the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Inversion of transfer zones in salt-bearing extensional systems: insights from analogue modeling
Elizabeth Parker Wilson
Pablo Granado
Pablo Santolaria
Oriol Ferrer
Josep Anton Muñoz
Abstract. This work uses sandbox analogue models to analyze the formation and subsequent inversion of a decoupled extensional system comprised of two segmented half-grabens with thick early syn-rift salt. The segmented half grabens strike perpendicular to the direction of extension and subsequent shortening. Rifting created first a basement topography that was infilled by model salt, followed by a second phase of extension and sedimentation, followed afterwards by inversion. During the second phase of extension, syn-rift syncline minibasins developed above the basement extensional system and extended beyond the confines of the fault blocks. Sedimentary downbuilding and extension initiated the migration of model salt to the basement highs, forming salt anticlines, reactive diapirs, and salt walls perpendicular to the direction of extension, except for along the transfer zone where a slightly oblique salt anticline developed. Inversion resulted in decoupled cover and basement thrust systems. Thrusts in the cover system nucleated along squeezed salt structures and along primary welds. New primary welds developed where the cover sequence touched down on basement thrust tips due to uplift, salt extrusion, and syn-contractional downbuilding caused by loading of syn-contractional sedimentation. Model geometries reveal the control imposed by the basement configuration and distribution of salt in the development of a thrust front from the inversion of a salt-bearing extensional system. In 3D, the interaction of salt migrating from adjacent syn-rift basins can modify the expected salt structure geometry, which may in turn influence the location and style of thrust in the cover sequence upon inversion. Results are compared to the northern Lusitanian Basin, offshore Portugal and the Isàbena area of the South-Central Pyrenees, Spain.
Elizabeth Parker Wilson et al.
Status: final response (author comments only)
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RC1: 'Comment on egusphere-2022-1461', Elena Konstantinovskaya, 23 Feb 2023
Review of the manuscript
Inversion of transfer zones in salt-bearing extensional systems: insights from analogue modeling by
Elizabeth P. Wilson, Pablo Granado, Pablo Santolaria, Oriol Ferrer, Josep Anton Muñoz
Submitted to EGUsphere
The submitted manuscript aims to demonstrate the variation of structural styles in salt-bearing segmented rift systems that underwent subsequent shortening and syn-shortening sedimentation. The sand-polymer-based analog experiments involved three models, in which two salt-bearing half-graben basins segmented by an intervening transfer zone experienced rifting (Model 1), rifting and inversion of the rift system (Model 2), and rifting and inversion with syn-contractional sedimentation (Model 3). The obtained results of analog modeling are compared to the geological structures of the Northern Lusitanian Basin, offshore Portugal and Isàbena area, South-Central Pyrenees, Spain.
Experimental setup and procedure are well described. Preparing and conducting experiments likely required a substantial amount of work. The manuscript represents a thorough analysis of experimental results and provides a new insight into the salt decoupled extensional and inversion systems.
It would be helpful if the following comments and questions might be clarified:
Lines 110-115: why Mylar sheet remains not deformable under the simulation settings of extension and subsequent shortening? The basal friction at the top of the steel plate and Mylar sheet is different. Did it influence the model deformation?
Lines 120-130: Is there any difference in mechanical properties of the basement and upper sand packs? In sand material itself or in a way it was packed? See comment for lines 485-490.
Lines 125-130: what procedure was applied to ensure that the triangular polymer prisms would not be deformed during the model buildup and burial of the prisms by 9 cm-thick sand pack simulating the basement?
Lines 130-135, 140-145, 170-175: The applied combined velocity during extension phases was set equal to 2.78 x 10-4 cm/s (phase 1) and 1.67 x 10-4 cm/s (Phase 3) and to 1.67 x 10-4 cm/s during the shortening (Phase 5). Please explain what does “combined velocity” mean. Deformation of the polymer is sensitive to applied strain rate. How the velocity of extension and shortening was chosen? Was there any sensitivity study performed to determine the range of applicable strain rate?
What was sedimentation rate of sand layers during syn-contractional deformation?
Line 170: Please clarify here which wall was pushed - the one at the side of the Mylar sheet or of the metal plate. It seems to be the right wall with attached Mylar sheet that was used for shortening (Figs 8 and 11).
Line 180-185: what steps were undertaken to facilitate the models’ slicing at intervals of 3 mm (!!) at the end of each experiment? Salt model polymer flows fast….
Line 200-205. Why MF1 attains more slip than MF2 in Model 1? The master listric faults MF2 and MF1 in Fig. 4a and 4c have different geometry, in particular degree of curvature and dip angle. Could these differences be related to the shape of triangle shape of salt model seeds in the basement? The seed SS2 after the extension generally preserves its triangle shape (Fig. 4a), while SS1 has migrated resulting in a weld between the basement sand layers and basal sheet (Fig. 4c).
