Charles Lewis

The planet beyond the plume hypothesis

Acceptance of the theory of plate tectonics was accompanied by the rise of the mantle plumerhotspot concept which has come to dominate geodynamics from its use both as an explanation for the origin of intraplate volcanism and as a reference frame for plate motions. However, even with a large degree of flexibility permitted in plume composition, temperature, size, and depth of origin, adoption of any limited number of hotspots means the plume model cannot account for all occurrences of the type of volcanism it was devised to explain. While scientific protocol would normally demand that an alternative explanation be sought, there have been few challenges to plume theory on account of a series of intricate controls set up by the plume model which makes plumes seem to be an essential feature of the Earth. The hotspot frame acts not only as a reference but also controls plate tectonics. Accommodating plumes relegates mantle convection to a weak, sluggish effect such that basal drag appears as a minor, resisting force, with plates having to move themselves by boundary forces and continents having to be rifted by plumes. Correspondingly, the geochemical evolution of the mantle is controlled by the requirement to isolate subducted crust into plume sources which limits potential buffers on the composition of the MORB-source to plume- or lower mantle material. Crustal growth and Precambrian tectonics are controlled by interpreta- tions of greenstone belts as oceanic plateaus generated by plumes. Challenges to any aspect of the plume model are thus liable to be dismissed unless a counter explanation is offered across the geodynamic spectrum influenced by plume theory. Nonetheless, an alternative synthesis can be made based on longstanding petrological evidence for derivation of . intraplate volcanism from volatile-bearing sources wetspots in conjunction with concepts dismissed for being incompatible or superfluous to plume theory. In the alternative Earth, the sources for intraplate volcanism evolve from the source residues of arc volcanism located along sutures in the continental mantle. Continental rifting and the lateral distribution of intraplate sources in the asthenosphere are controlled by Earth rotation. Shear induced on the base of the asthenosphere from the mesosphere as the Earth rotates is transmitted to the lithosphere as basal drag. Attenuation of the drag due to the low viscosity of the asthenosphere, in conjunction with plate motions from boundary forces, results in a rotation differential of up to 5c m yr y1 between the lithosphere and mesosphere manifest as westward plate lagreastward mantle flow. Continental rifting results from basal drag supplemented by local convection induced by lithospheric architecture. Large continental igneous provinces are generated by convective melting, with passive margin volcanic sequences following the axis of rifting and flood basalts overlying the intersection of sutures in the continental mantle. As rifting progresses, the convection cells expand, cycling continental mantle from sutures perpendicular to the rift axis to generate intraplate tracks in the ocean basin. Continental mantle not melted on rifting, or delaminated on continental collision, becomes displaced to the east of the continent by differential rotation, which also sets up a means for tapping the material to give fixed melting anomalies. When

Lithology identification of aquifers from geophysical well logs and fuzzy logic analysis: Shui-Lin Area, Taiwan

The purpose of this study is to construct a fuzzy lithology system from well logs to identify formation lithology of a groundwater aquifer system in order to better apply conventional well logging interpretation in hydro-geologic studies because well log responses of aquifers are sometimes different from those of conventional oil and gas reservoirs. The input variables for this system are the gamma-ray log reading, the separation between the spherically focused resistivity and the deep very-enhanced resistivity curves, and the borehole compensated sonic log reading. The output variable is groundwater formation lithology. All linguistic variables are based on five linguistic terms with a trapezoidal membership function. In this study, 50 data sets are clustered into 40 training sets and 10 testing sets for constructing the fuzzy lithology system and validating the ability of system prediction, respectively. The rule-based database containing 12 fuzzy lithology rules is developed from the training data sets, and the rule strength is weighted. A Madani inference system and the bisector of area defuzzification method are used for fuzzy inference and defuzzification. The success of training performance and the prediction ability were both 90%, with the calculated correlation of training and testing equal to 0.925 and 0.928, respectively. Well logs and core data from a clastic aquifer (depths 100-198m) in the Shui-Lin area of west-central Taiwan are used for testing the systems construction. Comparison of results from core analysis, well logging and the fuzzy lithology system indicates that even though the well logging method can easily define a permeable sand formation, distinguishing between silts and sands and determining grain size variation in sands is more subjective. These shortcomings can be improved by a fuzzy lithology system that is able to yield more objective decisions than some conventional methods of log interpretation.

