<<

RESEARCH

Craton formation: Internal structure inherited from closing of the early oceans

C.M. Cooper1 and M.S. Miller2 1SCHOOL OF THE ENVIRONMENT, WASHINGTON STATE UNIVERSITY, PO BOX 642812, PULLMAN, WASHINGTON 99164-2812, U.S.A. 2DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF SOUTHERN CALIFORNIA, 3651 TROUSDALE PARKWAY, LOS ANGELES, CALIFORNIA 90089, U.S.A.

ABSTRACT

The closure of ancient oceans created a dynamic setting suitable for craton formation via the thickening of continental material over a downwelling. This process subjected the thickening to extensive deformation, forming internal structure that can be preserved over the lifetime of the craton. Recent seismic imaging of cratonic lithosphere has led to observations of anomalous features colloquially known as midlithospheric discontinuities. These discontinuities are attributed to a range of sources, including the lithosphere- boundary, melt accumulation, and phase transitions. However, the internal structure imaged within these cratons might be refl ective of their formation. In particular, the orientation and nature of the variable depths of the midlithospheric discontinuities suggest a more complicated origin such as that which could be introduced during the formation and thickening phase of cratonic lithosphere. Here, we present geodynamic models demonstrating the internal structures produced during the formation of cratonic lithosphere as well as new seismological observations of midlithospheric discontinuities in the , together with reassessment of midlithospheric discontinuities observed in the North American, South African, Fennoscandia, and Australian cratons. We suggest that the midlithospheric discontinuities observed in these cratons could be remnants of deformation structures produced during the formation of the cratons after ancient oceans closed.

LITHOSPHERE; v. 6; no. 1; p. 35–42; GSA Data Repository Item 2014100 | Published online 17 January 2014 doi: 10.1130/L321.1

INTRODUCTION resulting from the cooling of Earth. In addition to intrinsic properties such as density, viscosity, and fi nite strength, thickness also may determine the The Wilson cycle model was introduced to describe the closing and stability and longevity of cratonic lithosphere. Therefore, thickness might opening of the Atlantic Ocean basin (Wilson, 1966). Wilson (1966) pro- not just be a characteristic of cratonic lithosphere, but a necessary require- posed that and their margins were continuously modifi ed by ment to help explain its stable nature (Lenardic and Moresi, 1999; Lena- collisional events that closed oceans and also by rifting events that then rdic et al., 2003; Sleep, 2003; Cooper and Conrad, 2009). opened ocean basins. The closing of an ocean basin sutures continents Models of the formation of stable continental lithosphere often invoke together and produces new continental material through the accretion of the thickening of buoyant material over a mantle downwelling (e.g., Jor- arc material. During the suturing process, both the new and preexisting dan, 1978, 1988; Bostock, 1998; Lee, 2006; Cooper et al., 2006; Rey and continental materials thicken and deform. In Wilson’s model, the newly Houseman, 2006; Duclaux et al., 2007; Gray and Pysklywec, 2010). These created larger will then proceed to apart and break up (pos- studies suggest cratons are formed either through the stacking of buoyant sibly in areas of preexisting weakness created during the collisional event) oceanic lithosphere (Hart et al., 1997; Bostock, 1998; Musacchio et al., until a new ocean basin is created, and the cycle begins anew. We ask the 2004; Cooper et al., 2006; Lee, 2006) or the progressive amalgamation of question: Can the process of closing an oceanic basin form thick stable island arcs (Jordan, 1978, 1988; Rey and Houseman, 2006; Duclaux et al., continental lithosphere that does not fall victim to the next stage of rifting 2007; Gray and Pysklywec, 2010). Regardless of the specifi c type of thick- in the cycle? Here, we present evidence that the closing of Paleoprotero- ening or the type of material thickened, both mechanisms provide a means zoic oceans caused the formation of the cratons in Canada, , Fen- for stabilization as well as introduce complex internal structure to the noscandia, West Africa, and South Africa, where there have been recent newly thickened cratonic lithosphere. In addition, both scenarios are likely signifi cant breakthroughs in seismic imaging. to occur during the closing of an ocean basin. The discrepancy between Cratons are regions of long-lived, stable, thick lithosphere (Jordan, the two models primarily comes from the need to explain the chemistry 1978; Pollack, 1986). Their stability comes primarily from their intrin- of cratonic lithosphere and, in part, by the presence of seismically imaged sic strength determined by enhanced chemical buoyancy, viscosity, and internal structure within the craton (e.g., Bostock, 1998; Musacchio et al., fi nite strength (Jordan, 1978; Pollack, 1986; Lenardic and Moresi, 1999; 2004; Olsson et al., 2007; Wittlinger and Farra, 2007; Ford et al., 2010; Sleep, 2003). Stability might also be ensured by proximity to weaker Fischer et al., 2010; Miller and Eaton, 2010; Yuan and Romanowicz, 2010; material that would preferentially localize deformation, protecting cra- Darbyshire et al., 2013; Snyder et al., 2013). For example, a midlitho- tonic lithosphere from destructive forces (Lenardic et al., 2003). In either spheric seismic discontinuity observed in the North American craton has case, craton stability is dependent on material properties to overcome the been interpreted as a remnant feature of craton formation (Abt et al., 2010; deforming forces exerted by the convecting mantle. Their long-term sta- Fischer et al., 2010; Miller and Eaton, 2010; Yuan and Romanowicz, 2010; bility suggests that the inherent strength of cratons must be suffi cient to Snyder et al., 2013). The presence and nature of the internal structure of offset deformation from initial stabilization forward; i.e., the craton must cratonic lithosphere should delineate between the two models and provide be able to sustain the ever-changing dynamic conditions within the mantle information on the formation and stabilization of cratons; i.e., the formation

LITHOSPHEREFor permission to| Volumecopy, contact 6 | Number [email protected] 1 | www.gsapubs.org | © 2014 Geological Society of America 35

