News Release 

New Geology articles published online ahead of print in February

Geological Society of America

Research News

p> Boulder, Colo., USA: Several new articles were published online ahead of print for Geology in February. Topics include stress in survivor plants following the collapse of land ecosystems, the Gulf of Aden, whether the Denali fault is still active, the first reported Burgess Shale-type fauna rediscovered, and redefining the age of the lower Colorado River. These Geology articles are online at .

Discovery of a large subsoil nitrate reservoir in an arroyo floodplain and associated aquifer contamination
Benjamin S. Linhoff; John J. Lunzer
Abstract: In an area of elevated nitrate (NO3) groundwater concentrations in the northern Chihuahuan Desert in central New Mexico (United States), a large reservoir of nitrate was found in the subsoil of an arroyo floodplain. Nitrate inventories in the floodplain subsoils ranged from 10,000 to 38,000 kg NO3-N/ha--over twice as high as any previously measured arid region. The floodplain subsoil NO3 reservoir was over 100 times higher than the adjacent desert (59-95 kg NO 3-N/ha). Chloride mass balance calculations of subsoils indicate arroyo floodplain subsoils have undergone negative recharge since 2600-8600 yr ago, while the surrounding desert has had negative recharge since 13,000-17,000 yr ago. Compared to the adjacent desert, plant communities are larger and more abundant in the floodplain, though subsoil NO 3 is apparently not utilized. We demonstrate that NO3 accumulates in the subsoil of the floodplain through evaporation of monsoon season precipitation funneled into the arroyo. Through a one-dimensional vadose zone model, we show that the NO3 inventories in the arroyo floodplain could be acquired 8 to 75 times faster than through atmospheric deposition through the lateral movement.
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Homeostatic response of Aptian gymnosperms to changes in atmospheric CO 2 concentrations
Germán Mora; Ana M. Carmo; William Elliott
Abstract: The sensitivity of plant carbon isotope fractionation (13Δleaf) to changes in atmospheric CO2 concentrations ( Ca) is the subject of heavy debate, with some studies finding no sensitivity, while others show a strong dependency. We tested the hypothesis of photosynthetic homeostasis by using δ13C of n-alkanes, cuticles, and bulk organic matter of gymnosperm-rich rocks (Arundel Clay) from two sites deposited during the Aptian, a time that experienced significant Ca variations. Our results show no effect of Ca on 13Δleaf, and a relatively constant Ci/Ca (0.64 ± 0.04, 1σ; i--intercellular space), a value that is similar to that of modern gymnosperms. These results suggest that Aptian gymnosperms used homeostatic adjustments with rising Ca, probably involving increased carbon assimilation and/or stomatal closure, a response also found in modern gymnosperms. The similarity between Aptian and modern gymnosperms suggests that the processes responsible for regulating CO 2 and water vapor exchange during photosynthesis have remained unaltered in gymnosperms for the past 128 m.y.
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The Parker Quarry Lagerstätte of Vermont--The first reported Burgess Shale-type fauna rediscovered
Giovanni Pari; Derek E.G. Briggs; Robert R. Gaines
Abstract: Soft-bodied fossils of Cambrian age, now known as Burgess Shale-type biotas, were first described from the Parker Slate of the northwest Vermont (USA) slate belt in the late 19th century, 25 years before the discovery of the Burgess Shale in British Columbia, Canada. Here, we report the rediscovery of fossiliferous horizons at Parker's Cobble, the site of the original quarry, which was thought to have been exhausted by excavation. New discoveries include a radiodont, multiple specimens of a new bivalved arthropod, a priapulid, and other undescribed forms. Pervasive soft-sediment deformation suggests accumulation near the toe of a steep unstable slope, similar to the setting of the Burgess Shale. Although fossils are rare, the exceptional preservation of some soft-bodied taxa suggests that recovered diversity was limited by transport into an inhospitable benthic setting rather than by decay, and this implies a potential for future discoveries of new taxa.
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Redefining the age of the lower Colorado River, southwestern United States
R.S. Crow; J. Schwing; K.E. Karlstrom; M. Heizler; P.A. Pearthree ...
