Studies related to the Charleston, South Carolina, earthquake of 1886; tectonics and seismicity

Professional Paper 1313




PART A: Major-, minor-, and trace-element compositions of lower Mesozoic basalts from three deep test holes near Charleston, S. C., are used to characterize magma type and o determine the tectonic setting of the volcanic rocks at the time of their eruption. Chemical and petrographic evidence indicates that slight to extreme oxidation and hydration in nearly all samples has caused widespread mobility of K, Na, and related trace elements. The minor elements P and Ti, the trace elements Th, Nb, Ta, Zr, and Hf, and the rare-earth elements (REE) show little or no variation regardless of the degree of alteration. The contents of these stable elements and the patterns of light-REE enrichment in basalts from the deepest of the test holes clearly show the presence of two chemical types that are strikingly similar to lower Mesozoic high-Ti, quartz-normative tholeiites and lower Mesozoic olivine-normative tholeiites exposed in eastern North America. The olivine-normative basalt is stratigraphically intercalated within a sequence of quartz-normative tholeiitic basalts. Quartz-normative basalts above and below the olivine basalt have nearly identical contents of most of the stable minor and trace elements, but the lower basalts have significantly more Cu and Ni and higher Ni/Co ratios. These differences are ascribed to pre-eruption separation of an immiscible sulfide melt into which Cu and Ni were strongly partitioned. The new chemical data clearly show that the olivine tholeiitic magma type does not necessarily represent the earliest stage of volcanism in the eastern North American, early Mesozoic tholeiite province and that the spatial distribution of olivine-normative magma types in the province is not related to any significant change in tectonic environment. PART B: 40Ar/39Ar total-fusion ages of three samples of basalt from Clubhouse Crossroads test hole #2, near Charleston, S.C., range from 182 to 236 m.y. (million years); only one of the total-fusion ages agrees within analytical uncertainty with conventional K-At ages of the same samples. Data from 40Ar/39Ar incremental heating experiments indicate that only one sample meets the criteria for a reliable crystallization age. The 40Ar/36Ar versus 39Ar/36Ar isochron age for this basalt is 184?3.3 m.y. This age is in good agreement with reliable ages of tectonically related lower Mesozoic diabase intrusions in eastern North America and Liberia. The ages of all these intrusions are consistent with their emplacement shortly after initiation of central Atlantic rifting about 190 m.y. ago. PART C: Paleomagnetic investigations have been undertaken on partially oriented basalt samples recovered from three USGS (U.S. Geological Survey) deep test holes at Clubhouse Crossroads near Charleston, S. C. The basalt unit lies at the base of the Coastal Plain sedimentary section; it is 256 m thick in test hole #3. It overlies and is partially interbedded with a sedimentary red-bed unit of probable Triassic or Early Jurassic age. On the basis of the paleomagnetic evidence and the geologic descriptions of the cores, 23 flows can be identified. Six of the flows have negative magnetic inclinations, which are interpreted as indicating periods of reversed polarity; one test hole contains a definite sequence of five reversed-polarity intervals separated by four normal-polarity intervals. The mean thermal remanent-magnetization (TRM) inclination for the 23 flows after magnetic cleaning is 35.4?3.2 ?. Comparison of this value with a paleoinclination curve for the Charleston area reveals that the age of the basalt has a 95-percent chance of being in the range 110-196 m.y. Comparison of the Clubhouse Crossroads basalt with other eastern North American basalts and diabases suggests that the true age is more likely to be in the older part of this range. PART D: In Clubhouse Crossroads drill hole #3, near Charleston, S. C., a minimum of 121 m of well-consolidated sedimentary red beds underlies 256

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Studies related to the Charleston, South Carolina, earthquake of 1886; tectonics and seismicity
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Professional Paper
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