Puna Plateau - Northwest Argentina

Much of my research over many years now has focused on the margins of the southern Puna plateau in NW Argentina. Our work in the Fiambalá basin documents its evolution from an open foreland in the late Miocene to a restricted basin today (Carrapa et al., 2008, 2010). One of my MASc students, Heather McPherson documented the ca. 4 Ma onset of Punaschotter conglomerate deposition, an important, regionally recognized unit, in the Fiambalá basin. In the El Cajon basin to the east, mapping, sedimentology, seismic reflection lines, detrital AFT data and U-Pb geochronology of ashes document out-of-sequence uplift of basement blocks and syn-depositional deformation of the intervening, segmented basin, identifying several phases of deposition and incision against a backdrop of constant, steady contraction (Mortimer et al., 2007). Two undergraduate theses further explored the El Cajon basin. Jonathan Pratt completed balanced cross-sections and participated in U-Pb analysis of intercalated ash beds and Hendrik Wulf mapped a large landslide in the northern part of the basin that may have been seismically triggered. In the Angastaco basin to the northeast, our work argues for the dominance of local climate change in dictating deposition in the Angastaco basin (Bywater-Reyes et al., 2010). Finally, my MASc student, James McCarthy, explored the evolution of the Pucara basin, a small, restricted basin west of Angatsaco and north of El Cajon. Our work is summarized in a capstone paper: Schoenbohm et al. in press. We demonstrate, based on the diachronous onset of Punaschotter conglomerates deposition in the Miocene and Pliocene, that initiation of coarse alluvial facies in this part of the Andes largely precedes the 2-4 Ma global climate change and emphasize the coupled nature of local climate and tectonics in controlling sedimentation. There are always more basins to explore around the margins of the Puna plateau, and I expect my research to continue there.

In the Puna plateau the argument has been made for late Miocene lithospheric foundering on the basis of geophysical (Tassara et al., 2006; Whitman et al., 1996) and geochemical evidence (DeCelles et al., 2009; Kay and Kay, 1993; Risse et al., 2008). However, the precise timing and location of potential lithospheric drips is largely unconstrained, particularly back in geologic time. Further, other lithospheric processes may be operating, such as gravitational spreading, lower crustal flow, and N-S synconvergent extension (Kley and Monaldi, 1996; Riller et al., 2012; Schoenbohm and Strecker, 2009), all of which complicate any potential foundering signal. A large portion of my academic career has been devoted to unraveling the history of deformation, changes in surface elevation and magmatism on the Puna Plateau in order to get at the history of lithospheric foundering.

One branch of our research is into the magmatic expression of lithospheric foundering. As drips descend, the drip itself can melt, producing pyroxenites. Additionally, asthenosphere flowing in to fill the space can experience decompression melting, producing peridotitic melts. Two papers which I co-authored with Mihai Ducea and his student explore the pattern and geochemistry of mafic volcanism in the southern Puna, using spatial clustering of pyroxenitic melts to argue for repeated, small-scale lithospheric drips (Ducea et al., 2013; Murray et al., in press). An additional study demonstrates the persistence of at least some lithospheric mantle beneath the Puna since at least the Ordovician, indicating incomplete removal of the mantle lithosphere through foundering, and suggesting the possibility of small-scale lithospheric “driplets” (Drew et al., 2009). One of my undergraduate students, J. Patrick Calhoun, also explored this topic, but from a more structural standpoint, investigating the alignment and elongation of cinder cones on the plateau in order to elucidate the strain record.

Another of my research branches has explored changes in surface elevation. Regional subsidence is expected during formation of a drip, followed by uplift (Göǧüs and Pysklywec, 2008a; Pysklywec and Cruden, 2004). My MSc student, Andisheh Beiki-Ardakani, tackled this problem using numerical modeling, demonstrating minimum surface uplift associated with individual drips. My colleagues and I have also documented inherited topography (Carrapa et al., 2014a) and the presence of an elevated plateau in the Puna since at least ca. 38 Ma (Canavan et al., 2014; Carrapa et al., 2014b), both of which suggest the plateau has been long-elevated, and recent lithospheric foundering has little influence on its modern elevation. This is consistent with the removal of lithosphere through small-scale, rather than large-scale drips.