Abstract: Sanidine dating and magnetostratigraphy constrain the timing of integration of the lower Colorado River (southwestern United States and northern Mexico) with the evolving Gulf of California. The Colorado River arrived at Cottonwood Valley (Nevada and Arizona) after 5.24 Ma (during or after the Thvera subchron). The river reached the proto-Gulf of California once between 4.80 and 4.63 Ma (during the C3n.2r subchron), not at 5.3 Ma and 5.0 Ma as previously proposed. Duplication of section across newly identified strands of the Earthquake Valley fault zone (California) probably explains the discrepancy. The data also imply the start of focused plate motion and basin development in the Salton Trough (California) at 6-6.5 Ma and relative tectonic stability of the southernmost part of the lower Colorado River corridor after its integration. After integration, the Colorado River quickly incised through sediment-filled basins and divides between them as it also likely excavated Grand Canyon (Arizona). The liberated sediment from throughout the system led to deposition of hundreds of meters of Bullhead Alluvium downstream of Grand Canyon after 4.6 Ma as the river adjusted to its lower base level.
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Subduction erosion and crustal material recycling indicated by adakites in central Tibet
Zong-Yong Yang; Qiang Wang; Lu-Lu Hao; Derek A. Wyman; Lin Ma ...
Abstract: Subduction erosion is important for crustal material recycling and is widespread in modern active convergent margins. However, such a process is rarely identified in fossil convergent systems, which casts doubt on the importance of subduction erosion through the geological record. We report on ca. 155 Ma Kangqiong (pluton) intrusive rocks of a Mesozoic magmatic arc in the southern Qiangtang terrane, central Tibet. These rocks mainly consist of trondhjemites and tonalites and are similar to slab-derived adakites with mantle-like zircon oxygen isotope compositions (δ 18O = 5.2‰-5.6‰), they display more evolved Sr-Nd isotopes and higher Th/La relative to mid-oceanic ridge basalts from the Bangong-Nujiang suture, and they contain abundant amphibole and biotite. These characteristics indicate magma generation via H2O-fluxed melting of eroded forearc crust debris with subducted oceanic crust at 1.5-2.5 GPa and 700-800 °C. In addition, the intrusions are exposed <20 km north of the Bangong-Nujiang suture. Given the formation of adakites, narrow arc-suture distance, migration of the Jurassic frontal arc toward the continent interior, and other independent geological archives, we suggest that the hydrated forearc crust materials were removed from the overlying plate and carried into the mantle by subduction erosion. Our study provides the first direct magmatic evidence for a subduction erosion process in pre-Cenozoic convergent systems, which confirms an important role for such processes in subduction-zone material recycling.
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Abrasion regimes in fluvial bedrock incision
Alexander R. Beer; Michael P. Lamb
Abstract: River incision into bedrock drives landscape evolution and couples surface changes to climate and tectonics in uplands. Mechanistic bedrock erosion modeling has focused on plucking--the hydraulic removal of large loosened rock fragments--and on abrasion--the slower fracturing-driven removal of rock due to impacts of transported sediment--which produces sand- or silt-sized fragments at the mineral grain scale (i.e., wear). An abrasion subregime (macro-abrasion) has been hypothesized to exist under high impact energies typical of cobble or boulder transport in mountain rivers, in which larger bedrock fragments can be generated. We conducted dry impact abrasion experiments across a wide range of impact energies and found that gravel-sized fragments were generated when the impact energy divided by squared impactor diameter exceeded 1 kJ/m2. However, the total abraded volume followed the same kinetic-energy scaling regardless of fragment size, holding over 13 orders of magnitude in impact energy and supporting a general abrasion law. Application to natural bedrock rivers shows that many of them likely can generate large fragments, especially in steep mountain streams and during large floods, transporting boulders in excess of 0.6 m diameter. In this regime, even single impacts can cause changes in riverbed topography that may drive morphodynamic feedbacks.
