Evidence for subduction in the ice shell of Europa

Transcription

1 SUPPLEMENTARY INFORMATION DOI: /NGEO2245 Evidence for subduction in the ice shell of Europa Simon A. Kattenhorn 1*, Louise M. Prockter 2 1 Department of Geological Sciences, University of Idaho, 875 Perimeter Drive, MS 3022, Moscow, ID , USA 2 Applied Physics Laboratory, Johns Hopkins University, Johns Hopkins Road, Laurel, MD 20723, USA This supplemental archive provides a detailed overview of the geology of the study region, as well as a stepwise explanation of the tectonic reconstruction sequence that resulted in the determination of the original geologic architecture shown in Fig. 2b. We also include a number of additional figures that complement the information provided in the main text (Supplementary Figs. 1 to 8). All citations are listed in the references list in the main text. Overview of Geology The map area contains numerous geological features with disparate ages based on crosscutting relationships. The features are mostly congruous with previously identified tectonic feature types on Europa 1, predominantly comprised of linear ridges and dilational bands, but also including arcuate cycloids and young linear troughs. We nonetheless identify two new classes of features previously not identified on Europa that are important for the identification of plate motions and subduction in the study region. We introduce the terms subsumption bands and transforms to refer to these features, which are described in the main text and shown in Figs. 2, 3, and Supplementary Fig. 5. We also identify features that we interpret as cryolavas (Fig. 2 and Supplementary Fig. 7). * Now at: ConocoPhillips Company, 600 N. Dairy Ashford, Houston TX NATURE GEOSCIENCE 1

2 2 For the purposes of this work, it was not necessary to create a detailed geologic map of every surface feature in the study region. Rather, we focused on the more prominent geological features (dilational bands and ridges), identified in Fig. 2 by a range of colors. The tectonic reconstruction (see next section) relies on the identification of numerous geological features that predate the interpreted plate motion sequence. Such features can be reconstructed to reveal their original architecture or distribution. Geologic mapping and identification of crosscutting relationships reveal that the majority of the geological features in the map area predate the tectonic plate motions that we reconstruct (see Supplementary Fig. 1). Furthermore, all features that postdate the reconstructed plate motions are ridges, cycloids, or troughs (no dilational bands). Interpreted cryolavas appear broadly coeval with inferred subduction. Features that predate the reconstructed plate motions are a mix of ridges and dilational bands (including both ridged bands and smooth bands 1 ). The ridges are mostly subdued and define a complex background of ridged plains that were subsequently dissected by dilational bands and more prominent ridges (Supplementary Fig. 2). The study area is almost devoid of endogenic disruption that forms lenticulae and chaos elsewhere on Europa, although there are some examples of pits. Notably, despite the approximately 134,000 km 2 study area, we have been unable to locate a single example of an impact crater at the resolution of the available images. Detailed Explanation of Tectonic Reconstruction The original geologic architecture of various mapped units (Fig. 2b) was reconstructed by progressively removing offsets along discrete boundaries, including dilational bands, strikeslip transforms, and tabular bands of inferred convergence termed subsumption bands (Supplementary Figs. 3 and 4). The margins of subsumption bands abruptly truncate older

3 3 geological features, possibly indicating that the matching equivalent on the opposing side of the band was relatively laterally offset during convergence. Convergent zones necessarily cause apparent lateral offsets of relatively older structures as a result of the associated area loss (Supplementary Fig. 3), particularly where the older features are oriented at a low oblique angle to the younger convergent zone 11, and increase as the amount of convergence increases. For example, in the mapped region, two old bands (green and dark orange, Fig. 2a) oriented at a high angle (75 ) to a relatively younger tabular zone (northern yellow tabular zone, east side of Fig. 2a) have no matching equivalents across the tabular zone within the field of view (~300 km along the strike of the tabular zone), implying a convergence of at least 80 km across the zone (Supplementary Fig. 3). In the palinspastic tectonic reconstruction, the removal of offsets was achieved in 4 discrete steps, although it should be noted that all reconstructions are inherently nonunique. For example, although the removal of dilations along specific boundaries, strike-slip offsets along different boundaries, or contraction across tabular bands are represented as separate steps in the reconstruction, it is not possible to resolve whether or not each of these events were truly consecutive, temporally overlapping, or broadly coeval. Nonetheless, the specifics of the reconstruction steps themselves are absolutely necessary to reconstruct the original geology. The final reconstructed architecture reveals an almost perfect match of geologic features at all scales, including the more prominent bands and ridges (colored on the geologic map in Fig. 2 and Supplementary Fig. 4) as well as fine details in the older background plains (mostly small ridges and very old, subtle background plains that may be ancient lineated bands). For the purposes of reconstruction and explanation, the study area was divided into 16 discrete plates (labeled A through P in Supplementary Fig. 4). The selection of plate boundaries was based on obvious mismatches of geologic features at plate edges or the