Lines 240-250: “Half-graben 1 propagated along strike across the whole model width (Fig. 4 a-c). This is probably related to the presence of the underlying velocity discontinuity (V.D. in Fig. 2) that favors extension localization, lateral slip transfer along the strike of the polymer seed, and the formation of the largest depocenter of Model 1. Conversely, extension along MF2 produces a more diffuse structural pattern, with one largest depocenter right of the master fault”.
Why MF2 and associated graben did not localize and propagate laterally to the same extent as MF1 and associated depocenter? Could it be that the behaviour of the V.D. between the rubber sheet and the Mylar sheet (MF2 system) was different from the V.D. between the steel plate and the rubber sheet due to the variation in the strength contrast between the basement materials: steel plate - rubber sheet - Mylar sheet? This suggestion is likely confirmed on lines 494-496.
Line 273: It is not mentioned here whether Model 2 was subjected to extension before the shortening. How steady are the results of extension obtained in Model 1 (Fig. 4)? Could it be expected that the results of Model 1 were repeated in Model 2 during the extension, and the shortening has started from the same or similar point as shown in Fig. 4?
Line 313. Replace SS2 by SS1 as Fig. 7c represents the section across the SS1 salt seed.
Line 310-315: If the results of extension in Model 2 repeated the results of Model 1, one could expect that the geometry of thrust fault systems in MF1 and MF2 during the shortening phase (Fig. 7a and 7c) would be controlled by the shape of listric faults MF1 and MF2 at the end of extension, which, in turn, might have been controlled by the behavior (degree of migration) of the model salt triangle seeds SS1 and SS2.
Models 2-3: Could the lower degree of shortening along MF1 be resulted from the greater distance between MF1 and movable backstop at the right, if compared to the shorter distance between the backstop and MF2? In both cases, most of the shortening is accommodated along MF2 that is located closer to the movable backstop, and the shortening structures progressed laterally across the model, accommodating most of the contractional deformation. As a result, shortening was more distributed in the section across MF1, and it contributed to reactivation of MF1 to lower degree.
Lines 485-490: “In our models, the syn-rift basin geometry and related sediment distribution are strongly controlled by the position of the underlying pre-salt basement faults, the amount and rate of slip attained by those faults, the original distribution and thickness of model salt, as well as the thickness and mechanical properties of the pre-kinematic sand pack”.
However, in the model setup, nothing is said about the difference of mechanical properties of the basement and upper (pre-kinematic) sand packs (lines 120-130).
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RC2: 'Comment on egusphere-2022-1461', Anonymous Referee #2, 24 Mar 2023
The manuscript presents results on a series of three analogue models of inversion basin tectonics including a viscous décollement layer (salt) and weak zones in the basement that simulate subsidenence at half-grabens in the basement that later also localize the thrust faults. The results of the study are clearly illustrated, and the manuscript is well written. The study is however very similar to another already published study of Dooley and Hudec (2020), who employed the same technique, and their map-view patterns and cross-sections and doubly-vergent thrusts are also similar, except the shape of the weak seeds in the basement layer that act as nuclei for developing normal and thrust faults.
The introduction mentions a wealth of literature, analog modeling studies on the inversion tectonics of salt basins, but I miss the definition of the clear objectives of the study. What are the unresolved scientific issues (e.g. fault and layer thickness patterns?) associated with inverted basins, especially those with segmented half-grabens? For example, how might the individual segments interact in terms of laterally migrating salt in the source layer? What are typical deformation patterns in the cover sequence in the segmented half-graben basins that might control on fault development and extrusion of salt in later stages (e.g. inversion)? This is difficult to imagine, becuase the Figure 1 only shows a single segment of the half-graben array. Are there any map examples that may have resulted in the inversion of the segmented half-graben basins? I am sure that some lessons about such systems can be derived also from the study of Dooley and Hudec (2020) - however there is no mention of this in the Introduction nor in the Discussion. How are your sets of experiments similar/different with respect to the latter study?
If you consider the Aras and San Juan basins (discussion), as equivalent transects inside a larger inverted basin with segmented half-graben, what can we learn from the similarity of the structures in these two transects with the vertical profiles accross your models? Is this comparison even possible given the large translation of the cover sequence above the basement?
Please find also some text corrections and suggestions in the annotated pdf version of your manuscripts (uploaded).
The viscosity value of the silicone is wrong everywhere in the text (and scaling table). It should be 1.6x10^4 ... (not -4).
Elizabeth Parker Wilson et al.
Video supplement
Inversion of Transfer Zones - Model 03 shortening with syn-contractional sedimentation Wilson, Elizabeth P. https://doi.org/10.5446/60622
Inversion of Transfer Zones - Model 02 shortening, no surface processes Wilson, Elizabeth P. https://doi.org/10.5446/60621
Inversion of Transfer Zones - Model 01 Extension Wilson, Elizabeth P. https://doi.org/10.5446/60620
Elizabeth Parker Wilson et al.
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