Icarus 306 (2018) 122–138 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus Evidence for triple-junction rifting focussed on local magmatic centres along Parga Chasma, Venus J.R. Graff a,∗, R.E. Ernst a,b, C. Samson a,c a Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada Faculty of Geology and Geography, Tomsk State University, 36 Lenin Avenue, Tomsk 634050, Russia c Department of Construction Engineering, École de Technologie Supérieure, 1100 rue Notre-Dame Ouest, Montréal, Québec H3C 1K3, Canada b a r t i c l e i n f o Article history: Received 10 May 2017 Revised 29 January 2018 Accepted 2 February 2018 Available online 7 February 2018 a b s t r a c t Parga Chasma is a discontinuous rift system marking the southern boundary of the Beta-Atla-Themis (BAT) region on Venus. Along a 1500 km section of Parga Chasma, detailed mapping of Magellan Synthetic Aperture Radar images has revealed 5 coronae, 11 local rift zones distinct from a regional extension pattern, and 47 graben-ﬁssure systems with radiating (28), linear (12) and circumferential (7) geometries. The magmatic centres of these graben-ﬁssure systems typically coincide with coronae or large volcanoes, although a few lack any central magmatic or tectonic feature (i.e. are cryptic). Some of the magmatic centres are interpreted as the foci of triple-junction rifting that form the 11 local rift zones. Cross-cutting relationships between graben-ﬁssure systems and local rift faults reveal synchronous formation, implying a genetic association. Additionally, cross-cutting relationships show that local rifting events postdate the regional extension along Parga Chasma, further indicating multiple stages of rifting. Evidence for multiple centres of younger magmatism and local rifting against a background of regional extension provides an explanation for the discontinuous morphology of Parga Chasma. Examination of the Atlantic Rift System (prior to ocean opening) on Earth provides an analogue to the rift morphologies observed on Venus. © 2018 Elsevier Inc. All rights reserved. 1. Introduction Venus and Earth are sister planets due to their similarities in bulk composition and physical properties, however, they are very different from a geological perspective (Phillips and Hansen, 1994; Donahue and Russel, 1997; Chaisson and McMillan, 2010; Head, 2014; Taylor, 2014). A new view of Venus was unveiled in the early 1990s when NASA’s Magellan spacecraft obtained high resolution (75–100 m/pixel) images for 98% of Venus’ surface using Synthetic Aperture Radar (SAR) (Young, 1990; Saunders et al., 1990, 1992; Saunders and Pettengill, 1991). Magellan SAR images have since provided planetary scientists with detailed information regarding Venus’ surface geology and have primarily been used to identify and catalogue tectono-magmatic structures and stratigraphic units (e.g. Saunders et al., 1992; Phillips and Hansen, 1994; Ivanov and Head, 2011, 2013). All basemap SAR and altimetry images presented herein were obtained during the Magellan mission and were compiled by the U.S. Geological Survey (USGS) Astrogeology Science Center (http://astrogeology.usgs.gov/). ∗ Corresponding author. E-mail addresses: email@example.com (J.R. Graff), richard.ernst@ ernstgeosciences.com (R.E. Ernst), firstname.lastname@example.org (C. Samson). https://doi.org/10.1016/j.icarus.2018.02.010 0019-1035/© 2018 Elsevier Inc. All rights reserved. Contrary to Earth, plate tectonics is thought not to be currently active on Venus due to a lack of observable plate boundaries (e.g. Phillips and Hansen, 1994). Instead, Venus is suggested to be operating predominantly in a single plate, stagnant lid regime, with all magmatism occurring in intraplate settings (Head et al., 1992; Grosﬁls and Head, 1994a,b; Solomatov and Moresi, 1996; Nimmo and McKenzie, 1998; Crumpler and Aubele, 20 0 0; Ernst and Desnoyers, 2004; Hansen and Young, 2007). Despite the lack of plate tectonics, however, Venus does experience major rifting. Rift systems are present in numerous regions of the planet, such as the Diana/Dali Chasma (Hansen and DeShon, 2003) and the Ganis Chasma (Brakenridge et al., 20 0 0) located to the southwest and northwest of the Atla Regio volcanic rise, respectively. They are most dramatically present, however, throughout the BAT region (Fig. 1) where they enclose a triangular region. The ENE-WSW trending Hecate Chasma system is 80 0 0 km long and connects Atla and Beta Regiones (Hamilton and Stofan, 1996). The NW-SE trending Parga Chasma system is 10,0 0 0 km long and connects Atla and Themis Regiones (Martin and Stofan, 2004; Martin et al., 2007). The N-S trending Devana Chasma system is 30 0 0 km long and connects Beta and Phoebe Regiones (Kiefer and Swafford, 2006), terminating a few hundreds of kilometres north of Themis Regio. Associated with these rifts are numerous coronae and volcanic cen- J.R. Graff et al. / Icarus 306 (2018) 122–138 123 Fig. 1. Sinusoidal projection of the entirety of Venus’ surface (central meridian at 180°E) showing major structures, including rift zones, large volcanoes, and coronae. The area conﬁned by the Beta-Atla-Themis (BAT) region is outlined by dashed lines; the location of the study area is indicated by the black square. Modiﬁed from Fig. 2 in Herrick (1999). All subsequent Venus maps in this paper are also in a sinusoidal projection with a central meridian of 180 E. tres (Senske et al., 1992; Stofan et al., 1995; Martin et al., 2007). There are also graben-ﬁssure systems radiating away from some of these tectono-magmatic centres (e.g. Grosﬁls and Head, 1994a; Ernst et al., 2003). It has been observed that many of the large volcanoes and coronae of the BAT region are often at the centre of triple-junction rifting (cf. Burke and Dewey, 1973; Torske, 1975) and have associated topographic and geoid highs. This has been interpreted to indicate that an underlying mantle plume caused domal