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/6/1/35/3050717/35.pdf by guest on 26 September 2021 COOPER AND MILLER

mechanism must produce structure that (1) can be preserved over billions and with geodynamic models that highlight the end-member processes of of years and (2) can be imaged seismically, which requires sharp velocity thickening of buoyant material over a mantle downwelling. contrasts or changes in anisotropy within the cold, thick craton keel. The process of forming cratons through the thickening of buoyant DEEP STRUCTURE OF CRATONS FROM SEISMIC material can be bracketed by two deformation mechanisms: thickening OBSERVATIONS through thrust stacking and localized deformation or viscous thickening and distributed deformation. These two end members call upon differ- Seismology-based estimates of the depth to the base of the cratonic ing physical mechanisms for thickening and stabilization. For example, lithospheric appear to have large variations in thickness (e.g., Nataf and if thick cratons form from thrust stacking of buoyant material, then that Ricard, 1996; Artemieva and Mooney, 2002; Rychert and Shearer, 2009; initial material must initially be highly viscous for deformation to primar- Eaton et al., 2009), but all fi nd that fast shear and compressional wave ily occur in localized shear zones (Cooper et al., 2006). Starting with a velocities overlie a lower-velocity region, and that boundary resides material with a high viscosity provides a means for stabilization and does between ~100 and 300 km. On a global scale, the continental keels are not require subsequent modifi cation of the material (Cooper et al., 2006). imaged with surface wave tomography as fast velocity perturbations (>1%) In addition, the localized shear zones have the potential to be preserved as in these depth ranges, although the depths to their bases are more diffi cult ancient suturing or deformation features that could defi ne the deep internal to determine due to the nature of the technique (e.g., Grand et al., 1997; seismic structure imaged within some of Earth’s cratons (e.g., Bostock, Mégnin and Romanowicz, 2000; Houser et al., 2008; Ritsema et al., 2011). 1998; Rychert and Shearer, 2009; Ford et al., 2010; Miller and Eaton, However, the locations of the fast velocities associated with the Precam- 2010; Darbyshire et al., 2013; Snyder et al., 2013). Conversely, if cra- brian cratons, in particular, those in Canada, Australia, Fennoscandia, West tons form through the viscous thickening of buoyant material, this results Africa, and South Africa, are quite clearly imaged by the global composite in continuous deformation over a broad zone (Dewey and Burke, 1973; mantle tomography model SMEAN (Fig. 1; Becker and Boschi, 2002). Jordan, 1978; Choukroune et al., 1995). The cratonic lithosphere then Recent regional surface wave tomography for these areas provides becomes viscously strong after a period of cooling, which then provides more detail on the cratonic structure (e.g., Bruneton et al., 2004; Fish- the necessary viscosity required for stabilization and longevity (Lenardic wick et al., 2005, 2008; Chevrot and Zhao, 2007; Eaton and Darbyshire, and Moresi, 1999). Formation of cratons through this type of thickening 2010; Fishwick, 2010; Darbyshire et al., 2013) and fi nds that the fast process would be less likely to produce deformational features that could velocity perturbations are greater (>3%) than those found in global mod- be preserved over a long period of time, but it still could introduce compo- els. Although the base of the craton remains challenging to defi ne using sitional variations at depth that could be seismically observable. surface wave–based techniques, these studies have inferred steps, gradi- While we do not rule out the possibility that cratons could be formed ents, and perhaps dipping layers at the edges and even within the craton. through an intermediary process between the two end-member cases, we These regional surface wave imaging studies suggest that the deep keels have highlighted these cases to provide a dynamic framework to guide appear to have a more complex internal structure than originally pro- our interpretation of seismic observations. In other words, in order to use posed or imaged in global studies. internal cratonic structure as an indicator of formation, stabilization, or Recent efforts in imaging the internal structure of the continents using modifi cation, it is important to consider what structure we might expect receiver function techniques have detected boundary layers within cratons given the dynamics of the proposed formation mechanisms. With that at variable depths between ~60 and 160 km (e.g., Bostock, 1998; Olsson et goal, we compare seismic observations of several cratons to each other al., 2007; Snyder, 2008; Hansen et al., 2009; Rychert and Shearer, 2009; Abt

Location of MLDs in the cratons dVs (%) Figure 1. Shear wave tomography model SMEAN (Becker and Boschi, 4 2002) at 250 km depth. The white tri- angles show the locations of inferred 3 midlithospheric discontinuities (MLDs) imaged in previous receiver function 2 studies at these broadband stations (Kumar et al., 2007; Wittlinger and 1 Farra, 2007; Olsson et al., 2007; Sav- age and Silver, 2008; Hansen et al., 0 2009; Ford et al., 2010; Miller and Eaton, 2010; Miller et al., 2014; Miller -1 and Becker, 2014; Snyder, 2008, 2013) and new measurements in the West African craton presented here (white -2 dashed box inset indicates the loca- tion of Fig. 2). 1(GSA Data Repository -3 Table S1).The blue lines are the plate 250 km depth boundaries, and the coastlines are -4 shown in gray.

1GSA Data Repository Item 2014100, Table S1, station locations with MLD signals from receiver functions mapped in Figure 1, is available at www.geosociety.org/ pubs/ft2014.htm, or on request from [email protected], Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA.

36 www.gsapubs.org | Volume 6 | Number 1 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/6/1/35/3050717/35.pdf by guest on 26 September 2021 Craton formation and midlithospheric discontinuities | RESEARCH