A final, major branch of my research is aimed at understanding the pattern of surface deformation, particularly horizontal extension, in order to locate past lithospheric drips. Normal faults should be radially arrayed around the locus of a drip, although pre-existing structural weaknesses, difficult access, and poor exposure make identifying this pattern a challenge. I’ve explored normal faulting along the margin of the plateau (Schoenbohm and Strecker, 2009). Additionally, Renjie Zhou completed an MSc thesis and manuscript (Zhou et al., 2013) quantifying extremely low rates of horizontal extension in the Pasto Ventura region. His PhD thesis will expand on this work both in space (encompassing Antofogasta region as well as Pasto Ventura) and time (he will attempt to unravel the history of sedimentary basins in both regions through detrital thermochronology and sedimentary petrography). His recent work has demonstrated out-of-sequence deformation (Zhou et al., in review). My former post-doc, Jason Dortch, will quantify horizontal extension on a number of structures on the western part of the southern Puna Plateau. His enthusiasms also led him to explore wind erosion in the Puna and efficient methods for dating terraces using cosmogenic surface exposure dating.

I have written a capstone manuscript which weaves together the various lines of evidence (numerical modeling, geophysical data, magmatism, uplift/subsidence, and particularly structural deformation) to argue for a record of at least three lithospheric drips in the Puna since ca. 20 Ma, alternating between the northern and southern Puna (Schoenbohm and Carrapa, in press).

 Bywater-Reyes, S., Carrapa, B., Clementz, M., and Schoenbohm, L., 2010, Geology, v. 38, p. 235-238.
Canavan, R., Carrapa, B., Clementz, M.T., Quade, J., DeCelles, P., and Schoenbohm, L.M., 2014, Geology, doi:10.1130/G35239.1.
Carrapa, B., Hauer, J., Schoenbohm, L., Strecker, M.R., Schmitt, A.K., Villanueva, A., and Sosa Gomez, J., 2008, v. 120, p. 1518-1543.
Carrapa, B., Hauer, J., Schoenbohm, L., Strecker, M.R., Schmitt, A.K., Villanueva, A., and Sosa Gomez, J., 2010, GSA Bulletin, v. 122, p. 950-953.
Carrapa, B., Reyes-Bywater, S., Safipour, R., Sobel, E.R., Schoenbohm, L.M., DeCelles, P.G., Reiners, P., Stockli, D., 2014a, GSA Bulletin, v. 126(1/2), p. 66-77.
Carrapa, B., Huntington, K.W., Clementz, M., Quade, J., Bywater-Reyes, S., Canavan, R., and Schoenbohm, L.M., 2014b, Tectonics, v. 33, 10.1002/2013TC003461.
DeCelles, P.G., Ducea, M.N., Kapp, P., Zandt, G., 2009, Nature Geoscience, doi: 10.1038/NGEO469.
Drew, S., Ducea, M.N., and Schoenbohm, L.M., 2009, Lithosphere, v. 1, p. 305-318.
Ducea, M.N., Seclaman, A.C., Murray, K.E., Jianu, D., and Schoenbohm, L.M., 2013, Geology, v. 31, p. 915-918.
Göǧüs, O.H., and Pysklywec R.N., 2008a, JGR, v. 113, B11404.
Kay, R.W., and Kay, S.M., 1993, Geol. Runcsch, v. 80, p. 259-278.
Kley, J., and Monaldi, C.R., 1998, Geology, v. 26, p. 723-726.
Mortimer, E., Carrapa, B., Coutand, I., Schoenbohm, L., Sobel, E.R., Sosa Gomez, J., and Strecker, M.R., 2007, v. 119, p. 637-653.
Murray, K.E., Ducea, M.N., and Schoenbohm, L.M., in revision, GSA COSA Volume.
Pysklywec, R.N., and Cruden, A.R., 2004, G3, v. 5, Q10003.
Riller, U., Cruden, A.R., Boutelier, D., and Schrank, C.E., 2012, Journal of Structural Geology, v. 37, p. 65-74.
Risse, A., Trumbull, R.B., Coira, B., Kay, S.M., and van den Bogaard, P. 2008, Journal of South American Earth Sciences, v. 26, p. 1-15.
Schoenbohm, L.M., and Carrapa, B., in press, GSA Memoir 212.
Schoenbohm, L.M., and Strecker, M.R., 2009, Tectonics, v. 28, doi:10.1029/2008TC002341.
Schoenbohm, L.M., Carrapa, B., McPherson, H.M., Pratt, J.R., Reyes-Bywater, S., and Mortimer, E., in press, GSA Memoir 212.
Tassara, A., Götze, H.-J., Schmidt, S., and Hackney, R., 2006, JGR, v. 111, B09404.
Whitman, D., Isacks, B.L., and Kay, S.M., 1996, Tectonophysics, v. 259, p. 29-40.
Zhou, R.-J., Schoenbohm, L.M., and Cosca, M., 2013, Tectonics, v. 32, p. 1-17.
Zhou, R.-J., Schoenbohm, L.M., and Cosca, M., in review, Tectonics.