uplift, radiating arms of triple-junction rifting (Senske et al., 1992; Smrekar et al., 1997; Herrick, 1999), and radiating graben-ﬁssure systems (i.e. dyke swarms) (e.g. Grosﬁls and Head, 1994a; Ernst et al., 2003), in addition to the voluminous magmatism of the individual volcanic ediﬁces (Senske et al., 1992). As long recognized, the rift systems in the BAT region represent attempted breakup but did not reach the stage of ocean opening (e.g. Head et al., 1992; Airey et al., 2017 and references therein). On Earth, rifting commonly occurs near volcanic centres of large igneous provinces and forms a series of fractures surrounding a spreading volcanic ediﬁce, or on a larger scale, associated with triple-junction rifting about mantle plume centres and associated large igneous provinces (Turcotte, 1995; Hazlett and Hyndman, 1996; Şengör, 2001; Şengör and Natal’in, 2001; Ernst, 2014). It has been inferred that the Venusian rift systems of the BAT region are, in a similar fashion, due to triple-junction rifting about the main volcanic centres located at the ends of the rift systems (cf. Stofan et al., 1995; Hamilton and Stofan, 1996; Herrick, 1999). However, there are considerable complexities in the pattern observed along the rift systems. In particular, as shown in this paper (based on rift segments extracted from the rift map of Graff et al., (2015)), the Parga Chasma rift system consists of 45 local segments rather than one continuous rift segment. Hecate Chasma consists of 13 local segments. Devana Chasma, however, only consists of two main segments, representing a fairly continuous rift system. An outstanding question related to rifting processes on Venus is the origin of the discontinuous rift pattern observed along Parga and Hecate Chasmata. Another question is the relationship of the coronae, volcanoes, and radiating graben-ﬁssure systems to the rift zones, both in terms of timing and genesis. Previous workers, such as Martin et al. (2007), provided an exhaustive study examining the spatial and genetic relationships of the coronae in the Parga Chasma region. Their study examined multiple characteristics of coronae, including topographic signatures and associated volcanism, located both in proximity to and away from the main rift branches of Parga Chasma. It revealed that coronae have roughly equal distributions both proximal and distal to the rift, and that any causal link between coronae and rift formation remained inconclusive. 2. Research objectives The overarching goal of this study is to identify and map the extensional lineaments associated with major graben-ﬁssure systems and rift zones in the BAT region. Speciﬁc objectives are: (1) to resolve the relative chronology of tectono-magmatic events along a section of the Parga Chasma rift system; (2) to characterize the morphological differences between local and regional rifting along Venusian rift systems; and (3) to unravel the potential genetic association between local rift zones and individual magmatic centres. To achieve the ﬁrst objective, a detailed study area (1.2 Mkm2 ) has been selected in the southeast portion of the BAT region, within the coordinates of 260–275°E and 25–33°S (Fig. 1). The study area includes a complex 1500 km section of the Parga Chasma rift system and is host to a wide variety of tectonomagmatic structures, including graben-ﬁssure systems, local rift zones, regional extension features, coronae, and large volcanoes. The following activities were performed: (1) Detailed mapping of the radar-bright lineaments representing individual graben-ﬁssures, and grouping graben-ﬁssures into separate systems of different geometries. (2) Mapping of extensional lineaments interpreted to represent both local and regional rift faults of Parga Chasma. 124 J.R. Graff et al. / Icarus 306 (2018) 122–138 (3) Interpreting cross-cutting relationships between grabenﬁssure systems and rift faults to produce a relative chronology of events in the study area using the regional rifting along Parga Chasma as a baseline. Since graben-ﬁssure systems are generally thought to overlie dykes (e.g. Grosﬁls and Head, 1994a,b; Ernst et al., 2001; Studd et al., 2011), the ﬁnal map of the study area will additionally become part of the Venus global dyke swarm map (Ernst et al., 2009; Studd et al., 2010a,b,c,2011). The second objective aims to characterize the difference in morphology observed among the three major rift systems of the BAT region. Most notably, Parga Chasma exhibits a signiﬁcantly more discontinuous morphology than either Hecate or Devana Chasmata, and contains an abundance of coronae within close proximity (<∼50 km) to several local rift segments. The detailed mapping and observations gathered to tackle objectives 1 and 2 form the basis for addressing the third objective and to test the hypothesis that the mapped section of Parga Chasma predominantly developed as a series of local triplejunction rifting events extending from individual magmatic centres. This model is then applied on a regional scale to interpret the rift segmentation extracted from the BAT rift map of Graff et al. (2015) for Parga Chasma as a whole and also for Hecate Chasma. The Atlantic Rift System (prior to ocean opening) is invoked as a terrestrial analogue to Venusian rift morphology. along steep slopes except perhaps for those dyke swarms associated with rift zones. Rift zones are composed of many broad and densely-packed extensional structures, and commonly overprint underlying unit morphologies (Ivanov and Head, 2011, 2013). On SAR images, extensional features which are grouped as sets of semi-parallel, quasi-linear radar-bright lineaments, and which align along ﬂanks of topographic troughs are typically interpreted to be rift faults (Stofan et al., 20 0 0; Martin et al., 20 07; Graff et al., 2015; Graff, 2016). The ﬂoors of these topographic troughs are often covered by lava ﬂows, while