Differential rotation of lithosphere and mantle andthe driving forces of plate tectonics

Uncertainties regarding the relative importance of basal drag and boundary forces in plate tectonic models are a consequence of ~awed assumptions imposed by the use of the hotspot reference frame[ Velocities of lithospheric plates are in~uenced not only by lateral boundary forces but also by basal drag forces resulting from Earth rotation[ Drag is exerted on the base of the asthenosphere and motion is transmitted upwards to the lithosphere[ However as the transmission of stress in the mantle is viscosity! dependent the reduction in viscosity through the asthenosphere results in plates su}ering a net westward lag[ This {di}erential rotation| e}ect causes continental plates to be more strongly coupled to the deep mantle as they are separated from the mesosphere by only relatively thin regions of asthenosphere[ For such plates the calculated drag forces are of the same order of magnitude as the boundary forces[ Intraplate volcanism during continental rifting and in opening basins is related to transverse convection cells set up where topographic structures in the lithospheric root oppose mantle ~ow[ The motion of oceanic plates is dominated by conventional plate boundary forces which may either reinforce or oppose drag from an eastward mantle ~ow[ Reinforcement e[g[ Nazca plate# gives rise to Couette ~ow in the asthenosphere[ Opposition e[g[ Paci_c plate# results in counter~ow[ Shear stresses in both regimes are concentrated in the upper asthenosphere and lead to melting of concentrations of hydrous minerals {wetspots|# derived from eroded continental mantle introduced by lateral asthenospheric ~ow[ Under a counter~ow regime melt collects in a stationary layer at shallow depth in the asthenosphere at the crossover point between plate! and mesosphere!induced ~ow regimes[ Release of melt to the surface is governed by lithospheric stress trajectories set up by convergence along plate boundaries[ Intraplate volcanism thus has a common source although due to di}erent interactions between the boundary and drag forces asthenosphere ~ow pro_les will di}er between basins giving relative motions between melting anomalies of a few centimetres per year[ The overall e}ect however is to give an illusion of a series of quasi!_xed melting anomalies though in reality all these and the lithospheric plates are moving relative to the deep mantle[ Þ 0888 Elsevier Science Ltd[ All rights reserved[ Corresponding author[ Tel[] 99775 5164 6464^ fax] 775 5163 9174^ e!mail] mochinmÝmail[ncku[edu[tw A[D[ Smith C[ Lewis : Geodynamics 17 0888# 86Ð005 87

Primary Geologic Sources of Arsenic in the Chianan Plain (Blackfoot Disease Area) and the Lanyang Plain of Taiwan

The area that includes Beimen and the immediate surrounding villages of southwestern Taiwan is recognized as the home of the worlds only well-documented Blackfoot Disease (BFD) occurrence—where extremely high concentrations of arsenic (As) have been found in the artesian wells from the Chianan Plain that were previously used as a drinking water supply. In the past decade, very high levels of arsenic have also been discovered in the potable water from the Lanyang Plain of northeastern Taiwan, but BFD does not occur there. From regional geological/geophysical field investigations in these two areas and a literature survey of arsenic problems in the drinking water of other countries, we have identified five potential primary geologic sources of the As for the BFD area and six sources for the Lanyang Plain. We also report measurements of arsenic in two new areas: (1) the sea water west of the BFD area (three samples with As levels of 2.32, 7.46, and 9.71 μg/L; and (2) stream bed deposits east of the BFD area (two samples with As concentrations of 0.00485 wt% and 0.23000 wt%). With the current information available, it seems most plausible that the primary source of the arsenic in the BFD area is from fluid escape from the accretionary prism of Taiwan or from in situ formation of arsenic-laden sulfide minerals in the water-rich coastal sediments, whereas the most likely sources of the high concentrations of arsenic in the Lanyang Plain are hydrothermal venting in the Okinawa Basin and/or weathering of sulfide minerals associated with gold and copper ores along the Lishan fault to the south. Although oxidation is known to play a role in the formation of some arsenic compounds, most of these 11 primary sources and their associated rocks and minerals are originally related to reducing environments; however, continued research is necessary to fully explain each sources expulsion mechanism (mobility), transport (if any), and final concentration or enrichment of arsenic in the groundwater of Taiwan.