et al., 2010; Ford et al., 2010; Miller and Eaton, 2010; Snyder et al., 2013), Miller and Eaton (2010) and Levander and Miller (2012). The S-receiver which are identifi ed by a negative-polarity pulse similar to the signal pro- functions were produced by deconvolving the SV component from the P duced by the lithosphere-asthenosphere boundary. Deeper (>150 km) dis- component in the frequency domain following Langston (1977) and were continuities are generally interpreted as the base of the lithosphere (for com- depth converted using the one-dimensional (1-D) velocity model ak135 pilations of previous results, see Rychert et al., 2010; Fischer et al. 2010), (Kennett et al., 1995), because it effectively represents the continental lith- although the shallower (60–160 km) features have also been suggested to osphere. Examples of receiver function gathers from two stations (MBO be the lithosphere-asthenosphere boundary (cf. Rychert and Shearer, 2009) in Senegal and IFE in Nigeria) are shown in Figure 2B. within the continental interior. Alternatively, this intermediate-depth feature We found that 10 of the 15 stations in Mali, Morocco, Ghana, and evident in the cratons has been termed the midlithospheric discontinuity, but Nigeria indicate internal cratonic discontinuity structures or midlitho- the origin of this feature has yet to be resolved. These structures appear to be spheric discontinuities (Table 1). We fi nd that the stations off the edge of present in many of the Paleoproterozoic- and -aged cratonic keels the craton have only a single clear lithosphere-asthenosphere boundary (Fig. 1), but they are not consistent features that are evident as a distinct signal with no deeper (or intermediate depth) signal from the midlitho- shear velocity decrease or a change in anisotropy from surface wave studies, sphere. The 10 stations that are clearly on the craton, as interpreted from making them a challenge to identify and interpret. fast shear wave velocities from SMEAN (Becker and Boschi, 2002), have Large-scale scattered imaging studies of the continental interiors, such a range of midlithospheric discontinuity signals between 75 and 160 km as those of Rychert et al. (2007), Abt et al. (2010), and Ford et al. (2010) for with an interpreted lithosphere-asthenosphere boundary signal extending most of cratonic and Australia, illustrate that the Archean to ~300 km (Fig. 2A). Examples of receiver function gathers from sta- and Paleoproterozoic terranes have internal cratonic structure as identi- tions IFE and MBO in Figure 2B show the existence and lack of mid- fi ed by midlithospheric discontinuities. Other studies, also using receiver lithospheric discontinuity signals, respectively. functions (Miller and Eaton, 2010; Snyder et al., 2013), have shown that Due to the oblique raypaths that S waves from earthquakes between the midlithospheric discontinuities beneath the Paleoproterozoic Superior 55° and 85° have, the locations of conversion points (or piercing points) craton appear to be dipping outward from the center of the craton, suggest- are at a distance away from the surface location of the station. Therefore, ing that they may be relict features resulting from the accretion of slabs the interpreted midlithospheric discontinuity depths are plotted at their and smaller cratonic terranes. These interpretations are also supported by estimated piercing points based on back-azimuth bins, which demonstrate more detailed and high-resolution array-based P receiver function (Bos- the variable depth range where the waves sample different parts of the tock, 1998; Snyder, 2008) and refl ection seismology imaging (Cook et cratonic lithosphere (Fig. 2A). Plotting the interpreted midlithospheric al., 1997), which also detected dipping lithospheric structures within the discontinuity depths in this way further shows that none of the stations Archean-age Slave Province. However, the apparent dip of these midlitho- is located very close to the continental shelf nor do those off the edge of spheric discontinuities in the younger Paleoproterozoic portion of the cra- the craton have clear internal lithospheric structure (MBO in Senegal and ton is away from the deepest part of the cratonic keel (~260 km), making it those in southernmost Ghana). In addition, all the stations in the interior diffi cult to reconcile with the model of ancient stacked or accreted oceanic of the craton, except TAM in Algeria, which is on the Hoggar swell (Liu slabs. This then leads to questions about whether these dipping structures and Gao, 2010), have midlithospheric discontinuity signals that are deeper could be another deformational feature, such as preserved sutures or shear than those placed on the edge of the craton (Fig. 2A). It is not surprising zones and whether they are actually continuous structures. that TAM lacks any midlithospheric discontinuity signal because it is set There have been a series of studies using receiver functions to image on a region of Cenozoic volcanism and in the middle of a massif with high cratonic structure in Fennoscandia and Africa that have also found internal elevation and relatively slow shear wave velocities (Liu and Gao, 2010). cratonic structures (Olsson et al., 2007; Wittlinger and Farra, 2007; Kumar However, we observe a general deepening of the midlithospheric disconti- et al., 2007; Savage and Silver, 2008; Hansen et al., 2009) through identi- nuities into the center of the craton, but due to the sparse stations, this can fi cation of a negative conversion at ~110–160 km depth (Fig. 1). The esti- only be a very broad generalization. mates of the base of the lithosphere are somewhat debated, but there are commonalities in all the receiver function images for these two regions, in which there are seismic discontinuities that produce the observed nega- TABLE 1. LOCATIONS OF BROADBAND SEISMIC STATIONS tive-polarity conversions internal to the cratons. Latitude Longitude Elevation Station Net MLD (°N) (°E/W) NEW RECEIVER FUNCTIONS FOR THE WEST AFRICAN CRATON 14.5 –4.01 0.321 KOWA* IU 6.19 –0.37 0.212 KUKU AF Previously, there had been no receiver function–based estimates for 14.39 –16.96 0.003 MBO GG lithospheric structure in the West African craton, although there have been 5.88 0.04 0.082 SHAI AF surface wave studies that target the region (e.g., Fishwick, 2010). Recent 28.49 –9.85 0.316 MTOR* IP results from the PICASSO experiment in Morocco have found that sta- 22.79 5.53 1.41 TAM GG tions on the craton (Fig. 1) have signatures of midlithospheric disconti- 10.99 8.12 0.882 TORO* NJ nuity structure (Miller and Becker, 2014; Miller et al., 2014). However, 6.87 7.42 0.43 NSU* NJ 7.55 4.55 0.289 IFE* NJ these stations are concentrated just on the northwest edge of the craton. 6.24 7.11 0.05 AWK* NJ Although there are very few broadband stations in West Africa, we were 10.44 7.64 0.668 KAD* NJ able to add additional observations across the region to help further under- 6.3 0.07 0.217 AKOS* GH stand the lithospheric structure of this large craton (Fig. 2A). For this 6.61 0.44 0.313 KLEF* GH study, we analyzed broadband seismic data for 15 broadband stations that 6.46 –1.44 0.361 MRON* GH recorded 36 earthquakes of magnitude 6 and greater at 55°–85° distance 5.59 –0.33 0.203 WEIJ GH that occurred between 2010 and 2013 with high signal-to-noise ratios. Note: Stations with an * indicate those with midlithos pheric We followed the S-receiver function methodology described in detail in discontinuities (MLDs).

LITHOSPHERE | Volume 6 | Number 1 | www.gsapubs.org 37

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/6/1/35/3050717/35.pdf by guest on 26 September 2021 COOPER AND MILLER 1.5 1 0.5 Moho LAB 0 −0.5 MLD −1 0 −50 −100 −150 −200 −250 −300 −350 (km) with the largest radii being the highest quality. being the highest quality. radii with the largest LAB Moho eric discontinuity signals. The background is the shear The background discontinuity signals. eric nterpretation of the Moho, midlithospheric discontinuity midlithospheric of the Moho, nterpretation MBO IFE Examples of two receiver function gathers for stations IFE for function gathers receiver Examples of two nctions at stations (shown by yellow triangles) in the West West in the triangles) yellow by nctions at stations (shown amplitude amplitude B −1 −0.5 0 0.5 1 1.5 (km) MLD d NSU KAD AWK TORO 250 km TAM IFE MLD depth KLEF AKOS KUKU 80 90 100 110 120 130 140 150 SHAI WEIJ MRON (%) S δv KOWA MTOR shear-wave velocity shear-wave −4 −2 0 2 4 station without MLD station with MLD 15˚W 10˚W 5˚W 0˚ 5˚E 10˚E MBO A omography model SMEAN (Becker and Boschi, 2002) at 250 km depth. The quality of the signal at each station is scaled by radius, station is scaled by The quality of the signal at each 2002) at 250 km depth. and Boschi, model SMEAN (Becker omography 0˚ 5˚N Figure 2. (A) Estimates of the depth of the midlithospheric discontinuity (MLD) boundaries (colored circles) from S receiver fu S receiver from circles) (colored discontinuity (MLD) boundaries of the depth midlithospheric (A) Estimates 2. Figure station that has midlithosph each bins for back-azimuth km depth for points at 100 piercing at estimated plotted craton African (B) functions. discontinuity signals in the receiver no midlithospheric are there locations where indicate triangles The green our i and labels show Arrows respectively. discontinuity signals, of midlithospheric and lack the existence show which and MBO, boundary (LAB) pulses. and lithosphere-asthenosphere (MLD), wave t 25˚N 20˚N 15˚N 10˚N