the exterior walls of the central valley tends to be elevated relative to the surrounding plains, representing rift ﬂank uplift (Mark et al., 2014). Quantitative criteria such as lineament thickness and sinuosity were also used to distinguish graben-ﬁssures from rift faults (Graff et al., 2015). Lineament thickness represents the magnitude of the exposed offset of either a rift fault or graben-ﬁssure wall. Sinuosity captures the departure from a linear trend and is calculated by taking the ratio of total length measured along the lineament over its end-to-end length. In general, rift faults (as identiﬁed by their association with topographic troughs) exhibit both larger thickness and sinuosity than graben-ﬁssures as shown in a comparative study of radar images from Venusian and terrestrial rift settings by Graff et al. (2015). On Venus, rift faults and graben-ﬁssures exhibit average thicknesses of 896 ± 263 m and 361 ± 101 m, respectively. The average sinuosity for Venusian rift faults and graben-ﬁssures are 1.057 ± 0.032 and 1.015 ± 0.009, respectively. 3. Mapping methodology 3.1. Data source and study area This research primarily utilizes Magellan SAR images (75– 100 m/pixel) imported into ArcGIS 10.3. All images displayed herein were projected using the GCS Venus coordinate system (cf. Davies et al., 1992). SAR images are greyscale images where pixel brightness indicates the intensity of radar signal return, due to either surface roughness or the orientation of a structure relative to the incident radar pulse. The Magellan mission also obtained measurements of Venus’ surface elevation using radar altimetry, which were compiled into both topography and metre scale slope images at a lower resolution of 3–5 km/pixel. The study area covers parts of the Helen Planitia (V-52) and Themis Regio (V-53) quadrangles, and comprises a 1500 km portion of the Parga Chasma rift system. Within the study area, there are 5 coronae as catalogued by the International Astronomical Union (IAU), 11 local rift zones of Parga Chasma, and 47 grabenﬁssure systems, exhibiting radiating, linear, and circumferential geometries. 3.2. Distinction between graben-ﬁssures and rift faults A challenging aspect of this research involves the identiﬁcation and detailed mapping of both graben-ﬁssures and rift faults, as well as the distinction between the two types of extensional features. On SAR images both graben-ﬁssures and rift faults are radarbright extensional lineaments with similar morphological characteristics. The following criteria assisted in making the distinction between the two features: geometric patterns and their relationship with topography, lineament thickness, and sinuosity. Qualitatively, graben-ﬁssures are grouped into separate systems with speciﬁc geometric patterns—radial, linear, or circumferential— that reﬂect the propagation of underlying dyke swarms, and are often found in spatial association with magmatic centres (e.g. Grosﬁls and Head, 1994a,b; Ernst et al., 20 01, 20 03; Ernst and Buchan, 2001). Graben-ﬁssure systems are not typically aligned 3.3. Cross-cutting relationships The relative chronology of tectono-magmatic events mapped within the study area was determined by the analysis of crosscutting relationships between interacting graben-ﬁssures and rift faults. Types of cross-cutting relationships that were most often observed included: (1) cross-cutting relationships between the lineaments of separate graben-ﬁssure systems; (2) cross-cutting relationships between graben-ﬁssures interacting with local rift zones and the regional rifting pattern along Parga Chasma; and (3) the effect of young lava ﬂows obscuring (or partially obscuring) the trace of both graben-ﬁssures and/or rift faults (Fig. 2). Crosscutting between interacting graben-ﬁssures and rift faults is observed in both direct and indirect ways. Direct interactions result when the lineaments from graben-ﬁssure systems or rift faults are observed to be physically cross-cutting the lineaments belonging to other structures/events (i.e. younger lineaments directly overprinting older lineaments).In such cases, younger lineaments can often be seen as creating disturbances or discontinuities in the trend of older lineaments. Indirect interactions result when the lineaments from separate graben-ﬁssure systems are observed to interact with a mutual geologic feature (e.g. a lava ﬂow), but not directly with each other. In addition, contrasts in radar backscatter intensity were sometimes observed when younger features partially or completely truncate the propagation of older systems. The relative chronology of events in the study area was developed by comparing the timing of graben-ﬁssure system emplacement relative to local and regional rifting along Parga Chasma. Graben-ﬁssures that overprint rift faults are interpreted to have been emplaced after rifting, whereas graben-ﬁssures that are overprinted by rift faults are interpreted to have been emplaced prior to rifting. When there is ambiguity between interacting grabenﬁssures and/or rift faults, synchronous emplacement is interpreted to have occurred. Age relationships were further constrained when the presence of lava ﬂooding obscured the radar signal of grabenﬁssures and/or rift faults. J.R. Graff et al. / Icarus 306 (2018) 122–138 125 Fig. 2. Characteristic traits observed when examining cross-cutting relationships. (Top) Trend of older E–W trending graben-ﬁssures offset by the emplacement of younger NW–SE rift faults; (bottom) older E–W trending graben-ﬁssures completely truncated by a young lava ﬂow, with E–W trending rift faults extending across the ﬂow, but still partially obscured, and therefore interpreted as older than the ﬂow. (a) and (c) simpliﬁed schematics with older lineaments in grey, younger lineaments in black, and a young lava ﬂow as a grey-ﬁlled polygon. (b) and (d) representative SAR images similar to schematics. The