Geochemistry of Metabasalts and Associated Metasedimentary Rocks from the Lushan Formation of the Upthrust Slate Belt, South-Central Taiwan

Pillow lavas and massive flow associated with metasedimentary rocks of the Lushan Formation near Baolai and Fangshan represent Miocene intraplate volcanism in the Upthrust Slate Belt of central Taiwan. The lavas are alkali-olivine basalts that have undergone variable degrees of seafloor hydrothermal alteration. Trace-element and isotopic compositions (Th/Ce = 0.07 to 0.08, (La/Yb)n = 10.5 to 13.0, εNd(15Ma) = +4.4 to +5.8, (87Sr/86Sr)i = ~0.7040), are consistent with derivation from asthenospheric and continental mantle sources and similar to those of Late Cenozoic volcanism in the Penghu Islands and northern Taiwan. Variable calcite in-filling of vesicles and the occurrence of chert within the volcanic pile indicates eruption around the calcite-compensation depth, placing the volcanism near the base of the continental slope along the northern margin of the South China Sea.

Magnetic Surveying of the Chaochou Fault of Southern Taiwan: Culmination of Basement-Involved Surface Thrusting in Arc-Continent Collision

Taiwans southernmost, best-defined onshore surface geologic contact, resulting from the collision of the Philippine plate against the Eurasia plate, is the NNE-trending, 80 km long, oblique-slip Chaochou fault. This major crustal break in the Eurasian plate has an extremely conspicuous surface expression noted from satellite imagery and topographic maps as well as distinct subsurface magnetic and gravity signatures, up to 40 nT and 50 mgal, respectively. From a regional total magnetic intensity survey that included seven east-west profiles, integrated with Bouguer gravity and seismic reflection data, we suggest that the nearly vertical dip of the Chaochou fault in the shallow subsurface results from basement-involved thrusting that changes to high-angle reverse faulting near the surface. The horizontal offset on this basement thrust probably involves shallow left-lateral strike slip motion, whereas the major thrust geometry is controlled by deeper seated normal faults in the pre-Miocene basement rocks. The net effect of all this motion produces the present-day surface escarpment over a distance of 80 km between the villages of Liouguei and Fangliao in southern Taiwan. The overriding east-dipping Chaochou fault and associated Wanlong splay are upthrusts that bifurcate from a basement-involved decollement at approximately 6 km depth. As such, these two faults represent the westernmost advance of basement-involved thrusting exposed at the surface in southern Taiwan. Linear regression analysis applied to results from detailed field surveying of the total intensity of the magnetic field is useful in locating both extensional and compressional fault regimes in basement rocks. This technique should be considered not only by plate tectonophysicists, but by those researchers involved in the mapping of active faults where seismic risk is high.

Life cycle assessment of fuel selection for power generation in Taiwan.

Abstract Life cycle assessment (LCA) was applied to performance data from 1997–2002 to evaluate the environmental impacts of the energy input, airborne emission, waterborne emission, and solid waste inventories for Taiwan’s electric power plants. Eco-indicator 95 was used to compare the differences among the generation processes and fuel purification. To better understand the environmental trends related to Taiwan’s electric power industry, three fuel scenarios were selected for LCA system analysis. Results indicate that there are differences in characteristic environmental impact among the 13 power plants. Scenario simulation provided a basis for minimizing environmental impacts from fuel selection targets. Fuel selection priority should be a gas-fired combined cycle substituted for a coal-fired steam turbine to be more environmentally friendly, particularly in the areas of the greenhouse effect, acidification, winter smog, and solid waste. Furthermore, based purely on economic and environmental criteria, it is recommended that the gas-fired combined cycle be substituted for the oil-fired steam turbine.