38 www.gsapubs.org | Volume 6 | Number 1 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/6/1/35/3050717/35.pdf by guest on 26 September 2021 Craton formation and midlithospheric discontinuities | RESEARCH

GEODYNAMIC MODELING A Model Setup Model Width = non-dimensional value of 2 To better understand how cratons formed and how internal structure can Top boundary conditions: isothermal, free slip velocity be created and preserved on a billion-year time scale, we use geodynamic buoyant lithosphere models to highlight the processes that occur during craton formation. The closing of an ocean basin introduces a dynamic setting in which two con- tinental masses are thickened over a mantle downwelling (initially existing hot mantle upwelling in the form of a subducting slab). It is this process that we call upon to form thick, cratonic lithosphere, and as such, we can model this by exploring the cold mantle downwelling side walls walls side

thickening behavior of buoyant material over a mantle downwelling. An boundary Side (scaled to 660 km) 660 to (scaled Model Depth Model non- = initial investigation addressed the dynamic feasibility of forming cratons Initial condition: well-developed convective flow pattern reflecting conditions: dimensional value of 1 of value dimensional via thickening processes (Cooper et al., 2006). We have expanded those Bottom boundary conditions: isothermal, free slip velocity models to track the predicted internal structure formed during thickening.

Model Description B Localized Thickening Simulations were run using the numerical code Underworld (Moresi et time al., 2007). The simulations were confi gured to isolate the key components driving the interaction between a chemically distinct layer and the convec- tive mantle. The model domain consists of a 2 × 1 box with resolution of 640 × 128 elements (for initial model confi guration, see Fig. 3A). Convection was driven from below with constant-temperature top and bottom boundar- ies. The side boundaries within the model domain are refl ecting walls. We included a temperature-dependent viscosity within the mantle such that the viscosity contrast between the coldest and hottest mantle material spans fi ve orders of magnitude. We started from a well-developed fl ow pattern and temperature fi eld and emplaced a buoyant multilayered lithosphere within the upper thermal boundary layer of the system. The initial length of the emplaced layer spanned 75% of the surface of system, and the initial thickness was set to 7.5% of the system’s thickness. The layer was centered over the box and resided over a predeveloped mantle downwelling. We used a viscoplastic rheology following that in Moresi et al. (2003). This rheology allows material to deform viscously as long as the local stresses are below a specifi ed yield stress. Once that yield stress is achieved, then plastic deformation occurs, following the continuum representation of Byerlee’s law (Byerlee, 1968; Moresi and Solomatov, 1998; Moresi et al., 2003). Thus, the potential viscoplastic response of the chemically distinct layer will depend on the assigned viscosity and cohesive strength of the layer as well as convective stresses within the model. The buoyancy and thickness of the chemically distinct layer will determine whether the layer will thicken or recycle over a downwelling. Following the regimes outlined in Cooper et al. (2006), we chose a combination of parameters that highlight either a distributed or localized thickening response. All model parameter values are given within the captions in Figures 3 and 4. The simulations were run for several mantle overturns. Figure 3. (A) Model setup: The protocratonic lithosphere (pink, yellow, The main goal of these simulations was to highlight the internal struc- and green layers) resides within the upper thermal boundary layer of a ture that each end-member deformation style produces (localized vs. dis- convecting system (where dark blue is the coldest region, and red is the tributed thickening). As such, though the lithospheric layers within these hottest region). The convecting mantle has a temperature-dependent vis- models are visually different (demarcated by different colors), their mate- cosity such that the viscosity contrast between the coldest and hottest rial properties are the same. While varying the buoyancy, viscosity, and mantle material spans fi ve orders of magnitude. The viscosity of the pro- tocratonic lithosphere is set at 1000 times that of the viscosity of the hot- yield parameters of each layer will infl uence the thickening of the model test regions of the mantle. The density of the protocratonic lithosphere is lithosphere, we left this added complexity for future work. Rather, we set to a nondimensional value equal to that of the hot mantle, ensuring chose uniform properties for all of the layers that we know will promote that it is always positively buoyant. The cohesion of the protocratonic either localized or distributed deformation. lithosphere is set at 1.0 × 105 kPa with an after softening value of 1.0 × 104 kPa. The mantle’s cohesion is set at 1.0 × 104 kPa with an after soften- 3 Localized Thickening as Applied to Craton Formation and ing value of 8 × 10 kPa. All other values are held constant between the two materials in the simulations. (B) Simulation of localized thickening. Stabilization This shows the progression of snapshots from a representative simula- tion that demonstrates localized thickening. The top frame is shortly after The underthrusting of buoyant material proves to be a viable option the beginning of the simulation. We have highlighted the regions of shear for craton formation. Our simulations confi rm that of Cooper et al. (2006) with pink dotted lines.

LITHOSPHERE | Volume 6 | Number 1 | www.gsapubs.org 39

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/6/1/35/3050717/35.pdf by guest on 26 September 2021 COOPER AND MILLER

and show that the material will thicken along localized shear zones (Fig. little to no yielding is observed (Fig. 4), thus setting the fi rst requirement 3). This behavior occurs if three conditions coexist: (1) Deformation must of this type of behavior—the layer must not be too viscous such that it be localized (i.e., the layer must possess a viscosity high enough such that would allow localized deformation to dominate. As before, this localized/ viscous deformation will not occur), (2) yielding must be possible, and distributed cutoff occurs when the layer is ~1000 times more viscous than (3) recycling into the mantle must be avoided. The fi rst condition was the mantle (as determined empirically by Cooper et al., 2006). Finally, constrained empirically by Cooper et al. (2006) and occurs if the layer’s the layer must be able to thicken over the downwelling but not recycle viscosity is ~1000 times that of the mantle. This is on order with the pre- wholesale back into the convecting mantle. This is determined by the force dicted viscosity contrast between the lithosphere and mantle if lithosphere balance between the layer’s intrinsic buoyancy and negative thermal buoy- is dehydrated of bound water (e.g., Pollack, 1986; Hirth and Kohlstadt, ancy of the downwelling as controlled by mantle dynamics. Craton forma- 1996). The last two requirements are simply stress and force balances set tion via viscous thickening of buoyant material assumes that the entire by the intrinsic properties of the material and the convecting mantle. In chemically distinct lithosphere remains intact throughout the thickening other words, a layer cannot yield unless the convective stresses exceed its process; i.e., the layer does not undergo any substantial dripping at its base. yield strength, and a layer will not recycle if its positive chemical buoy- However, we did observe small-scale dripping within those simulations ancy exceeds the negative thermal buoyancy of the downwelling. that highlighted the viscous thickening deformation response (Figure 4). Within the simulations, yielding occurred on newly formed, localized The occurrence of these drips is in large part a consequence of the lower shear zones and continued in those locations as long as the conditions for values of viscosity required to allow the material to deform in a distributed, yielding existed. (Note, we defi ned the locations of shear zones within the viscous manner. The development of these drips is potentially problematic simulations as regions that showed localized, linear regions of high strain because they counteract the thickening process. For example, Molnar et al. rate.) If those conditions were no longer met, then the previously active (1998) determined that a layer when thickened to twice its initial size could shear zone was considered “healed” within the simulation. Thus, though be thinned by potentially half in 10–20 m.y. through instability-driven drips. it does not remain a zone of long-lived weakness within the simulation, The size and presence of these drips can be suppressed by either a higher it can still introduce long-lived structure within the lithosphere. As such, initial viscosity or temperature-dependent viscosity (Molnar et al., 1998; yielding often occurred in conjugate bands and evolved in the simula- Conrad and Molnar, 1999; Dumoulin et al., 2005; Elkins-Tanton, 2007). A tions as the magnitude and orientation of the driving stresses also changed higher initial viscosity might be a feasible solution to avoid destructive thin- (lower panels of Fig. 3B). As stated in Cooper et al. (2006), the thickening process itself provides the stabilizing mechanism for a craton formed through the underthrust- Distributed Thickening ing of buoyant material. Given that the condition for yielding depends on lithospheric thickness, it is apparent how the thickening of a viscously time strong material can create a nondeformable feature; the increased thick- ness increases the overall fi nite strength of the material such that it can exceed the local convective stresses, thus making it stable.