location of part b is ∼266 E, 26 S, and for part d is ∼266 E, 29 S. North is approximately up on parts b and d. 4. Detailed mapping of graben-ﬁssure systems and rift zones 5. Tectono-magmatic history 4.1. Overview Following the approach of Studd et al. (2011) and using the relative age relationships developed in Supplementary File 1, a relative chronology of the tectono-magmatic events within the study area has been created. The age relationships between interacting radiating and linear graben-ﬁssure systems and/or the local/regional rifting of Parga Chasma have been integrated into a common timeline (Fig. 6). This approach (cf. Studd et al., 2011) assumes that the duration of emplacement for each graben-ﬁssure system was short (∼ a few Myr), based on terrestrial studies of dyke swarm emplacement (e.g. Ernst et al., 2001; Ernst, 2014). The initial regional extension and rifting of Parga Chasma is also hypothesized to have occurred as a singular event over a relatively short time span of 10–50 Myr, based on the terrestrial rift review and catalogue of Şengör and Natal’in (2001), and is placed in the centre of the timeline. This estimate of rifting duration along Parga Chasma, however, should be considered provisional pending more research on the similarities and differences between rifting on Venus and Earth. Local triple-junction rifting events have variable timing and, based on cross-cutting relationships, are genetically linked to the individual magmatic centres from which they are focussed on. In the timeline, horizontal lines are used to schematically represent the range of possible ages of formation of each system, with ages decreasing (becoming younger) towards the right. When graben-ﬁssures interacted with regional rift faults from Parga Chasma, the age relationships relative to this rifting could be determined and the system was provisionally placed on the left, centre, or right side of the timeline. When the graben-ﬁssures from different systems were observed to intersect, their age relationships could be obtained and the position of these systems on the timeline was reﬁned. Examination of the 40 systems displayed on the timeline (Fig. 6) reveals that 17 (42.5%) predate regional rifting, 13 (32.5%) exhibit synchroneity with regional rifting; and 10 (25%) postdate regional rifting. Of the 13 systems synchronous with regional rifting, Within the study area extensional lineaments interpreted to represent graben-ﬁssures and rift faults were systematically mapped. The tally of mapped features includes a total of ∼12,600 extensional lineaments, with ∼11,0 0 0 belonging to graben-ﬁssures and ∼1600 belonging to rift faults. The graben-ﬁssure lineaments were further grouped into 47 separate graben-ﬁssure systems and catalogued as 28 radiating, 12 linear, and 7 circumferential systems (Fig. 3; Tables 1–3). Over 10,0 0 0 graben-ﬁssures were assigned to the 47 grouped systems, while the remaining ∼10 0 0 mapped lineaments consist of isolated and/or unassigned sets of graben, ﬁssures, or fractures. These unassigned lineaments may belong to unidentiﬁed graben-ﬁssure systems or unrecognized distal portions to the aforementioned catalogued systems (cf. Ernst et al., 2003). Within the study area over 1600 rift-related lineaments were systematically mapped, corresponding to 633 regional rift faults and 1016 local rift faults (Fig. 4). Regional rift faults have a NW– SE orientation (catalogued as RF01) and are associated with the overall regional NE–SW extension of Parga Chasma. The majority of regional rift faults coincide with features previously mapped by López and Hansen (2008), who catalogued them as regional fractures. Local rift faults are distinguished from regional rift faults when they are found in association with individual magmatic centres. In the study area, 11 local sets of rift faults are identiﬁed (catalogued as RF02–RF12) and are often found in association with topographic troughs. 4.2. Relationships between graben-ﬁssure and rift fault systems Details on the cross-cutting relationships between interacting systems and associated magmatic centres are provided in Supplementary File 1. Five examples are discussed (Fig. 5) that feature major graben-ﬁssure systems and magmatic centres that are interpreted as the foci of local triple-junction rifting. 126 J.R. Graff et al. / Icarus 306 (2018) 122–138 Table 1 Characteristics of the radiating graben-ﬁssure systems in the study area. System Longitude (°E) Latitude (°S) Max radius (km) Arc (°) Number of mapped lineaments Centre elevation (km) Elevation above MPR (km) Centre within corona? R01 R02 R03 R04 R05 R06 R07 R08 R09 R10 R11 R12 R13 R14 R15 R16 R17 R18 R19 R20 R21 R22 R23 R24 R25 R26 R27 R28 269.75 267.50 275.75 273.00 262.00 262.50 268.00 269.50 272.25 272.00 268.25 261.75 262.50 271.25 268.25 265.50 266.00 275.75 269.50 273.25 269.50 263.00 268.00 267.00 267.75 266.25 266.75 267.00 28.00 27.50 31.75 27.25 28.00 27.50 30.50 30.75 27.25 32.00 30.75 27.50 28.00 28.00 30.50 28.25 24.50 31.50 29.00 27.25 29.75 28.00 23.00 30.00 31.50 27.00 29.00 23.00 747 560 475 471 448 352 280 232 230 226 221 213 203 203 197 196 186 175 154 152 148 114 111 107 99 99 57 46 360 210 190 320 50/45 250 335 60 75 40 95 50 90 65/10/5 60 85/70/25 330 80 55 85 60 65 60/35 90 60 35 40 70 733 763 700 634 344 711 834 218 162 117 237 136 172 112 106 309 391 302 122 75 132 63 110 74 104 85 38 38 6052.9 6051.8 6052.5 6053.2 6052.5 6052.2 6051.7 6052.2 6052.5 6052.1 6052.3 6052.8 6052.5 6053.6 6052.3 6052.4 6051.7 6052.6 6051.9 6053.2 6052.2 6052.3 6051.7 6052.8 6051.7 6051.5 6051.9 6051.9 1.9 0.8 1.5 2.2 1.5 1.2 0.7 1.2 1.5 1.1 1.3 1.8 1.5 2.6 1.3 1.4 0.7 1.6 0.9 2.2 1.2 1.3 0.7 1.8 0.7 0.5 0.9 0.9 Y N Y N Y Y N N N N N Y Y N N N Y Y N N N Y N N N N N N Associated structure Xmukane Corona Obiemi Corona Ts’an Nu Mons Kulimina Corona Kulimina Corona Kulimina Corona Kulimina Corona Chuku Corona Obiemi