In contrast to crustal deformation observed in the actively forming Himalayas, where shallowly dipping crustal detachments extend over hundreds of kilometers, prior work on the Paleozoic southern Appalachian orogeny inferred that the final continental collision occurred on a steeply dipping crustal suture, permitting collision models that are dominated by strike-slip motion. Here, we use scattered seismic phases to instead reveal the Appalachian (Alleghanian) crustal suture as a low-angle (<∼15°) southward-dipping interface that soles into a flat-lying mid-crustal detachment. The observed suture geometry implies more than 300 km of head-on shortening across a plate boundary structure similar to the Himalayan mid-crustal detachment, indicating that this mode of deformation has been fundamental in continental collisions over hundreds of millions of years.
Paleozoic and Proterozoic continental collisions are partially preserved in the eroded roots of ancient mountain belts, but prior studies of these structures have not produced examples of terminal collisions between deep continental crusts that occurred via processes analogous to the ongoing Himalayan orogeny. In the Himalayas, mid-crustal detachments that accommodate overthrusting dip at low angles (<20°) over hundreds of kilometers (Schulte-Pelkum et al., 2005; Caldwell et al., 2013; Gao et al., 2016). In other active or recent orogens such as the Alps, low-angle mid-crustal detachments have been inferred (Lacombe and Mouthereau, 2002) although not required, and sutures between continental crusts appear to be steeper in some zones (Pfiffner, 2016). In Paleozoic and Proterozoic orogens, earlier work has typically inferred steep dips (>30°) on the terminal contact between colliding continental crusts, permitting crustal rheologies that differ from those in the Himalayas; where low-angle detachments occur in old orogens they are restricted to detachments beneath thin sheets of sediments and arc terranes (Juhlin et al., 2000; White et al., 2000; Simancas et al., 2003; Korja and Heikkinen, 2005; Brown et al., 2006), or they do not involve the suture with the last-colliding continental crust (Hynes and Rivers, 2010). In contrast, new seismic imaging in the southern Appalachian Mountains, presented here, resolves a low-angle, deep crustal suture at the collisional contact between two continental crusts, supporting the existence of Himalayan-style deformation processes through time.
The Appalachian mountain chain formed in a series of Paleozoic accretion episodes culminating in the terminal collision between Laurentia (proto–North America) and Gondwana (proto–Africa–South America) at ca. 300 Ma (e.g., Hatcher, 2010). Although little of the final suture between the Laurentian and Gondwanan lithospheric plates remains on land today, it is partially preserved in the southeast United States. There, the Suwannee terrane, comprising Florida and parts of southern Georgia (Fig. 1), is associated with Gondwana based on borehole data including fossils, rock strata, and detrital zircon ages (Chowns and Williams, 1983; Hatcher, 2010; Thomas, 2010; Mueller et al., 2014). For simplicity, hereafter the Gondwanan-affinity Suwannee terrane will be referred to as Gondwana, and the Laurentian margin plus previously accreted peri-Gondwanan terranes as Laurentia.
Previous work defined the Laurentia-Gondwana deep crustal contact by dipping reflectors that were interpreted to be a steep crustal suture (dip >35°–45°) that extended to the Moho (the crust-mantle boundary) (McBride et al., 2005; Cook and Vasudevan, 2006) (pink dashed lines in Fig. 1). To the north of this apparent suture, seismic reflections also documented a shallow (<12 km) detachment, above which thin sheets of metasediment and arc rocks were thrust by hundreds of kilometers onto the undeformed Paleozoic sediments of the Laurentian margin (McBride et al., 2005; Cook and Vasudevan, 2006). The locations of the suture zone inferred from reflection profiles and from outcrop and borehole data (Thomas, 2010) differ (Fig. 1).
In addition to supporting a mode of crustal deformation during continental collision that differs from that of the Himalayan example, a steeply dipping Laurentia-Gondwana suture has been significant in defining the direction of relative plate motion during terminal Pangea accretion. A steep dip has been interpreted to permit strike-slip plate motion on the suture. This has led to two classes of models—those that invoke head-on shortening on the suture during the final stage of the collision (e.g., Hatcher, 2002, 2010) and those that infer extensive strike-slip motion (Pollock et al., 2011; Domeier and Torsvik, 2014; Mueller et al., 2014).
