broken Ridge were clearly separated by sea-floor spreading at the Southeast Indian Ridge. From 42–45 Ma to the Miocene, a major gap of sedimentation occurred. Later the Kerguelen-Heard Plateau was covered by pelagic sediments, interbedded with thick clastic sedimentary layers coming essentially from Kerguelen Island.

Landward vergence in accretionary prisms: The role of the backstop and thermal history

The accretionary histories recorded along the convergent margins in southwest Alaska and southwest Japan are punctuated by two anomalous events: near-trench magmatism closely associated in time and space with landward structural vergence. In both areas, the near-trench magmatism is believed to be related to the subduction of an active spreading center. It is postulated that these anomalous thermal events caused the backstop (i.e., mechanical boundary) within the prism to be reoriented to a seaward instead of a landward dip. This seaward-dipping backstop resulted in the development of landward-verging structures, for the only time in the almost 100-m.y. history of these two margins. A modern analog may be the Washington convergent margin. These examples illustrate the potential importance of thermal history and the dip of the backstop in controlling the style of accretion in submarine wedges.

Sources and hydrothermal alteration of organic matter in Quaternary sediments: A synthesis of studies from the Central Gulf of California

Deep sea drilling in the Central Gulf of California, a young and active spreading centre, shows that the high heat flow typical of these regions causes extensive alteration of sediment organic matter, especially near sills and above magma chambers where hydrothermal activity is concentrated. Even on the nearby passive margin, where there are no sills, heat flow is moderately high and hydrocarbon generation has begun in immature sequences. Migrating light hydrocarbons are detected especially where hydrothermal activity is concentrated. Thermogenic methane is more widespread, though not in the passive margin bordering the spreading centre. Despite the incidence of hydrocarbon generation and migration, the amounts of hydrocarbons involved are relatively small and apparently do not lead to commercially significant accumulations of petroleum. The organic matter in these sediments is mostly marine because the Gulf of California generally has low runoff from land and highly productive surface waters. Turbidites rich in terrigenous organic material are locally abundant in the mainly pelagic section in the Guaymas Basin. The highest concentrations of organic matter are found in laminated diatomites deposited on the Guaymas passive margin within the oxygen minimum zone.

The evolution of low-temperature convection cells near spreading centers; a mechanism for the formation of the Galapagos mounds and similar manganese deposits

The Galapagos mounds occur in two fields south of the Galapagos spreading center over crust which is between 500,000 and 900,000 years old. Detailed investigations of composition and chronology of these mounds (e.g., Honnorez et al., 1981; Lalou et al., 1984) have shown that their deposition started in 300,000-year-old sediments with the formation of hydrothermal sediments and ended with the recent formation of manganiferous crust. On the basis of numerical models the flow distribution in hydrothermal convection cells near active spreading centers is studied here as a possible formation mechanism for these and similar deposits.The flow at an active spreading center is dominated by a strong convection cell associated with the intrusion process at the ridge axis. This cell is stationary relative to the ride axis. All the other convection cells which form off-axis because of the heat input from the cooling plate are not stationary but move in general with the moving plate away from the ridge axis. As a consequence of this movement, a transition zone exists between the on-axis and off-axis convection where new off-axis cells are formed once the distance between stationary and moving cells has become too large. The beginning of low-temperature convection occurs thus at some distance from the ridge axis (ca. 10 km) in older crust which has in general a thin sediment cover. This convection can be related to the formation of the hydrothermal sediments in the Galapagos mounds.When the spreading rate is relatively low and the rate of fluid flow in the off-axis cells is high, merging of convection cells in this transition zone can occur. The merging episodes cause strong pulses of low-temperature hydrothermal convection, which are not related to off-axis intrusive events. As a segment of the crust moves through the transition zone, it can be exposed to several of these merging episodes. The evolution of off-axis convection cells near an active spreading center provides thus a mechanism which can cause one or more pulses of short-lived (ca. 10,000 years), low-temperature ( approximately 200 degrees C) convection in a segment of the crust. This activity has the characteristics necessary for the deposition of manganese crust or lenses of massive manganese ore associated with marine sediments. Calculated intervals between onset of convection and merging episodes are in good agreement with observations from the Galapagos mounds. Flow rates associated with the merging episodes are sufficient to explain the formation of sizable Mn lenses such as those found associated with the Appenine ophiolites and the Nicoya ophiolite.