Localized Thickening: Expected Internal Structure and Potential to be Seismically Imaged

This deformation response introduces regions of localized shearing (highlighted regions in Fig. 3). These shear zones could be the source of the variable-depth midlithospheric discontinuities imaged in the cratons (e.g., Olsson et al., 2007; Whittlinger and Farra, 2007; Ford et al., 2010; Miller and Eaton, 2010; Abt et al., 2010). Ancient deformational features have also been seismically imaged in other regions and demonstrate that sutures and shear zones can be preserved over geologic time (Cook et al., 1997; Magnani et al., 2004; Karlstrom et al., 2005). The intense and localized shearing during this thickening process could introduce anisot- ropy into the cratonic lithosphere, providing a potential source for the observed midlithospheric discontinuities. This type of deformation may preserve relict features that possess a dip. The orientation of the dip of the relict shear zone depends on the direction of convergence as well as on the occurrence of conjugate shear zones. Thickening via localized defor- mation introduces conjugate shear zones (Fig. 3; see also Cooper et al., 2006). This type of deformation can introduce structures with dips that are in the opposing sense of direction of convergence (Fig. 3). Thus, caution should be used when interpreting these relicts of ancient deformation. Figure 4. Simulation of distributed thickening. These images are snapshots of a simulation that demonstrates distributed Distributed Thickening as Applied to Craton Formation and thickening. All of the values are the same as in the simulation in Figure 3A except that of the viscosity of the protocratonic Stabilization lithosphere, which was reduced to a value 100 times that of the viscosity of the hottest regions in the mantle. In this The process of forming a thick lithosphere can also be achieved through simulation, the protocratonic lithosphere experienced no viscous deformation. During this thickening process in the simulations, internal yielding during the thickening process.

40 www.gsapubs.org | Volume 6 | Number 1 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/6/1/35/3050717/35.pdf by guest on 26 September 2021 Craton formation and midlithospheric discontinuities | RESEARCH