DATA AND METHODS
This study obtained new constraints on crust and uppermost mantle structure from the Laurentian interior to the accreted Gondwana terrane, including the geometry of the suture, using common conversion point (CCP) stacking of S-to-P (Sp) converted waves recorded by the 85 stations of the 2010–2014 EarthScope Southeastern Suture of the Appalachian Margin Experiment (SESAME), the EarthScope Transportable Array (TA), and other broadband stations (Fig. 1). Unusually fine resolution of crustal structure was enabled by the close spacing of SESAME stations (∼5 km above portions of the suture) and the retention of shorter periods (1–33 s) than is commonly possible with Sp imaging. (See the GSA Data Repository1 for more on methods). In the resulting images (Fig. 2), positive (red) phases correspond to a localized positive velocity gradient (PVG) with increasing depth; negative (blue) phases correspond to a negative velocity gradient (NVG).
The Sp CCP stack reveals the structure of the crust beneath the southern Appalachian Mountains. The PVG at 35–55 km depth represents the Moho, seen to deepen beneath the mountains (Fig. 2C, at 60–150 km distance; and the northern end of Fig. 2A), as predicted by isostasy (Parker et al., 2013; Schmandt et al., 2015). Crustal thickness variations agree well with previous P-to-S imaging using SESAME (Parker et al., 2013) (Fig. 2C) and TA data (Schmandt et al., 2015) (Fig. 3), demonstrating the ability of Sp waves to image crustal structure.
Within the crust, a large-amplitude PVG deepens from the northern near-surface limit of the Gondwana suture zone (Figs. 2A and 2B) with a south-southeastward dip (maximum of ∼15°) and levels out in the mid- to lower crust (18–22 km depth). This portion of the PVG is imaged over >360 km along strike (Fig. 1). The PVG lies at depths <3 km north of the suture. The PVG is typically underlain by a NVG, indicating that this structure is equivalent to a high-velocity layer.
THE LAURENTIA-GONDWANA SUTURE
Given that the southward-dipping portion of the PVG-NVG pair reaches the near surface within the suture zone defined by outcrop and borehole data (Thomas, 2010) (Fig. 1), we interpret it as the contact between the crusts of Laurentia and Gondwana. The near-surface intersection of the dipping layer also coincides with the northern edge of the Charleston magnetic terrane, the location of the suture as inferred by Higgins and Zietz (1983). We do not observe collisional structures within the mantle lithosphere that are analogous to the crustal structure (Fig. DR2 in the Data Repository).
The shallow dip of the PVG-NVG contrasts with the more steeply dipping suture structure (>35°–45°) inferred from reflection profiles (McBride et al., 2005). However, re-migration of reflection travel times with more accurate models of crustal velocity (Schmandt et al., 2015) demonstrates that they are consistent with the Sp CCP images of a much lower-angle structure (Fig. DR1). Moreover, the dipping reflections in eastern Georgia are at comparable depths to the subhorizontal portion of the PVG, resolving the longstanding discrepancy between suture zone locations in eastern Georgia inferred from geological data (Thomas, 2010) versus reflection profiles (McBride et al., 2005; Cook and Vasudevan, 2006) (Fig. 1). The Sp CCP suture is also comparable to that of more recent active-source observations. The deeper (>15 km) portion of the PVG coincides in depth with preliminary observations of wide-angle reflections from the active-source SUGAR (Suwannee Suture and GA Rift) experiment (Shillington et al., 2015), indicating that it is a sharp boundary in depth.
Although this region experienced extension during the Mesozoic, the observed dipping PVG-NVG is not consistent with an origin related to rifting. It does not connect to the northern bounding faults of the Mesozoic rift basins documented by reflection and borehole data (McBride, 1991) (Fig. 1), as would be expected of a rift-related crustal detachment (e.g., Gouiza et al., 2010). Neither can mis-migration of converted waves generated by the sedimentary basins in this region explain the observed dipping PVG or PVG-NVG pair (Fig. 3; Fig. DR4).
The shallow dip and down-dip extent of the suture indicate that the final crustal collision along the suture involved head-on shortening, with Gondwana overthrusting the Laurentian basement over a distance of at least 330 km. Such a geometry evokes the low-angle mid-crustal detachments imaged beneath the Himalayas (longer segments with dips of <10° bounded by ramps of <10°–20°; Schulte-Pelkum et al., 2005; Caldwell et al., 2013; Gao et al., 2016). This geometry also implies that any significant strike-slip motion during the late stages of the Appalachian orogeny occurred before this overthrusting, as suggested by Hatcher (2002), or outside of this suture zone. If strike-slip motion occurred on the dipping suture, it must have been limited such that no offset of this layer is visible. Low-grade, andalusite-bearing lower Paleozoic shales retrieved from Gondwana terrane boreholes above the suture (Milton, 1972) indicate that these now near-surface rocks were buried to no more than ∼10 km. Hence the flat-lying segment of the suture detachment (now at ∼20 km) was at a maximum of ∼30 km depth when active. A depth of 20–30 km is comparable to flat detachment segments in modern or recent orogens (e.g., 20–40 km in the Himalayas [Schulte-Pelkum et al., 2005; Caldwell et al., 2013]; 20–30 km in the Alps and Pyrenees [Lacombe and Mouthereau, 2002]).