The influence of plate movement on the evolution of hydrothermal convection cells in the oceanic crust

Two types of hydrothermal circulation can be distinguished in the oceanic crust, a very intense convection at spreading centers caused by the intrusion processes at mid-ocean ridges, and a much less vigorous, but more common convection in older, sediment covered crust driven by heat coming from the underlying, cooling plate. We investigated the relation between these two types of convection under normal spreading conditions, i.e. oceanic plate moving away from a stationary spreading center. In particular, we studied the stability of convection cells in a moving plate, the transition between on- and off-axis convection and the temperature distribution in these convection cells. The study is based on numerical calculations using the theory of flow through porous media. Our results show that convection cells associated with the intrusion processes in the accretion zone of a spreading center are stationary with respect to the ridge axis. Convective heat transport in the stationary, on-axis cells is sufficient to remove all the heat released at the ridge axis. The other convection cells, which are not immediately associated with these intrusion processes, are not stationary. Most of them, especially once they are more than 30 km away from the ridge axis, move with the moving plate. This movement occurs regardless of the permeability distribution in the crust. As a consequence, individual segments of the crust are exposed either to continuous upwelling or to continuous downwelling flow once they have left the vicinity of the ridge axis; high water-to-rock ratios can be reached in these long-lived cells in spite of the relatively slow flow velocities. Temperatures in the off-axis cells, even in close proximity to the ridge axis, are low, in general below 100°C. The low-temperature alteration found commonly in the oceanic crust is evidence for the widespread occurrence of these off-axis cells. More specifically, the distinct differences in degree of alteration observed in some closely spaced DSDP cores are in good agreement with the concept of convection cells attached to the moving plate. Because of the movement of the off-axis cells away from the stationary axial cells, a transition zone where new convection cells are formed exists at distances between 5 and 25 km from the spreading center. The formation of a new convection cell is accompanied by a maximum in low-temperature upwelling flow. When the heat transport by the plate and that by fluid convection is of similar magnitude, off-axis cells shift position with respect to the ridge axis and the moving plate. During this stage two cells can merge, which event causes a short (ca. 5000 years) episode of intense upwelling. These episodes, which are not directly related to any intrusive activity, can occur several times in or near a particular segment of the crust. Only during these short upwelling episodes are temperatures up to 200°C reached in the off-axis cells. Chains of hydrothermal mounds near active spreading centers as well as low-temperature hydrothermal deposits such as the Mn lenses commonly associated with ophiolite sequences may be related to the formation of these off-axis convection cells.

Seismicity and active tectonics of the Egyptian Red Sea margin and the northern Red Sea

An active median spreading center has been identified in the Red Sea south of 22°N, but from published reports, the northern Red Sea appears to be essentially aseismic. We have used microearthquake monitoring techniques along the Egyptian Red Sea margin to investigate the active tectonics in the region and have found that the northern Red Sea is not aseismic. Events recorded with epicenters in the Red Sea define an active zone extending south-southeast from the Gulf of Suez into the axial region of the Red Sea down to 25.75°N, with additional microseismicity between 24° and 25°N, suggesting active median spreading in the northern Red Sea. Two areas of intense microearthquake activity have also been identified; the first at the southern end of the Gulf of Suez and the second in the Egyptian Red Sea Hills, clustered at approximately 25.28°N, 34.52°E. The Gulf of Suez seismicity results from the adjustments in motion at the triple junction between the African and Arabian plates and the Sinai subplate. The source of the seismicity in the Egyptian Red Sea Hills is unknown, but a uniform fault plane mechanism is not indicated by first motion studies or the spatial distribution of the hypocenters.

Precious metals in ferromanganese crusts from the south-west Pacific

Little is known about the abundance, distribution and origin of the precious metals in marine ferromanganese materials1–4, although it has long been recognized that elements such as silver and gold are frequently enriched in ferromanganese nodules when compared with average crustal abundance5. Relative to crustal abundance, silver is enriched in nodules5 by a factor of between 50 and 100, and for gold enrichment factors of between 100 and 1,000 have been found3. We know of no similar data in the literature for silver and gold in ferromanganese crusts, although Halbach et al.4 have described enrichment of platinum in crusts from the central Pacific region. Elsewhere, precious metals show significant enrichment, possibly to economic grades, in massive sulphide deposits associated with active spreading centres such as the East Pacific Rise and the Red Sea6–8. We now report an unusual occurrence of silver and gold in ferromanganese crusts from a back-arc setting in the south-west Pacific; to our knowledge, the first report of material of this kind.