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External signal preservation in halokinetic stratigraphy: A discrete element modeling approach
Zoë A. Cumberpatch; Emma Finch; Ian A. Kane
Abstract: Subsurface salt movement in the absence of external tectonic forces can affect contemporaneous sediment deposition, mask allocyclic signals, and deform older strata. We used a discrete element model (DEM) to better understand salt-related modification of a sedimentary sequence with an increasing sedimentation rate. This permitted quantification of thinning rates and analysis of the lateral extent of synkinematic layers. Results show realistic evolution of salt-related faults, defining two salt-withdrawal basins, beyond which strata are undeformed. Thinning of stratigraphy is four times greater between the salt flank and crest than between the undeformed zone and flank, confirming an intense zone of halokinetic modulation adjacent to the diapir. Early, slowly aggrading layers are isolated within the salt-withdrawal basin and strongly influenced by salt growth, whereas later, quickly aggrading layers are more laterally extensive, matching inferences made from subsurface and outcrop data. Halokinetic modulation reduces up the stratigraphic section, mirroring observations around the Pierce diapirs, in the North Sea, offshore UK. Our DEM provides quantitative insights into the dynamic interplay between halokinetic and allocyclic controls on salt-stratigraphic relationships.
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Extreme Quaternary plate boundary exhumation and strike slip localized along the southern Fairweather fault, Alaska, USA
Richard O. Lease; Peter J. Haeussler; Robert C. Witter; Daniel F. Stockli; Adrian M. Bender ...
Abstract: The Fairweather fault (southeastern Alaska, USA) is Earth's fastest-slipping intracontinental strike-slip fault, but its long-term role in localizing Yakutat-(Pacific-)North America plate motion is poorly constrained. This plate boundary fault transitions northward from pure strike slip to transpression where it comes onshore and undergoes a <25°, 30-km-long restraining double bend. To the east, apatite (U-Th)/He (AHe) ages indicate that North America exhumation rates increase stepwise from ~0.7 to 1.7 km/m.y. across the bend. In contrast, to the west, AHe age-depth data indicate that extremely rapid 5-10 km/m.y. Yakutat exhumation rates are localized within the bend. Further northwest, Yakutat AHe and zircon (U-Th)/He (ZHe) ages gradually increase from 0.3 to 2.6 Ma over 150 km and depict an interval of extremely rapid >6-8 km/m.y. exhumation rates that increases in age away from the bend. We interpret this migration of rapid, transient exhumation to reflect prolonged advection of the Cenozoic-Cretaceous sedimentary cover of the eastern Yakutat microplate through a stationary restraining bend along the edge of the North America plate. Yakutat cooling ages imply a long-term strike-slip rate (54 ± 6 km/m.y.) that mimics the millennial (53 ± 5 m/k.y.) and decadal (46 mm/yr) rates. Fairweather fault slip can account for all Pacific-North America relative plate motion throughout Quaternary time and indicates stability of highly localized plate boundary strike slip on a single fault where extreme rock uplift rates are persistently localized within a restraining bend.
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Phanerozoic variation in dolomite abundance linked to oceanic anoxia
Mingtao Li; Paul B. Wignall; Xu Dai; Mingyi Hu; Haijun Song
Abstract: The abundance of dolomitic strata in the geological record contrasts with the general rarity of locations where dolomite forms today, a discrepancy that has long posed a problem for their interpretation. Recent culture experiments show that dolomite can precipitate at room temperature, raising the possibility that many ancient dolomites may be of syngenetic origin. We compiled a large geodata set of secular variations in dolomite abundance in the Phanerozoic, coupled with compilations of genus richness of marine benthic invertebrates and sulfur-isotope variations in marine carbonates. These data show that dolomite abundance is negatively correlated to genus diversity, with four dolomite peaks occurring during mass extinctions. Dolomite peaks also correspond to the rapid increase in sulfur-isotope composition (δ34S), an indicator of enhanced sulfate reduction, in anoxic oceans. These results confirm that variations in dolomite abundance during the Phanerozoic were closely linked with changes in marine benthic diversity, with both in turn related to oceanic redox conditions.
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Metal-induced stress in survivor plants following the end-Permian collapse of land ecosystems
Daoliang Chu; Jacopo Dal Corso; Wenchao Shu; Song Haijun; Paul B., Wignall ...
