TERRACED MARGINS ON THE INFLATED MCCARTYS LAVA FLOW, NEW MEXICO

The McCartys lava flow is a well-preserved compound tube-fed flow field that extends ~47 km from the source vent, with distal portions of the flow extending east within the valley of the San Jose River, in central New Mexico east and south of the town of Grants. Cosmogenic and radiocarbon methods both give an age of ~3000 yr for the McCartys flow (Laughlin et al., 1994). Flows that have undergone inflation sometimes have margins that display a 'terraced' topographic expression, with each successive level at approximately one-half the thickness of the flow section from which it emanates. At the McCartys lava flow, we investigated several terraced margins where two or three distinct elevation levels occur. We used Differential Global Positioning System (DGPS) data to obtain precision topographic profiles across terraced margins. Terraces are typically 0.4 to 0.6 of the thickness of the inflated flow section from which they originate; if a second terrace level is present, its surface level is similarly about half the thickness of the source terrace. We postulate tha the terraces represent discreet outbreaks from the inflated flow. At only one location were we able to identify the specific source of a terrace outbreak; a vertical fracture in the primary flow margin became a 'local vent' for a terrace extending approximately 200 by 80 meters from the breakout. The source fracture is only ~30 cm wide; the terrace material attained a level that is quite consistent over the entire terrace. A basaltic toe emanating from the margin of the terrace has approximately the same height relationship to the terrace as the terrace to the main flow, but flow into the toe ceased before more than a single digitate flow was produced. Field observations of terraces provide important constraints for interpreting similar features on Mars as a result of inflation.