A possible explanation of gravity anomalies over mid‐ocean ridges

Gravity fields over the crests of slow spreading mid‐ocean ridges are characterized by small amplitude free air gravity anomaly lows (30–70 mGal) with wavelengths of 40–60 km, flanked by smaller gravity highs. In general, the amplitude and wavelength of these gravity lows decrease with increasing spreading rate until, at fast spreading ridges (> 5 cm/yr), free air gravity highs (10–20 mGal) are observed. Previous explanations of these anomalies, which involve uncompensated ridges, thermally expanded ridges and elastic plate compensation models, are generally not consistent with the geological evidence regarding the nature of the crust and upper mantle beneath the ridge crest. In particular, the postulated presence of wide zones of partial melt in the uppermost mantle is inconsistent with petrologic data from mid‐ocean ridge basalts, which indicates that primary melts are in equilibrium with and extracted from residual mantle at depths of 30 km or more. In addition, similar gravity lows observed over extinct spreading ridges suggest that the anomalies are not mainly due to any dynamic aspect of the accretion process taking place at active spreading centers. Geometrical models of mid‐ocean ridges derived from detailed structural and petrologic studies of the Bay of Islands ophiolite complex have been used to generate gravity anomalies over ridges with various spreading rates. Modeling results show that four main sources serve to define the amplitude and wavelength of the gravity anomaly: the ridge topography, the crustal level magma chamber, the configuration of the isotherms near the ridge crest, and a low‐density, partially gabbroic cumulate root extending vertically into the mantle beneath the ridge. Systematic variations in the contributions from these sources with increasing spreading rate control the changes in character of the gravity anomaly from slow to fast spreading centers. Major contributions are found to come from the topography and from the lowdensity root, which broadens with slower spreading rates, resulting in both increasing amplitude and increasing wavelength with decreasing spreading rate. At extinct spreading centers, contributions from the magma chamber, isotherms, and topography gradually diminish with time once the ridge ceases to spread, whereas the low‐density root remains a permanent part of the oceanic lithosphere and continues its contribution to the gravity field. Seismic evidence from slow spreading centers appears to support the existence of the lowdensity gabbroic root zone inferred from the study of the Bay of Islands ophiolite and the gravity analysis presented here.

Accumulation rates of hydrothermal metalliferous sediments in the Lau Basin, S. W. Pacific

Five cores were recovered on a traverse in the Lau Basin at 18°30′S crossing a supposed active spreading center. The sediments were subjected to selective chemical leaching for Mn, Fe, Cu, Zn, and As. Accumulation rates were determined using14C. These rates increase from west to east, reflecting the influence of volcaniclastic inputs from the Tonga- Kermadec Ridge. All elements display highest non-detrital accumulation rates closest to the supposed spreading center, suggesting a hydrothermal input to the sediments there. Variable hydrothermal inputs also influence the other cores.

Symposium: Role of organisms and organic matter in ore deposition/Le rôle des organismes et de la matière organique dans la formation des gisements métallifères

The following seven papers were presented on May 16, 1984, at the Geological Association of Canada and Mineralogical Association of Canada joint annual meeting. The special session, organized by R. W. Macqueen and J. A. Coope, contained 10 papers and was sponsored by the Mineral Deposits Division of the Geological Association of Canada.Our objective in organizing the special session was to examine organically based processes and relationships that may be of major importance to the origin of ore deposits. As noted by Fyfe (1984), the concept of the geochemical cycle focuses attention on pathways of chemical elements and isotopes of the Earth's system during geologic history. It is clear from the chemistry of carbon-rich materials that a wide range of elements is concentrated directly or indirectly by biological processes operating as part of the geochemical cycle. Two of the papers of the special session examine some of these concentration processes, although definitive links to actual ore deposits cannot ...

Hydrothermal models for the generation of massive sulfide ore deposits

The discovery of massive sulfide ore deposits at certain sites at active seafloor spreading centers provides a basis for considering the energetics of processes that have concentrated similar large ore deposits that have been preserved in volcanic rocks on land. In this paper we construct transport models based on hydrothermal convection systems in the oceanic crust. We investigate models in which the circulation is driven by heat extracted from the permeable crustal rocks themselves as well as by heat conducted through the roof of a replenished, vigorously convecting crustal magma chamber. If the hydrothermal fluids carry 100 ppm dissolved iron as found in black smokers on the East Pacific Rise and exit at 350°C or greater, calculations show that the heat content of the oceanic crust is insufficient to account for ore deposits of 3 Mt or greater. Heat extraction through the roof of a magma chamber, in conjunction with the formation of a thin (∼1 km) layer of plated gabbro, may account for ore bodies of the order of 3 Mt, provided the permeability of the oceanic crust in the magma‐hydrothermal fluid contact zone is 10−15 m2 or less. If the conducting boundary layer were only 1 m thick, locally, for 103years, the permeability could be as high as 10−13 m2 The permeability in the discharge zone could, conceivably, be two orders of magnitude greater than the permeability at depth. More work is needed on the details of the effects of magma chamber replenishment and off‐axis dike injection and on the role of magmatic fluids. Very large ore deposits (i.e., ∼100 Mt) are probably formed in multiple hydrothermal cycles.

