GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 317-1
Presentation Time: 1:40 PM


AXEN, Gary, YAO, Shuoyu and VAN WIJK, Jolante W., Department of Earth & Environmental Science, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801,

The seismically imaged SMB (central Rio Grande rift) is a N-S elongate ~elliptical sill ~130 m thick at ~19 km depth, with area of ~3500 km2. Surface uplift occurs in a ~circular area over the northern 2/3 of the SMB (time-average maximum of ~2.5 mm/yr, ~1911 to now). Episodic earthquake swarms, in the shallower, co-located Socorro seismic anomaly, may reflect episodic inflation events on a decadal scale, similar to magma chamber changes in active volcanoes. Ongoing 2D numerical elastic modeling with conductive heat flow yields spatial-temporal uplift evolution to compare with geodetic results. SMB inflation and thermal expansion of host rocks are treated separately to reveal differences in their uplift signals.¬†Preliminary results are: 1. A sill the size of the SMB solidifies in ‚ȧ500 yrs, so the SMB is younger than that. 2. SMB inflation instantly drives uplift at a rate proportional to magma pressure increase rate, but only while P continues to rise. 3. The inflation-driven uplift profile is pseudo-conical in cross section with a narrow flat top, similar to the measured uplift profile. Width of the region that uplifts fast enough to be measurable geodetically (>~0.5-1 mm/yr) is similar to the SMB width. 4. Thermal expansion causes uplift soon after heating begins, at rates driven initially by expansion of host rocks near the SMB - close enough that T increase is fast and large. Uplift continues at a nearly constant rate while heating continues. Uplift rates are similar to the measured average rate, but drop to near zero soon after heating ends. If seismic swarms reflect episodic inflation events, then related thermal expansion would drive less strongly punctuated uplift events because rocks near the SMB are already hot and fully expanded, and slow heat conduction would smooth the temporal surface response. Advective heat loss (e.g., due to groundwater flow) probably would reduce expansion-driven uplift rates. 5. Expansion-driven uplift has a ~plateau-shaped profile that is wider than the SMB, wider than that due to SMB inflation, and unlike the measured uplift profile. These results suggest that the northern SMB is actively inflating and the whole SMB has existed long enough that the expansion-driven uplift profile is subdued. More detailed geodetic time series are needed to say if inflation is episodic or continuous.