Abstract: Teratological spores and pollen are widespread in sediments that record the Permian- Triassic mass extinction. The malformations are thought to be the result of extreme environmental conditions at that time, but the mutagenic agents and the precise timing of the events remain unclear. We examined the abundance of teratological sporomorphs and metal concentrations in a Permian-Triassic tropical peatland succession of southwestern China. We find a significant peak of spore tetrads of lycopsid plants (as much as 19% of all sporomorphs) coeval with increases in Cu and Hg concentrations above the main terrestrial extinction interval, which marks the loss of Permian Gigantopteris forests, increased wildfire activity, and the disappearance of coal beds. Thus, in tropical peatlands, mutagenesis affected only surviving plants. Mutagenesis was likely caused by metal toxicity, linked to increased Hg and Cu loading, but was not itself a direct cause of the terrestrial crisis.
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Inherited lithospheric structures control arc-continent collisional heterogeneity
M.S. Miller; P. Zhang; M.P. Dahlquist; A.J. West; T.W. Becker ...
Abstract: From west to east along the Sunda-Banda arc, convergence of the Indo-Australian plate transitions from subduction of oceanic lithosphere to arc-continent collision. This region of eastern Indonesia and Timor-Leste provides an opportunity for unraveling the processes that occur during collision between a continent and a volcanic arc, and it can be viewed as the temporal transition of this process along strike. We collected a range of complementary geological and geophysical data to place constraints on the geometry and history of arc-continent collision. Utilizing ~4 yr of new broadband seismic data, we imaged the structure of the crust through the uppermost mantle. Ambient noise tomography shows velocity anomalies along strike and across the arc that are attributed to the inherited structure of the incoming and colliding Australian plate. The pattern of anomalies at depth resembles the system of salients and embayments that is present offshore western Australia, which formed during rifting of east Gondwana. Previously identified changes in geochemistry of volcanics from Pb isotope anomalies from the inner arc islands correlate with newly identified velocity structures representing the underthrusted and subducted Indo-Australian plate. Reconstruction of uplift from river profiles from the outer arc islands suggests rapid uplift at the ends of the islands of Timor and western Sumba, which coincide with the edges of the volcanic-margin protrusions as inferred from the tomography. These findings suggest that the tectonic evolution of this region is defined by inherited structure of the Gondwana rifted continental margin of the incoming plate. Therefore, the initial template of plate structure controls orogenesis.
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Biomineralization of the Cambrian chancelloriids
Hao Yun; Xingliang Zhang; Glenn A. Brock; Luoyang Li; Guoxiang Li
Abstract: As extinct animals that flourished during the Cambrian explosion, chancelloriids have a unique body plan lacking guts but with a flexible integument and a suite of star-shaped, hollow sclerites. Due to this body plan, along with the paucity of knowledge on sclerite biomineralization, the phylogenetic position of chancelloriids within the Metazoa is still controversial. Integration of analyses of diverse fossils from Cambrian stage 2 to the Wuliuan Stage of China and Australia indicates that chancelloriid sclerites possess an encasement-like organic layer and a fibrous aragonitic layer. The organic layer is inferred to be a specialized trait derived from the epidermal integument of the animal body. The sclerites were likely biomineralized by using the outer organic layer as a template to absorb cations and precipitate crystal nuclei, reflecting a strategy adopted by a range of eumetazoans with a developed epidermis. Therefore, the hypothesis that chancelloriids represent an epitheliozoan-grade animal and an early explorer of template-based biomineralization is supported.
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High-precision U-Pb age constraints on the Permian floral turnovers, paleoclimate change, and tectonics of the North China block
Qiong Wu; Jahandar Ramezani; Hua Zhang; Jun Wang; Fangui Zeng ...