ning by drips; however, this runs counter to the lower viscosity needed to show complexity and dip and depth variations in midlithospheric disconti- resist localized deformation. Given that cratons presently reside in a cooler nuities may indicate that the craton was formed through processes capable thermal state (Jaupart et al., 1998), the increase in viscosity required to of promoting localized deformation. Regions of cratonic lithosphere with suppress dripping could be achieved by a temperature-dependent viscosity uniform midlithospheric discontinuity depths might be more indicative of depending upon the cooling rate after the thickening process. The increase formation through viscous thickening. in viscosity must suffi ciently suppress thinning via dripping before the layer The ability to delineate between the two interpretations allows us to thins below the critical thickness required for stability. use present-day observations to elucidate early Earth dynamics as well provide additional information about the type of material from which cra- Distributed Thickening: Expected Internal Structure and tons were formed. In particular, cratonic regions with present-day varia- Potential to be Seismically Imaged tions in midlithospheric discontinuity depth and dipping midlithospheric discontinuities could be the end result of the closure of ancient oceans. The This type of thickening is less likely to introduce deformation features midlithospheric discontinuities in this case could be remnant features of that could be preserved for long periods of geologic time. The lower vis- the suturing and thickening of ancient arc material possessing signifi cant cosity required to thicken the material in a distributed manner does not inherent strength to be able to experience and preserve localized deforma- promote any localized yielding and also may promote viscous relaxation tion. The complex midlithospheric discontinuities could also be recording of any localized features. Depending on the composition of the protocra- the stacking of subducting slabs (e.g., Bostock, 1998; Cooper et al., 2006), tonic material, this type of deformation could still provide a source for which also could provide material strong enough to deform in a localized the midlithospheric discontinuities, as it maintains compositional layer- manner; however, even buoyant oceanic lithosphere may not provide suf- ing (Fig. 4). If there is a signifi cant compositional change between the fi cient buoyancy to achieve long-term stability (Lee, 2006; Cooper et al., layers within the protocratonic lithosphere, then a seismic discontinuity 2006). Regions that show more uniform midlithospheric discontinuities could be induced. This discontinuity would follow the plane of composi- may be cratonic lithosphere that was formed from a weaker initial material tional change. Distributed thickening produces relatively horizontal layer- that underwent viscous thickening and obtained its stability during post- ing within the cratonic lithosphere (Fig. 4). As such, any compositionally formation cooling. Those regions formed via viscous thickening would be driven seismic discontinuity would be expected to also be relatively hori- more susceptible to weakening and destabilization due to metasomatism zontal. This could provide an explanation for regions that show relatively or other postformation modifi cation. uniform depth of observed midlithospheric discontinuities, particularly in Australia, although the station density there is the lowest when compared ACKNOWLEDGMENTS to the other cratons (Ford et al., 2010). C.M.C. was supported by NSF grant EAR-1112820 and M.S.M was supported by NSF grant EAR- 1054638. The authors would like to thank the anonymous reviewers for the helpful sug- gestions and timely edits. DISCUSSION AND CONCLUSIONS REFERENCES CITED Although station distribution is either quite sparse or concentrated Abt, D.L., Fischer, K.M., French, S.W., Ford, H.A., Yuan, H., and Romanowicz, B., 2010, North within a subregion in the Archean and terranes of Earth’s cra- American lithospheric discontinuity structure imaged by Ps and Sp receiver functions: Journal of Geophysical Research, v. 115, p. B09301, doi:10.1029/2009JB006914. tons, the presence of internal structure within the cratons suggests that the Artemieva, I.M., and Mooney, W.D., 2002, On the relations between cratonic lithosphere thick- midlithospheric discontinuities may be a common feature and possibly be ness, plate motions, and basal drag: Tectonophysics, v. 358, p. 211–231, doi:10.1016/S0040 relict internal structure from the formation of the oldest part of the conti- -1951(02)00425-0. Becker, T.W., and Boschi, L., 2002, A comparison of tomographic and geodynamic mantle nents. The range of depths of the midlithospheric discontinuities from cra- models: Geochemistry Geophysics Geosystems, v. 3, no. 1, doi:10.1029/2001GC000168. ton to craton and even within a single craton (Fig. 1) strongly suggests that Bostock, M.G., 1998, Mantle stratigraphy and evolution of the Slave province: Journal of Geo- the discontinuities are related to the internal structure of the lithosphere physical Research, v. 103, p. 21,183–21,200, doi:10.1029/98JB01069. Bruneton, M., Pedersen, H.A., Farra, V., Arndt, N.T., Vacher, P., Achauer, U., Alinaghi, A., Ans- rather due to any postformation modifi cations. Variations in the depth of orge, J., Bock, G., Friederich, W., Grad, M., Guterch, A., Heikkinen, P., Hjelt, S.E., Hyvönen, midlithospheric discontinuities also limit the possibility that the discon- T.L., Ikonen, J.P., Kissling, E., Komminaho, K., Korja, A., Kozlovskaya, E., Nevsky, M.V., Paulssen, H., Pavlenkova, N.I., Plomerová, J., Raita, T., Riznichenko, O.Y., Roberts, R.G., tinuities are marking a phase transition within the cratonic lithosphere. Sandoval, S., Sanina, I.A., Sharov, N.V., Shomali, Z.H., Tiikkainen, J., Wielandt, E., Wile- The present-day cool, thermal state of cratonic lithosphere (Jaupart et al., galla, K., Yliniemi, J., and Yurov, Y.G., 2004, Complex lithospheric structure under the cen- 1998) due to long-term cooling limits the presence of thermal perturba- tral Baltic from surface wave tomography: Journal of Geophysical Research–Solid Earth, v. 109, no. B10, B10303, doi:10.1029/2003JB002947. tions that would infl uence the depth at which the phase transitions would Byerlee, J., 1968, The brittle-ductile transition in rocks: Journal of Geophysical Research, v. 73, occur. Thus, the thermal structure of cratonic lithosphere could dampen p. 4741–4751, doi:10.1029/JB073i014p04741. any variations in the depth of seismic discontinuities produced by phase Chevrot, S., and Zhao, L., 2007, Multiscale fi nite-frequency Rayleigh wave tomography of the : Geophysical Journal International, v. 169, p. 201–215, doi:10.1111/j.1365 transitions. However, we cannot rule out that this could be the case in -246X.2006.03289.x. regions that show more of a uniform midlithospheric discontinuity depth. Choukroune, P., Bouhallier, H., and Arndt, N.T., 1995, Soft lithosphere during periods of Finally, it also seems unlikely that the midlithospheric discontinuities Archaean crustal growth or crustal reworking, in Coward, M.P., and Ries, A.C., eds., Early Processes: Geological Society of London Special Publication 95, p. 67–86. represent the lithosphere-asthenosphere boundary in these cratonic regions, Conrad, C.P., and Molnar, P., 1999, Convective instability of a boundary layer with tempera- where other techniques such as global surface wave tomography indicate ture- and strain-rate–dependent viscosity in terms of available buoyancy: Geophysical a much thicker lithosphere (Fig. 1). Rather, the simpler interpretation is Journal International, v. 139, p. 51–68, doi:10.1046/j.1365-246X.1999.00896.x. Cook, F.A., Van der Velden, A.J., and Hall, K.W., 1997, refl ectors beneath the that the midlithospheric discontinuities are relict features of the internal SNORCLE transect—Images of the base of the lithosphere?, in Slave-Northern Cordillera structure produced during the formation of a craton. Not all cratons may Lithospheric Evolution (SNORCLE) Transect and Cordilleran Workshop Meet- ing: University of Calgary, p. 58–62. form with the same or similar combinations of buoyancy, viscosity, fi nite Cooper, C.M. and Conrad, C.P., 2009, Does the mantle control the maximum thickness of cra- strength, and thicknesses, and, as such, different internal structures are tons?: Lithosphere, v. 1, no. 2, p. 67–72, doi:10.1130/L40.1. likely to be present in different cratonic lithosphere. This would also then Cooper, C.M., Lenardic, A., and Levander, A., 2006, Creation and preservation of cratonic litho- sphere: Seismic constraints and geodynamic models, in Benn, K., Mareschal, J.-C., and be refl ected in any patterns that we observe in the midlithospheric discon- Condie, K.C., eds., Archean Geodynamics and Environments: Washington, D.C., Ameri- tinuities within cratonic lithosphere. Regions of cratonic lithosphere that can Geophysical Union, p. 75–88.

LITHOSPHERE | Volume 6 | Number 1 | www.gsapubs.org 41

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/6/1/35/3050717/35.pdf by guest on 26 September 2021 COOPER AND MILLER

