Kevin G. Stewart

Plutonism in three orogenic pulses, Eastern Blue Ridge Province, southern Appalachians

Department of Geological Sciences University of North Carolina, Mitchell Hall, Chapel Hill, NC 27599-3315

Crustal-scale shortening structures beneath the Blue Ridge Mountains, North Carolina, USA

We present results from a new seismic data set that show evidence for crustal-scale shortening structures beneath the Blue Ridge Mountains in the Southern Appalachians. The data come from six broadband seismic stations deployed on a transect across the Piedmont and Blue Ridge of western North Carolina. The observed structures appear as both a Moho hole and doubled Moho in receiver function CCP (Common Conversion Point) stacks oriented roughly perpendicular to the trend of the Appalachian orogen. We interpret these features as evidence for tectonic wedging and associated delamination and underthrusting of Laurentian lithosphere beneath a crustal indenter. The Moho hole and underlying deeper Moho correspond closely to a significant regional Bouguer gravity anomaly low, which we interpret as being due to overthickened, normal-density crustal material. Beneath the indenter, we observe a double Moho, which may correspond to the partial eclogitization of the underthrust material. This would be consistent with the sharp increase in the observed gravity above this feature. In addition to these crustal structures, we see evidence for a mantle lithospheric discontinuity at 90–100 km depth. This increase in velocity with depth is spatially limited and may dip slightly to the west, though more data are needed to verify this result. We interpret this anomaly to be a fossil slab accreted onto Laurentian lithosphere. If the westward dip is accurate, this slab may be a remnant of a west-vergent subduction zone that was active during the accretion of Carolinia.

The use of fluid inclusions to constrain fault zone pressure, temperature and kinematic history: an example from the Alpi Apuane, Italy

The Alpi Apuane is a tectonic window that exposes ductilely deformed greenschist facies metaigneous and metasedimentary rocks beneath relatively unmetamorphosed, brittlely deformed sedimentary rocks of the Tuscan nappe. The brecciated fault zone, the ‘window fault’, separating the two tectonic units was originally described as a simple thrust fault, but has recently been interpreted to have been reactivated as a later extensional detachment. Although evidence for extensional faulting is seen above and below the window fault, the amount of extensional displacement along this fault is unclear. Fluid inclusions from veins cementing the fault breccia were used to estimate the pressures and temperatures during the last fault movement. Minimum pressure estimates obtained from these inclusions range from 105 to 240 MPa. Pressure-corrected trapping temperatures for these fluids range from about 300 to 345°C. These pressures and temperatures indicate that the fault was last active at a depth of about 10 km, assuming a geothermal gradient at the time of 31°C km−1. This rules out complete extensional unroofing of the Alpi Apuane by movement along the window fault. Fluid salinities increase abruptly from the footwall into the fault zone. This pattern suggests that fluids rose from the footwall, entered the fault zone and were channeled within it, leaching salt from the overlying evaporite. The lack of quartz veins above the fault zone indicates that these fluids did not circulate into the overlying Tuscan nappe.

The Burnsville fault: Evidence for the timing and kinematics of southern Appalachian Acadian dextral transform tectonics

There has been a conspicuous absence of documented Acadian structures in the Blue Ridge province of the southern Appalachians even though a growing body of evidence suggests that the middle Paleozoic Acadian orogeny affected the rocks of this region. New mapping along with structural, petrologic, and geochronologic data from western North Carolina show that the contact between the metasedimentary and metavolcanic rocks of the Ashe Metamorphic Suite and the Grenville basement rocks of the western Blue Ridge corresponds to a Devonian high-grade, dextral strike-slip fault zone, the Burnsville fault. Timing of motion on the Burnsville fault is constrained by field relationships, metamorphic fabrics and assemblages, and radiometric ages. A U-Pb zircon crystallization age for a pegmatite sheared by the Burnsville fault shows that the last movement on the fault must he younger than 377 Ma. Previously published 4 0 Ar/ 3 9 Ar cooling ages show that the Burnsville fault must be older than ca. 360 Ma. We have mapped the Burnsville fault for 100 km in northwest-ern North Carolina. The Gossan Lead fault of northwesternmost North Carolina and southwestern Virginia is the likely continuation of the Burnsville fault to the north-east. Southwest of Asheville, North Carolina. the Burnsville fault may connect to the Devonian Dahlonega shear zone, or may be cut by post-Devonian thrust faults associated with the Alleghanian orogeny, Diachroneity of Acadian elastic wedge, the presence of Silurian-Devonian pull-apart basins, the presence of Devonian high-grade, dextral shearing in the Inner Piedmont, and recent plate reconstructions for the middle Paleozoic, support the interpretation that the late phase of the Acadian orogeny in the southern Appalachians was primarily a dextral transpressional event. The Burnsville fault and the Inner Piedmont formed the boundaries of a dextral transform margin that may have extended the length of the Appalachian orogen. This interpretation requires that models for the Paleozoic tectonics of the Appalachians incorporate major pre-Alleghanian dextral displacement.

Computerized Geologic Map Compilation

Abstract Compilation and presentation of geologic maps are greatly aided by utilization of basic computer equipment and software. Geologic data in computer format can easily be modified to accommodate new field data. Cross sections are also easily constructed on the computer. Paper or photographic slide copies of a map can be conveniently and economically produced at various scales for use in the field or for communicating data. We scanned USGS 7.5 minute topographic base maps on a flatbed scanner. Each sheet was scanned in six 8.5 × 14 inch (22 × 36 cm) sections with 2 to 4 inches (5 to 10 cm) of overlap per section. The six scanned files were imported as PICT files into Canvas ™, spliced, and edited to eliminate overlap. Structural data, lithologic contacts, text, etc., were compiled into separate layers. The completed 7.5 minute quadrangle with all compiled structural and geologic data uses approximately 3.7 MB of disk space.