Abstract: The Permian marine-terrestrial system of the North China block provides an exceptional window into the evolution of northern temperate ecosystems during the critical transition from icehouse to greenhouse following the late Paleozoic ice age (LPIA). Despite many studies on its rich hydrocarbon reserves and climate-sensitive fossil flora, uncertain temporal constraints and correlations have hampered a thorough understanding of the records of geologic, biologic, and climatic change from the North China block. We present a new chronostratigraphy based on high-precision U-Pb chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) geochronology of tuffs from a near-complete latest Carboniferous-Permian succession in North China. The results indicate that the predominance of continental red beds, climate aridification, and the disappearance of coals and characteristic tropical flora were well under way during the Cisuralian (Early Permian) in the North China block, significantly earlier than previously thought. A nearly 20 m.y. hiatus spanning the early Kungurian to the mid-Guadalupian (or later) is revealed in the northern North China block to have close temporal and spatial associations with the closure and/or subduction of the Paleo-Asian Ocean and its related tectonic convergence. This long hiatus was concomitant with the prominent loss of the highly diverse and abundant Cathaysian floras and the widespread invasion of the monotonous Angaran floras under arid climate conditions in the North China block. Similarities in the floral and climate shift histories between Euramerica and North China suggest that aside from the regional tectonic controls and continental movement, extensive volcanism during the Cisuralian may have played a major role in the global warming and aridification in the aftermath of the LPIA.
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Apatite nanoparticles in 3.46-2.46 Ga iron formations: Evidence for phosphorus-rich hydrothermal plumes on early Earth
Birger Rasmussen; Janet R. Muhling; Alexandra Suvorova; Woodward W. Fischer
Abstract: Phosphorus is an essential nutrient that is thought to have regulated primary productivity in global oceans after the advent of oxygenic photosynthesis. The prime source of seawater phosphorus is regarded to be continental weathering of phosphate minerals. Ancient seawater phosphorus concentrations have been constrained using the phosphorus content of iron-rich chemical sediments--banded iron formations (BIFs); however, the removal processes and depositional phases remain unclear. Here we report that nanometer-sized apatite crystals (<500 nm) are ubiquitous in 3.46-2.46 Ga BIFs and cherts from the Kaapvaal (South Africa) and Yilgarn and Pilbara (Western Australia) cratons. The apatite is uniformly dispersed in a chemical sediment comprising greenalite nanoparticles, which were encased in very early diagenetic silica cement that limited compaction and chemical reactions. The lack of organic carbon (below detection; <0.3 wt%) and absence of primary iron oxides implies that the phosphorus was not derived from the degradation of organic matter or seawater scavenging by oxide particles. Instead, the occurrence of apatite in sediments derived from hydrothermally sourced Fe2+ and SiO2(aq) suggests that phosphorus too was derived from vent plumes. Today, seawater P is rapidly removed from vent fluids due to scavenging by oxidized Fe 2+. However, prior to the Great Oxidation Event (2.45-2.32 Ga), dissolved phosphorus released during anoxic alteration of seafloor basalts escaped the iron-oxidation trap. Our results point to the existence of a submarine hydrothermal flux of dissolved phosphorus that supplied nutrients to the early anoxic oceans. High amounts of seawater P may help to explain why phosphorus is ubiquitous in cell biology--it was not limiting during the origin and early evolution of life.
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Recalibrating Rodinian rifting in the northwestern United States
Daniel T. Brennan; Zheng-Xiang Li; Kai Rankenburg; Noreen Evans; Paul K. Link ...
Abstract: A lack of precise age constraints for Neoproterozoic strata in the northwestern United States (Washington State), including the Buffalo Hump Formation (BHF), has resulted in conflicting interpretations of Rodinia amalgamation and breakup processes. Previous detrital zircon (DZ) studies identified a youngest ca. 1.1 Ga DZ age population in the BHF, interpreted to reflect mostly first-cycle sourcing of unidentified but proximal magmatic rocks intruded during the amalgamation of Rodinia at ca. 1.0 Ga. Alternatively, the ca. 1.1 Ga DZ population has been suggested to represent a distal source with deposition occurring during the early phases of Rodinia rifting, more than 250 m.y. after zircon crystallization. We combined conventional laser-ablation split-stream analyses of U-Pb/Lu-Hf isotopes in zircon with a method of rapid (8 s per spot) U-Pb analysis to evaluate these opposing models. Our study of ~2000 DZ grains from the BHF identified for the first time a minor (~1%) yet significant ca. 760 Ma population, which constrains the maximum depositional age. This new geochronology implies that the BHF records early rift deposition during the breakup of Rodinia and correlates with sedimentary rocks found in other late Tonian basins of southwestern Laurentia.