Downslope transport of metalliferous sediments along the East Pacific Rise during the late Miocene

The distribution of metalliferous sediments next to active spreading centres has scientific and economic interest1. Although metal-rich waters emanating from active hydrothermal vents have been traced in intermediate level water masses far beyond the ridge crest2, the greatest concentrations of metal oxides in sediments occur near the vents3. There, however, it is conceivable that the oxides may be redistributed and possibly further concentrated by redeposition, resulting in misconceptions of the age and relative timing of hydrothermal pulses. Here, we document microfossil evidence of stratigraphical inversion and redeposition of Upper Miocene (Messinian) sediments cored at Deep Sea Drilling Project (DSDP) Site 599 on the East Pacific Rise (EPR). This suggests that where reworking can be confirmed, care should be taken not to correlate directly the occurrences of the metalliferous sediments with apparent coeval pulses of hydrothermal activity or enhanced sea-floor spreading rates.

The upper mantle beneath the north-east Pacific rim: a comparison with the Gulf of California

Summary. Seismograms from 22 earthquakes along the north-east Pacific rim recorded in southern California form the data set for investigation of the upper mantle beneath the Cascade Range-Juan de Fuca region, a transitional area encompassing both very young ocean floor and a continental margin. These data consist of 853 seismograms (6 < A < 42) which produce 1068 travel times and 40 ray parameter estimates. We compare these data directly to another large suite of records representative of structure beneath the Gulf of California, an active spreading centre. We use the spreading centre model, GCA, as a starting point in WKBJ synthetic seismogram modelling and perturb GCA until the NE Pacific data are matched. Application of wavefield continuation to these two groups of data provides checks on the model's con- sistency with the data as well as an estimate of the resolvability of differences between the two areas. The resulting NE Pacific model, CJF, is very similar to GCA. Disparities in travel times and waveforms from 16 to 22, however, are interpretable in Ierms of lateral structural variations between the two regions from 200 to 410 km depth. CJF features velocities from 200 to 350 km that are inter- mediate between the very low values of GCA and the more moderate values of T7, a model for the inland western United States. Models of continental shield regions have even higher velocities in this depth range, but all four model types are similar below 450 km. This regular pattern observed in the rate of velocity increase with tectonic regime suggests an inverse relationship between velocity gradient and lithospheric age above 400 km depth.

Structure of the Havallah sequence, Golconda allochthon, Nevada: Evidence for prolonged evolution in an accretionary prism

The Golconda allochthon of northern and central Nevada contains the Havallah sequence and correlative units of latest Devonian to early Late Permian age. The Havallah sequence is dominated by radiolarian ribbon chert and argillite associated with variable, but generally subordinate, siliciclastic, cal-carenitic, and volcaniclastic turbidites and slump deposits. Cherts of all ages (except Pennsylvanian?) locally rest depositionally on tholeiitic basalts. Some were altered and mineralized by ridge-type hydrothermal systems, suggesting deposition in an ocean basin containing active spreading center(s) for much of the upper Paleozoic. The Havallah sequence was thrust eastward along the basal Golconda thrust onto coeval sediments of the western North American shelf during the latest Paleozoic-earliest Mesozoic Sonoma orogeny. Detailed studies of the Havallah sequence in the Sonoma and Tobin Ranges and in Battle Mountain indicate the presence of complex diagenetic and structural fabrics that developed prior to the obduction of the allochthon. A large number of thrusts, composite thrusts, and shear zones slice the Havallah into numerous tectonic packets of contrasting lithology and/or internal structural style. Internal structures in chert packets include bedding-parallel and bedding-normal solution cleavage, solution boudins, three or more sets of east-verging folds of variable geometry, and features associated with high pore-fluid pressures such as crack-seal fractures, dilation breccias, clastic dikes and sills, and clastic intrusions along thrust surfaces. The fabrics suggest a general progressive evolution from deformation characterized by bedding-normal compression and slight east-west extension (D1) to deformation dominated by bedding-parallel, east-directed shear (D2). Local chert packets may have suffered alternating episodes of high and low pore-fluid pressure. The degree of development and the styles and orientations of various structural elements vary, sometimes radically, from packet to packet. The pre-Golconda thrust fabrics can be modeled as the result of imbrication and deformation of successive batches of ocean-floor sediments into the toe of an accretionary prism in front of an east-facing arc. Relatively undeformed Permian calcarenite units, interpreted as being trench-slope deposits, suggest that this prism was well developed by Permian time. Coarse slump deposits containing chert clasts with pre-depositional structural fabrics suggest a prism that was active long enough to recycle previously tectonized cherts. If we are correct, the classical Sonoma orogeny (D3) marked the culmination of a protracted structural evolution that may have spanned much of the upper Paleozoic.