4 4 presence of intervening dilational bands between plates. The reconstruction assumes rigid plate motions or rotations 26 and does not account for surface curvature effects given the scale of the study area. Nonetheless, slight discordances are expected latitudinally in response to the nature of the orthographic projection, which preserves distances along parallels but is not conformal. It should be noted that the reconstruction necessarily requires the edges of the study region to be treated as essentially free boundaries. In other words, defined plates within the high-resolution imaged area are permitted to move freely in order to reconstruct the original geology. The associated motions may move the plates past the original spatial limits of the imaged region. Such an assumption assumes that adjacent areas imaged in low-resolution move concurrently with the defined plates within the high-resolution area in such a way to facilitate the reconstructed plate motions. Details of the sequence of events and logic employed in the reconstruction are as follows: Step 1: A dilational band between plates C and N/O (i.e., the combination of plates N and O) is perfectly reconstructed by moving N/O back away from the tabular band along its southern margin along vector 294. This motion is guided along an identically oriented transform fault between plates N and C/H/G and results in an 11.5 km wide gap along the southern margin of N/O. Hence, missing material that originally existed within this gap was removed by convergence along the southern boundary of N/O (labeled as the southern subsumption band in Fig. 2). Step 2: Numerous dilational offsets are removed along a series of dilational bands (purple in Supplementary Fig. 4) separating plates B, C, D, E, F, G, H, I, and J. A total of 35.2 km of dilation was removed by simple translation along vector 081, resulting in a perfect reconstruction of the red band and the green ridge (relatively older features). These dilational bands were likely created in a manner analogous to tailcracks at the tip of a shearing

5 5 feature 28, whereby strike-slip motion along a fault causes dilational pull-aparts to one side of the fault tip. In this case, the rigid translation of plates A, B, and C toward the WSW along the transform boundary that separates them from plate N resulted in the dissection of plates B-J by dilational tailcracking. The acceptability of rigid body translations is particularly apparent here in that dilational offsets within these tailcracks are perfectly removed and piercing points matched across 5 individual bands in the northern plate (blocks B through J) using an identical plate motion vector of 081. Hence, the northern plate was moving generally westward at this time, simultaneously pulling apart dilational cracks in its interior. The transform boundary along the northern edge of plate N curves from an approximately E-W orientation between plates C/F/H and N to a more SW-NE orientation between plates A and N. The only reconstructed dilational zones located west of this abrupt change in orientation of the transform are the dark blue and pink dilational bands separating plate A from plates B and C, which were reconstructed by removing a strongly sinistral-oblique motion component across these bands, requiring 40.3 km of translation of plate A along cumulative vector 054. This motion parallels the orientation of the transform boundary between plates A and N, showing congruence between the kinematics and the plate boundary geometry. Also in step 2, a 10 km dilational offset between plates I and J is removed along vector 144. This vector differs from others, suggesting the timing of this dilational gap is in doubt. The gap created between plates I/J and D at this step is likely an artifact created by unaccounted internal deformation within plate D (a wedge-shaped dilational band that crosses only part way into plate D was not included in the plate boundary definitions or accounted for in the reconstruction). Ultimately, the exact timing and nature of the removal of the dilational band between plates I and J is irrelevant to the main elements of the tectonic reconstruction.