A single, continuous fossil shear zone would generate phases similar to those observed. The PVG and NVG could be the upper and lower boundaries, respectively, of a layer of radial anisotropy (Fig. 3), generated by a foliated fabric in a sheared/mylonitized layer. Wide mylonite zones (3–4 km) are observed in western Georgia near the suture (Sears and Cook, 1984). In this interpretation (Fig. 4), the shear zone corresponded to the Laurentia-Gondwana suture in the south, and transitioned into a shallow detachment in the north that accommodated the motion of thin, fault-bounded thrust sheets that are visible on reflection profiles (e.g., McBride et al., 2005). Such a model, in which deformation occurred in the mid-crust south of the suture, is consistent with largely undeformed shallow Paleozoic sediments in the Gondwana terrane (Chowns and Williams, 1983). Alternatively, portions of the PVG-NVG structure could be created by velocity contrasts due to rock type, such as a sliver of high-velocity mafic arc crust from the Carolina terrane along the dipping segment of the suture.
Not only is this detachment geometry similar to that observed today in the Himalayas, but estimates of peak metamorphic conditions are also comparable. Temperature-time paths for terranes directly north of the Laurentia-Gondwana suture indicate a decrease from peak metamorphic conditions (∼780 °C) at 300 Ma to ∼300 °C by 275 Ma (Steltenpohl et al., 2008), suggesting uplift and unroofing at the time of the collision with Gondwana (e.g., Hatcher, 2010). These high metamorphic temperatures are consistent with conditions reported for the Himalayas that have led to models involving mid-crustal flow evolving to localized shear zones (Parsons et al., 2016), in contrast to more rigid crustal rheologies.
We conclude that the final stages of the building of the southern Appalachian Mountains involved the north-northwestward overthrusting of Gondwana onto the Laurentian plate (Fig. 4). This period began when Gondwana (i.e., the Suwannee terrane) collided with the Laurentian margin. As the collision progressed, Gondwana overthrust the lower Laurentian crust along a localized mid-crustal shear zone at 20–30 km depth that ramped up at a shallow angle to the near surface. Additional shearing in the lower crust possibly occurred, but is not required. By the final stages of collision, Gondwana had overthrust the margin by >330 km (Fig. 4A, bottom panel). This interpretation implies that the underthrust Laurentian margin lies further south than previously inferred (Thomas, 2010). The geometry of the Laurentia-Gondwana suture precludes extensive strike-slip motion accommodated on the suture itself, negating models that include such motion (Pollock et al., 2011; Mueller et al., 2014), supporting those that instead invoke significant shortening on the suture (e.g., Hatcher, 2002, 2010), and placing new constraints on reconstructions of global plate motions (e.g., Domeier and Torsvik, 2014). While there may have been strike-slip motion accommodated elsewhere or earlier in time, the culmination of the Appalachian mountain building must have been driven by the near head-on collision of Laurentia and Gondwana.
The extensive, low-angle mid-crustal detachment imaged here in the southern Appalachian Mountains contrasts sharply with other Paleozoic and Proterozoic orogens where basement sutures between colliding crystalline continental crusts are inferred to be steeper and/or more complex (Juhlin et al., 2000; Simancas et al., 2003; Korja and Heikkinen, 2005). The Laurentia-Gondwana suture is instead similar to detachment structures in the Himalayas, indicating that conditions in the Paleozoic allowed deformation processes and crustal rheologies during continental collision that are comparable to those in the present day.
We thank the many people who made the SESAME array possible: the staff of the Incorporated Research Institutions for Seismology (IRIS) PASSCAL Instrument Center, the student/postdoctoral field crews (in particular, Horry Parker), and the landowners who hosted stations. We also thank Donna Shillington for sharing early results from the SUGAR experiment. Our thanks to Robert Hatcher and two anonymous reviewers for their helpful comments. The instruments used in the field program were provided by the IRIS PASSCAL facility, and waveform data were accessed through the IRIS Data Management Center. This work was funded by the National Science Foundation EarthScope Program under the American Recovery and Reinvestment Act of 2009 (Public Law 111-S) through awards EAR-0844276 (K.M.F.), EAR-0844186 (L.S.W.), and EAR-0844154 (R.B.H.).
- Received 7 August 2016.
- Revision received 5 October 2016.
- Accepted 6 October 2016.
- © 2016 Geological Society of America