Corona Kulimina Corona OtohimeTholus Table 2 Characteristics of the linear graben-ﬁssure systems in the study area. System Longitude (°E) Latitude (°S) Length × width (km) Number of mapped lineaments Centre elevation (km) Elevation above MPR (km) Centre within corona? L01 L02 L03 L04 L05 L06 L07 L08 L09 L10 L11 L12 266.00 264.50 263.00 273.75 274.00 260.25 275.50 262.25 266.25 264.25 266.25 271.25 26.50 29.00 29.75 27.75 31.50 25.25 30.50 27.00 30.50 26.75 30.75 31.00 393 × 148 343 × 134 341 × 104 195 × 43 187 × 73 164 × 56 152 × 54 150 × 54 143 × 58 108 × 54 92 × 40 88 × 107 248 87 133 79 122 119 194 84 236 82 49 157 6052.7 6052.4 6052.5 6052.3 6052.0 6052.9 6052.2 6052.6 6051.6 6051.8 6051.7 6052.5 1.7 1.4 1.5 1.3 1.0 1.9 1.2 1.6 0.6 0.8 0.7 1.5 N N N N Y N Y Y N N N N Associated structure Obiemi Corona Gertjon Corona Kulimina Corona Table 3 Characteristics of the circumferential graben-ﬁssure systems in the study area. System Longitude (°E) Latitude (°S) Max radius (km) Arc (°) Number of mapped lineaments Centre elevation (km) Elevation above MPR (km) Centre within corona? C01 C02 C03 C04 C05 C06 C07 269.00 262.25 265.75 275.75 273.00 269.50 269.75 25.25 27.50 24.50 31.75 27.50 25.25 28.00 138 121 75 74 70 52 39 360 130/90 320 280 300 330 340 273 89 119 45 30 97 34 6052.1 6052.6 6051.7 6052.6 6053.3 6051.3 6052.9 1.1 1.6 0.7 1.6 2.3 0.3 1.9 Y Y Y Y N Y Y Associated structure Hervor Corona Kulimina Corona Chuku Corona Obiemi Corona Ts’an Nu Mons Hervor Corona Xmukane Corona J.R. Graff et al. / Icarus 306 (2018) 122–138 127 Fig. 3. Distribution of radiating (R#), linear (L#), and circumferential (C#) graben-ﬁssure systems within the study area. Blue and yellow stars denote centres of radiating and circumferential systems, respectively; purple hexagons indicate central positions of linear systems. Number coding corresponds to the mapped systems that are catalogued in Tables 1–3. The location of topographic troughs, coinciding with Parga Chasma, is indicated in grey. The study area is indicated by the black rectangle. 7 (54%) exhibit a combination of younger and synchronous relationships, while the remaining 6 (46%) exhibit a combination of older and synchronous relationships. The results from this relative chronology and the detailed linework mapping have been superimposed onto a SAR image of the study area (Fig. 7). Graben-ﬁssure systems of all sizes are evenly distributed throughout the study area, but the largest ones are found in association with coronae (R01–R03, R05) or large volcanoes (R04). There are also some locations, such as within the areal extent of Kulimina Corona (e.g. systems R05 and R06) and in the vicinity of Cryptic Centre 1, that contains dense clusters of mostly smaller systems. These locations also host the majority of young systems interpreted to postdate regional rifting. Many of the largest systems are also among the youngest systems (with R03 as a notable exception). Importantly, all local magmatic centres that are interpreted as the focal points for local triple-junction rifting contain graben- ﬁssure systems that are observed to cross-cut regional rift faults. These age relationships provide evidence in support of multiple stages of Parga Chasma rifting – beginning from regional NE–SW extension between Atla and Themis Regiones and followed by local rifting events extending from individual magmatic centres. 6. Local triple-junction rifting Throughout the study area prominent extension has occurred, interpreted to be associated with both regional and local rifting of the Parga Chasma rift system. Regional rifting is interpreted to having resulted from large-scale NE–SW extension and observed as a pattern of NW–SE oriented rift faults extending across the study area (Fig. 8). Distinct from the regional pattern, local rifting is observed as individual segments (also marked by topographic troughs) extending from individual magmatic centres (Figs. 9–14, Table 4). 128 J.R. Graff et al. / Icarus 306 (2018) 122–138 Fig. 4. Distribution of rift faults within the study area. Topographic troughs are indicated in grey. Fig. 5. Locations of ﬁgures found in Supplementary File 1. J.R. Graff et al. / Icarus 306 (2018) 122–138 129 Fig. 6. Timeline of the tectono-magmatic events in the study area. The regional rifting of Parga Chasma is taken as the baseline (grey rectangle). Systems are numbered from top to bottom with horizontal lines representing their interpreted age ranges relative to each other and to the regional rifting. Older systems are placed towards the left side; younger systems towards the right. Systems interpreted to be synchronous with regional rifting intersect the baseline. Local rift fault segments are listed in parentheses next to the system(s) to which they are genetically linked. Numbering is the same as in Tables 1–4 and Figs. 3, 4, 7, 9–13. Within the study area, all local rift zones can be linked to individual magmatic centres acting as the focal points of triplejunction rifting. Some of these centres are coronae, while the rest are large volcanoes or cryptic centres, the latter only recognized as the loci of radiating graben-ﬁssure systems. In cases where there is no central volcano-tectonic ediﬁce or caldera above a centre, then we label this as a ‘cryptic centre’. Similar cryptic centres have been noted in Ernst et al. (2003) and Studd et al. (2011). Each of the magmatic centres (whether cryptic or not) are hypothesized to be formed from a mantle upwelling (Ernst and Buchan, 1997; Her- rick, 1999; Ernst et al., 2001).These upwellings may be linked to rising mantle plumes or originate from lithospheric delamination (e.g. Şengör, 2001; Şengör and Natal’in, 2001; Ernst, 2014). Mantle plumes are likely candidates based on plume-induced triplejunction rifting having been extensively observed and described in terrestrial settings (cf. Burke and Dewey, 1973; Şengör and Natal’in, 2001). Ts’an Nu