6 6 The end result of step 2 is that all of the youngest dilational bands north of plate N are reconstructed. Step 3: The reconstruction of matching features in plates A and B/D is accomplished via the removal of 54 km of sinistral motion between these plates along vector 030. Further south, matching features are reconstructed between plates N and H through a 4 counterclockwise rotation of plate N/O combined with a 42 km translation along vector 274. As a result, the gap between plates N/O and P widens further. Although steps 2 and 3 are shown as discrete steps for the purposes of illustration, it is kinematically and geometrically possible that the timing of specific plate motions shown in these two steps were either contemporaneous or broadly overlapping. Step 4: The original geologic architecture is fully reconstructed in this step via a 10 counterclockwise rotation of plates C, F, H, N, and O and translation of 63 km along vector 304. These motions result in a perfect match of the major geological features, such as the blue and red bands and the dark red and green ridges (Supplementary Fig. 4). Moreover, the two parts of the northern subsumption band (Fig. 2) that are currently offset along the boundary between plates N and K are perfectly reconstructed in this step. The end result is a gap between plates N/O and P that is up to 99 km wide. If we were to assume the 23-km-wide subsumption band alongside the missing surface area represents a convergent belt into which a 99-km-wide zone was simply contracted, it would imply a reduction of at least 81% of the total original surface width of this missing region. Such large contraction would necessarily be accommodated by thrust faulting and the creation of resolvable topography; however, no such evidence exists, implying that the original material present in this area has been physically removed along the southern subsumption band by subduction. The missing area amounts to ~20,000 km 2, or about 15% of the total 134,000 km 2 area of the study region.

7 7 This amount compares well to the ~10-40% of new surface area created by dilational bands in studies of both the leading and trailing hemispheres of Europa 2. Interestingly, the two laterally continuous subsumption bands preserved in the tectonically reconstructed geologic map (Fig. 2b and Supplementary Fig. 4) both extend beyond the limits of the 99-km-wide subsumed zone in the reconstruction, suggesting older periods of convergence along these laterally extensive boundaries. The northernmost of these may have accommodated >80 km of inferred earlier convergence. The southern subsumption band likely experienced an earlier phase of convergence prior to the events reconstructed here given that its lateral dimensions extend >1700 km beyond the bounds of the missing surface area in Fig. 2b. Plate L is left isolated in step 4 of the reconstruction because the fault along its western margin (which did not experience any motion up to this point in the reconstruction) most closely aligns with the transform along which the reconstruction of adjacent plates occurred in step 4. This reconstruction step also requires that plates N/O shift westward beyond the original edges of the mapped area, implying that adjacent areas imaged only in low resolution must move concurrently to facilitate the motion of N/O. Such motions require that the transform visible along the northern margin of N/O within the study area continue westward beyond the study area (not an unreasonable assumption given the 54 km of motion of plate A along this boundary in step 3). Also required is that the western margin of N/O (not visible) be effectively dilating in order for N/O to move eastward through time (not unreasonable given the high frequency of dilational bands within both this region and Europa in general 2 ). A final note: the reconstruction of features between plates F/H/N and A/B/E in this step is only possible if plate C is first rotated along with F/H/N/O to close the triangular dilational band on its northern edge (dark purple in Supplementary Fig. 4), with C then being translated 36 km along vector 256 to make room for the translation of F/H/N/O. The exact starting

8 8 location of plate C is difficult to constrain; however, it should be noted that in its location shown in step 4, the internal morphology of plate C shows a strong resemblance to the juxtaposed morphology within adjacent plate N. Cumulative plate motions The overall series of motions of the plates relative to a stable reference frame (e.g., plates P and K) indicate that plate A experienced a net motion of 82 km toward an azimuth of 238, whereas plates N and O underwent a net motion of 117 km toward 119. The relative motion between the northern plate A and the central (subducting) plates N and O was thus a relative sinistral motion of km in an approximately E-W direction. The removal of ~99 km of the central plate in the reconstruction implies that approximately half of the total relative motion between A and N/O was accommodated by the independent motions of each.

9 9 Supplementary Figures:

10 10 Supplementary Fig. 1. Study location and recent geology. (a) (Previous page) Regional context for the study area (solid line shows extent of Fig. 1a in main text) in false-color images of Europa s trailing hemisphere (source: LPL, University of Arizona). High-resolution RegMap mosaic coverage from E15 and E17 orbits shown. Area of (b) shown by dashed box. (b) Geologic map of all features that postdate the northern and southern subsumption bands (see Fig. 2), including several stages of ridge, cycloid, and fracture development with variable orientations. Features are arranged stratigraphically in the legend. The number of features relatively younger than the subsumption bands is small in comparison to the entire geologic history, suggesting geologic youthfulness for the proposed subduction systems.

11 11 Supplementary Fig. 7 Fig. 3c Fig. 3b Supplementary Fig. 5 Fig. 3a Fig. 3d Supplementary Fig. 6 Supplementary Fig. 2. Uninterpreted mosaic of study region. Orthographic projection mosaic of Galileo images from the E15 (228 m/pix) and E19 (200 m/pix) orbits, showing the extent of the region for which geologic mapping and tectonic reconstructions were performed (see Fig. 2 and Supplementary Fig. 4). Solid boxes show locations for Fig. 3. Dashed polygons show locations of Supplementary Figs. 5-7.