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Gulf of Aden spreading does not conform to triple-junction formation
Khalid A. Almalki; Peter G. Betts
Abstract: The Gulf of Aden represents an evolving example of a juvenile ocean system and is considered the most evolved rift arm of the Afar triple junction. We have undertaken analysis of recent coupled satellite and marine potential-field data to understand the first-order crustal architecture along the entire length of the gulf. Our interpretation suggests the Gulf of Aden has three domains with distinct free-air gravity and magnetic characteristics. These domains record a progression from active seafloor spreading in the eastern domain, through isolated and discontinuous spreading segments in the central domain, to active continental rifting in the western domain immediately adjacent to the Afar triple junction. Forward models suggest the presence of transitional crust, which displays linear magnetic stripe-like anomalies that bound oceanic stripes in the central domain and covering the majority of the western domain. Magnetic anomalies differ from magnetic stripes sensu stricto because they are discontinuous and cannot be correlated along the length of the gulf. Detection of northwest-southeast extension in the central domain based on magnetic stripe orientation is inconsistent with the regional northeast-southwest extension. Our observations reflect heterogeneous opening of the Gulf of Aden basins, in which spreading is migrating toward Afar as a series of isolated spreading segments, rather than initiating at the junction as proposed by classical platetectonic theory. This mechanism of ocean initiation is inconsistent with transtensional models that involve wholesale tearing of continental crust and contradicts conceptual models that rely on the Afar plume in initiating or driving the extension.
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Is the Eastern Denali fault still active?
Minhee Choi; David W. Eaton; Eva Enkelmann
Abstract: The Denali fault, a transcurrent fault system that extends from northwestern Canada across Alaska toward the Bering Sea, is partitioned into segments that exhibit variable levels of historical seismicity. A pair of earthquakes (M 6.2 and 6.3) on 1 May 2017, in proximity to the Eastern Denali fault (EDF), exhibited source mechanisms and stress conditions inconsistent with expectations for strike-slip fault activation. Precise relocation of ~1500 aftershocks revealed distinct fault strands that are oblique to the EDF. Calculated patterns of Coulomb stress show that the first earthquake likely triggered the second one. The EDF parallels the Fairweather transform, which separates the obliquely colliding Yakutat microplate from North America. In our model, inboard transfer of stress is deforming and shortening the mountainous region between the EDF and the Fairweather transform. This is supported by historical seismicity concentrated southwest of the EDF, suggesting that it now represents a structural boundary that controls regional deformation but is no longer an active fault.
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Wide-blocky veins explained by dependency of crystal growth rate on fracture surface type: Insights from phase-field modeling
Liene Sprueniece; Michael Späth; Janos L. Urai; Estibalitz Ukar; Michael Selzer ...
Abstract: Vein microstructures contain a wealth of information on coupled chemical and mechanical processes of fracturing, fluid transport, and crystal growth. Numerical simulations have been used for exploring the factors controlling the development of vein microstructures; however, they have not been quantitatively validated against natural veins. Here we combined phase-field modeling with microtextural analysis of previously unexplained wide-blocky calcite veins in natural limestone and of the fresh fracture surface in this limestone. Results show that the wide-blocky vein textures can only be reproduced if ~10%-20% of crystals grow faster than the rest. This fraction corresponds to the amount of transgranularly broken grains that were observed on the experimental fracture surfaces, which are dominantly intergranular. We hypothesize that transgranular fractures allow faster growth of vein minerals due to the lack of clay coatings and other nucleation discontinuities that are common along intergranular cracks. Our simulation results show remarkable similarity to the natural veins and reproduce the nonlinear relationship between vein crystal width and vein aperture. This allows accurate simulations of crystal growth processes and related permeability evolution in fractured rocks.
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