Theme: Starting Anew - The beginning of the first three long theme answers are synonyms of "New".
20A. Good reason to reopen a closed case : FRESH EVIDENCE
25A. Broad genre that began in Van Gogh's time : MODERN ART
49A. Original thought : NOVEL IDEA
54A. Title of Obama's 2009 Cairo speech, and what 20-, 25- and 49-Across have in common: : A NEW BEGINNING
Argyle here. Despite the theme title, I didn't find this to be FRESH, MODERN, NOVEL OR A NEW BEGINNING but it is a Monday after all.
Across:
1. Edible bow ties : PASTA
6. Largemouth fish : BASS and 16A. Shimmery sushi fish : OPAH
10. Places : PUTS
14. Twitter message : TWEET
15. Provoke : RILE
17. God of Islam : ALLAH
18. "Happy Gilmore" actor Sandler : ADAM
19. Song sung alone : SOLO
23. Intent : AIM
24. Former franc fraction : SOU. Alliteration in the clue.
31. Important purchase for a bride : DRESS
35. "__ Fideles": Christmas carol : ADESTE
36. Miami University state : OHIO
38. "Figured it out!" : "A-HA!"
39. Emeralds and diamonds : GEMS
40. Enlarge, as a road : WIDEN
42. Elvis __ Presley : ARON
43. "Who am __ judge?" : I TO. Unless your first name is Lance.
44. Unit of force : DYNE and 53A. Joule fraction : ERG. DYNE is defined as "the force required to accelerate a mass of one gram at a rate of one centimetre per second squared". A joule is equal to the energy expended in applying a force of one newton through a distance of one metre, or in passing an electric current of one ampere through a resistance of one ohm for one second. An ERG is the amount of work done by a force of one dyne exerted for a distance of one centimeter.(And I'm as confused as ever.)
45. Natural ability : TALENT
47. __ Rica : COSTA. Central America.
51. LAX or JFK, for American Airlines : HUB
61. Festive party : GALA
62. Tree house? : NEST
63. Spine-chilling : EERIE
66. Like Homer's "Iliad" : EPIC
67. Hudson Bay native : CREE
68. Furnish with more weapons : REARM
69. Eraser crumb site : DESK
70. Pay attention to : HEED
71. Eyelid woes : STYES
Down:
1. "Harper Valley __" : PTA. A country song written by Tom T. Hall and was a major hit single for country singer Jeannie C. Riley in 1968, and crossed over to the pop charts.
2. Hole-making tool : AWL
3. Ego : SELF
4. Rip : TEAR
5. Deep down inside : AT HEART
6. Rodeo bull : BRAHMA. Image. I don't think most bucking Brahmas are purebred but mixed parentage.
7. White House staffer : AIDE
8. Pole or Czech : SLAV
9. Big rigs : SEMIS
10. One who puts on airs : POSEUR. The French spelling gives POSER a more fraudulent touch.
11. Atop : UPON
12. Soft rock : TALC
13. Horse's footwear : SHOE
21. Muscular : SINEWY
22. Birdbrain, or extinct bird : DODO. Talk about a strange bird.
25. Hocus-pocus : MAGIC
26. Words that start many Keats titles : ODE TO
27. Band samples : DEMOS
28. Twisty road curve : ESS
29. Beverly Hills's __ Drive : RODEO
30. Alexander-Great link : THE
32. Like some seals : EARED
33. Glistened : SHONE
34. Rudolph's boss : SANTA. Ho-ho-ho! Rudolph has to be reminded who's boss sometimes, the poseur.
37. Apprentice : INTERN
41. B&B : INN. Bed and Breakfast
42. Baba who tangled with thieves : ALI. Yes he tangled with forty thieves but he wasn't one of them.
44. Slap on, as paint : DAUB. Not comfortable with this one. DAUB would indicate a small amount as opposed to "slap on" meaning an abundance of paint.
46. Casbah city : ALGIERS. Algeria's capital. Map.
48. Hitting sound : THWACK
50. Blew off steam : VENTED
52. Where second stringers sit : BENCH. First stringers are in the game.
54. Like fine wine : AGED
55. Scruff : NAPE
56. Yale alumni : ELIs
57. Richard of "Chicago" : GERE
58. Words of understanding : I SEE
59. Hair removal brand : NEET
60. Cloudy : GRAY
64. Anger : IRE
65. Letters after els : EMS
Argyle