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The Magdalena radial dike swarm (MRDS) is a large diameter (200- 250 km) radial array of basaltic-andesite dikes of Oligocene age broadly focused on the large volume (7000 km3) Socorro-Magdalena caldera cluster (SMCC) of the central Rio Grande rift. Five large overlapping calderas of the SMCC range in age from 32.5 to 24.7 Ma and show a pattern of migrating to the southwest over a distance of about 80 km during a period of about 8 million years.

The MRDS fans through 220° of arc from Pie Town clockwise to Truth or Consequences (TorC).  New Ar-Ar age determinations of long mafic dikes at Pie Town, Hickman, Jones Camp and Ft. McRae (Tor C) are analytically equivalent (at 0.4-1.0 % error) to the large volume La Jencia Tuff, which erupted from the Sawmill Canyon caldera at about 29.15 Ma (Chamberlin et al., 2009). Near horizontal magmatic lineations (elongate vesicles) and steeply inclined pahoehoe like folds (perpendicular to flow) observed on the chilled margins of these dikes indicate they propagated laterally as much as 100 km away from the SMCC.

Absence of significant contact metamorphism next to most dikes implies they did not reach the Oligocene land surface and did not erupt. In the modern world, seismicity associated with the lateral propagation of non-eruptive dikes would be considered to be “false alarms”. The few dikes that did reach the Oligocene land surface became feeders for basaltic lava flows. Deeply eroded feeder dikes are distinguished by 10-20 m wide contact metamorphic aureoles formed by relatively long term (weeks to months) of heat flow “pumped” into the wall rocks around the volcanic conduit.

Photo: Looking southwest along zig-zagging basaltic-andesite dike in the northern Joyita Hills. Dike cuts west-tilted sandstone beds of the Spears Formation and is buried by horizontal conglomerate beds of the upper Santa Fe Group that cap the mesa. Field relationships indicate this dike fed a 32-33 Ma basaltic-andesite flow and tephra beds that are well exposed behind the mesa.

Based on limited observations of the MRDS, we suggest the length scale of a dike-induced seismic swarm that propagates tens of kilometers (or more) outward from a restless caldera (e.g. Long Valley, 1980) is directly and roughly proportional to the pressure regime and potential eruptible volume of the underlying magmatic system. If this is true, a basaltic dike (and related seismicity)  that propagates 100 km (62 miles) outward from a restless caldera would suggest that the underlying magmatic system is under high pressure and relatively large in volume, perhaps capable of producing a caldera-forming eruption  (Chamberlin, McIntosh, Dunbar and Dimeo, 2012).


Selected references concerning the MRDS and other components of the SMMS are listed below.

MRDS— Magdalena radial dike swarm

  1. Chamberlin, R.M., McIntosh, W.C., and Dimeo, M.I., 2007, Geochronology of Oligocene Mafic Dikes Within the Southeastern Colorado Plateau: Implications to Regional Stress Fields of the Early Rio Grande Rift: Geological Society of America Abstracts with Programs, v.39, no.6, paper no. 182-14
  2. Chamberlin, R.M.,  Hook, S.C., and Dimeo, M.I., 2008, Preliminary Geologic Map of the Carbon Springs 7.5 Minute Quadrangle, Socorro County, New Mexico: New Mexico Bureau of Geology and Mineral Resources Open File Geologic Map Series, OF-GM-165.
  3. Chamberlin, R.M., 2009, Primary flow folds on the north margin of the Jones Camp dike: Evidence for easterly dike propagation and mafic magma transport: New Mexico Geological Society Guidebook, 60th Field Conference, Geology of the Chupadera Mesa Region, p.33-35.
  4. Chamberlin, R.M., McIntosh, W.C., and Peters, L, 2009, 40Ar/39Ar Geochronology of the Jones Camp Dike: An eastward projection of the Magdalena radial dike swarm from under the Oligocene Socorro-Magdalena caldera cluster: New Mexico Geological Society Guidebook, 60th Field Conference, Geology of the Chupadera Mesa Region, p.337-346.
  5. Chamberlin, R.M., McInstosh, W.C., Dunbar, N.W., and Dimeo, M.I., 2012, Ignimbrite calderas and a large radiating mafic dike swarm of Oligocene age, Rio Grande rift, New Mexico: Possible implications to restless calderas: Volcanism in the American Southwest: abstracts
  6. Dimeo, M. I. 2008, Geology, geochemistry, and geochronology of Oligocene mafic dikes rear Riley, New Mexico, M.S. Thesis, New Mexico Institute of Mining and Technology, Socorro New Mexico, 236p.
  7. Dimeo, M. I. and Chamberlin, R.M., 2006, Oligocene Mafic dikes Within the Southeastern Colorado Plateau: evidence of northward propagation away from a coeval caldera cluster And Implications to Regional Stress fields of the early Rio Grande rift: Eos Trans. AGU, Fall Meet. Suppl. Abstract V23D-0665
  8. Dimeo, M.I., and Chamberlin, R.M., 2007, Petrography and Geochemistry of Mafic  Dikes near Riley, New Mexico: A Guide to Magmatic Evolution under a Caldera Cluster of the Early Rio Grande rift: New Mexico Geology; v. 29, no. 2, p. 57-58.
  9. NMBGMR, 2003, Geologic Map of New Mexico, 1:500,000: New Mexico Bureau of Geology and Mineral Resources