Mons (Fig. 10) is interpreted to be the magmatic source of two separate rift zones, extending to the west (RF02) and to the southeast (RF03) of the volcano. Xmukane Corona (Fig. 11) 130 J.R. Graff et al. / Icarus 306 (2018) 122–138 Fig. 7. Magellan SAR image of the study area (50% transparency), superimposed with detailed linework of all radiating and linear graben-ﬁssure systems, which are colourcoded as in Fig. 3. System centres are identiﬁed by circles which are sized and coloured according to maximum system radius/length and relative age, respectively. Table 4 Local rift zones catalogued in the study area. Rift zone Number of rift faults Associated magmatic centre Trend (with respect to centre) RF02 RF03 RF04 RF05 RF06 RF07 RF08 RF09 RF10 RF11 RF12 52 64 202 23 75 136 67 51 174 43 129 Ts’an Nu Mons Ts’an Nu Mons Xmukane Corona Xmukane Corona Xmukane Corona Kulimina Corona Kulimina Corona Kulimina Corona Cryptic Centre 2 Cryptic Centres 1 & 2 Cryptic Centre 1 West Southeast Northwest Northeast South Northwest East East-southeast Northwest Southeast/northwest Southeast is the focus of three separate rift zones, extending to the northeast (RF04), the northwest (RF05), and the south (RF06). Kulimina Corona (Fig. 12) is at the centre of three rift zones, extending to the northwest (RF07), the east (RF08), and the east-southeast (RF09).Cryptic Centre 2 (Fig. 13) is centred on a northwest trending rift zone (RF10), in addition to a segment trending to the southeast (RF11)—this latter segment also connects with Cryptic Centre 1. Cryptic Centre 1 (Fig. 13) is also the centre of a rift zone extending to the southeast (RF12). Combining these observations at a local scale yields a summary map of the study area that displays the ﬁve major magmatic centres interpreted to be the foci for associated local triple-junction rifting (Fig. 14). The detailed mapping and analysis of the local rift fault pattern throughout the study area has revealed numerous local rift trends and provided the necessary information to group these rift faults into local rift zones. In turn, grouping these local rift zones allowed them to be linked to speciﬁc magmatic centres from which they extend. The magmatic centres are typically marked by coronae, large volcanoes, or radiating and/or circumferential graben-ﬁssure systems. Note that there are two radiating systems not associated with any observed centrally located volcano-tectonic structure and J.R. Graff et al. / Icarus 306 (2018) 122–138 131 Fig. 8. Magellan SAR image of the study area superimposed with linework corresponding to the regional rift fault pattern (light blue). The arrows denote the regional NE-SW extension of Parga Chasma. Fig. 9. Magellan SAR image of the study area superimposed with linework corresponding to the local rift faults which are further grouped into separate rift zones based on lineament orientation. Local rift zones are labelled according to the data catalogued in Table 4. 132 J.R. Graff et al. / Icarus 306 (2018) 122–138 Fig. 10. Local rifting associated with Ts’an Nu Mons. (a) Magellan SAR image. (b) Image overlain with mapping of local rift faults and segment lines (RF02, RF03) extending from Ts’an Nu Mons (dark blue). Arrows denote direction of opening. See Fig. 9 for image location. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.) Fig. 11. Local rifting associated with Xmukane Corona. (a) Magellan SAR image. (b) Image overlain with mapping of local rift faults and segment lines extending from Xmukane Corona (turquoise). Arrows denote direction of opening. See Fig. 9 for image location. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.) Fig. 12. Local rifting associated with Kulimina Corona. (a) Magellan SAR image. (b) Image overlain with mapping of local rift faults and segment lines extending from Kulimina Corona (green). Arrows denote direction of extension. See Fig. 9 for image location. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.) we have referred to these as “cryptic” (per the criteria mentioned above). When several mantle upwellings occur in a zone of regional extension and crustal thinning, these magmatic centres can collectively produce the morphology of a large, but often discontinuous rift system. 6.1. BAT region rift systems as collections of local rifting events In the previous section it was postulated that a major section of Parga Chasma can be interpreted as a collection of separate local rift zones, formed via triple-junction rifting from individual magmatic centres. This section explores the possibility that insights from our detailed mapping can be applied throughout the BAT region. Speciﬁcally, we consider the model that individual mag- matic centres can act as similar loci of triple-junction rifting along other parts of Parga Chasma as well as throughout other major rift systems of the BAT region. This hypothesis is formulated to explain the observed discontinuities in the morphologies of rift zones throughout Hecate and Parga Chasmata. Such discontinuities are apparent in the map presented in Graff et al. (2015) which was derived using tools in ArcMap to combine attributes from topography and slope data (zones of relatively steeper slopes and topographic lows) to automatically generate a rift map of the BAT region (Fig. 15). By contrast, the morphology of Devana Chasma can simply be explained as the result of triple-junction rifting extending from only the two major volcanic rises of Beta and Phoebe Regiones at the ends of this single rift system (cf. Kiefer and Swafford, 2006). J.R. Graff et al. / Icarus 306 (2018) 122–138 133 Fig. 13. Local rifting associated with the two cryptic magmatic centres. (a) Magellan SAR image. (b) Image overlain with mapping of local rift faults and segment lines extending from Cryptic Centre 1 (orange) and Cryptic Centre 2 (magenta). Rift zone RF11 connects the two centres and is coloured in both orange and magenta. Arrows denote direction of extension. See Fig. 9 for image location. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.) Fig. 14. Magellan SAR image of the study area superimposed by a summary of all magmatic centres interpreted to be the foci of individual triple-junction rifting; rift zones and centres are coloured-coded based on this interpreted association. Arrows denote direction of local extension. The procedure for identifying individual large-scale rift segments across the entirety of Hecate and Parga Chasmata is that of Graff et al. (2015) which is described in Section 3.3 of this paper. Augmented by the data obtained from the automatically generated ‘rift candidacy map’ (Fig. 15), the preliminary identiﬁcation of rift zones comes primarily by locating the large zones of topographic lows (indicating valleys) and relatively steeper slopes (marking the walls of rift bounding faults). Cross-examining these locations with the more detailed SAR images further allow us to locate rift zones by looking for the presence of radar-bright lineaments indicative of rift faults. With these locations identiﬁed they were then traced out to capture the overall trend of the individual segments. After each segment was identiﬁed, each end or junction point along its length was carefully examined for the presence of a volcanic or magmatic structure in the form of either a volcano or corona. The identiﬁcation of these structures was also further conﬁrmed by using a catalogue of geological features published by the USGS (http://astrogeology.usgs.gov/). In the rare instance that a junction point lacked a volcano or corona structure, close examination of the radar-bright lineaments revealed the presence of at least one radiating or circumferential graben-ﬁssure system, indicating a magmatic source in the nearby vicinity. Applying the methodology from the detailed mapping in the study area along Parga Chasma, we similarly interpret that Hecate Chasma is composed of 13 local rift zones (Fig. 16, Table 5), in- 134 J.R. Graff et al. / Icarus 306 (2018) 122–138 Fig. 15. Distribution of rifts in the BAT region of Venus (modiﬁed after Graff et al., 2015). Areas in red show the locations of major rift systems as automatically calculated by the ArcMap geographical information system, using the approach discussed in Graff et al. (2015), superimposed on a topography map at 50% transparency (source: USGS http://astrogeology.usgs.gov/). The black rectangle indicates the area of Parga Chasma mapped in detail in this study. Using tools available in ArcMap’s ArcToolbox function (such as ‘hillshade relief’, ‘raster reclassify’, and ‘weighted sum’ [ESRI ArcGIS Resources (2014) http://resources.arcgis.com/en/home/]), we were able to combine attributes from topography and slope data to delineate zones of topographic lows and relatively steeper slopes to automatically generate a map highlighting all candidate areas for rift system locations. Table 5 Rift zone groups along Hecate Chasma. Rift zone group Number of local segments Number of associated magmatic centres Type(s) of centre(s) RZG01(Atla Regio) RZG02(Beta Regio) RZG03 RZG04 RZG05 RZG06 RZG07 2 1 2 2 1 3 2 1 1 1 1 1 2 2 Volcanic rise Volcanic rise Graben-ﬁssure system Corona Corona Corona and graben-ﬁssure system Corona cluding those extending directly from the main plumes and associated triple-junction rifting of Atla and Beta Regiones at the ends of the overall rift system. We propose that Parga Chasma contains as many as 45 local rift zones throughout its full length (Fig. 17, Table 6). Most of these local rift zones are linked to a corona or large volcano, but some segments are observed to be isolated from any corona or volcano. However, in such cases, graben-ﬁssure systems mark cryptic magmatic centres from which separate local rift zones extend. 6.2. Terrestrial analogue to Hecate and Parga Chasmata The discontinuous morphology of rift systems, coupled with the interpretation of regional formation via local rifting from multiple magmatic centres, may not be conﬁned only to Venus. On Earth, the pre-spreading conﬁguration of the Atlantic Rift System exhibits local centres of rifting along its full extent that are similar to those observed along both Hecate and Parga Chasmata (Fig. 17). In addition, the rifting and subsequent opening of the Atlantic Ocean is linked to the arrival of at least three large mantle plumes, each of which resulted in the formation of a large igneous province (Ernst, 2014). Thus, there are many similarities between the terrestrial rifting (prior to ocean opening) of the Atlantic Rift System and the Venusian rifting of Hecate and Parga Chasmata can be observed. It is clear from comparing Fig. 18b and c and Fig. 17a that local rift centres are characteristic of both the BAT region and Atlantic Rift System, but the discontinuous aspect of rifting between the local centres in the Venusian rifts is not apparent for the Atlantic Rift System (Fig. 18c, modiﬁed from the schematic diagram of Sengor (1995)). However, such discontinuities in the Atlantic Rift System are implied by the existence of local centres (from which rifting should nucleate outward). This is well established for the J.R. Graff et al. / Icarus 306 (2018) 122–138 135 Fig. 16. Magellan SAR images of Hecate Chasma (centred at 240°E, 16°N). (a) Image overlain by ungrouped local rift zones (purple lines) and corona locations (red stars). (b) Local rift zones and magmatic centres organized into rift zone groups (RZG) and colour-coded to represent genetic association. Stars represent coronae, while circles denote radiating and/or circumferential graben-ﬁssure systems. Rift zone groups are catalogued in Table 5. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.) Table 6 Rift zone groups along Parga Chasma. Rift zone group Number of local segments Number of associated magmatic centres Type(s) of centre(s) RZG01 RZG02 RZG03 (Atla Regio) RZG04 RZG05 RZG06 RZG07 RZG08 RZG09 (Kulimina) RZG10 RZG11 RZG12 (Cryptic Centres 1 and 2) RZG13 RZG14 (Xmukane) RZG15 (Ts’an Nu Mons) RZG16 RZG17 RZG18 RZG19 RZG20 RZG21 RZG22 2 2 4 3 1 3 3 3 3 2 2 3 1 3 2 2 1 1 1 1 1 1 1 1 3 2 1 1 1 1 1 1 2 2 1 1 1 2 1 1 1 1 1 2 Corona Corona Corona and volcanic rise Corona Corona Corona Corona Graben-ﬁssure system Corona Graben-ﬁssure system Corona Graben-ﬁssure system Corona Corona Volcano Corona Corona Corona Corona Corona Corona Corona rift systems of the northern, central and southern Atlantic given the distinct differences in timing of rifting in these regions, c. 55, 190 and 120 Ma, respectively. Some rift discontinuities do seem to be present in the more detailed rift maps of Sengor and Natalin (2001), but a more comprehensive study of the entire Atlantic Rift System would be required to fully characterize the pattern of discontinuities along its length. To summarize, each rift system: (1) exhibits similar discontinuous morphology; (2) contains major plume-related magmatic centres, likely acting as a primary cause of rifting; and (3) contain smaller centres that display local triplejunction rifting. A difference between the Atlantic Rift System and the situation on Venus is that on Earth the various rift segments succeeded in opening the Central, Southern, and Northern Atlantic Oceans (at times, ∼190, ∼120, and ∼55 Ma), while the Venusian examples failed to progress to an ocean opening stage. Drawing further comparisons between terrestrial and Venusian rifting involves the age relationships between the rifting events and their interpreted magmatic centres. In the case of the Atlantic 136 J.R. Graff et al. / Icarus 306 (2018) 122–138 Fig. 17. Magellan SAR images of Parga Chasma (centred at 245°E, 20°S). (a) Image overlain by ungrouped local rift zones (purple lines) and corona locations (red stars). (b) Local rift zones and magmatic centres organized into rift zone groups (RZG) and colour-coded to represent genetic association. Stars represent coronae, while circles denote radiating and/or circumferential graben-ﬁssure systems. Rift zone groups are catalogued in Table 6. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.) Rift System each of the main plume centres were emplaced at different times over a range of ∼140 million years (200, 135, and 62 Ma, with ocean opening starting approximately 10 Ma later in each case; see Fig. 11.2 and text in Ernst (2014) for details). The separate triple-junctions focussed on each main plume centre are also interpreted to have formed synchronously (±∼10–20 Ma) with their associated plume centre (Ernst, 2014). With respect to Venusian rifting, the relative chronology of magmatic events within the study area leads a comparable result. Notably, the magmatic centres (and associated graben-ﬁssure systems) linked to local rift zones in the study area(Ts’an Nu Mons (R04); Xmukane Corona (R01); Kulimina Corona (R05); Cryptic Centre 1 (R07); and Cryptic Centre 2 (R16)) have all been dated via cross-cutting relationships as younger than the regional rifting of Parga Chasma and synchronous with formation with their associated triple-junction rift zones. 7. Conclusions While previous extensional lineament studies primarily focussed on graben-ﬁssure systems (e.g. Grosﬁls and Head, 1994a,b; Ernst et al., 2003; Studd et al., 2011), this study included the identiﬁcation of rift zones and detailed mapping of the associated rift faults along a 1500 km segment of Parga Chasma. Clusters of rift faults and graben-ﬁssures were grouped into 11 local rift zones and 47 graben-ﬁssure systems, respectively. Regional rifting of Parga Chasma exhibits an overall NW-SE extension pattern that is interpreted to have preceded local rifting events extending from individual magmatic centres located along the rift system. Each such magmatic centre was interpreted as the locus for the coeval development of a radiating graben-ﬁssure system and associated local triple-junction rifting. A similar model of overall regional extension and subsequent localization into local triple-junction rift zones is also applied to the rest of Parga Chasma and the entirety of Hecate Chasma. Comparable rift morphology is also observed in examples from Earth, notably along the ∼15,0 0 0 km Atlantic Rift System. The prespreading morphology of this giant terrestrial rift system exhibits similar discontinuities in the rift pattern to those observed along Hecate and Parga Chasmata. The rifting of the Atlantic Rift System (prior to ocean opening in each segment) has also formed from the arrival of at least three separate mantle plumes and several smaller local events, each causing local triple-junction rifting that collectively developed into the complex morphology of a complete rift system. This observation further supports our local rift model hypothesis, implying that major Venusian and terrestrial rift sys- J.R. Graff et al. / Icarus 306 (2018) 122–138 137 Fig. 18. Comparison between the Atlantic Rift System with the schematic morphology of Hecate and Parga Chasmata. Atlantic Rift System diagram modiﬁed from Fig. 11.2 in Ernst (2014) which was modiﬁed after Fig. 2.7B in Şengör (1995). The St. Helena plume centre is proposed after Hollanda et al. (2016). All three diagrams are at the same scale. The Hecate and Parga Chasmata maps are extracted from Figs. 16 and 17. tems can be identiﬁed as a collection of local triple-junction rifting events focussed on individual magmatic centres along a larger regional rift system. Acknowledgements This research was supported by a NSERC Discovery Grant awarded to Richard Ernst. In addition, Richard Ernst has been partially supported by Russian Government grant no. 14.Y26.31.0012. Two anonymous reviewers and journal editor Professor Oded Aharonson are thanked for their thoughtful suggestions. 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