Darbyshire, F., Eaton, D.W., and Bastow, I.D., 2013, Seismic imaging of the lithosphere beneath Levander, A., and M. S. Miller, 2012, Evolutionary aspects of the lithosphere discontinuity Hudson Bay: Episodic growth of the Laurentian mantle keel: Earth and Planetary Science structure in the western U.S.: Geochemistry Geophysics Geosystems, v. 13, Q0AK07, Letters, v. 373, p. 179–193, doi:10.1016/j.epsl.2013.05.002. doi:10.1029/2012GC004138. Dewey, J.F., and Burke, K.C., 1973, Tibetan, Variscan, and Precambrian reactiva- Liu, K.H., and Gao, S.S., 2010, Spatial variations of crustal characteristics beneath the Hoggar tion: Products of continental collision: The Journal of Geology, v. 81, no. 6, p. 683–692, swell, Algeria, revealed by systematic analyses of receiver functions from a single seismic doi:10.1086/627920. station: Geochemistry Geophysics Geosystems, v. 11, Q08011, doi:10.1029/2010GC003091. Duclaux, G., Rey, P., Guillot, S., and Menot, R.-P., 2007, Orogen-parallel fl ow during continen- Magnani, M.B., Miller, K.M., Levander, A., and Karlstrom, K.E., 2004, The Yavapai-Mazatzal tal convergence: Numerical experiments and Archean fi eld examples: Geology, v. 35, boundary: A long-lived assembly structure in the lithosphere of southwestern North Amer- p. 715–718, doi:10.1130/G23540A.1. ica: Geological Society of America Bulletin, v. 116, no. 9, p. 1137–1142, doi:10.1130/B25414.1. Dumoulin, C., Doin, M.-P., Arcay, D., and Fleitout, L., 2005, Onset of small-scale instabilities at Mégnin, C., and Romanowicz, B., 2000, The 3-D shear velocity structure of the mantle from the the base of the lithosphere and role of pre-existing lithospheric structures: Geophysi- inversion of body, surface, and higher mode waveforms: Geophysical Journal Interna- cal Journal International, v. 160, no. 1, p. 345–357, doi:10.1111/j.1365-246X.2004.02475.x. tional, v. 143, p. 709–728, doi:10.1046/j.1365-246X.2000.00298.x. Eaton, D.W., and Darbyshire, F.A., 2010, Lithospheric architecture and tectonic evolution of Miller, M.S., and Becker, T.W., 2014, Reactivated lithospheric-scale discontinuities localize the Hudson Bay region: Tectonophysics, v. 480, p. 1–22, doi:10.1016/j.tecto.2009.09.006. dynamic uplift of the Moroccan Atlas : Geology (in press), doi:10.1130/G34959. Eaton, D.W., Darbyshire, F.A., Evans, R.L., Grutter, H., and Jones, A.G., 2009, The elusive Miller, M.S., and Eaton, D.W., 2010, Formation of cratonic mantle keels by arc accretion: Evi- lithosphere-asthenosphere boundary (LAB) beneath cratons: Lithos, v. 109, p. 1–22, dence from S receiver functions: Geophysical Research Letters, v. 37, L18305, doi:10.1029 doi:10.1016/j.lithos.2008.05.009. /2010GL044366. Elkins-Tanton, L.T., 2007, Continental magmatism, volatile recycling, and a heterogeneous Molnar, P., Houseman, G.A., and Conrad, C.P., 1998, Rayleigh-Taylor instability and convective mantle cause by lithospheric gravitational instabilities: Journal of Geophysical Research, thinning of mechanically thickened lithosphere: Effects of non-linear viscosity decreas- v. 112, p. B03405, doi:10.1029/2005JB004072. ing exponentially with depth and of horizontal shortening of the layer: Geophysical Jour- Fischer, K.M., Ford, H.A., Abt, D.L., and Rychert, C.A., 2010, The lithosphere-asthenosphere nal International, v. 133, p. 568–584, doi:10.1046/j.1365-246X.1998.00510.x. boundary: Annual Review of Earth and Planetary Sciences, v. 38, p. 551–575, doi:10.1146 Moresi, L.-N., and Solomatov, V.S., 1998, Mantle convection with a brittle lithosphere: /annurev- earth-040809–152438. Thoughts on the global styles of the Earth and Venus: Geophysical Journal International, Fishwick, S., 2010, Surface wave tomography: Imaging of the lithosphere-asthenosphere v. 133, p. 669–682, doi:10.1046/j.1365-246X.1998.00521.x. boundary beneath central and southern Africa?: Lithos, v. 120, p. 63–73, doi:10.1016/j Moresi, L.-N., Dufour, F., and Muhlhaus, H.B., 2003, A Lagrangian integration point fi nite ele- .lithos.2010.05.011. ment method for large deformation modeling of viscoelastic geomaterials: Journal of Fishwick, S., Kennett, B.L.N., and Reading, A.M., 2005, Contrasts in lithospheric structure Computational Physics, v. 184, no. 2, p. 476–497, doi:10.1016/S0021-9991(02)00031-1. within the Australian craton—Insights from surface wave tomography: Earth and Plan- Moresi, L., Quenette, S., Lemiale, V., Meriaux, C., Appelbe, B., and Mühlhaus, H.B., 2007, Com- etary Science Letters, v. 231, p. 163–176, doi:10.1016/j.epsl.2005.01.009. putational approaches to studying non-linear dynamics of the and mantle: Physics Fishwick, S., Heintz, M., Kennett, B.L.N., and Reading, A.M., 2008, Steps in lithospheric thick- of the Earth and Planetary Interiors, v. 163, p. 69–82, doi:10.1016/j.pepi.2007.06.009. ness within eastern Australia, evidence from surface wave tomography: Tectonics, v. 27, Musacchio, G., White, D.J., Asudeh, I., and Thomson, C.J., 2004, Lithospheric structure and com- no. 4, doi:10.1029/2007TC002116. position of the Archean western Superior Province from seismic refraction/wide-angle Ford, H.A., Fischer, K.M., Abt, D.L., Rychert, C.A., and Elkins-Tanton, L.T., 2010, The lithosphere- refl ection and gravity modeling: Journal of Geophysical Research, v. 109, B03304, doi: asthenosphere boundary and cratonic lithospheric layering beneath Australia from Sp 10.1029/2003JB002427. wave imaging: Earth and Planetary Science Letters, v. 300, p. 299–310, doi:10.1016/j. Nataf, H.