A technique for improving pseudo-synthetic seismograms generated from neutron logs in gas-saturated clastic rocks

In clastic and carbonate rock sequences, the neutron and sonic log curves usually deflect in a similar fashion. Moreover, in some cases the two curves can be overlain and they generally appear to mimic each other, with variations between them only in the amplitudes of the two curves. This descriptive correlation is the basis of direct cross-plot techniques used to convert a neutron log into a pseudo-sonic log, which can then be combined with a density log to create a pseudo-synthetic seismogram. Unfortunately, the seismograms produced in this way may not match the standard synthetic seismograms produced from the sonic and density logs if the ?gas effect? is not taken into account. In order to correct for the gas effect, the inter-log correlations between the compensated neutron log (CNL) and the borehole-compensated (BHC) sonic log curves from a well in Taiwan were carefully examined. Then, we developed a technique for transforming the CNL log into a pseudo-BHC log by splicing together several continuous sandstone intervals in which the gas effect could be identified from the scattered data on the cross-plot of neutron porosity versus sonic interval transit time. Based upon our results, application of the new composite transform method yields a pseudo-synthetic seismogram that better matches the standard synthetic seismogram (made from the sonic and density logs) according to frequency, amplitude and polarity. This gas correction technique may be particularly useful in oil and gas exploratory and development areas where neutron logs are more prevalent than sonic logs or where sonic logs are scarce.

The Spontaneous Ray Log: A New Aid for Constructing Pseudo-Synthetic Seismograms

Conventional synthetic seismograms for hydrocarbon exploration combine the sonic and density logs, whereas pseudo-synthetic seismograms are constructed with a density log plus a resistivity, neutron, gamma ray, or rarely a spontaneous potential log. Herein, we introduce a new technique for constructing a pseudo-synthetic seismogram by combining the gamma ray (GR) and self-potential (SP) logs to produce the spontaneous ray (SR) log. Three wells, each of which consisted of more than 1000xa0m of carbonates, sandstones, and shales, were investigated; each well was divided into 12 Groups based on formation tops, and the Pearson product–moment correlation coefficient (PCC) was calculated for each “Group” from each of the GR, SP, and SR logs. The highest PCC-valued log curves for each Group were then combined to produce a single log whose values were cross-plotted against the reference well’s sonic ITT values to determine a linear transform for producing a pseudo-sonic (PS) log and, ultimately, a pseudo-synthetic seismogram. The range for the Nash–Sutcliffe efficiency (NSE) acceptable value for the pseudo-sonic logs of three wells was 78–83%. This technique was tested on three wells, one of which was used as a blind test well, with satisfactory results. The PCC value between the composite PS (SR) log with low-density correction and the conventional sonic (CS) log was 86%. Because of the common occurrence of spontaneous potential and gamma ray logs in many of the hydrocarbon basins of the world, this inexpensive and straightforward technique could hold significant promise in areas that are in need of alternate ways to create pseudo-synthetic seismograms for seismic reflection interpretation.

Generation of Pseudo-synthetic Seismograms from Gamma-Ray Well Logs of Highly Radioactive Formations

The conventional synthetic seismogram is created with a sonic and a density log; however, the sonic log can be replaced with the resistivity, neutron, gamma-ray or spontaneous potential log to produce a pseudo-sonic (PS) log. More recent techniques involve combining an SP log and a GR log to produce a PS log. In the past, a drawback in using GR logs for the PS is the presence of highly radioactive and often organic-rich layers possessing abnormally high GR readings. To improve the pseudo-sonic log produced from the gamma-ray log, a technique was developed to statistically treat the outliers from the wells in the Hugoton Embayment that encountered predominantly shale, sandstone, limestone, and dolomite and whose logged sections included both normal and abnormally high GR readings. To demonstrate a wider-range application of our method, the procedure was applied to wells from the Hugoton Embayment, Central Kansas Uplift, Sedgwick Basin, Salina Basin, Forest City Basin and Nemaha Uplift. The correlation coefficients between the PS and the conventional sonic for the six basins were 0.75, 0.92, 0.86, 0.91, 0.77, and 0.70, respectively. Also, the match between the resulting conventional synthetic seismogram and the pseudo-synthetic seismogram from a blind test well for each area was quite good. Provided the outliers have been properly treated, the GR log is a viable tool for creating pseudo-sonic logs and pseudo-synthetic seismograms for exploration in oil and gas basins where there are few wells with sonic logs or where sonic log quality is poor.