SMMS— Socorro-Magdalena magmatic system of Oligocene age

  1. Chamberlin, R. M., Chapin, C. E., and McIntosh, W. C., 2002, Westward Migrating Ignimbrite Calderas and a Large Radiating Mafic Dike Swarm of Oligocene Age, Central Rio Grande Rift, New Mexico: Surface Expression of an Upper Mantle Diapir?:  Geological Society of America, Abstracts with Programs, Vol. 34, No.6, p.438.
  2. Chamberlin, R.M., McIntosh W.C., and Chapin, C.E., 2003, Oligocene calderas, mafic lavas and radiating mafic dikes of the Socorro-Magdalena magmatic system, Rio Grande rift, New Mexico: surface expression of a miniplume?; abstract in The Hotspot Handbook, Proceedings of Penrose Conference Plume IV: Beyond the Plume Hypothesis, Hveragerdi, Iceland, August, 2003

SMCC— Socorro-Magdalena caldera cluster

  1. Chamberlin, R.M., 2001, Waning-stage Eruptions of the Oligocene Socorro Caldera, Central New Mexico: in Volcanology in New Mexico, L.S. Crumpler and S.G. Lucas (eds.), New Mexico Museum of Natural History and Science Bulletin 18, p. 69-77.
  2. Chamberlin,R.M.., McIntosh W.C., and Eggleston T.L., 200440Ar/ 39Ar Geochronology and Eruptive History of the Eastern Sector of the Oligocene Socorro Caldera, Central Rio Grande Rift, New Mexico: New Mexico Bureau of Mines and Mineral Resources Bulletin 160, p.251-279.
  3. Chapin, C.E., McIntosh, W.C., and Chamberlin, R.M., 2004,  The  Late Eocene-Oligocene Peak of Cenozoic Volcanism in Southwestern New Mexico: in  The Geology of New Mexico: A Geologic History  (G.H. Mack, K.A Giles and G.S. Austin eds.), New Mexico Geological Society Special Publication 11, p.271-293
  4. McIntosh, W. C., Kedzie, L. L., and Sutter, J. F., 1991, Paleomagnetism and 40Ar/39Ar ages of ignimbrites, Mogollon-Datil volcanic field, Southwestern New Mexico: New Mexico Bureau of Mines and Mineral Resources Bulletin 135, 79 p.
  5. McIntosh, W.C., Chapin, C.E, Ratte, J.C., and Sutter, J.F., 1992,  Time-stratigraphic framework for the Eocene-Oligocene Mogollon-Datil volcanic field, southwest New Mexico Geological Society of America Bulletin, v. 104, no. 7, p. 851-871, doi:10.1130/0016-7606
  6. McIntosh, W. C., and Chamberlin, R. M., 1994, 40Ar/39Ar geochronology of middle to late Cenozoic ignimbrites, mafic lavas and volcaniclastic rocks in the Quemado–Datil region, New Mexico: New Mexico Geological Society, Guidebook 45, p. 165–185.

RGR — Rio Grande rift of central New Mexico

  1. Chamberlin, R. M., 1983, Cenozoic domino-style crustal extension in the Lemitar Mountains, New Mexico —A summary: New Mexico Geological Society, Guidebook 34, p. 111–118,
  2. Chamberlin, R,M. and McIntosh, W.C., 2007, Chronology and Structural control of Late Cenozoic Volcanism in the Loma Creston Quadrangle, Southern Jemez Volcanic Field, New Mexico: New Mexico Geological Society Guidebook, 58th Field Conference, Geology of the Jemez Mountains Region II, p.248-261.
  3. Chapin, C. E., 1989, Volcanism along the Socorro accommodation zone, Rio Grande rift, New Mexico; in Chapin, C. E., and Zidek, J. (eds.), Field excursions to volcanic terranes in the western United States, Volume I—Southern Rocky Mountain region: New Mexico Bureau of Mines and Mineral Resources, Memoir 46, pp. 46–57.
  4. Chapin, C. E., and Cather, S. M., 1994, Tectonic setting of the axial basins of the northern and central Rio Grande rift; in Keller, G. R., and Cather, S. M. (eds.), Basins of the Rio Grande rift—Structure, stratigraphy, and tectonic setting: Geological Society of America, Special Paper 291-I, pp. 5–235

LJBA — La Jara Peak Basaltic Andesite (mafic lavas of the SMMS)

  1. Chapin, C.E., and Seager, W.R., 1975, Evolution of the Rio Grande rift in the Socorro and Las Cruces areas: New Mexico Geological Society Guidebook 26, Las Cruces Country, p. 297-321.
  2. Osburn, G. R., and Chapin, C. E., 1983, Nomenclature for Cenozoic rocks of northeast  Mogollon–Datil volcanic field, New Mexico: New Mexico Bureau of Mines and Mineral Resources, Stratigraphic Chart 1