12 Supplementary Fig. 3. Lateral offsets caused by convergence. Conceptual illustration showing apparent lateral offsets (z) in response to area loss induced by convergence or subduction. Only the cases of orthogonal convergence and subduction are illustrated. (a) Two older features crosscut by a developing zone of convergence. (b) Convergence C causes lateral offsets, z. Smaller offsets are associated with larger obliquities α for a given C. As C increases, z increases (z = C cot α). (c) Two older features crosscut by a developing zone of subduction. (d) Subduction by amount S causes lateral offsets, z. Smaller offsets are associated with larger obliquities α for a given S. As S increases, z increases (z = S cot α). (e) Configuration of the northern subsumption band (Fig. 2). In the field of view of the images (300 km wide), no matching features exist across the subsumption band, implying at least 80 km of contraction. The band is 22 km wide, indicating either significant contraction within the band (requiring mountain formation, which is not observed) or area loss by subduction. 12

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15 Supplementary Fig. 4. Palinspastic tectonic reconstruction. Progressive reconstruction of the motions of tectonic plates (identified using designations A through P) that resulted in the current configuration. (a) (Previous pages) White spaces in the current configuration are either young ridges that postdate the deformation sequence or data gaps. Geologic features were translated (transform motions) or removed (dilational bands) to reconfigure matching geologic features with a sequence based on crosscutting relationships. Moving backward through time, the reconstruction involved the following steps. (b) (Previous pages) Step 1: removal of the dilational gap between plate N and the plates to its north (A, C, and H). (c) (Previous page) Step 2: removal of dilational bands (purple, dark blue, and pink) in the plates north of plate N. (d) (Previous page) Step 3: removal of transform motion between A and B. Translation and rotation of N/O to reconstruct matching geology with H. (e) Step 4: reconstruction of original geologic architecture by rotation and translation of F/H/N/O along a transform along the northeastern boundary of these plates, and backward translation of C. A missing portion of the surface, ~99 km wide, occurs along the southern subsumption band. 15

16 Supplementary Fig. 5. Internal morphology of subsumption bands. The northern and southern subsumption bands have abrupt northern boundaries where preexisting geological structures terminate abruptly (arrows). There are no internal structures to suggest that the subsumption bands are composed of intensely deformed (contracted) background terrain; however, linear boundaries within the bands imply pervasively faulted material. There is no evidence of shadows within or along the margins of the bands that would be expected if high topography (> m) existed across the bands. For example, the dashed circle indicates an unknown monolithic feature that stands ~512 m above its surroundings based on the 3.3 km length of its prominent shadow and the 8.8 inclination angle of the sun above the horizontal from the east. Location shown in Supplementary Fig

17 Supplementary Fig. 6. Potential cryolavas. The southeastern margins of the subsumption bands are dotted by patches of rough hummocky terrain (delineated by white dashed lines) to a distance of at least 115 km from the proposed subduction boundaries. These patches stand higher than the surrounding terrain by up to 100 m and include at least one example of a flow-like lobe (arrowed and highlighted in the inset perspective view to the NE) emanating from a point source that resembles a circular vent. Location shown in Supplementary Fig

18 Supplementary Fig. 7. Cryolava and strike-slip faults on overriding plate. Potential cryolavas (pink patches) occur exclusively on the overriding plate, suggesting a thermal perturbation below in the zone of subsumption of the subducting plate within the ice shell. The overriding plate is also dissected by small, leftlateral strike-slip faults (dotted lines) that have a motion sense and orientation consistent with sinistraloblique convergence (orange and white arrows) along the convergent plate boundary. Location shown in Supplementary Fig

19 Supplementary Fig. 8. Conceptual model for subduction. Enlarged version of Fig. 4. Recycling of surface area through the subduction of a cold, brittle, outer portion of the ice shell into its warmer interior, where it is ultimately subsumed. Potential cryolavas are forced to the surface through the overriding plate. The higher density of the subducting outer ice layer obviates buoyancy-induced topographic relief at the site of subduction. Plate collision results in contractional deformation in the overriding plate along the collisional margin, creating the tabular subsumption bands. Image artwork created by Noah Kroese. 19