-C., and Ricard, Y., 1996, 3SMAC: An a priori tomographic model of the upper man- epsl.2010.10.007. tle based on geophysical modeling: Physics of the Earth and Planetary Interiors, v. 95, Grand, S.P., van der Hilst, R.D., and Widiyantoro, S., 1997, Global seismic tomography: A snap- no. 1–2, p. 101–122, doi:10.1016/0031-9201(95)03105-7. shot of convection in the Earth: GSA Today, v. 7, no. 4, p. 1–7. Olsson, S., Roberts, R.G., and Böovarsson, R., 2007, Analysis of waves converted from S to P Gray, R., and Pysklywec, R.N., 2010, Geodynamic models of Archean continental collision and in the upper mantle beneath the : Earth and Planetary Science Letters, v. 257, the formation of mantle lithosphere keels: Geophysical Research Letters, v. 37, L19301, p. 37–46, doi:10.1016/j.epsl.2007.02.017. doi:10.1029/2010GL043965. Pollack, H.N., 1986, Cratonization and thermal evolution of the mantle: Earth and Planetary Hart, R.J., M. Tredoux, and M.J. de Wit, 1997, Refractory trace elements in inclusions; Science Letters, v. 80, p. 175–182, doi:10.1016/0012-821X(86)90031-2. further clues to the origins of the ancient cratons, Geology, 25 (12), 1143-1146 Rey, P., and Houseman, G., 2006, Lithospheric scale gravitational fl ow: The impact of body Hansen, S.E., Nyblade, A.A., Julia, J., Dirks, P.H.G.M., and Durrheim, R.J., 2009, Upper- forces on orogenic processes from Archaean to Phanerozoic, in Buiter, S., and Schreurs, mantle low-velocity zone structure beneath the Kaapvaal craton from S-wave receiver G., eds., Analogue and Numerical Modelling of Crustal-Scale Processes: Geological Soci- functions: Geophysical Journal International, v. 178, p. 1021–1027, doi:10.1111/j.1365 ety of London Special Publication 253, p. 153–167. -246X.2009.04178.x. Ritsema, J., Deuss, A., van Heijst, H.J., and Woodhouse, J.H., 2011, S40RTS: A degree-40 shear Hirth, G., and Kohlstedt, D.L., 1996, Water in the oceanic upper mantle: Implications for rheol- velocity model for the mantle from new Rayleigh wave dispersion, teleseismic travel- ogy, melt extraction and the evolution of the lithosphere: Earth and Planetary Science time and normal-mode splitting function measurements: Geophysical Journal Interna- Letters, v. 144, p. 93–108, doi:10.1016/0012-821X(96)00154-9. tional, v. 184, no. 3, p. 1223–1236, doi:10.1111/j.1365-246X.2010.04884.x. Houser, C., Masters, G., Shearer, P.M., and Laske, G., 2008, Shear and compressional velocity Rychert, C.A., and Shearer, P.M., 2009, A global view of the lithosphere-asthenosphere bound- models of the mantle from cluster analysis of long-period waveforms: Geophysical Jour- ary: Science, v. 324, p. 495–498, doi:10.1126/science.1169754. nal International, v. 174, p. 195–212, doi:10.1111/j.1365-246X.2008.03763.x. Rychert, C.A., Rondenay, S., and Fischer, K.M., 2007, P-to-S and S-to-P imaging of a sharp Jaupart, C., Mareschal, J.C., Guillou-Frottier, L., and Davaille, A., 1998, Heat fl ow and thick- lithosphere-asthenosphere boundary beneath eastern North America: Journal of Geo- ness of the lithosphere in the : Journal of Geophysical Research, v. 103, physical Research, v. 112, B08314, doi:10.1029/2006JB004619. p. 15,269–15,286, doi:10.1029/98JB01395. Rychert, C.A., Shearer, P.M., and Fischer, K.M., 2010, Scattered wave imaging of the lithosphere- Jordan, T.H., 1978, Composition and development of the continental tectosphere: Nature, asthenosphere boundary: Lithos, v. 120, p. 173–185, doi:10.1016/j.lithos.2009.12.006. v. 274, p. 544–548, doi:10.1038/274544a0. Savage, B., and Silver, P.G., 2008, Evidence for a compositional boundary within the litho- Jordan, T.H., 1988, Structure and formation of the continental tectosphere: Journal of Petrol- spheric mantle beneath the from S receiver functions: Earth and Plan- ogy, Special Volume 1, p. 11–37, doi:10.1093/petrology/Special_Volume.1.11. etary Science Letters, v. 272, p. 600–609, doi:10.1016/j.epsl.2008.05.026. Karlstrom, K.E., Whitmeyer, S.J., Dueker, K.G., Williams, M.L., Bowring, S.A., Levander, A., Sleep, N.H., 2003, Survival of Archean cratonal lithosphere: Journal of Geophysical Research, Humphreys, E.D., and Keller, G.R., 2005, Synthesis of results from the CD-ROM Experi- v. 108, no. B6, 2302, doi:10.1029/2001JB000169. ment: 4-D image of the lithosphere beneath the Rocky Mountains and implications for Snyder, D.B., 2008, Stacked uppermost mantle layers within the of NW Canada understanding the evolution of continental lithosphere, in Karlstrom, K.E., and Keller, as defi ned by anisotropic seismic discontinuities: Tectonics, v. 27, TC4006, doi:10.1029 G.R., eds., The Rocky Region: An Evolving Lithosphere: Washington, D.C., /2007TC002132. American Geophysical Union, p. 421–441. Snyder, D.B., Berman, R.G., Kendall, J.-M., and Sanborn-Barrie, M., 2013, Seismic anisotropy Kennett, B.L.N., Engdahl, E.R., and Buland, R., 1995, Constraints on seismic velocities in the and mantle structure of the Rae craton, central Canada, from joint interpretation of SKS Earth from travel times: Geophysical Journal International, v. 122, p. 108–124, doi:10.1111 splitting and receiver functions: Precambrian Research, v. 232, p. 189–208, doi:10.1016/j /j.1365-246X.1995.tb03540.x. .precamres.2012.03.003. Kumar, P., Yuan, X., Kumar, M.R., Kind, R., Li, X., and Chadha, R.K., 2007, The rapid drift of the Wilson, J.T., 1966, Did the Atlantic close and then re-open?: Nature, v. 211, no. 5050, p. 676–681, Indian tectonic plate: Nature, v. 449, p. 894–897, doi:10.1038/nature06214. doi:10.1038/211676a0. Langston, C.A., 1977, Corvallis, Oregon, crustal and upper mantle receiver structure from tele- Wittlinger, G., and Farra, V., 2007, Converted waves reveal a thick and layered tectosphere seismic P and S waves: Bulletin of the Seismological Society of America, v. 67, p. 713–724. beneath the Kalahari super-craton: Earth and Planetary Science Letters, v. 254, p. 404– Lee, C.-T.A., 2006, Geochemical/petrologic constraints on the origin of cratonic mantle, in 415, doi:10.1016/j.epsl.2006.11.048. Benn, K., Mareschal, J.-C., and Condie, K.C., eds., Archean Geodynamics and Environ- Yuan, H., and Romanowicz, B., 2010, Lithospheric layering in the North American craton: ments: Washington, D.C., American Geophysical Union, p. 89–114. Nature, v. 466, p. 1063–1069, doi:10.1038/nature09332. Lenardic, A., and Moresi, L.-N., 1999, Some thoughts on the stability of cratonic lithosphere: Effects of buoyancy and viscosity: Journal of Geophysical Research, v. 104, p. 12,747– 12,758, doi:10.1029/1999JB900035. MANUSCRIPT RECEIVED 3 SEPTEMBER 2013 Lenardic, A., Moresi, L.-N., and Muhlhaus, H., 2003, Longevity and stability of cratonic REVISED MANUSCRIPT RECEIVED 2 DECEMBER 2013 lithosphere: Insights from numerical simulations of coupled mantle convection and MANUSCRIPT ACCEPTED 17 DECEMBER 2013 continental tectonic: Journal of Geophysical Research, v. 108, no. B6, 2303, p. 1–15, doi:10.1029/2002JB001859. Printed in the USA

42 www.gsapubs.org | Volume 6 | Number 1 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/6/1/35/3050717/35.pdf by guest on 26 September 2021